Ryan_M

I've been trying to find info on this and not having much luck. Shimano doesn't officially say it (that I've found) but I see from other sources that it should fit on a standard MTB HG hub, no spacers required. Another concern is cog spacing which I can't find anything about. Will it play nice with an M8000 RD and shifter? Can anyone confirm any of this?

Thanks!

cpach

It'll work OK. Shimano specs give 40t as the minimum large cog size on RD-M8000, so shifting may be slower than ideal, but the cog spacing is the same.
Are you just looking for narrower gearing? If you're worried about freehub compatibility all 11 speed Shimano cassettes will fit a 8/9/10 speed freehub body, the only HG body that is different is 11 speed road, which is 1.8mm longer.

alcjphil

11 speed sprocket spacing is the same for road and mountain bike cassettes. However, smaller sprocket tooth count 11 speed road cassettes are too wide to fit on a mountain bike freehub. Any 11 speed cassette with a large cog smaller than 34 teeth will not fit on a mountain bike wheel

Ryan_M

Pretty much, yes. There's a longer version of this story that you may not want to, or care to read about.... Currently I have a 38-28 / 42-11 M8000 drivetrain that works well for me. I don't use the 28:42 very often but happy to have it when I need it. I'm doing some serious upgrades - essentially building a new bike - that will get this drivetrain. After I'm done I'm hopeful I wouldn't miss the 28:42 if at all, but very much appreciate the tighter cassette.

I've already gotten a new 26 chain ring to test. Shimano specs say it "shouldn't" work but after a couple rides I notice no difference in shifting quality. So running it as 38-26 / 34-11 I'd only lose my lowest gear and have smoother steps especially in the meat of the cassette where I spent most of my time, and save 100g or so as a small bonus. Win-win.

I wasn't aware that RDs has a minimum large sprocket size so thanks for pointing that out. I'm curious about that though as Shimano's specs seem to be very conservative and sometimes so much so that they're contradictory. As I mentioned running a 12t difference on the cranks was no worse though they spec 10t max. But! I have the GS version of the RD. Their papers say it's designed for the drivetrain I have which would require a capacity of 41t, yet they list it as a 39t max. So running it on the drivetrain it was designed for is running out of specifications. Further, it dealt with the 26t chain ring just fine, OK the chain was a little slack at 26:42 but not like I'd run it there anyway except momentarily by accident. So I'm not sure how much weight to put into that 40t min large sprocket spec.

Im sort of inclined to try the 11-34 but I'd like to have a plan B if I'm not happy. However, there seems to be no RD that would play nice with that cassette at least on paper that also works with an MTB shifter. So I'm at a loss.

cpach

Shimano both has very high standards for shifting (which is great) and is pretty conservative about what works—if they didn't design it for a combination and didn't test it, they'll say it's not compatible. If you use a derailleur with a much smaller cassette than it was designed for it can be difficult to get the guide pulley as close to the cassette which means slower shifting than if the derailleur was so designed. It'll probably work fine.

Ryan_M

After doing some more digging this isn't going to work. Cog pitch on shimano 11 speed is different for road and MTB, 3.69mm and 3.9mm respectively. That's 2.1mm of cumulative error across the cassette. It sucks product info is so hard to come by but I'm assuming the CS-HG800 uses the road cog pitch as it's listed under the R8000 group.

jp911

I think the bigger issue is that the road cassettes are too wide for an MTB freehub body (by 1.8mm). When you put an MTB cassette on a road freehub you need to add a spacer.

redlude97

I think that is a mistake, the cog pitch should be the same, and it will work on a MTB hub without a spacer. Shimano 11 speed is interchangeable and the GRX line uses the MTB 11-40 and 11-42 cassettes for the 1x setups with the same cable pull for the RD.
I also use the 11-40 cassette with my RX800(ultegra level) RD along with 11-32 R8000 cassettes swapping back and forth regularly

redlude97

The HG7/800 specifically are narrower and the last cog is offset on the spider to fit the 10 speed cassettes and clear the spokes, they are the only "road" cassettes that do this hence the HG designation rather than the R7/8000 cassette naming

Ryan_M

Huh, that is promising! I've seen those numbers in a few different places now, perhaps one person got it wrong and everybody else is just recirculating that misinformation. Turns out a bike store near by happens to have the HG800 in stock. I'm going to pull my cassette and take some measurements, then take my gauges to the LBS and compare. If it's a match - as it sounds like it's going to be from your experience - then they got a sale. Thanks!

BTW nice machine 👍

Ryan_M

Confirmed, they are the same. I zeroed out on the large sprocket then moved up 2 cogs. Both measurements agree. Looks like both numbers off the net were wrong and actual pitch is 3.85mm.



CS-HG800 11-34 cassette

CS-M8000 11-42 cassette

Ryan_M

Now that I have a few hundred km on this I thought I'd follow up with the drivetrain choices. Using the 38-26 rings vs. the 38-28 is completely seamless, it's butter smooth either way. Setting up the M8000(SG) RD with the 11-34 cassette wasn't too bad. IIRC Shimano specs 6mm of gap between the largest cog and the guide pulley. IME this makes it a little harder for the chain to ride up onto the largest cog and a setting closer to 8mm works better. Setting up for this cassette with the B screw completely backed off I get 9-10mm of gap. Shifting is just fine. As an unexpected bonus the guide pulley tracks the smaller cogs better given the shallower slope on the cassete so nothing lost here either.

One thing I would mention if anyone else does this is I did the usual stretch the chain large to large then add 2 links to set the chain length. Just so happened this exactly lined up. Going large to large (not that I'd ever do this in regular riding) the RD is strangely stretched out, nearly parallel with the chain stay. Again it works fine just looks weird so I'd probably go for +3 or 4 links.