Frame crack on folding bike

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Richard of York
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Joined: 21 Mar 2021, 6:41pm

Frame crack on folding bike

Post by Richard of York »

Got an approx 17 year old Dahon Speed TR. SRAM gears, 3 SPD hub and 8spd cassette. Been a good commuter and workhorse. I'm retired. Bike needs fettling but wary of replacing parts as frame has a seat tube crack. It seems to be a design weakness. The seat post needs clamping tight to stop seat post movement. Over time this and the general leverage from ride with long seat post has caused a crack. I only occasionally use this bike locally now, things like when I drop car off for service or tootling with grandkids. Would a weld be a permanent repair or would cracking be a recurring issue. Trying to decide if I try get a repair or just use bike until crack becomes terminal.
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Cyckelgalen
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Joined: 21 Sep 2018, 11:29am

Re: Frame crack on folding bike

Post by Cyckelgalen »

I own a Speed TR, bought second hand, that I have used on a few tours, fully loaded with camping gear. I doubt your frame crack is a recurring issue or design flaw. Also, I know a few Speed TR owners and I have never heard of any frame failure, the only issue that I have seen reported online is frame hinges developing play and wear.


It is a great bike that rides better than most folders (apart from the fact that it weighs a tone),
so I definitely think it's worth repairing. Bear in mind that the interior of the seat tube has to be perfectly round. Any burrs or welding seams protruding on the inside of the tube with prevent the seat post from sliding in when you fold the bike. If not the most elegant, the easiest approach is probably to weld a sleeve on the outside of the seat tube. Being a common 4130 steel frame, you have no problems finding someone to do that job.
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531colin
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Re: Frame crack on folding bike

Post by 531colin »

I think its already a case of mend it or dump it....the crack already goes round half of the tube circumference.
If it was me, I would
drill holes a few mm diameter at the ends of the crack....this is said to help stop the crack extending.
make up a bit of steel (I prefer stainless) which is a good fit round the outside of the tube.
Silver solder the patch in place. viewtopic.php?t=49335&start=15
I think I could get it hot enough with 2 blowtorches burning MAPP gas....thats hot enough so the work melts the spelter, you don't want to be melting it with the flame.
Or, get a framebuilder to stick a patch on it with bronze brazing.
I think it will be easier to ream the seat tube after brazing than after welding, and you don't want to take off any metal you can avoid taking off! Brazing will give less distortion of the tubes than welding.
I would also be looking for a seatpost that was a bit better fit, or maybe stiffer. (you can carefully ovalise the bottom of the post a bit to stop it rocking as much)
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simonineaston
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Re: Frame crack on folding bike

Post by simonineaston »

It's impossible to know from the outside the full story of how or why the crack has emerged. Without knowing more, it must represent a risk. It might continue as-is for decades - or separate fully tomorrow.
S
(on the look out for Armageddon, on board a Brompton nano & ever-changing Moultons)
KM2
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Re: Frame crack on folding bike

Post by KM2 »

I think you can get it through welded and reemed.
fastpedaller
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Re: Frame crack on folding bike

Post by fastpedaller »

Is there also a crack where one of the seatstays joins the seat tube?
cycle tramp
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Re: Frame crack on folding bike

Post by cycle tramp »

I really rated the speed tr frame, for me it was a very capable folding bike let down by an over complicated hinge mechanism.
Personally I'd get the crack sorted and continue using it
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AndyA
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Location: Edinburgh

Re: Frame crack on folding bike

Post by AndyA »

Looks like there's a crack on the left seatstay/seat tube junction as well. It needs repaired or retired now

You could get someone to TIG or MIG weld the cracks, then file or ream away any burn-through on the inside of the seat tube.
For belt and braces, file down the weld then braze a patch cut from a tube offcut over the top (EDIT this repair has the best chance of outlasting the rest of the frame)

These frames (and a great many others for what it's worth) use a seat tube that's just a touch too light gauge
home
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Re: Frame crack on folding bike

Post by home »

Richard of York wrote: 2 Jul 2022, 10:42amI only occasionally use this bike locally now, things like when I drop car off for service or tootling with grandkids.
If it's a steel frame, get a steel seat post, set it to your favourite position (you're not going to grow more), and weld it all together. It's basically going to end up as what's called a rivet weld, i.e. you're going to drill holes at the end of the fracture, & weld through them into the seat post, as well as along the crack and up the expansion keyhole. Even round the top.

Someone will know better if it can be brazed in a similar way.

Bottomline is, it's not worth doing "properly", i.e. welded and reamed to size then repainted, so just weld into place a long seat post as a reinforcement.

