shimano grx

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

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onemanpeloton
Posts: 277
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: 67
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: 168
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: 67
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.

Bcoxa
Posts: 30
Joined: Fri May 26, 2017 8:44 pm

by Bcoxa

Just ordered the 812 and the double chainset. Not to use together, will use the rx800 I have with the double and switch my 1x setup to the 812.

But I am interested to see what the "real" limitations on the 812 are. And how it compares to the 800.

tomh79
Posts: 101
Joined: Thu Dec 27, 2018 9:50 pm

by tomh79

Bcoxa wrote:
Mon Jun 17, 2019 11:44 am
Just ordered the 812 and the double chainset. Not to use together, will use the rx800 I have with the double and switch my 1x setup to the 812.

But I am interested to see what the "real" limitations on the 812 are. And how it compares to the 800.
Did you order the 46/30 or 48/31 double?

Bcoxa
Posts: 30
Joined: Fri May 26, 2017 8:44 pm

by Bcoxa

46/30, didn't actually know there were options. But probably would still have gone with the same choice

tomh79
Posts: 101
Joined: Thu Dec 27, 2018 9:50 pm

by tomh79

Unfortunately the 46/30 is only availabe in the heaver 600 group. And the lighter 800 group only has the 48/31 - which needs the new frond derailleur (says Shimano...)

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TonyM
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Joined: Thu Jan 22, 2015 4:11 pm

by TonyM

tomh79 wrote:...which needs the new frond derailleur (says Shimano...)
Yes different chainline than usual

Bcoxa
Posts: 30
Joined: Fri May 26, 2017 8:44 pm

by Bcoxa

TonyM wrote:
Mon Jun 17, 2019 4:16 pm
tomh79 wrote:...which needs the new frond derailleur (says Shimano...)
Yes different chainline than usual
Considering this is going on a steel touring rig, not too concerned with a few grams. Yes is the 600 crank, should have said
I thought all had a new chain line? Ordered the new front derailleur too.

Shows how much I read...

tomh79
Posts: 101
Joined: Thu Dec 27, 2018 9:50 pm

by tomh79

Bcoxa wrote:
Mon Jun 17, 2019 4:28 pm
Shows how much I read...
not always a bad thing...I know I read too much and waste hours literally to research stuff....and in the end...I still don't know what to buy. Better go outside and ride. :lol:

Bcoxa
Posts: 30
Joined: Fri May 26, 2017 8:44 pm

by Bcoxa

According to this link
https://gravelcyclist.com/bicycle-tech/ ... omponents/

All grx cranks have a +2.5 chainline, and must be used with the new front mech.

tomh79
Posts: 101
Joined: Thu Dec 27, 2018 9:50 pm

by tomh79

Maybe a stupid beginner question: but how does this wider chainline affect the cassette? Compared to the 43.5mm I have now, the chainrings of the crank will be 2.5mm more to the right/outside looking down from above? Which would mean, more crosschaining in the lower gear cogs?
Last edited by tomh79 on Mon Jun 17, 2019 9:46 pm, edited 1 time in total.

by Weenie


Bcoxa
Posts: 30
Joined: Fri May 26, 2017 8:44 pm

by Bcoxa

This is something I am also interested in. Will report back what I find in the next few weeks when everything arrives and I get time to build up.

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