Hi folks.

I'm wanting to replace my gear setup. I have indexed 3 front / 7 rear Deore XT grouppo, and I'm finding the front simply doesn't need to be there. Current setup is 23-46 front (3 rings) and 12-21 rear. I want to replace this with a fixed front 34 tooth, and a 11-34 rear, 7 gear cassette (Shimano).

Anything I should worry about? I can still use my front setup on the middle ring setup, but I'm hoping there's no issue with just ordering a 7-speed rear with my calculations.

Any insight/advice appreciated. Cheers

I don't see a problem with what you're proposing. You did your research and figured that you don't need as much high end, and you can get a 34 tooth cog for low end.
You may want to add a chain-keeper or use a narrow-wide tooth chainring to help retain the chain in the extremes on your cassette. You can probably also use the front derailleur to keep the chain from dropping, rather than deleting it, or buying a retainer.
Check your rear derailleur's max teeth to make sure you have enough room for a 34t cog.

OK, your last point is what I was concerned about. I will be going from 21 teeth to 34 teeth. How can I check this allowance given my derailleur make/model?

Cassettes like that are annoying because one is too low and the next is too high.

I've used the full range, but I'm finding it's just not needed now. I can make this more simple while updating my drivetrain.

Simplicity is a good goal, but not at the cost of functionality. I think you may find that a 7-speed 11-34 cassette is going to have some massive jumps between gears that you may not like.
You will be giving up a lot of flexibility for the simplicity of a single front ring.

Then something more narrow?

What is lowest (easiest) gear that you need ?

With a more narrow cassette you will be giving up the high and/or low end. There is just no way to spread 7 gears across the range you can get from 21, even with some near-duplicates.
This is why they are needing to design cassettes with 11-40+ teeth, to get an acceptable range with a single front ring, and try to keep the jumps between gears acceptable by going to more and more rear speeds.
They are still hard-pressed to get the range and close spacing of a triple system.

That won't work with an indexed shifter.

I could say that my triple system has overlaps with its permutations.

Is there a way I could build my own cassette? Is there such a service, or would I have to build my own?

Shimano technical documents will have the information you need. Just type your model number in, and you'll have the exploded view, installation manual, and potentially dealer guide at your fingertips.

The other points made so far are good to keep in mind. You can estimate for yourself using a gearing calculator and your drivetrain's tooth counts, plus what you know about your riding style and cadence. There will be larger gaps between cogs for a seven speed, but you can use the calculator to see how that might affect you. My personal experience is that the wide range cassette and my pace conflict, so that my comfortable cruising ratio is unavailable while in my largest chainring. I'm left with a slightly too low or slightly too high gearing. That's on a 9 speed, there are more opportunities for that to happen on a 7 speed.

You CAN! It's really helpful to have extra parts around to do it, though. Can you access a bike co op or kitchen to find the uniglide or hyperglide pattern cogs?

I'm sure I can. A cassette is like $30 so I was hoping to bing bang boom this and off to the races.

I have no clue what model number this owuld be. Deore XT is all I see. This bike was purchased 28 years ago.

I should be able to eyeball this as the guide pulley demonstrates a fixed radius, am I right?

Nothing. It would fit right in. Make sure your derailleur cage is long enough for the gear range. Medium is very safe.

Here is mine. 11-40 with mediumcage.

You are definitely overthinking this. Get an 11-34 or a 12-32 cassette and throw it on and use your existing middle ring. This way you will know if you like the setup/gaps or maybe go with a 36 or 38 single chainring instead.

Personally I would try a 12-32. Your only bad gap is 21t to 26t.

Once you find the gearing you like, just make the change. If you are concerned about losing the chain off the chainring, go narrow-wide or use a chain guide.

John

I got a long cage.

I’m guessing you have an XT-737 rear derailleur. They made GS (mid cage) and SGS (long cage). Hard to tell which one you have, but if it is a GS the max is 32t, but you can probably run a 34t without any problems.

John

Edit: Changed from long/super long to mid/medium and long. Same as m900 XTR, which I have only seen in mid cage and I’m running with a 34t.

https://si.shimano.com/#/en/search/K...leur&name_web=

Yep that's where I looked. I couldn't find my model. Are you expecting it to be in there?

It is in there, but it's not my bike. You'll find the information you need in one of those manuals. Also, the EV exploded view documents can help you pinpoint which model you have, visually.

FWIW, shimano usually puts the model number on the wheel- facing side of the derailleur.

Bingo!

I love the simplicity of a single chain ring too. But when I looked for a new entry level bike in 2018, The one I liked used a Shimano 14 - 34 Megarange cassette. While I give them credit for keeping 6 of the 7 speeds well spaced and providing and adequate range (35 - 85 gear inches), That jump from 24 to 34T is WAY too much to be very practical with any single chainring. And if you spread them out, There're all to widely spaced.

The bike I choose has the same cassette, But thanks to a 28 - 38 - 48 triple chainring, I now have 12 closely spaced gears that cover every scenario I encounter. With a little patience and sometimes a small adjustment, I find the shifting to be reasonably smooth and reliable. Even with the low end Suntour crankset and Shimano Altus front derailleur.

Besides 90% of the time I can leave it on the 38T chainring if I choose. But sometimes it's advantageous to make a single shift of a chainring for a 2 - 3 step gear change.

FYI...

Sunrace makes a 12-34 8 speed cassette. I'm running it as a 14-34 7 speed (14-16-18-24-28-34) with a 30-38-48 (missed it by 2 teeth). Just break it open and use your 7 speed spacers or use with 8 speed spacing. I do have an added benefit of being able to thread on the 14t since my freehub body is UG/HG.

John

So it seems (thanks for the EV pointer), It's the Deore XT St-M737 with extra long cage. Boy, that was an exercise.

Total Capacity 38T or less
Largest sprocket 32T
Smallest sprocket 11T
Front chiainwheel tooth difference 22T

Not sure what Total Capacity means.

Total Capacity is (large chainring teeth - smallest chainring teeth) + (largest cog teeth- smallest cog teeth). It's meant to tell you how much chain the derailleur can tension.

https://www.bikeradar.com/advice/buy...r-derailleurs/