clarkbre

I am doing some revitalizing of my 21 speed hybrid to make it more like a flat bar road bike. So far I have changed out the shifters from grip to trigger, swapped seats to a firmer road saddle, and changed the tires from 700x35c to a set of 700x28c slicks. I like how it rides but feel that the gearing is extremely too high and too low for the around town riding I do.

In looking at the gearing the bike has a gear inch range from 23 to 102. For ease of cost and modification, i am looking to only switch cassettes from the current 11/28 to a 12/21. The new cassette would be 30 to 93 gear inches with closer gear ratios. The chainrings are 42, 34, & 24 tooth. The original gearing gives me chain wrap of 35 and the new gearing would be 27. So, my question is:

Will my current rear derailleur be able to handle the much smaller cassette or will I need to switch to a shorter cage derailleur?

TallRider

the derailer will easily be able to handle the narrower gearing range. It won't track as close to the cassette (because the cassette gears won't get larger at the same rate on a 12-21) but will probably shift fine.
The bigger issue with derailers is taking up chain slack. That's only a problem wiht a short-cage rear derailer not being able to take up enough slack in a wide-range drivetrain.
You could just keep the 11-28 cassette and set up the bike to only use the larger two chainrings (42 and 34).
But if you want a closely-spaced cassette, there's no problem in keeping your current triple and switching to the 12-21.

FBinNY 

Yes, any derailleur can handle anything within it's capacity. There's no such thing as too much capacity.

One way to think about this, is that if the RD can handle the 21t sprocket in the middle of the cassette, why couldn't it handle it if it were the largest?

Enjoy the new narrower gearing. If you ever trash the RD, that's when you can consider one with a shorter cage. The only advantage is a bit of saved weight, and higher ground clearance.

FBinNY 

This may or nay not be true. Yes, the narrower cassettes profile will be at a shallower angle, the RD won't be as close on the larger sprockets. OTOH, when derailleurs are offered with a choice of cage lengths, the pantograph angle is identical for all versions. So a short cage may not be any better that yours is in terms of matching the cassettes profile.

TallRider

I was assuming the hybrid came with a mtb-marketed rear derailer, whereas nearly all (Shimano) short-cage rear derailers are for road cassettes with an official max cog of 27 or 28t.

clarkbre

Awesome. Thank you all for the information. I knew about not using a short cage with large sprockets but wasn't sure if that also applied the other way around. I will change cassettes and report back....next step is to turbo the engine!

FBinNY 

Yes, which is why I left it as an open may or may not be the case.

clarkbre

Fyi the rear derailleur is currently a 7 speed shimano stx rd-mc32.

Bill Kapaun

You may be able to adjust the B screw to get a little better shifting.
Make sure it clears the largest AND smallest cog.

I did a similar change on my Globe.
Factory cassette was an 11-32 and I now run a 12-23 (also upgraded to 9 speed from 8)

RubeRad

The thing is, you might find at first that the RD appears too too big, because it is so slack, and the cage pointing way back and never getting stretched out, and never putting a lot of tension on the chain. If that's the case, you just need to shorten the chain.

eusebio

I've started using short cage RDs since this spring after finding long arms to affect my chain tension. I think if the chain coming out of the arm toward the chainring has to travel upwards (even slightly) the RD is not for your bike. This is my opinion though.

ThermionicScott 

Good luck with that! Switching to a cassette with smaller cogs, you'll want to play around with the B-tension screw to allow the jockey pulley to ride closer to the cogs.

clarkbre

Thanks. That's my plan. Install the new cassette and play with it a bit. If all else fails or I just want to upgrade I will switch it out for a short cage unit. Will let you know. I am pretty excited to get this bike going so I don't always have to use my road bike.

clarkbre

My new 12/21 7 speed cassette came in the mail yesterday and I got it installed. Initially I was having some trouble getting everything dialed in but after some adjustments, it started shifting better.

This morning I took my regeared hybrid out for a 10 mile ride. Here are my thoughts:
1. I should have regeared it sooner. The range of 30-93 gear inches within 21 speeds is great for riding around town. The closer ratios are more inline with my road bike and it puts the center gear of the chainrings and cassette a very comfortable gear inch.

2. Although the derailleur is shifting ok, I think there is room for some improvement. To get it set up correctly, I had to all but remove the tension b screw. The upper pulley is still riding a good distance away from the cassette so the shifting seems a bit slow/sloppy.

