Cyclocross

I wouldn't like to race 'cross on a modern road bike. You really need mud clearance, canti brakes and 32mm knobbly tyres. Lots of CX races will allow MTBs, so why not beg, borrow or buy a cheap one?

The more practical 'cross bikes like Kinesis, Kona can also be used as a winter trainer / light tourer so you could argue it can be a good idea. And anyway, you can never have too many bikes

The idea that you can convert a road bike for cyclocross comes from the days when road bikes, at least the ones ordinary amateur riders owned, were made of steel and had a bit of extra tyre clearance. You know, back in the mists of early pre-history - i.e. the mid-1990s.

As pward says, if you can find a local club with a cyclocross team, you might be able borrow one. Even if they don't have specific club bikes for newbies, most of the regional leagues have moved to separate races for vets and seniors, because of the explosion of popularity of the sport. So there is a reasonable chance of a spare bike being available, especially in September, before the courses get so muddy that pit-bikes are needed.

Also, Edinburgh Bicycle Coop have some very cheap 'cross bikes. They are so cheap that some of the components will probably break within a season if you actually race them without upgrading, but if you do one race and quit you'll have a decent commuter to either use or sell.

Converting a road bike is nigh on impossible, there simply isn't enough room for the tyres and then the mud would clog under the brake. Find a used CX bike and use it as your winter trainer.

Another thing i just thought of if you were going to use a road bike, get rid of any bottle cages cause your going to have it on your shoulder alot running uphill

Rather than trash your road bike, you might consider second hand or try to borrow a dedicated CX machine through contacting a local club that does this? They really can be(have) quite differently.... just a thought. Even if it's not for you, they make great Winter commuter machines.

You do see the odd person having a go on a road bike but it's mud clearance that's the real issue there. if it's dry, you'll get away with it if you can squeeze a cx tyre in.

if you have a mtb or a hybrid you're better off using that, most local races will have you whatever you're riding...

Bigger tyres with plenty of grip to them and some cantilever brakes are the main changes you'd want to make, but on road frames you'd have to modify the frame to take them

Road tyres aren't going to get you anywhere and road brakes will just clog