shimano grx

Especially for light weight issues concerning cyclocross / touring bikes & parts.

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onemanpeloton
Posts: 223
Joined: Wed Jul 12, 2017 11:30 am
Location: Edinburgh, UK

by onemanpeloton

jfranci3 wrote:
Wed Jun 05, 2019 4:56 pm
onemanpeloton wrote:
Wed May 29, 2019 4:36 pm
Specs for the 1x rear derailleur state:

Low sprocket: Max. 42T/Min. 40T

What does this mean? Does it mean it isn't designed for use with an 11-32 cassette?

I want to use 11-32 during cross races and 11-42 for gravel. Will this rear mech deal with both?

My other option is the older ultegra rx rear mech, but I'd have to fit a wolftooth roadlink to use the 42 cassette

Thanks
11-40 or 11-42 rear cassettes for the long cage RD. I havent' had an issue running a 28T on an RD set for 36t with a SRAM 1x setup. I don't imagine you'd have an issue with a 32t or 34t cassette.

The RX805 short cage (11-28 to 11-34 - 2x clutch - 278gr).
The RX810 med cage (11-30 to 11-34 - 1x clutch - 251gr)
The RX812 long cage (11-40 to 11-42 - 1x clutch - 264gr)

Diameter:
42t = ~85mm
40t= ~81mm
36t = ~73mm
34t = ~69mm
32t= ~65mm
28t= ~57mm
So, you're looking at (40t) 81mm - (32t) 65mm / 2 (diameter to radius) = 8mm extra spacing between the jockey wheel and the cog. You can take your current setup out with the b-screw padded out 8mm and see how things work. A 34t (48g penalty) or 36t would work slightlty better (40t cassette is 123g) .

If you don't want the rear weight, what I'd do is go for the shorter cage and just change front ring for the course. It might even make sense to have swap the whole crankset - get a beater CX crank with CX gearing and a nicer gravel crank with different gearing.
Thanks, thats all useful info!

I considered changing the chainring but keeping the gears as fast as possible is also a consideration for gravel. Speeds in gravel will be faster and slower than cx so a wider spread is preferable! Think I'll try the RX with an 11-40 and see how it goes
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by Weenie


Finx
Posts: 61
Joined: Wed Jun 15, 2016 1:14 am

by Finx

Jay Petervary won DKXL (350 mile version of DK) on Shimano GRX Di2 using the 41/30 chainrings...


emotive
Posts: 140
Joined: Tue Nov 01, 2016 10:40 am
Location: Melbourne, Australia

by emotive

Finx wrote:Jay Petervary won DKXL (350 mile version of DK) on Shimano GRX Di2 using the 41/30 chainrings...

Front chainrings are 48/31

Finx
Posts: 61
Joined: Wed Jun 15, 2016 1:14 am

by Finx

Yep,

Typo.

I'll leave it for posterity.

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bm0p700f
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by bm0p700f

In answer to an earlier question I will be swapping my grx rd rx812 to replace a 105 rd5800gs when the cassette wears out.

Currently i have a 42t single ring with a 12-34t cassette but will swap to an 11-42t which will help with the hills. This is on a road bike. For gravel I have an mtb.

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