Is a 21-speed Shimano Deore setup worth it?

Can I get some help from the experts here... or maybe just those who have messed this up before themselves?
I’ve got an MTB setup with a SRAM Rival Crankset (48/38/28) paired with a 7-speed Shimano Deore Cassette, 13-34 range—basically a standard 21-speed setup.

The rear derailleur is holding up okay, but my front Shimano Altus Component is absolutely trashed. It’s barely worth anything, but I can't get the indexing right for the life of me; it got bent during a crash and now shifting is a nightmare.

I have the chance to snag a Shimano Deore FD front shifter for free. According to the manufacturer specs, it isn't technically meant for this specific combination, so I'm wondering: will it actually work in the real world?
If anyone knows the answer, please share it with those of us still figuring things out. Thanks in advance!

Man, you’re seriously making a mountain out of a molehill here—you can pick up a front derailleur for next to nothing anywhere. I mean, you could probably find one sitting on a shelf at a local Walmart or even some random shop down the street. Just grab a used one if you want to save a few bucks, slap it on, and get riding.

Look, I don't know how much I actually messed up the setup, but if you end up getting that front shifter, then the rear gear count won't even matter—even if you managed to squeeze 14 speeds out of there instead of just 7. Just grab it. If it works perfectly, great. If it doesn't, at least you gave it a shot.

Sure thing, go ahead and install it

So, I just finished installing this Shimano Deore FD on my Trek.

If you're sitting there thinking about upgrading the derailleur on your old 21-speed bike, let me save you the trouble right now: don't bother. It’s a non-starter.😠

See, this Shimano Deore FD is designed for setups with 44-32-22T chainrings. Most 21-speed bikes run a 48/38/28 setup instead. What happens? When you try to shift into that largest front ring, the derailleur cage hits the teeth of the next one up. You could try to fix it by mounting the derailleur higher—maybe keeping the cage about 0.6 inches above the biggest ring—and sure, it might technically "work," but then you're asking for a chain to slip off during a shift. And trust me, having your chain jump while you're moving is a disaster waiting to happen.

I figured I’d post this just to save someone else from the massive headache.

John Lewis91 said:So, I went ahead and installed this Shimano Deore FD on my Trek.

And if anyone is thinking about upgrading the derailleur on their 21-speed bike like I did, let me save you the trouble right now: don't bother, because it’s fundamentally flawed.😠

This Shimano Deore FD is designed for setups with 44-32-22T chainrings. Most 21-speed bikes, however, run a 48/38/28 setup... which means when you try to shift into the largest front ring, the derailleur cage hits the teeth of the middle one. You could technically "fix" this by mounting the derailleur higher—positioning the cage about 1.5 cm above the largest ring—but while it might function, there's a high probability the chain will just drop off the teeth during the shift from the middle ring to the large one. That’s a recipe for a nasty crash mid-ride.

I just hope I've saved someone else the headache...

You've made several mistakes here, and you'll never get your derailleurs tuned correctly using that logic.
Your assessment regarding the tooth count versus the number of speeds isn't quite accurate, though that's beside the point.
Under no circumstances should you raise the front derailleur more than 5-7 mm above the largest chainring.
Regardless of how you tune a front derailleur, the chain should never jump over the top of the largest ring or fall below the smallest one. Those two small screws are limiters; once they are set, you shouldn't touch them again. Adjustment should be handled solely by adjusting cable tension, whether that's via the barrel adjuster on the cable or the fine-tuning dial on the shifter itself.
In any case, you've completely botched the setup. Time to go back and do it all over again.

Dana Reyes5 said:you've made a mess of this and you're never going to get those gears indexed working if you keep at it like this.
the math regarding front tooth count versus gear count isn't quite right, but whatever
you should never, under any circumstances, set that front derailleur more than 5-7 mm above the largest chainring.
No matter how much you tweak that front derailleur, the chain shouldn't be jumping off the top of the big ring or dropping below the small one. You use those two tiny limit screws for that—they are limits, plain and simple. Once they're set, you don't touch them again. Everything else is just cable tension, whether you're turning the barrel adjuster on the derailleur itself or using the fine-tuning knob on the shifter.
At any rate, you've completely botched it, so go ahead and try resetting it all over again.

