Motor Mount Metal Fatigue

[Timbone]

Please forgive the twisted pics - I consider it a website issue - but this does tell the tale.

I think making motor mounts is the most fun part of building a motorbike. There are no rules and its success or failure will lead to learning something important about building motorbikes.

KC gave the best tip ever: use U bolt muffler clamps to hold the motor to the frame tubes. Saves a million problems, immediate and down the road.

I've been using these thick pieces of steel angle 4" X 2" with good success. Drill some holes to connect big Grade 8 bolts and lock them down with red Loctite. Very strong.

My piece on the tire shows the piece in theory, while the pic above shows how a small crack can become a major fail. I noticed that the exhaust was shaking at high frequency and I saw a long crack in the metal. Released the mount from the downtube and it literally melted into two right in my hands. No pressure whatsoever - complete fatigue of the metal. (Notice how the crack was no where near a drilled hole. I would expect that.)

I got the idea to go bigger. I found these unistrut angles and hoped to use four of them. I lacked space. My original design was just a hair too tight to make proper fitment. After lots of measuring and configuring, I came up with a hybrid design, using the unistrut angle and the builder's angle with support coming from a 1/4" mild steel adapter plate and a crossmember on top tying the two single prices of angle on the motor.

I ground on the top half of the round fitting part of the muffler spacer so I can lay the adapter plate back as far as possible to get the piece perpendicular to the engine mount studs. Worked well.

The unistrut angles have big 1/2" holes so I cut bushings to cover the motor studs and filled the gaps between the 14mm studs and the 1/2" holes.

Front to back I have: 1) muffler mount; 2) 1/4" adapter plate; 3) builder's angle; 4; unistrut angles 2 pcs; and 8mm engine studs protected by 5/16"x 1/2" bushings.

My first 30 miles have been magnificent. The bend on the builders angle is where I expect any metal failure.

=Timbone=

 

[Frankenstein]

Is that stuff used for bringing studs together while building? The metal in that stuff probably isn't the best if so, not for vibration at least, maybe use some angle steel... I tried a few different things but only seems like angle steel has the guff to stay whole for motor mounts.

 

[Timbone]

The angle I have been using is the thick steel pieces made for framing. They are very clean and smooth and at least 1/8" thick. They are strong, but not crazy hard to drill holes in. I had another one fail a couple of years ago. Super easy to replace.

I am really interested to see how these unistrut angles hold up. They are thick and strong - I can't imagine them failing. What's interesting is that each motor stud locks into its separate angle piece and I tied them together on the top.

So far, so excellent. The bike is flying with very little vibration.

 

[Frankenstein]

The angle I have been using is the thick steel pieces made for framing. They are very clean and smooth and at least 1/8" thick. They are strong, but not crazy hard to drill holes in. I had another one fail a couple of years ago. Super easy to replace.

I am really interested to see how these unistrut angles hold up. They are thick and strong - I can't imagine them failing. What's interesting is that each motor stud locks into its separate angle piece and I tied them together on the top.

So far, so excellent. The bike is flying with very little vibration.

I trust your judgment to pick out a sturdy metal lol, I've just had bad luck I suppose with less sturdy bits.

That set up is close to what I did but without the muffler clamp saddle. I used an angle steel and drilled downward onto the front mount perpendicular to the original mount holes. Put 2 bolts straight though.

The front of the angle that was closer to being parallel with the downtube was anchored to the downtube with basically a u bolt. The prying on the metals I used probably couldn't handle that type if stress while the steel can.

The saddle does make a difference, if you have a really good line up you can pull it off rather well, some bikes though are just awful.