XRAY - Model racing cars

Hi all ,i just finished the car and would like to know how does the front upper arm position effect the car handling?
Thanks
Dan

I did a quick search and found this thread, it talks about forward or back on the arm position. is this the position you are curious about?

viewtopic.php?t=4350

i am talking about the upper front arm not the lower arm.

that would be your front roll center your´re talking about than.
it should be explained in your instructions manual.
take your time and read it i know i did. just every know and then i pop that manual out and start reading it just to understand perfectly what is what and the effect of changes.

As Keith said, you want your suspension to be free, so move it up and down a few times so the upper arms find a 'neutral' position, then insert the clips to take up the slack without moving the upper arms from this neutral point.

But to directly answer your question, the upper arms don't really do anything on our car...

On pivot ball cars they set the caster (angle of the steering block in relation to a line perpendicular to the surface) when you move them back and forth.

BUT, on our car you set the caster with with the eccentric pin holders that go in the outside of the front arms. But when you set the caster you have to slide the upper arm in the appropriate position to insure that the mechanism doesn't bind, which is what the gap and caster clips are for.

If you slide the upper arm back without adjusting it will just bind the suspension without changing caster...

I hope this helps!

Let me know if you have any further inquiries.

When you change the front of the front upper arm independently of the rear of the upper arm you are altering the angle/relation to the lower arm. If this angle matches and you just change one end of the front upper arm, you are just taking it out of line with the lower arm, and yes it will change the roll center if you want to get really technical about it.

To change the front roll center, you have to change the eccentric mounts the same amount. Example, you're running 1's down in the rear of the front upper arms and 0's in the front of the front upper arms, to change the roll center you would have to go to 0's in the rear of the front upper arm, and 1's up in the front of the front upper arm.

This can also be done to change the roll center in the front lower arm.

The main thing is you HAVE to change them the same amount front and rear, and not independently.

In short, altering the height of the front upper and lower arms without changing the angle between them will alter the roll center.

Ok that makes sense. But answer this question for me. I was running 1s up in the front of the upper arm and 1s down in the rear of the upper arm. Then I changed to zero in the front of the upper arm and zero in the rear of the upper arm. So the height at the front of my upper arm was lowered but the height at the rear of my upper arm was raised the same amount. Did my roll center change?

(Of course I made the necessary change to the kickup angle in case you were wondering.)

the roll center will come from the centre of the upper arm..so as long as that point remains the same height then the roll center will remain the same regards wether the rear is higher or lower

but as has been said,you have to try to match the angle of both the upper and lower pins,i use a vernier to measure the distance between the upper and lower pin at the front and rear of both pins,this gives you an accurate reading of what the upper eccentric bushings do inregards to the relationship with the lower arm.

dave