Following on from my comments on the KQ thread about gearing, I now find there’s another possible stumbling block!
My TT DuraAce cranks are 53/39 (or 55/42) with a 175mm crank.
My Ultegra cranks are a 52/36 and a 172.5mm crank.
My 105 cranks are a 50/34 and a 170mm crank.
Can I just swap them out with no discernible difference?
Is 5mm neither here nor there? Or would I notice a difference?
Cheers in advance
Just swap them, I doubt you’ll even notice. If you do it’ll last about 5mins before feeling completely normal.
In theory longer cranks could close up your hip angle on the TT bike and you should lower the seat by the difference. Shorter should open up your hip (raise seat) and current thinking seems to be that way for TT.
Yup, just read a load of links.
Shorter now seems to be better - as low as 165mm for my height!!!
Opening up that hip angle.
Shorter cranks = less revs = more energy for the run.
Also get a flatter back with shorter cranks.
At those lengths, do you think I’d need to adjust the saddle height?
170-175 is 5mm - is that noticeable???
Personally I wouldn’t bother, I’d just ride it and see if I noticed any difference at all. (But I’m not known for my delicacy and sensitivity)
If you go with the 52-36 (which I think gives the best of both worlds) you’re only talking 2.5mm, I’d be amazed if you could even tell.
My 11-30 cassette won’t arrive until Tuesday, which is when I’m looking at a double loop IMUK recce.
So the 50-34 should give a similar range?
Ps. You may be able to swap rings between the two smaller sets if you prefer 52-36 on the 170s for example.
Yeah, there’s a good gear calculator on Sheldon Brown if you want to work it out exactly.
I’ve switched to 165mm cranks, as you say shorter now is the consensus. Not raced in anger yet but done quite a few miles on the turbo and seems fine (had 172.5).
I used to ride 170 in track as that was the maximum allowed for bunch races but they changed rule to 165. I didnt even notice but you are at a high cadence generally anyway (120rpm average not uncommon) I have 175 on road and TT. definitely can get on top of a gear easier with that size over the track bike but I’m 8ft tall.
After reading a couple of articles saying it was vital to get it right, I found a GCN video / experiment that concluded there was no perceivable or measurable difference.
I had 172.5 on the TT bike (raced at IMUK). I had 175 on the road bike on which I did about half of my training (the other half was on the turbo on the TT bike. I can’t feel the difference between them. If you hold your finger and thumb 5mm apart, ask yourself will you really notice if your thigh’s highest point and foot’s lowest point changes by 5mm. I’d say not…! You probably change your geometry by more than that by shifting fore and aft on the saddle. I’d definitely ride a 50-34 at IMUK. A 105 set of cranks weighs 713g and the ultegra weight 674g. You’ll not notice the difference! When I was looking at buying a stages power meter I worried about the extra weight it would add and whether I would feel the imbalance (I can’t). Try the 50-34 with an 11-28 and see if you can spin up comparable IMUK hills at 90rpm and 210 watts. If you can, great. If not, put a 30 on the back. You can overthink these things!
Cheers!!!
I’m riding the loop again on Tuesday, so will give that a rasp 
No problem. I hope the race goes ahead for you…!
Are you sure?
Seems counter intuitive but my natural 90rpm on 172.5mm is now 80rpm on 165mm.
That’s interesting. I always “over-rev” when racing. Quite happy at 100rpm and above and always have to tell myself to dial it back. I’d sooner do intervals at 100rpm than 90rpm. Did that just “happen” when you shortened the cranks? More of a shortening than my 2.5mm between road and tt bike but still less than a centimetre…
Crank length was slowtwitch’s favourite topic for a while.
For the same speed, yes.
Circumference of a circle innit!
One pedal revolution on 165mm cranks in less distance than that of 175mm cranks.
2πr or πD
R = length of crank
D = 2 * crank length
So…at 90rpm, in one minute:
A 165mm crank travels 93.3174m
A 175mm crank travels 98.973m
For a six hour IM ride;
165mm = 33.594km
175mm = 35.63km
Therefore, a 175mm crank travels 2,036m further.
Or 1,852 revolutions more.
As the gearing is staying the same - you’ve pedalled far less with the shorter cranks.
Think of all that energy saved for the run!
There’s nothing natural about cadence.
It’s a learned behaviour.
But I get what you mean 
Now, if you could get that 165mm crank back up to 90rpm, think of all the extra speed
Ah but is it really energy saved? If you’re outputting the same power, then each stroke at faster rpm will be slightly less powerful than each stroke at slower rpm so it all balances out at the same power. It’s like driving a car in 4th versus 5th at 50mph. Most people think fuel consumption is higher at higher revs, but less fuel is being burned per rev and so again it pretty much balances out…