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Old 01-21-2003, 03:00 PM   #1
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Default Roll Center question

Hi all,

I was hoping someone could clear up a query I have reagrding 'roll center', which seems to be the latest buzz set-up term.

The question I have is what exactly are people referring to when they say 'raising' or 'lowering' the roll centre?

If I am correct in my thinking, the 'height' of the roll centre means nothing in isolation, only its position relative to the centre of mass of the car is important.

With that in mind, when someone says they are raising the roll centre, do they mean they are moving it closer to the centre of mass and hence shortening the roll coupling? Conversely, lowering the roll centre takes it further away from the centre of mass as the roll coupling is longer.

Further, am I right in assuming that a short roll coupling ('high' roll centre) will have an effect such that the chassis rolls around less, with a long roll coupling ('low' roll centre) having the opposite effect?

Lastly, if the chassis is rolling less with a 'high' roll centre, is this force now being distributed to the tyre? Is that the conventional wisdom?

Hope this all makes sense. Just need someone to confirm if I am correct or way off... most probably the latter!

Cheers,
Scotty P
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Old 01-21-2003, 03:10 PM   #2
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My brain is hurting after reading that one scotty

The only way i understand it is for the next 30 seconds after reading through the XXX Main book, if you have not seen it come take a look on the weekend.

Roll Centers seem to be everyones biggest mod lately, although i believe they are of little importance when things like tires, springs oils etc are not right.
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Old 01-21-2003, 03:38 PM   #3
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Roll centres are a very useful tuning aid.

Raising the roll centre basically makes the car roll less due to a lower roll moment force.

Lowering roll centre makes the car roll more.

A higher roll centre will make the car lean less through a corner completely independant of springs, swaybars etc. So the net result is you can use softer springs and bars to control the roll, and achieve a higher overall car grip, but still maintain the benefits of a flatter handling car.

Basically it is the other way besides lowering the CG to make a car handle flatter thru the bends.
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Old 01-21-2003, 04:15 PM   #4
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A roll centre is an imaginary point in space, look at it as the virtual hinge your car hinges around when its chassis rolls in a corner. It's as if the suspension components force the chassis to pivot around this point in space. The theory is the theorem of Kennedy and tells us that if three objects are hinged together (I have heard that you should now all about this stuff scottyp).
The swing axle length, together with the angle, determine the amount of camber change the wheel has during compression. A long swing axle will cause very little camber change and a short one will cause a lot.
The roll centre will move when the suspension is compressed or lifted, that's why it's actually an instantaneous roll centre. It moves because the suspension components don't move in perfect circles relative to each other, most of the paths of motion are more random
The dampers determine the speed at which roll happens. So for a given spring stiffness a big roll moment will make the chassis roll very far in the corners, and a small roll moment will make the chassis lean over less. This also explains why a vehicle with a high CG has a tendency to lean very far in a corner, and possibly tip over.
So at any given time, the size of the roll moment is an indication of the size of the torque that causes the chassis to lean over while cornering and the torque to the tires. But the location of the roll centre changes when the suspension is compressed or extended, most of the time it moves in the same direction as the chassis
An upper link that is parallel to the lower A-arm will make the RC sit very low when the car is at normal ride height, hence the initial body roll when entering a corner will be big. An upper link that is angled down will make the RC sit up higher, making the initial roll moment smaller, which makes that particular end of the car feel very aggressive entering the corner. A very long upper link will make that the roll moment stays more or less the same size when the chassis leans over; that end of the chassis will roll very deeply into the suspension travel. If not a lot of camber is used, this can make the tires slide because of excessive positive camber. A short upper link will make that the roll moment becomes a lot smaller when the chassis leans; the chassis won't roll very far. It a fine line to sort it but I hope this has given you something to work on. As far as if you have spring/ oil/ wheel rates and things like that are correct you can really change the car just by a camber link change.
Most people forget that changing the roll centre also changes the Roll pitch which is the relation to the front to the rear. But that’s another long chapter.

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Old 01-21-2003, 04:44 PM   #5
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Thanks for your help guys. Think I'm getting there!

Just wish Sydney had a permanent accessible venue so I could go and do some testing on these kinds of things! No wonder the VICS are putting us to shame.

