Why are crank arms longer these days?

good morning,

Firstly i'd like to apologise if i'm just reigniting an old subject, but can anyone enlighten me as to why crank arm lengths have got longer?.

My 80's bike has 170mm arms, and a lot of chainsets that i see on ebay from this period and earlier are the same. However my road bike from the early 00's has 172.5mm arms and i regularly see 175mm arm lengths on new chainsets.

So my question is, what benefit is gained by the extra few millimetres?, and will i notice the difference if i fit 175mm cranks to my 80's bike?.

Thanks for your time. Matt

There certainly were longer cranks used in the old days too. Indurain used to ride 180mm cranks. I've used everything apart from 180s, and prefer 175, but then I do have an inside leg of 35"... I can tell the difference between 170 and 175, but not between 170 and 172.5.

As to benefit, longer cranks give you more torque (force applied x distance from the axis of rotation) but are harder to spin than shorter cranks. For this reason, people quite often use a longer crank in time trials. I find 175s better for climbing than 170s, but I'm not a high cadence climber.

On the downside, on a standard frame, you get less pedal clearance with longer cranks, which can be an issue with both toe overlap with the front wheel (especially on a tight frame) and cornering while pedaling. Usually not such an issue on larger frames.

Shorter riders on small frames often benefit from shorter cranks than 170. This used to mean going for TA, as Campag etc. didn't accommodate either short legs or small chainrings...

I don't know if 172.5/175 really are more common now than they used to be. If so, my guess is that it might have filtered through from mountain biking, where cranks longer than 170 are common due to the low cadence high grunt nature of cross country (coupled with the higher bottom bracket for clearance).

Last word: In my experience, yes, I would predict that you will tell the difference between 170 and 175, at least at first. Whether it is a positive change will probably depend on you leg length. If you've got long legs, I think you might find the longer cranks liberating, like I do. If not, you might find them a bit unwieldy.

Nick

Thanks for the quick and informative response Nick, exactly what i was after Matt

I was thinking about this yesterday. The shimano cranks I took off an old shot to pieces Raleigh were 160s, the Athena cranks I have to go on my bike at some point are 175. Ideally I think I need something a bit smaller than 175, I've got short legs!

My nearly 7 feet friend has 182mm cranks fitted to all his bikes :shock:

Ian Raleigh":2eis349t said:

My nearly 7 feet friend has 182mm cranks fitted to all his bikes :shock:

Click to expand...

That would like me at a bit below average height using 100mm cranks, or something like that. Those cranks will be very short for a person of that height, with the pedal at the top of the stroke, I bet their thigh will be at 45 degrees.

If you look at someone riding a bike, you can easily tell if they're tall or short by the range of movement of their legs. That's the result of everyone from 1.5m to 2m all using 165-180mm cranks.

fiks":2oyvpbw1 said:

Ian Raleigh":2oyvpbw1 said:

My nearly 7 feet friend has 182mm cranks fitted to all his bikes :shock:

Click to expand...

That would like me at a bit below average height using 100mm cranks, or something like that. Those cranks will be very short for a person of that height, with the pedal at the top of the stroke, I bet their thigh will be at 45 degrees.
If you look at someone riding a bike, you can easily tell if they're tall or short by the range of movement of their legs. That's the result of everyone from 1.5m to 2m all using 165-180mm cranks.

Click to expand...

My friend has very long legs so i reckon thats why he has long crank arms.

Modern crank arm sizes vs bike -

165 - super small/women's/kids
170 - small
172.5 - medium
175 - large
Over that - ridiculous sized giants of people!!

Fixed road bikes generally use quite short for the high cadence, clearense through corners and for toe overlap

My 54cm modern bike has 172.5 crank arms
My old road bike has 165, and to me they feel loads better, can just spin them up so easy

Raleighracer":3mbix5ts said:

...Fixed road bikes generally use quite short for the high cadence, clearense through corners and for toe overlap....

Click to expand...

Also because they are related to track bikes. Where shorter cranks are more common, due to the cornering.

Ian Raleigh":2p0zlm29 said:

fiks":2p0zlm29 said:

Ian Raleigh":2p0zlm29 said:

My nearly 7 feet friend has 182mm cranks fitted to all his bikes :shock:

Click to expand...

That would like me at a bit below average height using 100mm cranks, or something like that. Those cranks will be very short for a person of that height, with the pedal at the top of the stroke, I bet their thigh will be at 45 degrees.
If you look at someone riding a bike, you can easily tell if they're tall or short by the range of movement of their legs. That's the result of everyone from 1.5m to 2m all using 165-180mm cranks.

Click to expand...

My friend has very long legs so i reckon thats why he has long crank arms.

Click to expand...

They might be "long" cranks as in longer than 170mm, but in proportion to his leg length they are still short.

The riders who are actually using long cranks in proportion to their leg length are shorter people. A 155cmm person using 165mm cranks will be using longer cranks than a 200cm person using 180cm.