Snapped Rear Mounting Studs (3rd time!)

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motorizedmatt

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Aug 31, 2010
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Hi Guys,

I cannot convey how aggrivated I am at this time. I just snapped my third rear mounting stud in about 2 weeks. The first one I can understand as I used to stock studs, but the second two I used upgrade bolts which I ordered from Sickbikeparts and they still snapped! Can anyone explain to me why this keeps happening? The motor is mounting correctly and is extremely sturdy except the studs keep snapping after I get about 20 miles on them, and I am not exceeding 28mph even.

Is my best option just to drill the stud holes bigger and use a larger bolt?

Edit: the bolt keeps snapping right at the motor and all three times it has been the bolt on the opposite side of the motor chain.
 
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azbill

azbill

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May 18, 2008
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the only reason for snapping studs (in my experience) is mounts are NOT seated to the frame correctly, causing the engine to flex side-to-side with the torque of the engine
 
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motorizedmatt

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azbill said:
the only reason for snapping studs (in my experience) is mounts are NOT seated to the frame correctly, causing the engine to flex side-to-side with the torque of the engine
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Thing is the rear mount is 100% flush, and when mounted I cannot move the motor, however the bike vibrates ALOT anything above 20mph. Attached are a few pics of my mounts.
 

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motorizedmatt

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Nougat said:
I know what your problem is. You've drilled a hole in the frame (which by itself makes me cringe) -- but moreover, you've got the mounting bolt through the frame and into one of the stud holes instead of using the three-hole adapter plate. Your engine is all cockeyed in the frame.

EDIT: Oh hold on, never mind. I see the plate now. The first picture there is at just the wrong angle.
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At first I was hesitant about drilling that hole too, however the front mount is extremely sturdy and none of those studs have snapped yet.

I have put countless hours into building this bike (I'd estimate around 40) yet I only have 100 miles on it so far :-||
 
corgi1

corgi1

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Aug 13, 2009
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Are you positive the rear wheel sproket is centered perfectly no excentric movement (tighten and loosining of the chain as the bike rolls forward) and the sproket teeth accept the chain seating firm and even all the way around and no bent teeth
 
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motorizedmatt

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corgi1 said:
Are you positive the rear wheel sproket is centered perfectly no excentric movement (tighten and loosining of the chain as the bike rolls forward) and the sproket teeth accept the chain seating firm and even all the way around and no bent teeth
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Honestly, the rear sprocket is offcenter ever so slightly, maybe 1-2mm. It doesnt make the chain go tight loose, tight loose though. It's really only noticeable if your going above 10mph and look down at and you can see it moving very little. And there are no bent teeth and the chain fits well on it.
 
killercanuck

killercanuck

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2mm can be alot at 20mph+, add that to the 'pivot' front mount and that can add serious stress to your poor rear mount. Have you thought of getting a universal U-bolt type front mount? Can you change your current front plate for a U-bolt?

At least your case isn't snapping in half :) If you have the tools, going to a larger bolt could work, just remember you're transferring that stress to your case then.

gl.
 
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motorizedmatt

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killercanuck said:
2mm can be alot at 20mph+, add that to the 'pivot' front mount and that can add serious stress to your poor rear mount. Have you thought of getting a universal U-bolt type front mount? Can you change your current front plate for a U-bolt?

At least your case isn't snapping in half :) If you have the tools, going to a larger bolt could work, just remember you're transferring that stress to your case then.

gl.
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The front mount has nearly no give though, I really doubt the front mount is the problem. I can't use the U-bolt because it isn't big enough and if I get a bigger one I am fairly confident there will be more play than there is now.
 
Kevlarr

Kevlarr

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Is there any space between the frame and the mount before you tighten it? In my experience if you have to pull the motor to the frame at all when you tighten the mount your studs are going to break.

Try adding a pair of 1/4" flat washers between the mount saddle and the block next time.
 
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motorizedmatt

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Can someone recommend a stronger mount design? I need to try to fab something up tonight as I use my MB as my sole mode of transportation. Or should I just try not tightening down the rear bolts as much?
 
killercanuck

killercanuck

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k, another option might be your hole through the frame is a little off where it should be, causing undue stress? Know what I mean? If its too low or high they(your bolts) would be under tension just sitting there. Does that sound right guys?

If its too low, try adding a washer or two between the plate and the frame, to raise it a little bit. If its too high, some grinding of your plate might be in order. Wait for the guys first though, to make sure.

Or it could be something completely different ;) Just getting some ideas out in case its happened to someone else before.

gl.
 
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motorizedmatt

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Kevlarr said:
Is there any space between the frame and the mount before you tighten it? In my experience if you have to pull the motor to the frame at all when you tighten the mount your studs are going to break.

Try adding a pair of 1/4" flat washers between the mount saddle and the block next time.
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There isnt any space between the mount, it sits perfectly flush on the tube. Could 2 washers really make that big of a difference though?
 
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motorizedmatt

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killercanuck said:
k, another option might be your hole through the frame is a little off where it should be, causing undue stress? Know what I mean? If its too low or high they(your bolts) would be under tension just sitting there. Does that sound right guys?

If its too low, try adding a washer or two between the plate and the frame, to raise it a little bit. If its too high, some grinding of your plate might be in order. Wait for the guys first though, to make sure.

Or it could be something completely different ;) Just getting some ideas out in case its happened to someone else before.

gl.
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I know what you mean however everything lines up perfectly and the back mount is pretty much perfectly vertical with the tube.
 
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motorizedmatt

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kipharley said:
It's in the Mounting Techniques Thread 2nd or 3rd page.Under Motor Mounting Help.Kip.
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I couldn't find it.

I replaced the stud tonight and didnt torque it down quite so hard, and used a washer between the block with 8.8 grade bolts. Rode it about 1 1/2 miles and so far so good. If it can make it past 50 mile mark then i'll be stoked .shft.
 
mapbike

mapbike

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Mar 14, 2010
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motorizedmatt, you are doing what I was going to post before I saw that you already had the idea, and YES over tightening those studs can stress them to the point that the added pressure from the vibration can cause them to break, what I do is make my studs long enough so that I can just torque them down good and snug and then I give-em just a little tweak past that point and then with the added length I install another nut against the first one as a jam nut which will ensure that nothing comes lose you just need 2 10mm wrenches to get them tightened down against each other, and I highly recommend you use BLUE Loctite on the studs as well, I also had studs breaking and also falling out on me until I started doing what I just described and I have never had another problem since and I have almost 800 miles on one bike and 66 miles on another I just built have not even had to retighten one bolt or nut on the new one yet because of how I secure everything from the get go.
Good luck on resolving this issue and happy safe riding.
Shan
 
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Chalo

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Aug 10, 2010
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motorizedmatt said:
Honestly, the rear sprocket is offcenter ever so slightly, maybe 1-2mm. It doesnt make the chain go tight loose, tight loose though. It's really only noticeable if your going above 10mph and look down at and you can see it moving very little. And there are no bent teeth and the chain fits well on it.
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The amount of eccentricity that causes tight and loose spots on a fixed gear or single speed bike is usually not even visible. You'd express it in tenths of a millimeter at most. But if the tight spots are too tight, they can cause noticeable resistance in the pedals, break axles, wreck freewheel bearings, etc.

If you must have runout in your drive sprocket, bias the chain tension towards looseness, so that it does not tighten excessively at any point in the wheel's rotation.

Chalo
 
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