Chopping out a Seatstay bridge, sensible?

I’m currently hacking about an old steel (kona) MTB frame, it’s going to be used as a Road/towpath tourer/commuter/winter road bike.

I’m welding on Disc tabs and grinding off the old (26″) canti’ mounts, I’ve also offered up a couple of 700C wheels/rims in the frame and at present it looks like I would be limited to 622x32mm which is adequate but being able to fit something like a 40mm tyre would be nice.

The only thing preventing a bigger tyre being fitted is clashing with the seatstay bridge, part of me is wondering just how essential this element of the frame really is from a structural point of view, and if I can get away with chopping it out to increase clearances… Before I get busy with the angle grinder offer me some reassurance (or not)…

Kona obviously just put it there because they had a bit of extra material lying around and the welders were bored. The only thing that could possibly go wrong would be the frame cracking and needing to be binned. I say, grind away!

PP Shans don’t have a seat stay bridge, and they’re designed to be flung around. If it’s just a pub bike, why not?

seatstay bridges were critical when rimbrakes were a big deal – I can’t imagine they do a huge amount on a mountain bike being ridden to the pub

If you are welding on disk tabs, could you not modify and raise the stay bridge a little further up?

While you have the welding kit out and the frame on the bench like.

What Jef said, why not just move it further up?

It probably ads a smidgen of stiffness to the rear triangle so without it the flex might cause something to crack in the seat collar area.  It’s only a might though as the seat stays were probably heavier gauge than required in order to deal with the canti brakes.

Moving it would cause more stress than removing it.

I’d also think about 650B wheels and tyres instead, would allow you to use bigger volume tyre without upsetting the geo/raising the BB as much. 584 x 38 or 42 should fit within the available space, keeping outside rolling diameter in check but giving more cush. Just a thought…

I cut out my chainstay bridge on the cx for mud clearance with no ill effects

Seat stay bridges very useful for:-

mounting rim brakes

mounting mudguards

stopping stays bowing apart under canti brake loads

If you don’t need to do any of that then they aren’t needed.

Welding it back on a bit further up is 100% more effort than just cutting it out, grinding the tube smooth and re-painting (rattle can) the stay.

This is just going to be a rough and ready mod, nothing fancy and I’m removing the canti bosses too, it’s only going to be used on towpaths and roads. Of course having something there to fix a guard to would be beneficial, but that could be done with a simpler plate or other ways.

The extra clearance is really just a “would be nice” rather than a vital need.

I think I’ll leave it for now, I’ll probably only fit 28mm tyres to start with anyway…

stopping stays bowing apart under canti brake loads

Wouldn’t the stays bow apart under vertical loads also? As in due to the forces from the rider?

Seems a sad thing to do to a nice old frame for 8mm of tyre, 26″ wheels will actually run fine BTW.

The stays don’t do mcuh tho, likely OK to do.

It’s only a fire mountain and using it with 700c wheels is the plan to allow for a bit of interchangability with my other bikes.

Would the seat stay brace also not help to prevent differential shear of the combined seat/chainstays, or is the triangulation of each side sufficient to stop this? I’m just wondering if you’d get more disk brake rub when cornering or leaning th bike to one side.  I feel a need to FE it.

I think it may well be a bit of a “convention” in that frames have alway had bracing local to the wheel, in road bikes it supported a caliper, on canti/V braked frames it provided some bracing for reaction forces when the brake was applied, and in all instances its a handy place to attach a mudguard…

Like others I also now have an MTB which has “braceless” stays obviously as it is disc only and not intended to have mudguards that makes sense…

A couple of cable ties should sort it.

cynic-al   Member

Replace it with a spoon

Yeah, do that.

I would imagine (IANAE) that the uneven forces from the added disc brake would be the bigger issue. I’d question whether the seatstay bridge would help balance that assymetry…

Frame builders love seat stay bridges as they hold the rear triangle together when attaching them to the front triangle. I expect the chain stays on a Fire Mountain are pretty beefy, but it still might be worth fitting a brace into the NDS rear triangle to help distribute the stresses.

I knew someone who was an ex-pro XC rider who modded his ex-team, rim-brake race bike by having disc tabs welded to the seat stay without accounting for it being constructed from super-skinny steel tubing. Took his bike to Moab, first ride snapped steatstay, OTB and an expensive visit to hospital

but it still might be worth fitting a brace into the NDS rear triangle to help distribute the stresses.

Yep, I’m planning on doing this, seems to be the accepted wisdom when doing this sort of thing to older frames…

If it were mine I’d probably chop it out.

Biggest problem you might find is a rusty hole in the seatstays if water has collected in the bridge tube. Had this when doing the same (disc tabs etc) to a friend’s Raleigh 853 frame. Brace also had to come off anyway as that had rotted. Just brazed a couple of little oval patches over the stay holes and all has been good for a few years.

Cut out the seatstay bridge, then use it as the NDS brace.

That way you gain a NDS brace and haven’t lost the seatstay bridge – sort of…

Remove the brace, but put an old brake-booster onto to the v-brake posts. Does the same thing structurally but with way more clearance.

I am not a welder.