preach (dudeist priest)
preach (dudeist priest) SuperDork
2/26/23 7:31 a.m.

I have a plan, not the parts yet so I can't touch, for a great car. Problem is I want to do everything with it.

This post touches on what I have in my head.

My car will be IRS, rwd, very light to power. In my experience, if you mess with the IRS too much so you can launch from a dig at a drag race you lose the handling of the rear. If you don't do anything to it, as my Cayman is, you cannot launch hard without a bunch of transaxle breaking hop. My Cayman would be a high 12sec car if I could launch it, but I am not changing anything on that an lose the canyon/dragon ability that it was made for.

So, in my muddled half-crazed brain, I have been thinking of a bolt-on jig(?) for the control arms that will add a straight axle feel and drag shocks while not berkeleying with the IRS set up. Think nocones Dan whipping out an thing and bolting it to his rearend and making the LMP360 hook to a 10sec run.

A little foreshadowing here but here are a couple pics of what I am trying to convey:

IRS rear:

Drag style set up:

I am sure there is a better example for a rear drag set up maybe for a Corvette but that gets the gist. 

That what should not be?:


Driven5 UberDork
2/26/23 1:28 p.m.

Adding a Z-bar that's neutral at ride height will resist both bump and droop travel, reducing pitching and (more importantly for drag racing) speeding the longitudinal weight transfer, all without affecting roll stiffness or handling balance. This could even be applied both front and rear for maximum effect.

Trent PowerDork
2/26/23 2:03 p.m.

So, you want a bolt on, temporary conversion from semi trailing arm IRS to a DeDion rear suspension?


It seems to me that the drag race launch benefits of a solid axle are more about the large amount of anti squat that can be dialed in with it than the fact that it is a rigid member.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
2/26/23 2:56 p.m.

That "drag style setup" is a solid axle, no IRS.

The biggest issue with IRS is the instant center can never be very good.  Whether this is important or not for a mid engine vehicle is debatable.  Wheel hop is usually because of play in the motor mounts and suspension bushings.

I liked Leon Drake's end run around that problem on his Caddy build.  16v Volkswagen engine.  He put slapper bars on the engine that worked against the firewall.  Under acceleration the mounting went solid, no wheelhop.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
2/26/23 3:02 p.m.
Trent said:

It seems to me that the drag race launch benefits of a solid axle are more about the large amount of anti squat that can be dialed in with it than the fact that it is a rigid member.

The axle is also the suspension, so the axle torque is loaded through the suspension.  You can get over 100% antisquat this way.

Whether that much is desirable is also debatable smiley. But it also leads to brake hop when you get extreme.

Serious solid axle cars mount the brakes on sliders so they are decoupled from the axle, have their own reaction linkage to the chassis. 

I liked the solution BMW came up with for the E36 318ti: trailing arm suspensions' instant center is defined largely by how high up the pivot is.  The diff was mounted to the beam and the beam was attached with bushings with a lot of vertical compliance.  Under acceleration the differential would torque the beam up, moving the IC more favorable for acceleration.  Braking torque would move it down, for reduced brake hop.  Those were the only cars I saw BMW make with a CV joint on the driveshaft instead of a Guibo smiley

preach (dudeist priest)
preach (dudeist priest) SuperDork
2/26/23 5:07 p.m.
Pete. (l33t FS) said:

That "drag style setup" is a solid axle, no IRS.

Yes. It was for illustration purposes, thus the corvette mention. Also not looking at just trailing arm irs. Just exercising the thought here.

Thinking if I could run Import Night on a friday at the strip, then run a club day at a road course Saturday after an hour of taking a axle harness off it would be super cool.

frenchyd MegaDork
2/26/23 5:14 p.m.

There is a conflict in geometry.   The arc or the wheel in compression and the arc of the drag link. 
  To eliminate that conflict most suspensions use flexible ( rubber)  coupling.  Jaguar mounted the rear assembly in rubber. And the Drag link in rubber. 
   Racers brought the drag link forward at an angle  to intersect with the inner pivot point.   

STM317 PowerDork
2/27/23 5:38 a.m.

Not sure what's breaking, or why on a Cayman rear suspension but plenty of high hp Mustangs, Supras, etc still have an IRS. When they break, it's usually because of axle hop. They typically just remove soft rubber mounts and replace with harder, less compliant materials. In some cases, external supports are added. None of that would negatively impact cornering ability.

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