Why does the oil tank bracket break?

concours · Jul 25, 2018

maylar said:
The rear oil tank bracket on my 74 850 has broken off multiple times since I've owned it. It's been welded, reinforced, epoxied, and still breaks. I replaced the tank with one from my buddy's parts stash and that one too has broken. Twice.

Why? What are the stresses that cause that piece of metal to fracture? My buddy's 72 uses the factory rubber mounts like I do and his has never broken. What is it that's different from one bike to another that puts these things under stress sometimes and other times not?

Yes, I know I can send it to CNW for a permanent solution - but I want to understand what the underlying problem is with the factory mounting.

Design sucks. Bracket too weak, flexes along with the rubber.

marshg246 · Jul 25, 2018

maylar said:
The rear oil tank bracket on my 74 850 has broken off multiple times since I've owned it. It's been welded, reinforced, epoxied, and still breaks. I replaced the tank with one from my buddy's parts stash and that one too has broken. Twice.

When my rear mount broke I ordered a strip of steel about 50% thicker than the original and cut it longer than standard so it went further down the tank. Then I brazed it on. I feel like brazing causes less brittleness. Then I used a new standard mount and formed the front and rear top mounts so that there was no stress on either when the tank was mounted. I believe mine broke because the front mount was solidly mounted to the frame by the PO and the rear mount was a little loose. For it to break where it did meant that I was getting front/back movement.

texasSlick · Jul 25, 2018

General rules of thumb to prevent/reduce vibration stress fractures:

1) increase gauge of metal used for bracket
2) stiffen bracket by welding on a rib
3) use a generous radius on bends ... 1/8" minimum
4 do not weld on edge with a radius ..... doing so reduces the radius
5) temper material after welding
6) no sharp corners, nicks, or cuts ...... i.e. stress risers

Also, isolate source of vibration using rubber. Strive to eliminate relative motion (flexing) between bracket and tank.

Slick

fredful · Jul 26, 2018

Have a look to see if the seat base is resting on top of the oil tank when you sit on it, I had the same problem till cut 1/4" off the edge of the seat base so it was not supporting my weight as well as the tank itself.

Chris Zet · Jul 26, 2018

did the same as marshg246 said....thicker metalstrap....running longer down the rearside of the oiltank .....spot welded....welding spots every 1-2cm..... mine broke also from now and then when i fixed it as it was original. the why that Maylar is asking here is imo: the construktion and the bracket is to weak but stiff....and when there are tensions - it breaks...it`s welded all the line down so it can handle the movements/vibrations so it breaks

maylar · Jul 26, 2018

ludwig said:
Stop welding . Remove the bracket and replace with a clamp around the filler neck :

Ludwig, I've seen your posts on this before, and I've not been able to figure out how to get a strap around the filler neck with the breather pipe in the way. Do you have any photos of how you accomplished this?

maylar · Jul 27, 2018

ludwig said:
Well maylar , you've got me there !
I never gave that much thought , because on my alloy oil tank , there is no vent tube in that location ( 1st pic ) .
So I took an old oil tank and made a clamp .
About 20 mm wide , but above the vent tube reduced to 10 mm .
It does not interfere with the tank cap .
For demo only . With a little extra time , you can do better .. :

Thanks. You've given me some ideas.

Why does the oil tank bracket break?

concours

marshg246

texasSlick

fredful

Chris Zet

maylar

maylar

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