shimano grx

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

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onemanpeloton
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by onemanpeloton

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
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by Weenie


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onemanpeloton
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by onemanpeloton

Think I found my answer. Here it is for anyone else who's wondering:

Want to drop some weight, simplify your shifting, and still maintain a versatile range? Consider the 1x11 GRX option with either Shimano’s 11-40 or 11-42 cassette and a 40 or 42-tooth equipped crankset, shifted by a RD-RX812 or Di2 RD-RX817 wide ratio rear derailleur. You’ll lose a little, absolute top speed, but still love the 1:1 low gear ratio. If you live in an area without sustained or steep climbs, you might love the smaller gear steps of that same 1x11 front chainring with an 11-34 cassette. Options abound.
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fourfa
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by fourfa

I've been using both 11-32 and 11-36 with the original RD-RX800. I leave the B-screw set for the 36, and the upper pulley is fairly far from the 32 but the shifting is just fine. But set for a 42, it'll be really far away from the 32 and I'm guessing the shifting will be poor - I'd expect you'll need to tweak the B-screw when you change wheels. And you'll have to account for the change in chain length when you do.

edit - I mean that proper chain tension for 42 will likely be too loose with 32. Have to find some relatively happy medium when you cut your chain.

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pdlpsher1
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by pdlpsher1

I'm using a RD-8050 on a 11-40 without a RoadLink. You might be able to use a 11-42 on a RX800 without a RoadLink. But I would prepare two chains of different lengths, one for 11-32 and one for 11-42. And write down the B-screw settings between the two. I have another bike where I switch between a 11-32 and 11-34. I use the same chain but note that I have to adjust the B-screw by two full turns between the two cassettes.

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FIJIGabe
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by FIJIGabe

The GRX has the same minimum limits as the XT M8000 series GS rear derailleurs, with a 40T min (and 46T max for 1x). I have run as low as a 28T on my CX bike with no ill effects on shifting.
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by Ritxis


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onemanpeloton
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by onemanpeloton

Thanks for the feedback guys

I intend to have a chain of the correct length for each cassette.

I was planning to use the 11-42 cassette because I already have one, but maybe it makes more sense to buy an 11-40, ditch the road link and just change the B-screw setting when swapping cassettes and chains. Do the 11-40 shimano mtb cassettes fit shimano road freehubs?
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pdlpsher1
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by pdlpsher1

Yes. The 11-40 is similar to the 11-42 in that it’s designed to fit a ten speed mtb hub. If you are using a 11 speed road hub you need to add the included spacer behind the largest cog.


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onemanpeloton
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by onemanpeloton

pdlpsher1 wrote:
Fri May 31, 2019 12:20 am
Yes. The 11-40 is similar to the 11-42 in that it’s designed to fit a ten speed mtb hub. If you are using a 11 speed road hub you need to add the included spacer behind the largest cog.


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boots2000
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by boots2000

I was looking at this group in the flesh yesterday.
Things I liked:
1.) Rear derailleur and cranks look great
2.) 1x specific left lever without shifter guts

Things I didn't like.
1.) Shifters look blocky and cheap- More like 685/785 dhifter than Dura-Ace or Ultegra.

bm0p700f
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by bm0p700f

the 11-34T is meant fro double setups. I got my GRX RD today. Looking forward to fitting it.

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TonyM
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by TonyM

bm0p700f wrote:the 11-34T is meant fro double setups. I got my GRX RD today. Looking forward to fitting it.
But the 11-34 could also make sense for 1x when you don‘t have many hills.

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FIJIGabe
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by FIJIGabe

TonyM wrote:
Wed Jun 05, 2019 12:43 am
bm0p700f wrote:the 11-34T is meant fro double setups. I got my GRX RD today. Looking forward to fitting it.
But the 11-34 could also make sense for 1x when you don‘t have many hills.
I mean, technically he's right, since the 11-34 (HG-800) cassette was released with R8000, which is a 2x group. Even the Ultegra RX derailleur is listed as 2x (although it is 1x capable, from what I've seen). That said, Shimano's GRX group can be 1x or 2x, and Shimano lists the 11-34 as compatible with the GRX group. Also, there's nothing in the literature for the HG-800 cassette that limits it to only 2x drivetrains. Those drivetrains just weren't officially sanctioned by Shimano, up until now.

Where I live, in SE Texas, where there aren't many hills, I use an 11-32 or 11-34, depending on the terrain (I use the same cassettes for CX racing as I do for gravel). Using an XT rear derailleur on my bike, I can fit up to an 11-46, but I've never seen the need.
Madone 9 - https://bit.ly/2Nqedbn
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Crockett - https://bit.ly/2Xem4sk

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jfranci3
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by jfranci3

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.
Last edited by jfranci3 on Wed Jun 05, 2019 5:21 pm, edited 1 time in total.

by Weenie


rides4beer
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by rides4beer

bm0p700f wrote:
Wed Jun 05, 2019 12:12 am
the 11-34T is meant fro double setups. I got my GRX RD today. Looking forward to fitting it.
Do you know if this would be a straight swap for a 105 RD?

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