Brompton project

[JhnBssll]

These past few days have been very wheely The new rims showed up today so I spent a few hours in the garage this afternoon. I'd already bought a dynamo front wheel but had no rear rim so decided to buy two so I had a matching pair. The front wheel was built in a single cross pattern but I decided to go for 2-cross and sized new spokes to suit.

I started with the rear as it was most likely to cause a problem with the two cross pattern due to the wide hub flanges. I needn't have worried because it build up easily enough 😊

Buoyed by my success I took the plunge and stripped down the brand new front wheel 😬

With the hub harvested I went about rebuilding it with new rim and spokes...

I'll likely rebuild the old standard front wheel that came with the bike with this new rim and either keep it as a spare or sell it on to recoup some costs

Both wheels done and just enough time to pop them in the frame for a test fit before I head out for dinner

I'll likely fit tape, tubes and tyres later

 

[JhnBssll]

Had another play when I got in from dinner. First up I refitted the brakes to check everything lines up correctly and rechecked the alignment, she's still straight as an arrow

The new easywheels arrived this morning so I popped those on and refitted the crank...

Decided to build it up a bit more so on went tyres and tubes...

And while I was at it I figured I'd pop the stem back in and fit the new brake levers to the bars

This is the first time in my ownership that she has sat on two wheels, that has to be progress

The lights and chain tensioner are due to arrive tomorrow so more bits to play with

 

[SkipdiverJohn]

@JhnBssll , as an engineer, do you regard Brompton's use of hi-tensile in the main frame as parsimonious penny-pinching - or just sensible considering the main tube is curved and has flanges attached to it?
I can't see that much opportunity to save weight here with butted tubing, nor on any of the generic 20" wheel "shopper" bikes.

 

[rualexander]

How much tyre clearance at the chainstay bridge then?
Can we have a close up photo of that please.

 

[FrankCrank]

@JhnBssll , as an engineer, do you regard Brompton's use of hi-tensile in the main frame as parsimonious penny-pinching - or just sensible considering the main tube is curved and has flanges attached to it?
I can't see that much opportunity to save weight here with butted tubing, nor on any of the generic 20" wheel "shopper" bikes.

...I heard recently that the Brommie design patents had expired, and that clones were being made in China, some of which had titanium frames. Might be worth delving on the web to see what shows up. My understanding is that the same compact frame would not work in ally, as it would need tubes with greater wall thickness, and the whole job would be that much bulkier, and maybe not much lighter. There was a lot of kerfuffle a few years back about a titanium bike called the Helix, with 24" wheels, and claims it would be the smallest folder on the market. Another kickstarter project that went cold........

 

[Pale Rider]

Have you tried any test folding?

Clearances on Bromptons are notoriously tight.

I would be concerned something will foul where you least expect it.

 

[JhnBssll]

@JhnBssll , as an engineer, do you regard Brompton's use of hi-tensile in the main frame as parsimonious penny-pinching - or just sensible considering the main tube is curved and has flanges attached to it?
I can't see that much opportunity to save weight here with butted tubing, nor on any of the generic 20" wheel "shopper" bikes.

In this instance I would say hi-tensile steel was a wise material choice. It's easy to overlook steel as a 'low-tech' and antiquated material but as you know it has some excellent qualities that make it very well suited to some roles, this being one.

How much tyre clearance at the chainstay bridge then?
Can we have a close up photo of that please.

There's not a huge amount of clearance, but clearance there is I have no idea how much you would expect on an unmodified rear triangle as I had no rear wheel to check with until now

Excuse my holey shoe at the bottom there, they're too comfy to throw away and perfect for dirty garage jobs

Have you tried any test folding?

Clearances on Bromptons are notoriously tight.

I would be concerned something will foul where you least expect it.

Point taken, I've just done a test fold as you had me worried Happy to report everything seems to fold in to the right place, but I won't really know until I've got the proper hooks and catches fitted. A good start and hopefully nothing some minor fettling won't be able to fix though

Ignore the wonky looking rear wheel in that photo, it wasnt tight and slipped in the dropout

 

[JhnBssll]

Dynamo lights are here I've just been looking at possible mounting points and cable routes and have come up with a cunning plan. Well, a plan anyway

Front light will need a new bracket made to bring it closer to the head tube and tyre to prevent fouling:

The rear light is designed to sit on a mudguard but I won't be fitting any. I chose it instead in the hope that it would fit up between the easywheels and it seems I was spot on - again it will need a bracket made but that will be fairly straightforward 😊 The key here will be making sure its high enough for maximum visibility but low enough so as not to hit the floor when folded

Cable routing is planned as below - rear light cable (red line) will run down inside the seatstay then out and in to the chainstay. The Di2 cable will enter the chainstay at the same point (yellow line) and both will exit in close proximity to the rear brake cable, another reason for mounting the V-brakes underneath the stays. All three cables will then be heatshrunk together and follow the normal cable routing from there to the bars. I'll then run the light cable back down the front brake cable to pick up the front light. I figure this will give maximum protection against repeated folds and snagging.