Thanks for the input, Dana Reyes5. What you said is technically true, but I already mounted a compatible 48T setup on my bike and it works fine, even if the build quality is nowhere near that Deore level. I actually put a Deore LX setup on a friend's Trek, and it runs like a dream for him... everyone's happy...🙂...even though I nearly lost my mind trying to install it on my own ride...
I trust you know your stuff... but honestly? I don't think even you could pull this off... unless you grind down about 1.5 mm from the bottom of the cage on this LX... just compare the shape of the bottom of the Deore cage against this 48T one when you're hanging out with the crew.

The posts are recent enough that I’m hoping for a quick response.
I’m struggling with indexing my front and rear derailleurs. They used to shift perfectly, but now they’re definitely lagging. I took a pretty hard fall recently, which might have knocked things out of alignment, though they worked fine immediately after. Then winter hit, and the bike sat in the garage collecting dust and old, gummy grease. I tried cleaning it up, but it’s still not hitting every gear correctly 😕 Chatter is happening, and I’ve tried adjusting the limit screws, but nothing seems to sync up. It works for some gears and fails on others. I don't have time to haul the bike to a local Bike Shop; I want to get this dialed in tonight so I can head out for a ride tomorrow. Is there any hope here? Any advice would be appreciated 😉
Waiting on a reply ☕ THANKS!

Elizabeth Myers32
Just take it into a shop—they'll have you back on the road in like 10 minutes. Honestly, the odds of you fixing this yourself are about the same as hitting the Powerball.

Jeffrey Hall5, thanks for the encouragement 😉 though it looks like I might need to win the lottery first 😁
I managed to get it tuned, but everything is off by one gear. It won't catch the largest cog on the rear derailleur, but I don't really climb hills in first gear anyway... Since I'm the only one riding this bike, it’s enough that I know how it works 😂 Heh, so now I have a 24-speed bike without the first three gears 🤣
Thanks, that was a quick response...

I managed to tune the but everything feels off by just one gear. It won't grab the biggest cog on the rear derailleur—not that I’m exactly grinding it out in first gear anyway...

It kind of reminds me of that old TV glitch—you know, where the thing works fine, but there's zero sound and the picture keeps jumping all over the place.😂

But seriously, no jokes—it's awesome that you're actually putting in the effort to fix this yourself. Honestly? Just ride it as is for now until you have some downtime to hit up a shop. Once you drop it off at a local bike mechanic, they'll have it sorted in a few minutes flat.

Here’s a quick video tutorial on how to tune up those derailleurs, it’s actually pretty easy... there are tons of videos about this on YouTube 😉

http://www.youtube.com/watch?v=xkGBajG4TPc

http://www.youtube.com/watch?v=t7KKQy0OkUs

Thanks 😉
These YouTube tutorials are top-notch 😲 I watched them all, followed the steps, and now everything is running perfectly 🙂

Dana Reyes5 said:No matter how much you fiddle with the front derailleur adjustment, there is absolutely no excuse for the chain to jump over the largest sprocket or slip underneath the smallest one. Those two tiny limit screws aren't just there for decoration; they serve as physical boundaries specifically designed to prevent that exact kind of mess. Use them as intended. Once you get them properly tuned, there’s really no need to mess with them ever again. Adjusting the gear linkage is strictly a matter of tensioning the cables. You can either go the blunt route by shortening the cable at the adjustment screw itself, or you can opt for more precise, fine-tuned regulation directly at the shifter.

Elizabeth Myers32 said: I’ve spent quite a bit of time tinkering with those adjustment screws, tightening and loosening them in an attempt to get everything perfectly aligned, but I simply cannot get them to sync up.It’s hit or miss depending on the speed. For some velocity profiles, it performs adequately; for others, it fails entirely.

Well, I’m glad you managed to snag that deal—it was a smart move on your part. Just a bit of advice for next time: if you read a little more closely, you’d realize they’ve actually laid out most of those details quite clearly already.
When you finally get those two little adjuster bolts dialed in, for heaven's sake, just leave them alone. It seems like everyone’s first instinct is to rush over, grab a wrench, and start tinkering—tightening this bit, loosening that bit—until they inevitably cause the chain to jump the sprocket. And then, suddenly, everything is ruined. Let's just remember what these things actually are: they are limit screws, not toys. Set them once, set them right, and walk away.