Cracked chainstay bridge weld

[Yellow Saddle]

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:

Clearly the finger crossing worked. Nice job.

 

[Rohloff_Brompton_Rider]

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.

As a big powerful guy I know that the chainstay bridge is absolutely necessary for fixed riding. Bikes with no bridge flex so much the rear wheel rubs. Bikes with don't. Of course this is anecdotal and circumstantial - however, there is so much anecdotal evidence along the similar experience I've had its difficult to ignore.

 

[Yellow Saddle]

As a big powerful guy I know that the chainstay bridge is absolutely necessary for fixed riding. Bikes with no bridge flex so much the rear wheel rubs. Bikes with don't. Of course this is anecdotal and circumstantial - however, there is so much anecdotal evidence along the similar experience I've had its difficult to ignore.

Where does the wheel rub? By "fixed riding" do you mean fixed gear riding and if so, why is that different from other riding? How similar were the two bikes you rode where one rubbed and the other didn't?

 

[Rohloff_Brompton_Rider]

This framebuilder / tester agrees that chain stays improve ride and stiffness...


http://www.kirkframeworks.com/blog/2012/02/27/product-testing-or-not/

 

[Rohloff_Brompton_Rider]

Where does the wheel rub? By "fixed riding" do you mean fixed gear riding and if so, why is that different from other riding? How similar were the two bikes you rode where one rubbed and the other didn't?

Really or are just being obtuse?

Fixed means higher gearing and lots of standing up pedal mashing. It rubs on the chain stays.

 

[Yellow Saddle]

Really or are just being obtuse?

Fixed means higher gearing and lots of standing up pedal mashing. It rubs on the chain stays.

No not intentionally obtuse, just trying to get insight into what's happening there. I had no idea what "Fixed" means and before speculating, I'd rather ask.

A cursory look at the link you provided gave me no idea of the test method nor any readings, I'll read it properly later. However, you've given me enough to work on. You say the wheels rub on the chainstays. This means that all the movement is in the horizontal plane. Otherwise, you would have reported brake rub or in the case of no brakes, chainstay rub.

Now that we have the movement isolated, lets envisage what's happening if the wheel touches the chainstay when you pedal. It is not the flexing of the seat tube or downtube that's the cause, but the compression and elongation of the chainstays. We know that, because you would have reported rub on the FD. Therefore movement in your frame is isolated in a way I describe, in the horizontal plane. If you examine that plane, it is a perfect isosceles triangle with the chainstay forming two legs and the wheel axle the third. For that structure (a truss) to flex so that you get no brake rub, the two legs either have to bow or compress and stretch, with each pedal stroke. We can choose the original gap between wheel and chainstay - 1mm, 2mm? Working backwards from that figure we can calculate the amplitude of the strain in the triangle. In addition, the axle/frame interface is dynamic (not fixed) and by changing the shape of the triangle you will have movement between jamb nut and frame and this will manifest as rouge, especially if one of them is aluminium. If there is no rouge, it means that the axle is flexing. This manifests as a broken axle between the freewheel bearing and wheel bearing.

None of this is likely which in my mind then only leaves wheel flex and very specifically at the rim, not the axle. However, you report no brake rub (perhaps you have discs), which leaves the source of your chainstay flex a mystery to me. Perhaps someone else has some input.

What brakes do you have there on the back? Disc or rim and if the latter, are the dual-pivot or single pivot?
When these chainstays touch the wheel, do you get brake rub in front or perhaps in the case of disc brakes, rub against the fork legs?
What wheels were on that bike that flexed so badly?

The solution has to be elsewhere as a rigid truss just doesn't stiffen up with a cross-member so close to its sharp end.

 

[Smurfy]

I think it's fairly obvious that if the rear wheel is displaced sideways, then the two chainstays will move relative to each other. Just imagine the place where the chainstays meet the BB shell as a very stiff hinge, and it's immediately clear.The chainstay bridge increases back end rigidity by making it harder for the chainstays to move relative to each other.

With respect to wheel removal, there are much cheaper and easier methods of stopping a wheel jamming during wheel removal than carefully welding or brazing a tubular chainstay bridge. Also, my old Trek steel-framed mountain bike has a chainstay bridge, and that certainly wasn't installed for mudguard fitment. It's also got vertical dropouts, so it isn't to stop tyre jamming either. In the case of mountain bikes, to run a triple chainset requires a long BB spindle, or long chainstays to prevent the chainset fouling the chainstays. In this case a chainstay bridge is highly desirable to reduce the extra back end flexibility due to the long chainstays.

Oh, and I wouldn't go TIGing in chainsuck gouges on any frame, unless a weld procedure has already been fully tested, and found not to adversely affect the base material.

 

[Yellow Saddle]

I think it's fairly obvious that if the rear wheel is displaced sideways, then the two chainstays will move relative to each other. Just imagine the place where the chainstays meet the BB shell as a very stiff hinge, and it's immediately clear.The chainstay bridge increases back end rigidity by making it harder for the chainstays to move relative to each other.

With respect to wheel removal, there are much cheaper and easier methods of stopping a wheel jamming during wheel removal than carefully welding or brazing a tubular chainstay bridge. Also, my old Trek steel-framed mountain bike has a chainstay bridge, and that certainly wasn't installed for mudguard fitment. It's also got vertical dropouts, so it isn't to stop tyre jamming either. In the case of mountain bikes, to run a triple chainset requires a long BB spindle, or long chainstays to prevent the chainset fouling the chainstays. In this case a chainstay bridge is highly desirable to reduce the extra back end flexibility due to the long chainstays.

Oh, and I wouldn't go TIGing in chainsuck gouges on any frame, unless a weld procedure has already been fully tested, and found not to adversely affect the base material.

Well, be more specific as to where the movement is. Like I say, it is a truss and movement in a truss is well understood. Saying it displaces sideways o relative to each other merely repeats what Brompton has said. I have narrowed down the possibilities, which one do you choose or do you want to add others?

Give us some examples of these cheaper and easier methods to prevent a wheel from jamming.

 

[Globalti]

Oh, and I wouldn't go TIGing in chainsuck gouges on any frame, unless a weld procedure has already been fully tested, and found not to adversely affect the base material.

The frame was un-saleable with the gouges so my reckoning was that Mr Harris couldn't make it any worse! I sold it with that photo and clear description of what I'd done and the buyer was very happy with it. It was a Kona Hei Hei.

 

[Bodhbh]

Well it's booked in at the local bodyshop to get brazed this Saturday. If I remember, I'll try to post a pic of their handywork. I'm agnostic on the structural role of the bridge - but I plan on riding it loaded now and then so may as well err on the side of caution.

Irritating thing is, it will get taken to a proper framebuilder at somepoint. Once the stock BB fails, I'll like to have the shell faced rethreaded to to take standard BBs (it's 76mm and 26tpi atm), prolly some other work too, then ofc a powdercoating. But right now, I just want to ride it some and also paying bills is more of a priority.