Crank length?

[John Bozi]

Chalo, 120 is now the lowest I have heard of thanks to you.

Problem is I have 4 joining holes (cant think of the bike vocab for them) as in mountian bikes these days.

the shorteners for tandems are not a way of avoiding pedal strike. They still have the same length of crank just move the holes up to make it easier to pedal.

I need ground clearance

 

[Chalo]

the shorteners for tandems are not a way of avoiding pedal strike. They still have the same length of crank just move the holes up to make it easier to pedal.

I need ground clearance

Shorter cranks make it harder to pedal, by reducing the amount of mechanical advantage you have available. To turn a shorter crank, you have to push it harder, for any given load, compared to a longer crank.

If what you need is total ground clearance, there are many factors involved. Bottom bracket "drop" (distance below a line connecting front and rear hubs), fork length and travel, rear suspension travel or lack thereof, wheelbase, and wheel outside diameter all play a part. To disregard all these other factors, in favor of changing the cranks to child sized parts, is just neutering your bike's ability to be operated as a real pedal bicycle. I guess I don't see the reason to keep any functional pedals on your bike in such a case.

"Pedal strike" usually refers to touching down a pedal while leaned over and pedaling through a turn. Moving the pedal inward towards the crank spindle reduces this tendency, even if the crank is the same length. Reducing pedal strike is why road bike pedals have gotten much narrower (and bottom brackets higher) over the last 40 years or so.

In the '70s, Schwinn used 165mm cranks along with very high (about 12"/305mm) bottom brackets on their inexpensive Varsity and Continental road bikes. I think this was an early attempt at idiot-proofing, like their requirement that all "Schwinn Approved" rear derailleurs have integrated bash guards.

 

[docnjoj]

Her we go again! The simple fact is that if you find shorter cranks help your knees, as I do, you need to gear down to reduce the amount of force you use to push those cranks. No question, if you don't gear down and spin faster, you need more force than with longer cranks. That is what gears (and motors) are for.
otherDoc

 

[John Bozi]

No questions about efficiency of pedalling Chalo. This is an experiment into what the maximum torque is I can get onto this bike (without adding more electric power) and still remain possible to pedal home if it fails.

going down from 26" to 19" rear should be roughly 30% more torque - that and the venting I plan to do, I should see 30% less heating up, which means for me riding 30% longer. Will be losing a couple of kilos.

I will miss the extra range I get from pedalling at about 20-30kmh - I dont mind going slower especially in the more beautiful parts of my rides.

The pedalling will have to be on my non electric carbon mountain bike - gathering dust in the corner...

This is roughly what my cranks and chainring will look like.

mine will be 120mm 30t black race face wide narrow (below pictured they are 150mm)

edit: after everything is installed I plan to closeup slowmotion ride buys with the gopro to get real measurements of the clearance between the 30t chainring and erosion humps which are all over the mountain paths I ride. If there is enough space there I will probably go down to 24" front ( I doubt any smaller would be possible). Also depending on how the gearing works I might change my rear cassette - maybe single speed. Not sure yet and no rush.

 

[Offroader]

I see Sinz makes a range of crank length sizes from 115mm to 180mm for only $49.95, in both ISIS and square tapper

I was thinking about purchasing a 115mm because my knee hurts a bit from the 165mm cranks that I have.

I was wondering if 115 could be too small? I don't really pedal just constantly sit and stand hundreds of times per ride and the wide stance makes it harder on me knee to do this I believe. It gets to the point sometimes that I move the pedals to the 12 and 6 oclock position to stand up.

 

[Eskimo]

Just installed 152mm cranks on my velomobile. Will see how that works out soon. On a velomobile or recumbent you spend certain amount of energy by just moving your big legs horizontally. Shorter crank reduces this load i think. 170mm cranks make your legs move all over the place quite a bit. Crank arms that would have multiple threads on different places would be good. You would just need to switch pedals to the new position.

 

[docnjoj]

Eskimo pretty much got it right. Shorter cranks allow less knee motion, which is where the stress is. You do gotta gear down and use the motor to help get off, but one you are moving 80-90 cadence works good at least for me. Slower is also good.
otherDoc

 

[ddk]

I see Sinz makes a range of crank length sizes from 115mm to 180mm for only $49.95, in both ISIS and square tapper

I was thinking about purchasing a 115mm because my knee hurts a bit from the 165mm cranks that I have.

I was wondering if 115 could be too small? I don't really pedal just constantly sit and stand hundreds of times per ride and the wide stance makes it harder on me knee to do this I believe. It gets to the point sometimes that I move the pedals to the 12 and 6 oclock position to stand up.

