Choice of cassette

[Smoothhound]

Hi Folks,

Hopefully a quick and easy one: I need to replace my 8 speed cassette on my commuter hybrid (shimano HG50), even if I stick with the HG50 it comes in a variety of teeth combinations, would I be alright with any of them?

My current is 12-25 and I never use the big sprockets, so I was thinking I might go for the 12-21 and have the gears 'closer'?

Apologies if this has been answered before but I couldn't find anything conclusive when I searched.

Cheers

 

[jimboalee]

What rings do you have?

Chart every combination and see where the chainring change is.

There should be a smooth, progressive increment in gear lengths.

Some cassettes give a less than perfect series of ratios.

 

[andy_wrx]

You will be alright with any Shimano or SRAM 8sp cassette, you don't need HG50 specifically.

Yes, you can replace with 12-23 or 12-21 or whatever and you'll lose your smallest hill-climbing gears, but you'll have smaller gaps between those you have left.

12-21 has 12, 13, 14, 15, 16, 17, 19, 21
12-23 has 12, 13, 14, 15, 17, 19, 21, 23
12-25 has 12, 13, 15, 17, 19, 21, 23, 25

If you swap to a 12-21 from 12-25 your chain will be a little bit long, but particularly if you have a triple with a long-cage rear derailleur ('mech') it will take up the slack.

I run a SRAM 12-26 on my 8sp Winter bike for general riding, but a 12-21 Shimano cassette on my TT bike as I do flat TT courses rather than hilly ones so don't need the lower gears and want less gaps between gears so as to get the right cadence.

 

[Smoothhound]

Thanks for the info,

My commute is pretty flat so I've no problems with losing a couple of hill climbing gears and it is indeed a triple with long cage mech so technically it should be fine

However I'm not sure I understand the bit about how the chainrings affect the best choice of cassette, I've got a 48-38-28 which, using Sheldon Browns gear length calculator (in inches) for the proposed 12-21 gives me...

107.0 84.7 62.4
98.8 78.2 57.6
91.7 72.6 53.5
85.6 67.8 50.0
80.3 63.6 46.8
75.6 59.8 44.1
67.6 53.5 39.4
61.2 48.4 35.7

Versus this for my current 12-25

107.0 84.7 62.4
98.8 78.2 57.6
85.6 67.8 50.0
75.6 59.8 44.1
67.6 53.5 39.4
61.2 48.4 35.7
55.8 44.2 32.6
51.4 40.7 30.0

And I'm not sure how to tell which is better?

Cheers

 

[andy_wrx]

No, I don't understand what he's on about either...

If the discussion was about the pro's and cons of a compact chainset vs. a triple it would make sense....except that's not what you're asking !

One of the arguments for a compact is that if you run say a 50/34 compact chainset and 12-27 cassette, you have kind-of the same gears at the extremes as you would with a triple with 52/42/30 and a 12/23 cassette , but it's only a compact so it's lighter, easier to set-up and 'looks nicer'.

But the counter argument is that a 12-27 cassette has bigger gaps between the cogs than 12-25, so you might find yourself missing that 'in-between' gear you haven't got, where the one above is slightly too high yet the one below is slightly too low.

And - this is the point (finally !) - there's a big gap between 50 and 34 on the chainrings, so with the 17 on the back dropping from the 50 chainring to the 34 chainring would be like staying in the 50 but changing from the 17 to the 25 on the cassette, i.e. a big jump of 3-or-more gears-worth on the cassette
- thus on a compact you can find yourself needing to change from the 'big ring' to the 'little ring' but also having to shift up a couple of gears on the cassette so that your legs don't suddenly start spinning madly (in practice it's less of an issue changing up the other way).

But I don't see how that would happen with you changing from a 12-25 cassette to a 12-21 : you'll have smaller gaps between cogs on the cassette, changing from one chainring to another will mean you have less of a jump than you had before.

You will end-up with lots of duplication, i.e. you'll have gears covering 85-61 inches on both outer and middle rings, 62-48 on both middle and inner, but you may or may not think that's a problem.

Where a problem could lie is if e.g. you were changing-down the cassette in your big ring, went down to the 75.6-inch gear, then needed to change down further
- in theory you shouldn't use the extreme-opposite gears, i.e. largest cogs on the cassette v.s big chainring and smallest cogs on the cassette v.s small chainring, because you're pulling the chain diagonally-across and it doesn't like that, increases wear on the chain and gearteeth, could cause the chain to fail if you were really heaving on it, etc
- so you now need to change down to the middle chainring, which gives you the 59.8-inch gear : is that low enough ? If not, you might need to change down again to the smaller ring - but that'll be a 44.1-inch and if that's too low you'll end up changing up the cassette....

Thus perhaps now whilst you might think you never use your lowest gears, in fact you may never be using the little ring at all, you might find you end up having to use all three rings and shift up and down the cassette a bit as you do it.

Maybe...

But if you're using it as an urban commuter on the flat, I can't see it - you'll probably just stick it in the middle ring and leave it there...

 

[swee'pea99]

All this stuff about compacts and inches has always been way over my head, but two 'dumb' thoughts that may be relevant or even helpful...

I went to some lengths to change from a five to a seven speed block on my back wheel, and it's been a massive boon - the closer ratios (aka smaller increments) between the 'middle' gears makes it much easier to keep a good cadence at and around 'average' speeds (almost always on the flat); but you can have too much of a good thing: for a while I rode with a 'racing block' on the back, that had just one-tooth increments, as against the two-tooth ones I'm used to, and I found it frustrating rather than helpful - the difference between one gear and the next was just too small. It wasn't a smoother ride so much as a more irritating one, having to change all the time.

FWIW. Probably bugger all.

 

[Smoothhound]

Thanks for all the info, I'm beginning to get head round the pros and cons of various set ups. I am planning to buy a road/audax very soon - so this is also very useful for understanding the gearing choices.

I've been paying attention to which gears I actually use over the last few days and it's looking like I'll probably go for the 12-21 as I almost always stay in the 60 to 90 inch gears and I'd end up with more choice in this area. I have been working on keeping my cadence up so hopefully I'll actually notice the difference

 

[MadoneRider1991]

i would go 12 - 23

 

[PpPete]

I almost always stay in the 60 to 90 inch gears

In that case you might be better off with bigger chainrings - here's why:

A triple gives you the opportunity to minimize double shifting - if you set it up so that you can get your most commonly used ratios whilst staying on middle chainring.
With your 48-38-28 chainrings you are always going to be changing both front & back to find the best ratio within your 60-90 inch range - whatever cassette you put on the back.

For a commuter bike you may find its better to simplify things (esp. thru traffic ?) by eliminating double shifting and not being fussed about having smallest possible gaps between gears.

For a road bike (or indeed a tourer) the ability to get the absolutely spot on gear is maybe more important than eliminating all the double shifting.

BTW - what happened to the fixie crowd ? someone's usually along to suggest that "route to paradise" just as soon as we all start discussing ratios, inches & the like.