Hello,

I need to widen a Brompton rear triangle from 112mm to 135mm to fit a Shimano gear hub.

A local bike maker isn't too enthusiastic about cold setting, and recommended welding new dropouts.

Before giving an OK, any idea what it involves?

Removing the existing dropouts, and welding new ones somehow?

Thank you.

Well, 112 mm to 135 mm is quite a "stretch", particularly on something like a Brompton with its short chainstays so i can see why cold setting isn't a good option. I don't know what the bike builder has in mind but if he is going to replace the dropouts with wider spaced ones you should get a better explanation as to how this is going to be done. Also, you will require a repaint after the change so factor that into the cost.

Yeah, that's not gonna be easy.

Sounds like putting a F-350 rear end into a Cooper Mini. With enough time and money it can be done.

You should perhaps look to see if the cassette you plan to use will clear the chainstay with that new the welded dropout. THe second or third cog might have to be fairly small. (Unless either some bending is going to be done or some of that inboard chainstay corner gets cut off.)

It's hard to judge based on the picture alone but I'm wondering if even changing dropouts will be enough unless they're a very different design.

Why the change to such a wider hub?

Yes, this is not just resetting the stays. You'll have to ensure alignment, add a connection for a derailleur hanger. Given the frame geometry, the dropout is unlikely to be standard (but perhaps someone makes them?). Also: Brompton claims on their site that the frames are brazed. If so, there's a reason. Perhaps they're made of chrome-manganese steel (like 531) and welding is problematic. So make sure that your guy uses the right technique to make the joint.

I take this to mean a Shimano internal gear hub. Correct?

You can always look up the specs to see just how narrow the IGH hub you'd like to use can be made. An appropriate axle replacement may be possible/necessary & wouldn't be too difficult for a quality bike shop if the job requires more than spacers & bearing cones.

I know for a fact that a Nexus 7 can be made as narrow as 126mm & is easy to do. Maybe narrower depending on vintage/date of manufacture. Sheldon Brown would know for sure. Edit: 124mm is minimum & all it entails is thinner locknuts & leaving off the optional brake unit you weren't going to use anyway.

The cog location in a 135mm drop out spacing also would impact the chainline further outboard considerably. How's your crankset gonna line up? New wider bottom bracket in your future, maybe?

Why not a 130mm instead of 135?
What's the goal?

I agree that this is probably a bad idea. Welding will be an awful lot of work--removing the old drop outs, welding, some significant repainting, and realigning the dropouts. I haven't spent much time with Bromptons but I'd also be concerned it'd compromise the fold. Have you considered getting a Sturmy Archer? They make a 120mm spaced hub: https://www.treefortbikes.com/Sturme...F8-8-Speed-Hub . It looks like you may be able to narrow it a little by reducing the size of the spacer on the NDS, and then that should be in a sane range for cold setting.

As someone who builds bicycle frames, I would not be able to make that frame take a 135mm hub without also replacing the stays. Placing the dropouts at 135 spacing puts them outside of the current chain and seat stays with nothing to attach them to. You could make something fit by making a sharp bend in the dropout to angle over to the stays, but that presents other issues with the chain and cogs.

Thanks everyone for the feedback.

Fitting a cassette is not an issue, since the goal is precisely to remove the two-cog cassette + derailleur and replace the three-speed Sturmer-Archer hub with a Shimano geared hub.

It appears that the Nexus 8 needs 132,5mm while the Alfine 8/11 require 135mm (Source). The eight-speed Sturmey-Archer gear hub has a bad reputation, which is the reason I'd rather go Shimano.

Brompton seems to use HiTen steel, and I guess it's brazing not welding.

incidently, Juliane Neuss in Germany has been cold setting Bromptons for years, but it seems only doable right by someone experienced with Bromptons.

I might need to change the bottom bracket to get a correct chainline.

This just sounds like a bad idea. If you want a bike with an 8 speed IGH get a bike that takes an 8sp IGH. We all think we can "make it work" but attempting to modify a frame so that it will take this hub is the tail wagging the dog. Just get a bigger dog.

Would it be possible to create "adaptors" from mild steel (flat or angle) that could bolt to the outside of the existing dropouts with spacers/washers to widen the OLD? Perhaps extending straight back? It would increase the wheelbase slightly but not change headtube angle. It might need another smaller bolt to prevent rotation around the larger bolt: perhaps in the square hole that is already there?). This wouldn't be real pretty but it would not alter the frame and dropouts much at all.

Any decent machine shop should be capable of making an adapter to do exactly this!i

Any decent machine shop should be capable of making an adapter to do exactly this!i

This thread makes reference to an 8-speed IGH that is specially spaced for Bromptons, at 115mm OLD. Is that what you’re after?

8 speed brompton!

EDIT: Oops, that’s the SA hub you’re concerned about, never mind...

Here is a thread that references a Shimano Alfine in a Brompton, and it says “as usual” about the new rear end, so maybe this is a common mod? Clearly a new rear end is in order, though...

Modified Brompton with Shimano Alfine hub

Yes, Fudge Cycles in Kensal Rise, London is another shop that cold sets Brompton triangles, but you have to be used to working with Bromptons to do it right, which isn't the case for the bike maker over here who never worked with them, and who'd rather braze new dropouts.

Based on how a Brompton folds — the rear wheel pushes against the seatpost to keep the whole thing closed —, I'm concerned bolting new dropouts to the existing ones might impact the fold by adding ~1-2cm to the wheelbase, but I'll certainly suggest the idea. Thanks!

I would suspect chromemoly which would have superior strength/weight ration compared to HiTen. You can weld chromemoly, if you are willing to heat treat the whole frame afterwards. Otherwise, the steel in the heat-affected zone ends up being either too hard and brittle and/or too soft and weak. Neither good for bicycles. Which is why they use brazing, which is lower temperature and doesn't cause changes in metal temper.

I used "welding" because I didn't know there were different ways to "glue" metals. Yes, Brompton uses brazing instead.

I got a reply from Andrew from Fudge Cycles in London who cold set at least one triangle back in 2009: "We can no longer do that due to health and safety insurance and Bromptons terms and conditions."

There's obviously no way to know how many Brompton users have successfully been riding their cold set bikes for years… and how many broke theirs in the process.

So brazing an extra pair of dropouts might be a safer option… provided it doesn't impact the fold and the ride.

Bromptons are made from chromoly steel. They are brazed because that's just how Brompton has chosen to join them. Chromoly, 4130, 531, 631, 853 and all of the other supersteel alloys used in bicycle frame building can be TIG welded with no heat treating afterward. There are a few BMX companies that heat treat their production frames after welding, but none of the custom framebuilders I know heat treat their steel frames. It's just not necessary to produce a frame whose strength far exceeds the loads placed on it.

Thanks for the infos.

Considering those two Chinese companies also offer 135mm-wide triangles in titanium for $500, it's odd they haven't thought of also offering 135mm steel triangle for around $100, but they might someday.

Sturmey FW or FM plus the two speed derailleur gives 8 gears. No fuss beyond locating a good condition older hub. The 4 speed Sturmey were very good.

Thanks for the idea, but those are no longer manufactured, and a 52T chainring + 18T cog only gives 4.7m development on the top speed, which is way too low, as I use this bike for touring — I know it's not its nature, but it rides well enough and it's more convenient to combine with trains/buses.

Too bad Sturmey-Archer' X-RF8(W) is heavy, noisy, and a bit unreliable; Otherwise, it'd be an easy solution to the problem.