Ignorant question about changing tyre sizes

[jnb]

I've just got a bike (Giant expression DX) and it's the first time I've been on a bike in a long time so everything has changed since I last looked so anything I used to know is well and truly obsolete.

First thing I thought I should do is get hold of a spare inner tube just in case of punctures, so I duly stop by a branch of Halfords (I know, but they were on route and they're a lot less obnoxious and arrogant than my LBS) only to discover that apparently I can't get inner tubes for the tyres on my new bike. The bike has 700 x 42C tyres (well that's what it says on the side wall anyway) and the local halfords has loads of inner tubes in sizes up to 700 x 35 but not above that. Also I notice that the tyres I have are a lot chunkier and knobblier than the alternatives I saw. Which makes me think that I have tyres more geared to off road use than I need. So ...

Can I (should I) use a different size inner tube to tyre? I'm thinking not but I have no experience and there seemed to be ranges of tyre sizes listed on each inner tube.
Can I change tyre tread and more importantly size to something smaller without changing the wheels, i.e. how much does the wheel rim dictate what tyre size I can use? Most of my riding will be on tarmac with the occasional use of bridalways.

and is there a numpty's guide to newbie cycling here somewhere?

 

[BSRU]

I use Schwalbe SV17 inner tubes, they can cope with 45's, just buy them of EBay.

Use Sheldon Brown's website

Rim/Tyre Sizes

to see what size tyres your rim will take, a nice table near the end will help.

 

[andrew_s]

Inner tubes are stretchy and will fit a range of tyre sizes.
This may be expessed as a list of separate sizes, or as a range - eg 700x25-32 (suitable for tyres between 700x25 and 700x32), or some combination. Anything marked 27" or 28" will fit 700c too.

Any tube you can fit will work, but...
If you try to fit a large tube (eg 42c) in a narrow tyre (eg 23c), it will be awkward to avoid trapping the tube between tyre bead and rim, and you may get a bit of tyre folded over that chafes through in a month or two.
If you fit a small tube in a large tyre, the tube will just stretch to fit the tyre. However, because it's stretched it will puncture more quickly than a large tube would have (it will go flat straight away, whilst large tubes can fairly often give a slow puncture that you only find after you've got home). Stretching 35c to 42c isn't a problem, but an 18-23 tube would probably warrant swapping to a bigger tube when possible.

Whatever tube you buy, make sure you get the correct valve type, and make sure the valve stem is long enough to poke far enough out of the rim to get the pump head on (i.e. at least as long as on your current tubes).

If you want to swap tyres, you are safe going down to 700x32c, or as big as will fit the frame