can anyone explain and how to sort it on 2wd sapp?

 

I am no expert...
Iwent through ths a while back, There are others that can help better than I but here goes.. Others might flame me as its what worked for me... but might not be what they reckon...

Here is a tech reference I found

http://www.neohio-scca.org/comp_clin...namics2007.pdf

I assume you are talking on the front of the car, the rear I did a bunch of things to trailing arms...

What I found on the sierra was that the hubs on three doors and sapphires have different TCA mount points, and that there are some that have a lower mount point which is about 25mm lower than the standard 2WD one... I found three different sizes actually. The one I used was claimed to be RS500 hub but you never see it mentioned, It did not worry me as it does what it needed to.

I have some of each which I can photograph... regardless it enabled the car to have a better TCA alignment when the car is in its ride position (the tca is more parrallel to ground rather than on an angle)allowing the roll centre to be set up better.

As I said I am sure there are others than can give better advise, this might tempt them to chip in...

 

The roll centre is an imaginary point which the car effectively pivots on (longitudinally).. There is a point calculated at the back, and at the front, and a line drawn through them (not on the below pics, you'll have to imagine that)..

Here's a piccy..



You have to create a reference point in space (instant centre) from the angle of the upper and lower arms and then draw this back to the centre of the tyres contact patch (C).. where THIS line crosses the middle of the car, thats the height of your roll centre (at the front or rear etc).. If you lower your car, your roll centre falls at an exponential rate (depending on the suspension design type).. this piccy demonstrates quite well:



Fractionally lower springs completely changes the roll centre.. this is why lowering on standard suspension and geometry is (generally) a bad idea..

Hope this helps..

 

Excellent explanation, now a question relating to where the roll center should be? any ideas?

 

Nice pics, thats similar to the URL i posted.. but more concise...
Note that on front suspension that moving the top mount has a significant effect on role centre. I have seen a few race cars with the shocks moved out a bit for this reason... Naturally they are adjustable...