CRANK ARM LENGTH AND TEETH

Discuss light weight issues concerning road bikes & parts.
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apricotwalnut
Posts: 36
Joined: Fri Aug 04, 2017 3:49 pm

by apricotwalnut

I'M CURRENTLY USING 170MM AND 50-34 COMPACT, BUT I'M THINKING TO USE 165MM AND 52-36 MID-COMPACT.
IS IT GOOD FOR PERFORMANCE OR WAY TO GO HELL? ;;;

AJS914
Posts: 2259
Joined: Tue Jan 28, 2014 6:52 pm

by AJS914

No one can say if it will be good or bad for your performance. Most people are liking it when they go from longer to shorter cranks.

One consideration is that with a shorter crank arm and less leverage you might want an easier gear on the cassette in the rear. But maybe you live in a flat area and it wouldn't matter?

BTW, YOU DON"T HAVE TO YELL.

by Weenie


audiojan
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Joined: Fri Jan 27, 2006 1:38 pm
Location: New Hampshire

by audiojan

There's no correlation between crank length and chain ring size... It's impossible to say if changing from a 170 to a 165 is going to be good or bad for YOU. Shorter cranks will allow a more open hip angle, but will slightly reduce leverage. Changing from a 50/34 to a 52/36 all depends on terrain, type of rider you are, cassette, etc. Personally, I like 52/36 with something like a 12-29 cassette. Gives me plenty of options. But I could just as well use a 11/27 with a 50/34 and end up with about the same gearing.
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Mr.Gib
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by Mr.Gib

For many riders, the shorter the crank, the faster the cadence. That means that at a given speed the rider will use a smaller (easier) gear. This suggests that shorter cranks should be paired with a drive train that includes easier gears. For example moving from an 11-26 cassette to a 12- 28. However terrain is a better determinant of gear choice and a 5 mm difference in crank length is probably not enough to require a change in cassette. I switched from 175 to 170 and I never felt the need to alter my equipment. On average I probably spin a little bit faster and ride one cog easier.
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morganb
Posts: 585
Joined: Wed Mar 15, 2017 5:30 pm

by morganb

Shorter cranks increases your effective gearing so I would consider going the same rings you are already running unless you are finding yourself consistently undergeared. I went from 172.5mm to 155mm and had to switch from 53/39 to 52/36. I run compact on my winter bike now.

pdlpsher1
Posts: 1687
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Location: CO

by pdlpsher1

155mm? Where do you find such a short crank? And what is your inseam?


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cyclenutnz
Posts: 772
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Location: Cambridge, New Zealand
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by cyclenutnz

7.5mm on the crank arm is the same gearing effect as one tooth on the rear.
If you go shorter you need an easier gear, longer you need a bigger gear.
Hence the old TTer thing of thinking that longer cranks gave them more power because they were in bigger ratios (or if someone bought Zinn 200mm cranks) when really the complete gearing (which is from pedal to where the tyre touches the road) stayed the same and all they did was close up their hip angles.

Rotor make a 155 but it is hard to find. Lightning do a 160. Cobb do 145, 155, 165.

If your saddle height is less than 65cm you will definitely need cranks shorter than 165 (to open the min knee and hip angles).
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JoO
Posts: 59
Joined: Thu May 04, 2017 7:30 am

by JoO

If would recommend reading the article of "gain ratio" by the late sheldon brown.
https://www.sheldonbrown.com/gain.html

Several studies have examined the effect of crank length on power output.
(I would have to google them again)
conclusion. no effect (crank length between 140 and 200 if memory serves me well)

the advantage of shorter shorter cranks would be:
more comfort due to the more open hip angle
or keep the hip angle and get more aero.

I have also tried 155 cranks and they worked
Stronglight impact kid (square taper realy cheap)

I would like some 167,5 cranks to complement my campagnolo builds. but they are very rare it seems.

bm0p700f
in the industry
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by bm0p700f

Adding crank length does not change torque at the chain ring probably because you maintain the higher torque provided by the longer crank arm for a shorter time. therefore the overal power output remains the same as with a shorter crank.

therefore increasing crank length is not like adding a extra tooth on your rear cassette. I have a variety of crank lenths from 165mm to 177.5mm and my ability to turn all of them is the same. On my aero bike I need to change the zed3 cranks to 170mm as that would help me stay in the drops. my power output might even improve.

by Weenie


cassard
Posts: 95
Joined: Wed Jan 24, 2018 12:31 am

by cassard

to add to joO,

Zani, Gonzales and Hull evaluated that crank length will have and inverse correlation with your cadence. So in the size recommendations for a crank length that is depending on your inseam and/or femur length ( Rochefort and al. made a correlation between the femur length and crank arm length.

So in all, find the crank arm length around your inseam and depending on your cadence, you can go one size up or down. You don't need all the weird formulas you find here and there.

The only other point that could justify a crank arm length change is anything pathology related. (Hip or ankle problems for example).

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