chris1548

I want to use GRX 2x10 on a bike that can take up to 50mm tires. What are the largest tires anyone has run with GRX 2x10? Shimano says the RX 400 front derailleur is compatible with up to 42 mm tires. If I can't go bigger I may not be able to use the 2x10, which I would really like to. I've got some idea why they may say that, what with the bottom bracket and trying to keep their chainline at 47mm. but I'm not totally sure. What has been your experience? Any thoughts? Thanks.

mstateglfr 

? This is a mindblow. Why would the rx400 FD be limited to 42mm tires? It looks to have the same design as all the 11sp FDs now have- the long arm was removed because that actually did limit tire clearance.
I have a 105 5801 FD on a my gravel bike with 43mm tires and there isnt an issue. There wouldnt be an issue even if the tires larger either. A year or so ago I put an Ultegra R8000 FD on a bike with 1.95" 26" tires that shifted just fine.


Sorry, no help on your question, but this really was surprising to read. The RX400 description on Shimano's site says it plain as day. Crazy.

grolby

Basically this depends on chainstay length more than anything. On a gravel bike with manipulated chainstays to allow big tires in a short rear-center, the tire can come quite close to the seat tube and there might just not be space for the cable and derailleur arm to fit as well. This is one reason why some gravel bikes like this are 1x only. So, I assume the limitation given by Shimano assumes a certain chainstay length. On my custom CX with 425 mm chainstays, the leading edge of the tire (38 mm) overlaps quite a bit in the front-to-back axis with the Ultegra Di2 derailleur, but of course the tire isn’t wide enough to actually make contact (currently the bike isn’t home to double-check but I don’t think it’s all that close).

I can definitely imagine a point where a wide enough tire would start to impinge on the derailleur, and of course a mechanical derailleur needs more space than an electronic one. On a bike with long chainstays, though, the tire doesn’t really intrude into this zone, so you can basically do what you want. Hence the example of the bike with 26x1.95” tires - anything old enough to use 26” wheels was probably designed before the current trend of keeping the rear-center as short as possible, clearance for wide tires and triple cranks was achieved mostly by having a long chainstay. So the key question is: what’s the bike and how long are the chainstays? If they’re under about 440 mm, you might have a problem. If they’re under about 430 I’d say there’s very little chance at all it would work. If they’re nice and long, though, it should be fine.

edit: the lengths I give assume 700C wheels and are basically guesses, not definitive! Caveat emptor.

chris1548

Thanks, Grolby. Your point about chainstay length is probably key. This frame has a 435mm. chainstay and the frambebuilder says it will take up to 50mm. tires. I want to use GRX 2x10 because I want close steps in the rear cassette but still have a good low gear. I also want to run 700c most of the time, but realize that using 650b with a 50mm tire would eliminate the problem. That means 2 sets of wheels, not ideal budget-wise.

I know Shimano is usually pretty conservative, so the spec may be worst case, say for a 425mm chainstay. I'll keep working on it.

Any other thoughts anybody, or any experiences with GRX 10 speed and wide tires. Thanks.

Litho dbh

I've got an All City space horse with the grx set up. Came from factory with 47mm wide tires. There's room for a little bit wider as well. 2 x 10. Works most excellently. Wheels are 650b.

pipeliner

With 650b wheels, you have much more room. You'll see a bunch of frames that can run 45mm 700c and 2.1 to 2.2" (52mm) tires with 2x. I doubt you re going to be able to run much more than a 47mm 700c on any 2x11 setup.

tofudog415

My Canyon Grizl came stock with GRX 2x10 and 650b x 47mm tires that measure out to a hair shy of 50mm. This is on a frame with 420mm chainstays that is designed only to be used with 650b, so this is not a 700c situation where I'm downsizing to 650b. There are no clearance issues whatsoever.