XRAY - Model racing cars

Here is something I have been trying to figure out for a while. I know you can change the front roll center with the upper arm but let's say you start with zero's in the front and zero's in the back of the upper arm. That gives you a certain roll center. Now suppose you change to 1s up in the front and 1s down in the back for the upper arm. Did the roll center change at all?

The reason I am asking is because the roll center changes when you either raise or lower the upper arm. But in my example above, I raised the front of the link and lowered the back of the link the same amount. But then again, maybe it is only the front of the upper arm that matters since the rod is nearer to the front rather than in the center. Does anyone know?

Curious!!! I tought the same thing today, and I was going to post the same question.

Hope someone can explain that to us.

no, it doesnt changes the roll center.
the only time you can change the front roll center is when the lower arm bushings are the same value (1 down front/rear, 0 front rear). when both lower arms bushings is 1 up the arm will contact the outdrive.

the reason for this is when the lower arm bushing are the same value you can change the upper arm bushings without cause bind. (again same value front and rear)

so, you can understand that when you are using full or minimal kickup you forced to adjust the upper arms as well, and therefore you cannot change roll center.

i forgot to mention, roll center is changed only when the angle between the upper and lower arms is changed.

I was thinking that the front bushings changes the roll center. the rear bushing just keeps the arms from binding in comparison to the lower arm.

Great explanation dotan and you are correct but that is only if you want to prevent binding. Of course you would want to prevent binding but for the sake of understanding let's assume you don't change the lower arm position. Would the roll center have changed in my example?

well, if the the upper joint on the c hub is absolut vertical to the upper arm hinge pin, and is right on the middle of it, the angle of the upper arm in relation to the lower arm will not change, so as well the roll center.
if it vertical to the hinge pin but closer to the back of the arm and you are using 1 down in the rear, the roll center will be higher, because the angle between the two arms changed, if you are using 1 up, the roll center will be lower. the same will be true if it closer to the front.

try to think of it as a strait line between the joint on the c hub and the hinge pin, and i'm sure it will be clear.

BTW, that is the reason i replaced the upper arm with single link, just like in the rear suspension. now i can change kick up and front roll center without worring about binding. and as a plus my turnbuckles do not bend.

That still does not answer the question though. I will state the question again and this has to do only with the position of the upper arm:

What if you have zero in the front and zero in the back. Then you change to 1 up in the front and 1 down in the back. Did the roll center change?

I don't think so, as long as you change your lower and upper arm the same amount, and keep the 'planar' distance between them the same the roll center should no be effected.

maybe i wasnt clear enough.

when you change to 1 up in the front and 1 down in the rear, the center of the hinge pin stays in same place. so, if the joint on the c hub i exatly in the middle with realtion to hinge pin, it like you have not changed any thing in realtion to the lower arm.

if the joint is more toward the rear of the hinge pin, it will make acutal angle of the arm steeper because at that point the hinge pin is lower than 0 in front and 0 in rear situation. same case if the jiont is more toward the front, but this time is the opposite.

sorry for leting you make all the calculations, but as i sayed on my car i am using different arms, so i dont know the position of the hub in relation the the hinge pin.

also sorry if the explansion isnt very clear, as my english is not the best.

Ok I see where you are getting at. The Chub is more to the front of the hingepin so I guess in that case the roll center does change in my example.

just remember that the c hub chenges with caster
too much calculations, i think i will stick with driving, its more fun