Question about crank length

Discuss light weight issues concerning road bikes & parts.
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eugkim
Posts: 4
Joined: Sat May 03, 2008 8:59 am

by eugkim

I just picked up a great deal on a Trek Domane Koppenberg. The only drawback was the crank length of 175. I've always used 172.5. Based on 2 calculations (9.5% of height, inseam x 1.25 +65) I should be using 170. I've never had an issue with the 172.5, so I'm not really interested in going down in length. I decided to try the 175s, hoping I wouldn't notice the difference. I immediately found that my cadence went down by about 8 rpms. I also had to lower my seat, and I seemed to develop a sore on my right buttock that prevented me from riding a few times. I assumed that, with lowering my seat and the extra crank length, I was becoming too flexed at the top of my stroke and I may have rocked back and forth.

Based on my experience, I found the same crankset (DA 9100) in a 172.5 that I was confident would solve my problems. In the meantime, I continued using the 175s after taking a few days off and switching to an old, comfortable saddle from another bike (Selle Italia SLR Gel Flow). I went gingerly for a couple rides, and now I've become accustomed to the longer cranks. I also started core stretching to help. I found that I can ride in the drops without any discomfort.

So my question is this - should I leave well enough alone or go with the same setup as all my other bikes. I'm pretty happy with the way this bike feels. The pressing issue with me is that I also purchased a Pioneer power meter - I have the box and need to send my crankset in for installation. So I have to make a choice.

Have others messed with different crank lengths?

by Weenie


alcatraz
Posts: 1298
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Put up an ad maybe, requesting a swap for the crank you want.

In the meanwhile ride it.

If you don't find anything in a while and still would like to switch just get a used crank at an ok deal and sell your old crank.

I found that here in China. A lot of people sell their 9100 cranks because they want to upgrade to power meter cranks, and they sell them quite cheap. I think the 9100 came standard on the bike they ordered.

/a

dastott
Posts: 120
Joined: Wed Jun 17, 2015 12:35 pm

by dastott

I'm 185cms and have ridden 175mm since 2012. Recently got a bike with 172.5mm cranks. Felt tougher on the climbs at first, but that might have been because it's winter and I was having to fiddle with my position on the new bike. Getting used to it now. If only we could swap...

pdlpsher1
Posts: 1717
Joined: Thu Jul 19, 2012 6:09 pm
Location: CO

by pdlpsher1

What's your height and inseam? Is it about 178cm and 84cm? I'm 170cm and 80cm, and I use a 170mm crank. If I apply the formulas I get 161.5mm and 165mm, which I believe are too short for me. With 170mm cranks I have no issues spinning at a decent cadence. On a moderate 5% climb I average about 90rpm. On steeper climbs usually a bit less. I'd like to find out what crank lengths other people are running.

dastott
Posts: 120
Joined: Wed Jun 17, 2015 12:35 pm

by dastott

My inseam is 90cm. Think I would still prefer 175mm but it's too much hassle and expense to change from 172.5mm TBH. The bike with 172.5mm is also an aero bike for flat and rolling rides, and my lightweight climber has 175mm so will probably just leave it like that. Yes, 90rpm on 5% is about what I do too, with either crank length.

pdlpsher1
Posts: 1717
Joined: Thu Jul 19, 2012 6:09 pm
Location: CO

by pdlpsher1

dastott wrote: ↑
Tue Feb 20, 2018 6:46 am
My inseam is 90cm. Think I would still prefer 175mm but it's too much hassle and expense to change from 172.5mm TBH. The bike with 172.5mm is also an aero bike for flat and rolling rides, and my lightweight climber has 175mm so will probably just leave it like that. Yes, 90rpm on 5% is about what I do too, with either crank length.
Thanks. When I posted my message I was directing my questions to the OP and I didn't see your post. I think for your height 175mm seems morel like the norm. I don't feel the crank length formulas are accurate.

