Cracked chainstay bridge weld

[Rohloff_Brompton_Rider]

It's only brazing. If you know a plumber, borrow their gas gun. Then goto B n Q and spend £3 on brazing fillet rods (no need for flux as their rods are coated).

Get on YouTube, after ten minutes you'll know what to do.

Get into the garage / back yard and after another half an hour, including prepping, you'll be proud as punch that you've done the job yerself....

....it really is that easy.

 

[raleighnut]

The chainstay bridge is not a structural element of the frame and a crack there is neither here nor there. I don't know if that bike is alu or steel, but welding/brazing the crack involves a repaint as well. If it is aluminium, just leave it. If it is steel, you want to rust-protect it somehow. If it is cracked through and causes noises when you pedal hard, you will want to consider a repair.

You don't say whether you re-aligned the drop-outs. If you didn't, you may have premature hub failures as the hub axle now has to bend into a slight bow when you tighten the QR. Further, tightening the skewer/nut on such a frame with non-parallel dropouts alters the rear derailer alignment. You can see this happening as you tighten. Get yor drop-outs sorted. You get special frame alignment tools for the job but since this is a DIY job, do it by eye. Use a large shifting spanner to slightly (do the math and you'll see it is slight) end the drop-outs inwards. Now use the wheel as a guide - fit the wheel and start to clamp it with the QR whilst watching the gap between the drop-out and the hub's jamb nut. The space should be parallel and there must be no movement as you tighten the QR. If you can achieve that, you've done a perfect job on the frame.

I love the euphemism for "bending the frame open" namely "cold setting". I used to have "Cold Setting" as a labour item on my workshop invoicing system. Somehow I didn't like the term "Frame Bending". You can charge much more for a Cold Set than a Frame Bend.

The OP is fitting a Nexus HUB gear.

 

[Bodhbh]

The chainstay bridge is not a structural element of the frame and a crack there is neither here nor there. I don't know if that bike is alu or steel, but welding/brazing the crack involves a repaint as well. If it is aluminium, just leave it. If it is steel, you want to rust-protect it somehow. If it is cracked through and causes noises when you pedal hard, you will want to consider a repair.

You don't say whether you re-aligned the drop-outs. If you didn't, you may have premature hub failures as the hub axle now has to bend into a slight bow when you tighten the QR. Further, tightening the skewer/nut on such a frame with non-parallel dropouts alters the rear derailer alignment. You can see this happening as you tighten. Get yor drop-outs sorted. You get special frame alignment tools for the job but since this is a DIY job, do it by eye. Use a large shifting spanner to slightly (do the math and you'll see it is slight) end the drop-outs inwards. Now use the wheel as a guide - fit the wheel and start to clamp it with the QR whilst watching the gap between the drop-out and the hub's jamb nut. The space should be parallel and there must be no movement as you tighten the QR. If you can achieve that, you've done a perfect job on the frame.

Regarding the bridge not being a structural element - I noticed on some of the fixed/singlespeed forums it seems to be par for the course to remove it to keep the frame minamalist and preserve the 'clean lines' of the bike. The crack is gonna bug me tho! The bike is steel and I know it'll wreck the paint round there, but tbh it has a rat look already and I'm fine with that.

I didn't re-align the drop outs. Alot of people upgrade R20s to modern internal geared hubs with wider spacing and the impression I get is it's not a big deal to align the dropouts after spreading them. This is just me reading the R20 forums and upgrade blogs, not personal knowledge. I'll probably have a go now you mention it's a wrench job.

It's only brazing. If you know a plumber, borrow their gas gun. Then goto B n Q and spend £3 on brazing fillet rods (no need for flux as their rods are coated).

Get on YouTube, after ten minutes you'll know what to do.

Get into the garage / back yard and after another half an hour, including prepping, you'll be proud as punch that you've done the job yerself....

....it really is that easy.

I honestly don't doubt you - I'd have a go. Me being a pen pushing biochemist I don't know any plumbers! If there's any YouTube videos of someone fixing it with Bulldog Clips and A5 envolopes, I'm there! What I'll do, is there's a bodyshop about 5mins ride away that's open on Saturdays. I'll phone up and see if they'll fit it in and just ride down.

 

[Bodhbh]

They might prefer it a bit less fully constructed.

Yup, back wheel, mudguard and gear cable need to come off, I think it's a 5mins job. Will do it on the spot if needed.

 

[Bodhbh]

What about cranks and chainring? Would they be in the way?

Looking at it I don't think so, but good point. Best check they come off easy, as they're one thing I've not touched.

 

[Smurfy]

I was spreading the dropouts on my Raleigh Twenty 'ala Sheldon' from 120mm to approx 135mm to take a Nexus 8-speed hub and managed to partially crack the chainstay bridge weld. I knew this was a risk, but didn't have the gear to stop it happening (i.e. a couple of clamps and few of lumps of drilled out pine). The rear end is now 135 and aligned fine (again checked ala Sheldon), but I'm stuck with a cracked bridge.

Questions -

a) is this a big deal in terms of riding it?
b) it's gonna bug me even if it's fine, so I'd like to get it fixed - where's the best place to get a quick weld? Car mechanic? Correct me if I'm wrong - as it only needs a few globs of weld so far as I can see - but I can't see it's worth the attention of a frame builder.

a. Yes, it is a big deal. That little bridge may look small, but it adds a lot of strength.
b. This is a job for a highly skilled TIG welder or a frame builder. The tubing is quite thin, and getting the heat input spot on to obtain fusion, but without blowing a hole is definitely not for a beginner, part timer or someone used to welding garden gates.

