How do you know when to replace a tire
 

Due to skidding and such. Do you just watch for it to be worn through to the layer underneath the rubber surface? Also, my rear wheel has numerous visible cuts due to glass, etc. Does this merit replacement?

the tread will square off/flatten in the spots that are in contact with asphalt when skidding.

one way to lengthen the life of a tread is to get more comfortable with the rhythms and flows of traffic around you. In doing so, you can better adjust speed/control by 'seeing ahead.' Long skids are pretty much for fun/competition/bizarre mating rituals. Most effective is skipping to reduce speed .
*good tips here: http://www.63xc.com/ *
This is by no means a static event - I think anyone that has ridden a bike on a street numerous times get a feel for the overall flow. Watch out for armored columns of Soccer moms and unseen teenage nervebags. Or just anyone for that matter come to think of it. Awareness in your surroundings when riding is key, regardless of choice of bike. Some fix/streettrack artists (ha!) in here often dub this sensation "zen" when in fact I think that can be dangerous. To be so wrapped in the excitement of no hearing derailleurs of brakes grated on machined sidewalls is a wonderful thing. I like the simplicty as my equipment is less prone to failure (less maint to perform allows me the ability to become obsessive/compulsive. I clean my chain like I was brusing and flossing teeth!). I like hearing beyond the ride at hand: I am aware of what is behind me without looking and an get a good idea of open lanes and traffic. Not a solid idea, but a better one than if a freewheel/cassette is buzzing in my ear. But everyone has a preference: tires, fixed, track, geared, sober, Jesus, chocolate, good whiskey, lemons, wool, etc.
Wow - hangovers make me ramble...

But there isn't a 'tread' on most road tires...

Hmmm...so what are they called then?
A surface pattern?
Crop circles?
Weird formations naturally occuring via electromagnetic discharge during the sales process?

trust me, there are treads on tires. Some will be smooth, some will be more aggressive, some will look like grandma or grandpa. But they are there.
Friendly example:
Treads now available!

The tire will have a harder (typically "carbon") rubber coating the skin wall. When you start seeing the skin wall through the rubber it's time to start saving up for a new tire. Gradually you'll find that tire starts to feel pretty squirrely, that skin wall isn't so great for control. Find your own comfort level when it comes to replacing.

BUT if you start to see the tire threads, the gigs up and you must replace.

Geeze!

Flippin' idiot!

uh!

A primary concern should be that you have grip on the road. If there's not grip, get a new tire.

Yessssss

tee hee

What about the little cuts?

Is debris going to sneak inside and give me flats?

YES
GET A NEW TIRE.

See? That was simple! And you'll be better off if the tire has that much wear.
Or just carry a spare tire (yup, you read it right - spare tire) in whatever you carry crap around in on a bike.

Do the little cuts go all the way through the tire or are they just in the surface? If it's just the surface, no worries. If it pierces the skin wall, stuff may find its way in or your tire could even come apart.

Roadie tyres get slashed to hell on city streets. Just watch for really ugly cuts: if there's bulging like the stuff inside wants to get out, it's time to replace the tyre. And if you've skidded the gip surface off and can see the casing, it's past time!

Meantime, swap front and rear tyres around to get the most out of them ;)

Don't swap your tires.

You're taking a tire off the rear because you're afraid that it's past its useful life and you're putting it up front on your control surface? Genius.

I like to switch front to rear and then get a new one for the front. This has always worked for me.

A good way is if your slicks no longer have a rounded profile, such as having the tire worn enough that you have a visually identifiable flat portion along the center of the contact patch.

At least that's how I go about reaplcing tires on my roadie.

Thanks guys.

Yup, it's fine to shift the front to the back, but you shouldn't do a straight up swap between the two.

only if money is tight and you are doing it for a few days.

Pop a Band-Aid over the cut. Preferably a cute one with disney pics.

just not pinocchio, you know the ink pattern on the pinochio one doesnt offer the superior traction of the lion king band-aids :p

Actually, booting a tire is a useful short-term fix, but sadly the boot goes on the inside, between the tube and cut tire casing. So if you do it right, you don't get to see your cut band-aid at all. So sad.

I ride mine till I see the threads through the treads or if cuts go through. Rotating your tire to change skid locations helps. Look on the inside of the tire if the cuts don't go through then your fine...

Moral of the story, tires are cheap but broken bones and funerals are expensive.

I replace my tires when I can see the tube through holes/abrasions/wear/skids in the tire... :o

Right. If you have cuts that are through the cords, change the tire right away. If the nicks don't go to or through the tire, you're probably fine. If a tire is looking pretty rough, move it to the back and put a new one on front. If a front tire blows while you're cruising, your SOL, but if a back one goes, you'll probably be okay. You'll just have to walk it home.