I use 89mm cranks on my e-trikes. Whatever turns your crank-(lol)

I previously used 150-120mm. Non-powered bikes I used 185-165mm cranks- all at the same cadence -same effort- (faster when I was younger because big cranks+big gearing ratios.)
The difference being aged joints and... old guy issues.

BTW I can pedal one trike (un-powered) home because of I have the gearing ratio to do so. @5-6mph=not too shabby when you consider I'm using 89mm cranks and the trike (with batteries mounted) weighs about 90kg and as previously mentioned old guy (I am)

 

[Chalo]

Why bother with pretend cranks? ADA says you can use a scooter if you need one.

Rode my chopper bike to work today, with 190mm cranks. I need to get much longer ones, because the BB on that bike is really high.

 

[ddk]

Why bother with pretend cranks? ADA says you can use a scooter if you need one.

...

I believe it's referred to as:
exercise

 

[Chalo]

Exercise benefits from real cranks. Would you consider it "climbing staircases" if the stairs were all 1.5" tall?

W = F x D

 

[tahustvedt]

Exercise benefits from real cranks. Would you consider it "climbing staircases" if the stairs were all 1.5" tall?

W = F x D

Make up your mind. A few posts back you said shorter cranks means you have to push harder.

Obviously, if you pedal at the same cadence as with longer crank arms. With shorter crank arms, shifting down and increasing the cadence achieves the same thing at the rear wheel, and you move your feet in a circle with the same velocity (not rpm) and push with the same force. I can see a benefit with a recumbent, as you don't have to waste as much energy lifting your legs up and down with the increased leverange of having your legs pointing straight forward.

 

[Chalo]

I can tell you from experience that it's easy to pedal 90 RPM with 200mm cranks. How easy is it to pedal 180 RPM with 100mm cranks? How much energy do you have to use just reciprocating your legs in such a case?

I push on the ground with a force of 330 pounds when standing still. That doesn't mean it's exercise.

 

[tahustvedt]

I can tell you from experience that it's easy to pedal 90 RPM with 200mm cranks. How easy is it to pedal 180 RPM with 100mm cranks? How much energy do you have to use just reciprocating your legs in such a case?

I push on the ground with a force of 330 pounds when standing still. That doesn't mean it's exercise.

A similar comparison is with 150 mm vs 300 mm. All within reason.

The weird thing I keep reading about is how someone will shorten their crank arms more and more without positive results and not comcluding that it doesn't do them any good.

 

[docnjoj]

I can tell you from experience that it's easy to pedal 90 RPM with 200mm cranks. How easy is it to pedal 180 RPM with 100mm cranks? How much energy do you have to use just reciprocating your legs in such a case?

I push on the ground with a force of 330 pounds when standing still. That doesn't mean it's exercise.

Getting back to us old guys, those of us that had knee therapy (not replacement) were given recumbent exercisers in the rehab gym that had 150 mm cranks. We were told to keep our cadence above 75 and the pressure device (a clamp on a wheel like a bike brake) was kept loose (without braking force). I asked my PT guy about this and he said that for training of folks with good knees they can and do adjust the pressure up when the electronic program puts in a hill. For old guy rehab, however, minimum pressure. They also had some upright trainers with 175 cranks, and the bike racers did use them. Their resistance was wind driven.

The bottom line seems to be: Regular healthy bikers can subscribe to Chalo's (and others)formula for increasing the crank length the longer your legs are. Once your knees are broken or painful, you want to minimize motion and pressure. Hence the need for shorter cranks and higher cadence with less pressure. Volt up/gear down. (Holmes Hobbies)
otherDoc

 

[John Bozi]

BEcause this topic has gotten a bit of attention thought I'd update where my cranks are.

As I added the mid drive to the bike I now had all the unicycle options open up.

The toughest lightest cheapest and coolest looking That I could find are these Nimbus Venture:

http://www.municycle.com.au/catalogue/cat/CranksCOT.html

Mine were these ones in 114mm, a few mm shorter than my previously posted spider above. They've got smaller and bigger too.

Nimbus Venture2 cotterless cranks. Upgrade your unicycle with these robust, durable high quality and lightweight cranks.

Machined 7075 T-6 aluminium cranks, available in 114mm length. Sold in pairs.
Product Description
Cotterless cranks are often produced as cheaply as possible and consequently are of poor quality. The better cranks are only found on ISIS hubs. Nimbus Venture2 cotterless cranks are the exception.

