Lower gears - change cassette or inner chainring?

[Tim O]

Hello All, I want to get lower gearing on my tourer. Not sure whether to go for a cassette with 36T lowest cog or an inner chainring, say, 24T, on the triple. Are there any mechanical issues that I should be aware of before buying? I.e., which route do you reckon would cause less set up problems? I've got a long throw rear mech and I'm inclined to go for the cassette option on the basis that the tolerances on the existing front mech seem a tad restricted: I've got to ensure I plan ahead for changes to the inner ring as it is - or won't the size of the inner ring make any difference? Existing set up is 32T lowest cog on the cassette and a 28T inner chainring.

Any thoughts or experiences would be much appreciated.

 

[fossyant]

What's the other chain rings - 53, 39, 28. It will be cheaper to replace the inner ring and a 26 or 24 will give you a lower gear than changing a cassette to a 36.

You might just get away with it. Changing a cassette will also mean bigger tooth jumps.

 

[ChrisEyles]

I'd be tempted to swap the granny ring on the front - that way you keep the (slightly) closer ratios for normal riding but you've still got the super low gearing for when you need it.

If your front mech can cope with a 39-52 shift, I guess it should manage a 38-24 middle to bottom shift equally well. Like you say since you won't be speeding when you need these gears it's not really a problem to prepare for the shift and gently ease in/out of the granny ring.

 

[Tim O]

Thank you to both of you - very helpful. That's just why this forum is so great: techie question answered authoritatively. Chris, that's re-assuring about the inner chainring coping with the shift. And that was a very quick bit of maths fossyant - re changing the chainring giving a lower gear - I always get confused over calculating the ratios! Looks like I'll go with the chainring option then. Cheers.

 

[ChrisEyles]

You'd definitely need to double check the capacity of the rear mech if you wanted to do both of these - an 11-36 and 53(?)-24 is a total of 54t!

Having said that, you can cut down on the big chain ring size a touch to reduce the difference (I don't think I've ever needed the 48/11 top gear on my tourer, even), and you *could* run a slightly short chain if the mech is unable to take up all the slack..... but only do this if you are very confident you won't ever accidentally shift into any of the "big-big" chain ring/sprocket combos though, or you could be looking at a broken chain and knackered rear mech/drive side spokes!!

 

[ChrisEyles]

One thing to check before going ahead - when in the granny ring at the moment, how close is the chain running to the bottom of the front derailleur cage?

A smaller granny ring will drop the chain a touch lower, and assuming your FD is set up nicely so the cage is just above the chain ring when in the biggest ring, you won't be able to lower the FD clamp on the seat tube.

(Going from a 28t to 24t granny would lower the chain line by almost 15% of the 28t granny ring's radius)

 

[lpretro1]

Also be careful that a very small inner ring doesn't foul on the frame - all depends on the angle of the chainstays. If you have too big a jump down from bi to small ring it may well just start chucking h chain off. Is it not easier/cheaper to just get a bit fitter for those hills - I'm no great climber and by no means a youngster but I manage alpine type cols on a compact front with a 11-32t cassette on the rear?

 

[raleighnut]

I'd go with @ChrisEyles comments on this

One thing to check before going ahead - when in the granny ring at the moment, how close is the chain running to the bottom of the front derailleur cage?

A smaller granny ring will drop the chain a touch lower, and assuming your FD is set up nicely so the cage is just above the chain ring when in the biggest ring, you won't be able to lower the FD clamp on the seat tube.

(Going from a 28t to 24t granny would lower the chain line by almost 15% of the 28t granny ring's radius)

you may need to change the whole crank, or all 3 rings.

 

[Tim O]

Again, thank you all. Chris, I've just checked the gap between the chain (when on the inner ring) and the bottom of the front cage and it's a good 2 cms. Enough? Also, I've got bar end, friction shifters which, of course as you know, give a certain latitude for micro-adjustment on the bike.

