Couple of Quick Questions About Shimano GRX...

[XXtwindad]

I want to go 2x on my new build.

1) I thought they had an 11-36 cassette, but I don't see it advertised.
2) Is the RX812 (max low 42 tooth) compatible with a 2X? If not, which derailleur do I run a 40t cassette with?

Thanks.

[eddief]

I think 36 tooth 11 cassette available from SRAM, not Shimano.

Watch this guy do 11-40.

https://www.youtube.com/watch?v=wNeYYM9_aeM

Originally Posted by XXtwindad

I want to go 2x on my new build.

1) I thought they had an 11-36 cassette, but I don't see it advertised.
2) Is the RX812 (max low 42 tooth) compatible with a 2X? If not, which derailleur do I run a 40t cassette with?

Thanks.

[Clean39T]

Originally Posted by XXtwindad

I want to go 2x on my new build.

1) I thought they had an 11-36 cassette, but I don't see it advertised.
2) Is the RX812 (max low 42 tooth) compatible with a 2X? If not, which derailleur do I run a 40t cassette with?

Thanks.

I'm putting 11-36 w GRX 2x Di2 RD on my Ellis mated to 31/48 up front - will report back if it seems like there's capacity to go to 40. I can't imagine climbing anything that requires a 31/40 low gear though......can barely imagine anything I need a 31/36 for.....but it seems worth trying.

[eddief]

interested to hear results.

Originally Posted by Clean39T

I'm putting 11-36 w GRX 2x Di2 RD on my Ellis mated to 31/48 up front - will report back if it seems like there's capacity to go to 40. I can't imagine climbing anything that requires a 31/40 low gear though......can barely imagine anything I need a 31/36 for.....but it seems worth trying.

[XXtwindad]

Originally Posted by eddief

I think 36 tooth 11 cassette available from SRAM, not Shimano.

Watch this guy do 11-40.

https://www.youtube.com/watch?v=wNeYYM9_aeM

Exactly what I was looking for. Thanks.

[XXtwindad]

Originally Posted by Clean39T

I'm putting 11-36 w GRX 2x Di2 RD on my Ellis mated to 31/48 up front - will report back if it seems like there's capacity to go to 40. I can't imagine climbing anything that requires a 31/40 low gear though......can barely imagine anything I need a 31/36 for.....but it seems worth trying.

Should've specified. Mechanical shifting. Also, we have 15% grade "gravel" here in the Bay Area. More than happy to have any "bail out" gear short of a motor.*


* not that there's anything wrong with that

[prototoast]

Originally Posted by Clean39T

I can't imagine climbing anything that requires a 31/40 low gear though......can barely imagine anything I need a 31/36 for.....but it seems worth trying.

Come visit this neck of the woods some time and you'll see. The fire roads around here are steep and long. I usually opt for my rigid mtb just because I have 32/50 gearing on that. With my 36t cassette in my gravel bike, I'm walking a lot more segments.

[Clean39T]

Originally Posted by XXtwindad

Should've specified. Mechanical shifting. Also, we have 15% grade "gravel" here in the Bay Area. More than happy to have any "bail out" gear short of a motor.*





* not that there's anything wrong with that

Sorry, didn't mean to sound judgy -- would love to come ride w y'all down there...

Sent from my Pixel 2 XL using Tapatalk

[HTupolev]

Originally Posted by XXtwindad

I want to go 2x on my new build.

1) I thought they had an 11-36 cassette, but I don't see it advertised.

Not for GRX.

The chain wrap ratings that Shimano supplies for their derailleurs are usually pretty accurate. Many people choose to exceed them, but doing so *does* result in bad behaviors in one or both cross-chained areas of the drivetrain.