These cranks are produced by a different process and create an excellent balance between weight, strength and toughness. Although the original Venture cranks are good... these are 40% stronger!

Material: 7075 T-6 Aluminium.
Color: Black.
Length: 114mm.
Weight (pair): 341g
Type: Cotterless
Q-Factor: Almost Zero Q-factor - This is the amount a crank bends outwards. Zero means it doesn't lean outwards at all. The straighter the crank the better for speed, the wider the better for ankle clearance and greater maneuverability.
Note: Crank length is measured from the center of the axle hole to the center of the pedal hole. Sold in pairs.

Sure at first riding it feels a bit like a clown in the circus. But After a while it's normal and after a year I hate the feeling of long cranks. My knees feel much better.

 

[recumbent]

170mm cranks are a common size where I live, and found them a bit cumbersome even before I went electric 14 years ago.
Now with my new build, 155mm crank arms work well, down from 165mm with my 5 year old recumbent. I'm 6 feet tall with a hip replacement and other minor leg issues from 60 years of active living.
Thanks for the links to the suppliers of shorter cranks BTW, because I had to pay an extra premium for shorter cranks on my new "Lightfoot" trike made in Montana.
Will post pictures of the build when completed, haven't seen anybody use the "Lightfoot" frames here on this forum that I know of.

 

[docnjoj]

170mm cranks are a common size where I live, and found them a bit cumbersome even before I went electric 14 years ago.
Now with my new build, 155mm crank arms work well, down from 165mm with my 5 year old recumbent. I'm 6 feet tall with a hip replacement and other minor leg issues from 60 years of active living.
Thanks for the links to the suppliers of shorter cranks BTW, because I had to pay an extra premium for shorter cranks on my new "Lightfoot" trike made in Montana.
Will post pictures of the build when completed, haven't seen anybody use the "Lightfoot" frames here on this forum that I know of.

I've seen one Lightfoot down here and they are "the bees knees"! Really well made and in America to boot. Quality as good or better than my Steintrike. If I have another life...............I'l definitely get one.
otherDoc

 

[recumbent]

Here is a picture of the one I bought, hope they don't mind. Great for mid drive mod, but I went 2WD, one rear wheel motored with 10 turn "Mac" and other side is peddle power.

 

[docnjoj]

That is interesting. Can you show more pictures of how you mounted the motor. My wife's USX delta trike has front wheel drive, but has a rear split axle. I would like to see how the rear wheel drive is set up. I makes a lot of sense.
otherDoc

 

[recumbent]

Just to be clear, The picture I posted was from the "Lightfoot site" I purchased the exact same model. My trike has the 10T Mac motor mounted to the right rear wheel, which is isolated from the left chain driven (from factory) wheel.
Here is a bottom shot, of the same trike, from their site, so you can see it better.
Sorry if I misled anyone.

 

[Seatec]

I just cut a piece out of my cranks and re welded them, they are now 100 mm, very easy to do f they are steel, easier than taping I think.
Just got the cranks to make it legal, will not be pedaling as long as I have battery power.

WAYNE

 

[docnjoj]

So you just mounted a motor wheel to replace one of the rear wheels? I would love to see that. I really like the jackshaft arrangement. Lightfoot does fine work. I'm guessing the motor would go on the right side? How did you join the shafts of the bike and the motor?
otherDoc

Edit: I just noticed the "dropouts on each rear axle. Cool

 

[recumbent]

So you just mounted a motor wheel to replace one of the rear wheels? I would love to see that. I really like the jackshaft arrangement. Lightfoot does fine work. I'm guessing the motor would go on the right side? How did you join the shafts of the bike and the motor?
otherDoc

Edit: I just noticed the "dropouts on each rear axle. Cool

Yes, you spotted the two pairs of drop-outs, no mechanical connections and open storage.
The engeneering of the trike is great for a less-than-perfect-design to begin with. The rear subframe is only connected by two cr-mo pipes that are part of the front framing members. This allows the rear to twist, somewhat, in a very limited but beneficial manner when riding on uneven surfaces. I'm a strong supporter of Cr-Mo steel frames as you can tell. I purchased an aluminum bike years ago with front suspension, and the thing rattled my bones, even on pavement.
I promise to post pictures and such on this great forum when finished, 98% done.

 

[Offroader]

Since I have an 83mm bottom bracket width, I can't buy a square tapper bottom bracket.

This leaves me only an ISIS 135mm crank as the smallest I can find.

Is it even worth dropping from 165mm to 135mm? Seems like you guys like less than 100mm cranks.