Suspension Roll Centres
After following a post on Xweb (Scuderria Ferric Oxide-Steve C) discussing Roll Centres (RC) I wondered if that may be why my cars handling on track is not as good as I would like it. After a few goes on the online VSusp calculator I eventually came up with some numbers that I think are as accurate as I can get them. It showed that with my 2.5 deg of body roll when cornering my front RC was moving from about 50mm above ground to 36mm below ground and outside the track width of the tyres. By all accounts this is not a good thing to be happening.
After much trial and error on VSusp I settled on moving the top of the struts inwards by about 20mm each side, limiting body roll to 2 deg with stiffer springs, and raising the ride height slightly to get the Virtual lower control arms to slope up towards the chassis a bit more.
I cut out the centre of the top of the front strut towers, moved them over and rewelded. I added a 4th stud to the stock top mount and rotated it 180 deg to get it as close as possible to the inside of the strut tower. I have fitted 500lb/in springs in the front and 375lb in the rear. I have set the corner weights and adjusted F and R ride heights to try and get the best compromise with roll centres with the 2 deg body roll I am hoping to achieve. I have set the rear ride height slightly lower than the front to get the rear RC at 54mm, 9mm lower than the front RC of 63mm. At 2 deg body roll the RC drops to 30mm, same as front RC at 2 deg. Camber adjusted using camber bolts at the strut to knuckle joint, -3 deg F & R.
Steve H has said that he likes to have the rear RC lower than the front RC, I’m not sure if this is mainly for autocross driving or does it apply equally to road course use? Just wondering as I have seen where Porsche 911s as standard generally have the rear RC higher than the front, I thought with their rear weight bias they may need similar balance to an X1/9, but I guess the different suspension design and spring rates etc would have a big bearing?
VSusp (vehicle suspension): Front View Suspension Geometry Calculator/Simulator
www.vsusp.com
P.S. You can change the roll angle from 0 deg to 2 deg and see where the RC moves to in these VSusp links.
Drawings of my suspension, based on the factory X1/9 chassis drawing and my measurements.
Looking at the rear susp on the 2D Vsusp, I'm not sure how true it will be, it really needs a 3D program. Unlike the front susp the pivot points for the rear are nowhere near parallel to the car centre line. I did a CAD drawing using the factory chassis drawing as the basis, for the virtual LCA I used the length from the ball joint to 90 deg to the line between the pivots, but then turned to put it at 90 deg to the centreline to input into 2D Vsusp. Also, the factory drawing shows the F and R pivots at the same height, on my car there is a 6mm height difference.
VSusp (vehicle suspension): Front View Suspension Geometry Calculator/Simulator
www.vsusp.com
I have also fitted new Toyo R888R 185/60 R13 to the front.
Unfortunately the track event I was intending to try it out at this weekend at The Bend has been cancelled due to covid, but there is another sprint scheduled for the end of the month, but at a tighter track.