Replacing 1x SRAM chainset with a Shimano chainset

Question

I've just got a Genesis Datum 2019 bike on clearance.

The spec wasn't exactly what I would want—for a start it has SRAM rather than the Shimano we have on other bikes in our household—but more importantly it has 172.5mm cranks whereas I have reduced hip mobility and 165mm cranks are much better for me.

I therefore need to swap out the chainset, but wonder if it's possible to switch over to a Shimano chainset at the same time?

Since this is 1x, there is no front derailleur compatibility to worry about, just the chain and the bottom bracket. I assume the chain will be fine, since Shimano and SRAM 11-speed cassettes are inter-changeable.

So then it's just a question of the bottom bracket, which I suspect won't be compatible.

I'm quite used to doing maintenance on bikes, but so far I've been lucky to only work on BSA threaded bottom brackets, with either square taper or Hollowtech II cranks, so don't know anything about the press-fit type (86mm shell width, I understand) on the Datum.

The same frame has been sold with Shimano's BB-RS500-PB bottom bracket, so presumably if I get this and a 165mm Shimano 1x chainset I will be good to go?

Or will a 1x Shimano chainset work with the BB used by the SRAM chainset?

And are there other compatibility issues I need to worry about?

Answer 1

Unfortunately, the bottom bracket compatibility is an issue.

The Genesis Datum appears to be a gravel bike, mainly sold in the UK. Here is one retailer page containing specifications for a 2019 Datum with SRAM Rival 1x. If this page is correct, you have a Rival crankset with a GXP spindle.

Shimano's cranksets use 24mm spindles. GXP, a standard specific to SRAM, uses a stepped spindle that's 24mm in most places, stepping down to 22mm on one side (I forget which). The bike is listed as having a SRAM bottom bracket, so I'd presume this is a GXP bottom bracket. You can easily adapt a Shimano BB to GXP with the use of a spacer on the 22mm side. If you do indeed have a SRAM GXP press fit BB, then you can't adapt that to a Shimano crankset.

You would need to have someone press out the BB and install a new one to make the Shimano crankset work. You would look for a BB86 bottom bracket. However, note that all BBs are made to accommodate a number of spindle diameters, and you have to specify the spindle type as well as the BB shell type. There are straight 24mm (or similar) spindles and 30mm (or similar) spindles. One issue arises with the "or similar" bit - SRAM have gone to a spindle standard called DUB, which uses a 28.99mm spindle. This gives more room for seals and bearings, and they believe it improves durability versus a 30mm spindle, especially on some BB standards; however, you can generally find reducing spacers to adapt BBs for 30mm spindles to the DUB spindle. The fact that you need to specify two separate characteristics is initially confusing. However, you would specify something like BB86 24mm or BB86 Shimano to look for a BB86 for a Shimano-style straight 24mm spindle. BB86 30mm would pull up results for bottom brackets that fit your shell but are too large for your intended crank.

So, if you were getting a new SRAM crankset, you would need a DUB BB. Your possible options include:

• If you intend to mount a Shimano GRX 1x crank, you'll need a BB compatible with Shimano spindles. Aside from that, you should be able to simply swap the cranks in. There shouldn't be any material issues with the chainline, and the chainrings will be compatible. If you are using a standard Shimano road double crank, then the chainline may not exactly match SRAM's intended 1x chainline, but the difference may not be material.

• You could find 1x GXP crankarms or a whole 1x chainset in your desired length, whatever the model. SRAM is moving away from GXP as a standard. It's possible that this may impact the availability of new parts, but I think Rival hasn't transitioned to the DUB BB standard yet. It's possible that people are looking to sell off their old GXP stuff. You might consider adjacent models, e.g. Apex cranks. You should be aware that some SRAM cranksets might have 30mm spindles, usually suffixed with BB30. If you get one of these, you'll also have to change your bottom bracket.

• I would normally assume that if Genesis specced Rival, they would have used the Rival GXP press fit BB. If they somehow used the Shimano BB with a spacer with an otherwise Rival setup, then you can ignore what I wrote above. However, I don't see this as likely. Component manufacturers offer favorable pricing based on bulk buys. It seems a bit unlikely that Shimano would have offered Genesis good pricing to buy a bunch of orphan RS500 BBs, and Genesis would still have to had to source spacers.

Answer 2

Why do you want a shimano chainring up front ,If it is 1X then the only parts you need to change are the rear mech and shifter.Shimano mechs and chains run perfectly on sram cassettes and viceversa.The chainring is irrelevant as you have no front shifter.Why not just buy sram short cranks,is it really such an issue ? Most of my bikes run a combination of shimano and sram,front mechs and shifters are cross compatible,only rear mechs need the same make shifters.