How best to determine crank length?

[Kenneth]

Today i spoke with Mark the Bikesmith. He says my cranks can be shorted to whatever length i want from153 to 80 mm
The question is what would be the best length for me?
I've been running 48/38chainrings on the Surly and have added a third ring at 30 to this for the S40.
Today i sourced at a local bike co-op a used Sinz 135 mm crank set that will take the 48/38 rings. I'm thinking i will start with these to try to get an idea before i send mine off to be cut.
Any input would be appreciated
Also I'm 6"1" with a 34 inseam
Regards
Kenneth

 

[Gary123]

Interesting to see if you're at all comfortable with 135's. Seems very short but might work.

 

[ratz]

6"1' probably 153-165 will be the sweet spot.

 

[cpml123]

I posted a similar question before and summarized the suggestions. This may help.

http://cruzbike.com/forum/threads/knee-issue-need-suggestions.12068/

 

[Markopolo]

Still waiting for my S40 but here are my thoughts. Have a Catrike I've been riding, I'm just a little over 6'1" and been playing with the boom. I think taller people can go with longer cranks. The Catrike uses 170. Long legs, long crank, advantage. Short legs, short crank, disadvantage. More or less strokes to the mile, depending on the legs. I've found the sweet spot for my boom, moving it 1/4" at a time can really tell the difference, then finally 1/8" and still tell the difference, finally finding where the most power is made. Going too long makes for a lot of effort with no gain, shorten til the power picks up and effort drops off. I think the DF riders go with longer seat/pedal length then bents need. The boom gets too long and your power stroke tends to the bottom of the stroke, shorten the boom and the power moves back up the arc. But in the end it is all personal, find where your power is made, just a little movement can make a big difference. Then let your legs get used to it. I think the S40 comes with 165's, if I was to build it I'd use 170's or 175's which are on my DF and like. Previous DF had 165's and moving to the 175 could tell the difference, though not for better or worse.

 

[Markopolo]

Re the gears, my Catrike uses 30/39/52. It can be hilly around here and when it is use the 30 and big gear on the back, any smaller and don't think you'll have the speed to stay upright on 2 wheels, about 5 mph on the trike. Otherwise on smooth ground rolling on the 39, never use the 52. Have a DF Sequoia with 30/46 and really like that over previous 34/50. I think something like a 30/39/46 would be sweet or smaller on the 30.

Re crank length, if you go shorter on the crank you'll just be bringing the boom in shorter to pull the power up to get the angularity, too long on the crank and you'll have more angularity then you can stand no matter how far out you push the boom, but that would apply more to short people.

 

[ratz]

Long legs, long crank, advantage. Short legs, short crank, disadvantage.

While this seems obvious and many of us started with those thoughts as well, it turns out to not really be true. It has actually been scientifically tested and proven misleading. The body adapts and the cadence will change to match the change in leverage and power will remain the same, velocity takes up the slack. There a discussion here on the board that reference the research. I'll see if I can find then. In trike world with it's much heavier vehicle it is more noticeable at first, probably what you are seeing, but the cadence will adapt as you put in the miles. unless you are an infrequent rider.

Re crank length, if you go shorter on the crank you'll just be bringing the boom in shorter

This is backwards, use to confuse me all the time when I'd think about it. A Shorter crank means a longer boom so that you can get full extension of the legs. The longest position of the pedal gets closer to you so you have to make the boom longer to maintain proper full leg extension.

The reason many of us go shorter on the cranks than we do on a DF is:

• To lengthen the boom to get more knee clearance at the handle bars without putting the handlebars too far back which causes handling issues especially for new riders. 5mm makes a huge different in handling under high power.
• To reduce knee bend for safety if we are at that age or have knee issues
• To reduce hip "throw" and rotation

Purely by reviewing the long standing riders

5'0" - 5'5" ===> 145-155mm
5'5"- 5'10" ===> 150 - 165mm
5'10"- 6'3" ===> 155mm - 170mm
6'3 + =====> 165mm - 180mm

Leg length relative to the body of course affects where you fall in that chart.

Rules that seem to hold true

• It's hard to go too short
• It's easy to go too long
• Trike vs DF usually your going to shorten about 10mm, 172.5 on a DF will usually like 165mm on the trike, but can go shorter.
• CB vs DF usually 15mm shorter is very effective for most people

@LarryOz can probably chime in to at 4'2" and 115mm cranks he's done the most testing with his variable length crank set.

 

[Don1]

also ... power is force x distance. shorter cranks mean more power to the point you cant spin em fast enuff. also you need a higher vo2 max to run longer cranks. my df has 175 and i run 10 bpm heart rate higher compared to 172.5 on a bike with my bike fit over it.

