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Author Topic:   Front and Rear Roll Centers
irace74
Dirt Forum Racer

Total posts: 80
posted March 28, 2005 07:38 AM  
Do the front and Rear roll centers need to be at the same elevation in a modified? If not what kind of front to rear roll center elevation difference is desirable?

Should the front roll center be to the right of center or left of center? ( the rear is almost universally right of center).


wfoondirt
Dirt Forum Champ
Total posts: 505
posted March 28, 2005 12:11 PM  
99.9% of any kind of car like the rear roll center higher. Exactly how much varies but 4-6" difference will get you close to start.

I like to keep the rc's centered and stable. Alot of people don't aggree with this but it works for me and I think it makes the car more predictable. On an asymetric car the rc's aren't as other aspects of front end geometry.

Whats your reasoning behind the rear rc being universally right of center?

NJantz
Dirt Forum Champ
Total posts: 755
posted March 28, 2005 12:30 PM  
A lot of text books tell you that the roll center heights should be the same but I've also found what WFO said to be true.

irace74
Dirt Forum Racer

Total posts: 80
posted March 28, 2005 02:19 PM  
I say that it is universally to the right based on using a J-bar type panhard bar. From the information provided by Mark Bush at afco, the roll rear roll center is generally located in a window 2-8" to the right of the pinion when considering a J-bar. Like you, my understanding of rear roll center location was based in long panhard par theory where the rear roll center is located 1/2 way between locating points. I have found no reference or definitive information on rear roll center location when a j-bar is employed. I can only find statements which state " the rear roll center is located closer to the stiffer spring stc. etc." Mark, in his Racewise workshop shows a location to the right of the pinion and states that it is somewhere in that window.

His generalized comment helps but, why? Is there a definitive way to calculate it? If it is closer to the pinion then it is even higher then I think. If you raise the angle on the panhard bar then are you not raising the roll center? Practical history has shown that raising the bar on the chassis has tightened the car. Was this due to increased rear bite, left side shift of the rear end or lowered roll center? Probably a combination but, without a way to calculate the rear roll center who knows. Lots of questions, even more confusion.

The front roll center is very straightforward. The car I have has the front roll center located very low, at grade or 1" below depending on measurement accuracy. My thought is that this point should be 3-4" above grade. I believe this is consistant with your thoughts.

I think that the extremely low roll center in the front of the car makes the car inconsistant in that the front is biting the track too differently then the rear resulting in incinsistant handling. What are your thoughts?

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted March 28, 2005 04:42 PM  
3-4" above ground is what I normally hear for the front RC. I recently measured mine and found it to be almost 2" below ground. The car was VERY inconsistent. I relocated it to 3.5" above ground and the car handles alot better. I've been told 4" above ground and 2-4" to the left on the front. But I read an article in Circle track magazine and my understanding was that the height was more important than the left to right location. As long as it fell between the front tires.

The rear roll center I have seen explained plenty different ways. I have heard 1/2 way between the mounting points even on the J-bars. So I'm not sure on that but it makes sense I guess. I've heard the deal about the springs affecting it but I would personally think that it wouldn't because it doesn't affect the front roll center..... I have a roll center program on my computer and the springs can be changed and the RC never moves.

I would definately like to hear more on the rear RC. I also read in Circle Track that the front and rear RC's being as close as possible to the same location was better because it helped balance the car.

[This message has been edited by FlyNLoIMCA17 (edited March 28, 2005).]

Istock66
unregistered Total posts: 753
posted March 28, 2005 06:30 PM           send a private message to FlyNLoIMCA17   Edit/Delete Message   Reply w/QuoteBBBBBB
On front 3-4 inches above ground, and 2-4 to the right is what I came up with as optimum when reading everything I could about it.

The lower roll center in front the more vertical the loading of the outside tire and the better the outside tire sticks!!

The higher the more horizontal loading will occur, causing the tire to shear at the contact patch. push!

The lower roll center the less camber change also.
The rear being lower and higher has the same effect on as on the front end. lower=more body roll.

Rear roll with a j bar is still center of the mounting points and its height is at that center also.

But it is usually offset in relation to the cars centerline, thats why the rear springs come into play. The side with the stiffer spring moves the roll center towards it even more.

I think you could use the same formula used to locate your upper 3 link mount side to side to see how much it is offset?

THE HARDER THE TIRE THE LOWER ROLL CENTER WE NEED TO CREATE MORE DOWNFORCE AND BITE.

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted March 29, 2005 12:32 AM  
also remember that the front RC CAN be too low. I'm not sure what exactly is considered too low but I know that anything below ground is NOT good.

Kromulous
Dirt Forum Champ
Total posts: 796
posted March 29, 2005 06:55 AM  
2 to 4" inches to the right? Looking from what driection? standing in front of it, or sitting in the seat?

irace74
Dirt Forum Racer

Total posts: 80
posted March 29, 2005 07:36 AM  
I don't believe the roll center is between the mounting points when a j bar is employed. This would place the rear roll center at around 12" above the ground with little hope of getting it any lower. I do think that maybe we could get it down to 10" or so but we would be in the bottom hole of the mounting plate and little to no angle in the bar. I do also think that the spring mounting positions have an impact on roll center since they are the connections where the rear end is rotating about the roll center.

All of the input has been great! Eventhough the higher front roll center will not "bite" into the track as hard as a lower one, I think that the relativity of the front to the rear roll center heights being more equal will improve overall handling consistancy. The word balance may come into play here.

Rook78
Dirt Maniac

Total posts: 106
posted March 29, 2005 09:32 AM  
According to what I have read and understand of rear rc it is in fact half way between mounting points straight line from pinion to chassis.(Bar type does not change that) The springs do affect rear rc because of the fixed axel housing. Springs do not affect the front because of independent suspension. My understanding anyway.
Rook

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