Crank length

[Pale Rider]

Does 5mm in crank length make any difference?

A pedal went a bit wobbly earlier today, and while it appears to have nipped up tight, there was a bit of swarf which makes me think I may be on borrowed time with that crank.

Replacement choice appears to be 170mm or 175mm.

The existing crank is stamped 170mm.

Is there any benefit in buying 175mm?

 

[S-Express]

None whatsoever. Unless your leg length and biomechanics would be better suited to 175. But if you've been happy on 170s then there's no reason to change.

 

[ColinJ]

I have a friend who swears that crank length makes a huge difference. He insists that he simply must ride 175 mm cranks.

I think that (within reason) it doesn't matter much. I can certainly notice the difference on my gym bike which has tiny 140 mm cranks but I can still spin away on it.

I usually ride 175 mm cranks but have also ridden 170 mm cranks without problems. I cracked the 175 mm LH crank on my Basso. I had my old 170 mm cranks in my junk box so I put the LH one on the Basso. It was intended to be a short-term measure but my left leg is significantly shorter than my right and I found that it suited me to have a shorter crank for it so I kept it on and used the bike for many years with that 5 mm difference between the cranks. (If I choose the saddle height to suit my longer leg then I have too far to reach down with my left leg if the cranks are the same length.)

I now ride 172.5 mm cranks on my CAAD 5. Spa Cycles didn't have 175s in stock when I bought my triple chainset and I was sure that 2.5 mm wasn't going to make much difference to me. It didn't!

I have 175 mm cranks on my MTB and CX bikes.

I still have 175 mm and 170 mm cranks on the Basso, which is now permanently on my turbo trainer.

I am getting rid of the old gym bike. Not because of the short cranks, but because I don't need it now I have the turbo trainer.

I think if you were going for the world hour record or riding any other top level event then it could be worth pursuing the tiny fraction of one percent difference that the crank length might make. For the rest of us, not really!

It's an individual preference. It isn't something that I worry about.

 

[midlife]

I've ridden 165 to 180 and not noticed a whole lot of difference. Seems that the gearing made more of an impact....

Shaun

 

[Sharky]

In terms of leverage it wont make any difference. The longer the crank, the larger the gear you can push, but the pedal circle is bigger, so cadence is reduced. Net effect flat.

But with changes in crank length, to keep the same leg extension, you should adjust saddle height. To change from 170 to 175, the saddle needs to be lowered by 5m. Again 5mm is a very small change so probably wont be noticed.

 

[gazza1286]

Fore-aft saddle position will alter slightly too. Longer crank - saddle would need to be further foward.

 

[S-Express]

Fore-aft saddle position will alter slightly too. Longer crank - saddle would need to be further foward.

There's no particular reason why it should. Saddle position fore/aft is generally set in terms of position behind the BB - crank length is not really a consideration.

 

[Sharky]

Fore-aft saddle position will alter slightly too. Longer crank - saddle would need to be further foward.

Would be very slight, but should it be a move back if saddle is lowered to preserve distance to the bars?

 

[gazza1286]

The Fore-aft position of the saddle is determined (broadly speaking) to ensure that the rider's knee is directly above the pedal spindle when the cranks are horizontal. There's stuff on YouTube which fully describes the measurements and logic.
Consequently a longer crank would obviously affect this.

 

[S-Express]

The Fore-aft position of the saddle is determined (broadly speaking) to ensure that the rider's knee is directly above the pedal spindle when the cranks are horizontal. There's stuff on YouTube which fully describes the measurements and logic.
Consequently a longer crank would obviously affect this.

That's only relevant if you believe KOPS actually matters. Lots of people think it doesn't. It certainly wouldn't be top of my list of priorities..

 

[Simon Head]

I think a 175mm crank is easier to turn due to the increase in leverage. However your cadence is usually a bit less initially.
the biggest disadvantage of these longer cranks is that they are closer to the ground so during a criterium around lots of tight corners you will be a bit slower than you be because the pedals run the risk of hitting the road.
In fact as I am only likely to do TT's this year and no crit's I may try some 175mm cranks but I do like to try to spin.

 

[S-Express]

I think a 175mm crank is easier to turn due to the increase in leverage

Technically correct in terms of physics, but the benefit is so miniscule as to be practically irrelevant..

 

[chriscross1966]

I didn't notice the difference in crank length going from the standard 165mm to the 175mm when I switched my Brompton to Campagnolo Ultratorque BB and centaur crank, or if I did it was lost in the wonderful smoothness of the BB and the radical change in number of available gear ratios that went with the near simultaneous fitting of an 11-speed rear hub. I was looking at a set of 180mm PMP's for a future project, but tha twas simply because they're batshit-ridiculous things to look at....

 

[Simon Head]

I will have to try 175mm crank and see if I can easily push a bigger gear with the same cadence. I suspect I will be able to as I can actually reach much higher speeds on the turbo's if I change up. I do have an injury, https://shedsweb.wordpress.com/, that has robbed me of most of my pedalling technique anyway so I now "push" rather than "spin"

 

[boydj]

I ride 165 cranks because I'm a short-arse. For me the important thing about crank length is the range of motion your legs go through, so for me the shorter cranks are more comfortable and less tiring on longer runs.