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siauragama
Posts: 17
Joined: Fri Dec 30, 2016 9:33 am

by siauragama

eugkim wrote: ↑
Mon Feb 19, 2018 9:54 pm
Have others messed with different crank lengths?
I'm 172cm with 78.5cm inseam, so short legged, but I have large feet. My shoe size is 45.
I've ridden 170/172.5/175mm. 172.5 mostly, last season switched to 170.
Looking to invest into new Stages power meter and I'm considering 172.5 because they feel OK and (in my mind) have greater potential to sell themselves when I'm done with them (as opposed to 170s).

I could say I felt "fluent" on all of them, without any discomfort, but I could bend most with 170s.

TobinHatesYou
Posts: 1887
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

5'10"/178cm 32"/81.5cm inseam. I'm currently on 170mm cranks. I kind of want to go back to 175mm. I rotate my pelvis forward a lot in the saddle and my hip flexibility is good, so that gives me more range before closing up my hip angle. The added leverage would be great.

NickJHP
Posts: 125
Joined: Wed Feb 14, 2018 2:22 am

by NickJHP

My inseam is 88cm and I've used 165, 170, and 175mm cranks - 165mm on the track and 170 and 175mm on the road, and I presently have 165mm on my touring/commuting/general mucking around bike. Can't say I really notice any difference between any of them after the first minute or two when I swap from one length to another.

eugkim
Posts: 4
Joined: Sat May 03, 2008 8:59 am

by eugkim

Thanks for the replies. To be clear, I already bought a 172.5 crankset that's sitting in a box. I have to decide whether to send the 175 or the 172.5 in to Pioneer to get the power meter installed.

I'm 177 cm with an 80 cm inseam.

To complicate matters, this new bike is a compact with an 11-30 cassette. I'm used to 53/39, and my area is very hilly. so my cadences have gone up on the climbs. I was so used to smashing the pedals and suffering, so my satisfaction with the longer crank arms may be muddied somewhat. I definitely feel like the sweetspot for spinning is about 83, while I used to prefer about 89.

ancker
Posts: 35
Joined: Sat Dec 31, 2011 9:29 pm

by ancker

Get a professional fit. There's no formula that will tell you the ideal crank length for you.
The formulas say I should be on 172.5 which was perfect since that came on all the bikes I ever bought. However I was always fighting discomfort in my knees and hips.
I messed with saddle height/fore/aft and pedals forward/back/in/out/shims, nothing helped.

Got a pro bike fit and swapped down to 165mm. I've never felt more comfortable and relaxed on the bike. Saddle height went up a bit and bars down, so double win on the 'look' as well.

alcatraz
Posts: 1298
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Under effort I seem to automatically default to high cadence. (small guy)

I'm thinking if I during an ftp test for example default to 110-115 (simply based on comfort). Does that mean my cranks are too short?

I see my heart rate decrease slightly when I force myself to stay under 110 like 105 but my legs start to ache a bit. I get that feeling of unsustainability.

What do you think? I'm new to structured training. Got two seasons behind me and maybe 20.000km ridden.

/a

TobinHatesYou
Posts: 1887
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

110-115rpm is quite a high cadence. I only know a couple people who default that high.

For an FTP test, it depends which test protocol you are doing. For a 20min test, I can't imagine staying at 110rpm the whole time...I want my skeletal leg muscles to be dead at the end of the effort. Basically I watch my HR after about 5min and see if it plateaus. If it pegs itself at LTHR then that's great...if it dips and my legs still feel heavy, then I shift to a lower gear ratio and up my cadence until the final 3 minutes or so, where I want my legs to fall off.

pdlpsher1
Posts: 1717
Joined: Thu Jul 19, 2012 6:09 pm
Location: CO

by pdlpsher1

It seems like everyone here is running quite a short crank for their height. I'm 170cm with an inseam of 80cm. I'm on a 170mm crank. It feels perfect for me and I can't imagine using something like a 165mm crank. But I've never tried it. It would be quite expensive and troublesome to try it as I have a Quarq powermeter. My avg. rpm is 85 to 95. I spin a lot and that's the reason why I have extremely short gearings (50/34, 11-34).

by Weenie


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