Miradaman

Can one upgrade a cassette to a wider gear range while keeping the stock rear derailleur? My ’89 Trek currently has a 7 speed 13-28 with Shimano Sport LX RD. Would, say, a 7-speed 12-32 just fit right in and work with the rest of the drivetrain remaining as is?

sdmc530

According to my friend the bike mech he says yes, 32 will fit. you will need a new chain though.

Miradaman

Thanks. That's actually what precipitated the question, I need a new chain so thought I might upgrade the cassette while I'm at it...

Iride01 

Is this what you have? https://si.shimano.com/pdfs/si/SI-R-45-000-00-ENG.pdf

Looks like a pretty short cage and mechanism. The specs show it only for a 28 tooth large on the rear. Shimano is conservative most of the time.

dsbrantjr

Understand that you will have larger jumps in the midrange gears with the new cassette; this is the tradeoff for extending the high and low ends. It is only an upgrade if these extensions are what you need. Also understand that you are using the derailleur outside of its intended range so there may be some tradeoffs/issues there as well.

Here is a calculator which will enable you to compare the two setups: https://www.kstoerz.com/gearcalc/compare/

Miradaman

Appears similar but not quite the same.

Miradaman

So, would it not be worth upgrading do you think? Do larger jumps make it shift rougher or poorly? Would RD possibly not even have capacity for larger range as Iride01 pointed out in his link?

Miradaman

This is the RD in question by the way. Sorry for the poor quality. Branding is obscured by glare but as I said is Sport LX from 1989.

dsbrantjr

Only you can decide if the gears you are giving up are worth losing to get the lower and higher ones. If you rarely spin out in your 13 or struggle in your 28 having higher and lower cogs will not be helpful. The larger jumps do not affect shifting much but going past the derailleur's capability might. You will lose some ability to finely adjust your gearing due to the larger steps. This is why I pointed to the calculator, you can see how the shift will affect your commonly-used gears.

Iride01 

I think about what gears I ride the most in and what gears to I need to do the climbs that I will normally encounter on some or all rides during the year. If those two conditions are the same gears, then you might need to think about doing other things.

On a seven speed rear, if you normally ride in 4,5 and maybe 6 of the back, then a freewheel or cassette that has gears with those same tooth counts and then 1, 2 3 have bigger spacings, it's not a big deal, to me. Those are gears for climbing and acceleration. If you aren't racing then who cares if you might not have gears spaced to let you make maximum speed from the power you are able to put out during that moment in the conditions you are in.

If you normally ride in the 1,2 and 3 rear cog.... well, somethings got to change. Probably need a new bike with more gears as well as more lower ratios. Possibly or probably smaller chain wheels. As well, your whole riding conditions, style and other things needs to be known to discuss it adequately here.

Retro Grouch 

Essentially, the benefit you will get is one additional easier hill climb gear.
What you give up is closer spacing in mid range gears. If you are riding on a long and relatively flat road I find it comfortable to have a lot of gears spaced close together. That lets me keep my pedaling in the comfortable "sweet spot" neither having to push too hard nor having to spin too fast.

Miradaman

As is it's great on hills. What I was hoping for was to maybe eke out a higher average speed with the higher gear...

Miradaman

Meant lower gear btw...

Retro Grouch 

Unless you are spending a LOT of time in the highest gear that you have now, and also spinning at a reasonable cadence, probably not.

Iride01 

If you are looking at the high end or small cogs, then can't you find something less than the 32? Then you won't be taking as big a risk by going outside specs.

However, you may not get as much as you are wanting tooth smaller. That is probably less than two mph difference at 70 to 80 rpm cadence, depending on front ring sizes.


Lower gears to me are the big rears. Lower ratio (front teeth ÷ back teeth) = lower gear.

Miradaman

OK, that's what I thought but second guessed myself. Gearing is still very confusing to me I must admit...

Miradaman

OK, so my takeaway here for cassette upgrade is minimal (if any) performance gains in terms of higher average speed, with potential performance loss due to wider mid-range gear spacing and the stock RD being out of its range. I think you guys have talked me out of it. I'll replace chain and leave it at that. Thanks as always for everyone's expertise!

Iride01 

This is a case where changing the front rings or crankset might give you more. Changing the bike, might give you a lot.

Miradaman

True, but so far I'm enjoying the challenge of upgrading a (formerly) beater bike on a budget...

70sSanO

I looked at that cassette and the only bad gap is a 21t to 26t. I’ve ridden 26t to 32t and that is no issue since the 32t is kind of a bailout. But that 21t to 26t would not be enjoyable for me.

You should take your bike out and see how comfortable you feel with your current setup to see if you can dump your current 24t. As others have said, how often are you going to push a 12t.

John