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How much Mechanical Trail

tilly819 - 13/4/11 at 10:09 PM

Hi all

Bit of background first.

Car details

weight 600kg
split 40F 60R

Kpi 0 deg
Scrub radius 26.65mm
King pin Offset 0mm
tire width 215mm

Front tire radius 313mm

Now then i am in the process of deciding how much caster to incorporate and this will determine the trail since im not using any king pin offset or (pin lead)
Obviously the tire radius is quite large so i do not need much caster to create the trail, and dont want to much caster to prevent weather cocking as someone kindly pointed out to me in another post. however i need some self centering, essentially enough to pass IVA
so the question is how much mechanical trail??

cheers tilly

blakep82 - 13/4/11 at 10:50 PM

have you read this book?
click

i still reckon you need KPI! kpi helps lift the side of the car when you turn the wheel (ie lifts the right side when you turn right) and the weight helps self centering. well, kpi and castor, but i don't think castor itself will. reckon you're onto plums without kpi, thats all i'm saying on it.

tilly819 - 13/4/11 at 11:34 PM

i understand you comment about kpi however the way i see it, kpi's conns outweigh its pros's, i dont want this to come across wrong as it is very easy to sound rude when typing but that is not my intention i assure you. i would like to point out that the lifting of the car due to its KPI is not a desirable characteristic.

the trail should be able to provide adequate self centering, think of a caster wheel on a trolly it has no KPI and no caster however it uses a King pin Lead to create mechanical trail which allows it to self center, i now this is a rather extreem example but it shows the concept.

cheers tilly

britishtrident - 14/4/11 at 05:58 AM

Castor & KPI are like Morcambe & Wise

KPI is the best source of steering self-centering.

The trail will create weather cocking even if the actual castor angle is low, how much will depend on the moment of th lateral force on the tyre contact about the virtual king pin.

Doctor Derek Doctors - 14/4/11 at 07:09 AM

Not meaning to sound rude but you will need some KPI, there is a reason that virtually every single road and race car ever designed has it.....

Also in general Mechanical trail is usually left as a product of the caster angle and wheel size, there are very few cars were the wheel hub centre is not on the centreline (between the pivots) of the upright.

One thing (amongst many others) that I have learnt when at uni' studying motorsport design and out in the real world designing race cars is that there are many things that seem like a good idea on paper or after reading a book (no KPI, Roll centres under ground, solid axles) but in real life they make horrible handling cars that don't win races or get sold to the public.

tilly819 - 14/4/11 at 08:32 AM

quote:
Originally posted by Doctor Derek Doctors
you will need some KPI, there is a reason that virtually every single road and race car ever designed has it.....

Also in general Mechanical trail is usually left as a product of the caster angle and wheel size, there are very few cars were the wheel hub centre is not on the centreline (between the pivots) of the upright.

could you possibly expand on your first point Ie What is the reason?? since the only positive i can see to KPI is that it helps with self centering but this is at the expence of some jacking when the wheel is steered.

I agree that mechanical trail is ushaly a result of caster and in my case it will be since as i said abive i will not be using any pin lead.

thanks
tilly

v8kid - 14/4/11 at 08:36 AM

DDD

Sorry to pick on you but you were the last contributor! If everything remains the same we will never progress. I guess that's OK if you only want to be one of the crowd but it does stifle things a bit don't you think?

Sure a lot of progressive ideas have turned out to be poo but by analysing the reasons we progress just as much as the ideas that work straight out of the box.

So why is no kpi such a bad idea? Come to that why is "the wheel hub centre not on the centreline (between the pivots) of the upright" a bad idea. Seems to me that Tilly is balancing pro's and cons to come up with a design and is being open about it. If there is a logical engineering reason why not I think we have a duty to spell it out!

For my part I don't know of one apart from it's jolly difficult to package all the stuff into the wheel and get zero kpi - I know I've tried and failed! I've seen Tilleys drawings and it seems to be possible due to a clever twist. But what is this overarching reason not to have zero kpi?

Cheers!

v8kid - 14/4/11 at 08:43 AM

Check out this link to a forum with quite a respected contributor on the subject

http://www.eng-tips.com/viewthread.cfm?qid=113181&page=8

Cheers!

britishtrident - 14/4/11 at 10:43 AM

KPI causing the front of the car to rise when the steering is turned a bit of a red herring because (ignoring effects of wheel width) castor causes the nose of the to fall as the steering is turned.

The relationship between camber castor and kpi and the weight/download carried by the front wheels needs careful consideration. A major factor involved that has to be considered are the tyres.

