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What type of bolt for rear shock studs?

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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Some of you may remember that I managed to twist off the threaded portion of a rear shock stud on my '80 1000GL over the winter. To resolve the problem, I sawed off the remaining threaded portion, drilled about half an inch into the stud and threaded to M8x1.25. I have fitted a bolt and washer to retain the shocks.

Right now I've just got a standard bolt in there, but it occured to me that since the studs are hardened steel, it would be best to use a hardened bolt in there. I suspect that the number stamped on the head of the bolt indicates hardness, but other than that I am without a clue. What type of bolt should I use in the stud? Best place to get it? Thanks!

Michael
 
Look for "Fasteners" in the book and see what you can find in your area. If you are lucky they will have a hardened steel stud and an accorn nut. If you explain to them now hard the orgional stud was and give them the dimensions you MIGHT be able to get even tougher hardware to compensate for the smaller diameter.

Steve
 
Correct me if I'm wrong -- just thought of this recently. Harder steel is less succeptible to change of shape over time, but also is more brittle. This makes it appropriate for something like a shock stud, which needs to keep its geometry, but not so much for other fasteners on the bike, for which a more reslilient, softer, steel is more appropriate.

I was planning just to fit a bolt in the hole I've drilled and tapped in the remains of the original stud. If I choose too soft a bolt, I will lose some of the rigidity of the original stud. But if I choose a bolt which is harder than the original material, I run the risk of having the piece shatter, right?

So, I'm not sure what you mean by "tougher" hardware -- it's a tradeoff, right? Any idea what hardness the original steel is? Do I want to go harder than that? Softer? Thanks!

Michael
 
Yes hardened steel is more brittle, but it depends how hard. What hardend steel does in this application is raise the shear strength. This means if you would grab the bolt and tear it apart sideways. With a sideways torsion on a soft bolt, the bolt would bend, but not with a bolt that was heat treated in a way to accomidate the intended application. Like I mentioned before this situation, it is a tricky one. You have to find out some info on the stock stud, like the rockwell hardness. Then you can get an appropriate bolt.

Take your time and figure this one out properly, the last thing you want is to have the stud break on a bump at highway speeds, this could be devestating. Not trying to scare you just trying to make you aware of the importance of this little stud.

Andre
 
Michael, You drilled the hardened shoulder and screwed in a bolt, right?
Then the load of the shock-absober is taken on the shoulder, all the bolt is doing is keeping the shock on the shoulder, which is not a lot of load. Think of how hard it is to pull shocks off stud without releasing the lower fixing.
As long as you are not using a really soft bolt you will be OK.
Here in UK our bolts are marked with hardness/tensile strength - 8.8/10.8 etc, the higher the numbers the harder the bolt, if no numbers it is a soft furniture bolt. Or just use an allen bolt (socket head cap screw) - they are always toughened
 
Michael, Paul is right. I completly stripped my shock studs from many cycles of removing and installing a rear rack (hate the look but damn are they handy). I was originally planning to have new studs welded on but a welder friend of mine explained that this would only weaken the frame (the less heat cycles the better). All the force is vertical. I was told you could almost cotter pin this type of connection with a washer! The stud is designed to have very little flex. If it flexed, the bolt would need to be much stronger as the vertical load would transfer into it.
 
Paul, I think we're talking about the same thing. I sawed off the threaded portion of the stud (up to the shoulder where the shock sits) and then drilled and tapped about half an inch into the shoulder (maybe a quarter or a third of the distance to the frame). I am comfortable with the strength of the bolt -- as you mention, there's not a lot of force on the bolt.

I also figured most of the strength of the stud needs to be at the shear point where the shock buts up against the frame -- and this part has not been affected. There should be no bending force in the area where I drilled, since that area is completely surrounded by the upper end of the shock. Does that sound right?

Currently I've installed unmarked hex-head bolts from the hardware store -- same as are used on most of the bike. But, I figured if there was any chance of a bending force in the drilled area, it would be worthwhile to try to match the original material as closely as possible. Any idea what number that would be?

Thanks for all the responses, guys -- this is the first somewhat structural mod I've done to a bike, and I understand it's a very important stud. I definitely want to get it right, and to make it as educated a decision as I can.

Michael
 
If you just broke off the threaded part, you have nothing to worry about. Like Jethro said a cotter pin could hold it on. I apoligize I thought you buggered the whole stud. The stud where the shock bushing fits over is the structurally important part, with this intact you are good to go.

Happy riding!

:P Dr. Dre :P
 
it would be worthwhile to try to match the original material as closely as possible. Any idea what number that would be?
Click to expand...

We have a rockwell hardness testor at work. If I remember, I will see if our QA will run a test on my shock mount.
 
For hardened bolts I use grade 8 which have 6 lines on the head, they are usually a gold color and resist bending. A grade 5 will bend then snap, a grade 8 will usually just snap. But they will take more torture than a grade 5, I have used them as hitch pins on the tractor when I don't have a real one on hand.
 
Michael: I was referring to the tensile strength of the bolt. Considering what I now know about your project it doesn't really matter what you use. I have in fact stripped one of these bolts on my trailers shock mount so I just drilled the bolt and stud to fit a cotter pin.

Steve
 
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