3.5 inches of pure fun. ;0)

I’ve just put my new 89 mm cranks on my 24 inch Nimbus. The results have been er… surprising.

Get the maths out of the way first: 89 mm = 3.5 inches, so the ‘ratio’ is 29%. That’s equivalent to a Coker with 133 mmm cranks (150s are standard, mine has 125s); or equivalent to a 28 with 102 mm cranks.

Mounting requires care, otherwise the uni runs away from you. Riding on the flat is easy, cruising without effort at 8 - 10 mph. Although I haven’t done any serious speed testing, my gut feeling is that there is little top speed advantage compared to the 102 mm cranks I had on before, because of the reduced control and increased fear of UPD. Idling is reasonably easy, but requires care, otherwise it overshoots. It has very little hill climbing/descending ability at all!

The distinctive thing is how easy it is for the pedal to overshoot top dead centre when mounting, idling or decelerating. This leads me to think that there is more to this short cranks thing than simple ratios. I think the absolute length of the crank matters too.

I think this is because you can only exert maximum torque on the pedal within a very narrow angle as the crank rotates. Assuming that this angle is the same for all crank lengths (a gross simplification) then the actual distance moved by the pedal as it crosses that angle will vary with the crank length. An 89 mm crank is 52% as long as a 170mm crank, so the pedal is only in the ‘maximum torque’ position for 52% of the distance. Assuming constant footspeed then the foot is only in position to exert maximum torque for 52% as long.

If so, then this brings in reaction times, skill, and ‘muscle memory’. All three of these can be improved with the dreaded P word, but as the 89mm crank is always 52% as long as the 170mm crank, even if you get twice as ‘good’ through practice, the 89s will only give 52% as much control as the 170s.

And if you think about it, if the 52% is a constant, and your skill ‘doubles’, then the gap between the levels of control on the two crank lengths doubles - as far as these immeasureables can be measured. Er…

But in simple layman’s terms… 89s on a 24 are really really silly, totally impractical, but jolly good fun, and mean that you can find challenging riding on the simplest of routes. Everyone should try it once. :0) Ultra short cranks are a fine way of taking yourself back to those heady days when simply riding the uni on an uneven surface was an achievement.

And finally, thanks to Roger for his usual excellent and speedy service. I can’t recommend unicycle.uk.com enough. The steel 89 mm cranks are under 10 quid including postage, as are the 102s and 110s. Put some short cranks on your Christmas list NOW.

That sounds cool. I’ll have to get a pair of 110s though before i go for 89s! by the way, The name of this thread sounds kinda sick untill you read it :smiley: just kidding. So how fast can you maintain with those cranks?

-Ryan

Re: 3.5 inches of pure fun. ;0)

Mikefule, thank you for your continued effort in sharing your crank
length obsession with us. This was joking and serious at the same
time.

>my gut feeling is that there is
>little top speed advantage compared to the 102 mm cranks I had on
>before, because of the reduced control and increased fear of UPD.
May get better with getting used to. We wouldn’t want to trash the
CFSH just for some silly cranks would we?

>The distinctive thing is how easy it is for the pedal to overshoot top
>dead centre when mounting, idling or decelerating. This leads me to
>think that there is more to this short cranks thing than simple ratios.
>I think the absolute length of the crank matters too.
I think the easy overshooting is purely due to the lower torque you
can exert with the shorter cranks.

>I think this is because you can only exert maximum torque on the pedal
>within a very narrow angle as the crank rotates. Assuming that this
>angle is the same for all crank lengths (a gross simplification) then
>the actual distance moved by the pedal as it crosses that angle will
>vary with the crank length. An 89 mm crank is 52% as long as a 170mm
>crank, so the pedal is only in the ‘maximum torque’ position for 52% of
>the distance. Assuming constant footspeed then the foot is only in
>position to exert maximum torque for 52% as long.
I think your assumption of constant footspeed is not valid for this
reasoning. Especially for mounting, it would rather be that rotational
speed is constant, and hence footspeed is lower (proportional to crank
length). This would be almost equally valid for idling, maybe even
MORE valid because the smaller cranks make it more difficult (read:
slower) to effect the oscillatory idling motion. Only for decelerating
I can see it, but just when assuming that you are delerating from
riding about as fast as the small cranks allow.

>And finally, thanks to Roger for his usual excellent and speedy service.
>I can’t recommend unicycle.uk.com enough. The steel 89 mm cranks are
>under 10 quid including postage, as are the 102s and 110s. Put some
>short cranks on your Christmas list NOW.
I was (and am) planning to buy the whole set from Roger, but just a
little bit later when the weather will be better for fast road riding.

If more riding with the 89’s changes your view, please post a
follow-up.

Klaas Bil

Yak’s milk is pink.

3.01 miles today with a recorded time of 26:30 which = an average speed of 6.81mph. Top recorded speed is 10mph, and whilst I didn’t ‘go for it’ (it was dark, and the track was more or less uniform in colour, but not texture :astonished: ) I was a bit disappointed to see that the figures are very similar indeed to what I would expect on the 102mm cranks on the same route.

Even on this short route, though, there was some noticeable improvement, and I ended by climbing a short sharp hill which used to be a challenge on my 26 with 150s a few months ago.

The ride is pretty similar to my 28 with 110mm cranks, which has a broadly similar gearing. I reckont he 28 is that bit faster though, and surprisingly, easier to idle.

I do think absolute crank length has an effect, irrespective of the ratio of crank length to wheel size. I can’t quite put my finger on why. The simple ‘lack of torque’ answer isn’t it, because it feels different. The Coker (28%) lifts me out of the seat, whereas the 24in/89mm combination (29%) just sort of flips past my foot without warning. It does seem to be a ‘response time’ problem.

Either way, it’s fun, and will be a good summer booby trap for precocious kids who once rode a 20 and think they can show me how it’s done on this one.:smiley:

Re: 3.5 inches of pure fun. ;0)

On Wed, 11 Dec 2002 12:45:46 -0600, Mikefule
<Mikefule.fifxy@timelimit.unicyclist.com> wrote:

>I do think absolute crank length has an effect, irrespective of the
>ratio of crank length to wheel size. I can’t quite put my finger on
>why. The simple ‘lack of torque’ answer isn’t it, because it feels
>different. The Coker (28%) lifts me out of the seat, whereas the
>24in/89mm combination (29%) just sort of flips past my foot without
>warning. It does seem to be a ‘response time’ problem.
Possible alternative explanation that corroborates your “absolute
crank length” idea: the 89 mm cranks may be just too short to lift you
out of the seat. I presume that that short distance is taken up by
slack in bones, joints, muscles and sitting flesh (no attempt was made
at being polite here, but no offence intended either). With the Coker
combination the first (say) 89 mm you’re also not lifted, but the long
(150 mm) crank carries on and there you go.

>a good summer booby trap for
>precocious kids who once rode a 20 and think they can show me how it’s
>done on this one.:smiley:
Hah!

Klaas Bil

The flushing toilet was invented by Thomas Crapper.