Using the Cortina Mk3+ as an example the tyres were much heavier loaded than on any car we are likely to design so nearly all of the steering self-centering and stability came from the self-aligning torque generated by the deflection caused in the radial tyres.

Getting a near zero scrub radius with wide wheels and a low KPI is very difficult because of wishbone clearance and steering lock considerations. Interestingly , production cars (Audi) with strut suspension can actually have negative scrub. Back in in the 1970s and 1980s racing saloon cars were built with a ridiculous ammounts of scrub radius because racers were using very wide wheels on suspensions (for example Capri) that were designed before tyre widths increased.
An increased scrub radius is not desirable but not a major problem in tarmac use.

Incidentally the only cars I can think of that had the stub axle axis offset from the king pin centreline to give mechanical trail were the Hillman Imp and the rear engined Fiats ----- both types very prone to weather cocking.

[Edited on 14/4/11 by britishtrident]

tilly819 - 14/4/11 at 01:29 PM

Right i was hoping that this wasnt going to turn into a you need KPI you dont need KPI but it has.
This discution has happen a couple of times already and since knowone has been able to give a definitive "you need KPI because" or "Zero KPI is a definate No NO because " then i see no reason why my choise to have no king pin inclination should be recived so badly..

I have given the basic geometery in my first post and what i was hoping for was some responces to establish some aproxermations of a suitable caster angle and resulting trail, now i have made a staring estimate of 5.5deg caster which results in 30mm mechanical trail. this seems reasonable to me since i am awere of someone who has build a similar weight car the the same aproximate weight distribution with about 25mm of trail and there steering is very light and has some self centering

thanks
tilly

Doctor Derek Doctors - 14/4/11 at 01:33 PM

quote:
Originally posted by v8kid
DDD

Sorry to pick on you but you were the last contributor! If everything remains the same we will never progress. I guess that's OK if you only want to be one of the crowd but it does stifle things a bit don't you think?

Sure a lot of progressive ideas have turned out to be poo but by analysing the reasons we progress just as much as the ideas that work straight out of the box.

So why is no kpi such a bad idea? Come to that why is "the wheel hub centre not on the centreline (between the pivots) of the upright" a bad idea. Seems to me that Tilly is balancing pro's and cons to come up with a design and is being open about it. If there is a logical engineering reason why not I think we have a duty to spell it out!

For my part I don't know of one apart from it's jolly difficult to package all the stuff into the wheel and get zero kpi - I know I've tried and failed! I've seen Tilleys drawings and it seems to be possible due to a clever twist. But what is this overarching reason not to have zero kpi?

Cheers!

Because it just doesn't work. I know that doesn't sound right but 50+ years of motorsport developement have always led to the same conclusion. I could spell it all out here but there are plenty of massive books on the subject as well as 24 F1 cars that demonstrate it better than I can in my short lunch brake....

....but think about this, a shopping trolley self centres the wheels as it goes forwards (from the froward motion), but there is no resistance to side loading except from the steering and as you increase the mechanical trail the lever ratio of this load is increased meaning it is harder to counteract this load. This load in corners will come from the centripetal acceleration (centrifugal force to the lamen) of the car rotating around the corner, thus the more mechanical trail you have the harder is will be to steer around a corner and it won't just be trying to self centre it will be trying to follow the load path thus steering in the opposite direction! It will also be a nightmare to resist side loading from side wind and bumps. With with KPI on the other hand the it self centres to the chassis via gravity and the centripetal acceleration acts directly on the centre of hub so it is not trying to steer the car, the increased resistance you feel is from the increased load on the outside wheel working harder to centre the wheel because of KPI.

To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are pushing them

When I was a wide eyed design student we read the books and weighed up the pro's and cons and decided that we knew better and had new ideas.... and for every idea there was always an example of why it didn't work. For instance placing the IRC under the ground seems like a great way to counteract the CoG and stop the car leaning in a corner, in theory it works, in reality it has been tried to death and it always causes odd handling characteristics.

As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.

v8kid - 14/4/11 at 01:36 PM

Interesting I thought the Hillman Imp has front swinging axles with a true king pin - basically a beam axle cut in half. Blessed if I can see how they get any spindle offset with this setup you don't have any pics do you I'm very interested?

Also can't find any reference to Imps "weather cocking" what do you mean by it - as I remember the handling was OK unless you got a bit overenthusiastic!

Cheers1

tilly819 - 14/4/11 at 01:45 PM

quote:
Originally posted by v8kid

Also can't find any reference to Imps "weather cocking" what do you mean by it - as I remember the handling was OK unless you got a bit overenthusiastic!

