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Old 08-10-2012, 09:20 AM   #1
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Questions?? Affects of Roll Center

Hi guys,


I've been hearing alot about the importance of roll center recently. What does changing it do and how can i change it on my 1/8 buggy?


Cheers
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Old 08-10-2012, 09:42 AM   #2
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http://home.scarlet.be/~be067749/58/c2/index.htm
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Old 08-10-2012, 09:45 AM   #3
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Lets discuss roll center in offroad.
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Old 08-10-2012, 09:58 AM   #4
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i think for alot of people, including myself, the concept is quite difficult to understand...i have to double check myself when explaining it too haha, the wording alone makes it confusing. lets see if i can try and get it right this time

in general, when you move your top and lower arms or links closer together (vertically) you make the roll center HIGHER.
I just think off the total amount of "car" that will now be above that imaginary centre line. think about it this way....a double decker bus has a HIGH roll centre because its swing arms are basically on road level and the rest of it is 5m in the air and therefor top heavy...it will fall over easier.

the inverse is true for lets say a supercar which is very low to the ground for example and alot of the actual car is closer to the ground and that imaginary centre line.

but that alone does not determine what happens when we change it. it basically comes down to how the geometry of the car is changed and used to transfer the weight of the car around a centre line to generate side traction around corners.

ok lets try lol
So, moving the top and lower arms or links further away from each other and making the gap bigger (vertically), LOWERS your roll centre because less of the actuall car is above an imaginary line somewhere in the centre of the car.

in general:

FOR the FRONT:

LOWER roll centre (arms/links further apart vertically) = in general makes the car more responsive.

HIGHER roll centre (arms/links closer together vertically) = in general it makes the car less responsive


In the rear it is similar in terms of the above info but a little more complicated because we can play with link arm lenghts as well, changing the geometry a little.

raising the rear link = larger vertical gap between top and lower arm/link
= LOWER roll center
= less rear traction/more steering

lowering rear link on tower = smaller vertical gap between top and lower
arm/link
= HIGHER roll centre
= more rear traction

rear link length has the following effects depending on where you put them on the hubs and tower in the rear:

SHORTER = less rear traction
LONGER = more rear traction

hope that helps...still pretty confusing if you ask me

PS: most of this info is in the HUDY setup book for 1/8 off-road vehicles (somewhere near page 20 i think)
you can download it from xray's website
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Last edited by Sideshow Bob; 08-10-2012 at 10:09 AM.
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Old 08-10-2012, 12:48 PM   #5
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Quote:
Originally Posted by Sideshow Bob View Post
....a double decker bus has a HIGH roll centre because its swing arms are basically on road level and the rest of it is 5m in the air and therefor top heavy...it will fall over easier.
You have the basic concepts down, like moving in upper inner link positions down raises the roll center and vice versa. You'd be surprised how many people inverse those. But your above statement isn't really correct. Many people inadvertently confuse the CG with the RC, but it is their relationship to one another's position that actually affects the way the car handles.

In your example, we really wouldn't know anything about the position of the bus's RC unless we knew some details about the geometry of the suspension. But we can assume it's somewhere and for the purpose of this explanation, assume it's located at ground level. Now, the reason the double decker bus tips or rolls in turns is because it has a high center of gravity, and the distance between the CG and the RC is large. If I was to adjust the bus's suspension and raise the RC closer to the CG, the same bus would actually roll less in the turns.

The CG always acts about the RC, so the closer the two are together, the less the CG will seem to act. Think of it like a torque wrench-if the handle is long and you exert a 10lb force, the bolt moves easily. But if you shorten the handle and apply the same 10lb force, it's much more difficult to move the bolt. The moment arm (distance btwn the CG and the RC) is what you're actually changing when you move your RC. As you get further away from the CG (lower RC) the car rolls more. The closer you get (high CG), the less the car rolls.

There are a whole host of other things to consider also, like the relationship between your front and rear roll centers (roll axis), roll stiffness, dynamic roll center, etc. but that's a topic for another day.
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