Need somebody smart to do a calculation for me...

[Formerly Steve Wilkinson]

A company sent me a set of "air alert tire-valve caps" to try out and ideally write about. They're big and heavy, because they each contain a battery and a red LED that blinks if the tire pressure falls by four pounds. Very nice, but they weigh 0.35 ounce apiece, and I wouldn't dream of putting them on a fast car for fear of snapping the vale stem off at speed.

Can somebody good with numbers tell me what their effective weight would be at, say, 120 mph (a speed I'm sure a lot of us have done at one time or another...maybe more) on a 19-inch wheel? I want to be able to sound as though I know what I'm talking about when I tell the company that makes them that they're dangerous--which I think they are.

[cstreit]

I think you'd be in the 300 RPM range or so at 120mph... Ran a quick calculation I think it's like 600 G's or some ridiculous thing.. in other words, it weighs a LOT.

[cstreit]

((feetpersec * feetpersec) / 32.2) / radiusinfeet


feetpersec= 120mph*5280ft / 60min/60sec = 176

((176 * 176) / 32.2) / 1.58 = 608 G's...

[Formerly Steve Wilkinson]

Which means each valve cap effectively weighs 13.3 pounds at that speed, right?

Question for anybody, does that sound like weight that would do damage to a tire stem?

[cstreit]

I don't think I'd put it on my rubber valve stems...

[EarlyPorsche]

No I dont think that would be much of a problem. The tire monkeys have long levers to remove the stems. I think somewhere over 20 pounds and it never tears them no matter how old they are. Certainly this should be installed on shorty stems and not the long truck stems. Furthermore it would require a rebalance of the wheel. The steel stems would probably be best.

[cstreit]

Porsche saw fit to put stem supports on even basic rubber valve stems,... I won't even run them on the racecar ... Seems risky to me.

[450knotOffice]

608 G's! Wow.

.35 ounces X 608 G = 212.8 ounces = about 13.3 pounds.

Mind you, the tip of the valve stem is significantly inboard of the outer radius of the tire so the G- forces wouldn't be quite as high as indicated above (this was based on the rolling radius of the tire), but it is still probably on the order of 1 pound. If the weight acting on the stem is not perfectly centered, centrifugal force will tend to force the valve stem out toward the edge of the tire, possibly leading to valve stem failure where it attaches to the rim. However, if it's one of the bolted-in types, that one pound or so of force probably would be insignificant.

This is just a guess on my part, btw.
(edited for a stupid math error)

[rick-l]

I am an EE so I need practice. Somebody check

weight-mass
0.35 ounce = 0.021874997 lbs mass = 0.000679898 slugs

velocity
120 mi/hr = 176 ft/sec

radius
19 in dia = 9.5 rad in = 0.791666667 ft


F= m v^2 / r 26.60277889 pounds

[BK911]

I did a real quick calc assuming an 18” rim with a 20” rolling diameter and came up a centrifugal force of about 22#’s each at 120mph.

It’s been awhile since I’ve done math so….

[rick-l]

We have 3 answers now, is there a prize?

A 19 inch wheel would have an 9.5 inch radius right?

Lets ask Wayne. Those MSMEs are supposed to know this crap.

The problem I would see with this is every time you went 120 mph the stem would flex over against the rim and then return again when you slowed down. How many times would a valve stem do this?

[BK911]

I think we are close enough Rick. I used 18" as where the stem would actually be, for r = 9", and used 20" as the rolling diameter to determine W. I used a different formula though, I used:

F = m w^2 r

[Formerly Steve Wilkinson]

Interestingly, I queried the company about this, and they told me hundreds of thousands of these units have been sold in Germany and they have never had a valve-stem failure even in Autobahn use.

[Bill Verburg]

I came up w/ an answer close to CS's


here are my assumptions
235/35x19 Michelin PS2 26" OD 799rev/mi @120mph
valve cap is 9" from the axis orf rotation

answer
~653g

The cap will weigh ~653x it's static weight

[skinnerd]

Just stick them on the racing steel valve stems, and you'll never need to worry about them....

[Formerly Steve Wilkinson]

I'm sure they'd be fine on racing steel valve stems, but how many ordinary consumers who will buy them at AutoZone even know what a racing steel valve stem is?

[BK911]

Ok, this bothered me so I used Bill's numbers in:

F = m w^2 r

and came up with 13.44 lbs, which is what CS said an hour ago. I like his method better.

[deanp]

If they were sold in Europe/Germany, don't they have to pass TUV testing?

[Formerly Steve Wilkinson]

TUV: yes, that's what I assumed, and I told the guy that if they were good enough for the TUV, they were good enough for me...

[Dixie]

but they weigh 0.35 ounce apiece

You know, wheel weights are typically in 0.25 oz increments. The caps weigh 40% more than that!

I can see it now. Joe Average buys the caps at Pep-Zone and installs them. Now his Ford Taurus has a shimmy at 80. He takes it in to the shop. They take the wheel off the car, take the cap off, check the air pressure, and spin the wheel. Hmmm.... it's in balance.

Eventually it dawns on someone (probably Joe) that the caps are the problem. After all, replacing shocks, ball joints and tie-rods didn't work. After 30 minutes, the tire jockey decides to humor Joe and balance the wheel with the caps on. Viola! The problem is resolved.

On the ride home, Joe looses one of the caps...