Bike Frame Repair

[Gizmo.Y]

I have been given a bicycle frame to repair. the guy cut the center post open to get the seat post out.

He has cut 2 x 2mm wide slots, about 8" long down either side of the tube.

Issue is, the tube still needs to be perfectly round inside for the new seatpost to slide in an out. I need to back the tube with something to stop the weld building on the inside of the tube.

Any recommendations?

 

[p0689109]

Copper tubing/bar to act as heat sink?

 

[Mr shiney]

Copper.

 

[grim_d]

Assuming the frame is aluminium, wouldn't a steel bar/tube the correct size work?

Cheaper than copper if it needs to be made or would steel affect the arc somehow?

 

[Gizmo.Y]

Its certainly steel.

 

[hotrodder]

Run, don't walk, RUN away. Whether it's a cheap clunker or (was once) a nice high end frame it's now scrap or a liability waiting to happen.

When making frames from scratch things like seat tubes need reaming afterwoods even if expanding heatsinks were used. Slipping a bit of copper or ally tube/bar inside what's left of the seat tube ain't gonna do jack to keep the thing round let along even close enough size wise... fractions of a mm too big or small and the seat post either won't fit or will keep slipping until the monkey overtightens the binder enough to break the frame again

 

[slim_boy_fat]

Would an expanding reamer work in this application?

 

[hotrodder]

Expanding reamers is what I (and everyone else i know in the bike trade) used to size seat tubes whether it was a new frame or a repair. Seat posts go in 0.2mm increments and 0.2mm undersize is enough to have a post that keeps moving or a broken frame if the binder is overtightened to try and clamp the post up. In theory it'd work for this case but you'd have to wave several bin liners full of £50 notes, a disclaimer witnessed by a herd of lawyers and a plane ticket at me to even consider touching this

If it's a cheap frame then it's not worth repairing, if it's a decent frame the seat tube will probably be butted (externally at that end)*, the tube won't be low carbon steel but one of various different alloy steels some of which are heat treated. Also we don't know if the frame was welded origionally or brazed... if the later then welding it anywhwere near a brazed joint is not gonna end well and while brazing is often done for aesthetics/various reasons sometimes it's because the steels used don't like being welded

* with brazed frames it's not uncommon to sleeve the post end of a single butted (at the bottom bracket end) seat tube instead as you can make a feature of it. Decent steel frames probably talking wall thickness of 0.6mm or less for a seat tube once away from the butted zones. A cheap (but not clunker cheap) plain gauge tube set like Columbus Gara is gonna have a wall thickness of about 0.8mm for the seat tube so not exactly plenty of material to remove to make the seat tube round again after fixing the mess

ETA if it's a proper cheap and nasty clunker that the owner is irrationally attached to then about the only i'd (not personally, just in theory) consider doing is making a steel seat post, setting it at the right height and welding the lot together

 

[Chris Stevens]

I have been given a bicycle frame to repair. the guy cut the center post open to get the seat post out.

He has cut 2 x 2mm wide slots, about 8" long down either side of the tube.

Issue is, the tube still needs to be perfectly round inside for the new seatpost to slide in an out. I need to back the tube with something to stop the weld building on the inside of the tube.

Any recommendations?

Yes.......... tell him the frame is scrap..............

 

[Mr shiney]

Well...... at the risk of incurring the wrath of the naysayers. ...... I have actually done this with my neighbours kids bike. I just shoved some copper pipe down and Brazed it. Still in use now........

 

[brightspark]

Well...... at the risk of incurring the wrath of the naysayers. ...... I have actually done this with my neighbours kids bike. I just shoved some copper pipe down and Brazed it. Still in use now........

well done u seem to have done the impossible

 

[Gizmo.Y]

I should of took some pictures.

He hasn't cut the tube it. he has cut a 1mm slot down either side so he could remove the old seized seatpost, he just wants the slots welding up - But i need to prevent penetration into the tube so the new seat post slides into the tube.

 

[Mr shiney]

well done u seem to have done the impossible

What's the worse that can happen? They can always breed a new kid.

 

[Mr shiney]

I should of took some pictures.

He hasn't cut the tube it. he has cut a 1mm slot down either side so he could remove the old seized seatpost, he just wants the slots welding up - But i need to prevent penetration into the tube so the new seat post slides into the tube.

Just stick some copper pipe down it and go for it. It's a push bike not the space shuttle.

 

[Chris Stevens]

I should of took some pictures.

He hasn't cut the tube it. he has cut a 1mm slot down either side so he could remove the old seized seatpost, he just wants the slots welding up - But i need to prevent penetration into the tube so the new seat post slides into the tube.

If he hasnt cut the tube, why are you asking about welding it up?

 

[Gizmo.Y]

hes cut the tube about 8" along the length of the tube to remove the old seized seat post. he has left a 1mm slit, 8" along down the tube. he wants it welding up so its flush like it was before.

 

[brightspark]

make some collars and sleeve it on outside

 

[sardine]

He should have cut the seat post from the inside rather than the tube from the outside.

Big copper bar turned to seat post diameter to keep it true, and weld it one tack at a time to limit the heat ?

 

[Chris Stevens]

hes cut the tube about 8" along the length of the tube to remove the old seized seat post. he has left a 1mm slit, 8" along down the tube. he wants it welding up so its flush like it was before.

I would say its scrap. Remember if you repair it and there is some sort of accident if it fails, you would be held liable................simply not worth touching! Wonder if its worth risking all that for maybe £50?

 

[hotrodder]

Welds shrink. They're gonna shrink no matter what. If it's a cheap clunker a chunk of copper bar is probably worth more than the frame. Turn that down for a snug fit and you end up with a lump of expensive copper bar stuck inside a knackered bike frame i.e. back at square one. If whatever is stuck inside the tube isn't a tight fit (so it stands a chance of coming out easily afterwoods) then it'll do absolutely nothing to keep the seat tube round