How to stiffen the suspension on a Brompton folding bike

Brompton folding bikes have a miniature shock absorber between the seat post and the rear wheel. This suspension block, in Brompton parlance, is a solid cylinder of hard, black, rubber-like material that’s about an inch and a half (38 mm) in diameter and just under two inches (50 mm) long. When you go over a bump, it squishes a little, expands in the middle, and smoothes out the ride a bit. Even though I got the firmer of the two suspension blocks that Brompton offered, it still squished too much for my tastes and gave the Brompton a bouncier ride than I wanted.

Brompton suspension block

My local Ace Hardware had the solution to this problem back in the plumbing section: flexible coupling, which is a hollow tube of black, rubber-like material that I could slide over the suspension block and secure with a circular clamp. (At Ace, they call it tubular drain connector, but the standard plumbing term is coupling.) The clamp reduces the amount by which the suspension block can expand in the middle when you go over a bump; reducing the amount of expansion reduces the amount of bounce. The clamp alone, without the black tube, would have solved the problem, but I was afraid that, over time, the sharp edges of the clamp would cut into the suspension block.

Coupling

Couplings come in lots of sizes, including some that are labeled with the same dimensions but that nevertheless have different inside diameters. The important part is that the opening (the inside diameter) must match the outside diameter of the suspension block. My suspension block is an inch and a half across, so I got a coupling whose opening is an inch and a half across:

Inside diameter of coupling

If you can’t find a coupling of the correct size, you could probably find other materials that would serve the same purpose, for example, radiator hose for a car and a hose clamp.

The coupling I bought was too long, so I lopped off the ends:

Cutting the coupling

and ended up with the one on the right. That’s actually one that I’ve had on the bike for three years, so you can see that it’s quite durable.

Couplings before and after

To install it, you slip the coupling over the suspension block:

Installing the coupling

slip the clamp over the coupling:

Installing the clamp

and tighten the clamp until it’s snug:

Tightening the clamp

Don’t get carried away in the tightening department, or the clamp will cut into the coupling and maybe, eventually, cut into the suspension block.

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