Solid axles - what's REALLY the problem in mid-corner bumps?

[mikedc]

Conventional wisdom says a solid axle jumps sideways because of the camber change on both rear wheels whenever one wheel is bumped. Is this really the primary reason?

Or is it more a product of the heavy unsprung axle weight, which forces the rear springs & shocks to be too stiff for the job of absorbing bumps?




Can anyone answer how much of the problem remains on "dead" rear axles? That would be revealing about the unsprung weight issue alone.

DeDion rear axles were common on the earliest sports cars 80-100 years ago. But tires were so thin & crowned in those days that it probably negated most of the camber issues.

I've never driven a DeDion setup. And it's been too long since I've driven a dummy-rear FWD car very hard.

Anybody else have a comment on this?



Any thoughts?

[iadr]

I've run into at least two cases where there was way too much rebound damping on the shocks.

[Ron Sutton]

I've run into at least two cases where there was way too much rebound damping on the shocks.

Do you know what piston speed the shocks were too stiff at ?

[iadr]

No sir.

I wasn't just doing a fly by post, I didn't think I had all the info to write a long one.

Both cars were early/mid 1960's Volvo's. They have a Dana 27 axle (very similar to a Jeep Dana 30), and weight around 2300-2650 lbs with fairly close to half on the back axles.

I'm in Canada, and we frequently see frost heaves in the asphalt. That's where a crack forms, then the edges left up- as much as several inches.

The one car had Koni street/resto model shocks, and the rears were adjustable from maybe close to the right amount of rebound damping, coupled with a little bit too little compression...all the way to rock solid rebound damping...and the compression was not adjustable at all.

The rear axle is located with a panhard rod. The factory geometry is questionable, plus it's lowered (more on the drivers side-oops). I haven't examined the possiblity of relocating the mounts, but should because it feels asymmetrical in many, many motions.

Going down the highway, hit a frost heave at 60 mph, and the car jumps right, and there is a real "crash" felt on "landing".

I initially thought the limit straps looked too short, and made my own longer ones, but it seemed to be 75%+ in the shock damping.

I put a different pair- non gas OE's I think, or Bilsteins to match the fronts, and the problem went about 70% away.

I need to level the rear end, and maybe send the Koni's away to be revalved- about 20% more in compression and about 35% less in rebound, then see where I am at. I've also thought to get a front and rear of the OE "Fitchel and Sachs" non gas, non performance, and have them "dyno'ed" then use those settings to spec my revalve (@ 15-20% stiffer or whatever).

In consulting with a racer of Volvo's with an active website of vintage volvo parts and car prep (John Parker), he said he was trying to do a group buy to have Bilstien recalibrate the rear shock they offer. I think the problem is the manufacturers making non adj apps that share part numbers with much heavier cars.

[regal454]

Have you verified the thrust angle is perpendicular to the centerline of the car?