Fitting a Shimano GRX 2×11 gravel chainset to an Ultegra road BB.

Have ordered a Planet X Tempest with 2 x 11 Ultegra groupset. I am over 50 with some health issues and was looking to reduce the gearing. My current Audax bike has a 28/38/48 triple with 12-30 cassette and is perfect. (108″ top and 25″ bottom gear).

One possible option for the Tempest is to fit an SLX 11-40 cassette as Google suggests the Ultegra mech can cope well beyond the stated 34T max capacity. (11-40 giving a 23″ bottom gear, rather than 27″ with Ultegra). What I’d rather do, if possible, is swap the Ultegra 2×11 50/34 chainset for a GRX FC-RX600 2×11 46/30. (overall reduction, closer gears and 24″ bottom gear) I understand the Ultegra front mech would have to move down the seat tube a bit to maintain crisp shifting but can it also cope with the 2.5mm wider chainline of the GRX BB spindle? Presumably the external BB bearings are compatible?

Anyone done this without fitting the GRX front mech, or is a larger cassette the best way to go? I don’t need a 123″ top gear!

TIA

2.5mm may adjust out with the limit screws but I wouldn’t want to trust it. Mixing some bits from the Shimano catalogue can work, more often than not it leads to gnashing of teeth and tears of rage.

That’s what I’m worried about. The cost of a GRX front mech and chainset may make the cassette option more obvious, provided SLX and Ultegra play nice together!

The bottom brackets are the same on the road and grx groupsets. What changes is the front mech – the grx ones are designed to move further out to cope with the wider chainline. Some people have had success using the standard road ones but that is no guarantee that it will work.

FWIW I looked at this too and opted for the 11-40T cassette with 50/34 cranks* and a R8000 rear mechm

* I opted for 105 rather than Ultegra, but just because it was available in silver

Your rear mech needs to have capacity for both the big cassette and the front rings

46-30=16 plus 40-11=29, so 16+29=45 total capacity

Not sure what your mech is exactly, but the one below has a capacity of 33 listed – your rear mech might not be able to take up all the slack of a chain that will fit a 40t rear and a 46t front

https://www.mantel.com/ie/shimano-ultegra-6800-11-speed-rear-derailleur?spec%5B%5D=17700&spec%5B%5D=7015&spec%5B%5D=1743&utm_source=google&utm_medium=shopping&gclid=CjwKCAiA35rxBRAWEiwADqB37zx5m39wQTqAQQfntkw1CM35a_WxjdwY4ImtaO2QGTe33i8Ap1L8iRoCzXgQAvD_BwE

Edit – sorry – I see you mention capacity in your original post, so sorry if you’ve already considered this.

says here: https://bike.shimano.com/en-EU/product/component/ultegra-r8000/FD-R8000-F.html

that the R8000 front derailleur is good for big rings between 46 and 53 teeth, with a maximum of 16t between rings

which makes a 46-30 just fine

which makes a 46-30 just fine

As long as the mech can actually move far enough across to get to the 46t chainring.

wind off the limit screw, grab hold of the cable and see how much further she’ll move. i suspect with a big fat seat tube like the tempest has it’ll be alright.

As long as the mech can actually move far enough across to get to the 46t chainring.

Yep. GRX rings are further outboard than road rings.

Thanks for clarifying re the BB compatibility. I’m happy with the overall capacity as the original 50/34 and proposed 46/30 have the same 16T front difference. It’s the front mech sideways adjustment that worries me most. I may go for an SLX cassette instead and only change the front as parts wear out.

Colin, does the rear mech cope fine with the extra 6 teeth of the 11-40 cassette? Would a roadlink / goat type adapter help?

I have swapped from a 50/34 chainset to GRX600 46/30 and used an R8000 front mech. It coped with the GRX chainrings being 2.5mm further out without issue. Shifts perfectly. Really leased with the GRX chainset so far.

I did have a problem moving the front mech down the seat tube to fit the smaller big ring due to a bottle boss exactly where I wanted the mech clamp to go so swapped the mech for a braze on and used an adaptor clamp with a slotted mounting hole. Think I picked up a new R8000 mech for £12 off ebay which was a lot cheaper than a GRX front mech.

When we wanted lower gears on our ultegra set-up, we bought absolute black oval chinrings in 46-30 flavour. They work ace!

Andy, that’s perfect.

Keir – Good point. Given that the Tempest has a 31.6 seatpost, the seat tube will probably take a 34.9 band on mech, as opposed to the shimmed 31.8 for 27.2 posts. Unless Shimano make totally different front mechs for larger diameter seat tubes, rather than just a bigger band, it is likely that the 34.9 mech sits 1.55mm further out, leaving only an extra 1mm to find within the adjustment range to ensure the mech clears the chain. Will wind off the limit screw and give it pull.

Doesn’t that cost about £150 in chainrings though? GRX double chainset can be had for £99…

Colin, does the rear mech cope fine with the extra 6 teeth of the 11-40 cassette? Would a roadlink / goat type adapter help?

Aye, it just works. A Roadlink was my next move if it hadn’t. I had to use 118 Links in the chain but that’s a factor of chainstay length as much as anything else.

Aye, it just works. A Roadlink was my next move if it hadn’t. I had to use 118 Links in the chain but that’s a factor of chainstay length as much as anything else.

Cheers.