Piston Rod Material for Extreme Abuse

[kritter]

If money was no option what would be the shaft of all shafts to resist sand blasting, rock dings, and general abuse suffered in an offroad application?

If money was important in the same application what would you choose and why?

Currently for street applications the material is:
1045/1050 CHROME PLATED BAR
TENSILE STRENGTH = 115000 PSI
YIELD STRENGTH = 100000 PSI
ELONGATION ( % IN 2 " ) = 19-26
SURFACE HARDNESS = HRc 48-54
SURFACE FINISH = 4-6 RMS
CORE HARDNESS = HRc 30<MOD-PM> 3

When this material is used in an off road application it gets dinged, chips the chrome and then destroys the seals, obvioulsy a failure.

Currently we have looked at our competitors who are using:
15-5 induction hardened.040" deep 55-60Rc
17-4 H900 with .0005" chrome 55-60Rc
100ksi Nitro Steel
4340 chromed and heat treated to a 68-72Rc

and the powers that be have decided 17-4 H900 without chrome is the way to go since there is no chrome to chip and rip seals(my argument is that it will dent instead and slowly leak out and its worse off then we currently use other then tensile which is not currently an issue). Me looking at the specs disagree due to the 42Rc surface hardness and would at least like to chrome it to bring the surface hardness up a bit, but I need some good support for my argument.

Is 17-4 H900 chromed or not chromed even a good idea?

 

[Fabrico]

Are we to assume you are referring to shock absorbers?

I am curious as to what steered them to the H900. You might try a search on this site for that alloy as it has been discussed by the materials experts before. Or state your question there. Using 17-4PH plated to 55-60Rc is fairly common among race shocks.

If money is no object, you can have the shafts burnished. Diamond burnishing can give you a surface finish of down to around 4 microns or even less. This will make them much smoother which would help them tolerate dust and last longer. It will also add some surface hardness.

There are coatings that can run up to around 80-90Rc, but then again check with specialist in that area.

 

[GregLocock]

I suppose gaiters over the rods are too sensible?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warpspeed]

My thoughts exactly.....

 

[kritter]

People dont always use them or they fall off or rip off...either way, its a failed shock which can cost a race.

100mph through the desert terrain does wonders to equipment.

 

[GregLocock]

I'd look at what OEMs use for shock rods.

Gaiters aren't always the answer, apparently you get a fine paste of oil and dust on the rod that never gets cleaned off, which wears the steel. I don't think that would matter much in 1000 miles.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NakedTestMonkey]

Given the frequency of oscillations wouldn't a gaitor also promote shock overheating to a small degree?

 

[kritter]

Delphi is the only tip I have got. These are not your everyday shock absorbers. 3" rocks coming off the front tires bashing into anything in its way rearward means stuff has to be hard.

OEM would be a good start, but any engineer at an OEM designing shafts to hold up to the abuse desert racing offers wouldnt have a job very long! Or maybe thats why they are in the whole so bad...make there cars durable enough to go desert racing!

I dont think the OEM had anything like this in mind other then ford. Ford posts contingency in the stock off road classes for R&D purposes..basically the only part you cant touch in the stock class is the shocks for the obvious reason that they would last 2 miles in a desert race before they failed.

Notice a couple hundred thousand dollar race car and no gators...
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http://www.mysite.com/images/happy.gif

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another with no gators...9 of 10 race cars dont run them is my guess.

You can see how much shaft is exposed and how close it is to the tire. I think gators collect too much dirt and then ruin the seals way too fast.

Shaft speeds are high, heat is high, dirt, dust and rocks everywhere...

 

[kritter]

Greg

ford stated that 1000 miles offroad racing is equivalent to 100,000 or 1,000,000 I cant remember which and that is why they have an offroad race program for stock vehicles...cheap R&D. They give a few free vehicles away every yea that are bare bone and supply OEM parts to the teams...pay out is 10k for a win per race. I dont think you can get any better R&D then that for the price.

 

[kritter]

ford backed stock vehicle other then tires, shocks and added safety equipment.

http://videos.desertracing.com/videos/2003/v2r/wmv/93.wmv

 

[GregLocock]

Yeah, I'm aware of off-road buggies, my ex-mechanic races one rather successfully.

I don't think you perhaps appreciate quite what a durability test for an Australian OEM shock involves. We don't do it for fun, we do it to reduce warranty costs. If it turns out that it is the same spec as you currently use then fair enough, but until you've checked then I think you are missing a useful source of information.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

I have worked on several very successful off road race cars.

We used a polyurethane bellows shaped cover over the shaft area and small attachment areas. We held them in place with cable ties.

I presume this is what Greg calls gaitors.

We removed the covers after every race to clean and inspect the shafts.

We used Bilstiens and Old Man Emu brands (I think manufactured by Munro Whylie) depending on size available and sponsorship arrangements. I was not directly involved and it was 15 or 20 years ago, but I think OME custom made long travel big bore shocks with a remote reservoir and a Schroeder valve for pressurising.

I think a few degrees hotter is a better result than a shock with a leaky seal.

The body work was designed to at least minimise the exposure of the shocks to debris.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

I'm only familiar with the Fox shocks with external valving - a very neat concept which looks tailor made for a remote heat exchanger.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

The buggy I worked on was a VW aftermarket type suspension with extra long trailing arms.

I think we had 18" front travel and 24" rear travel.

It was powered by A VW golf motor with a small roots type blower and Bosch/Kuglefischer mechanical fuel injection.

We never had a shock failure.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[kritter]

Greg:

Where would I find such sources. I came here to ask since I couldnt find much OEM data available.

 

[Fabrico]

To further address the problem instead of the question...many desert race cars run either bellows, fixed plastic, or floppy type covers to protect shock shafts. This is especially true on the hand built, one-off's, and gas-sprung shock absorbers, such as those built by Nye Frank or Robby Gordon. They are simply too expensive to let rocks ruin them for no good reason. Bilstein off-road shocks, and many motorcycle suspensions have used bellows type, or rigid plastic shields for many years with great success. PPI used plastic and other shielding to protect shock shafts and many other components on their famous Toyota/Ivan Stewart Trophy-Trucks.

You can lesson damage by using harder materials, but no exposed shaft of any material will hold up to solid rock hits. Gaiters, covers, or whatever, are a proven sensible solution used by proffessionals and factory designers, but are often avoided by the rank and file purely because of cosmetic reasons.

 

[TestDriver]

Hello,

I'll second Greg's comment on OEM durability concerns and I agree with Fabrico's suggestion of using hard plastic covers.

In my opinion, if you used something similar to modern dirt bike fork shields, it would be an attractive and elegant solution.

Ramon Mendoza