Calling all engineers. Wheel flex.

I was reading the UDC (UK) site today. Ahem.

And I notice that the new breed of super Cokers have extra wide hubs and 48 spokes and stronger frames to reduce flex.

And I noticed that the purpose made super lightweight racing uni has an extra narrow hub (to minimise Q factor and wobble) and only 24 spokes.

Now it occurs to me that someone who has a purpose made racing uni is likely to be putting maximum effort in most of the time, whereas someone who is cruising on a Coker will only put maximum effort in from time to time.

So why is the racing uni designed for lightness rather than rigidity? Why is the cruising uni designed for rigidity rather than lightness? There seems to be a contradiction here.

Here’s me with a 700c with a lightweight custom wheel, high pressure tyre, and aluminium cranks, looking for the “pure unicycling experience” and wondering about that racing uni…

The new 36" rim (nightrider) is 36 spoke, so the nimbus and KH 36" unicycles will be 36 spoke. It’s only the ‘coker cokers’ and the qu-ax 36 that are 48 spoke.

Also, if you look at the fastest non-geared coker riders, they have super lightweight wheels, including drilling, using silly tubes, shaving off the tyre etc. Quite a few racers at RTL also used the narrow standard hubs rather than the extra wide hub. I think for the really fast people, they’re doing pretty similar things on the big and small wheels.

As for why to not worry so much about strength for a unicycle you’ll race on - a non-geared unicycle race is primarily about how fast you can spin - lightweight wheels are everything for this. The small wheel ones are run on tracks / smooth surfaces, so they can really push it strength wise. Big wheel races are run on roads, which may be more bumpy, and pretty much everyone also trains on real surfaces, so you can’t go so far on the strength thing.


Re: Calling all engineers. Wheel flex.

On Fri, 15 Aug 2008, Mikefule <> wrote:
> So why is the racing uni designed for lightness rather than rigidity?
> Why is the cruising uni designed for rigidity rather than lightness?

Possibly it’s designed for durability - the racer wants lightness
above all else and will be prepared to retrue the wheel more often.
Also, the racer has more tightly defined operating conditions, so you
don’t need as big a safety factor. Finally, if something untoward
happens to the racer you lose, but you’re not stranded miles from
where you want to be.

Taken to extreme in motorsport - you don’t mind rebuilding the engine
every few hours running if it gets you first place.

regards, Ian SMith

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So is this your latest reason for why riders shouldn’t wear helmets? :stuck_out_tongue:

Of course, at 45 I won’t be racing anyone - and as for tightly defined operating conditions, I ride my 700c/114mm cranks, 28mm tyre (120psi) cross country as often as not. It’s the “how much can I achieve that’s down to me, not the machine?” thing. The KH24/165mm seems to do half the work for me.

I have one of those super lightweight racing unis myself, on which I competed in unicon just a few weeks ago. For racing (mostly done on a smooth racetrack) you will primarily need rigidity in the drive direction. Lateral stiffness is desirable but the wheel doesn’t need to be very strong laterally. Yes you do exert “maximum effort”, but that mostly means spinning fast; it doesn’t translate into much torque except at the start. A narrow wheel can handle torque as well as a wider wheel. The narrowness of the wheel, on the other hand, is very beneficial to fast spinning. A wider wheel would generate more wobble because the leg mass (and the pedals and cranks but they weigh less than legs) is farther away from the wheel centre plane.

A bigger wheel (think 36") has more tendency to give laterally. Partly because of how people use the big wheels (a few do “Coker trials”, some more do offroad, even more do tightish turns in traffic etc), and partly just because the wheel is bigger. Quite a few old narrow Coker wheels tacoed when put under lateral strain. A wide hub helps to stabilise the wheel laterally. You pay with more wobble at high rpm’s but Cokers are rarely ridden at very high rpm’s so the wobbling tendency is not so pronounced anyway. For Coker racing though, again you want a narrow wheel to maximise spinning.

Why ahem? Are you secretly in the market for something?

