Two rides in a row I've had pinch puncture style blowouts on the front tyre, both caused by hitting something relatively hard like a root or rock and the bike decelerating somewhat. But both times the tube has failed at the bottom of the valve stem. Is this a common failure and is it just happening because of tube age?

It's only happened since putting a Speed King 2.3 folding on the front, however it's more probably related to me being a new returnee to mtbing and now that it's drying out and I'm chasing faster better riders I'm hitting obstacles harder. Is there any way to stop these blow-outs happening?

The tyre is slipping around on the rim. Talc inside the tyre (reduces how much the valve is dragged around when the tyre slips), you can run higher pressures, crosshatch the rim with sandpaper, or try different tyres.

I had it loads with Panaracers, much less with Contis and Schwalbe.

I get it less with tubes with nuts to hold them in place, and if they'r big valve holes in the rim have you get the correct spacer for the presta valve?

Also find that using mini pumps tends to cause them (especialy in lightweight tubes). Not had any bother with track pumps. Might be making small weaknesses wich get riped open on the trail?

I've had this problem before. Do you tighten the valve nut before you inflate the tube or after? I found finger tightening the nut before inflation stretched the tube to the point it became weak and burst.

check valve hole for burs, file if needed.

I agree with TheBrick

Exactly what tron says – I had this when I first fitted a Panaracer Fire XC Pro to my front wheel with a disc brake. The Panaracer went on the rim easily enough, but the tradeoff seemed to be that it was sufficiently loose to slip around and rip the valves off the inner tube.

In no particular order, I tried: reinforcing around the valve stem with some old inner tube; talcing the inside of the tyre extensively; trying different inner tubes; replacing the tyre with a Bonty Jones ACX. All except the last worked somewhat (as in seemingly alleviated it for a couple of rides but still led ultimately to the same problem).
Having said that, the Bonty *did* rip one valve off an inner tube, but only once. Weird. The bead of the Bonty is much tighter than the Panaracer, so that may be part of its strength

In summary then, talc, perhaps try some brand-name inner tubes, try to check how tight the bead of the tyre is on the rim, and if all else fails, try a different tyre.

I had a lot of trouble with it last year with new wheels and tyres on the downhill bike, swapped the tyres for this season and no probs yet, it was the tyres slipping on the rim

I've had this happen quite a bit, always with Panaracer tyres on XC717 rims. It is the tyre dragging around the rim. I tried the talc thing, not great, in fact if anything tyre slipped more. I also tried setting the valve back at an angle so there was space to move with the tyre. That more or less worked but I was always worried that I'd reach the limit and tyre would blow. Now I've fitted two rim tapes to each wheel and it seems to have cured it. I also swithched from presta to schreider type valves at the same time, seem to be tougher where the valve is welded into the tube for some reason.

My mate kept having this problem and when we had a good look, there was some really quite nasty sharp edges on the valve hole. So +2, TheBrick. 2 mins with a rat-tail file and it was sorted.