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Crank Arm Length
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I need the help of ST. I have a Cervelo C3 2017 (Ultegra) , size 51. It has a Rotor 3D30 BBright 50/34 crankset. Neither arm has the length on it as expected. Anyone know what the length would be? Where would I find it. I looked on the inside near the pedal mount and on one side it says 700va, on the other, 70ODPB. Any help would be greatly appreciated as I need the crank arm length to set to configure my Power Meter pedals.
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Re: Crank Arm Length [velocomp] [ In reply to ]
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Just measure from the centre of your bottom bracket axle and pedal axle with a tape or ruler. If it is on the crank arm it is usually on the back side around the pedal thread hole.
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Re: Crank Arm Length [velocomp] [ In reply to ]
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It's a 170mm
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Re: Crank Arm Length [Shambolic] [ In reply to ]
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Shambolic wrote:
Just measure from the centre of your bottom bracket axle and pedal axle with a tape or ruler. If it is on the crank arm it is usually on the back side around the pedal thread hole.

Yeah, but accuracy between 170, 172.5 and 175 mm Is too small to measure accurately...
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Re: Crank Arm Length [Nonojohn] [ In reply to ]
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Nonojohn wrote:
It's a 170mm

Thanks, that is what I thought.
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Re: Crank Arm Length [Nonojohn] [ In reply to ]
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Nonojohn wrote:
It's a 170mm

Yep. 170mm is the standard crank length on a stock Cervelo 51cm frame.
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Re: Crank Arm Length [velocomp] [ In reply to ]
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It's a 170mm//

And it should be a 150 on that size bike, 155 tops...
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Re: Crank Arm Length [monty] [ In reply to ]
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monty wrote:
It's a 170mm//

And it should be a 150 on that size bike, 155 tops...

That would be interesting... It would be fun to try.
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Re: Crank Arm Length [velocomp] [ In reply to ]
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velocomp wrote:
monty wrote:
It's a 170mm//

And it should be a 150 on that size bike, 155 tops...


That would be interesting... It would be fun to try.

It would also probably be the most useful piece of equipment to change. 170s belong on maybe the largest sized tri bikes for riders well over 6 feet or well over 80cm of seat height.


3 Months of Paradigm Shifting Swim Instruction for Cheap // Your Professional & Private “Critique my Fit”

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Re: Crank Arm Length [FindinFreestyle] [ In reply to ]
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FindinFreestyle wrote:
velocomp wrote:
monty wrote:
It's a 170mm//

And it should be a 150 on that size bike, 155 tops...


That would be interesting... It would be fun to try.


It would also probably be the most useful piece of equipment to change. 170s belong on maybe the largest sized tri bikes for riders well over 6 feet or well over 80cm of seat height.

Except it's not a tri bike, it's a C3. Unless the OP has set it up as a tri bike (not indicated in the OP) I don't see much wrong with 170's on that size frame for a road bike. Could be a touch shorter, but nowhere near as critical as a tri bike IMO.
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Re: Crank Arm Length [el gato] [ In reply to ]
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Agree 100%. I ride 160 on my tri bike and 170 on my road bike. I tried 165 on my CX bike and hated it. If you don't have a closed off hip crank length is nowhere near as critical and I found what felt too short.
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Re: Crank Arm Length [el gato] [ In reply to ]
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Except it's not a tri bike, it's a C3. Unless the OP has set it up as a tri bike (not indicated in the OP) //

That's a good point, I was imagining that it was a gravel bike of some sort. Which brings up a great topic, I know if I were to build my dream gravel bike, I would have clip-ons attached. I would for sure be riding a lot of those races by myself, so I would want an aero position available, and shorter cranks to go with it. I think they are good for clearance too, which I come across on my MTB quite often...
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Re: Crank Arm Length [monty] [ In reply to ]
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monty wrote:
That's a good point, I was imagining that it was a gravel bike of some sort. Which brings up a great topic, I know if I were to build my dream gravel bike, I would have clip-ons attached. I would for sure be riding a lot of those races by myself, so I would want an aero position available, and shorter cranks to go with it. I think they are good for clearance too, which I come across on my MTB quite often...

Well, that's a good point too! :-) I have a C5, and while Cervelo markets the C series as gravel(ish) bikes I don't see it as a 'real' gravel bike. I use it as an uber-comfortable road bike. It's perfect for all the crappy backroads of the Santa Cruz mountains. I guess if one was to use it as a gravel bike your suggested setup with clip-ons and short cranks sounds pretty cool.
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Re: Crank Arm Length [el gato] [ In reply to ]
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el gato wrote:
FindinFreestyle wrote:
velocomp wrote:
monty wrote:
It's a 170mm//

And it should be a 150 on that size bike, 155 tops...


That would be interesting... It would be fun to try.


It would also probably be the most useful piece of equipment to change. 170s belong on maybe the largest sized tri bikes for riders well over 6 feet or well over 80cm of seat height.


Except it's not a tri bike, it's a C3. Unless the OP has set it up as a tri bike (not indicated in the OP) I don't see much wrong with 170's on that size frame for a road bike. Could be a touch shorter, but nowhere near as critical as a tri bike IMO.

Trick question!


