Instant Center, Anti Squat and Roll Steer

[The WidowMaker]

im working on the rear suspension for my 70 chevelle right now and its gotten out of control. im running the c6z brakes and because of the caliper offset, the wheels would need to push the spokes outward and they would appear to have a small lip. since i can move everything inboard 4" before the caliper hits the frame, ive decieded to narrow the rear. but when i do this, i must move the lower shock mount. since im doing that and the rear is so low (id need to cut my already short 11" springs), ive decided to run coilovers.

so heres the dilema; right now my ic is 50" at 8" above the ground and my antisquat is ~85%. but, that is with the current tires (new will raise the rear 1")( 2" dia change). doing so would increase my AS since the cross would be at 9" above the ground and the neutral line would stay the same. since im going to coilovers and will have more control of the rear, id like to lower it another inch to tuck the tires in a little more. this would still give me 3" of bump travel which should be more than enough with the stiffer springs. but this also puts me at ~150% AS at 30" out.

so heres the question. if i drop the rear and angle the lca down towards the front 2*, in a turn with one bar at 0* and the other at 4* (the worst case scenario) one bar will be about .020 longer. how much roll steer am i going to experience? this would get my AS down to about 100%.

my other option is to move the front mounting pt of the UCA up. id have to make some mounts, a new crossmember or modify whats there.

how does a long IC differ from a short IC? what would happen if the ic was in front of the car? where do you measure the intersection of the front wheel veritcal line when the IC is in front of the car? half way inbtwn the two lines? how much roll steer before it makes a difference? ive dropped the rear a ton to get it wheres its at. this car stock must have had at least 10* of upward angle, so i dont see how 2* down is anything other than an improvement.

Thanks, Tim

[deuce_454]

.. no idea! you should however look into an SC&C watts to keep the rear rollcenter fixed! and what are those adjusters on top of the rear springs??
speedway motors??

[The WidowMaker]

speedway motors adjusters. but, id never use them with the stock setup unless i cut the springs too far on accident.

right now its taking 500lbs in the trunk to get it to ride height. even with a fuel tank, glass and interior im not going to be there. plus, with the springs being so short, they dont seem to fit correctly, and even if they did theyll pop out under droop.

Tim

[deuce_454]

the reason i ask is my bu is so low in the back that the arms ara above horizontal and my exhaust catches everything unless i have some 2 inch blocks inthere (made them on the lathe) but they are hardly adjustable..

but with regards to the rolcenter you will be much better off with a watts.. the rollcenter on a k link like on our cars is in the intersection of the upper arms.. so somwhere in the trunk.. and it moves arround when the suspension moves thru its travel... a framemounted watts will keep the rollcenter fixed in relation to the centre of gravity.. thus keeping the rear predictable.. which you will feel if it isnt... especially now that your front suspension can actually handle

[The WidowMaker]

thanks for that info. ive done a lot of research on the watts and may end up going with one.

im still interested in how to calculate roll steer. i plan on using about 2-3% roll understeer, but i dont know what that is a percentage of????

[Marcus SC&C]

Roll steer is kinda like bumpsteer. You just want it gone. That said most setups will have at least a little. How it affects the performance of the car will vary from car to car and setup to setup. Some it won`t be very noticable,another car it may be more so even with the same amount. The bottom line is that if you get the LCAs level at ride height you`ll have minimized it a great deal. The easiest point to move is the rear lower mount and you can even use readily available bolt/weld on mounts to do it (of course we carry them,shameless plug!). The upper arms define (albeit very poorly) the rear roll center. Lowering the car too much (more than 1" or so) can put the RC higher than the CG which is a no no. The moment arms should form inverted pendulums both front and rear. The UCAs also provide almost all of the lateral axle location. Again they do it very poorly and even the best aftermarket arms need to strike a delicate balance between eliminating bind and providing good axle location. These issues will cause you a lot more grief than a small amount of roll steer caused by the LCA angle.
That`s were the watts comes in! It takes over the lateral axle location and RC location dutys from the UCAs leaving them with nothing to do but provide pinion angle control. Leaving rubber bushings in the upper axle mounts allows plenty of compliance to let the watts do it`s job smoothly and consistently. It also lets you tune the car by adjusting the rear RC height. Here`s a sneak peek of the prototype watts on a G body (shown at full droop) . Production G body bolt on watts are shipping now. Weld in Fab Fit watts are also available now (cut the crossmember to length,weld it in and the rest bolts in) and bolt in A body watts should be available by the end of July. Mark SC&C