My guess is you used the seat post at almost full extension, therefore it was wobbling back and forwards a bit.

If you're really tall, & need a full extension, you can use a simple steel tube, & then clamp a small, short seat post into the top of it. Then paint.

"One can"/some will weld stainless to mild steel (not all welders will, you need special rods) but the latter way would also allow you a degree of adjustment & finish, e.g. using a very short protruding alloy seat post in a 'welded into place' [stainless] steel tube.

But you probably don't need to. Sort of thing a local car exhaust shop could do. No need to go get ripped off by a bicycle builder who doesn't really want the work.
Last edited by home on 5 Jul 2022, 6:58pm, edited 3 times in total.
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531colin
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Re: Frame crack on folding bike

Post by 531colin »

If you are going to permanently fix the seatpost, then braze/silver solder is the obvious way to do it because you can run the spelter in through a small hole and get a huge area brazed together. Won't help the OP get it in his car, though.
As somebody said it might be possible to permanently fix one bit of tube in the frame (again, I would braze, for the reasons given) and have a smaller diameter seatpost working in that. But you would need standard tube sizes to work for you, unless you get something turned.
And you CAN braze/silver solder stainless and mild steel together.

Heres a steerer I extended, sleeved internally with a bit of quill stem

Image005 by 531colin, on Flickr

Image003 by 531colin, on Flickr
bgnukem
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Re: Frame crack on folding bike

Post by bgnukem »

The crack has extended around roughly half of the seat tube circumference, which would be the half placed in tension by the bending force applied by the seat post, so I think the crack may not extend much further. Drilling small holes through the ends of the crack to stop further propagation, as Colin suggested, might be a good idea. Be careful that the ends of the crack may be beyond the visible rust / paint cracking so use a magnifier to check and perhaps polish up the area with some emery paper.

You would want to check that the crack does not continue to propagate. The seat post itself should help to hold things together in the worst case scenario that cracking continued further around the circumference of the seat tube, but if the crack encircles the seat tube and/or reaches the seat stays then I'd scrap or repair the frame.
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Chris Jeggo
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Re: Frame crack on folding bike

Post by Chris Jeggo »

How far down the seat tube does your seatpost go? How good a fit is it? It might be that a seatpost that goes at least 3cm below the top-tube/seat-tube joint and that is a really good fit might work.
Just a thought.
home
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Re: Frame crack on folding bike

Post by home »

Can the OP measure or show us a picture of his seat post and seat tube? Including internal and external diameters. I think I'd try for a lot more than 3cm. It's a v short frame, so try going all the way to the bottom/
531colin wrote: 5 Jul 2022, 9:50amBut you would need standard tube sizes to work for you, unless you get something turned ...
I've use a grinding stone, or flap wheel to re-size IDs a little. For just such jobs, I got one welded onto a long rod so I could run it down inside tubes to, say, remove the internal seam on cheap tube. I'd pay the extra and get seamless tube.

But is this what it looks like? What's the ID of the seat post?

May be you could use it, cut it's top off, slide it all the way to the bottom of the frame. Then put an adjustable seat post into its top? It's alloy though, so you couldn't weld it to a steel frame but you could epoxy glue it.
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Last edited by home on 6 Jul 2022, 2:21am, edited 1 time in total.
fastpedaller
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Re: Frame crack on folding bike

Post by fastpedaller »

home wrote: 5 Jul 2022, 3:10am "One can"/some will weld stainless to mild steel (not all welders will, you need special rods) but the latter way would also allow you a degree of adjustment & finish, e.g. using a very short protruding alloy seat post in a 'welded into place' [stainless] steel tube.

Is the above true? I have welded a stainless car exhaust pipe to a mild steel exhaust pipe using regular 6013 arc welding rods - It has lasted several years without fracture or leak, so I guess it's worked. I wouldn't profess to being a skilled welder either,
Back to the bike..... not a candidate for arc welding I'm sure, and the above suggestions of brazing or silver soldering seem good. I think I'd bin it though, otherwise the next failure could cause injury!
home
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Re: Frame crack on folding bike

Post by home »

home wrote: 5 Jul 2022, 3:10am"One can"/some will weld stainless to mild steel (not all welders will, you need special rods)
fastpedaller wrote: 5 Jul 2022, 11:19pmIs the above true? I have welded a stainless car exhaust pipe to a mild steel exhaust pipe using regular 6013 arc welding rods - It has lasted several years without fracture or leak
It's a common question on welding forums, E309 or E309L welding rods is a typical answer. I think 312 too.
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