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Centre of Gravity Q's
Rodeo - 29/12/11 at 07:19 PM

Hi All,Im a newb to the site ,cant see the newb intro area ,so thought I would dive straight in here....Apologies if this isnt the site etiquet!
Ive just completed and MSVA'd a new build Reliant engined trike with tube frame etc,but am now thinking I would like to build a 2 seater reverse trike .Im only at the toe dipping stage at the moment. I read with interest a few of the posts on here regarding CofG . I believe below G.L. is best according to the info on here. How do I work this out please?
Thanks

designer - 29/12/11 at 07:29 PM

quote:

I believe below G.L. is best


You better tell us what G.L. is!

Rodeo - 29/12/11 at 07:32 PM

lol! Ground Level

JoelP - 29/12/11 at 07:40 PM

think you mean roll centre re below ground level, i cant see how centre of mass can be below ground when all the mass is above ground!

Basically, CoG wants to be low, and in a reverse trike, id imagine quite forwards for stability.

Rodeo - 29/12/11 at 07:43 PM

Yes,sorry,I got the two things mixed up!

Chippy - 29/12/11 at 11:36 PM

With regard to roll centre calculation, this diagram may be of assistance. HTH Ray

[img] Roll centre diag
Roll centre diag [/img]

daviep - 29/12/11 at 11:42 PM

quote:
Originally posted by Chippy
With regard to roll centre calculation, this diagram may be of assistance. HTH Ray

[img] Roll centre diag
Roll centre diag [/img]


Can you explain for me please

Chippy - 29/12/11 at 11:50 PM

Ermmm! whats to explain, thought the diagram was self explanatory. Cheers Ray

speedstar - 30/12/11 at 01:16 AM

quote:
Originally posted by daviep
quote:
Originally posted by Chippy
With regard to roll centre calculation, this diagram may be of assistance. HTH Ray

[img] Roll centre diag
Roll centre diag [/img]


Can you explain for me please


Take a line from each of the wishbones straight out to the side of the car. They SHOULDN'T be parallel (thats bad) and this means they will intersect.

From the point they intersect, draw another line to the contact patch of the tyre.

Repeat for the opposite side. Where the two contact patch lines cross is your roll centre.

And it IS possible to have a roll centre below ground level, if your wishbone line intersection point is below ground level. You need really funky set up wishbones for this, and am yet to understand its advantages but some old school single seaters ran that.

Regarding CofG, an IDEAL situation would be to have it at exactly that same point as your roll centre. This means you get zero roll. This has other repercussions which can be detrimental (its never easy, is it?). If your CofG is i]below your roll centre, the car will actually roll the opposite way you would expect (weight transfer stays the same though).

Happy planning

Hector.Brocklebank - 30/12/11 at 03:41 AM

quote:
Originally posted by Chippy
Ermmm! whats to explain, thought the diagram was self explanatory. Cheers Ray


+1

snapper - 30/12/11 at 08:32 AM

Just to add a bit of info, on a 4 wheel vehicle if the roll centre is below GL ground level then the car is more inclined to slip sideways.
It was always considered best to have the roll center a little above ground even at full suspension compression.
Well that's what I was told.

Rodeo - 30/12/11 at 08:43 AM

Chippy and speedstar,thanks for the info

[Edited on 30/12/11 by Rodeo]

[Edited on 30/12/11 by Rodeo]

Halemini - 30/12/11 at 08:45 AM

quote:
Originally posted by speedstar
quote:
Originally posted by daviep
quote:
Originally posted by Chippy
With regard to roll centre calculation, this diagram may be of assistance. HTH Ray

[img] Roll centre diag
Roll centre diag [/img]


Can you explain for me please


Take a line from each of the wishbones straight out to the side of the car. They SHOULDN'T be parallel (thats bad) and this means they will intersect.

From the point they intersect, draw another line to the contact patch of the tyre.

Repeat for the opposite side. Where the two contact patch lines cross is your roll centre.

And it IS possible to have a roll centre below ground level, if your wishbone line intersection point is below ground level. You need really funky set up wishbones for this, and am yet to understand its advantages but some old school single seaters ran that.

Regarding CofG, an IDEAL situation would be to have it at exactly that same point as your roll centre. This means you get zero roll. This has other repercussions which can be detrimental (its never easy, is it?). If your CofG is i]below your roll centre, the car will actually roll the opposite way you would expect (weight transfer stays the same though).

Happy planning



Ah 3 wheelers! Sorry Speedstar, This is very close to my heart, so I must disagree here! The reason Morgans were so successful was because of their stability and speed! This comes from parallel cross tubes and sliding pillar front suspension!

Dc

MikeRJ - 30/12/11 at 10:19 AM

quote:
Originally posted by Halemini


Ah 3 wheelers! Sorry Speedstar, This is very close to my heart, so I must disagree here! The reason Morgans were so successful was because of their stability and speed! This comes from parallel cross tubes and sliding pillar front suspension!

