britishtrident
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| posted on 14/4/11 at 04:12 PM |
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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
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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blakep82
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| posted on 14/4/11 at 04:14 PM |
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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
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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Doctor Derek Doctors
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| posted on 14/4/11 at 04:18 PM |
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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.
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tilly819
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| posted on 14/4/11 at 04:43 PM |
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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]
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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v8kid
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| posted on 14/4/11 at 04:46 PM |
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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'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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britishtrident
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| posted on 14/4/11 at 05:04 PM |
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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.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Doctor Derek Doctors
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| posted on 14/4/11 at 07:04 PM |
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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.
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Doctor Derek Doctors
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| posted on 14/4/11 at 07:14 PM |
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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.
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tilly819
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| posted on 14/4/11 at 08:17 PM |
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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.
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
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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tilly819
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| posted on 15/4/11 at 11:38 AM |
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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]
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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Doctor Derek Doctors
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| posted on 15/4/11 at 01:13 PM |
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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.
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Doctor Derek Doctors
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| posted on 15/4/11 at 01:23 PM |
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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.
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ffrgtm
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| posted on 18/4/11 at 08:13 AM |
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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)
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Doctor Derek Doctors
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| posted on 18/4/11 at 09:36 AM |
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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.
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v8kid
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| posted on 18/4/11 at 10:51 AM |
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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
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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Doctor Derek Doctors
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| posted on 18/4/11 at 11:49 AM |
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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.
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