Just some questions on the rear suspension rubber bush: You mention it's more progressive. Now I know what that means but I fail to see how it is much different? Seems to me one rides with the bush under a certain compression and therefore a certain spring force; road movement would result in deviations around that operating point just as with a normal bush. The only difference I see is with a depression in the road which would cause the bush to more gently decompress and with longer movement... a sinusoidal movement would result in a lop-sided sinusoidal response... but the catch is the catch - it stops response beyond a certain point and so lops off the lop-sided sinusoid resulting in a little jar as the catch catches? (Sorry for all the double-talk.)
Over the standard polyurethane:
1) for a given upwards acceleration of the wheel, the bush compression is greater, therefore the acceleration of the rider is reduced. In other words, the suspension has a greater effective travel under upwards bumps.
2) when the wheel meets a depression, it can travel further downwards without the catch catching, because the static deflection of the bush under the load of the rider is greater. In other words, in a static condition the suspension sits 'further through its travel' than it does with the poly bush. You can allow it to do this without running out of travel under bump because the rubber spring, with its convolutions, progressively hardens as it compresses.
With the ex-Daewoo rubber bush fitted, with about 50 psi in the tyres, and measuring from the rear carrier to the ground, about 70mm of rear suspension travel can be measured when the rider bounces up and down on the seat. With the red poly bush fitted, I measured 25mm movement under the same test conditions. So with the rubber bush fitted, the rear suspension has 2.8 times the effective travel!
I measured 25mm static suspension compression with the red poly bush and 40mm static compresison with the rubber bush. So at this ride height, natural frequency of the rear suspension with the poly bush is 4.1Hz and with the ruber bush, 2.5Hz.
Note that also I apply some preload, ie the unrestrained rubber bush is a little longer than the distance provided by the catch.
Anyone who thinks the standard poly bush is too soft will, I would think, intensely dislike the action of the rubber bush. But I am used to riding a self-built recumbent suspension trike with a lot of suspension travel and the Birdy immediately struck me as having rather ineffective rear suspension (but I think the front suspension is pretty good). The trade-off for the greater rear suspension effectiveness over bumps is that you need to pedal smoothly.
[Edit: looking at the above figures, you can see that with the rubber bush the static deflection is 40mm and the total travel 70mm, so the bike is sitting 57 per cent through its suspension travel. However, as the spring rate is non-linear, this is probably fine. Note also the measured 25mm static deflection AND 25mm total deflection measured with the red bush (anyone want to check these figures?) which indicates how incredibly steeply the spring rate of the red bush rises under compression.]
The rubber bush out of the bike:
