XB Rear Engine Mount Warning

[ElliottM]

Hey all,
I am studying mechanical engineering currently and recently we did a Finite Element Analysis on the rear engine mount used in all XB's. (We were able to do this since the company is no longer producing the motorcycles).

Basically, the rear engine mount is prone to failure, sometimes catastrophic. For the normal everyday rider this is no problem, however if you like to do wheelies or happen to get into a front end collision you may want to take this into consideration.

The yield strength for standard aluminum alloys is approximately 40 ksi. In my analysis it was found that doing wheelies at 30 mph has the potential to surpass this yield strength, meaning the mount will begin to permanently deform. This is not good.

For the most part, the mount performs fine. However I would suggest getting it checked out every once in awhile if you enjoy your trick riding. This way you won't end up breaking the mount while riding it and end up taking it down on the road, which has happened.

After I had told a friend of mine about this, he suggested I should post something on here just to let people know. There is no reason for us to stop riding our Buells. I am keeping my 05 Firebolt till the day I die, however I am glad to be at least aware of the rear engine mount.

On side note, this was an undergraduate project that I worked on, but I am by no means an expert in the field. My professor, however, is an expert in the field and makes loads of money because he is good at what he does. He has tried many times to get me to stop riding my XB12R, I just refuse.

 

[konarider94]

wanna post some screen shots of the mesh and the von mises stress. im curious to see what you did. what program did you use? ive used abaqus quite a bit.

im curious how you determined the load on the mount while doing a wheelie. sounds like a pretty cool project.

 

[ElliottM]

I was trying to figure out how to get pictures up... as soon as I can I will post them. I used ANSYS Workbench.

 

[konarider94]

click the my pics tab up near the top and upload some, they have to be jpeg. then in the post click add your photo and all of your uploaded pics will come up. just click what you want to post. and what school do you go to?

 

[Livers]

Great work. I seem to remember threads across the boards maybe last season about failure of these mounts. After a lot of discussion and even more speculation, it came down to 100% of the failures occurred on bikes that had been wrecked.

 

[squidbuellie]

so basically your saying that our bikes are great looking engineered pieces of artwork. that can only handle casual bike riding? or that

it came down to 100% of the failures occurred on bikes that had been wrecked

 

[bfletcher]

My professor, however, is an expert in the field and makes loads of money because he is good at what he does.

There is an old saying, "Those who can't do, teach" Careful holding him on the pedestool.

 

[kyle_dewald]

I do know the rear mount will fail when a car hits you in the right rearset and swingarm, I did a study on it but mine was real world. haha I don't do wheelies and all that but it's good to know what can happen.

 

[Livers]

Here are the discussions I remembered:

BuellXB

Buelletinboard

And BadWeb here and here.

 

[ElliottM]

Here are a couple screenshots of my analysis using ANSYS Workbench...

@konarider94
I am a senior at Milwaukee School of Engineering in Milwaukee, WI. The loading due to wheelies was based off of assuming the engine mount experienced an acceleration due to the weight of the engine while moving at certain speeds. (i.e. 1.5Gs)

@squidbuellie
I am just saying it is something that riders should be aware of. Anyone with a mechanical engineering background will look at the engine mount and wonder why the engineers allowed for massive stress concentrations on a vital part of the motorcycle.

@bfletcher
Haha I am very familiar with that saying, especially at my school. However this one professor I know for a fact is a very successful engineer and therefore he taught for about 3 years and is now vice president of a company that deals in engine design. (Admittedly, I forget the name of the company...[smirk])

Also, you will notice that only half the part is shown in the screenshots. This is due to the symmetry of the part that was accounted for in the analysis.

 

[Baal]

This is pretty cool. I appreciate the heads-up.
[up]

 

[onelogue]

The yield strength for standard aluminum alloys is approximately 40 ksi.

Whats ksi? Is this number a constant? Because i figures the units would be (... /...)

 

[RudiRulez]

I can already envision a fix, slightly longer bolts and a bridge that joins A to B and could possibly even slot into the recess below C.

That way not all the stress is concentrated on the inside corners next to the bolt holes.

Hmm, wonder if its worth firing up the old CNC mill!

 

[dave_xb12r]

Why not just make one out of steel?

 

[RudiRulez]

yeh can do if someone can recreate the casting or has a 3+ axis mill , i only have 2 axis on my mill so wouldnt be possible for me.

 

[Livers]

Guys, it's $64 retail, still available. If in doubt, replace it.

http://www.lshd1.com/servlet/Detail?no=776258

 

[squidbuellie]

right on thanks for the heads up.

 

[ElliottM]

Yeah there are a couple ways to go about redesigning it, and personally I made a redesign with this project that could be cast (Lost Foam) and require little machining other than bolt holes.

Also, ksi is 1000 psi (lbs/in^2).

You should be able to check it fairly easily, and I would look right around the recesses for the bolts for any signs of cracking or failing. And of course if in doubt it might be better to replace it.

 

[Stevenc150]

Been here since '08 and a member on 3 Buell forums. Seen LOTS of wrecks caused by cars, other bikes, trick riding, flat out stupidity, etc. - have yet to see or remember seeing a single case where specifically, the rear engine mount suffered a catastrophic failure or otherwise.

Don't get me wrong, I agree it's good to be informed and this post is definitely worth a read. But I'm also a Mechanical Engineer that started as a Machinist. One thing you Cannot predict with exact accuracy, is how metal (especially alloys) will act in the Real World. It's like trying to predict weather or the wave height of a tsunami - there's simply too much information that isn't considered or isn't even yet to be understood.

 

[americanthunder]

sweet post thank for the heads up [up]