I have an old Carlton Grand Prix, i can't find too much information on it but i beleive it is from late 70's early 80's. It's pretty sluggish and heavy, but the steel frame feels pretty sturdy.

Anyway, on my commute this morning i took a detour and ended up off road and down some muddy trails.

It was a bit sketchy on slick tyres, but both me and bike survived, and i must admit it was good fun.

So if i was to fit some knobbly tyres or even semi slicks would it make a suitable cx bike, or am i asking for trouble?

calliper brakes and mud clearence would be the biggest problem imo.

As thomthumb says you need to check the clearances – not just on the brakes but on the frame as well. On my Peugeot road frame (1985 vintage) 25s are the widest tyres I can get in the back.

If you can fit fat tyres in, check out this for info on caliper brakes off-road.

A mate uses his old Pug road bike on the HONC etc every year. Chaff all clearance for even the slimmest of knobblies, but it seems to roll on just fine.

I often use my hack with 23mm slicks to pootle around off road. Tightens up your handling skills no end, you have to be super wary or the tyres will shred though.

If you're lucky it'll have Weinmann Vainquer top-pulls or similar and tonnes of clearance. Quite a lot of mid-market things from that era assumed you'd use it off-road in the winter I think.

Possibly alright, there's one guy or lad in our league that races on an old road bike.
I had a 75 crosser and that had very little clearance, I think even old cross tyres back then were very narrow.

Don't know how good it would be for cyclocross racing, but long before the Americans invented the concept of mountainbiking, we were riding bikes like that with drop bars all over the mountains. The worst that could happen is that you might bend it.

Just don't try jumps 🙂