So I currently have a Bontrager 53/39T (12-26 out back) with a 170mm crank length, now CRC have a good price on some Ultegra’s at the moment and the cheapest is a 50/34 and comes in a 175mm length.

Now I am in the process of upgrading everything so I can go for a 11-25 out back to compensate for the compact but not sure on crank arm length.

I know on MTB I have had ground clipping issues with longer crank arms but what effect on the road bike will it have going from a 170 to a 175 (other than droping the saddle a fraction)

What’s you’re height/inside leg?

5 10 (ish) 30 inside leg

170s

I’m 5’8″ with a 31.5″ inside leg, bought a frame that had a SRAM Red chainset and ceramic bottom bracket, but had to flog it as it was 175.

Some of the six footers use 175s, but 170 will be far nearer optimal.

Cadence is more important than leverage on a road bike, and shorter cranks help you maintain a higher cadence.

As said above, if you’re 6ft plus maybe give 175’s a try, but otherwise stick with 170’s.

Also try it with your current cassette. How often do you need to use 53/12? 50/11 will be pointless unless you regularly spin out right now, but you may prefer the slightly lower ratios overall by keeping your current cassette.

I very much doubt you’ll notice the difference in crank length or new crank, sorry, spend your money elsewhere.