 

[Rohloff_Brompton_Rider]

a. Yes, it is a big deal. That little bridge may look small, but it adds a lot of strength.
b. This is a job for a highly skilled TIG welder or a frame builder. The tubing is quite thin, and getting the heat input spot on to obtain fusion, but without blowing a hole is definitely not for a beginner, part timer or someone used to welding garden gates.

It may have been welded originally, but brazing will be good enough and doesn't risk 'blow through'.

 

[Smurfy]

It may have been welded originally, but brazing will be good enough and doesn't risk 'blow through'.

Totally agree, and of the two techniques, welding and brazing, I think brazing is what you're most likely to get at a frame builder.

 

[Yellow Saddle]

a. Yes, it is a big deal. That little bridge may look small, but it adds a lot of strength.
b. This is a job for a highly skilled TIG welder or a frame builder. The tubing is quite thin, and getting the heat input spot on to obtain fusion, but without blowing a hole is definitely not for a beginner, part timer or someone used to welding garden gates.

Please explain where it adds strength and what will be compromised by removing it. While you're at it, please explain what its original purpose was and why modern frames no longer have it.

 

[Smurfy]

The strength comes from the additional bracing. I'd guess modern manufacturers don't like it because it adds to costs. I doubt many custom steel frames are made without this component, where customers are paying a premium for a high quality product.

 

[Yellow Saddle]

The strength comes from the additional bracing. I'd guess modern manufacturers don't like it because it adds to costs. I doubt many custom steel frames are made without this component, where customers are paying a premium for a high quality product.

Nope.

 

[Smurfy]

I've bought two new bikes recently, one in 2013 (aluminium), one in 2011 (steel), both of them have a chainstay bridge in the frame. One might wonder what Yellow Saddle means when he says 'modern frames no longer have it'.

I think it's obvious to anyone who's ever done any kind of structural analysis, that the chainstay bridge increases strength by helping to resist forces that change the tracking of the rear wheel (i.e. side to side movement of the rear wheel). Yes, not all frames have this additional bracing, but those frames may be inferior, or compensate for the lack of a chainstay bridge elsewhere (e.g. a lugged frame with a very rigid BB shell). Other frames may be a compromise, in that the frame geometry requires the wheel to be so far forward that there is no room for a chainstay bridge.

 

[Pale Rider]

The chain stays are triangulated by the bottom bracket shell and the hub axle.

I can't see how the bridge adds any structural strength.

Then there is the application to consider.

It might be worth worrying about the crack on an extreme downhill bike, but not on a shopper.

 

[Yellow Saddle]

I've bought two new bikes recently, one in 2013 (aluminium), one in 2011 (steel), both of them have a chainstay bridge in the frame. One might wonder what Yellow Saddle means when he says 'modern frames no longer have it'.

I think it's obvious to anyone who's ever done any kind of structural analysis, that the chainstay bridge increases strength by helping to resist forces that change the tracking of the rear wheel (i.e. side to side movement of the rear wheel). Yes, not all frames have this additional bracing, but those frames may be inferior, or compensate for the lack of a chainstay bridge elsewhere (e.g. a lugged frame with a very rigid BB shell). Other frames may be a compromise, in that the frame geometry requires the wheel to be so far forward that there is no room for a chainstay bridge.

I see you won't reply directly to me yet you will make statements without substantiating them. Answering my question as to why a bridge increases strength by saying it does to because it is a brace is a cop out.

Yes it is a brace but a useless "brace". These bridges were fitted not to brace the bike, the triangulated structure already did that very well, but in the days of (near) forward facing drop-outs, when removing the wheel, the tyre jammed in the corner of the triangle. Bridges were then fitted to prevent that. Today's frames have near vertical drop-outs and we don't have the problem of wheels jamming in the frame anymore. However, they make very effective mudguard mounts and are therefore still preferred on some bikes that will ostensibly be fitted with mudguards.

Modern bikes have no space for a bridge and therefore no space for a mudguard. But, they are stiff enough to get away from a bridge as were all the steel and alu frames before the days of carbon.

As for the OP's dilemma - will be frame be OK without the bridge. The answer is yes, if the question is structural and no if he has a mudguard that relies on the bridge for mounting.

 

[Globalti]

This is correct. A chainstay bridge is so far away from the ends of the chainstays that it has no hope of exerting any control over the stiffness of the rear traingle, especially as a small degree of flexibility is designed into the stays. It can only be there to mount a mudguard.

I needed some deep chainsuck gouges repaired on a Ti frame I wanted to sell so I used Yellow Pages and eventually found a welder who thought he could do it. Went I went to see him, the foreman took me over to a corner of the workshop where some lads were having a lunch break and introduced me to "Mr Harris". Any fan of the film Apocalypse Now will remember the scene in the madness of the front line where a Vietcong soldier with a loud-hailer is berating the Americans in the night with tirades of abuse and they go to find the man with the rocket-launcher. A completely stoned dude gets up, takes his launcher, listens for a moment, takes aim and fires into the sky... a few seconds later there's a loud bang and then blissful silence. This was what happened when Mr Harris folded his Sun and stood up, rows of beads around his neck and a spaced-out look in his eyes..... he took the frame, glanced at the gouges and just said: "Come back tomorrow lunch time". I left with my fingers firmly crossed and this is what I came back to find the next day: