Centrifugal forces on tire stem valves

[BillV39]

thread404-18490

I've been looking at the centrifugal forces on motorcycle valve stem components at high speeds and found this old thread, which provides a basis for what follows. The specific thrust of this is that I've heard that the centrifugal forces can be high enough to unseat the valve and release air from the tire (very undesirable).

At 200 MPH the rear tire, which has a diameter of 24 7/8", rotates at 2702 RPM or 283 radians/sec. The internal radius of the wheel is about 7.5". So the G-force on the stem components is F/m = w*w*r, which comes out to 1556 G's.

I've made some measurements of a typical stem valve (the poppet weight was estimated from its dimensions as I didn't want to cut the valve up).

Total weight: .79 gm
Force necessary to just lift the popett: 400 gm
Weight of poppet: .3 gm

So this valve would start bleeding air at about 1200 G's, which is reached at about 175 MPH. Note that this ignores the centrifugal force on the valve spring itself, which would reduce it's force on the poppet. A safe limit with this valve is likely to be more like 150 MPH.

This was something of a shock to me. Have I made an error?

P.S. There were numerous errors made by the various posters to the old thread. I suggest that you familiarize yourself with the calculation before responding.

 

[MintJulep]

So make some special valve stems.

The density of brass (I'm assuming that is what the stock stem is made of) is about 8500 kg/m^3.

Titanium is only 4500kg/m^3.

That should give you all the margin you need.

 

[BobM3]

Looking at the picture from BillV's link, it looks like the spring is a compression spring. If so, then the spring and the centrifugal forces are both tying to open the valve. So it must be only DeltaP that keeps the valve closed.

 

[ivymike]

um, no, the valve is somewhat difficult to open even when tire pressure is very low. try it.

 

[patprimmer]

delta P will hold the valve closed but will not actually close it.

I sincerely doubt the wikipedia diagram is accurate as it takes downward force against a spring pressure to release a schrader valve.

Regards
Pat
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[GregLocock]

I strongly suggest you pull the valve core out of an old inner tube if you are unsure of this. The wiki illustration is incorrect, the spring helps to close the valve (obviously).





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[byrdj]

 

[patprimmer]

By coincidence I just looked in my tool box and saw a packet of Schrader valves. The spring seat sits on a shoulder in the valve stem which preloads the spring to help closing the valve.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[MintJulep]

The wiki illustration is incorrect[/quot]

What?!?!?!?

Something on Wikipedia is wrong?

How can that be?

It's on the internet. It HAS to be right.

 

[swearingen]

Hence my tag, below...


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