Anyway, thats the plan for now - these things have a habit of changing so I'm open to other options

 

[SkipdiverJohn]

I do like a good tinkering thread, even though it's not the sort of bike I could live with riding. The thought that goes into deciding what to do and how, makes it fascinating to me.

 

[JhnBssll]

I do like a good tinkering thread, even though it's not the sort of bike I could live with riding. The thought that goes into deciding what to do and how, makes it fascinating to me.

Thanks John! In that case I think you'll like this next update too

Chain Tensioner!

The snazzy aluminium tensioner is here, I'm rather pleased with it It's designed for a standard brompton rear hub with multiple sprockets so some level of modification was inevitable but having stripped it down I've got a good idea of what I'm going to do

The first set of modifications are to make it actually bolt on to the bike - quite important In this case it will have to interface with the Alfine hub bolt and anti-rotation spacer. This spacer (green in the pics) has a small shoulder on the outer edge which I noticed already fits very snuggly in to the hole of the chain tensioner...

It does however stand proud by a little over 2mm when pushed fully home:

I'll pop the spacer in the lathe at work and move the shoulder down ~2mm so that just the teeth and anti-rotation tab of the washer protrude. With this done I'll use a low profile nut to bolt the wheel and washer in place, and then a larger nut and washer to clamp the tensioner over the top. This way the tensioner itself is not holding the wheel in place so it should all be fairly robust. It will make the wheel a little more tricky to remove but these things are a fiddle at the best of times

The second set of modifications are to align the idler sprockets with the chain. I was a little more concerned about this topic as I didn't know the construction of the tensioner when I ordered it, I just went on the assumption that something made of metal will be easier to modify than something made of plastic On this tensioner the idler's are able to slide laterally to allow the chain to move between sprockets when changing gear on a standard Brommie. There is about 15mm of travel (estimated, not measured) and the photo below is taken in the fully extended position.

As predicted, due to the spacing of the Alfine hub allowing for the Di2 motor, the chainline is further inboard from the dropout than is currently allowed by this tensioner. Ignore the pulley top left, it looks miles out but this is just a trick of perspective. Behind the chainstay you can just make out the lower pulley, I estimate it will need extending inboard by an extra 5 or 10mm. I don't have the sprocket yet so it's hard to tell

To find out what changes are needed I first had to take it to bits to find out how it's constructed I don't know if you've noticed yet, but I quite like taking things to pieces... I was pleasantly surprised to find a very simple construction which requires only minor modifications to fit my requirements

The bolt and spacer tube in the centre of the picture are what set the maximum extension range of the idler. By adjusting the length of this spacer tube and bolt I can fine tune its lateral position. I'll also make a second, larger diameter spacer to put behind the idler assembly to fix it's position so it can no longer slide in as I don't need this function with my single rear sprocket 😊

So that should be relatively straight forward but I can't start until the sprocket arrives and I can measure things up properly I'll have to get to work on the light brackets instead for now

 

[ColinJ]

This is a very interesting project (which I am enjoying following), but... what happened about the cracked seat tube? That looked like real show-stopper to me!

 

[JhnBssll]

This is a very interesting project (which I am enjoying following), but... what happened about the cracked seat tube? That looked like real show-stopper to me!

A valid point - it's still on the list of things to do but I want to get the paint off the main frame first so I can keep an eye on the brazed joint while I weld it up. I've shown it to a few people, including a Brompton dealer, and theyre confident the cracks can be fixed. I'll document my efforts when I get to that bit Some searching revealed Tredz appear to sell replacement main frames for £150 so if the worst were to happen I know I can replace it and the project will live on

 

[12boy]

I, too, was wondering about the seat tube. I am also unclear on the rear brake routing and have some concern for too many curves n the cable. Fenderless Bromptons seem to be unusual although the Brompton Explore is one such. What are you planning to do with this bike since it seems your work has a definite purpose?

 

[JhnBssll]

I, too, was wondering about the seat tube. I am also unclear on the rear brake routing and have some concern for too many curves n the cable. Fenderless Bromptons seem to be unusual although the Brompton Explore is one such. What are you planning to do with this bike since it seems your work has a definite purpose?

I don't really have a plan for the bike once it's finished, it's a project to keep me entertained more than anything else. I am considering keeping it under my desk at work to whip in and out of town at lunchtimes I'll probably use it to commute on occasionally but I have other bikes for that already that are more suitable for the task I just want it to be a little different and will have some fun making it such

On that note I've been doing some CAD modelling - I've measured up and created a short section of seat stay and modelled the rear light so far, now for the fun bit - designing the mount ready for 3D printing

 

[rualexander]

There's not a huge amount of clearance, but clearance there is I have no idea how much you would expect on an unmodified rear triangle as I had no rear wheel to check with until now

View attachment 493040

Thanks, that's more clearance than I was expecting from what I've read previously elsewhere. Might be tempted to try and put my Alfine 8 into my Brompton.