I’m currently running 172.5 Ultegra, and would like to try 167.5 (realistically it might end up being 165, as 167.5s seem rare) on my existing bike (in advance of speccing them on a new one).
What is the cheapest and easiest way to do this? I don’t fancy dropping £200 ish on a new ultegra set (in case the experiment doesn’t work), and 165mm second hand in good condition are pretty hard to find.
Rotor or Praxis seem to mandate a new bb, so I’m a bit stuck at the mo.
Any suggestions welcome.
Honestly think holding out for secondhand is your best bet. Can’t imagine there is a pressing need to switch? Wait for secondhand, then you should be relatively cost neutral I.e. You’ll have a secondhand set of cranks in 2 sizes and can recoup most or all of your cost regardless of which set you keep.
This is the reason most people are rising around on 170-175mm cranks, because that’s what comes on most bikes and switching on even one bike is expensive! Switching on multiple bikes doesn’t bear thinking about. If it’s any help though, I switched to 165mm on my TT bike for aero reasons, but kept 172.5-175mm on my 3 other road bikes (turbo, winter/commuter, race) for cost reasons, and found there is almost no adaptation period required. I.e. The first ride on the 165s felt fine straight away, no loss of power, just a more open hip angle which is what I was after.
It’s a change in hip angle - partly to address a longstanding hamstring issue - that I’m after. I don’t have terribly long legs (just over 80cm inseam), and I’m currently riding 172.5.
Suspect holding out is the best bet.
I went to 165mm from 172,5mm
Much more comfortable in aero position.
I paid 100 quid for some 105 cranks to try
Went from 172.5 to 165mm for TT bike. Amazed at the difference in room and less cramping of hip angle. It also seems to have improved my cadence by about 10rpm.
I went 2nd hand off ebay with Shimano 105 cranks as I wanted to use my 130bcd 55t q ring about £100 as I remember.
Agreed on waiting it out on the used market. However, for a “faster” alternative that’s in your control and also simple + cost-effective, is replace with a Shimano 105 in 165mm.
Install the crank at home for free. Many Youtube instructional videos. Other materials are hex keys, torque wrench, and some grease. If you change your chainring size, just need to adjust the FD a nudge. Only a 30g penalty from Ultegra.
And off you go with your new experiment for under $200. Another benefit of 165mm is it’ll be easier to resell if your experiment doesn’t pan out compared to 167.5mm.
Edit: Should have run the numbers in £, but the takeaway is look into 105 for on-the-cheap. 
I just bit the bullet and did it. 175 to 165. No regrets.
In any case, it’s not something you’ll slap on and decide whether you like it or not during your first ride. It’ll take many rides, which requires a level of commitment beyond just the monetary expense.
Thanks. Have changed out cranksets before so happy with the process. I suspect I’ll just bite the bullet if nothing turns up in the next week or so.
One thing to consider is that with 165 there maybe a different fit. Saddle height and position may need to be alter. It caused many pains until I figured it out. May want to talk with a fitter to find the correct position.
Agree in fit. I put the saddle up 7mm as the pedal was ‘nearer’ at the bottom of the stroke other than that everything else remains the same.
Partially related, but this kind of inquiry makes a really good argument for modular crank systems (SRAM DUB, Rotor Aldhu, Easton, etc.) when and if you settle on a crank length, or want to have a few options. A few bolts removes/replaces crank arms on the existing spider, and keeps you from having to invest in an entirely new crankset. This makes particular sense if you already have a power meter spider.
Cam has a really good YouTube channel, he switched from 172.5. There are some recent videos from a bike fitter as well that you bet find useful.
Yeah, the whole bike fit series with Neill is pure gold. There is a lot of great, detailed info in there, and he’s really generous to share all of that.
If you don’t mind dropping down to Tiagra to give it a try there’s this. If it works look out for an Ultegra one and sell the Tiagra on eBay when you find one.
https://www.ebay.co.uk/itm/Shimano-Tiagra-4700-10-Speed-Double-4-Bolt-110-BCD-Crankset-Cranks-165mm-/174502427567?_trksid=p2349624.m46890.l49292
Search for Shimano FC-RS510.
The cheapest 11-speed crank option IMO. Not very lighweight, but otherwise top performance for the price. Also available in 165mm.
Sells for 87€ (100$) in Europe: https://www.bike24.de/p1223878.html