XRAY - Model racing cars

Tony V,
I know that changing the upper link angle changes the roll center but it also changes camber gain. If you lower the inner pivot of the link you raise the roll center but you get more camber gain. If I lower the inner pivot and get more steering what gave me that result ? Higher roll center or more camber gain ?
Thanks !

When you raise the roll center it moves up compared to the CG. Whether or not it is below or above the CG is not really important, the key thing is that if you raise the roll center the chassis roll at that end of the car is reduced! In principle a higher roll center also results in less grip on that end of the car.

So how do you raise the roll center? There are two ways to do this. First you can raise ( ) the inside of the lower suspension arm (i.e. lower hole for the inner pivot holders), or lower the inside mounting of the top link (alternatively raise the outside mounting of the top link).

When you move the inside of the lower arm up or the inside of the top link down you also get a second effect. The more the top link and lower suspension arm are angled towards each other the more the camber will increase when the suspension is compressed. Usually this effect is less effective as the reduction in roll resulting from the raised roll center will reduce chassis roll and thus reduce how far the suspension compresses partially negating the increase in camber change.

Since 1jag8 did not indicate whether he lowered the inside mounting of the top link at the front or the back it is difficult to determine what was the biggest factor here. If he made the change at the rear the main effect is most likely the raised rear roll center. If he made the change at the front the increased camber change had the bigger impact.

So,if you move the inside of the lower arm down(at the rear)you get more grip?

Hey Tony,
I raised the outside of the front upper link by 2mm. to get the additional sterring. I have used a 1mm shim under the outside rear upper link and found that it settled the rear end nicely.
Thanks for taking the time to reply !

Hi guys,

Sorry for the errors I posted yesterday. I think I have got my head round it now....

James

Hi again,

Just been re-reading the above posts and some of is not makinf sense.

"the key thing is that if you raise the roll center the chassis roll at that end of the car is reduced! In principle a higher roll center also results in less grip on that end of the car.

So how do you raise the roll center? There are two ways to do this. First you can raise ( ) the inside of the lower suspension arm (i.e. lower hole for the inner pivot holders), or lower the inside mounting of the top link (alternatively raise the outside mounting of the top link). "

And then another post says that having spacers under the outside top links front and back gives more grip.....?

I now understand the theory of how the relative positions of the roll centre and the CoG affect the amount of body roll and grip, but exacly what effect do the angles and lengths of the upper and lower arms have on the roll centre?

Help!

James

I was wondering when someone would notice that.

There are multiple answers on why 1jag8 felt the higher roll centers had this effect.

(1) The increased camber change was more important than the higher roll center.

(2) While a higher roll center decreases the traction on the ouside wheel it increases traction on the inside wheel (less chassis rol = more weight on the inside wheels!). Normally the increased traction is less than the loss of traction on the outside wheel meaning less overall traction. But, depending on the circumstances this does not allways hold. It is similar to running harder front springs and gaining steering as a result.

(3) The car had too much grip to begin with, causing the car to be close to traction rolling. This will make the car feel very unstable (although usually not understeering...). Reducing chassis roll will reduce the traction to the point that car does not drive on the edge anymore, making it feel more stable.

In this case my bet is that the increased camber change did the trick.