I've been thinking about occasionally commuting to work on my mountain bike. If so, I would need to switch out the knobby tires for some slicks.

In this vein, I have also been toying with the idea of keeping two sets of wheels: one with knobby tires and the current mtb cogs and one with street tires and a slightly higher set of gears for the commute on the flat road. I figure that way I can commute to work during the week and still go trail riding on the weekends without having to change the tires repeatedly. (I can just switch out the wheels instead)

Aside from the expense of new wheels and gears, the only other drawback that I can see is that the chain might wear more quickly being switched betweein two different sets of cogs. Is this a legitimate concern?

Any comments or suggestions are appreciated.

Cotton - I think it would be better/cheaper to invest in a second bike, plus you may have chain length problems switching between different rear cogs depending how different the cogs are. You would need a chain big enough for the largest cog on the biggest set. A second set of wheels, tires, and cogs is expensive and a hassle to always switch in/out. You could probably pick up a used Hybrid or road bike fairly inexpensive. A hybrid or road bike will move you into 700c wheels that are faster than mtb 26 inch wheels with slicks. My 2 cents.

Whilst switching wheels is not a bad idea, you may want to commute on a practical machine, fully equipped with lights, fenders, luggage rack and reflectors. It can be a real hassle swapping all this out for some weekend fun.
Getting a beater-bike for the commute is probably a more sensible idea. Roadified MTBs make good commuters, if that is the style you like. In practical terms, there is little difference in efficiency between a mid-sized 700c commuter tyre and a good slick for 26".
Beater bikes don't have to be poor quality ones. If you can pick up an old MTB that was high quality when new, it will still ride well.

Using your MTB for commuting is not such a bad idea. MTBs are pretty tough and should handle the 'mean streets' pretty well. The larger diameter tires will give you a comfy ride too.

I would not go to the trouble of getting a second set of wheels. Assuming you don't need to switch tires on a daily basis, just get a set of slicks (I like Hemispere or Nimbus Armadillos) and swap them out on Fiday and Sunday nights. You'll probably find that your current gearing is fine for commuting.

However, if you really do want a second set of wheels, go for it. The last thing I'd worry about is a little extra chain wear. I doubt that you'll have any problem with that anyway unless your current cassette is already in need of replacement.

I have two sets of wheels for just such a reason. I have a friend who likes to ride the paved trails regularly, so I ride with him on my MTB, rather than smoke him on the road bike.
I don't like commuting on an MTB though, the sitting up in the wind is too much of a beating for me.

It's funny how things always seem to happen in pairs or threes. I had a guy at work today ask me exactly the same question.
I also think that you would be better off thinking about getting a second bike rather than a second set of wheels. I advised a friend of mine to get a second set of wheels for his mtb a few years ago. He switched them once or twice and then just stuck with his first set of wheels. Actually changing the wheels was more trouble than it was worth (double the trouble if the rims are not the same and you have to adjust your brakes as well).
If you don't want to get a second bike and if you are not going to be commuting every day then I wouldn't worry about riding on the knobbies especially if you get a semislick tire instead of one with a full aggressive tread. I commute to work most of the time on a touring bike, but once or twice a week I get a lift from work out to some good mountain biking trails so I have to ride my mountain bike in to work. I really don't notice the tire difference as much as I notice the fork bobbing around.
Anyway whichever you think you might do, new tires, new wheels, or new bike, try doing the commute for now on the equipement that you already have.
Cheers, Dave

I'm of the two sets of wheels crowd. Last year I swapped tires back and forth on one set of wheels and that got old in a hurry. I purchased a nice set of Mavic CrossMax wheels that I put my MTB tires on--definitely and upgrade from the stock wheels. I then put my XT cassette on the Mavic rear wheel. On the stock rear wheel, I put a 105 road cassette and some 1.5 inch slicks. Both cassettes are Shimano 9 speed so they work great with the indexed shifting. I run a tight set of gears for the road/commuting 12/24 and a wider set on the MTB wheels, something like 11/35.

Now when I get home from a commute, it only takes me about 2-3 minutes to switch wheels and head out on the trail! Plus, I feel like I have two really high quality bikes that both fit me perfectly, for much lower cost and I don't have to do maintenance on two bikes.

I also don't see chainwear as an issue. The only problem you will have is if your current cassette is in bad shape--and then you need to replace it anyway.

cotton, i would go and invest in a second bike. i have Rockhopper that i'm gonna soon convert it into slicks (as soon as I am able to find one that i could use for trailing too) and use it as my commute from M-Th. then on friday, i would treat myself by taking out my lightweight Sirrus and ride to work.

One potential issue of wheel swapping is that the brake shoes may need adjustment if the rim widths are different. A difference of 2mm is probably OK, but from experience, I know that going from 28mm width on the knobby wheels to 22mm on the slick wheels requires a brake shoe adjustment on my MTB (cantilever brakes).

Winter commuters sometimes switch between studded and nonstudded tires using the wheel switching method you describe. It is less trouble than swapping just tires.

In practice you will probably find you switch alot less than you figured at the start. Lights and fenders needed for a commuter don't provide much value on the trail either.

You can get a new trek 820 cheap to commute on for not much more than what you will spend on wheel, tires, and cassettes. Used beaters are even less and make good commuters.

Dan

for me... i went with the Specialized Nimbus EX 1.5" wide tires. rode my rockhopper to work today. it's a bit bumpy at 80psi... but shaved off 5mins of commute with these tires compared to the original 1.95" Cheng Shin tires.