Frame crack
 

Hello everyone.. newbie here with a question regarding my Raleigh Tandem bike. I'm beginning to learn more and more about bicycle terminology so bear with me if I sound amateurish. The other day the front crank assembly came loose. I don't know how these cranks are held down, but I did noticed two bolts underneath the bike so I flipped the bike around to see if tightening these bolts would do the job. That's when I discovered a crack running along the center of the frame directly underneath the crank assembly. My question is this a common type of crack, looks more like a stress crack. The bike is aluminum so can this crack be repaired? I don't see it being a part of the main frame looks more like a weld that supports the bolt retaining area. I will try to post a pic later on because as they say a picture is worth a thousand words. Thanks all. Will

In this day and age why don't you post a photo with your question?

If you had just held off posting your question until "later on" when you had the photo you would have made it easier for everyone and not squandered good will right off the bat. No one can possibly tell whether a gap in a bike frame is a crack or is supposed to be there without being able to look at it.

A photograph would be very helpful. Front bottom bracket shells on tandems are commonly split to allow timing chain tension adjustment using the eccentric front bottom bracket. Your description is vague enough that I can't be certain that the "crack" you refer to isn't the split shell referred to above.

posted the pic, yes I should've posted the pic right off the bat. So whatcha all think a crack or something else. Thanks all.

It looks like a crack in the weld. Is the block covering a seam in the bottom bracket shell or is it just holding the eccentric holding bolts. if there is no open seam in the bottom bracket shell you might be able to just replace your eccentric with an expanding type. While aluminum can sometimes be welded it is cost prohibitave to temper the joint and this repair would require tempering.

Thanks for you response. "I think" its just holding the bolts, but as I said I'm really don't know my way around the mechanics of the bike. I'm learning though. Added more pics and hopefully will be a little more clearer to figure out the crack. Thanks.

I do believe that crack is a frame killer. You might find a good aluminum welder in the area and have it welded but if the frame is heat-treated you will destroy the heat-treatment and there is no telling how long the weld would last. It is better/safer to just find a new frame.

First of all, Raleigh frames usually have a lifetime warranty and this might qualify for free replacement. That's your first and faraway your best option. Failing that next best is to have a pair of steel collars made and fitted around the BB shell on both sides to support the shell and keep it closed properly. You might be able to use a split collar like one of those pictured here which would be better yet. I've had success with the single split style.

With any collar you'll have to get a diameter to match the shell and file notches on the collar to clear the downtube, seattube and chainstay welds. You'll also have to file away some of the plate welded on the bottom also.

As I said at the outset, it should be handled as a warranty issue and I'd pursue that aggressively before moving to a repair.

Thanks guys for your input. The collar idea sounds like something I could do. I'm not the orginal owner of this bike, bought it second hand so not sure if the warranty will cover me w/o a receipt. Hey FB.. how about two radiator clamps? Or is that too hokey?

Radiator clamps won't be nearly strong enough, especially after you notch out the clearances. Shaft collars have substancial wall thickness and structural strength which you'll need to get things to work.

BTW- if you go the the re-weld route don't have it repaired using the same design, where the set screw pressure stresses the shell. Instead have the crack cut open cleanly with a gap if 2mm or so, and have substantial ears welded on either side (possibly 3 pairs rather than the normal 2) so it can be clamped closed. This is the time proven design that's been used on the front BB of tandems for eons. of course, as others have said, a decent weld repair involves a number of complexities so make sure you go to someone who knows what he's doing.

Here is the style of eccentric I was reccomending http://www.cycle-frames.com/bicycle-...-BUSHNELL.html. you will need one that fits the ID of your existing shell.

+1 I would try to get it covered by the manufacturer. Frame is toast. It manufacturer refuses to cover it, I would start hunting for a good frame builder in your area. Repair may well exceed value of the frame. If so, I would start looking for a good replacement, and swap parts.

Thanks all for your advice, not sure which way I want to go now. I was hoping for the best, but preparing for the worst.

The steel-collar method to clamp the bottom-bracket shell around the eccentric will work only if the shell has a slot cut into it, which doesn't seem to be the case here. What I see is a billet of aluminum welded to a solid shell, into which set screws have been threaded. In this design the ends of the set screws pass through holes in the shell to bear against the eccentric and when tightened prevent it from rotating. What appears to have happened to the OP's bike is that the weld has failed under reaction stress from the set screws as FBinNY says. (As the screws are tightened, they want to lift the billet off the shell, and only the weld prevents that.) Our old Santana uses a similar design except that the the shell is steel and all one piece, just formed with a thicker "rib" for the set-screw threads, no welds to fail.

I agree with FBinNY's suggestion to cut a gap in the shell and weld two pairs of threaded "ears" each side of the gap. Two bolts that bridge the gap will compress the shell around the eccentric. Many tandems use this design and it works well. My only quibble (to avoid confusing the OP) is that using a steel collar to accomplish the same thing will not work until the gap is cut. Really, all depends on whether the crack we see in the photo is just the weld failing between the billet and the shell, or whether it is a crack through the shell itself. I think it is the former. The frame could be salvaged (if not replaceable under warranty) if it could be welded successfully as FBinNY recommends.

Agree also that Bushnell eccentrics are excellent but unlikely to be cost-sensible on an entry-level tandem even if one would happen to fit the OP's frame. The ears and bolts compressing a split shell are the way to go.

I'm hoping FBinNY will give the last word on this since s/he has more experience in fixing this sort of thing. My input is merely as a tandem driver.

I posted about using collars based on my interpretation that the crack was through the shell at the weld. For the plate to crack off at the outer edge of the fillet taking the entire fillet off intact it would have to be an incredible job of making the fillet with no bonding to the shell itself.

If the shell is indeed intact, then the fix is far easier. Simply use an expanding eccentric, or some bonding to keep it in place. Or possibly a partial split of the shell at the ends and a hose clamp might have enough force to provide traction to keep the eccentric from moving.

BTW- Conspiratemu1 is right that for the collars to work the crack would need to sawed open to about a 2mm slot to provide for compression, same as I described if ears for bolts were to be welded on.

For anyone else with this design BB. Though I think it's a lousy design, it's probably OK as long as users understand that the bolts need to be tightened only enough to keep the eccentric from shifting, and they should avoid overtightening the bolts at the risk of causing this problem.