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Old 12 Jun 2008, 10:41 (Ref:2226777)   #1
proutyc
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Roll Centre - Good and Bad

I'm building a sports car to race in new Zealand and I'm hoping someone here is happy to share their knowledge / opinions here.

I've read plenty on this site about what is recommended by several re roll centre heights.

My question is.

Is it bad to have a roll centre that switches from above to below ground during some period of its suspension movement and what about when we add in roll (twist). The numbers that to me looked best were when I set the front RC at ground level, when I did this and dived the car 1 inch it obviously stays below ground, when I then add roll the roll centre height does not move and the side roll centre is only 5 inches. Is this good?

If so is it a problem that when the dive comes off (transition to power on / weight transfer to rear wheels) that the side RC is at 100 inches at static ride height but then when the ride height increases .1 inch the side RC moves 100 inches to the oposite side.

Am I heading down the right track or am I thinking about this wrong (I read that some recommend reducing level of vertical change of RC, but dont see how this can be done with my short wishbones)??
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Old 12 Jun 2008, 20:21 (Ref:2227299)   #2
Tony C
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You nearly gave me a heart attack! If the RC is moving 100 inches then you have some work to do. I have found that vertical movement is not significant because it doesn't move far, but lateral movement is critical to balance. Try to keep the lateral movement to a minimum (within reason) and you need to look at front and rear at the same time, They work together.
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Old 12 Jun 2008, 22:11 (Ref:2227366)   #3
Goran Malmberg
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Hi!
"Roll centre" is a tricky question to explain in a few words. We have another questioner a few treads back asking a similar Q. One problem is that we cant ask for what is best, if the car for example have aero down force, another roll centre philosophy is to be used. Apart from this we must divide Rc in to two parts. Rc 1, = intersection of the forcelines and Rc 2, = the spot the car actually roll about. As for Rc 1, all four wheels is having their own forcelines and therfore their own jacking effect=geometric weight transfer. If we want the car to have camber compensation, then the Ic distance will become shorter which has a number of side effects. I have used quite heavy camber compansation curve some year ago, and it worked well during cornering but braking an acceleration suffered from squat and dive camber.
I prefere having as little geometric weight transfer as possible, and that the sum of the jacking forces will stay as constant as possible during roll. This makes it easier for the amateur racer to tune the car on the track. Usually the outer pair of wheels will have +jacking and the inner pair - jacking.

The RC 2, is dependent on weight transfer, no matter where the intersection of the forcelines are located. Like taking 100p of the left side of the car and put it on the right side. In other words, if this Rc 2 stays pretty much in the centre of the car. So, if the jacking forces is keept low, the equation of roll becomes quite simple.
Regards
Goran
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Old 12 Jun 2008, 23:45 (Ref:2227425)   #4
proutyc
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Thanks for your comments.

Goran the car does have aero. The body is a modified Saker but runs front diffuser and also tunnels from mid point of wheelbase.

I'm pretty happy with the rear end but the front I'm worries about. What concerns me the sudden change of RC from above the ground to below the ground when in roll. What I notice is the the side RC jumps from right to left around the crossover of vertical RC. I tried plotting my Static RC around zero to see how it performs. numbers looked good other than both -.1 inch and +.1inch dive where I saw big swing in RC side and move from left to right. My head (As you can tell from these silly questions I'm no expert) suggest that it'll have a very bad balance in the car in transition mid corner (picking up the throttle)

My car has short arms similiar to what you would see on a radical (car fits 2 people) Do you think my prioirity should be to focus on the change in side RC or the vertical?

Thanks again Goran etc for thoughts
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Old 14 Jun 2008, 08:10 (Ref:2228262)   #5
Goran Malmberg
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Quote:
Originally Posted by proutyc
Thanks for your comments.

Goran the car does have aero. The body is a modified Saker but runs front diffuser and also tunnels from mid point of wheelbase.

I'm pretty happy with the rear end but the front I'm worries about. What concerns me the sudden change of RC from above the ground to below the ground when in roll. What I notice is the the side RC jumps from right to left around the crossover of vertical RC. I tried plotting my Static RC around zero to see how it performs. numbers looked good other than both -.1 inch and +.1inch dive where I saw big swing in RC side and move from left to right. My head (As you can tell from these silly questions I'm no expert) suggest that it'll have a very bad balance in the car in transition mid corner (picking up the throttle)

My car has short arms similiar to what you would see on a radical (car fits 2 people) Do you think my prioirity should be to focus on the change in side RC or the vertical?

Thanks again Goran etc for thoughts
First, I am now reffering to Rc as beeing the intersection of the forcelines.
There has been discussion about using higher Rc at the rear, and this is becouse we got more rear geometric weight transfer then, giving a faster weight transfer responce to balance turne in. This tells what happen if Rc height is altered during cornering, it affect the balance. For a downforce car I should recomend longer and more parallell A-arms, not to much camber compensation, (not to get downforce camber). Try to keep Rc height the same during both roll and heave. And if it moves sideways, it should be in direction to the inside wheel, but within the Tw. This makes for two advantages, 1 you keep geometric load transfer the same during spring movement and 2 you equlize the +- jacking forces. You need stiff spring settings along with it.
Goran
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