past 6 months, ive started getting these really bone (rubbish) punctures.
under heavy braking, the front front tube gets a nick or cut in it at the stem, or the stem comes clean off.

ive tried filing the hole so its a rounded surface, but it still happens.
obviously, having the stem come off the tube means every time this happens its a new tube :'(
(im a tube mender, not a tube binner like some of you lot ;))

someone mentioned to me this past weekend, that it is caused by the tube being stuck to the tyre. over time, the tyre becomes a bit sticky inside (reusing tubes wont help i guess), so the tube sticks to the tyre, the tyre moves under braking a little (im using conti folding tyres on 717 rims, so they are VERY easy to get on and off, not tight at all) and this causes the tube to get moved with the tyre.

subsequently, he reckons that talcing the tyre inside, will prevent it from moving the tube (as much?).

does anyone else do this?

a quick google indicates some do, most dont, wondered on what you locals thought.

Talcing may help it. Personally I'd get the tyre to stick to the rim better.

I do just because it makes it easier to get the tube out for quick changes.

I always add some talc when changing tubes/tyres, and never had the problem you describe. Baby talc is nice and cheap – just sling a bit in and coat the whole inside of the tyre.

I talc my tyres and inners. I understood it stops that inner tube sticking to the inside of the tyre, dunno if it makes much (or any) difference though. Just do it out of habit. Rarely get anything other than thorn punctures though

tubeless now, but i did when useing tubes and it did make a difference

I take you are running a rim strip? put a strip of tape over the valve hole, duct tape would be the best, re fit rim strip, then force a valve through the tape, that should cover any edge you have around the hole, try not to remove the valve, and re fit your tyre should be easy enough with folding tyres.

But yes talc is good, for stopping the tyre getting stuck to the tube.

Edit:- are you using a presta valve in a shrader sized hole?

I used to get this badly on my old rear wheel and tyre. I had a presta valvle in a schrader hole allowing the valve to go at a funny angle. Put in some french chalk/talc and also had a reducer so that the valve was a tighter fit. Didn't ahve the problems after that. I still ahve soem slight slippage with my Bonty ACX tyre on my 717 rim but hasn't ripped a valve off yet but the beading is quite loose around the rim itself.
Is the inner tube size compatable with the tyre? Have you also hceked the pressures?

I've had this problem quite a bit over the years – it happens most with combination of big, grippy tyres and relatively low pressures, as obviously the tyre has less pressure holding it onto the rim.

Logically I can't see that talc can make much difference, given the pressure between tube and tyre and the surface area involved. However, I talc and it does seem to make a difference.

I also make sure the rim is nice and clean, and I try and make the bead as grippy as possible. Rubbing it down with some sandpaper if it's "glazed" and wiping over with something like meths seems to help.

Also I always over inflate until I'm sure the bead is completely seated on the rim, and then let down to required pressue.

What talc for DH tubes? 😉

I had exactly the same issue with Fire XC Pros on 717 rims. I used talc, and also tried to reinforce the stem area by putting a square of inner tube around the valve/ tube join.
To be brutally honest, it worked for a week or two for me, then it'd be just as bad. In the end I got new tyres (Bonty Jones ACX) – not only do they not rip the valves off, I find them much better for grip than the Fire XCs, so never went back.
YMMV, though – good luck with it