Even the Brompton’s worst enemies have to agree that it does a good job of folding. It’s also pretty nippy on the road.Where it’s not so hot is in those tricky off-road situations – loose rocky surfaces and mud in particular. As anyone will know who’s tried to haul one along the wrong sort of track, mud soon builds into a sticky poultice between the tyre and mudguard, and it can take weeks to shake out the last remnants.

The Brompton isn’t an off-road machine of course, and we’re not suggesting for a moment that it would be the best tool for serious mud-plugging. But many folding bike riders commute for all or part of their journey on tracks and trails, where the smooth tyres, limited mudguard clearance and high gearing are a positive hindrance. Most small- wheelers have 305mm or 406mm tyres, and there’s a reasonable range of knobbly rubber available in both these sizes. In the Brompton’s 349mm (16″ x 1 3/8″) size, there are none, and as far as we know, there never have been any. But even if a good tyre was available, you’d be left with the high gearing and the limited clearance.

…Slip on a tube and a new knobbly tyre and you’re off…Well, not quite…

The answer is to convert the bike to take the smaller 305mm tyres, as used by Dahon and most Far Eastern manufacturers. Although tricky to do, the conversion solves all three problems at once: a pair of knobbly tyres cut through the mud, and the smaller tyre diameter increases clearance and reduces the gearing.The operation is reversible too, so it would be quite practical to convert the bike for winter riding, or even for an off-road holiday, then swap all the bits back in fairer weather.

At this stage, we should point out that our donor bike is a fairly old three-speed Sturmey Archer model.The 5-speed offers a wider gear range, but the hub is a bit frail for off-roading and the gear change rather vulnerable.We don’t know how easy it would be to convert a post-Summer 2000 bike fitted with a 3-speed SRAM gearbox, although most of the information below will remain the same.

Front Wheel

Needless to say, the operation requires a few bits and pieces and a few workshop days, because the new tyres won’t fit the old rims, the old brakes won’t (necessarily) contact the new rims, and the Brompton uses special narrow axles…

We bought a 305mm front wheel at a local cycle shop – these are relatively common wheels, fitted mainly to childrens’ bikes, but look for something with proper spokes and a substantial rim. A Dahon wheel is ideal. Unfortunately, the hub will be too wide for the Brompton forks, so it’s necessary to have the wheel rebuilt onto a spare Brompton hub. For the enthusiastic amateur wheel-builder, this is an easy task, but a professional rebuild shouldn’t be too expensive,  because the Dahon spokes should fit straight into the new Brompton hub.

Slip on a tube, and a new knobbly tyre and you’re off…Well, not quite, because the Brompton brakes have a ‘drop’ of about 60mm, and the smaller wheel needs a drop of 80mm to reach the rim. As the new tyre is wider, and we’re particularly interested in creating extra space around the tyre, the answer is to raid your local cycle shop for an old-fashioned calliper brake.We found something called a Chang-Star Deluxe, with a width of 65mm and drop of 70-90mm. Inefficient, not very attractive, but ideal for the purpose. Plumb in the new calliper, taking care to follow the standard Brompton cable run, and the front of the bike is ready for action.

Rear Wheel

Finding a three-speed 305mm wheel proved surprisingly difficult, and we ended up approaching Dahon UK.The only complication here is that Dahon, like most cycle manufacturers used the standard Sturmey-Archer axle, whereas the Brompton has always used a special narrow-axle version.The easiest way round this problem is to swap the Dahon hub internals for the Brompton ones. If you know how to do this, great – if you don’t, pop into a dealer with the correct equipment.The conversion requires a giant spanner and takes just a few minutes (in other words, it should cost very little).

At this stage, it’s necessary to do something rather cruel to the Brompton rear frame, but it’s unavoidable.The Brompton frame is offset to one side for various technical reasons, but the wheel is not. A close inspection will reveal that the frame tube on the right hand (chain) side is slightly squashed, to give sufficient tyre to frame clearance. Unfortunately, our new tyre is a little wider, and it also meets the frame in a different place, so it’s necessary to re-squash the tube some 20mm closer to the hub.We did this by holding a suitably-shaped tube against the frame and gently hitting it. Don’t hit the frame directly with a hammer, and don’t overdo it…

With the tyre and tube in place, the wheel can now be fitted and the gear cable and tensioner assembly refitted as normal. But once again, we’re up against the brake problem. Standard- issue Brompton rear brakes have a drop of about 50mm, according to model. Most designs are also a tight fit around the tyre, causing many of the problems off-road.

No doubt there are a number of suitable callipers around.We managed to squeeze the required 67mm  clearance out of the Alhonga dual-pivot design.This was a common Brompton upgrade before the company introduced its own dual-pivot brake a couple of years ago, and should still be available from folding bike specialists. After grinding the slots, we just produced enough clearance for the pads to work effectively.

Gears, brakes, clearance… have we finished? Not if the bike has a dynamo, because this will now be in the wrong place, but it’s easily sorted with an extended bracket.The final adjustment is to the folding stop on the rear brake assembly.This device must be accurately set for the bike to lock together properly when folded.The smaller tyres allow the bike to fold a little lower, necessitating some adjustment of the stop to keep everything together. Once again, this was just possible on the most extended setting.

On the road, the first impression is of high rolling resistance – only to be expected from tyres of this kind. Gearing on older 3-speed Bromptons is generally 48″ – 83″, which is far too high for off-road use.The new smaller tyres reduce the ratios to 44″ (first), 59″ (second) and 77″ (top), which is better, but still too high.

On & Off Road

We tried fitting a 44-tooth chainring and 14-tooth sprocket, a combination supplied by Brompton for those preferring lower gears.This gave ratios of 36″, 48″ and 63″. Not ideal, but a good compromise, offering reasonable performance off-road, plus a useable top speed on tarmac. A smaller chainring will help off-road, but limit top speed.

How does the bike perform? Obviously braking and acceleration on loose, sandy or muddy surfaces is much improved. Indeed, the off-road Brompton has yet to get stuck anywhere. Even with the mudguards in place (we still view this primarily as a commuter bike, like any other), there’s some 20mm clearance around the tyres in most places, although things are a bit tight near the rear brake.When mud does start to accumulate, it’s much easier to hook, or wash, it out.

A pleasant surprise was the extra efficiency of the mudguards.These normally create quite a bit of spray, but the smaller 305mm tyres reduce this, even under the most extreme conditions. Surprisingly, the conversion proved quite popular on tarmac too. Provided you aren’t aiming to break any speed records, the off-road Brompton plugs along perfectly well. One point to watch is that brake efficiency may be considerably reduced, particularly if you have fitted nondescript callipers and/or steel rims (ours has a steel rim on the front, aluminium at the rear).

Conclusion

Is this conversion really worth all the effort? That depends on your circumstances. If you regularly have problems with poor off-road traction, and clumps of mud that take days to work their way out of the mudguards, a 305mm conversion should more or less cure the problem.

Cost will vary a great deal, depending on how much you do yourself. Our bike cost about £80 in parts, but you should expect to pay at least two or three times that for a professional conversion.

Reversing the process takes only a few hours, although there is such a wide variety of 305mm tyres available, it might be easier to stick with the smaller tyres, but keep a summer and winter set, swapping them according to the weather conditions.

Most good Brompton dealers should be able to make the conversion. Kinetics of Glasgow offer a mail- order service – both wheels rebuilt with 305mm rims for £65 plus postage. Tel 0141 942 2552