Roll Centre

[Risking]

Loffe are those your simulations or did you have them drawn up for you??

A few of your values that have big effects on RC's are substancially different to original.

Did you end up correcting the upper control arm angle or spacing the lower arm only?

[loffe]

Hi

I made this calculations today.

To be honest i have played a little with the data (just like the meteorologists ) and i can have put

some of the data wrong when i have changed back, i havent controlled that yet, therefore i wrote

close to original setup. Which data do you think is wrong?

On my car i have

RC distance with camber increased under spindle,

i have raised upper inner linkmount 25mm.

i have shortened upper link.

i have shortened hight on spindlearm.

trackwith is 1618mm

camber 4º

caster 7º (somewher close to)

I have calculations on that to, but on my other computor.

In this ridehight i have 6cm workingdistance inwards and 4cm outwards on coilover.

I´m not an expert, i have this as my hoby, but i would like to get better.

/christofer

Edited August 9, 2010 by loffe

[loffe]

Youre right Risking, it was bad data, i have found my original data now and see some foults.

Except from those faults..

One measure that i dont have is original hight on inner lower mount from ground, does anybody know hight?

Do you think this look better?

GTR org setup

Roll

Edited August 9, 2010 by loffe

[Risking]

Stuffing around with measurments is the best way to learn that's for sure.

Have you used susprog or just casim??

Is that last simulation your actual Car and how it's been built, is the center of gravity you have inserted the actual figure from weighing the vehicle or a calculated input??

In my oppinion for a gtr with a CG of 300mm a 14mm roll center is far too low. The roll couple is alot larger than a factory gtr.

Work backwards to get the roll center up around the 170mm mark front and 260-270 rear. Depending on your cars actual COG of course. Unless you have lowered the weight into the chassis the center of gravity will typically lower in acordance to the chassis height you lower the suspension by.

Also with your roll simulations remember you are only simulating the front and not factoring in the horizontal COG along

the car. Your asumimg it's a 50/50 weight split which it probably is not. Rear roll center and roll couple also effect the

roll you will see at the front like a post from above mentioned.

I don't think casim can do such simulations??

By bringing the RC closer to the CG you can control the body roll without stiffer springs and roll bars which can limit traction.

Edited August 10, 2010 by Risking

[loffe]

Its just casim.

The CG is just set from a quess.

The simulation i did with original spec, problem is i don´t know factory car hight, i maesured on my car and it wasn´t original hight from start, so if i make it higher than on calculation it raises the Roll centre. Good tip, i can try and see what happens if i goo back, so i try to get 80mm RC. CG of 170 sounds very low?

But maybee it isnt so important, i don´t have a factory spec car any way, it was just to give a view in what happens in an easy to see way, for those

who doesn´t can visualize that in mind.

I do the calculations just to compare, what happens, i have not made it together with rear end, that i can not do, have to try it on road instead.

Edited August 10, 2010 by loffe

[loffe]

This is how my car looks like today Today setup RC 33

i´m thinking of raising inner lower link 20mm, then it would be like this

Next step? RC 96

What do you think?

Edited August 10, 2010 by loffe

[Risking]

Ah hu now I understand what you have done I think....

You don't want an 80mm odd RC you want a 170mm odd RC. The closer the roll center is to the CG the less roll you will have with the same springs and bars. But if it's too close you will get ubdersteer and oversteer.

It's a balncing act that requires experiabce to get right there is no formula or realationship between the two that I know of. It's a drivers personal taste that determines what happens.

I'd recommend for your own good you have your car corner weighted and find a set of scales that can quickly and easily calculate an accurate vertical center of gravity for you to use.

Like you said what you have done is a good illustration but If you want to learn about it more you need the right info to begin with. CG is a very important input when playing with dynamic geometry like you are.

[loffe]

do you think its a good way, if i make the inner lower adjustible, so i can adjust it maybee from RC80-RC180.

At other places i have read it is very extreme to have such high RC that is so close to CG

I think you are totaly right, i should find out what my CG really is.

here is an interesting comparison ive found on another cite:

Now we may have 4 basic positions:

1) Roll center height (from now on RCH) = center of gravity (CoG)

No pivoting here, it means that there is no roll. Its like trying to spin a door applying force in the hinge. The car is turning, lateral force is applied, but there is no roll. Hence, all the force is "catched" by the wishbones. This makes the car as hard as a rock, as spring/dampers doesnt work here. Its good for nothing.

2) RCH between CoG and the ground. Depending the percentage of that height you distribute how much force goes through the wishbones and how much through the spring/dampers. The range between 15% and 30% of RCH compared to CoG is the most common place to locate it in many racing cars.

3) RCH = ground height. All the lateral forces passes from the chasis to the wheels throught dampers/springs, so virtually the wishbones makes no force under pure lateral load condition.

4) RCH below ground. More force than whats actually transferred passes through the spring/dampers, so that the wishbones is loaded unders "a negative" force. This means outer top wishbone for example is not under compresion, but under traction.

This is the case of tourism racing cars that have to maintain the suspension geometry from the original street car when you reduce their ride height, there you have to find the best compromise between what you gain from aero and reduced CoG height and what you loose for poor suspension geometry. Here you dont have jacking, but the contrary. Also it is the case of heavily "tuned" street cars... puaj!!!... .

Edited August 10, 2010 by loffe

[loffe]

i´m having it cornerweighted when i think i´m close to what i think is final solution

[Risking]

Ummm the first two points are exactly what I posted earlier in this thread, I left RC at ground level and underground level out.

How are you intending to work out the "ideal" roll center height when you don't know the the vertical center of gravity???

I personally don't always agree with the 15-30% guidline for roll center heights. I have my reasons which are far to indepth for this thread. there is no perfect RC height nor is there a perfect formula to calculate one.

I'm still not sure how you plan to work out your final solution without all the real information you need not substituted guess'??

You need to know what the car weighs! Find someone with good corner weights and they can also tell you a very accurate vertical and horizontal CG to save you alot of calcs.

You won't get that range of adjustment out of just the lower arm without some massive control arm angles. You must use both arms otherwise your going to run into all types of trouble with camber gains, bump steer etc etc.

I never said anything about an extremely high RC that's close to the CG......

I said the closer it is the less roll. 170mm is a recommended starting point from my experiabce and what I know is the

typical CG of an r32 GTR, it's far from extremely high. as opposed to your current 14mm roll center which will roll like a boat.

If you plan to use your work on a real car like I think you plan to then you really need the right information and to incorporate all aspects of geometry not just roll centers.

[loffe]

ok, thanks for your input, i will try to find some way to figure out the CG and go from there.

[Risking]

Have it corner weighted, and make sure the corner weights are capable of giving a CG figure.

My intercomp weights are capable of giving both vertical and horizontal.

I wish you bes of luck with it. Good to see Simone playing around with the "black art"