Dc


I strongly suspect any positive handling attributes they had was in spite of, rather than because of the sliding pillar system. How many modern cars use the sliding pillar design?

randombloke - 30/12/11 at 11:22 AM

quote:
Originally posted by speedstar
quote:
Originally posted by daviep
quote:
Originally posted by Chippy
With regard to roll centre calculation, this diagram may be of assistance. HTH Ray

[img] Roll centre diag
Roll centre diag [/img]


Can you explain for me please


Take a line from each of the wishbones straight out to the side of the car. They SHOULDN'T be parallel (thats bad) and this means they will intersect.

From the point they intersect, draw another line to the contact patch of the tyre.

Repeat for the opposite side. Where the two contact patch lines cross is your roll centre.

And it IS possible to have a roll centre below ground level, if your wishbone line intersection point is below ground level. You need really funky set up wishbones for this, and am yet to understand its advantages but some old school single seaters ran that.

Regarding CofG, an IDEAL situation would be to have it at exactly that same point as your roll centre. This means you get zero roll. This has other repercussions which can be detrimental (its never easy, is it?). If your CofG is i]below your roll centre, the car will actually roll the opposite way you would expect (weight transfer stays the same though).

Happy planning


The CofG is difficult to calculate but can be estimated to be somewhere in the middle of the car. This is dictated by how the vehicle is built, where the components are mounted on the chassis and how high the chassis sits.

The vertical location of the roll centre dictates what the car does under load. With a roll centre above ground the car will squat under load whereas a roll centre below ground level will jack the car under load (Useful for drifting). Theoretically a roll centre at ground level would be ideal. However because the CofG will be above the ground, the car will want to roll, the further away the RC from the CofG the more it will want to roll. Having the RC at the same point as the CofG means no roll but will have other unwanted effects on suspension geometry so a compromise is required that puts the RC somewhere between the ground and the CofG.

What is also important is the distance from the instantaneous roll centre (Marked B on Chippy's diagram) to the wheel contact patch (A). This is called the Swing Axel Length (SAL), short SAL gives very good geometry in corners but creates too much camber in bump and droop, long SAL is the oppersite so you need to compromise based on what you will be using the vehicle for.

It is quite common practice to have the lower wishbone paralel to the ground. You can then decide your own compromise of SAL and RC height. Then you just need to choose your upright and everything else falls into place. Hopefully?

Suspension is all about compromise which is why Lotus developed active suspension that just hydraulically adjusted the geometry to suit every situation. And like everything that works well, it was promptly banned from F1

minitici - 30/12/11 at 11:54 AM

Found this interesting article which may be of assistance?
3 wheel stability article

speedstar - 30/12/11 at 12:54 PM

quote:
Originally posted by Halemini

Ah 3 wheelers! Sorry Speedstar, This is very close to my heart, so I must disagree here! The reason Morgans were so successful was because of their stability and speed! This comes from parallel cross tubes and sliding pillar front suspension!

Dc


What exactly do you disagree with?

Chippy - 30/12/11 at 11:48 PM

quote:
Originally posted by speedstar
And it IS possible to have a roll centre below ground level, if your wishbone line intersection point is below ground level. You need really funky set up wishbones for this, and am yet to understand its advantages but some old school single seaters ran that.
Happy planning


Whilst i agree it is possable to design a suspension system with the roll centre below ground, it is generaly considered to be bad practise, due in part to the very poor handling that this can give. It is generaly considered that the roll centre should be below the C of G and above ground level, as this configeration will give feel as to what the car is about to do, "ie" loose traction or break away. The further that the roll centre is from the C of G the more the car will lean in corners, adversly the closer to the C of G the less it will lean. To give an example the SS100 Jag had its roll centre almost exactly where the C of G occured, this car had no roll in corners at all, BUT, would swop ends without the slightest sign that it would do so, very unnerving for the driver, :-). Cheers Ray

Dusty - 1/1/12 at 03:08 AM

Er theoreticaly if the roll centre is above the CofG the car will lift an outside wheel. Is that possible?

I do have a serious question Chippy. The diagram says choose the height of your roll centre. (By using your diagram and adjusting the geo of the wishbones I can see how you do this) But How do you choose a roll centre. Whats the ideal height for the front roll centre in a sevenesque car?

Chippy - 1/1/12 at 02:05 PM

quote:
Originally posted by Dusty
Er theoreticaly if the roll centre is above the CofG the car will lift an outside wheel. Is that possible?
I do have a serious question Chippy. The diagram says choose the height of your roll centre. (By using your diagram and adjusting the geo of the wishbones I can see how you do this) But How do you choose a roll centre. Whats the ideal height for the front roll centre in a sevenesque car?


I know that if the RC is above the CofG then the car will lean into the bend, not sure about if it would lift an outside wheel though, :-).
I believe that this however does not give the same indication that you are aproching the limits of grip in the way that having the RC below the CofG does, not sure on that but apears logical to me.
With regards the RC heights for a seven type car, its pretty much up to the individual. Mine for instance is 3" at front and 4" at rear, but then with the weight of my engine etc. the CofG is going to be in the region of 15" or 16", (by my very rough calculations). I think though that generaly the sort of height used is 1" or 2" front and 3" or 4" rear, (rear should always be slightly higher than the front). HTH Ray

randombloke - 1/1/12 at 03:14 PM

quote:
Originally posted by Dusty
Er theoreticaly if the roll centre is above the CofG the car will lift an outside wheel. Is that possible?

