chephy

There is a very simple formula to figure out what sort of numbers different combinations of cogs and chainrings give you. It is: (d*r)/c where d is wheel diameter in inches, r the number of teeth on chainring and c the number of teeth on the rear cog. The result is referred to as gear-inches. So for example if in some gear you get 70 gear-inches, that means a full pedal cycle (one rotation of pedals) in that gear will move the bike as far as one rotation of a 70-inch wheel (namely pi*70 inches).

So, the bigger the number, the harder the gear and the smaller the number, the lighter the gear. Knowing this simple formula you can calculate and compare gear-inches yourself and see exactly whether replacing the cassette or one of the front rings will give you better ranges and by how much.

WillisB

I have a 28/38/48 crank and 11X34 8-speed cassette on my commuter. I couldn't ask for anything better. My 40 mile round trip includes a total of 3125 ft of climbing. There are times when I am tired that I love that 28X34 Granny!

George

Hi WillisB,did you have to add links to make it work.Thanks

WillisB

I had a wheelset on this bike with an 11X28 cassette. The new cassette is on my new wheelset with no changes in chain length. However, I cannot use the big ring - big cog combination. The chain is too short for that. (Not that I would anyway). I will eventually add a link to take care of this problem as a just-in-case.

cyccommute 

Everything I've ever read, and my own experience, has always said to change the chain and cassette at the same time. Even short duty cycles on my chains (changing length with gearing, upgrading chains, etc.) of only a few hundred miles have resulted in chains that skip on certain cogs, i.e. the ones I use most. Until now I've never run across anything that said otherwise. Based on my experience, I still feel that carlton is lucky to not have a chain that pops all the time. Perhaps where he rides, he doesn't put as much torque to the chain as I do because he doesn't have to climb mountains on a regular basis.

I don't use chains that are excessively worn since I don't relish the idea of walking out of the woods while mountain biking nor trying to ride a single speed while touring. I don't keep track of how long I use them since I have several bikes that I use all the time but I do check them for wear on a regular basis.

Al1943

Not only will you need a long cage derailleur but if the cassette has a cog larger than a 30 you will need a mountain type derailleur. The longer cage is to handle the additional chainwrap, the mountain type derailleurs have upper parallelagrams designed to handle larger cogs, two different issues.

Al

carlton

George-- If you want to try out a differrent cassette go for it. If you don't have a bike stand just flip the bike over on the seat and handlebars and run the bike through all the gear combinations after changing the cassette. If it doesn't bind up and shifts ok give it a spin around the block. If your chainrings are not 3 seperate rings bolted together you will probably be better off changing the cassette. The hill country will be fun. Hope you got some of this 70 degree, bright sun, no clouds, no wind weather over your way today. Happy New Year.

George

Hey thanks carlton,my son works down in your area sometimes and he wants me to go down there,but I looked for state park bike paths, but the ones I found weren't very long. One more question before I order the new cassette. How can I tell if the chain is to short,(still learning) I'm thinking I'll try the 11-34 or the 12-34. I just put a Sram PC 48 chain on, what size would I need to do this thing right. Thanks everybody for your replys and have a happy new year,George

Retro Grouch 

Here's the problem with the too short chain. It'll only bite you on what otherwise would have probbly been the best cycling day of your life.

Say you're breezing along with a tailwind in your big chainring. Everything is going your way. You come to a little hill and you decide to just sprint up it. You shift down and feel a little resistance but attribute that to the hill so you bear down a bit more.

SURPRISE!!! That resistance that you felt was your chain not wanting to go onto the biggest cog. Something had to give and the easiest thing was the derailleur cage which is now lodged among your bent and broken spokes.

carlton

George--I believe you current setup is a 11/30. If your current chain is the correct length and it should be if every thing shifts right and you go with a 11/34 you would need to add 4 links to your current link count. 5 extra links should do it on the 12/34. Buy a 8speed chain since you have a 8 speed cassette. Most new chains are to long when you take them out of the box. You will need a chaintool to trim it to fit. Your LBS will trim it for a couple of $ if you don't want to do it yourself. You will also need a couple of tools to remove the cassette. A chainwhip and a cassette lockring remover. If you go to Park Tools website they have an excellent do-it yourself section. It shows how to do it and what tools you need.

I ride mostly on the street here in CC. Along the waterfront for the most part. There is an area just below Lake Corpus Christi that some of the locals go to trailride. Sorry I don't know much about it.

BluesDawg

This goes against all I have heard or experienced. Definitely change the chain whenever you change the cassette or freewheel, but if you change the chain before it stretches, you should go through a few chains before needing a new cassette or freeewheel. Stretched chains are what wears out cogs.

carlton

I see my good neighbors to the north have also figured out the effective, cheap, and ez route to driveline maintence.

