deflating tyre at speed
deflating tyre at speed
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Discussion

mel

Original Poster:

10,168 posts

291 months

Tuesday 25th January 2005
quotequote all
We had a conversation in the pub the other night when a friend of mine said he had a front end wobble at about 150 ish come on, when he stopped he had lost a fair amount of pressure in the front tyre. He blew it up and wipped it down the tyre place to get them to check it, no puncture its fine sir, rides again for a bit and it stays up, but when he winds it up to 150 ish a week or so later the same thing happens, so back to tyre shop. This time they replace the valve and dust cap and when he explained the whole story they said the venturi (sp?) effect on the wheel at those speeds had forced the valve in and allowed it to deflait. Now although I've never heard of this before or experienced it (and we never bothered with dust caps on race bikes) I'm open minded enough to believe it (sort of) but what I can't get my head round is if the rotational effect of the wheel is strong enough to force the valve in surely that same effect is going to be in place and prohibit air from escaping by forcing the air into the tyre, so by that logic surely the tyre would inflate ??? or am I missing something.

italiano

8,352 posts

248 months

Tuesday 25th January 2005
quotequote all
This is new to me - I would have thought that it's more likely that the centripetal effect is 'pulling' the tyre away from the rim...even this seems unlikely though.

Venturi effects are normally the result of differing pressures over a surface - I can't think how that would happen with the valve to be honest. I'm really intrigued though - lets hope someone else can make sense of it for us!

BliarOut

72,863 posts

255 months

Tuesday 25th January 2005
quotequote all
I would have thought if it was a likely explanation it would happen to most bikes. Sounds quite unlikely to me, as it would have happened on the old tyre if they didn't change the valve.

Sounds more like a manufacturing defect in the tyre.

tim2100

6,287 posts

273 months

Tuesday 25th January 2005
quotequote all
Similar thing happened to a mate on his Busa. He put the bike on a rolling road at an open day we went to, the guy cranked the bike up and at ~190 mph there was a pop from the back tyre, the forces had blew a hole in the tyre.

My mate was well gutted as he had only just brought the bike!!

Still the bike pulled 199mph and produced 149.5bhp at the back wheel, with the bike running very lean at top revs.

Tim.

pesty

42,655 posts

272 months

Tuesday 25th January 2005
quotequote all
I had this.

The cause for me was the valve. due to cetrafugal force at 1.60 kleptons and no o-ring in the valve cap the tyre went down.

didnt notice until I got off the motorway (sorry autobahn) and tried to go round a roundabout! thing just went streight on.

Sorted by screwing valve in tighter with a tool specific for it and getting proper valve caps

DennisTheMenace

15,605 posts

284 months

Tuesday 25th January 2005
quotequote all
ZX12R's were recalled to have bolt on tyre valves IIRC the standard rubber jobbies would bend due to the centrifugal (sp?) force which would let the tyre deflate

Edited cos im a numpty

>> Edited by DennisTheMenace on Tuesday 25th January 16:29

italiano

8,352 posts

248 months

Tuesday 25th January 2005
quotequote all
What speed do you reckon this could start happening at? I had a flatty last night after a little burst of speed and haven't had a chance to investigate yet. The tyre was brand spanking so I'm guess they fiddled around with the valve at some point.

DennisTheMenace

15,605 posts

284 months

Tuesday 25th January 2005
quotequote all
Not sure i have had the ZX6R pretty much flat out and it was fine , so i would say anything thats capable of 160+

italiano

8,352 posts

248 months

Tuesday 25th January 2005
quotequote all
Thats me safe then!

Yoda954

2,260 posts

264 months

Tuesday 25th January 2005
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Had this happen on my old FZ750 with virtually brand new Avon Super Venoms (I think). The back lost pressure at about 130 which started a 'slow' weave from the rear end. Pulled over to hard shoulder gradually without incident....luckily
Cause of puncture was a screw in the tyre which must've been qute happily keeping the air in until the speed/heat/forces reached a certain point ?



>> Edited by Yoda954 on Tuesday 25th January 20:12

sierranut

21 posts

255 months

Wednesday 26th January 2005
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angled valve stems? I'm sure they do them for some alloy wheels. would solve all porblems.

eliminator

762 posts

271 months

Wednesday 26th January 2005
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Your wheel is just like a centrifuge. Faster you spin it the more centrifugal force you get. With a straight tyre valve then eventually you have enough weight in the valve body to overcome the back pressure from the tyre (which is how you blow a tyre up). Then air leaks out.

Valve caps with seals help as the air leaking past the valve cannot escape past the cap hence tyre stays up - you only lose a cap-full of air.

Go for angled tyre valves if you want full solution.

bennyboysvuk

3,494 posts

264 months

Thursday 27th January 2005
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Hmm, it seems rubbish that a valve would fail like this, but I can accept that it might. Oh well.

I once had a rear tyre suddenly lose a lot of pressure whilst wheelying at around a ton. It weaved a fair old amount so I put her down.