Cheers1

Weather cocking is a tendency to turn into wind when exposed to a sidewind /crosswind

tilly819 - 14/4/11 at 02:06 PM

quote:
Originally posted by Doctor Derek Doctors

quote:
Originally posted by v8kid
DDD

Sorry to pick on you but you were the last contributor! If everything remains the same we will never progress. I guess that's OK if you only want to be one of the crowd but it does stifle things a bit don't you think?

Sure a lot of progressive ideas have turned out to be poo but by analysing the reasons we progress just as much as the ideas that work straight out of the box.

So why is no kpi such a bad idea? Come to that why is "the wheel hub centre not on the centreline (between the pivots) of the upright" a bad idea. Seems to me that Tilly is balancing pro's and cons to come up with a design and is being open about it. If there is a logical engineering reason why not I think we have a duty to spell it out!

For my part I don't know of one apart from it's jolly difficult to package all the stuff into the wheel and get zero kpi - I know I've tried and failed! I've seen Tilleys drawings and it seems to be possible due to a clever twist. But what is this overarching reason not to have zero kpi?

Cheers!

Because it just doesn't work. I know that doesn't sound right but 50+ years of motorsport developement have always led to the same conclusion. I could spell it all out here but there are plenty of massive books on the subject as well as 24 F1 cars that demonstrate it better than I can in my short lunch brake....

....but think about this, a shopping trolley self centres the wheels as it goes forwards (from the froward motion), but there is no resistance to side loading except from the steering and as you increase the mechanical trail the lever ratio of this load is increased meaning it is harder to counteract this load. This load in corners will come from the centripetal acceleration (centrifugal force to the lamen) of the car rotating around the corner, thus the more mechanical trail you have the harder is will be to steer around a corner and it won't just be trying to self centre it will be trying to follow the load path thus steering in the opposite direction! It will also be a nightmare to resist side loading from side wind and bumps. With with KPI on the other hand the it self centres to the chassis via gravity and the centripetal acceleration acts directly on the centre of hub so it is not trying to steer the car, the increased resistance you feel is from the increased load on the outside wheel working harder to centre the wheel because of KPI.

To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are pushing them

When I was a wide eyed design student we read the books and weighed up the pro's and cons and decided that we knew better and had new ideas.... and for every idea there was always an example of why it didn't work. For instance placing the IRC under the ground seems like a great way to counteract the CoG and stop the car leaning in a corner, in theory it works, in reality it has been tried to death and it always causes odd handling characteristics.

As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.

Thanks thats a very good answer with some good technical input thank you.
I would like to respond by saying that in my oppiniun in a very lightweight road car which will not be experiancing massive lateral acceleration that i am willing so sactifice some steering weight for the beinfits of the camber change due to caster rarther than KPI. my philosophy has always been to keep the tyre vertical for the longest duration possible.
In a 600Kg car with a 40F 60R weight distribution exposed to a 1G lateral acceleration with a small amout of mechanical trail (aprox 30mm) the steering effort should be perfectly managable

tilly

Doctor Derek Doctors - 14/4/11 at 02:10 PM

quote:
Originally posted by tilly819

quote:
Originally posted by v8kid

Also can't find any reference to Imps "weather cocking" what do you mean by it - as I remember the handling was OK unless you got a bit overenthusiastic!

Cheers1

Weather cocking is a tendency to turn into wind when exposed to a sidewind /crosswind

Which supports what I was saying about mechanical trail trying to self centre to the outside forces applied and not to the chassis direction

blakep82 - 14/4/11 at 02:14 PM

quote:
Originally posted by tilly819
This discution has happen a couple of times already and since knowone has been able to give a definitive "you need KPI because" or "Zero KPI is a definate No NO because " then i see no reason why my choise to have no king pin inclination should be recived so badly..

sorry, you've received plenty of reasons why having no kpi is such a bad idea, i've even linked to a whole chapter in a book by des hamill about it and you choose to ignore it lol.

trail works on shopping trolleys, becasue, as mentioned above, the wheels go in the direction the trolley is being pushed in by the person pushing, this isn't self centering. if you push a trolley sideways, at no point will it try to go forwards. if you push it round a corner and let go, it will continue to go round a corner, and not straighten up.
trail is on trolleys an castor wheels to make sure that when you push, the wheels go the way you want them. imagine trying to push a trolley with no trail, and the wheels are pointing across you. it won't go anywhere.

what are these pros and cons of kpi you've identified? i saw you'd mentioned somewhere that it affects the camber of the wheel and thats bad. its not bad at all! if you have absolute zero body roll, then ok, but thats hardly ever going to happen unless all your suspension is solidly mounted with no shocks or springs. once you get a bit of body roll, you need a little bit of camber change to make sure the wheels stay flat on the ground. with no change to camber, the car will be trying to balance on the inside of one wheel and the outside of the other. you want maximum contact with the road for most grip, so you need a small change toward positive camber to correct this. if you don't you'll put additional stress on the ball joints

