Thanks chaps. Can you give me some other names so i can check their tools out? you mentioned Draper which i have heard of, would the Halfords professional ones be any good or are these for use on a car?
Try looking for Norbar - they make them rather than just putting stickers on them, unlike Park. Torque wrenches are either ratchet or beam type - the beam are slightly more accurate, but cost more and not really needed for bikes.
It's also worth deciding whether you need a torque wrench at all. There's only two types of bolt on a bike - those that mustn't be over tightened and those which really need cranking up.
For the first, develop a degree of sensitivity and always use a short(ish) allen key. Nearly everything that uses a 5mm allen key only needs 'firming' up. For critical things, like stem / handlebar clamps, use blue loctite rather than risk over tightening them.
For fitting bottom brackets (especially Italian threaded) and crank nuts, I don't think you can over tighten them with even a standard length mechanics spanner. Just heave till your eyes pop.
It's also easy to use the 'foot-pound' values to make sure you are in the right ball park. Forty foot pounds of torque is a sweat to achieve with only a six inch spanner. It's the same effort as lifting an eighty pound weight. Whereas five foot pounds with a long 12 inch T- allen wrench is no more effort than lifting a couple of bags of sugar.
Depends on the materials of frame and parts to an extent, but think I'm going to buy one just forpiece of mind. The more cost effective ones are a better option that scrapping a £50+ carbon post (or even worse a £1,000 carbon frame) because a bolts been overtightened.
Torgue wrenches vary greatly in accurassey. Practical Classics did a test on the smaller ones, what they found was that a lot where accurate in the mid range, but few where accurate in the lower ranges, which is where you will need it for the bike bits which are critical to easily stripped threads!
I agree that it is beneficial to develope a 'feel' for tightening up the variuos parts of bikes. Relying on an innacurate torque wrench (perhaps one from argos or somewhere similar) to tighten up a 5mm bolt into an alloy thread could well mean a stripped threa while waiting for that click. A skilled mechanic should have a feel for just tight enough and a bit more to lock.
Torque wrenches are very critical in automotice engine building and such like, where you are mating large flat faces together under a lot of pressure and subject to warping, such as alloy cylinder heads.
I use torque wrenches for vehicles regualarly, have never used one on my bike.
We have torque wrenches to assemble elements of our instruments, but they cost well into the 6 figure bracket, so a few hundred quid on a decent tool is nothing. On a bike I do it by feel, and I've never stripped a thread (famous last words!)