I do have a serious question Chippy. The diagram says choose the height of your roll centre. (By using your diagram and adjusting the geo of the wishbones I can see how you do this) But How do you choose a roll centre. Whats the ideal height for the front roll centre in a sevenesque car?


There is no simple answer to roll centre height and in fact RCH is just a consequence of SAL and ride height. With the assumption that you have chosen your uprights and that their geometry is fixed the only way to change the RCH is changing the SAL and RH. Ride height changes the CofG, lower being better but lower effects ground clearance, SAL changes how dynamic geometry handles cornering versus bump and droop (See my post above). So it comes down to which compromise suits what you want from the car.

randombloke - 1/1/12 at 03:22 PM

quote:
Originally posted by Chippy I think though that generaly the sort of height used is 1" or 2" front and 3" or 4" rear, (rear should always be slightly higher than the front). HTH Ray


A general rule of thumb the rear should be higher than the front. Having different heights allows weight transfer from the front to the back and vice versa, which effects whether the car oversteers or understeers. The same can be achieved by using different spring rates.

Chippy - 1/1/12 at 10:25 PM

quote:
Originally posted by randombloke
There is no simple answer to roll centre height and in fact RCH is just a consequence of SAL and ride height. With the assumption that you have chosen your uprights and that their geometry is fixed the only way to change the RCH is changing the SAL and RH. Ride height changes the CofG, lower being better but lower effects ground clearance, SAL changes how dynamic geometry handles cornering versus bump and droop (See my post above). So it comes down to which compromise suits what you want from the car.


Whilst I agree with you in part, this only applies if you are using a chassis that somebody else has designed, ("ie" Locost, MK, MNR etc. etc.). Starting with a blank sheet of paper then you can make your own mind up as to where things will be, and where your pickup points will be located. This also gives you the opertunity to adjust the length of your wishbones to give the geommetry that you require. As my chassis was home designed I used the stick and pin principal to see where my designed RC and length of wishbones would place the wheels in regard to body roll, thereby getting the wheels to stay somewhere in the ball park that I calculated they would perform best. This has appeared to be well founded as the car is directionally stable, and only has a little overstear, (but that could be from a heavy right foot and 238 bhp). Cheers Ray

Dusty - 2/1/12 at 02:55 PM

Ok, that's clearer on roll centre but chucking in swing arm lengths is a bit mean. No definitive answers to any of these questions? I take it short medium and long all have their + and - points so I'm not going to ask that question.
Instead it looks as if the roll centre leaps up and down on suspension movement. What would you do to stop this? Having chosen where I want the little bleeder I would like it to stay there. At least I think I would unless there is some advantage in having it move.

Would it be more helpful to put together a list of things you definitely don't want to do with suspension?

[Edited on 2/1/12 by Dusty]

Chippy - 2/1/12 at 04:28 PM

Hi Dusty, well it's pretty much impossable to not have any movement of the RC, the trick is to try to get as little as possable. Using a long SAL is the easiest way to attain this, (reason for the length stated in my diagram), but you still need to play around with wishbone, WB, lengths to try to get the least movement. With mine I started with a lower WB of 18" and upper at 11", and after some experimentation ended up with 16" and 10", which gave me the least movement of the RC. Really if you want to get the full SP on suspension there are some very good books that go into it in detail, though some are very maths orientated, (not my strong suite), I found "Andre Jutes" book, "Building Special Cars", very informative, not only for suspension but the complete design of a car. Regards Ray

randombloke - 4/1/12 at 12:57 PM

Hello Dusty sorry no definite answer to RC height but as for what stops it moving when you’re driving the answer is most definitely stiff springs!

The problem with roll centre is that it doesn't just move up and down but side to side as well. This is why the SAL is important. IMHO the roll centre height isn’t important but keeping it from moving is.

My design process started with the things that I could define so I started with the wheel size, track width and chosen upright, with this I had the location of the outer suspension connection points. Next I chose the ride height, my car is for road with very occasional track so I decided on 100mm to keep it low but give reasonable clearance. Because I didn’t want to modify the steering rack from my donor the lower front chassis width was dictated by the rack, so I now had the lower inner pickup points. I decided to go for a medium length SAL of 1700mm because I wanted to keep the outer suspension geometry from gaining positive camber when cornering but also wanted the car to be stable in a straight line. Then all I had to do then was draw a line from the IRC to the upper outer pickup point. With this geometry the RCH is about 75mm but I only bothered what the height was once I had the rest of the geometry in place.

After drawing my design in Autodesk Inventor I needed tiny modification of wishbone length to stop the geometry gaining positive camber but now the outside wheel is at zero camber whilst the chassis rolls to nearly 4 degrees.