This is more complicated than I know. Because I don’t know. Don’t really understand it. The gearing maths is simple enough. But the sums for working out the number of skid patches available for a given ratio baffles me. Baffles me still, even after having it explained repeatedly by the impossibly patient editor of this online magazine.

I don’t have a maths exam pass to my name. Never will have. The gearing thing is easily do-able online, ratios, gear inches, all that stuff. Everybody knows 69” is the go for the road, but when you want to get involved in voluntary skidding for laughs it pays to not have the same piece of tyre visiting the tarmac every time. That leads to blow-outs after a very short time.

So, in order to maximise the quantity of available rubber to erase while performing unnecessary manoeuvres (assuming you still run a front brake) one has to use maths and a brain.

It goes like this: with 48/16 ring and sprocket making a gear of 81″ then 48 divided by 16 equals three, so the rear wheel rotates exactly three times per pedal revolution. Therefore if you lock the cranks at exactly the same point every time then the same patch on the tyre will be in contact with the asphalt and will wear. Then explode.

Much better (and at the optimum 67”) is 48/19: this works out, says the ed., as 2.52631578 wheel revs per pedal rev. To get the same point on the tyre at the same place relative to the crank, you have to do 19 pedal revs. This is because 0.052631578 etc is 1/19th so 2.52631578 is 2 + 10/19ths. So, after one pedal revolution, the wheel has done 2 + 10/19ths revolutions and the contact patch is now 10/19ths of the circumference further round the wheel. One pedal rev more and the patch is yet another 10/19ths further on, i.e. one 19th past the original contact point. So it goes on until after 19 pedal revs and 48 complete revolutions of the sprocket, during which the contact patch has been at 19 separate points on the circumference of the tyre, we are back where we started.

I still don’t really understand.

[Ed’s note… Erm… Apart from the obvious, and all we’ll say on that is that Mark does have a front brake, the idea is apparently not to wear through the tyre any faster than necessary. With exactly three wheel revs per pedal rev, the contact patch is in the same place every one pedal rev and that’s where the tyre will wear. In 48×20, every five pedal revs the wheel will have done 12 and the contact patch will be back in the same orientation to the pedals after having been in four other places.

Of course, each skid removes rubber along a longish patch of the tyre and the patches are bound to overlap, so no gearing combo will prevent the ultimate – and untimely – demise of any tyre. However, of more interest to most fixed gear riders will be the fact that, since pedalling produces a cyclic variation in torque as the feet press on the pedals, the thrust generated against the road by the tyre will also be cyclic, peaking twice per pedal revolution. In a gear where there is one contact patch per pedal position for a skid, assuming the skid is done with the same foot forwards, there will be two patches on the tyre, one for each foot, that transmit thrust and these will wear faster than the rest of the tyre. Luckily for those of us who ride 48×19, this means that there are 38 thrust patches and a very low rate of wear spread nicely around the tyre. ]