KPI and castor together is what gives you self centering! you need some built in, without any kpi at all, you'd need something like 15 degrees castor, which i'm not completely sure is practical.

do the top and bottom ball joints of the uprights make a perfect line to the centre of the tires? thats essentially what you're aiming for.

still, if you deside to continue without kpi, good luck lol. i'd love to be proved wrong, but i don't think i will be... think the handling will be great, when the cars parked in the garage

[Edited on 14/4/11 by blakep82]

Doctor Derek Doctors - 14/4/11 at 02:18 PM

quote:
Originally posted by tilly819

quote:
Originally posted by Doctor Derek Doctors

quote:
Originally posted by v8kid
DDD

Sorry to pick on you but you were the last contributor! If everything remains the same we will never progress. I guess that's OK if you only want to be one of the crowd but it does stifle things a bit don't you think?

Sure a lot of progressive ideas have turned out to be poo but by analysing the reasons we progress just as much as the ideas that work straight out of the box.

So why is no kpi such a bad idea? Come to that why is "the wheel hub centre not on the centreline (between the pivots) of the upright" a bad idea. Seems to me that Tilly is balancing pro's and cons to come up with a design and is being open about it. If there is a logical engineering reason why not I think we have a duty to spell it out!

For my part I don't know of one apart from it's jolly difficult to package all the stuff into the wheel and get zero kpi - I know I've tried and failed! I've seen Tilleys drawings and it seems to be possible due to a clever twist. But what is this overarching reason not to have zero kpi?

Cheers!

Because it just doesn't work. I know that doesn't sound right but 50+ years of motorsport developement have always led to the same conclusion. I could spell it all out here but there are plenty of massive books on the subject as well as 24 F1 cars that demonstrate it better than I can in my short lunch brake....

....but think about this, a shopping trolley self centres the wheels as it goes forwards (from the froward motion), but there is no resistance to side loading except from the steering and as you increase the mechanical trail the lever ratio of this load is increased meaning it is harder to counteract this load. This load in corners will come from the centripetal acceleration (centrifugal force to the lamen) of the car rotating around the corner, thus the more mechanical trail you have the harder is will be to steer around a corner and it won't just be trying to self centre it will be trying to follow the load path thus steering in the opposite direction! It will also be a nightmare to resist side loading from side wind and bumps. With with KPI on the other hand the it self centres to the chassis via gravity and the centripetal acceleration acts directly on the centre of hub so it is not trying to steer the car, the increased resistance you feel is from the increased load on the outside wheel working harder to centre the wheel because of KPI.

To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are pushing them

When I was a wide eyed design student we read the books and weighed up the pro's and cons and decided that we knew better and had new ideas.... and for every idea there was always an example of why it didn't work. For instance placing the IRC under the ground seems like a great way to counteract the CoG and stop the car leaning in a corner, in theory it works, in reality it has been tried to death and it always causes odd handling characteristics.

As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.

Thanks thats a very good answer with some good technical input thank you.
I would like to respond by saying that in my oppiniun in a very lightweight road car which will not be experiancing massive lateral acceleration that i am willing so sactifice some steering weight for the beinfits of the camber change due to caster rarther than KPI. my philosophy has always been to keep the tyre vertical for the longest duration possible.
In a 600Kg car with a 40F 60R weight distribution exposed to a 1G lateral acceleration with a small amout of mechanical trail (aprox 30mm) the steering effort should be perfectly managable

tilly

You seemed to have missed the point though that Mechanical trail does not give self centreing to the chassis, it gives self centreing to the outside forces applied. You will spend your entire time fighting to keep the thing in a straight line as every change in the road and the wind pushes the front of the car around with the mechanical trail helping it out and working against you. In a very light car this effect will be even worse as the caster won't be helping as much.

You can build it if you like but I bet you end up changing back as everyone else who has designed cars is of the opposite opinion.

v8kid - 14/4/11 at 02:33 PM

quote:
Originally posted by Doctor Derek Doctors

To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are pushing them

As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.

Nice summary thanks. Easy to see the argument about KPI when you put it that way and I can see the point now.

Can you do the same for "there are very few cars were the wheel hub centre is not on the centreline (between the pivots) of the upright" please? Certainly thats true but are there could be considerations other than handling ones that cause it. Why is it a no no after all it seems to comply with your points already made.

quote:
Originally posted by Doctor Derek Doctors

As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.

I've lost the plot a bit here that was the very point I was trying to make - unless we try something different there will be no progress - anyhow no matter

Cheers!

blakep82 - 14/4/11 at 02:36 PM

quote:
Originally posted by v8kid

quote:
Originally posted by Doctor Derek Doctors

As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.

I've lost the plot a bit here that was the very point I was trying to make - unless we try something different there will be no progress - anyhow no matter

Cheers!

i think he means these are designers at the top of their game, and if 0 KPI was a good idea, they would be doing it but they don't. doing it now would be doing something different, but not progress

Doctor Derek Doctors - 14/4/11 at 02:45 PM

quote:
Originally posted by v8kid

quote:
Originally posted by Doctor Derek Doctors

To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are pushing them

As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.

Nice summary thanks. Easy to see the argument about KPI when you put it that way and I can see the point now.

Can you do the same for "there are very few cars were the wheel hub centre is not on the centreline (between the pivots) of the upright" please? Certainly thats true but are there could be considerations other than handling ones that cause it. Why is it a no no after all it seems to comply with your points already made.

quote:
Originally posted by Doctor Derek Doctors

As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.

I've lost the plot a bit here that was the very point I was trying to make - unless we try something different there will be no progress - anyhow no matter

Cheers!

Well the point about Brawn (hero) and Newey (legend) are that they are always trying to do something new but they still use double wishbone suspension with KPI, Caster and Camber like most other race cars, just alot more refined and optimised..... because it is the best way. But yes sorry it may have come accross as a little rude.

As for the hub off centreline, this I know less about but it seems from experience (and design) that if you use a calculated amount of Caster and KPI the mechanical trail that leaving the hub on the centreline gives is just about right for handling, grip and feel, I think its just one of those lucky coincidences. Especially when you consider that its no harder to make an upright with the hub off centre that know one does it.

Suspension design is bloody hard, really really hard in fact and the best place to start is by looking at what works and start with that.

v8kid - 14/4/11 at 02:53 PM

Ta - Good series of posts here it has clarified my thoughts quite a bit.

Cheers!

britishtrident - 14/4/11 at 03:15 PM

With a light front end you need a fair ammount of caster and kpi if you dial in caster alone you end up with a silly ammount of negative camber at full lock.

If you look at the Lotus Elise forums you will find that although the Elise comes with a fair ammount of both caster and kpi as standard owners are adjusting the suspension to give more kpi and caster to get sharper turn in.

As to Hillman Imp front ends the swivel hub casting was arranged so the king pin passed about 25mmmm ahead of the stub axle centre, this had the advantage of allowing room for the cable drive for the speedo to pass through the stub axle.

Imps used a lot of both caster and KPI, istr the caster was about 11 degrees and on a racer the kpi about 7-9 degrees depending on camber, this combination would result in too much negative camber on hairpins so quite often the leading wishbone pivot point was lowered 25mm giving anti-dive and reducing caster.

Imp handling when lowered was nothing short of fantastic BUT the driving experience in motorway cross winds more like tacking in a sailing boat than driving most owners always carried something heavy in thre friont boot to ballast the front and move the cg forward of the centre of pressure.

[Edited on 14/4/11 by britishtrident]

tilly819 - 14/4/11 at 03:58 PM

OK

thank you all for your input it has been invaluable as i am no expert.

Just to clear a few points up:

I can see the value of some KPI to prevent the effects on the steering from crosswinds and road defects.

A large amount of negative camber on the outside wheel under full lock dosnt bother me at all since under normal driving conditions this scinario will never happen, only when doing low speed opperation such as parking will it become relevelnt.

I am not trying to achive centerpoint steering and would like a small amount of scrub radius.

I have indeed read the book a link was provided to along with many others ( the problem i find is that information in one book contradicts another and finding out which is correct/best can be difficult)

I am coming round to your way of thinking that a small amount of KPI should be included to prevent the weather cocking and aid self centering. the explanation by DDD was very good. I am still however reluctant to include a large KPI angle due to its adverse camber effects on the outside wheel, I am thinking after some very quick maths that an angle of about 3deg would not sacrifice to much outer wheel control, how does this sound?? this would also bring my scrub radius down to the 10mm range

thanks again
tilly

v8kid - 14/4/11 at 04:06 PM

Sounds like compromise is the order of the day! Would that take your caster up from 7 to around 9 then? Back to the original question then trail will be around 45 -50mm without stub axle lead.

