Deadly valve caps...

[Disco Dan]

Was riding a borrowed bike this evening, the gorgous ZX11 and as I was travelling along the mway (listening to the ram air system sucking like two giant straws over the roar of the engine - pure music!), the front tyre went down - almost flat. Making it almost impossible to turn... as I was almost at a bend, lets just say it got 'interesting'. Pulled over, to see the front tyre had gone down almost flat. Checked for any foreign objects and around the valve for problems but nothing.

...hobbled and wobbled to the nearest gas station, again checked the tyre, and inflated it. Seems alright....

..until I noticed the valve cap. As you can(t) see from the picture, they are little brass caps. But they have a rubber seal ring inside and a 'button' on the top; when pressed it pushes a pin, that pushes down on the tyre valve pin letting out air. "weird".

They only thing I can think of in the absence of any visible punctures... (and the fact it has not gone down again) that the centrifugal force of the wheel spinning 'spun' the weight of the button on its spring, onto the tyre valve needle and actually deflated it ! Is that possible???

WHY? Why have a valve cap that deflates your tyre when you go fast??? How smegging dangerous!!!!!!! What possible purpose could they solve??? Am I missing something important???? Valve extenders maybe????

[bert_is_evil]

That damn penguin is at it again - to the bat mobile boy wonder!

[Fat Tony]

Possibly designed for fitment to car tyres rather than a bike mate??

There are some 'bling' valve caps available in the UK that light up with the centrifugal force (). I've heard reports that these also have been responsible for deflating tyres when fitted to bikes.

[jim.cox]

Was riding a borrowed bike this evening, the gorgous ZX11 and as I was travelling along the mway (listening to the ram air system sucking like two giant straws over the roar of the engine - pure music!), the front tyre went down - almost flat. Making it almost impossible to turn... as I was almost at a bend, lets just say it got 'interesting'. Pulled over, to see the front tyre had gone down almost flat. Checked for any foreign objects and around the valve for problems but nothing.

...hobbled and wobbled to the nearest gas station, again checked the tyre, and inflated it. Seems alright....

..until I noticed the valve cap. As you can(t) see from the picture, they are little brass caps. But they have a rubber seal ring inside and a 'button' on the top; when pressed it pushes a pin, that pushes down on the tyre valve pin letting out air. "weird".

They only thing I can think of in the absence of any visible punctures... (and the fact it has not gone down again) that the centrifugal force of the wheel spinning 'spun' the weight of the button on its spring, onto the tyre valve needle and actually deflated it ! Is that possible???

WHY? Why have a valve cap that deflates your tyre when you go fast??? How smegging dangerous!!!!!!! What possible purpose could they solve??? Am I missing something important???? Valve extenders maybe????

Ouch - add that to life's experiences.

Could be the valve leaking (maybe due to pressure on the needle), but the air being held by the cap - but for only SO long.

I've also seen air escape a tube, but be held by the tyre until pressure on the side wall in a turn was enough to cause deflation along the bead.

In both cases you fill the tyre agian - and can find no obvious problem

Till it happens a second time

Take care

=mjc=
.

[vifferman]

You're on the wrong track - it's "the ram air system sucking like two giant straws" that sucked the air out of the tyre.

I know that there have been other valve stems that have caused problems on bikes. On one of the VFR forums I am on, people who fitted right-angled stems for easier access had them break due to the forces acting on them.

[gijoe1313]

Crikey! That doesn't sound like a good feature Anyhow, I think the real reason is you've been stealing all the meat pies causing overloading on the whole system. Poor bike can't handle all that!

[Disco Dan]

It may be a faulty valve cap mate, or something may have become lodged in the valve mechanism.
Yes it is true, valves can depress with centrifugal force, I have had this happen to me on my TLR with no valve cap.
Tyre manufacturers will specify certain rated valve spring strengths for this very reason... resisting centrifugal force. High speed race bikes have stronger valve springs than a GN 250 for example.
The valve cap is really only designed as a dust cover, and to stop foreign objects contaminating the valve mechanism, but a properly sealed dust cap will form a vacuum within the airspace in the cap to also resist centrifugal force.

It was the valve cap intself - it was a heavy brass thing, with its own spring and needle inside - and a brass button on the top. This extra mass would have been enough to push down on the tyre valve needle!

I took it off, and did 'the same thing' and it was fine!

What a stupid invention!!!

[Kickaha]

The valve cap is really only designed as a dust cover, and to stop foreign objects contaminating the valve mechanism, but a properly sealed dust cap will form a vacuum within the airspace in the cap to also resist centrifugal force.

Metal valve caps with the rubber o ring in them are best, no leaks from them even with no valve core in

A valve cap is essential on a Schrader valve because if one is not fitted, dirt and water can enter the outside of the valve, potentially jamming it or contaminating the sealing surfaces and causing a leak. Metal valve caps usually have a small rubber insert to permit a good seal against the valve body; a cap of this kind also helps to prevent air escaping from a slightly leaking valve. However, the vast majority of Schrader valves used for tires are fitted with plastic caps which effectively serve only to keep contaminants out of the valve stem

It was the valve cap intself - it was a heavy brass thing, with its own spring and needle inside - and a brass button on the top. This extra mass would have been enough to push down on the tyre valve needle!

They're not actually valve caps, they are valve extensions

[paturoa]

Sometimes the valve itself wont or isn't screwed all the way in the stem and the little pointy bit stick up proud of the valve stem. Then when the cap is screwed on it pushes on the valve and causes it to leak. The current front tube in my KDS does it.

[Blondini]

HECTOR Disco Glad you stayed upright Never had that problem on the FXR????? :spudwave:

[Disco Dan]

They're not actually valve caps, they are valve extensions

just as I thought!

Why would someone have them on a motorbike! Shit for brains if you ask me!!

HECTOR Disco Glad you stayed upright Never had that problem on the FXR????? :spudwave:

hmmm... yeah... a few hundered cc's diference and an over enthusiastic wrist....

[Big Dog]

Possibly designed for fitment to car tyres rather than a bike mate??

There are some 'bling' valve caps available in the UK that light up with the centrifugal force (). I've heard reports that these also have been responsible for deflating tyres when fitted to bikes.

Can vouch for that.
Had a set and the rear spat the valve almost clean out under the pressure.

Metal caps of any sort are dangerous and should be illegal.
I have heard of some allows that are lighter than plastic but why take the risk.

[Big Dog]

http://www.kiwibiker.co.nz/forums/sh...&postcount=118

[Kickaha]

Why would someone have them on a motorbike! Shit for brains if you ask me!!

They're generally fitted to make it easier to pump up the tyres

Metal caps of any sort are dangerous and should be illegal.

Absolute rubbish they are far superior to the plastic "dust" caps as they will seal even without a valve in, due to the rubber o ring in them, I wont use plastic caps on any bike I ride

[MacD]

Yep, one of the first things I do on any motorcycle new to me is fit (lightweight) metal caps on the valves as they provide a secondary seal. The bike Disco was riding had valve extensions, all together a different and heavier deal.