96 Chev PU - Suspension Problem?

it's a v6 short bed. When I brake fairly hard, the back end feels like
I am going across a washboard road. Light braking may only produce a
few bounces. My first thought was brakes, but they have been rebuilt.
When I rebuilt them, I thought the rear axle bearings felt a little
"dry", but I could never decide if the roughness was the axle bearings
or the differential. Anyway, I am wondering if anyone has encountered
this before, and if there are ways to diagnose it without just swapping
parts. Just bouncing the shocks tests out OK. So I can't figure out if
it is the shocks, leaf springs, or possibly some kind of drag in the
axle.

Thanks,
Roscoe

Out of round rear drums?
> wrote in message
oups.com...
> it's a v6 short bed. When I brake fairly hard, the back end feels like
> I am going across a washboard road. Light braking may only produce a
> few bounces. My first thought was brakes, but they have been rebuilt.
> When I rebuilt them, I thought the rear axle bearings felt a little
> "dry", but I could never decide if the roughness was the axle bearings
> or the differential. Anyway, I am wondering if anyone has encountered
> this before, and if there are ways to diagnose it without just swapping
> parts. Just bouncing the shocks tests out OK. So I can't figure out if
> it is the shocks, leaf springs, or possibly some kind of drag in the
> axle.
>
> Thanks,
> Roscoe
>



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Knee-on-the-bumper-bouncing only tests the lightest control area of the
shock.

Bounce-testing - for the lack of a better term - produces shaft velocities
of around one to two inches-per-second.

Driving down a smooth road will produce shaft velocities around four to six
i.p.s.

An extremely rough road with potholes can easily go past 10 i.p.s.

The shock valving that controls low shaft velocity is different from the
valving that controls moderate shaft velocity - which is different from the
valving that controls high-speed shaft velocity.

Most cheap shocks have three valving stages.

More expensive shocks add valving stages - up to a dozen or so - which is
good for boulevard gliders, but the cheap shocks work better on oval-track
race cars when stock shocks are required by rulebooks.

Low-speed bouncing is controlled as much by fluid leaking past the shock
piston, through designed bleed ports, and past seals as it is by any
low-speed valving.

When you start increasing shaft velocities, different valving comes into
play.

Low-speed valving might be okay while intermediate or high-speed valving is
junk - or any other possible good stage/bad stage combination.

You "test" moderate and high-speed shock valving by driving the vehicle
over both smooth and bumpy roads.

IOW - the "Bounce test" is NOT indicative of how a shock will perform while
traveling down the road.

Try taking the truck around a curve on a rough road. Does the rear end
"chatter" and want to wash out?

If the wheels are leaving the ground, it is likely that the shocks are not
controlling spring oscilations.

IOW2 - Put a new set of rear shocks on.

If I suspected you lived anywhere near me, I would offer to run your shocks
on my shock absorber dynamometer........just to prove my point.

> wrote in message
oups.com...
> it's a v6 short bed. When I brake fairly hard, the back end feels like
> I am going across a washboard road. Light braking may only produce a
> few bounces. My first thought was brakes, but they have been rebuilt.
> When I rebuilt them, I thought the rear axle bearings felt a little
> "dry", but I could never decide if the roughness was the axle bearings
> or the differential. Anyway, I am wondering if anyone has encountered
> this before, and if there are ways to diagnose it without just swapping
> parts. Just bouncing the shocks tests out OK. So I can't figure out if
> it is the shocks, leaf springs, or possibly some kind of drag in the
> axle.
>
> Thanks,
> Roscoe
>


Several possibilities come to mind.

1. Your rear drums may be warped or bent. They can be checked with a brake
drum micrometer, or if you adjust your rear brakes to drag slightly, you can
feel the warp as a tight then loose spot when rotating them by hand.

2. You may have malfunctioning shocks. Bouncing the rear bumper may not be a
conclusive test. Look for signs of oil leaking out of the shocks or just
replace the old suckers for good measure. If you have someone follow behind
you while driving down a bumpy road, they may be able to see the rear wheels
hopping due to bad shocks.

3. Tires. Broken belts, out of round, cupping, etc. Swap front to rear and
see if there is a difference.

4. Normal operation of rear wheel antilock brakes (if that's what you have).
Some types of rear wheel antilock brakes produce a rapid pulsation in the
rear brakes when activated, and sometimes the RWAL brakes activate too soon,
which is a whole nuther post on how to diagnose that.


--
Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green