3

Following a TT Bike Fit I have been advised to change from a 170mm crank length to 160mm. I currently have a near immaculate Ultegra 6800 53-39T 170mm fitted and I am trying to identify the simplest and cheapest way of solving this.

I believe that I cannot simply swap out the crank arm only, as the right side is bonded on to the chain ring and that I therefore need to look at swapping the entire crankset? That being the case, I believe that a 160mm crankset is not made in the Ultegra range and that I therefore need to look up and down the range?

If I am correct, and I manage to source a Shimano FC-R7000 160mm, will I be able to swap this in? I believe that the bottom brackets are the same? Or is there a newer Shimano 160mm crankset that will fit?

Or....is there another brand compatible with the existing bottom bracket?

Thanks so much for taking the time to read and thanks, thanks, so much for any wisdom shared!

Trekking Tracey
  • 31
  • 2
  • The chainrings are bolted on, not bonded on. You can replace just the crank arms. – MaplePanda Feb 02 '22 at 18:54
  • 2
    Unfortunately you have come up against the one thing the cycling industry has yet 'to get' Rider heights vary from around 1.5 to 2 meters (25%), bike frames are made to suit this wide range, yet any crank that is not 170mm or 175mm (3%, 10th of rider height range) is considered abnormal. One would think providing a range of crank lengths to suit rider biomechanics would easily be a profit center. – mattnz Feb 02 '22 at 22:56
  • 1
    @mattnz I think there is one discussion of crank length somewhere. However, I think that the current state of science is that most riders should be able to adapt to many crank lengths. However, shorter cranks can enable riders to avoid hip impingement at the top of the pedal stroke. This can enable a more aero position for those interested. That said, cranks are expensive to change, and road cycling has been relatively slow to change in general, so shorter cranks are only just starting to take root. – Weiwen Ng Feb 05 '22 at 00:26
  • Yep - " shorter cranks can enable riders to avoid hip impingement at the top of the pedal stroke" is exactly the issue for me and why it has been recommended at my Bike Fit.... – Trekking Tracey Feb 06 '22 at 07:40
  • Size selection was common long ago in the days of standardized square taper. But, public demand(Whatever the driver, likely just ignorance.) seems shaped to favor mass production of one size with lots of extraneous gimmicks, rather than a simple design in a wide range of sizes. Moving to 165 was great for me. I used some very old ultegra cranks I found at a swap meet, but they worked well with the age of the vintage bike. – Max Power Feb 09 '22 at 04:54
  • @mattnz specifically femur length. Unfortunately I still hear the old urban myths about selecting for leverage, which is bio-mechanically irrelevant, especially when combined with adjustable sprocket ratios. (5mm is only about 1 tooth on the big ring anyway) – Max Power Feb 09 '22 at 05:02

1 Answers1

3

You are correct that you'll need to replace the whole crankset assembly (though you should be able to reuse your chainrings if you find a crankset without them). The FC-R7000 should be a drop-in substitution that requires a minimum of tweaking. It's also cosmetically a good fit, if that matters. This is probably your best option.

160-mm cranks are fairly exotic—I'm a little surprised that they're available in the 105 groupset. As far as I can tell, neither Campagnolo nor SRAM nor FSA offer 160s; I checked a few boutique manufacturers, and they don't either. Rotor does have 160s and shorter—it looks like theirs are available with a 24-mm spindle that should fit your existing bottom bracket. They're expensive, though. If you feel like you need to explore other options just for due diligence, it is my understanding that triathletes generally favor short cranks, so possibly ask around on the slowtwitch forum; Lennard Zinn also sells custom cranks at extreme lengths.

Adam Rice
  • 29,997
  • 1
  • 47
  • 102
  • Are 6800 chainrings compatible with R7000? I heard from somewhere that Shimano changed the angle between the bolts between the generations. – MaplePanda Feb 02 '22 at 18:55
  • You might be right. – Adam Rice Feb 02 '22 at 19:24
  • Adam - thank you so much! That's great - I have found one (Shmano 105 160mm R7000) and will order it :-) – Trekking Tracey Feb 02 '22 at 19:56
  • Oh no! Only just seen the comment from @Maplepanda! I've ordered it - will have to see (she says with crossed fingers)! – Trekking Tracey Feb 03 '22 at 11:22
  • 2
    @MaplePanda it’s not the angle between the bolts. It’s the spacing between the rings, and the shape of the rings where they meet the spider. Also fyi, 12s Shimano have 160mm as an option in Ultegra and DA. Otherwise, yes, 160mm is niche. – Weiwen Ng Feb 05 '22 at 00:22
  • Grx 600 is available in 165mm. Not what you're looking for but might be useful for someone looking for a shorter crank that's easy to find and compatible. – shox May 05 '23 at 06:57