Thanks again,
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Old 01-21-2003, 05:26 PM   #6
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Whoa, thats way over my head, i just pull the trigger and turn the wheel, and the thing with the aerial sticking up goes, cya
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Old 01-21-2003, 05:55 PM   #7
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Coxy, if I raise the roll centre on my car, will that mean that it won't roll over when I hit something?

The big problem with lowering Bersercoe's and My roll centres, is that our bellies will scrape the ground
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Old 01-21-2003, 05:59 PM   #8
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Scotty P,

Without getting into Coxy's detail about roll moments and changes in C of G, Yes your basic assumptions are correct.
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Old 01-21-2003, 06:05 PM   #9
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ive built a REAL formula ford style open wheeler from scratch and roll centers are very important and very hard to get right but once they are it is great. its the imaginary line drawn out from the top and bottom A arms, where they intersect is the roll center. now the trick is to get it as close as possible to the CG (center of gravity, once this is achieved u have perfect suspension symerty. which means u dont need roll bars!!! roll bars are a quick fix to fix a suspension design problem, yes they do add another tunable option but once again if u dont understand it probley its another thing that can go wrong. another thing its a bit useless and its a lot of trial and alot of error fiddeling with roll centers unless you know ur CG, now this can be a achieved by doing a tilt test, we at uni did this with our race car and it is a good and FUN test to do and well worth it, if u want to now more read up on suspenion geomerty books, i can help with some titles of great books if people are interested.

ive learnt from experience with our REAL race car is that u dont start fiddeling with set up unless u understand whats happening, it really annoys me when people say im going to do this and that, u ask them 'hows that goin to change or improve the car' and they carnt answer the question or they say 'well so and so did it', its like the blind leading the blind sometimes
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Old 01-21-2003, 06:10 PM   #10
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ooooohhhhhh pass the panadol.....

I agree with NOFX, whack a pack in turn it on wait for the hooter, slam full throttle until the buzzer goes again. and steer.

Works for me. Well most of the time.
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Old 01-21-2003, 06:49 PM   #11
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Woodsy144,

You mean it's like asking

Wyle E Coyote.............Super Genius!!

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Old 01-21-2003, 07:09 PM   #12
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Snoopy,

Thanks... just wanted to confirm I was on the right path.

Woodsy,

I think you calculation for finding the roll centre is incorrect. From the point you describe, you then have to draw another line back to where the tyre contacts the ground (mid-point of the tyre). Where this line intersects the mid-point of the chassis (represented by a vertical line) is the roll centre. This method assumes your car is balanced left to right.

Happy to be corrected, but as far as I am aware, this is how to determine roll centre position.

To throw the cat amongst the pidgeons, what if the upper and lower control arms do not intersect ie they are parallel?

I'm with bersercoe now... getting a headache!

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Old 01-21-2003, 07:20 PM   #13
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Very good question. You end up with a car that handles like crap. ie you get neg camber to positive camber change.

I will try to explain how to get your CG and Roll centre in my next post. I have to draw a diagram. And yes you have to messure from the oiuside hinge pin and inner hinge pins. You have to get your "'instantaneous centre'or "I" point and then from the interecting I points back to the centre of the tire contact point you get the roll centre.

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Old 01-21-2003, 07:40 PM   #14
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Coxy - You really need to get a job where this sort of enthusiasm is put to practice.
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Old 01-21-2003, 07:46 PM   #15
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Ok I will try this without a drawing. Measure all you can from the rear of your car and write them down. Pin point these on a large bit of paper. Maybe start with a ground line and a centre line and work of these lines makes it easier. If you have CAD then perfect. I did this to a car that I was driving and having trouble with and it all seemed very simple in the end.

Point A = top hinge pin (or link) on tire
Point B = Inner top Hinge pin or link
Point C = Lower Hinge pin on Tire
Point D = Inner lower Hinge pin.

Continue all these lines a long way past these point so you can intersect these lines later.
Draw a line through A to B
Draw a line between C to D
Where they intersect = I point.
Do this in a different colour pen. Now take the left “I point” back to the right wheel centre point of the tire where it contacts the ground. Do this to both sides of the car left to right and right to left wheel and “I” point. Where these new lines intersect is your roll centre.

Now if you have a car with multiple camber links do it for all and see how it affects the car.

Note this then has to be done in a series of drawings on each so you can see the camber change. So I did this based on 5mm ride height then 4 ,3, 3,1. If you link them together and play them back and forth you can see the camber change take affect.

If I don’t see you at the track on the weekend I know where you will be.

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