172mm length cranks

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trek_dan
Posts: 1338
Joined: Thu Jul 19, 2012 16:32 pm

172mm length cranks

Postby trek_dan » Thu Sep 12, 2019 13:06 pm

What are my reasonably priced options for 172mm length cranks? When I was building my other bike I luckily managed to score some second hand XTR cranks which seem to be one of the few options around. Can't see any up for sale this time. Do any other brands make them?
And before anybody tells me I won't be able to tell the difference between other lengths I absolutely can.

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steve_sordy
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Joined: Tue May 29, 2018 08:38 am

Re: 172mm length cranks

Postby steve_sordy » Thu Sep 12, 2019 19:03 pm

I thought the standards were 170 and 175. I just Googled and I can see 172 mm and 172.5 mm. I didn't even know that they were made, so I've learned something there. :)

You are correct, I'm surprised that you can tell the difference between 170/175 and a 172 mm crank.

The gearing benefit between them is 1.18% more torque for the 172 vs the 170 crank and 1.17% less then the 175 crank.

The shift between gears is typically 15%, and as we know that is only an average, it varies enormously from one end of the cassette to the other. One average shift on the cassette is bigger by a factor of almost 13 times. So I guess it's not the torque difference you are feeling; so it must be the foot position. I have difficulty getting my foot in the same place twice running on my flat pedals, so if you can tell between 2 mm and 3 mm then you must ride clipped in or you would have no chance at all.

Impressive though! I sure couldn't do it. :(

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BillyCool
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Joined: Mon May 21, 2018 20:30 pm

Re: 172mm length cranks

Postby BillyCool » Fri Sep 13, 2019 08:30 am

Ditto - I didn't know you could get any other than 170 and 175 as standard sizes.

Not sure if they are so niché that hardly anyone makes them.

Hope you manage to sort something.
"Ride, crash, replace"

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steve_sordy
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Joined: Tue May 29, 2018 08:38 am

Re: 172mm length cranks

Postby steve_sordy » Fri Sep 13, 2019 08:52 am

BillyCool wrote:Ditto - I didn't know you could get any other than 170 and 175 as standard sizes.

Not sure if they are so niché that hardly anyone makes them.

Hope you manage to sort something.


When I Googled them, Shimano make them, mostly for road at first glance. And there were a few other makers that I had never heard of) but they may be well known suppliers to the roadies.

trek_dan
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Joined: Thu Jul 19, 2012 16:32 pm

Re: 172mm length cranks

Postby trek_dan » Fri Sep 13, 2019 17:37 pm

I started life as a roadie and have used 172 cranks forever, feels very strange if I use anything else. I'll keep looking out for a set of used XTRs.

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steve_sordy
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Joined: Tue May 29, 2018 08:38 am

Re: 172mm length cranks

Postby steve_sordy » Fri Sep 13, 2019 19:45 pm

trek_dan wrote:I started life as a roadie and have used 172 cranks forever, feels very strange if I use anything else. I'll keep looking out for a set of used XTRs.


I feel weird! I went from 175 to 170, back to 175 back to 170 and now I'm on 165 and I can't say that I noticed them feeling any different.

There was a long thread on another forum about crank lengths. It boiled down to ground clearance when the bike was sagged under your weight (we assumed 30% sag). Whichever bike we looked at, whatever the suspension travel, and allowing for the difference between hardtails and full suss, the axis of the pedal on a new bike (ie no mods by owners) was always the same within a few mm (140mm if I recall correctly, I can’t be arsed to do the calcs all over again). It was as though it was a target by the bike designers.


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