Do GRX (810) Cranks use "road" or "mountain" bottom brackets (BSA)?

The shell width on Chris King’s original threaded bottom brackets, is 12mm for “road”, and less for mountain. (The newer combined mountain/road one is 10.5mm for the shells).

I know that the “road” version will not work with mountain cranks like XTR, as the cups are too thick.

What I can’t answer, is will a threadfit road bottom bracket work with GRX, or does GRX use a “mountain” bottom bracket? An easy way to answer this, would be if anyone is running a Dura-ace or Ulegra bottom bracket with their GRX cranks (in a BSA bike), or is anyone running an XT or XTR bottom bracket?

I don’t have the cranks (back ordered) and haven’t placed the order for a frame (as I’m hopeful the Santa Cruz Stigmata will be updated this summer for a new 2023 model … but I could be wrong on that one), so I can’t just try yet.

I have a road Chris King bottom bracket (they discontinued having separate road and mountain ones, and now just have a general one that can be spaced for either road or mountain bikes). If the road bb will fit GRX, then I’ll keep it. If not, then I’ll sell and look for a GRX compatible Chris King bb.

Thanks.

Road

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Thank you!!

I will be keeping my Chris King road bottom bracket then.

Yep, it’s road. My Stigmata GRX BB says road on the shell.

Your terminology confused me there:

Cup = the bits of the bottom bracket holding the bearings
Shell = the threaded bit of the frame that you thread the cups into.

Here’s Hope’s instruction manual - https://www.hopetech.com/_repository/1/documents/24mmThreadedBB-EN_FR_DE.pdf

Shimano and Hope have the same sized cups for both road and MTB, the only difference being the length of the centre tube. I’ve moved BBs between road, MTB and fat bike with no problem.

Never had Chris King BBs so can’t comment on those. Shimano only produce 24mm spindle cranks, they don’t do 30mm or anything else like GXP (24 -22mm) or DUB (28.99mm) so if you’ve a Shimano crankset it will fit any of their bottom brackets (assuming Hollowtech 2), the only thing to be concerned about is the width of the shell on the bike and using any spacers to get the chainline as you want it.

Thanks. Chris King had separate mountain and road cups. I asked them at one point if I could use my road ck bb on a mountain bike I was building a few years ago, and they said no, because the cups are thicker.

CK Road = 12mm
CK general (a single model that replaced road and mountain variants) 10.5mm
Shimano XTR (I think 980, it doesn’t have a model number on it) 10.35mm
Shimano ultegra SM-BBR60 6800 11.38mm

So a “mountain” bb can have spacers added (1mm thick ones) to make it work in a road setup, but the thicker road ones look like they can only be used in a mountain setup if you are doing a narrower mtb setup. So if it was going to be a mtb bb (10.35mm) + a 2.5mm spacer, then you could do a road (11.35mm) + a 1.5mm spacer to achieve the same thing. And I’d have to make sure I add spacers in such a way to not mess up the chain line.

So in my case, I have a road/thick cup bb, and if the Stigmata with GRX required a mountain/narrow cup bb and no spacers, then I’m out of luck. If it required a mountain/narrow + spacers, then I could probably find some 1mm spacers to make it fit.

But, luckily for me, sounds like GRX is in the road family, and not in the mountain family. So my CK Road thread fit 24mm BB is exactly what I’ll need.

I wasn’t finding the info, as Shimano just seem to say GRX cranks, use a GRX bb. Rather than saying GRX cranks work with 105, ultegra, dura-ace or GRX bb’s, but not Deore, SLX, XT, XTR, Saint.

Thanks.

I don’t know the reason why but most MTBs have a slightly wider BB shell width (73mm vs 68mm for road bikes) though you can find MTBs with 68mm shells. So you need to add the width of the cups plus any spacers to get the actual width of “the system”.

MTB cranks are thus slightly wider, i.e. have a longer spindle, than road cranks, it’s actually the spindle length that matters when fitting. The crank arms also give a wider Q-factor so they can clear the wider spaced chainstays.

I work out what spacers I need for a particular crank/BB combo then arrange them drive side/ non-drive side to give the best chainline.

I agree. You can add spacers in most cases. There is potentially a case where you need a narrow cupped mtb bottom bracket without spacers. So if you only have a wide cupped road bottom bracket (accepting that each cup is only 1mm wider than required), then you will have a system 2mm wider than design. Does 2mm less left crank arm engagement onto the axle matter (thinking hollowtech2 here) … ?? Maybe doesn’t matter, but I’m not wanting to set up my bike with less engagement only to find it causes issues when I’m away from home for a vacation or race or something.

The only other issue I’m seeing, is that the common spacers are 2.5mm wide, and the difference between road and mtb cups (from measuring the ones I have sitting around), is 1mm per cup. So it could be that you can’t easily find the exact spacer widths you need, or you have to add all the spacers to one side rather than even or something. Bottom line, a little pack of spacers at 0.5mm, 1mm and regular 2.5mm would be pretty useful in cases where you’re not using the “correct” bb for the crank/frame combination.

(I started the thread just to confirm that GRX is a child of the road side, and not a child of the mtb side. I’ve got the answer, and luckily it means I have the right bb already for my setup, and won’t have to get odd ball spacers or setup with less than the designed left arm to spindle engagement. Happy days for me).

chain stay clearance from the chainrings

Yes, because the tyres are wider.

Except that back in the day they weren’t any wider than what are fitted to current day gravel bikes.

For some reason we’ve a pair of 26" x 1.75" tyres hung up, no idea how long they’ve been there or what bike they were bought for. They are marked up as 47 - 559 so 47mm wide. Quite a lot of 47mm 650b tyres around these days on bikes with “road” BB shells.

Gravel bikes now are dealing with contrived chainstays to clear chainrings with mtb sized tires built round road shells and chain lines. GRX pushed the chain line out with a normal road BB she’ll width which is another option but it’s not like this wasn’t done arbitrarily. They are addressing a real problem