Seat tube motor bolts keep breaking?!

[soccerrj88]

Well I have an aluminum bike that has oversized down tube and seat tube. Well the front tube i make a motor mount for that looks just like the ones you can buy online and the back, I did pretty much the same thing. A piece of aluminum with a muffler clamp under and engine studs over. (see pic for all this might help) Well the origninal studs lasted about 450 to 500 miles before they snapped off. And they broke right between the motor and the spacer that is supposed to clamp to the seat tube. Well I upraded to 8.8 metric bolts that i turned into studs and those broke after1 ride. Well then I drilled and tapped to 1/4 20 with grade 8 bolts. They snapped after one ride! What is going on here? Anybody else have these problems.

[Venice Motor Bikes]

Get rid of the rubber engine mounts! (they allow the engine to vibrate harder & move around in the frame)

[2door]

Originally Posted by Venice Motor Bikes

Get rid of the rubber engine mounts! (they allow the engine to vibrate harder & move around in the frame)

Exactly what Venice said. Mounting the engine with pliable rubber mounts will cause the problem you're having. These engines are designed to be rigidly mounted. Get rid of the rubber and if that aluminum piece is in any way flexable get rid of it too and use steel. The more rigid you can mount the engine the less problems you'll have with failing fasteners.
Tom

[bairdco]

in case you're still not convinced, i agree with 2door and venice.

rigid is the way to go.

work on getting the other parts (fenders, chainguards, gastanks, anything else,) of your bike to stop rattling and vibrating, and you're in for a smoother ride.

[BarelyAWake]

Given you have an aluminum bike, if yer concerned with kinking the tubing with the pipe clamps you could always make steel sleeves to spread the load out a bit;

[FileStyle]

I concur to all who have posted! I also have a ride with oversized tubing (downtube,seat stay) and instead of rubber I use leather! (from a belt) the reason you are breaking bolts in my opinion is 1) chain is too tight, 2)rear sprocket is not mounted true center or 3) the motor is not quite straight in the frame! I too had that problem one time and it turned out the reason was because I had moved the motor in the frame and the sprocket was not dead center and I had tightened the chain when the rear sprocket was on the short side, so when I took off for a ride there was a terrible vibration, unlike before and before I made it back home, the left rear bolt had sheared! these were socket head bolts from my local bolt Co. not the factory stock studs. after I relieved the chain of the tension and readjusted my rear sprocket I have no problems.

[K.i.p]

Ok, this is some seat of the pants engineering that will be a little difficult to explain. Here goes. I am suspect of the thickness of the plate you are using too unify the motor and "U" clamp.

It is possible that as you tighten down the two nuts on the U bolt and against the plate it is warping inward as you achieve a grip on the seat tube. Although the warping in itself is tolerable, the two engine studs may well be being torqued laterally by the warped plate. Studs have strength longitudinally but they are not designed to counter shear forces.

And actually plate thickness issues and warping aside, your studs are not being used longitudinally at all regardless. The motor weight is resting in a shearing state across the studs. Lets add some constant vibration. Metal fatigue. I'm not trying to be a smart arse I just like to think (too much).
****
I just noticed the shear point which eliminates the warped plate theory. However the motor is still relying on the shear strength of the studs unlike a typical clamp set up where forces are distributed by well fitting blocks that can not rock by virtue their fit and the studs lengthwise strength.

[soccerrj88]

Ok. Thank you. I like the idea of the plates to spread the load. The original bolts must have been so cheap that they flexed and bent more before snapping rather than the high strength ones. Back to the drawing board to figure out how to mount it to fit! Does anyone know where I can get the wide steel plates. That's the reason I used the aluminum for the mount because it was the only thing that was 3in wide and the only steel plate I have is 1" flat stock. My Christmas list has a welder on it. Maybe it's time I get myself a gift and start fabricating my own stuff. lol If you have some pictures of anybody that has mounted to a oversized seat tube that would be awesome! I'm a visual kinda person! Thanks again.

[K.i.p]

Well I could be wrong, definitely consider all posts. It is just an opinion based on your pic and common sense mechanical interpretation. Good luck and hopefully there might be some more thoughts posted here that will help you.

[soccerrj88]

Well I agree eliminating the vibration may be just the token, i was just curious if anyone had pics of their's and where to get wide steel pieces.