[Edited on 14-4-11 by v8kid]

tilly819 - 14/4/11 at 04:11 PM

I hate compromise i like to acchive perfetion with evrything but this is one of those things that it just dosnt seem to to posible there has to be a compromise like you say, i had actualy considered bringing the caster down to 5.5 with 0 kpi but with 3 kpi i think 6-7 caster might work il have to have a play with it late when i get home

tilly
compromise GRRRRR why carnt it be perfect lol

britishtrident - 14/4/11 at 04:12 PM

Lotus Elise ( some variation on specs found on the web)

Front suspension
roll centre height 30mm
travel 50mm bump / 60 mm rebound
camber gain in bump 0.31 degrees per inch
frequency 90cpm
*******KPI 12.0 degrees**********
Castor 4.25 degrees
Trail 4mm
Scrub radius 10.5mm

blakep82 - 14/4/11 at 04:14 PM

from most examples i've seen, when you add the castor and kpi angles together you get 15 (or very close) so if you go for 3 degrees kpi, expect about 12 degrees castor. thats not an exact rule, but will be about that

Doctor Derek Doctors - 14/4/11 at 04:18 PM

quote:
Originally posted by tilly819
I am still however reluctant to include a large KPI angle due to its adverse camber effects on the outside wheel

I am struggling to understand this statement.

You will need a camber increase in cornering, this goes without saying. If you are designing the car then it is a simple job to design the inner wishbone mounts in a position to give the correct camber change for any amount of KPI. You could even have 8� of KPI and have no camber change at all.
Off the top of my head (I'm not the suspension leader and dont have the CAD at my fingertips) our RGB Race car will have about 7-9� of KPI, my personal race car has Cortina uprights with only 4.25� of KPI which is the 1st thing that will be changed to give more.

tilly819 - 14/4/11 at 04:43 PM

quote:
Originally posted by Doctor Derek Doctors

quote:
Originally posted by tilly819
I am still however reluctant to include a large KPI angle due to its adverse camber effects on the outside wheel

I am struggling to understand this statement.

You will need a camber increase in cornering, this goes without saying. If you are designing the car then it is a simple job to design the inner wishbone mounts in a position to give the correct camber change for any amount of KPI. You could even have 8� of KPI and have no camber change at all.
Off the top of my head (I'm not the suspension leader and dont have the CAD at my fingertips) our RGB Race car will have about 7-9� of KPI, my personal race car has Cortina uprights with only 4.25� of KPI which is the 1st thing that will be changed to give more.

When the wheel is steered KPI pushes the outside wheel in a positive camber direction *BAD*
I agree that this can be countered by inter pivot point position to create a negative camber movement however this makes the camber curve more "curved" under bump conditions or more relevantly in the case of the front suspension the breaking condition the wheel will be loosing camber (going negative) due to suspension compression thus making the tyre no longer flat to the road and reducing available breaking grip.

cornering grip is a trade off against acceleration/breaking grip or at least thats the conclusion i have come to

also you will more than lightly find that if you design a system that has a KPI of 8 deg as you say and no camber change due to the inboard points the lateral scrub would be very large

[Edited on 14/4/11 by tilly819]

v8kid - 14/4/11 at 04:46 PM

No DDD to do this you end up with a swing arm length so small it makes the Imp look positively modern! Also the scrub introduced by the track change becomes very significant.

Compromises again but that's too far in the opposite direction for me!

Cheers!

Oops crossed posts again

[Edited on 14-4-11 by v8kid]

britishtrident - 14/4/11 at 05:04 PM

Short virtual swing axle is no problem provided the roll centre is low enough and the roll stiffness is mainly generated from the oposite end of the vehicle.

Doctor Derek Doctors - 14/4/11 at 07:04 PM

quote:
Originally posted by v8kid
No DDD to do this you end up with a swing arm length so small it makes the Imp look positively modern! Also the scrub introduced by the track change becomes very significant.

Compromises again but that's too far in the opposite direction for me!

Cheers!

Oops crossed posts again

[Edited on 14-4-11 by v8kid]

You don't need a short swing arm length to have zero-camber change in bump. Its a product of relative wishbone length and mounting point height., doesn't matter if they're 30mm 30cm or 30m long as long as the ratios are correct.

Doctor Derek Doctors - 14/4/11 at 07:14 PM

quote:
Originally posted by tilly819

quote:
Originally posted by Doctor Derek Doctors

quote:
Originally posted by tilly819
I am still however reluctant to include a large KPI angle due to its adverse camber effects on the outside wheel

I am struggling to understand this statement.

