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get's a bit confused talking about roll centres and roll steer all at once like that. But assuming you're mainly talking about roll centres and roll axis - all roll centres are dynamic things, they never stay in one location (height) in any production suspension configuration. Sliders are probably worse than hangers for maintaining roll centre height because they fix the roll centre to the body. As the body leans while cornering and gets closer to the ground, so does the roll centre The roll centre height changes exactly witht he height of the body at that side. With a shackle as the body leans, the spring compresses and (typically) lengthens, pivoting the rear hanger back and upward, giving back some of the lost roll centre height. But changing shackle angles also change spring rate... And that is the main advantage of sliders - consistent spring rate.

If you're worried about changes in roll axis during cornering, the strut front end is going to be the main player - they have notoriously poor dynamic control of roll centres.

On the idea of front and rear eyes at the same height - leaf spring setups always handle best with the rear eye higher. And it wouldn't stop twist anyway - any time there is roll, the twisting of the live axle has to cause leaf spring twist. Its just inevitable.

thanks for the info tom.
very much appreciated!

Benny,
Agreed this would in theory produce a roll neutral setup, whilst going straight. But these cars are far from ideal stock and given the spring is an arch, as it compresses you will still likely end in either roll understeer or roll oversteer at some point in the corner, if that make sense. I.e the roll axis will change angle as the car rolls. The only way i know to avoid this is to use rear sliders, in lieu of shackles.
I guess the idea is to optimise an element of the corner to produce the best characteristic, potentially at the expense of the other aspects. As you know its always a compromise.

I think most people extend the rear hangers because that is the only option they have.
Rear hangers will help to ensure the angle change on the rear is minimised, thus keeping the spring at a more constant rate. Given that it stiffens/softens with changes in rear spring shackle angle.
We have a little luxury with the coupe (should the rules allow) to play with both front and rear easily. therefore playing with the front can minimise the impact of playing with the rear shackles only and may help fix an underlying issue. I guess it depends on what you are trying to fix or optimise for.

Tom, Often we would cut the floor/trunk pan above the H190 pumpkin (for travel) and make a removable cover out of aluminum. Make sure it has nothing to hit. Peter

Why would you lengthen the front spring hangers, Wouldn't that just exaggerate the issue. I was under the impression that in an ideal setup, both ends of the leaf spring should be at the same height above the road surface and therefore roll would transition parallel to the road rather then twisting.

Im not sure how much travel you would need in both rear wheel bump (worst senario i guess, in roll i would have a guess that 45mm at the centre is plenty).
I would have thought this bump senario would be limited by correctly setting the rear shock abosrbers high speed bump or if you are really worried, I'd add a small/stiff/progressive bump stop in the centre.
This should allow enough freedom in roll and control the amount of bump movement (These bump stops could be incorporated into the shocks like Bilstien do).

If we are talking a 1200 coupe and your regulations allow, I would look at lengthening the front spring hangers as this will point the rear roll axis down further and produce better roll geometry (roll understeer if i remember correctly) and also increase the rear ride height.

The rear ride height is now at 165 mm by moving the rear spring shackle position 57 mm (down to the lowest hole). The diff is now at 45mm from the floor (at the reenforcing plate for seat belts) I knocked off the weld nuts so that issue is gone, As for suspension travel it's now 2.5 inches (actaully 66 mm)

Keep in mind that the diff snout is roughly at the same point as the front mont for the traction bar so theoretically as the axle rises the snout would pivot away from the body..................of course it's not that simple as the upper link in the system is the leaf spring itself.

I am going to run the car at a local autocross this weekend end see if there are any issues.

a12grunt: as for the fitment issues yes most of it is due to fitting the XXL H190 into H145 hole. The Nismo springs will lower the car near 3 1/2 inches / 90mm with the standard spring shackles. The bushes I'm using are straight out of the suspension manual on this page. Aluminum at the front frame rail and urethane at the leaf spring. The traction bar uses rod ends.

Tom

From the back of the rear arch on my ute, it is sitting 270mm

hey rallytwit,

are you having issues with the nismo leafs because of running the "larger" h190 diff?
also which bushes do u run with the nismo leafs?and how much mm do they lower the rear end?
any info will be appreciated as i will also be running nismo leafs but with a nismo h165 but not fitted prob until late this year.

cheers

I`ll measure mine and post it up. I`d think its a bit higher than yours tho, as I have about 3.5" of travel between diff axel tube and the rails. Also have adjustable shackles.