Cassette Replacement

Typical Road.cc forum. You start off going to the shop looking for Oranges, but when you get there everyone starts telling you that you actually wanted guttering.

Just replace the rear cassette with whatever you have now (Shimano, SRAM, Campag etc). You're likely to get better quality if you pay more too - for price durability I would recommend Ultegra.

From what you have said though, it does sound like maintenance is an issue. Clean and oil your chain every 1-2 weeks (if not more - mileage is probably a better test of when, but regular and often). Dirt gets on oily chain and wears it, which then wears the gear too.

It sounds like you are changing down too quickly to your 11 if it is the worn gear, and as everyone has said, if your cadence is low and you are stressing the chain/gears you will get more wear from that. The base guideline is that you dictate to the gear, rather that the gear dictate to you. I always drop down at least 2-3 gears coming to a junction/traffic lights, if not a whole ring. It actually gets you out of the blocks quicker and at the correct cadence ASAP. If you spin out and then gear up you'll be breaking your 18mph average in no time. Your knees and bike will thank you for it.

If you are in London there is no way you can average 18mph. You may do between traffic lights, but the whole system is rigged to mean that you can go no faster than 12.5mph.

Your car analogy doesn't hold up though - in a car, it's more fuel efficient, up to a point, to drive at lower revs. It's not on a bike. The very fact that the 11 tooth sprocket is the only one that's worn out and you're averaging just 18mph is proof that you can dramatically improve the way you ride... unless you're saying that all your commute is a fairly steep downhill all the way, save for a long period where you're not moving at all? Maybe it's a ladder to get up to the top of the hill.

50x11 on a commute averaging 18mph is like going in 5th gear in car around shopping centre.focus on high cadence.i stick middle ring on a triple on my commute and average 23mph with cadence always around 90rpm

I normally change the cassette with every second chain. I replace the chain when it is close to or at 0.7 mark on a Park CC-3 and if it jumps then the cassette gets swapped too.

for 7 speed cassettes I have used Shimano 12-28 HG50 and HG20 on my commuter but there is a 11-28 version too (HG30, I think). All are solid, durable items. Keeping the chain and cassette reasonably clean and chain lubed will extend lifespan of both. IME the KMC Z82 chain is better than the Z51 and SRAM PC830, both of which must be made of cheese, as they doesn't last long

I can't imagine many people being undergeared with 50x11, but there are plenty who claim to be 'spinning out' on the flat into a headwind and dragging a 18kg trailer with this combo (or whatever). Yeah, right! Make sure you keep your mobile phones on, Dave Brailsford is sure you be calling you with an offer very soon...

That's about 51rpm! Nice quick spin of the gears is the order of the day, or your knees aren't going to thank you, in old age if nothing else.

It might seem difficult at first because you will start stressing the cardio system more than previously, but no worries it's all good.

Unless you live somewhere really flat, I think a compact is better for most of us. You'd need to be pretty competitive to need 53/39.

Fuck, if you're riding 50-11 and averaging 18mph then I fear for your knees! As well as hammering your joints, grinding out the miles at 50rpm also contributes to excessive chain and cassette wear. Riding at a higher cadence does a lot of things - it's more efficient, it's better for you in terms of fatigue and cardiovascular health, and it doesn't wear your components out as quickly.
You need to be spinning at about 90rpm... if memory serves, on the flat in the big ring, you'll be going at around 20mph in the third sprocket down from the top.

Bit confused when you say its on the 'smallest' gear and 'top' gear at the front but you're not crossing it- are you saying you're spending most of your time on 50 at the front and 11 at the back as thats a huge gear especially for commuting?

Either way as its a 7 speed I'd suggest going for a 'tighter' block and sticking with the compact chainset. Maybe 11-23 or even 11-19?

How often did you replace your chain? (or clean it, for that matter?) If you keep using a cassette with a worn chain, the cassette will wear too. You can keep using the same cassette for much longer if the chain is in good nick. I don't think you'll find that different cassettes wear much better or worse than what you had (except that a 10 or 11 speed are thinner and will wear out faster if mistreated)

More on this topic here:
http://sheldonbrown.com/chains.html#wear

Other things that I'd wonder about - this is the smallest gear? So you spend all your time in top gear on a compact? Either you should be on normal 53/39 cranks (giving you more gear options at the top) or you're riding that cross geared (34x11) which would cause excessive wear - you'd be able to hear this!