Inspired by the success of my recent conversion from 110 mm cranks to 102 mm cranks on the 28, recent discussions in this forum about weight saving with lighter tubes on Copkers, and a conversation I had with the man in the local bike shop, I am once again toying with the idea of building the Road Razor: a super lightweight, super skinny 700c.
As a gesture of goodwill to the man in the shop who spent some time advising me about rims, even though he clearly thought I will never buy one, I bought a new tyre: a Continental Ultra GatorSkin, 700 x 28c. That’s instead of the 700 x 32c fitted as standard to my Nimbus 28.
I removed the old tyre and made a quick weight comparison using the kitchen scales. I reckon the saving was something in the region of 275 grammes. That’s not a lot when you’re picking the unicycle up, but it’s a lot less when you think that the tyre travels further than any other part of the unicycle, and has to be accelerated and decelerated more - and when you consider the gyroscopic effect on steering.
So, I swapped the tyres.
A scientist should only change one variable at a time, so that the effect of that one change, in isolation, can be assessed. However, I was impatient. As a nod in the direction of scientific analysis, I rode the uni up and down outside my house a little bit and decided it was “obviously loads better”. Ahem!
I then swapped the cranks, down to my old pair of Dotek 89s. I freemounted first time, and realised that the thread in one of the Doteks is damaged and the pedal was at an angle. Clearly, I’d need to change. Off came the 89s, and on went some cheap steel 90s from the bits box.
And then I was ready, so off I go down to the Water Sports Centre.
For the first time in a long time, I am quite nervous about riding. The 28 on 102s can be a bit temperamental, and the 90s are over 11% shorter. (Note to mathematicians: yes, I know there isn’t a unit of shortness, and that statement is therefore meaningless, but you know what it means.)
So, I freemount and step off again within about 2 metres. I stop, made as if to check my shoelaces and adjust the seat (just in case anyone is looking) then I freemount again and set off. So far so good - until 30 metres into the ride, I’m on a patch of ballast, I hit a stone that doesn’t move, and I’m on the floor. Elbow and knee bruised, wrist guards earning their keep. I remount and continue with great caution, until I’m on the tarmac again.
The first slope is a challenge, and I ride it more slowly than I would on 102s or 110s, because I know that I lack the braking power if I spin out of control. Then I’m on the flat lap of the lake. I stop briefly to tighten the crank nuts then continue.
It feels slower than on the 102s. Experimentation shows that it takes me about 10 pedal strokes to slow down to walking pace from somewhere near top speed. Every bump is a challenge. I find myself riding well within the comfort zone. On my first ride on the 102s, I found myself drawn forward as if by magnets, spinning faster and faster. Clearly, at my current level of ability, the 102s represent some sort of optimum, and the 90s are so short that they are counterproductive.
Stopping only once for a phone call (this time, a welcome one from a friend I’ve not seen in months) I do an easy 5 km lap of the lake. It takes virtually no effort. The tyre feels smooth and light, and is almost silent on the tarmac. However, the 90s make complete concentration an absolute necessity.
After the first lap, I decide to see what it can do by riding some of my usual easy cross country sections. Here, the shortness of the cranks is a problem, and it’s made worse by the very skinny tyre. Sometimes you need the confidence of long cranks to get you over some rough ground; sometimes you need the cushion of a soft tyre. The machine in its current set up is pure road, and although I can ride most (but not all) of my regular sections, it is too challenging to be unalloyed pleasure.
Back to the car park, I practise some slow speed stuff and idling. Idling it left foot down is achievable with great care. Right foot down is a bit of a lottery. (For comparison, I can idle my 20 (on 110s) for several minutes at a time, either foot down, or one footed.)
What I do notice is that the unicycle turns really nicely. The narrow tyre has a smoothish surface with a few narrow grooves cut into it for grip. The old 32mm tyre had a moulded tread. The new tyre makes the uni much nicer to turn on the spot, and small figure eights are easy.
Back onto the circuit, I keep to the rougher sections, and find that the uni is very temperamental indeed. The tyre has no cushioning effect if I hit a stone, and the cranks are too short to give me that sudden burst of control I need to recover from a surprise. However, I do manage to ride up and down the zig zag path - that’s no great achievement in itself, but it’s a section that I use as a yardstick because I can never be 100% certain of riding it on any set up.
Over to the canoe slalom course, I find it impossible to ride up the smallest of kerbs onto the footbridge - and I attribute this to the very narrow high pressure tyre - and I UPD as I drop down the very small kerb at the other end - this being more to do with the lack of control because of the short cranks. (I’m not making excuses here, just saying that I know my abilities well enough to recognise which of the two new factors was more significant in each case.)
Back to the car, and a bit more slow speed stuff, then home to write this up.
Conclusions?
Riding a 28 on 90 mm cranks is easy enough on the flat, but it offers no speed advantage (to me) over 102s. (102s do offer a speed advantage (to me) over 110s.)
On 90 mm cranks, the margin for error is very much reduced, and there are safety considerations on hills, unpredictable surfaces, and near traffic or crowds.
The 28 mm tyre is nice for road riding, and slow speed control and manoeuvrability, but makes the wheel very vulnerable to “tripping up” if it hits something that doesn’t have any “give” in it.
Lightening the tyre (or, presumably, the tube, wheel rim, rim tape and spokes) does make a significant difference to idle-ability and the effort required to ride.
I will stick with the idea of getting a really nice light wheel, but for now, 102s for speed, 110s for control, 90s only for novelty factor and practice.