Happy New Year.

carlton

"Stretched chains are what wears out cogs." It would be pretty hard to say it any better than that.

Happy New Year.

George

I called a few places and all I could get is the 11-32. I'll put it on tomorrow and I hope it works out. Thanks for all the help.

Michel Gagnon

I read the original message. You live in Texas, which, if my memory is correct, is rather flat or gently rolling, but with a few steep climbs here and there.

I would probably keep the original cassette, unless it's worn out. If you want to change the cassette, check that the upper pulley of your rear derailleur can be adjusted to float down 4-5 mm away from the "30". My guess is that you might not have enough clearance there to run a 32 without lots of noise, and certainly not a 34. To use these larger cogs, you would likely need a mountain derailleur (ex.: LX or XT), which also has a long cage, but which moves DOWN as it moves inboard.

On the other hand, I would suggest you replace the small ring with a smaller one. You could go for 26 or even for 24. I see four benefits of going that way:

- No front or rear derailleur compatibility issue. IOW, no more money to spend

- Your chain will be long enough. No need to change it, until it's worn and needs to be changed (i.e. as usual).

- You'll get much lower gears that way. Basically, with your present 28/30, you have 25,2 gear-inches. Alternative gearings would give you:
28/32: 23.6 gear-inches (front / rear, assuming 700c wheel)
28/34: 22.2 gear-inches
26/30: 23.4 gear-inches
24/30: 21.6 gear-inches

- By having 48-38-24 in front, you'll keep relatively close gears for flat or gently rolling terrain, which is likely to represent most of your riding. And you'll have the really low gears when you need them.

Barabaika

I think the algorithm of selecting a cassette should be like this:

1. Find a "sweet spot" - the most used chainring/cog combination in your current cassette.
2. Find "extremes" - the maximum and minimum chainring/cog combinations that you use for ascending and descending the steepest hills in your area.
3. Get the gear ratio from www.sheldonbrown.com/gears

What we get for your bike:
For 700 X 35 / 35-622 tire with 175 mm cranks
With 8-speed "ak/an" 11-13-15-17-20-23-26-30 Cassette
48 38 28
11 8.6 6.8 5.0
13 7.3 5.8 4.2
15 6.3 5.0 3.7
17 5.6 4.4 3.2
20 4.7 3.7 2.8
23 4.1 3.3 2.4
26 3.6 2.9 2.1
30 3.2 2.5 1.8

So your lowest gear ratio is 1.8.

4. If you get an 11-34 cassette, the lowest gear ratio will be 1.6, not a big difference.
5. If you buy a new mountain crankset 42/32/22, the lowest gear will be 1.3.
6. Now you must consider the "sweet spot". For example, you like the 48/20 combination. The gear ratio for it is 4.7. It's better to have it in the middle (in other words, to create a straight chain line from the front chainring to this cog) of a new cassette and don't have big gaps between it and the adjacent bigger and smaller cogs in the number of teeth.
7. The highest gear ratio is 8.6. Do you really use it? Maybe, it's better to get a cassette that starts from a 13t cog.

George

After getting the new 11-32 cassette I was anxious to put it on. The specs on my bike said I had a 11-30 on my bike,well it had a 11-32 on it already. All I can think of is the LBS upgraded it to the terrain we have around here. I took the other cassette back and talked to the mechanic. He seemed to think that I could put a 11-34 on with no new chain or adding links because the chain I put on was setup for an 11-32 anyhow. So I guess I should try what I have in the hill country and see how I do. If it doen't work out then I'll try the 11-34, if that don't work I'll start on the chain rings. They told me that I couldn't just change the cogs out, but the one I took back fell apart when I was putting it back in the box. That got the wheels in my head going again. If I knew what gearing would work smooth I'd probably try and build my own. I better just leave it alone. Thanks again everybody.

chephy

I believe you but I guess we have completely different sources and experiences, because everything I've read and my own experience has always said to change chains before they stretch too much just so you don't have to change the cassette at the same time. Sheldon Brown, CAN-BIKE cycling manual and various bike mechanics books I own all basically say that you'll only have to replace the cassette if you've left the chain on for too long. Given how chains are much cheaper than cassettes, it makes sense to replace them more often (it's also less work, of course).

I've yet to encounter any book or website about mechanics or cycling in general that recommends always replacing chain and cassette together. Maybe that's the only thing that works for your situation (lots of hill, lots of mashing, lousy chains, lousy cassettes?) - I am not telling you what you should do. But this chain + cassette simultaneous replacement is far from the general rule you're making it out to be.

Count me another lucky one. The cassette on my current bike has had who knows how many chains on it... at least two were put in by me and I bought it used, so the previous owner probably used up a few chains on it as well. Runs very smoothly.

Perhaps. That's the case for me as well. Toronto doesn't have mountains. Even when climbing I often spin, and that's good for the drivetrain.