Centrifugal forces on tire stem valves
Centrifugal forces on tire stem valves
(OP)
thread404-18490: Calculating Centrifugal Force
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.
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.





RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
The centrifical force is outward, i.e. away from the center. This will try to open the valve.
RE: Centrifugal forces on tire stem valves
Ted
RE: Centrifugal forces on tire stem valves
In any case, a normal Shrader valve can be sealed with a metal cap, instead of the usual plastic/rubber one.
TTFN
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RE: Centrifugal forces on tire stem valves
"F/m = w*w*r, which comes out to 1556 G's"
. The tire radius sets rotation speed based on tire OD.
Spring pressure on the poppet pushed against the centrifugal force acting at the smaller radius of the inside of the tire, inside the valve at the actual poppet position.
RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
Regards
Pat
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RE: Centrifugal forces on tire stem valves
http://tinyurl.com/lpjz6t
(Right angled valve stems)
RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
I think your numbers look good. It was an interesting question and I spent some time searching Google. The subject comes up in number of high speed bicycle racing forums. The numbers they come up with are similar to yours. No one on those forums posted any definitive answers. I went to Schrader's web site but didn't see anything there about high speed valves. Perhaps Pat and Brian's responses show the work around the high speed racing guys have come up with.
RE: Centrifugal forces on tire stem valves
ht
I'd assumed that this was rather well known.
Most but not all caps have a rubber seal of some sort, though it seems to be discussed as a dust rather than a pressure seal. What I don't like about them is that it's very difficult to confirm that it's sealing since the valve core seal will normally prevent air from getting to the cap seal. Also the sealing surface is at the end of the stem, which is vulnerable to being damaged. I guess you could spin the tire up while the bike is on a stand but this isn't practical in the real world.
The stems on motorcycles almost always enter the wheel along the radius rather than at an angle like cars. I've been using the angled stems mentioned by BiPloarMoment but they're normally discussed in terms of providing easier access to the stem rather than avoiding air loss at speed. Also, contrary to the listing, they are 83° rather than 90° so they reduce the axial force by a factor of 8.2. They're rather pricey. That and the occasional mention of the air loss issue is what prompted me to look into this in the first place, wondering if it was a real issue or advertising hype. The 8.2 factor translates into 2.8 times greater speed before unseating which provides the needed safety factor.
I wasn't aware that bicyclists worried about this because of the lower speeds. I further gather that they often use a different type of valve (Presta) with a retaining nut on the stem. I've also been on the Schrader site but found it to be not very informative, though they did list some valve cores with different opening pressures.
What bothers me about this issue is that there are several street-legal/commonly-available sportbikes that can easily reach these speeds and are sold with standard stems. Yet the owner's mauals make no mention of their vulnerability to air loss at speed or to the importance of the valve cap.
RE: Centrifugal forces on tire stem valves
Regards
Pat
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RE: Centrifugal forces on tire stem valves
So, per the picture provided by BillV39, the valve's acceleration must be relative to the mounting interface of the return spring on the valve.
TTFN
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RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
Cheers
Greg Locock
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RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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RE: Centrifugal forces on tire stem valves
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.
RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
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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RE: Centrifugal forces on tire stem valves
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Centrifugal forces on tire stem valves
RE: Centrifugal forces on tire stem valves
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Centrifugal forces on tire stem valves
What?!?!?!?
Something on Wikipedia is wrong?
How can that be?
It's on the internet. It HAS to be right.
RE: Centrifugal forces on tire stem valves
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
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