a design detail not worth repeating

[e.Gellie]

This busted-off chunk of a frame has floated around my workshop since I repaired the rest of the frame. It's a high-end mass-produced MTB frame from eighties. The larger sizes of these frames did not have any failures to my knowledge.

Discuss.....

busted.jpg

Ewen Gellie

[anthonymaietta]

What design detail are we suppose to look at?

[steve garro]

I will NOT miter a tube to a tube, ever. YRMV - Garro.

[anthonymaietta]

I will NOT miter a tube to a tube, ever. YRMV - Garro.

DT/ST joint?

[EricKeller]

not a tt/dt/head tube, that's for sure. I take it this is prompted by the thread on mtbr?

This failure is strange, it almost seems like there isn't a lot of deformation of the tt/ht joint

[Kevin Maxwell Ostrom]

What did the down tube look like? Was part of the top tube still attached to the down tube? Was this excessive amount of corrosion present on when it came apart? Could we see pictures of the Down tube?

Steve,
If the down tube was full welded/brazed to the head tube then had the top tube welded/brazed over that would that be better?

Kevin Ostrom

[EricKeller]

the two tubes being separated by some distance really helps take up the moment the fork puts on the head tube. Having the tt/dt mitered together also reduces the redundancy of the joint. MTBs take a lot of shock, having any sort of change in compliance of around the joint could cause an unexpected load path which will promote crack initiation.

[edoz]

Steve,
If the down tube was full welded/brazed to the head tube then had the top tube welded/brazed over that would that be better?

Kevin Ostrom

I'd say yes, but not nearly as good as both tubes fully joined to the head tube. With the head tube taking the kind of levering action that it does, especially on a mtb, I'd lean towards getting all the support I could get.

[progetto]

Aren't we talkin about a small frame here that maybe couldn't avoid having DT - TT overlap. Done this plenty of times on small road or Time trial frames but fully weld DT to HT then mitre in TT. Different situation , I know. Having a second look at the photo,maybe the DT - TT butt together and when welded at the intersection of the three tubes there wasn't any penetration with the head tube. Can you shed some light here Ewen.
Bill

[e.Gellie]

I reckon you guys have got this sorted out. I didn't know of an MTBR thread....

This frame was from the horizontal top-tube era, so the head-tube was really short on this 15" rigid-fork frame

The downtube to headtube weld really hung on...., the corrosion wasn't a factor there, since there wasn't any weld where the top-tube concealed the DT. The DT was mitred to the HT, then the TT mitred to the HT AND DT. It was welded in one go, so the DT to HT weld was never complete... a crack waiting to happen.

Kevin: the DT came away complete, and it took with it the bit of the TT that was welded to it. (It was complete enough that I repaired it, keeping the original DT with only a few mm lost off its length, and only new TT needed. Fillet-brazed back together and solid).

Yep, this is similar to and lots more critical than a ST to BB where it's concealed by the DT.

For a MTB frame, I'd fully weld the DT, and keep the TT separate, with a slightly longer HT.

For road, from what I've seen, the TT to DT mitre would be OK if the DT was fully welded.


Since this is similar to the set-up at the BB, it poses the question, anyone seen or had success or failure with not welding the DT complete at the BB? I've a couple of early frames out there, and no problems......so far....

Cheers,
Ewen

[e.Gellie]

And I reckon this track frame was never designed to run a front brake...Yikes!

There are times when maybe it's just not worth trying to achieve a horizontal top-tube.......

I love seeing all this off-the-wall stuff....a great way to learn.

short.jpg

[Tyler Evans]

We have been repeating this "design detail" for the last 15 years and never had a failure**. In some cases it is a necessity for proper fit. Works fine in both titanium and steel, at least when it is made properly. IME, ATME, BTW, FWIW.

[edoz]

Frame Failure, Be Careful - Mountain Bike Forums

This is the MTBR thread that was referred to I believe. Scary, and educational.

[Dornbox]

I don't think either of the images/bikes has/had a gusset... I'm more concerned about that detail than the mitering of the DT/TT/HT. As a compilation, it just seems stronger to me to have everything working as one larger unit if properly done. I feel that it's a weakest link thing.