I'm on holiday this week! Back next Tuesday...

Brompton Wheel Sizes

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.
All Brompton wheel sizes including the G Line

On Standards

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 Upgrades By Model

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:

A and C Line
All Older Models apart from X
All conversions are possible, by replacing the rear triangle and/or forks
P and T Line
Older X Superlight Models
Hub gear and belt drive are possible by modifying the titanium rear triangle

All conversions are possible, by replacing the rear triangle and/or forks
G LineHub gear and belt drive are possible
ElectricHub gear, rear disc brake and belt drive are possible

Wheel size change is possible with a front V-brake instead of disc

G-Line vs MTBrompton vs 18″ Brompton

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.

Brompton
G-Line
Kinetics MTBromptonKinetics
18″ Brompton
Folded Size?730 x 690 x 402 mm (0.2 m3)750 x 680 x 320 mm (0.16 m3)650 x 630 x 310 mm (0.13 m3)
Weight?13.9kg (no rack or mudguards)
14.85kg to 15.25kg
(with rack and mudguards)
14.1kg (no rack or mudguards)
14.75kg (integral rack and mudguards)
12.9kg (no rack or mudguards)
13.5kg (integral rack and mudguards)
Disc Brakes?Tektro hydraulicTRP Spyre
Magura MT4, MT5 etc
Hope XCR
TRP Spyre
Magura MT4, MT5 etc
Hope XCR
Hub Gear?Alfine 8-speedAlfine 8-speed or 11-speed
Rohloff 14-speed
Enviolo
Alfine 8-speed or 11-speed
Rohloff 14-speed
Enviolo
Derailleur Gear?4-speed on Electric modelsUp to 12-speedUp to 12-speed
Drive?Chain driveChain or Belt driveChain or Belt drive
Wheel Size?20″ up to 2.1″ wide20″ up to 2.4″ wide18″ up to 2″ wide
20″ up to 1.35″ wide
Electric Assist?Yes, rear hub motor with 4-speed derailleurYes – any front hub motor system, or Pendix mid driveYes – any front hub motor system, or Pendix mid drive
Lighting?Optional battery lightsOptional battery lights or hub dynamoOptional battery lights or hub dynamo
Rack?Optional rear load platform for top bagsOptional integrated rack for panniersOptional standard Brompton rack
Optional integrated rack
Colours?Adventure Orange, Traildust White, Forest GreenAny standard Brompton colour
Custom colours at extra cost
Any standard Brompton colour
Custom colours at extra cost
Luggage?Brompton-specific front bags, small rack-top bag at back.Brompton-specific bags, or full-size panniers front and backBrompton-specific bags, or full-size panniers front, and rack top bag
Material?Steel mainframe and rear triangle, aluminium fork and stemSteel mainframe, stainless rear triangle, Columbus steel forkSteel mainframe, stainless rear triangle, Columbus steel fork
Geometry?Bottom bracket height 290mm
Wheelbase 1115mm
Standard geometry:
BB height 350mm
Handlebar height (S) 1030mm
Head angle 70 deg
Seat angle 72 deg

Dropped geometry:
BB height 300mm
Head angle 66 deg
Seat angle 68 deg

Wheelbase 1118mm
Bottom bracket height 300mm
Upgrade?All new bike, existing bikes cannot be upgradedExisting Bromptons can be upgraded from £1435Existing Bromptons can be upgraded from £1275
Made?Made in the UK and other countriesHandmade in ScotlandHandmade in Scotland
Cost?From £2499 (with rack and mudguards)From £2635From £2660 (with mudguards)
*measurements and weights are for a G-Line with rack and mudguards, and a MTBrompton with 2.4″ tyres and wide wide bars

Brompton Hub Comparisons

With a bunch of different hubs available for the Brompton, I thought a comparison would be a good idea:

KindernayRohloffAlfine 11Alfine 8Enviolo
NuVinci
Sturmey 8
Number of gears1414118n/a8
Gear range543%525%405%307%380%325%
Weight1.4kg1.7kg1.6kg1.7kg2.4kg1.8kg
Gear ratios18″-98″19″-100″24″-99″24″-74″25″-95″29″-95″
ShifterThumbTwist
or Thumb
Lever*Lever*TwistTwist or Thumb
EfficiencyVery goodVery goodGoodMediumGoodMedium
AxleSolid 10mmQuick releaseSolid 10mmSolid 10mmSolid 10mmSolid 13/32″
Disc brakeYesYesYesYesYesNo
Kit cost£1795£1595£795£725£725£395

Gear ratios are the most common possibilities – with the Rohloff and Kindernay, they are with the smallest possible rear cog and largest Brompton front ring (54t). With the Alfine and Enviolo, it’s 18t/50t. So getting higher gears is possible on the Alfine and Enviolo hubs, more difficult with the Kindernay and Rohloff.

I know it seems a bit odd that the Kindernay has a wider range than the Rohloff but has very similar gear ratios, but it’s because the difference between 18″ and 19″ at the bottom end is a much bigger difference than between 98″ and 100″ at the top end.

Other factors are harder to quantify in a table: In terms of fitting, the Rohloff is easiest – it’s a simple quick release hub (though I usually use an Allen key skewer) and cable fitting is very simple. The Alfine and Enviolo hubs are nutted, so need a spanner to fit, but cabling is pretty simple. The Kindernay is the most complex, with a nutted through axle, and hydraulic shifting that has to be cut and perhaps re-bled to fit.

*A rapid-fire shifter is available, but I strongly recommend the Jtek shifter for the Brompton handlebars.