3 Months of Paradigm Shifting Swim Instruction for Cheap // Your Professional & Private “Critique my Fit”

The Swim Help Compilation Thread

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Re: Crank Arm Length [FindinFreestyle] [ In reply to ]
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I decided to give 170 cranks a try for the past three months, I have always been on 172.5. Initially I like the feeling that I could "get over" the gear a bit easier and coming out of the off season it was kind of nice. However, I found my bike splits in our short course season (i am in Australia) were lacking behind the guys I usually ride with and I felt my fitness was fine. I've decided to go back to 172.5 and right away felt I could get more power and ride at my old cadence and bike split times. There was no real reason I went shorter initially, I was just following a lot of discussion on shorter cranks. In issue for me is I started riding 22 years ago when I was 13 and we had 172.5 cranks and I've always had that crank length. I ride about 85-90 cadence and am not really able to change that as its just what i do, so I am not capable of change, I'm too stuck in ways so 172.5 is where it is at for me.

Also a few years a go I bought a new road bike, it came with 175 cranks, I lasted about 3 months on those, I actually almost wanted to give up cycling as could not get comfy on that new expensive bike, then I put some 172.5s on and I was happy.

I am actually amazed in both instances (longer and shorter) that 2.5mm was even noticeable and annoyed me so much.

Anyways, it just shows this crank length stuff is really personal.
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Re: Crank Arm Length [Gilliga] [ In reply to ]
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I have 155's on my TT bike, 165's on my commuter and 170 on my road bike. I wish I could say I could tell the difference between them. I feel like less of a cyclist when I say they all feel the same to me.

However, I'm about as flexible as a piece of rebar, so I have to say I would be concerned about going with longer cranks on my TT bike.
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Re: Crank Arm Length [Gilliga] [ In reply to ]
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2.5mms on both sides. There’s a really great write up on it in this forum. I also ride a 51 Cervelo and use 170 for my R5 and 160 on my P4.
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Re: Crank Arm Length [Gilliga] [ In reply to ]
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With all due respect, I find it impossible to believe that a 2.5mm change had that much effect on you, especially since you say it's true going either up OR down by 2.5mm). If it was one direction only, maybe there was something you were just about getting away with that you suddenly weren't, but both directions? - Don't think I can take that at face value as a physical reality. In fact I'd be skeptical most people would even detect a 2.5mm change in a blind test on a correctly adjusted bike unless they were looking for it.

Did you adjust the saddle height correctly to compensate when you switched crank length?
A few mm in effective saddle height I'd find more believable.

I started off using 175mm, changed to 170mm after a few years, and tweaked the saddle and bar height to match - felt like I noticed a very minor difference but to be honest I couldn't tell you if it was just because I knew they'd changed. A while later I got a new bike which came with 172.5mm cranks. My position changed a bit so it would be silly to say I noticed or didn't notice a change int eh crank length.....but I didn't ;)
Then last year, I changed from 172.5mm to 160mm. I noticed. That is 5 times the magnitude of your change and even so while noticeable and significant, it was, for me, an easy adaptation.... oh and my typical cadence hasn't changed at all.
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Re: Crank Arm Length [velocomp] [ In reply to ]
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velocomp wrote:
Yeah, but accuracy between 170, 172.5 and 175 mm Is too small to measure accurately...
If you can't measure it accurately, then you are not using the right tool or are doing something wrong. 2.5mm difference between the values is pretty large.
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Re: Crank Arm Length [RichardL] [ In reply to ]
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The problem would be measuring the exact center to center of pedal hole. That would be hard to do easily with crank arm on Bike.

But we did determine its 170. So all done.
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Re: Crank Arm Length [Gilliga] [ In reply to ]
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Gilliga wrote:

I decided to give 170 cranks a try for the past three months, I have always been on 172.5. Initially I like the feeling that I could "get over" the gear a bit easier and coming out of the off season it was kind of nice. However, I found my bike splits in our short course season (i am in Australia) were lacking behind the guys I usually ride with and I felt my fitness was fine. I've decided to go back to 172.5 and right away felt I could get more power and ride at my old cadence and bike split times. There was no real reason I went shorter initially, I was just following a lot of discussion on shorter cranks. In issue for me is I started riding 22 years ago when I was 13 and we had 172.5 cranks and I've always had that crank length. I ride about 85-90 cadence and am not really able to change that as its just what i do, so I am not capable of change, I'm too stuck in ways so 172.5 is where it is at for me.

Also a few years a go I bought a new road bike, it came with 175 cranks, I lasted about 3 months on those, I actually almost wanted to give up cycling as could not get comfy on that new expensive bike, then I put some 172.5s on and I was happy.

I am actually amazed in both instances (longer and shorter) that 2.5mm was even noticeable and annoyed me so much.

Anyways, it just shows this crank length stuff is really personal.


I could sneak into your garage and put the 170s on and you wouldn't notice a thing if you didn't know they were there. Buying a set of cranks with a 2.5mm change in either direction is largely pointless. It's not enough to really matter, and the change is so subtle, compounded by the extended time between trials....it's really just a waste of time and money.

Get on a fit bike and have a competent fitter take you to the limit of your existing crank length. Get off the bike for 90 seconds. Get back on to a crank length different by 5 to 10mm and a position which is the same with the correct saddle and bar adjustments. Ride the same electronically controlled resistance level. Repeat that 2 to 8 times. The results will be very enlightening.


3 Months of Paradigm Shifting Swim Instruction for Cheap // Your Professional & Private “Critique my Fit”

The Swim Help Compilation Thread

Last edited by: FindinFreestyle: Feb 12, 19 18:01
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