OMG! Broken vertical frame tube!!!

[tfhudson]

Okay,

This upsets me. My bike broke down tonight in the dark; it felt like the engine immediately locked up really tight. Fortunately, I was just starting from a stop, so it didn't result in a crash.

I was a couple blocks from home, so I ditched the bike in some bushes without trying to diagnose the problem, and walked the rest of the way back to get my truck.

When I returned to my bike and shined my headlights on it ... I saw the vertical tube on the frame had totally SNAPPED IN HALF where the engine mounts to it! It's an 80cc kings motor, mounted on a quality (Gary Fischer Aquila) frame, and I confess I drive it pretty hard. But hard enough to snap the vertical frame post?!

The likely cause: During installation I over-torqued one of the bolts that holds the motor block to the DIAGONAL frame tube. The bolt snapped off, with part of it in the block. Being in a hurry, I tightened a 6" hose clamp around the back of the block and around the diagonal tube, to substitute for the front mount. This may have loosened with the motor vibration, allowing the whole block to vibrate up and down on the back mount. Sudden acceleration/decelerations may have down their part to yank on the block via the chain.

But I can only imagine that is a partial cause. Will post more as I investigate the damage tomorrow.

 

[RATRODER]

tfhudson,whre in AR do you live? I lm just north of Russellville,maybe I can help.louis

 

[tfhudson]

You gonna buy me a new bike frame?

I live in LR. I was thinking about sliding larger piece of steel tube over the sheered ends of the other tube, and having a muffler shop weld it there. (The next step would have to be milling out the rear block mount to match the larger tube diameter, or otherwise fabbing a way to reattach the block to the vertical post...) Any ideas?

 

[Large Filipino]

You have any pics? Is it a steel frame or aluminum?

 

[tfhudson]

Steel frame, from a quality manufacturer. Tube sheered straight in half. Pics in a few moments....

 

[5-7HEAVEN]

I have a suggestion.

on your next frame, shove a seat post down into the vertical tube, at least to the point where you bolt the engine on. It will strengthen that area.

In fact, maybe all us frame-mounted members should proactively do this. the post is a press fit, and should substantially strengthen that part of the frame. i'll be doing that, when i work on my twin-engined project.

Myron

 

[tfhudson]

Wow, great suggestion, Myron! Great ideas are always simple like that!

I would highly recommend that for any frame-mounted adrenaline plant.

And to think I was over here drawing up external reinforcements to be welded to the frame....

(Sorry for the lack of pics - can't get them of my camera so will use another cam when I go home...)

 

[JemmaUK]

Hi,

Not that I have experience of the engines concerned but one thing has always bothered me...

Its basically the problem that you have an engine with an inherant vibration problem in (often) a frame that is overengineered for the stresses of the average bike rider but most definately not for a motorbike. If you want to understand what I mean, sit on the seat of an idling Field Marshall... We then tend to take a drill to the frame and dry bolt an engine to it, usually running the thing on wide open throttle... not a good mixture because it stresses parts of the frame that are designed to transmit stress not deal with it (those areas being the joints of the frame).

While there is always going to be some twisting stress on the engine mount and therefore to the sections its attached to there is very little that can be done with the current configuration and whatever the system there will always be torsional stress of some sort on the frame..

However - as regards vibrational stress - this can be reduced - namely by the use of the engine fin inserts for a start and also even the most minimum of rubber bushing between the mount and the frame. Granted that this may make the chain drive more mobile... but given the choice of chain slap or meeting the gravel face first at 40+mph i know which I would pick, not to mention the fact that a catastrophic failure may mean being punted into the course of an oncoming car..

just a thought ...

Jemma xx

 

[tfhudson]

Hi Jemma,

good observations, for sure. I'm definitely gonna reinforce the frame on my next build (or this one, if repairable).

So how might we arrive at mentioned rubber bushings? I've been contemplating taking sheers to the soles of some old tennis shoes...

Seems the seat post idea should do a great deal to reinforce the area, and I'm even thinking about filling the tube with a heavy resin of some sort to add inertia/ absorb vibes.

 

[ocscully]

Seatpost solution?

I don't know the particular bike frame you mentioned , but it is very probable that the seat tube is a double butted tube. This means that the ends of the tube have a thicker wall than the middle section of the tube. On most mid to high end mtb's the part of the seat tube that matches the the diameter of the seatpost is the top 1-2 inches? So slipping an extra seat post down to where the engine mounts won't do the trick, unless the frame tub is straight guage or mild steel. You should be able to measure the ID of the tube at the break to confirm the diameter but I'm pretty sure that you will find that it is just enough larger than the seatpost and will rattle a bit.

ocscully