 

[Markopolo]

Have to agree there, cadence is too much of a variable, how the legs feel, temp, speed, barometric pressure, incline, which gear. Can't imagine 5 or even 10 mm having much of an effect, my arse moves in the saddle more then that. And I would think effortless spinning can fall anywhere along a broad continuum of boom or crank length.

Power used for acceleration is a different animal.

Power gets you there, spinning keeps you there. But I'm not a scientist.

 

[tiltmaniac]

I have a set of powercranks that allow one to set a new crank length by loosening a bolt on each crank arm.

Good for playing around with crank lengths.

 

[Kenneth]

Thanks for all the input.
My reasons for going to shorter cranks
1. The general consensus that it better for recumbent/CB
2. Learning that it's better for your knees,
less bending of them anyway ( my knees are starting down the path of no return)
3. The clearance gained at the handle bars

After considering the imput given i feel comfortable sending off my cranks to be cut down to 155

Thanks
Kenneth

 

[ratz]

also ... power is force x distance. shorter cranks mean more power to the point you cant spin em fast enuff. also you need a higher vo2 max to run longer cranks. my df has 175 and i run 10 bpm heart rate higher compared to 172.5 on a bike with my bike fit over it.

That is interesting never checked that or heard that would have expected the opposite. Interesting

 

[Markopolo]

Kenneth, you might see if u can test ride or borrow a catrike expedition. It has the 40 degree seat and 170mm cranks. The boom has quick release adjustment so you can make infinite adjustments while out riding. And it is a trike so stable.

 

[LarryOz]

Have to agree there, cadence is too much of a variable, how the legs feel, temp, speed, barometric pressure, incline, which gear. Can't imagine 5 or even 10 mm having much of an effect, my arse moves in the saddle more then that. And I would think effortless spinning can fall anywhere along a broad continuum of boom or crank length.

Power used for acceleration is a different animal.

Power gets you there, spinning keeps you there. But I'm not a scientist.

I have tested various crank lengths from 90mm to 160 based on my 29" inseam and being about 5'7"
The 4 key factors for me are:

• Less knee bend during the back-stoke: knee bend greater than 90 degrees causes me knee pain, plus when your knee is bent father than 90 degrees you are unable to make as much power when started to un-bend it into the power stroke.
• Less leg movement during each spin, especially noticeable during 95+ rpm, but really noticable at 120 (try it)
• More aero-dynamic since you are cutting a smaller hole in the wind at the front of your bike
• Easier and faster to spin up to a high cadence.

The "only" downside I have ever noticed is that with extremely short cranks pushing at really high power and low cadence seems to cause me some knee pain, other than that it is a, win, win, win, win!

I ended up really liking cranks in the 145-155mm length.

 

[Mathew Fy]

Cobb’s cranks are on sale. If I didn’t already have their 155 mm. alloy crank I’d buy the carbon one.
https://cobbcycling.com/collections/cranks

 

[super slim]

Is the fact that when riding a recumbent, the body to legs is not at 90 degrees for a DF rider, but at 135 degrees, for a 45 degree seat back, and 165 degrees for a 21 degree seat back, effect the amount of power that can be applied to the pedals, using the seat back.

When pedalling with a shorter crank, the same crank power is achieved, at a proportionally higher RPM for the same pedal load!
Going from 175 mm crank to 153 mm crank, with a 46" x Seam, was the best thing for my knees, and climbing ability!

 

[LarryOz]

Is the fact that when riding a recumbent, the body to legs is not at 90 degrees for a DF rider, but at 135 degrees, for a 45 degree seat back, and 165 degrees for a 21 degree seat back, effect the amount of power that can be applied to the pedals, using the seat back.

That is the way I see it and have tested it. That is why you can make more power easier on a Silvio compared to a Vendetta (at least I can). Of course you do not have the aero.

 

[Kenneth]

Kenneth, you might see if u can test ride or borrow a catrike expedition. It has the 40 degree seat and 170mm cranks. The boom has quick release adjustment so you can make infinite adjustments while out riding. And it is a trike so stable.

I might do that if i find one in my neck of the woods

 

[rx7mark]

I'm 6' 2" and have a 34 inch inseam also. The crank length calculator told me a 150mm - 165mm crank would work, I went with a 155mm on my T50 and find it an ideal compromise. My knee does not go passed 90 degrees, and I have just a little clearance to the bars. I have no erg to try anything longer, it just works and feels perfectly natural.

Mark

 

[Kenneth]

Interesting to see if you're at all comfortable with 135's. Seems very short but might work.

I've been riding for a week or so now on the 135's and they seem fine and natural enough,certainly don't feel bad or ackward. Going to a CB which is so different from a DF and changing the crank length at the same time has made it hard for me to judge what the effect is really. I still haven't sent off my triple to be cut down, but i need to make up my mind as to length as i really want the granny gear for climbing the hills here about, which I've avoided knowing i wouldn't be able to climb a 10% grade on the 38T