Ah, then someone who knows. Is it significantly better than a less specialised uni for the purpose for which it is designed, or is it a bit of a gimmick?

I weigh a bit more than I’d like (something like 170 Pounds/ 75 Kilos) would it stand up to uneven but not extreme surfaces 9for example, a bit of river bank)?

I should never look at UDC’s website. It plants seeds. I have been stripping my fleet of all the redundant stuff and am down to 20, 24, 26 and 28 (and UW) but I sometimes miss the Coker, I was inspired by the recent post about the 43" uni, and then the racer caught my eye.

It is significantly better. Firstly, for a so-called “standard unicycle” the IUF allows a maximum wheel diameter of 618 mm, which is almost equal to 24.33 inch. Most race unis have a 26 x 1" tyre which I think is less than 1 mm smaller than 618 mm. Secondly, at racing speeds gyroscopic forces play a role in rounding the bends in a race track. The heavier the wheel, the more it wants to go straight. Then, the narrowness of the wheel and the lightness of the cranks (aluminium) and pedals reduce wobble, and hence allow higher rpm’s. Finally, the lightness of the frame and seat helps in accelerating at the start.

The wheel (rim and spokes) would probably stand up, if you keep it true and tight. The tyre is sensitive though, especially if you keep it at its max pressure (mine is rated for 125 psi). During last month’s unicon, about 3 km of the 10 km race was on a gravel road. About 30 (thirty!) race tyres were ruptured on that piece, in a field of maybe 200 of them. (400 ppl competed but not all on those race tyres.) It would probably be less puncture-prone if you deflate it somewhat but that would defeat the purpose, wouldn’t it? Bottom line: a race uni is good for track racing, but sub-optimal for almost anything else.

So if I may ask, why are (or were :)) you ogling the race uni in the first place?

I disagree, if you are running cranks any shorter than 125mm on a 36er, the narrower hub makes spinning easier and more comfortable. If I’m correct, you can tighten the spokes, and it can almost compensate for the narrower hub. Plus, isn’t race pace how fast everyone normally rides?:wink:

Think flywheel. A heavier rimmed flywheel will take more time to come up to speed than a lighter one. Coming off a starting line, the lighter uni wheel will accelerate up to speed much quicker than a heavy wheel.

Out on the track the lighter wheel can make quicker speed changes for the same reason above for passing or a last second burst across the finish line.

Do you mean
“I disagree WITH THE STATEMENT THAT if you are…”
“I disagree; MY OPINION IS THAT if you are…”
Usually people mean the latter. But that’s what I tried to say too: narrower hub makes spinning easier.

Agreed, if you have a sloppy wheelbuild on the wider hub to begin with.

Disregarding the smiley: no.

I don’t want to speak too soon, because I’ve never ridden a 24" or 700c racing uni, but I’m thinking that the sturdiness of Cokers nowadays is really not necessary on a such a small 1:1 wheel (low gear ratio) because no matter how hard a rider pushes on the wheel, the frame will never be under very much torque…

On a short-cranked 36 or geared 36, you can really put some muscle into the pedals, and that’s (methinks) where the frame/wheel flex comes in.

(and yeah, I don’t know why Coker uses 48 spokes in their wheels; also, aren’t Coker’s hubs still narrower than the UDC extrawide?)

I tend to favor stiffness of my uni over its weight, because I really don’t like it when things bend on me. (that’s why I love the KH36 so much; the frame is so durn stiff! [and it’s also really light!])

Lighter is better, but if a wheel is too flexy, then it’s almost worse than having a heavier wheel.

However, I’ve never had any trouble with wheel flex on a std UDC narrow hub Coker. Narrower hubs are easier to spin fast. I’d stay well away from the extra-wide hubs unless you are extremely heavy or a wiggly rider. The 48 spoke hubs seem complete overkill to me.

If you are referring to racing Uni’s (IUF limited 24"/125mm), then I think the wheel is so small that you shouldn’t have much noticeable wheel flex unless you have a very sloppy wheelbuild or cheap lightweight rims.