A question of cassettes
 

I took my bike into the shop for a tune up. They said I needed a new chain, cassette, and one new chain ring. I agreed and they did the work. We decided to replace the worn cassette with a 12-28 PG-950. After I got home (after the shop was closed) I did some research on the PG-950 and found out that it can be very hard on hubs with an alloy freebody. I went back to the shop today and asked them to put on a PG-970 or Shimano cassette instead, cause I was worried about the PG-950 damaging my freebody. They didn't have any PG-970s in stock, only PG950s or 990s, and a bunch of Shimano mountain cassettes. I was told they could order a PG970 in and it would arrive on Tuesday or Wednesday, but I use this bike to commute to work 5 days per week, my wife is going to her parents for Christmas on Wednesday, so if the part had been late at all, I'd be both without transportation to work (I suppose I COULD take public transit!), I'd have no way to get my bike home, and then I'd have a hard time getting to the in laws (I'm planning on biking there, it's only about 60km). They did have a Shimano HG80 11-34T (identical to the cassette that came off the bike), so I got that put on. I didn't really want quite such a wide ranging cassette (I rarely use the 30 or 34 tooth cogs), I often find the spacing between cogs a little too wide. I wanted something like a 12-28T, but at least this way I wasn't worried about it damaging my bike. So my question is: Would you have just kept the PG950 and hoped that it didn't damage the freebody?

For what it's worth, I got very good usage out of that chain, chainring, and cassette. They have between 5500 and 6000 kilometers on them, and the last two months have been on rainy and frequently gritty roads. I don't feel that any of those parts owe me anything; but this has increased my resolve to buy a bike repair stand and some tools an do the work myself.

Cheers,
Brad

I would put the old one back on until the one you want arrives.

Seems like low miles for replacing a cassette and chainring. I have 20 year old bikes with 10's of 1000's of miles on them that have never had a chainring replaced. I get about 8-10,000 miles out of a cassette, usually only the "most used" cogs are worn. Chains I'll get 4-5K out of, although I have moved up to 10s on my commuter and will find out next season what kind of life I get from those.

First off, you're best off asking these kinds of questions in Bicycle Mechanics forum.

You have an aluminum alloy freehub? What kind of hub is this? Aluminum freehubs are a bad idea with most cassettes. For a short while Shimano used an aluminum freehub for their upper-end 10-speed stuff, but the splines were taller to prevent the cogs from digging in as much. These were only compatible with 10-speed cassettes because they contained corresponding deeper grooves. I would guess that a cassette with a carrier would minimize the larger cogs from digging in, but the PG970 doesn't have a carrier, it's just a collection of loose cogs like the PG950. (Edit: I can't tell from pics online whether the PG970 has a carrier.)

Which cassette did you have on it before?

Both the 950 and the 970 have partial carriers, the 'top' two cogs and lockring come loose. Have run both, with no problems.

I have YET to have a cassette ruin a freehub, either.... I'd be suspicious of your 'source' on that.

+1 Damaging a freehub is not a real issue. BHDavis1978, what hub do you have on the rear?

What happens to the cassettes that shops replace after a few thousand miles? I'm sure there's a reason to do so, but I'm also sure I could be happy using cassettes that shops consider worn out for the rest of my life. I've been riding about 4000 miles a year for the last 4 and a half years, and while those miles are spread out over a few different cassettes, I have yet to replace a cassette because I thought it was worn out. I'm more conservative with chains, replacing those every 2500 miles or so (based on wear measurements), but the cassettes seem fine to me.

My bike is a 2009 Jamis Aurora Elite, so I believe the hubs are Shimano Tiagra 32H hubs. The previous cassette was a Shimano M770 11-34T, so the replacement cassette has exactly the same configuration, just a lower end model I think. I believe I read about the problems with the PG950 cassettes off bikeforums.net from a google search, but I could be incorrect.

Anyway, the guys at the shop said that the cassette was okay, but that the chain was really stretched, and that if I changed the chain then I needed to change the cassette and the chain ring at the same time, otherwise it wouldn't mesh well.

I don't understand this line of (common) LBS reasoning. Some tell you to replace your chain often so that it won't wear out the more expensive drive train parts, which makes sense to me. Others tell you that you should replace everything at the same time, in which case I would think you may as well run it all until it won't shift tolerably.

I admit that my understanding is thin, but I don't understand why a cassette that isn't badly worn wouldn't work well with a new chain.

Did they give you any trade-in value for the old cassette?

http://sheldonbrown.com/k7.html#aluminum

Your freehub is steel, as most are. You can run any compatible cassette and not be concerned about damaging the freehub.

Chains should be replaced before 12 full links stretches past 12 1/8". Doing so will allow your cassette to wear less quickly and it will last through several chains. Elongated chains will grind away at cog teeth so that they mesh better.

http://sheldonbrown.com/chains.html#wear

I have 28,788 miles on my bike since 5/15/06......I replaced the large chainwheel at 13,765 miles and had to again at 24,938 miles...I usually get about 2,000 miles on a chain and usually get 3 chains per cassette. This is on Dura Ace 7800 drive train (triple) ten speed....

probably what I would have done too.

Sounds like a beautiful job of upselling by the bike shop.

Just a suggestion, measure your chain monthly and replace it when it exceeds 12 1/8" for 12 links. The cassette lasts a lot longer that way, and by now you should have an idea of the chain / cassette price ratio.

Agreed. The chain mileage seems reasonable, but it seems low to need a cassette replacement (although wet road grit can be a significant wear factor). Best way to tell is to put on a new chain and then go for a ride and see if it skips on the old cassette. If it does then replace the cassette, otherwise it's good to go until you replace the chain again. I just make sure I have a spare cassette available when I replace the chain so I'm not stuck without a bike in the meantime. Finally had to replace a chainring, but that was with almost 150,000 km on it

Yes, I would've kept the PG950. Any cassette with at least one thin cog can dig into an alloy cassette body, which includes the PG970, so you wouldn't have accomplished anything. The 950 actually has all its thin cogs bolted together, too (1.5mm hex key from the backside) which helps transfer the load from any single cog to its neighbors.

Anyway, moot point with a Tiagra hub. As the others noted, it's got a steel freehub body.