I am presently running a 29" wheel with 125mm cranks.
My average speed is 10 km/hr.
Moving to a 4" crank what average speed should I expect?
Theoretically 12km/hr, in reality probably slightly less.
As a rough guess, 10km/hr * (125mm/102mm) = 12.25 km/hr, once you get used to them. 125-102 is a fairly big jump on a 29er if you haven’t ridden super-short cranks before, so it may take you a while to get used to them.
Thanks for replies
Thanks for the replies
Sorry for mixing the metric and standard sizes.
I figure after 450km on my 125 cranks, it time for something new.
I hated 102’s for down hill and decelerating. I like 110’s on a 29er for road riding.
Yeah I am thinking of getting some 4" cranks for my 29er…But I think for now I will hold off.
I like my 5" right now
Qualify on 29"
For me its all about qualifing for Ride The Lobster.
I want to squeeze every last speed advantage for distance out of my 29" so I can qualify for Ride the Lobster.
If I can’t qualify on my 29" I won’t buy a 36" for the race.
Right now my average speed is only 10km/hr. This is my present max average speed for long distance endurance racing on a 29". I recognize this speed will be higher on a 36" but no sense in buying 36" if you don’t qualify.
This is discouraging because it equates to 8+hrs to do 87km per day qualification.
Waah
I still haven’t put the 102’s on my coker yet… I keep meaning to do it but have been to lazy.
I suspect that, given enough time, you’d improve your speed on 125mm cranks noticeably as you get the hang of spinning quickly but smoothly.
Crank lengths do make a direct difference in leverage but they’re also down to personal preference. You may find that 114mm or 89mm is optimum for you, so it can be worth playing about. I once put some 89mm cranks on my 29er and hit 16mph - but it was really scary 
For normal riding I didn’t find the 89mm cranks improved my average due to lack of control but that probably would have changed if I’d bothered to get more used to it.
Basically, it’s worth varying crank length a bit to find something that works for your body and style of riding. Shorter cranks aren’t necessarily an improvement but the Right Cranks should be pretty good once you find them!
Do you not have any hills where you are? We did two days of riding, 68km and 51km, and that was enough to qualify. One of the people who qualified only learned to unicycle three months ago. Qualifying really isn’t difficult, it just takes a month of dedication to increasing your distance and saddle time.
This is entirely untrue. The reason to buy a 36" is because they’re a gas to ride. It should have nothing to do with RTL.
The idea that you can accurately calculate a maximum speed with a configuration that you have never tried is kind of a stretch, too. Short cranks help you spin faster but the revs are not inversely proportional to the crank length in any linear manner. With a faster spin you give up an element of control and with shorter cranks you give up torque. Both of these sacrifice stability. You have to be able to spin fast without wobbling. And you have to stay on top.
More gear-inches, not shorter crank length, increases speed. If you don’t believe this, look at the problem in the limit. If you believe the inverse proportionality myth then reducing crank length 20% increases speed by 25% (do the fractions). Following this argument you would be able to go infinitely fast with cranks of zero length.
Buy a Coker. They’re fun.
which is why you upgrade crank length and then you practice spinning fast with little wobble, then speed is about as fast as you can keep you legs going.
I feel comfortable on 29"
I feel really comfortable on 29", freemounting, dirt trail, etc. Its very versitile.
Its difficult to say if I wobble at 14km/hr. But at that speed its a result of down hill, or short quick take off.
Over prolonged distance I pace myself and found that speed is 10km/hr.
I travel round trips of 14km / day commuting to work.
Not sure how to get the hang of spinning the wheel quicker. What type of practice do you suggest?
Ride with faster riders. Or have a dog chase you.
Go with the dog. I have found them to be more reliable, personable, and conversational than faster riders.
Need a leash
Need a leash for the dog, or a unicyclist.
Pushing yourself to ride faster (but still safely) can help get yourself acclimatised. I tried to spin with minimal pressure on the pedals and making smooth motions - no “stomping” on the pedals as is the temptation when pedalling fast, that just reduces your efficiency. I find the limiting factor becomes how fast I can lift and drop my knees (whilst maintaining control) rather than how hard I can push.
Using a smaller, lighter, slower wheel can apparently be good to get you used to spinning quick without such a risk of high speed falls.
Boy, do I agree with this. Legtod - considering your apparent enthusiasm, do yourself a favor in this life - buy a Coker!
Finally tried my 102 cranks on 29"
Went on empty parking lot with few icy patches.
Was able to crank my speed up to 18.1 kph on first try and 18.3 kph at end of night.
This speed represents max speed or burst of speed not average speed.
My average was 11.8 kph. The slower average was due to circling arround the parking lot. I expect my average speed straight line would be 12 - 13 kph.
So the net difference in average speed between my 125 to my 102 is 2 - 3 kph
What difference did I notice?
My peddling felt smoother at higher revolutions, not as woobley during pedal strokes but took more effort on inclines/hills.
Braking/Slowing down harder.
Idling was to hard to control to a point I could only do a few strokes.
Conclusion: Since my 29" isn’t my speed/long distance machine. I’m putting my 125’s back on. I prefer control over speed on my 29" Besides, I’m only giving up a couple of kph between the two.