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-   -   Crank length fixed gear without freewheel (https://www.bikeforums.net/bicycle-mechanics/1185076-crank-length-fixed-gear-without-freewheel.html)

koga Mika 10-05-19 05:49 AM

Crank length fixed gear without freewheel
 
Hey guys,

I'm building my own fixed gear bike with no freewheel and I was wondering what crank-length I should choose. I need short cranks so I don't scratch the pavement in the turns but long enough so I can generate a good amount of power. I'm going to use the bike for regular city-use with not many hills.

Thanks for your help
Mika

flatrockmobile 10-05-19 05:59 AM

I have 165mm on one beach cruiser and 185mm on another. I much prefer the 185's They just feel right but I do make sure the cranks are horizontal going into a 90deg turn because they can scrape if vertical.

JohnDThompson 10-05-19 06:02 AM

Track frames designed to be ridden on a banked velodrome have fairly high bottom brackets to prevent pedal strike. If this is a converted road frame, the bottom bracket may be lower than that on a true track frame, so shorter cranks will help prevent pedal strike. Shorter crank arms are also easier on your knees at high RPMs; something that becomes significant on a fixed gear riding downhill or with a tailwind.

I run 170mm arms on my road bikes and 165mm arms on my fixed gear bikes and really don't notice any difference in power or leverage.

dsbrantjr 10-05-19 06:09 AM


Originally Posted by JohnDThompson (Post 21150891)
I run 170mm arms on my road bikes and 165mm arms on my fixed gear bikes and really don't notice any difference in power or leverage.

The difference is about 3%

HillRider 10-05-19 06:30 AM


Originally Posted by flatrockmobile (Post 21150889)
I do make sure the cranks are horizontal going into a 90deg turn because they can scrape if vertical.

Can't be done on a fixie. I agree, on a road frame with its normal height bottom bracket, short crank arm (say 165 or 170 at most) are the way to go.

koga Mika 10-05-19 09:58 AM

thanks for your help guys
 
,,

Originally Posted by flatrockmobile (Post 21150889)
I have 165mm on one beach cruiser and 185mm on another. I much prefer the 185's They just feel right but I do make sure the cranks are horizontal going into a 90deg turn because they can scrape if vertical.


Originally Posted by JohnDThompson (Post 21150891)
Track frames designed to be ridden on a banked velodrome have fairly high bottom brackets to prevent pedal strike. If this is a converted road frame, the bottom bracket may be lower than that on a true track frame, so shorter cranks will help prevent pedal strike. Shorter crank arms are also easier on your knees at high RPMs; something that becomes significant on a fixed gear riding downhill or with a tailwind.

I run 170mm arms on my road bikes and 165mm arms on my fixed gear bikes and really don't notice any difference in power or leverage.


Originally Posted by dsbrantjr (Post 21150895)
The difference is about 3%


Originally Posted by HillRider (Post 21150909)
Can't be done on a fixie. I agree, on a road frame with its normal height bottom bracket, short crank arm (say 165 or 170 at most) are the way to go.


Reynolds 531 10-05-19 10:17 AM

165mm. Spinning out a long crank is worse than pushing up a hill on a short crank.

operator 10-06-19 10:11 PM

The real reason for a shorter crank is hip angle in regards to bike fit.

In this scenario with a high BB, people still want max clearance when cornering, so shorter cranks ~ 165mm range.

Next person to say it's because of leverage needs to choke some puppies

Andrew R Stewart 10-06-19 10:35 PM


Originally Posted by operator (Post 21152926)
The real reason for a shorter crank is hip angle in regards to bike fit.

In this scenario with a high BB, people still want max clearance when cornering, so shorter cranks ~ 165mm range.

Next person to say it's because of leverage needs to choke some puppies

Slow down here. Only two people have mentioned leverage. The first was the OP, wondering about what crank arm length would be best for him and his concerns about the efficience of shorter arm lengths. The second was John who said he couldn't feel any real difference in leverage. I take this to reinforce your view of leverage not being what's it about.. So who are you so seeming annoyed with?

koga Mika- There's no one answer for everyone. Some riders are good at avoiding pedal strikes, others not so much so. Some are good at high rpms and don't need short arms for that aspect. Some riders find 165 arms to be right for all riding (I have many smaller friends who this applies to.) Some riders (the big and tall often) find 170 arms to be really short.

Also you haven't provided all the data we might want to know before we suggest our opinion of your need. How tall are you? how well do you spin? Are you an aggressive rider and will corner hard? If you were to scrape a pedal what would you do/feel/conclude? Are you clipped in or running flats? How big are the pedals? Are you running hand brakes?

My set up has the same arm length that I use on all my bikes, 170mm (including my two mountain bikes). I've had a track/rollers.fixed bike since the late 1970s but have also run hand brakes since the mid 1980s. I spin very well for an old guy. I stand 5'6" tall.I run small pedals (Speedplay X). And the last two frames for this function I built had 6.5 or 7cm BB drops. Pretty middle to low a BB height for a track bike. But I don't do much open road riding on these things often. I find the connection of a fixed cog to no longer hold some sort of draw for me and like having gears to make riding easier. Andy

bargainguy 10-06-19 10:47 PM

I'm a shorty and have experimented with really short cranks. I also have a subinterest in folders, where short cranks come in handy for packing.

About a decade ago, I had Mark Stonich at BikeSmith Design and Fabrication modify a couple of Shimano cranks, a double 105 and a triple Ultegra, from 170 to 148mm. I put those 148s on a couple folders and non-folders and rode them around for awhile, then switched both cranks back to original so I could study the transition back after the 148s.

My conclusion: Great for spinning, lousy for power. Any time I had a strong headwind or a steep hill, it just felt like I was spinning and not getting anywhere.

The 148s just don't do it for me. I don't mind 165s and have them on a few bikes, mostly folders and my 650c wheel frames, whereas I save 170 for my 700c frames.

63rickert 10-12-19 09:35 AM

Stop worrying about pedal strike. Back when racers all used wide Campy 1037 pedals we all ground the outside corner of the pedal into the pavement on every corner. And never noticed a thing. It wasn't that the pedal strike didn't much bother us, we simply never noticed. When the pedal was worn so far the bearings were exposed it was time to replace the pedal.

There are two ways to induce pedal strike. One is to use an extra wide pedal. Don't do that. The other is aberrant cornering technique. A small (real small) subset of riders go into a corner and - how to explain - drop and flop the bike to the side to make the thing corner quicker. Basically they throw the bike out of control entering the corner and then try to pull it back into control exiting the corner. If pedal strike occurs in that circumstance sure there could be a problem. Throwing the bike out of control makes anything a problem.

Most modern bikes already have high bottom brackets. Whining and wheezing about pedal strike got the attention of the lawyers some time ago. So most bikes on the road are already high off the ground. Clipless pedals are so small and narrow it would take some very extreme cornering technique to ever ground a pedal. Many flat pedals are plenty narrow enough to evade pedal strike. If you ride monster beach cruiser pedals, have size 15 feet, and ride in galoshes you could hit something. And further, fixed gear riders on the street have to work very hard to corner as aggressively as is possible while coasting.

Who started this pedal strike panic? I am an old man and do not need to fall. One of my vintage bikes has a very low bracket, lower than anything you can buy new and lower than 99.9% of vintage bikes. Sometimes I ground a pedal. Nothing ever happens. Were it not for all the chatter about pedal strike I'd probably not even notice.


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