Ipretro1, point taken about the fouling on the frame, thanks. Also you're a fitter man (or woman) than I am! I've just got this bike and its heavier, being steel, than my other road bike which is aluminium and set up for sportives. I wanted something I could haul stuff around on and do light off-roading and it is pretty superb in this respect. Here on the Isle of Wight the hills can be pretty steep but not, I grant you, like an Alpine Col. You must be weighed down if you're touring and I simply can't imagine doing it with that ratio! I'm afraid to say I hit the big 7oh next year and the knees aren't what they once were. You'll probably make me feel even worse by telling me you're 80!

Cheers to all, Tim.

 

[ChrisEyles]

Let's see.... bike chains have a 1/2" pitch, so:

Current 28t chain ring has a circumference of 14" and a radius of 2.23"
New 24t chain ring circumference 12", radius 1.91"

Difference between the two radii is around 0.3" or 8mm - this is how much your chain line should drop if you swap over the chain rings. So you should still have plenty of clerance between the chain and bottom of the FD cage.

Nothing wrong with having a nice low gear in reserve to get you up the steep bits without your knees popping right off! I'm only in my 30s but I've still been very grateful for the 28/32 low gear on my tourer at times

 

[slowmotion]

I went for a smaller chain ring, from 30 to 28 teeth, and it works well. I also went for a different cassette but, bang per buck, the chain ring was the better option. I'm absolute rubbish on hills and I needed help. You must check that the new chain ring has the same bolt circle diameter (BCD) as the old one, otherwise it won't fit. A really good source of chain rings (and all things touring) is Spa Cycles. You can usually break the rules a bit as regards tooth capacity of the derailleur. Good luck.
http://www.spacycles.co.uk/

 

[Tim O]

Thanks for that calculation Chris. That's really helpful. I'll post how it goes. And thanks to everyone.

 

[PpPete]

Again, thank you all. Chris, I've just checked the gap between the chain (when on the inner ring) and the bottom of the front cage and it's a good 2 cms. Enough?

I'd say so - I had this problem changing my triple (for reasons that are no longer important a 53-42-30) to a smaller granny. A 28t cleared the cage by a few mm, but a 26t fouled it when towards the bottom of the cassette.
I changed to a bespoke 50-38-26 later and was able to move the mech down enough to cope with it.

 

[lpretro1]

Again, thank you all. Chris, I've just checked the gap between the chain (when on the inner ring) and the bottom of the front cage and it's a good 2 cms. Enough? Also, I've got bar end, friction shifters which, of course as you know, give a certain latitude for micro-adjustment on the bike.

Ipretro1, point taken about the fouling on the frame, thanks. Also you're a fitter man (or woman) than I am! I've just got this bike and its heavier, being steel, than my other road bike which is aluminium and set up for sportives. I wanted something I could haul stuff around on and do light off-roading and it is pretty superb in this respect. Here on the Isle of Wight the hills can be pretty steep but not, I grant you, like an Alpine Col. You must be weighed down if you're touring and I simply can't imagine doing it with that ratio! I'm afraid to say I hit the big 7oh next year and the knees aren't what they once were. You'll probably make me feel even worse by telling me you're 80!

Cheers to all, Tim.

Female 5ft 3" - not 80 - but not that far behind your good self. Normal road riding - compact 11spd with 32t lowest sprocket. For touring I have confess to resorting to my Shimano 105 triple (50-39-30t) with a 28t sprocket I live in Lancashire where we know hills very well so when I'm off for any alpine stuff I just spend my time going up and down em' - its' the only way - changing bike around can only do so much

 

[Tim O]

Just an update in case anyone is thinking about doing the same thing. I had a TA 24 T inner ring put on yesterday and went for a decent spin. Just what I needed. It works perfectly. No problems shifting between the rings. Thank you all for your very useful contributions.

And Ipretro, I know you're right about fitness and I am genuinely in awe of your gear ratios living in Lancashire. My mother was a Lancashire lass and I know you lot are built tough up there (not like us southern softies)! Cheers, Tim.