Why didn’t they discuss what happens on the rear wheels
My theories as to why the effect is not noticed on rear wheels:
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more air in the rear wheels, so any loss is not noticeable in the few seconds at high speed.
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acceleration is less. Acceleration is V*V/R where R is radius of valve from hub center and V is velocity of valve. On front wheels, valve is near outer edge of tires so R is the same as the wheel and V is about the same as the speed of the dragster. On rear wheels, R on the rim where the valve is located is probably half or a third of the rear tire outer radius but probably about the same R as the front wheel outer radius so R is the same as the front but the V is half or a third so acceleration at the rear valve is maybe one fourth to one ninth (V squared) of that at the front.
I’m not buying this Puzzler at all, especially since TomRay said they didn’t know if the solution was right. Anyway, wouldn’t it be safer to use solid tires on the front wheel if the goal is to reduce rolling resistance? No chance of a blowout.
Calling bogus.
This problem is mentioned in the 1958 Hot Rod Magazine (pg 249 in this summary edition):
It’s all academic. If they put a cap on the valve, it will be forced down tighter to close off the leak. Aircraft style schrader valves are turned off with a wrench. It’s all good science.
Hello All…
Visualize the forces on the Schraeder valve under high centrifical speed and you will realize that the valve is forced closed not open. The tire air pressure exerts the same force as the centricical force against the valve. The addition of a o ring in the valve cap is totaly unnecessary and serves only to add confusion.
Just kidding…Lets see if anyone reads this stuf.
Two forces tend to keep the valve closed: the spring that is part of the valve asembly and the air pressure in the tire/tube.
Wow! I just did the calculation. Assuming a 1 foot radius, at 200 mph that valve stem is experiencing almost 2700 g’s. I don’t know what the specs are for that spring, but it probably isn’t up to the job of keeping the valve sealed.
Even at 80 psi the counter force supplied by the air is miniscule when you consider the small cross section of the valve surface.
I wonder if they consider these effects with those large diameter wheels de rigeur on fancy, high performance cars today.
Note that the valves in the small tires on high-end bicycles don’t have the spring, They’re called Presta valves.
I looked into this about a year ago and posted some numbers here:
http://www.eng-tips.com/viewthread.cfm?qid=248379&page=1
I was looking for some reasoned comments from other engineers with supporting calcs like mine and, but for a few responders, got a lot of non-information/idiocy instead.
The issue is real. It doesn’t affect the dragster’s rear wheels because the rear stems come in at an angle nearly parallel to the axle rather than along a radius.
Valve caps work but have the disadvantage that there’s no practical way to check that they’re working. I’ve checked the major motorcycle racing rulebooks and see no requirement as was claimed.
I continue to be suprised that the owner’s manuals for high performance motorcycles (which top out at 300 KPH/187 MPH) don’t mention the importance of having a working valve cap.
OK I’ll play the dumb card. I can totally see your point but cannot figure out the just kidding.
2carguyz:
I question the magnitude of your figure of 2700 g's. Please show your work.Radial acceleration is Vsquared/R
V=5280feet*200/3600/3= roughly 98 meters per second
98squared/one-third meter=roughly 29000 meters/second squared.
Gravity is 10 meters per second squared. So, roughly (I didn’t use the exact conversions from English to metric) 2900 g’s. I’m assuming 2carguyz used the correct conversions and his figure is more accurate.
Scrabbler
Thanks, this appears to be correct. As you and carguyz know, this is the acceleration, and assumes a constant speed of 200 mph; we can ignore that so as to avoid having to calculate the Euler Force based on assumptions about changes in the rate of acceleration over the racecourse. If we assume a .25 gram valve stem, this gives a centrifugal force of 6.5 Newtons, probably enough to compress the spring. Not bogus.
Yes, of course, I cheated:
“Why doesn’t the air leak from the rear tires?”
The valve for the rear tires does not come straight up as on a bicycle or motorcycle valve stem. So, the centrifugal force is sideways to the valve axis. Even if the stems are slanted, the centrifugal force will lay the stem down horizontal making the centrifugal force orthogonal to the axis of the schrader valve. IIRC rear floppies used a tube to help keep the air in when the walls distorted on launch.
I agree with the puzzler and the solution proffered. Good one guys – but too easy. Now if it had been “a dark and stormy night at the strip” that would have made it a true Car Talk Puzzler.
In addition to the previous answers, remember that the rear wheels are also much larger diameter than the fronts, and so will turn fewer revolutions to cover the same distance (disregarding the burn-out starts). Fewer revs will produce less centrifugal force.
Aircraft also have to use special sealed valve covers for their Shrader valves.
Number of revs is irrelevant, RPM is relevant. Are the rear wheels really significantly larger in diameter than the fron when it comes to the distance of the valve stem from the hub? Or is the orientation of the valve stem more important. I think if they were similarly oriented you would also lose aire from the rear tires. We’re probably talking about a pound of force on the valve stem.
Thanks. I thought I did pretty well for a humanities major who last saw the inside of a physics class 15 years ago. 
Scrabbler
Despite lower air pressure adding to holding valve closed, in the rear there is: lower RPM due to greater diameter, valve farther from circumference do to hight of tire, valve orientated more horizontally, and therefore less down force of valve pin.
Thanks for the calculation. Of course, the valve pin force must overcome not only the spring, I believe, but the pressure in the tire contributes to forcing the valve closed as well.
Good you mentioned Presta valves that have no spring, and a nut on the pin that would keep the pin from being pushed in.
Answer to this puzzler came to me when recalling an account of John Howard’s drafting bicycle speed record of 150 mph. During attempts, the problem arose of the tires going flat as that speed was approached. It was finally resolved by discovering that the Schrader valves were opening under centrifugal force.