Broken Damper Piston Rod -Penske Quarter Midget

[rmk36]

Originally posted by mdavis:
(although I heard something about aluminum piston rods being used in some dampers, which worries me)

Until just recently, the Kaz 7800 piggyback dampers have been running anodized aluminum shafts with no failures that I've heard of. The externally adjustable model has a hollow shaft as well. They just switched to steel, interesting.

[Gabriel Dessureault]

rmk36, you are not right in saying that the there were no failures with the aluminum shafts... In total, we had 6 shaft failures on our adjustable 7800 Kaz dampers. On the shaft shown in this case, you can see a reduction in the diameter of the shaft right where it sheared off. Additionally the adjustable dampers have inner thread to further increase stress concentrations.

As far as I know, we were the only team failing those shafts without an obvious design problem (ie: induced bending on the shafts). All our failures happened on the rear shaft, caused by vibrations (single cylinder....). Our shafts were replaced for steel ones as soon as they were released and we have had no failure since.

Without more information on your setup, it will be hard to suggest any solutions. Without any doubt, they are good parts supplied by very competent people. I do not think the design is ideal (too many stress concentrations if you ask me), but Kaz has very little say on that part of the process, since most components are off the shelf Penske. This being said, and for any parts on FSAE cars, I would try to get spare parts, to at least get you going quickly when this happens. We got really good with changing shafts by the end of last year, and learned a ton in the process. Maybe they actually did this on purpose, so you can learn about the inner workings of your shocks!

[Richard Pare]

Looks like typical torque moment bending concentrated at the root of the first thread of the shaft.

You get a couple of torque moments expressed on shocks - a moment due to friction in the spherical bearing in the end eye, and one produced by the uneven spread of forces produced by the spring against the spring retainer.

Do a study of the frictions produced at the spherical - you will be surprised at how high they can be. There are many ways that they can be reduced, and this is something that you will want to do to maximize tire grip, and is an area of a lot of research by the top professional teams.

Not much you can do to reduce the uneven face loading of the spring - something that is inherent in all coil springs, regardless of the manufacturer and one of the main reasons why F1 went to torsion bars - except to use the Hyperco hydraulic spring perches. Unfortunately, we do not make any for that shock.

The spring design can/will influence how much side load it produces (because of the uneven face loading) and rarely is it a design consideration with the inexpensive spring manufacturers, so don't expect those $12 springs from XYZ company to be anywhere near optimized!

Regardless of the spring design, they will also produce extremely high side loading once the coils start to bind - somewhere around 70-80% of available travel, depending on the design. So, make sure that you are not regularly forcing the spring to run in coil bind.

PS - if the alu shafts were anodized after threading, they were guaranteed to fail - anodizing reduces the fatigue strength by about 50%, and once it cracks, it starts a beautiful little stress riser.

[sbrenaman]

anodizing reduces the fatigue strength by about 50%, and once it cracks, it starts a beautiful little stress riser.

I would absolutely love to see the paper/study associated with this statement. Do you have a link/paper number?

Thanks

[Nick Renold]

Originally posted by sbrenaman:

I would absolutely love to see the paper/study associated with this statement. Do you have a link/paper number?

Thanks

There are plenty of them:

From the S-N curve, it is evident that there is a relationship between the fatigue life
and the thickness of the anodize coating. First of all, when comparing the 0.0-0.2 mil
coating thickness, to the 0.3-0.5 mil coating thickness, it is clear that 0.0-0.2 mil
specimens have a significantly longer fatigue life. Although not as significant, this trend
extends to the 0.5-0.7 mil coating thickness as well, which has an even further reduced
fatigue life. The 0.7-0.9 mil specimens do not follow the trend of reducing fatigue life.
Thus, the data suggests that increasing coating thickness beyond 0.7 mil does not have a
further degrading effect on the fatigue life of the specimen.

source: htt p : / / bit. ly / LCxco5

In an excellent text, titled Fatigue Design of Aluminum
Components & Structures, Sharp, Nordmark and Menzemer, a
chart shows decrease in fatigue life due to pre-cleaning
as well as the affects of Alodine and a couple of different thicknesses of anodic coatings, (page 110).
At a stress level of 35 ksi, (1 ksi = 1000 #/ sq
inch), the fatigue life of a non pre-cleaned, un-coated
sample was around 0.2 million cycles. Just the use of
a pre-cleaning compound labeled C2X showed a
fatigue life of 0.055 million cycles or a 3.64 factor
reduction. With no pre-cleaning and an anodic coating of 0.002 inches, the fatigue life was reduced to
0.035 million cycles or a 5.7 factor reduction.

source: htt p : / / bit. ly / MrDkyh

[sbrenaman]

Very cool, thanks.

[Z]

Abarth478,

Fuzzy photo, but that looks like a classic fatigue failure.

Maybe caused by the left end of the approx horizontal rod being shaken up and down over bumps, due to the actute angle between rocker-arm and rod (inertia of the spring-damper resists shaking, hence cyclic bending). Any binding, or BJ friction, or non-central spring load, makes things worse (as noted above).

Solution? Better design!

Z