Re: 97 Camry - When to replace struts?

Jason James <vellicet@bigpond.com> wrote in article
<37pjquF5ilkqmU1@individual.net>...[color=blue]
>[/color]
[color=blue]
>
> If the struts have substantial wear either in the shock-absorbing part or[/color]
in[color=blue]
> the ram-bush your car will start to get unstable. Often the strut shock[/color]
can[color=blue]
> be deteriorated but not be obvious to the driver. There is a test rig[/color]
which[color=blue]
> involves a chart graph which records how many rebounds or ocillations the
> strut shock allows. This a test for damping. >[/color]


The "test rig" that Jason refers to is known as a shock absorber
dynamometer.....and I own one.

Basically, it gives you a graph of the pressures produced as compared to
the shaft velocities at which they are produced when the shock is moved at
different speeds - ranging from a shaft velocity of one-inch-per-second to
20 i.p.s.

Basically, a shock that creates 200 pounds of resistance pressure while
moving at a shaft velocity of five i.p.s will better control a car than a
shock that only produces 100 pounds of resistance pressure at the same
shaft velocity.

We use these graphs a bit differently in racing applications to "fine-tune"
the suspension with shocks, but the above information is pretty much all
you need to know for standard passenger automobiles....more shock pressure
at a given shaft velocity controls better than less pressure at the same
velocity.

When internal valves and springs weaken and wear out (imagine how many
cycles a shock valve control spring experiences in 50,000 miles of
compressing to open and close the valving each time the shaft moves in or
out) , they allow fluid to pass more easily at lower pressures - usually
with no external leakage to suggest that any sort of problem exists.

The so-called "bounce test" only tells you if a shock will control a car
while negotiating "Mickey D" parking lot speed bumps at less than five mph
with a carload of rug rats and Happy Meals.

"Hand-testing" a shock off the car moves the shaft at a velocity of
approximately one-half i.p.s.

A shock can "feel" good at slow "bounce-test" or "hand-test" speeds of one
i.p.s. or less because it is only passing fluid through its designed,
low-speed, bleed orifices and/or bypassing the seals, but be a complete
failure at higher shaft velocities once it gets up onto the
valving....sometimes, actually providing less resistance at five i.p.s.
then at "bounce-test" velocities once the valves open up.

On a smooth road, the shocks will likely be working in the 2-6 i.p.s. shaft
velocity range....which simply cannot be duplicated by bouncing on the
bumper of the car.

Best test for a shock (short of dyno-testing) is to drive it fairly
aggressively - but carefully - over rough road. If the car remains under
control, then the shocks are, likely, okay.

If one end or the other tends to "wash out", then new shocks (or struts)
are indicated.
[color=blue][color=green]
> >
> > How much do they affect your wheel alignment?[/color][/color]
[color=blue]
>In severe cases the road wheel
> will have 'cupping or scalloping" wear points on the tire concerned.[/color]

A shock that doesn't help control the unsprung weight (basically,
everything that moves up and down with the wheel) will allow more than
anticipated suspension movement at the extremes, which allows excessive
camber/caster, gain/loss and bump steer....in addition to allowing the
wheels to bounce and create the cupping pattern.

It CAN contribute to tire wear.

--
Bob Paulin - R.A.C.E. <><
Race Car Chassis Analysis & Setup Services
Chassis Blueprinting Services (as in engine blueprinting)