How much faster and I supposed to go?

General
1

Remember those threads about crank length and speed?

http://www.unicyclist.com/forums/showthread.php?s=&threadid=30994&highlight=170mm
http://www.unicyclist.com/forums/showthread.php?s=&threadid=30734&highlight=170mm

Well, recently, I decided to try getting more speed by changing the crank length. With that I ordered 145mm cranks. What I found out was surprising:
I only went a tad faster with 145mm cranks. I rode around a .60 paved loop - as fast as I could - 4 times.

The result, my average speed and top speed were less than 1 mph faster with the 145mm cranks - and I didn’t have much more endurance. In other words, I peddled as fast as I could - even when it hurt. Top speed was slightly faster (.6 mph) on the 145mm cranks.

My average speed with 145mm was 7.35 mph
My average speed with 170mm was 7.00 mph

How much faster am I supposed to go with the shorter cranks?
(I wasn’t paying attention in math class to learn how to make this basic calculation)

2

Ok, i’m far far from an expert, but I had some thoughts when I read this.

I have a 24 inch and a 36 inch unicycle. I’ve been riding the 24 for awhile, so i’m pretty good at riding, forwards anyways. I got the Coker recently, so I haven’t been riding for awhile. I haven’t timed myself, but I think I probably don’t go a lot faster on the Coker, than my other one. I think the reason it cause i’m just not used to going fast on a unicycle, I pedal a lot slower on the coker than my 24. Now If I ride the Coker more and more and get used to it, i’ll probably fly compared to my 24.

So your shorter cranks i’m thiking are kinda the same, just maybe not as big as a difference, but your still kinda pedaling less, or at least making a smaller circle. Maybe your muscles need to adjust to the difference and get more used to the motion of a smaller circle. I don’t know how long you’ve been practicing, but i’d say compare the times later when you’ve practiced more.

Like I said i’m just hypothesizing, it makes sense to me, but I could be wrong, have a good day.

Andrew

3

I believe you are correct KJ-52.
However, my question is:
How much faster SHOULD I go IF I pedal at the same speed?

I am not afraid of the speed I go with the 145mm (whereas I WOULD be afraid peddling that fast on a Coker). I am going “the same speed” and working “just as” hard. I expected to go at least 1 mph faster with the shorter cranks. :frowning: What I have found so far is that I gain little speed and lose abilities like getting into idling, going backwards and forwards, stopping, descending, etc.
I am hoping someone like newtouni will calculate how much faster I “should go” (on flat pavement). Seems like there are a lot of people who can calculate it on this forum.

4

We’ll base these calculations on the constant foot speed hypothesis. This hypothesis is based on the idea that your foot speed turning the cranks will stay constant for different crank lengths. Foot speed is calculated based on how fast your foot is moving in a circle. When spinning 170 mm cranks at 100 rpm your foot speed is going to be faster than when spinning 145 mm cranks at 100 rpm.

So we can figure out what your foot speed is with 170 mm cranks when going 7 mph. Then we can work backwards and figure out what speed you’d need to go to get the same foot speed with 145 mm cranks.

This is a job for UniCalc. Plug in the numbers and use trial and error to find the results.

Figure for a 24x3 muni your wheel diameter is about 25 inches.

Going 7 mph your cadence is going to be 94 rpm and your foot speed with 170 mm cranks is going to be 5.49 ft/sec.

Now we’ll figure out how fast you’ll need to go with 145 mm cranks to keep your foot speed at about 5.5 ft/sec.

Experiment with the numbers in UniCalc and you find that you’ll be pedaling at 110 rpm and have a speed of 8.18 mph with the 145 mm cranks while maintaining the 5.5 ft/sec foot speed.

So you can figure to gain 1.18 mph by switching to 145 mm cranks. Assuming that the constant foot speed hypothesis is true.

Of course there is more to it than just constant foot speed. It depends on how smoothly you can spin the pedals. How much wasted effort you have in making balance corrections (by varying your pedal speed). And other factors. If you’re a perfectly efficient unicyclist then the constant foot speed hypothesis is probably closer to the truth. If you’re not quite a perfectly efficient unicyclist yet then the constant foot speed hypothesis is not going to be as close to reality.

5

It’s much simpler than that.

NEW CRANKS DIVIDED BY OLD CRANKS, MULTIPLIED BY SPEED.

If you accept the basic idea that your feet move at the same speed, then shorter cranks mean your feet can do more revolutions per second, whilst moving at the same speed.

If your cranks are half as long, your feet have half as far to go, so they can do twice as many rpm.

If you do twice as many rpm, then you go twice as fast.

So, say you went from 6 inch cranks to 3 inch cranks, you’d be halving your cranks and doubling your speed.
6/3 is the same as 2/1. (Just do 6 divided by 3 = and you will get the answer 2)

So, going from 170 top 145, you use the same formula: original cranks divided by new cranks:

170 divided by 145 = 1.17

Your new speed will be 1.17 x whatever it was before.

Or, if you went the other way, 145 divided by 170 = 0.85, so cahnging back makes your speed 0.85 x whatever it is on the shorter cranks.

So, to calculate the approximate change of speed:

NEW CRANKS DIVIDED BY OLD CRANKS, MULTIPLIED BY SPEED.

That’s all there is to the maths.

But what is speed? Maximum speed? Cruising speed? Average riding speed over a mixed terrain journey? Shorter cranks will not affect all of these identically, because other factors like skill, control, acceleration/deceleration, and nerve come into it.

The “constant footspeed hypothesis” is a jokey name we give to a rough and ready rule of thumb. I have ridden wheels sized 20, 24, 26, 28 and 36, and have used various lengths of cranks including 89, 102, 110, 125, 150, 170, and have put some effort into making comparisons. The constant footspeed hypothesis is no more than an approximation, and only really works for adjacent crank lengths (say 125 and 150) with all other things being equal (wheel, tyre, terrain, rider etc.) and when the rider is confident and in control.

The rider and cranks are the engine. The wheel is the gear. Choose the cranks to suit the rider (we are all different sizes) and the wheel to suit the speed or terrain. A big wheel is the way to go fast, a small wheel is the way to go up hill.

6

From a lot of riding on my muni with 150s and 29-er with 125’s, with some minor experimentation with a muni with 125’s and 29-er with 110’s; my conclusions about crank length in regard to speed are: -

  • a bigger wheel size has more affect on speed than crank length

  • the speed/smoothness benefits of a bigger wheel/shorter crank manifest mainly in constant cruising i.e. I can go as fast on the muni for short bursts, but for going fast for a sustained longish distance the 29-er is much better

  • I’m much happier going full blast on the muni, as I know I’ll be OK if something goes wrong; on the 29-er I have little desire to go at max speed because it will be problematic if I come off while doing it

  • in terms of getting somewhere fast (and I’m not the first to point this out), sometimes the slower uni can work out faster. For example, for a ride to the shopping center in Sheffield, the 29-er should be fastest, but, on the muni I can utilise it’s greater control to make full use of pavements/short cuts and be very confident around pedestrians; whereas, with the 29-er, it’s either struggle on the pavements, with lots of dismounts, or go on the roads, with traffic lights etc.

Similarly, people with Cokers often point out that, on rougher terrain, 150’s work out faster than 125’s because of more control and less UPDs.