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differences between low gears and long cranks?(4 posts)

differences between low gears and long cranks?DougSloan
Nov 12, 2003 9:06 AM
Thinking of building another fixie, and crank length/gearing are concerns.

I realize there are two ways that can help climb a hill, longer cranks (more leverage) or lower gearing. Assuming you geared either way, with longer or shorter cranks, is there an advantage one way or the other in biomechanical terms? Shorter cranks have less leverage, but then you can gear down to compensate. Shorter cranks also allow faster spinning on the descents and better cornering clearance.

However, what are you giving up with shorter cranks? Are you sacrificing some climbing power? Seems to me that climbing should be the priority vs. descending or cornering, as you spend far more time going up than anything else on most roads. Thanks.

Here are a couple thoughtsPdxMark
Nov 12, 2003 11:27 AM

My original thought follows, but I think I've dropped it in view of the following.

I now think there's no down side to short cranks for a fixed gear. You get the increased clearancve for cornering, but thy also ease the gearing a bit for climbs. (See reference to Sheldom below.) For easy grades, that means you can spin up them easier. For steep grades (>10%) the easier gearing of a shorter crank would only help.

I'm thinking that regular theory of wanting leverage to climb on a on multi-gear bike assumes you can spin at whatever grade you're on. In a fixed gear slog up a 14% grade at 12 rpm, I'm maxed at what I can turn. Easier "gearing" through crank length would only ease the burden.

**Discarded original thought**

In one of his examples, bike with 175 cranks has gearing that is 3% lower than a bike with 170 cranks.

From talks with my fitter regarding a multi-gear bike, crank length could be adapted to whether you wanted to climb or spin the flats. Long cranks helped climbing (giving leverage on the pedals), short ones helped spinning (reducing the extent of motion per revolution).

I'd opt for climbing power in the context of fixed gear cornering. In my case, that means a 170 fixed gear crank rather than the 172.5 on my road bike. For me, it's not the mild-grade fixed gear climbs that matter, it's the steep grades (>10%) that push me. Really short cranks would make those climbs

It seems like a classic engineering balancing act.
getting thereDougSloan
Nov 12, 2003 2:09 PM
I'm trying to zero in on the biomechanical differences. In other words, what difference does it make to your legs whether they are pedaling a 165 mm radius circle or a 175 mm circle? I realize the latter has more leverage, but in other ways is it "easier" for your legs, even if the resulting "gain ratio," as Sheldon puts it, would be the same (adjusting gears to equalize)?

With a longer crank, the legs move further up and down, and each down stroke takes longer; your muscles are acting over a longer period of time, but with less intensity. However, that angle of your knees at the top becomes more acute, assuming you set your saddle to pedal (at bottom of stroke) distance the same.

If you extrapolate for demonstration purposes, you can see that if you made the cranks very short, say 30 mm, you'd have to apply enormous force over a short time to make any speed. However, you'd be able to spin like Dremel tool. If you made the cranks, say, 300 mm (almost a foot), pushing down would be a breeze, but you knees would slam into your chest (and the bb better be very high).

Within the relatively narrow window of lengths available and reasonable, is there an advantage of one over the other?

Oh, the deciding factor may ultimate be durability -- of your knees -- over the long run. I can see how long cranks and the more acute knee angle might cause problems.

Still thinking shorter is better - but to what bottom limit?PdxMark
Nov 12, 2003 3:20 PM
as just a seat of the pants (not based on actual data) opinion....

In comparing a 165mm crank and a 175mm crank with matched "Sheldon-esque" gearing (incorporating crank length), the leverage of the 175mm crank is offset in the gear ratio. There would be no difference and only the detriments you mention of the longer crank.

For them same actual gearing (not incorporating crank length), I'm thinking that the increased leverage with a longer crank doesn't necessarily help a fixed gear in slow-grind climbing. The extra leverage, by Sheldon's thinking, increases the effective gear ratio. In slow-grind mode, the last thing any of us needs is stiffer gearing.

So then then the question is whether there is a bottom limit for biomechanically effective crank length. That might depend one whether one is an adherent of the knee-over-pedal measurement. If so, tibia & femur lengths might define an optimal crank length for a person. But maybe the 5 mm difference between a 170mm crank & 165mm crank is within KOP measurement accuracy.