Category: Technical Info

I now offer six different wheel sizes for custom Bromptons, based on three frame sizes – it’s maybe a bit complicated, so hopefully this will help you choose the right size!

Changing between the two different wheel size in each frame size isn’t a big deal, it’s just a matter of rebuilding the hubs into different rims (assuming you have disc brakes).

Want to change the wheel size on your Brompton? You’ll need a rear gearing kit (Alfine, Rohloff, or Enviolo), and front and rear disc brakes.

Standard frame sizes:	These frame parts are dimensionally identical to original Brompton ones, so the bike folds exactly as normal, and normal spares and accessories will fit with no problems.
	16″ (305): This is the smallest rim size, and it’s usually combined with fat 2″ Big Apple tyres – these are comfortable and less skittish on loose surfaces. The Big Apple tyres are becoming hard to find, but there are a few alternatives still available.
	16″ (349): This is the normal Brompton wheel size, so this size will take all normal Brompton tyres. This is the only size that is compatible with the standard Brompton brakes rather than disc brakes.
	16″ (305) Fat Bike: This is a special I’ve built a few times – it uses the smaller 305 wheel size, so can also use those tyres, but has special wide forks and rear triangle to use the unusual 16×3.0″ fat bike tyres.
Stretch frame sizes:	These frame parts are longer than standard, for larger wheels – so the folded size is a bit larger, and standard accessories like mudguards either won’t fit or have to be modified to fit. The riding position is a bit different with these frames, the bottom bracket (where the pedals attach) is about 3cm higher than standard.
	18″ (355): This size is shared with the Birdy folding bikes, and can use either the fat 2″ Big Apple tyres, or knobbly mountain bike tyres. Standard racks and mudguards still fit okay.
	20″ (406): This is the largest wheel size, so rolls very well, and is great for road riding – the fattest tyre that can be fitted is a 1.3″ slick. Because of the larger diameter, gearing is increased – this is especially useful if you want higher than standard Rohloff gearing. Standard mudguards fit, with modification.
	MTBrompton 20″ (406): This size uses the same rim diameter as the 20″ option above, but with fork and rear triangle made even larger to accommodate up to 20 x 2.4″ tyres! The bottom bracket height is 350mm, good for ground clearance – standard rack and mudguards will not fit. More about the MTBrompton here.

I thought I should write a bit about Brompton rear pivots, as a few people have asked. Brompton rear pivots are all the same, going back 30+ years. Now I’m going to immediately contradict myself – there are two types 🙂

A reasonably common question I get asked about my Brompton frame parts is what standards are they built for (and why)?

All my forks are 100 x 9mm QR, rear triangles are 135mm x 10mm, and disc mounts are IS (International Standard), intended for 160mm rotors.

Some people do ask about through axles, flat mount, and other standards – so why do I stick with these older standards? There’s a bunch of reasons:

• Compatibility: a few hub gears can be made to work with through axles, but everything works with 135x10mm, and it can also work with derailleurs and pretty much anything else. At the front, 100x9mm works with disc hubs, dynamo hubs, and it works nicely with the fork hook. For disc mounts, IS allows a wide choice of brakes.
• Ease of manufacturing: Not a trivial concern when I make several hundred of these every year! Flat plate dropouts and disc mounts are much quicker to manufacture than through axle or different disc mounts.
• Cost: A connected reason (because time is money) but different standards would either need more expensive parts, or would need custom parts made via more expensive processes (CNC machined vs laser cut).
• Being ornery: New standards come and go, sometimes the new standards actually make something better, sometimes they’re just a marketing thing, sometimes they’re a good idea but not for little folding bikes. Bromptons don’t need the stiffness of through axles or the aerodynamics of flat mount brakes.

Brompton’s naming conventions are a bit confusing, and have changed several times, so I thought I’d put together a simple table to show you what upgrades are possible on the different models:

With the launch of the new Brompton 20″ G-Line, I’ve put together a table comparing the G-Line to the MTBrompton. I’ve also added a comparison with an 18″ Brompton, because I think that is actually more like what the G-Line is like.

Belt drives have been around on bikes for a while, after several false starts – they’re now very common on electric bikes, cargo bikes, and I’ve been fitting them to Bromptons for years. I get quite a few questions about belt setup, especially on the Brompton, so this article lays out how belt drives work.

Go SwissDrive, the manufacturer of electric bike kits, went bust in 2018 – but there are still some kits in use. Sometimes they get a calibration error, which you need the Dealer Service Tool to fix.

The zero-hassle guide to removing Brompton rear pivot bolts – these can sometimes be a nightmare to get out, but this technique never fails.