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How/why does it happen and how can I stop it? Haven't got much money so tubeless isn't really an option at the moment. Just happened on my ride this evening
tyre grips the inner tube under braking and pulls it around. talc can stop them sticking together.
tyre/tube moving around whilst tube stays where it is held in the rim.
Try talcum powder inside the tyres before you put the inner tube in. Small amount in bottom and rotate the tyre to get an even layer around the inside of the tyre. insert inner tube inflate and ride
Normally it only happens it you are running low pressures in the tyres - but then you probably want to do that for grip and all
It happens when you brake. The tyre moves on the rim, drags the tube, rips the valve.
Fixes: Some talc on the tube to stop it being dragged or a couple of dots of superglue on the tube to stick it to the rim or higher tyre pressure or don't brake
I had this a few times when running tyres too big for the rims . I was running 2.25 29er tyres on a 16mm rim. Since I got new wheels its stopped. Also running thinner tyres on the old rims on my cx and its Also stopped value ripping . So could well be that
I use to get punctures around the bottom of the valve on a regular basis (more of a split actually). One day I decided to not do up the valve collar all the way, allowing for it to move around a bit: Since then I haven't had a single problem.
Your problem sounds a bit more severe than that though.
as m74, on dirt motorbikes you would leave off the valve thread bolt and watch for creep (valve starts to lean - then deflate, realign, repump before it leans too far - as it then tears off)
is they tyre really old and been on/off lots as it may have loosened up
some of my dirt bikes wheel had tyre clamps (extra valve hole, and a 3" shoe with a bolt/nut - this was used to squueze tyre to rim via rim)
Happened to me on the roadbike recently with a cheap tube, pumping it up, roadside with a mini pump, replacing a tube that had totally blown out. Suffice to say, when the replacement tube failed with the valve stem snapping, I was in the proverbial. No puncture repair kit would have repaired either one! I think it could have been caused by me stressing the valve stem during pumping, but I don't think I was being too rough with it - maybe similar for yours? I feel your pain... 😕
Track pump for pumping up tyres whenever possible should help (saves moving the valve around too much with your hand pump).
Usually talc but probably a while since that tube was out, was at a bit of a crucial point and it nearly put me on my arse (only allow myself to brake after a certain point on the trail and when I did whumpssssssssssssshhhh). Think low pressure was the problem, is quite gloopy at the moment!