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In a 2/6sp Brompton, the tensioner pulleys are challenged when you try to use them with 3 rather than 2 cogs and snowy conditions and very likely in those muddy. The Brompton pulleys are made as a single entity with central teethed wheel and two side wheels with small side openings to let the dirt out. The snow or mud can easily pack the space between these component wheels, forcing the chain out, see the photo below. The lower pulley, closer to the ground, is obviously more vulnerable. If such lower pulley fills up, you drop the chain. However, if the upper pulley fills up, the chain will try to force its way out of the Brompton pusher. In the best case, you will need to take the tensioner off on the road to put back everything in order. In the worst case, the chain will bend the plastic tensioner too far, damaging it irreversibly. Yes, it happened to me and I had to replace the tensioner.

The side wheels help to keep the chain on the pulleys. In the upper pulley, they are moreover needed for the pusher to move the pulley sideways, pulling the chain from one cog to another. Pusher tweezers grab those side wheels to accomplish the move. However, after the move, the tweezers should stay away from the side wheels as those sides will get damaged when the wheels constantly rub against the tweezers, see another photo below. When you try to extend the cog number from 2 to 3, on the rear wheel hub, to get more speeds, the tolerances get tighter and it gets harder to avoid the rub.

The Advance Derailleur tensioner arm in the P/T lines has the same snow/mud problem with its pulley, as far as I can tell. The upper, derailleur pulley seems OK.

Problems with snow or mud could be alleviated by the sides of pulleys moving relative to the center and by larger openings in the side wheels. Moving sides could also help with rubbing against the pusher tweezers. Most aftermarket pulleys have that and the central toothed wheels often move on ball bearings. Unfortunately, these pulleys have been, in my experience until last year, designed so that they could not shift over 3 cogs. Typical issue was of the pulley getting stuck during sideway movement, due to too narrow central channels, with sharp edges there that would bite into the tensioner side. Also, rotation of the pulley as a whole has a benefit of facilitating the sideways movement of the pulley in shifting, as it is easier to shift the pulley that is already moving relative to its axle than one with a part that sits idle on that axle. Still, some of the tested aftermarket pulleys could function in the lower position, as longer time taken by a pulley to get into a right placement could be tolerated there. Some pulleys, albeit expensive, had larger side openings so could be additionally good for snow at that lower position.

When I went out checking aftermarket pulleys this year, I realized that the market has moved, so you could get a better finish for less money. One set of pulleys felt immediately better in the hand, due to softened edges. When I put the pulley in the upper position, there was play. To my amazement to pulley shifted fine over 3 cogs. Because of independent movement of the center and sides in the pulley there has been no issue of rubbing against the tweezers of the pusher. The independent movement helped with the snow, but bigger side holes could work better. In fact, against the snow, that pulley worked just OK in the lower position. However, there I could put another pulley from the new crop, that would not work in the upper position, but was fine in the lower position even when riding straight into a deep powder snow. The last photo below shows a compromise that shifts well over 3 cogs and is fine in the snow too.



Chain climbs out of a Brompton pulley filled with snow


Edge of a Brompton pulley eaten up by the pusher


3 Pulleys: (left) original Brompton, (center) aftermarket with big side holes, good with snow, but no cap, (right) soft-edge pulley that shifts fine over 3 cogs and does not suffer due to pusher rub



Compromise setup, that shifts well over 3 cogs and does not suffer due to to snow

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One of the aftermarket pulleys in the utilized set comes without a plug, to protect the axle on which it sits, from dirt. The hole that can accept the plug is a tad wider than in the Brompton pulley, so the original Brompton plug will not hold. On other occasions, I thought about changes to the original Brompton plugs, in combination with the original pulleys, to accommodate longer axles than original.

When you look for some plugs, other than original, to use with the pulleys, this looks like a simple task. After all, there are plenty of different plugs used in other contexts in everyday life. In practice, you find it practically impossible to get hold of a plug that will actually properly fit. One way to solve the problem is to use a 3D printer. Another way is to use ABS that is easy to cut and glue using acetone. Specifically, you dissolve some ABS pieces in acetone and use the solution as a glue. Making the plug below took me literally just a few active minutes, with the circle cut out with a circle punch.