Max cog capacity is more complicated.
The max cog that Shimano lists for a given rear derailleur is usually the largest cog in the largest cassette that they ship in a group with that derailleur. The biggest cassette that Shimano offers in a 2x GRX800 configuration is 11-34, so that's the max cog that they list for the RD-GRX810.
Obviously you can often get a rear derailleur to clear a bigger cog, by manipulating the b-pivot screw or - if that's not enough - by using a hanger extension like a Road Link. Sometimes this can cause balkier shifting in the smaller cogs, but usually everything works out fine outside of extreme cases.
An 11-34 cassette paired to the 48-31 chainring option is already at the nominal wrap limits of the RD-RX810 derailleur, which is probably a big part of why Shimano doesn't permit a wider cassette in their specs. Plenty of people are using drivetrains that exceed those specs, though. If you do it, just choose your chain length carefully and be aware of what parts of your gearing range your drivetrain exhibits poor behaviors.

2) Is the RX812 (max low 42 tooth) compatible with a 2X?

The RD-RX812 is specifically designed for 1x applications.

This is most visible in the design of the pulley cage. When you shift to a bigger cog, the pulley cage has to let out chain, and it rotates counterclockwise (when viewed from the drive side of the bike).
On the RX812, there is a large distance from the cage pivot to the jockey wheel, so this counterclockwise rotation has the effect of lowering the jockey wheel away from the cassette. This is reasonable behavior, because you just shifted onto a bigger cog which needs the jockey wheel to be lower!
However, front shifts also affect rotation of the pulley cage. If you were to use an RX812 derailleur in a 2x drivetrain, shifting from the small ring to the big ring would also cause the jockey wheel to move away from the cogs, which can result in balkier shifting.
Obviously you don't want front shifts to affect rear shifting. Derailleurs that are designed for multi-chainring drivetrains usually avoid the issue by using a smaller distance between the cage pivot and the jockey wheel. In the case of the RX810, the cage pivot is actually concentric with the jockey wheel, so cage rotation does not affect the position of the jockey wheel at all.

Also, although the RX812 is designed to clear a bigger cog than the RX810, it actually has less chain wrap because it's not intended to wrap chain from front shifts. The RX810 is the longer-cage model.

So, definitely use the RX810 in a multi-chainring drivetrain.

Originally Posted by Clean39T

I can't imagine climbing anything that requires a 31/40 low gear though......can barely imagine anything I need a 31/36 for.....

All depends on the regional terrain. There's one forest road near me that slices an extremely sharp path up a foothill, averaging 15% over a two-mile stretch. That's a bit extreme: most climbs will have half-mile or 1-mile stretches at 12% or so, but even then, I end up spending a lot of time in my 19" gear.

[joosttx]

Sram PG-1170 11x36 is what you want and compatible with Shimano 11 speed free bodies.

I have on lightly used that I would let go for $40.00 PP. https://forums.thepaceline.net/showt...=1#post2635332

[Hindmost]

Originally Posted by XXtwindad

* not that there's anything wrong with that

Where are we in that picture? Wildcat canyon?

[XXtwindad]

Originally Posted by Hindmost

Where are we in that picture? Wildcat canyon?

Bingo

I would actually use my MTB for much of Wildcat. I was trying to find pics of the Canyon Trail in Redwood Regional Park, which is the connector trail. I would use my gravel bike on that, and man, is that a steep sucker...

[R3awak3n]

I thought you were going to ask... is GRX really that ugly? which I was going to say, yes, yes it is

[weaponsgrade]

I'm running an RX800 RD, which was the precursor to GRX, 11-40 cassette, and 2x, 31/48. The RD is super stretched out in the big-big combo, but otherwise works great.

[charliedid]

Originally Posted by XXtwindad

I want to go 2x on my new build.

1) I thought they had an 11-36 cassette, but I don't see it advertised.
2) Is the RX812 (max low 42 tooth) compatible with a 2X? If not, which derailleur do I run a 40t cassette with?

Thanks.

GRX 10 spd advertised 11-36

I have not tried it but I am 93.5% certain 11 spd will run an 11-36