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.

 

[israelkk]

I think you forgot that the pressure inside the tire is the one that pushes the stem and creates the sealing force. This force is is probably much larger than the spring force that just creates the starting sealing force. Secondly centrifugal force pushes in the direction that add force to sealing force if I am not wrong.

 

[BillV39]

I hadn't considered the force associated with the air inside the tire. My valve has about a .090" opening, which at 35 psi is about 100 gmf. This helps but isn't enough.

The centrifical force is outward, i.e. away from the center. This will try to open the valve.

 

[hydtools]

I don't see an error. It looks like you need a valve stem cap that seals as a backup to the stem valve.

Ted

 

[IRstuff]

The force on the valve is NOT from that centrifugal force, but from the difference between the base and the tip of the actual valve. There's a difference of about 100 g across a 0.5-inch change in radius. This may still be an over-estimate of the length of the spring that holds the valve against its seal.

In any case, a normal Shrader valve can be sealed with a metal cap, instead of the usual plastic/rubber one.

TTFN

FAQ731-376

 

[racookpe1978]

Look at your "g" force assumptions and, in particular, write out each of your units as you stuff them in the formula:

"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.

 

[BrianPetersen]

In roadracing, it is a tech-inspection requirement to use metal valve caps with a redundant O-ring seal because of this.

 

[patprimmer]

Also valve stems mostly lay over at an angle to the radial axis.

Regards
Pat
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[BiPolarMoment]

Or an even greater angle:

http://tinyurl.com/lpjz6t

(Right angled valve stems)

 

[chicopee]

BillV39, if you drew a free body diagram showing all forces and their directions associated on this valve stem ad all related math, you may get your answer. Of course you could post your results for our scrutinies.

 

[BobM3]

BillV-

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.

 

[BillV39]

A picture of a tire valve showing how it works is at:

http://en.wikipedia.org/wiki/File:Schrader_valve_opening_and_closing_on_a_tire.gif

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.

 

[patprimmer]

Maybe that is because they have no incidents of tyres going flat at high speeds, and surely it would very quickly become apparent as a problem if it was in fact happening.

Regards
Pat
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[IRstuff]

Again, if everything is at the same acceleration, then, there cannot be separation. Only when there is differential acceleration can there be relative motion.

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

FAQ731-376

 

[ivymike]

uh, IR, the wheel holds the body of the valve in place, so its acceleration is pretty well constrained to make it follow the wheel trajectory. The poor little spring-a-ling has to try to hold the poppet against the seat, with a little help from the air pressure inside. If the springy isn't up to the task, then there is differential acceleration. In other words, all of the force to make the poppet accelerate radially has to come from the spring+gas.

 

[GregLocock]

Otherwise rims wouldn't come off flywheels. It is the absolute acceleration that is important, a force has to be applied to the mass of the valve insert to continuously accelerate it around the axle.

Cheers

Greg Locock

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[HDS]

Is this mentioned in one of the Carroll Smith books?

 

[Ralph2]

I have a hard time accepting that centrifugal forces on the valve stem are sufficient to overcome the spring tension and release air. If... it is, then there are a multitude of other issues that should also be considered.. the centrifugal forces on the air in the tire will create an uneven internal pressure, possibly aproaching a negative next to the rim. The rim / bead of the tire will increase in diameter.. the rim of the wheel itself will change in diameter. The profile of the tire will change and quite likely would run just fine with no air at all in the tire..... just don't slow down or stop.

 

[ivymike]

With a quick calc (holding density constant), I only get about 8 kPa pressure variation within the tire - so not a huge effect.

 

[patprimmer]

The very strong steel cables in the bead of the tyre are much stronger than the springs in schrader valves. Same deal for the rim materal. It has much higher capacity to resist the centrifugal force.

Regards
Pat
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