This is a whimsy I have been pursuing for some time, and it may now be superseded by the availability of the deluxe Coker, but here we go…
What is distinctive and fun about the Coker? It has the massive wheel which as someone pointed out, gives it unbeatable aesthetic appeal. It also looks good. It has this amazing ‘flywheel effect’ that means it feels like you’re pumping extra energy into a flywheel, rather than simply balancing a unicycle. You sit high up and sway gracefully along. It’s fast at even moderate cadences. It rolls over things without tripping up.
But… the wheel is fragile, and the build quality is poor, and the tyres are expensive, and take about a year to pump up with a handpump. It’s not awfully manoeuvreable, although some would say it has excellent directional stability.
So how can we make a Coker with all the advantages and none of the disadvantages?
Contrary to popular opinion, I think the flywheel effect is little to do with the weight of the wheel. I know that the rotating mass of the wheel is ‘worth more’ than the mass of the frame, seat or rider, in terms of momentum, but if that were everything, then we’d all be fitting extra weights to the wheels on our other unicycles to give them more momentum. Clearly this would be ridiculous.
I think it’s largely to do with the leverage. A 20 inch wheel has a 10 inch radius, and with 5 inch cranks, you are pedalling with a 1:2 leverage ratio. Similarly with a 24 inch wheel with 6 inch cranks. On the Coker, you have an 18 inch radius and 6 inch cranks (as standard) giving a ratio of 1:3. That means that accelerating or decelerating the wheel is harder, and when you are going fast, and there is only a small part of the pedal stroke where you can exert maximum torque, stopping the wheel is difficult, sometimes taking 2 or 3 revolutions.
On a 700c (28 inch approx.) wheel, the same ratio could be achieved with a crank which is 1/3 of 14 inches long: approx 4.7 inches, or around 110 mm. So my guess is a 28 inch uni with 110 cranks would feel similar to a Coker at least in terms of ‘flywheel effect’.
What about seat height? The seat has to be a certain distance from the bottom of the pedal travel. The ‘certain distance’ is a constant (K) for each rider. Seat height will be K + radius - crank length. K+R-L.
So on a Coker, seat height will be K+18-6 = K+12.
On our 28 incher with 4.7 inch cranks, seat height will be K+14-4.7 = K+ 11.3.
This means that on a 28 with 110 cranks, the seat height is less than an inch lower than on a Coker. And this Cokerette gives us greater choice of tyres, probably a quicker maximum cadence, and it’s easier to pump the tyres up, but we retain the stately seat height and the exciting ‘riding by kind consent of the unicycle’ flywheel effect.
Of course, we lose the gyroscopic effect of the big wheel (we wouldn’t really weight the smaller wheel to reproduce this) and we lose the distinctive ponderous manoeuvring (or you could say that we gain more manoeuvrability).
We also lose the high speed at ‘reasonable’ cadences. We probably don’t lose top speed, but have to pedal frantically to achieve it: about 30% more rpm.
The only other way to get high speed at reasonable cadences is to use gears - either the Uni.5, or a chain drive. The uni.5 has been discussed elsewhere, so I won’t go into it here.
So, chain drive: a small giraffe. This gives us a choice of wheel sizes. To retain any of the ‘rollover factor’ of the large Coker wheel, we’d need at least a 24 inch wheel, and probably a 26. In terms of seat height, there would only be 1 inch difference, so I’d say a 26 will be nearer to what we want.
Now, the seat height on a giraffe is not related to the radius of the wheel. It is related to the diameter of the wheel, plus an allowance for the distance from the top of the wheel to the bottom of the bottom bracket. The choice of cranks is not dictated by the wheel size, and I’d suggest going back to 6 inch cranks and choosing the gear ratio to reproduce the 36 inch Coker feel. We could even consider a slight increase, but that’s another discussion.
So, the centre of the bottom bracket will probably be 4 inches above the top of the wheel. With a 26 inch wheel, that gives us a bottom bracket 30 inches up, and with 6 inch cranks, a seat height of K+24 which is way higher than the Coker, but nothing compared to a conventional giraffe. With a seat height about twice as high as a standard Coker, mounting would be a real challenge requiring a different technique. Probably not good for MUni, although I’m sure someone will tell me otherwise.
So this small giraffe gives us more than the Coker in terms of swaying elegantly along looking down at our fellow men, the same in terms of ‘flywheel effect’ and is probably about as manoeuvrable with it’s smaller wheel but greater height overall. We have a wide choice of tyres, the option to change wheel size down to 24, or to adjust the gearing to taste. It may have some of the ‘unbeatable aesthetic appeal’ of the Coker; it’ll certainly turn heads.
Both of these options give the builder the ability to choose from a wide range of rims, spokes and so on, so we lose the Coker’s iffy build quality and fragility. The 28 inch Cokerette would be by far the more practical option of the three. The Giraffe (Giroker? Goker? Joker?) would be more challenging, a bit less practical, and probably more fun.
What do you think?