You will need a camber increase in cornering, this goes without saying. If you are designing the car then it is a simple job to design the inner wishbone mounts in a position to give the correct camber change for any amount of KPI. You could even have 8� of KPI and have no camber change at all.
Off the top of my head (I'm not the suspension leader and dont have the CAD at my fingertips) our RGB Race car will have about 7-9� of KPI, my personal race car has Cortina uprights with only 4.25� of KPI which is the 1st thing that will be changed to give more.

When the wheel is steered KPI pushes the outside wheel in a positive camber direction *BAD*
I agree that this can be countered by inter pivot point position to create a negative camber movement however this makes the camber curve more "curved" under bump conditions or more relevantly in the case of the front suspension the breaking condition the wheel will be loosing camber (going negative) due to suspension compression thus making the tyre no longer flat to the road and reducing available breaking grip.

cornering grip is a trade off against acceleration/breaking grip or at least thats the conclusion i have come to

also you will more than lightly find that if you design a system that has a KPI of 8 deg as you say and no camber change due to the inboard points the lateral scrub would be very large

[Edited on 14/4/11 by tilly819]

You want a change towards positive camber on the outside wheel.

You seem to have become completely hung up on one minor part of the system that has been studied and tested to death by people over a 60 year period with names like Chapman and Brawn and they have come to pretty much the same conclusion every time.

I'm starting to think that a little knowledge is a dangerous thing here, I've been running and designing race cars for years now, I have a degree in M'sport engineering, design my own race cars and work in F1 but I still appreciate that I don't know better than all of that experience, computer simulation and race data.

I've said my peace, design a KPI less system if you like and then have fun re-designing it later.

tilly819 - 14/4/11 at 08:17 PM

From this i gather you did not bother reading my last couple of posts in which i clearly stated that it was going to be nessasery to incorperate some KPI on YOUR recomendation. Unlike yourself i do not have a motorsport engineering degree or years of experiance, and you may well think i am stupid for my ideas and my efforts, however the stupid person is the one who dosnt ask for help and dosnt ask the potentualy stupid questions and just blindly goes ahead anyway, my aim it to further my knolage on a very complex subject and i am bound to get things wrong however i do not apretate your what apears to be arogant and sarcastic responce in this post. I am a very open minded person and like to consider all of the options to be able to engineer the best solution to the problem. As you yourself said this is a bloody hard subject, so please dont shoot me down for trying. I do not meen for this to come accross as a personal attack but it will probly read that way.
I more than welcome your input as good informed information is dificult to get hold of, however not being an expert on the subject i sometimes find it dificult to understand and some things counter intuertive. certainly to me wanting the outside wheel to go into positive camber in a corner seems like the opposite of what i would think since it would be running on the outside edge of the tyre?

sorry of this is abit like a rant but im putting alot of time and effort into this and its not easy

Tilly

tilly819 - 15/4/11 at 11:38 AM

Having had a nights sleep and re-read the entire thread threw this morning i realise that i may have come accross a little strong in my last post to DDD so i am sorry of this cause any offence it was not ment to. All of your input has been greatfully recived and i can now see the value of the king pin inclination.

Put in simple terms I was wrong simple as that.

Now i after doing some more thinking (dangerous i now) i can see no problem designing a system for KPI and caster. this still however leaves the question of trail. In an eirler post by british trident the lotus elise geo was listed now this has a trail of 10mm therefore with a caster 4.25deg and a tyre radius of APROX 290mm (about 195/50/15 or 205/50/15) this must use a king pin lead of about 10mm to accive that trail of 10mm or ease the trail would be about 21mm this seems like a good idea, what are people thoughts????

thanks tilly

[Edited on 15/4/11 by tilly819]

Doctor Derek Doctors - 15/4/11 at 01:13 PM

quote:
Originally posted by tilly819
Having had a nights sleep and re-read the entire thread threw this morning i realise that i may have come accross a little strong in my last post to DDD so i am sorry of this cause any offence it was not ment to. All of your input has been greatfully recived and i can now see the value of the king pin inclination.

Put in simple terms I was wrong simple as that.

Now i after doing some more thinking (dangerous i now) i can see no problem designing a system for KPI and caster. this still however leaves the question of trail. In an eirler post by british trident the lotus elise geo was listed now this has a trail of 10mm therefore with a caster 4.25deg and a tyre radius of APROX 290mm (about 195/50/15 or 205/50/15) this must use a king pin lead of about 10mm to accive that trail of 10mm or ease the trail would be about 21mm this seems like a good idea, what are people thoughts????

thanks tilly

[Edited on 15/4/11 by tilly819]

No Probs.

Without trying to sound patronising can I pass on a brilliant piece of advice I recieved from a very knowledgable race car engineer:

"Find out what currently works, copy it and then build plenty of adjustment into the system"

This way you take a baseline that you know is already correct and you can then tweak and optimise it from there.