Moving forward, I will be looking at swapping the original STX extra long cage derailleur for something more in tune with the small cassette and chainrings. From my research it looks like I could easily go with a medium length but would maybe like to try a short cage. The gearing is within the overall capacity for the derailleur, the only problem might be an 18 tooth difference on the chainrings but shimano says the max is only 16t.

sreten

Hi,

The cassette change is good. Given that many folders with
a single front come with derailleurs that can handle a triple
you are worrying about nothing. Reach is mainly about the
front range not the rear, and you basically cannot buy a
derailleur for 21 max, they all start at 27/28 AFAIK.

Folder should come with short reach, but cheap ones don't.

rgds, sreten.

The B screw I understand is set in the smallest rear, so
there is some change from 11 to 12 *. I further understand
its proper setting can be compromised to accommodate
bigger lowest gears, but that is hardly the issue here.

Both my derailleurs are fixed at 14 to 28.

* You can't reduce it to better suit 21, it needs to be set for the
12. I understand its often stated it is to be set for the largest rear
sprocket, which is true with a big sprocket, here it is very wrong.
In setting it properly for the 12, biggest cog size goes up.

Unless :

Basic derailleur geometry is 14 to 28. With 11 to 28 you would
set it on the biggest sprocket and 12 to 21 you would slacken
off the B screw, but would be setting it to the smallest sprocket.

Bill Kapaun

You are overthinking this!
basically set it as close as you can and make sure no cogs rub.
Simple!
Sounds like he CAN'T set it too close-

OP, if you plan on sticking with thie cassette, I'd recommend shortening the chain appropriately.
That "might" help things a bit. It might also move the guide pulley a bit closer to the cogs.

Also, when making B screw adjustments, I adjust, ride, going through all the gear ranges, and than check it again.

I would expect that when you get it "dialed in", you may actually have slightly better shifting due to smaller jumps between the cogs.

gregjones

I guess time marched on!!!

I just got a new 105 and it's listed at 30t here, in the spec chart.

clarkbre

Correct, I can't get it too close to the cassette right now. I ran through an online chain length calculator and the manual calculations and both came out to needing a 52" chain. The derailleur cage looks like it's slanted forward a bit too much so I'm thinking with the proper length cage everything will tighten upwards towards the cassette.

Either way, I am very happy with the regearing. Thank you all for your help.

ThermionicScott 

Glad it worked out. I, too, have regeared bikes to tighten up the range and enjoyed it as a result. "Road" derailleurs will generally ride closer to the cogs, and I've gone so far as to remove the B-screws from my bikes and put them into a labelled Ziploc baggie for safekeeping since they weren't needed at all.

HillRider

Excuse me but this is nonsense.

FBinNY 

There was one tiny element of truth, which is where I though the poster was headed before he veered off into the Twilight Zone.

With the same tension spring, a longer cage will have lower chain tension than a shorter one. This can be corrected in the design, by moving the spring's engagement point, or using a stiffer spring, but to my knowledge, no maker does either on short and long cage versions of the same RD.

clarkbre

I actually thought about that. Would it be possible to reindex the spring by drilling another hole? Would this help tilt the derailleur up making the pulleys closer to the cassette?

FBinNY 

You could drill a hole to increase spring tension, but that has no effect on the height of the RD. If you've backed out the B-screw all the way, and still want it higher, you might be able to file some from the front of the stop cam (if the RD has one) which engages the stop on the hanger.

It's a fairly safe effort since if you file too much, you can use the B screw to push the RD down.

rydabent

The derailer will work just fine. Just adj the "B" screw to get the jockey wheel to the proper clearance of the largest sprocket. That way the shifts will be better.

clarkbre

The problem is/was that even with the "B" screw turned all the way out, the jockey wheel still was not close enough to the sprockets. It's almost as if the rear derailleur's return spring didn't have enough tension.

That being said, I stopped by the LBS yesterday and found a lightly used Tiagra 9 speed long cage derailleuer for $12. The cage was about 2cm shorter than my original so I gave it a try and it works perfect. The jockey wheel now rides very close to the sprockets (not touching) and all my up and downshifts are very crisp and quick. It's amazing how changing a few parts (cassette, derailleur, tires) turns an old, sluggish hybrid into a nimble, flatbar road bike.

Again, thank you all for your help and suggestions.