As many of you will know, on my recumbent pedal trike I use a Firestone airbag for rear springing. This air spring has major advantages over other springing approaches but as it has little intrinsic damping, external damping is needed.

The rear damper is an ex-R1 Yamaha motorcycle steering damper. This is an unusual design for a motorbike steering damper in that it runs an external passage connecting the sides of the piston. The piston is a loose fit in the bore. The damping action in standard form is provided by the oil passing through the bypass passage, and also making its way past the loose piston. (I assume that the steering damper can be tuned in its action by placing restrictors in the bypass passage.)

To make the steering damper suitable for use as a suspension damper, I modify a plug in the external passage and insert in this passage a one-way valve. This allows free-er flow of oil on bump and more restriction to flow on rebound. Bump damping is therefore provided by the oil flowing in the bypass passage around the open valve and also around the piston, and rebound damping by the oil flow past the piston only.

This gives the desired asymmetric bump/rebound damping.

For the one-way valve in the bypass passage I previously used a rubber disc backed by a spring. I used ‘fork oil’ (a very light weight oil designed for motorcycle fork dampers) and I thought the result pretty good.

However, when carrying a touring load (the trike is pictured above being ridden by my wife Georgina and with a full touring load), the rear damping proved insufficient. The result was that when cornering on pattery surfaces, the rear end could hop.

(But why should the load change the required damping? The reason is that the air spring needs to be inflated to a much higher pressure – and so is much stiffer.)

What was needed was a stiffer damper that could damp the spring even when it was inflated to a higher pressure – but without wrecking the ride when the load wasn’t there. To achieve this, the asymmetry between bump and rebound damping needed to be increased, so that bump damping was the same (or even softer) and the extra stiffness came from increased rebound damping.

Using thicker oil in the damper would increase rebound damping – but also bump damping. So I decided to improve the one-way valve so that oil flow past it would occur more easily, so reducing bump damping. With thicker oil used in the damper, the result would be similar bump damping but stiffer rebound damping.

I went to a bearing shop and got a selection of different size ball bearings (the actual balls, not complete sealed bearings). With the use of the correct size ball and a suitable spring, the one-way valve could be made to flow much better when open but have a low cracking pressure when the flow was the other way. I then filled the damper with gearbox oil – much thicker than fork oil.

The result is (so far) excellent – a better ride quality and a more progressive rear suspension extension under brakes. I now need to do some testing when heavily laden, but I am sure the results will be better there as well.

It’s interesting that this most simple of dampers can have its behaviour changed so dramatically by varying the oil viscosity and the shape of the one-way valve. In earlier testing, I found in fact that the internal fluid flow dynamics behaved in some quite unexpected ways – fluid flow inside the damper can be very fast indeed.

It’s an interesting thing to experiment with…