Research the data and geo' from as many similar cars as possible, spread sheet it all up and then compare it to the theory and work out an 'average' you will spot patterns and direction of what works best startiing to emerge. If you then build to those specs but have a good range of adjustment on all of you parts (not to hard to do) you can then test, tinker and tune to your hearts content knowing that you haven't started a million miles away from whats correct.

Doctor Derek Doctors - 15/4/11 at 01:23 PM

Oh and as for the mechanical trail, I would just let it be a product of the caster angle and tyre size, it seems to be one of those happy coincidences that doing it that way seems to work just fine for virtually all types of car.

One thing you also need to take into account is 'rake' you may end up with X� of Caster but with 2� of 'rake' in the car the caster will in reality be X-2� and so your true mechanical trail will be reduced.

If you figure 2� of rake into your system it reduces the trail to 10.8mm anyway, this is why most publications recomending a caster angle higher than you need to compensate for the rake of the chassis.

Personally I would start with at least 7� of caster (reduced to 5-6� with rake) giving a trail of.... 25.4mm.... ooooh that a nice number for an OCD sufferer like myself.

ffrgtm - 18/4/11 at 08:13 AM

quote:
Originally posted by v8kid
Check out this link to a forum with quite a respected contributor on the subject

http://www.eng-tips.com/viewthread.cfm?qid=113181&page=8

Cheers!

Please note that in your link he is talking about a strut type suspension. In a strut type suspension, the higher SAI (steering axis inclination... believe this is what you are calling kpi) the more the wheels camber OUT as you turn the steering wheel. Basically the opposite effect of caster (in this regard).

I came across this issue when noticing all of these people in their mcpherson strut cars (stis, evos, m3s) would throw adjustable camber plates on the top of the struts. They would crank them way in and then not understand why their cars actually had worse camber in corners (looking at photos people had taken ect.). I came to the conclusion that you can not use camber plates to adjust strut type cars and must use the eccentric bolts unless you want to lose your dynamic (in the sense of steering angle) camber.

I hope you aren't trying to put a mcstrut type suspension on a locost though.... should really use sla (double a-arm)

Doctor Derek Doctors - 18/4/11 at 09:36 AM

quote:
Originally posted by ffrgtm

quote:
Originally posted by v8kid
Check out this link to a forum with quite a respected contributor on the subject

http://www.eng-tips.com/viewthread.cfm?qid=113181&page=8

Cheers!

Please note that in your link he is talking about a strut type suspension. In a strut type suspension, the higher SAI (steering axis inclination... believe this is what you are calling kpi) the more the wheels camber OUT as you turn the steering wheel. Basically the opposite effect of caster (in this regard).

I came across this issue when noticing all of these people in their mcpherson strut cars (stis, evos, m3s) would throw adjustable camber plates on the top of the struts. They would crank them way in and then not understand why their cars actually had worse camber in corners (looking at photos people had taken ect.). I came to the conclusion that you can not use camber plates to adjust strut type cars and must use the eccentric bolts unless you want to lose your dynamic (in the sense of steering angle) camber.

I hope you aren't trying to put a mcstrut type suspension on a locost though.... should really use sla (double a-arm)

The other problem with Macphearson Struts is what happnes when you lower the car (as most EVO/M3/STi type cars are). Macphearson struts need the lower wishbone to be drooped downwards so that the track widens as the suspension compresses, this increases the camber (desired affect) as you lower the car though this effect is decreased and if you pass the horizontal point of the wishbone it actually starts to decrease the camber as the suspension copresses which is a bad thing.

Many MPS cars though don't have any adjustment between the hub and the strut or on the inner end of the wishbone, the MK2 Golf has a nice set up using a slotted bolt hole between the upright and the strut for camber adjustment.

I'm pretty certian Tilly won't be using MPS suspension though.

v8kid - 18/4/11 at 10:51 AM

Hi chaps,

McP is really the devil in disguise

However if you read the discussion in the link it is generic and not specific to struts

Have to go only 4 days to my dissertation cut off date!

Cheers

Doctor Derek Doctors - 18/4/11 at 11:49 AM

quote:
Originally posted by v8kid
Hi chaps,

McP is really the devil in disguise

Cheers

MCP while not as good as Double Wishbone is good for the demands of a road car and a well designed and set-up MCP can lead to great handling, some of the best handling FWD and RWD cars of all time have been MCP. VW Corrado, Integra Type R, BMW E30 M3, Porsche 968 etc etc.

Its also a neet little package, shame it just lacks the geo' control and tunability of Double Wishbone.