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Is it true?


BMMW

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Tire guy, at a trackday, told me that centrifugal force draws the valve stem core down and air escapes from the tire. Is it true?

He replaced my plain plastic valve stem caps with metal ones. Indicated that the rubber seal inside keeps the air in.

Seemed kind of odd.

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Just read that somewhere the other day, may have been Click and Clack, the CarTalk guys.

They said you need a cap, didn't say anything about steel > plastic, though.

*** edit: I done seen it in the June 2011 Motorcyclist, page 119, the MC Garage section:

Missing valve stem caps can be a bigger deal than you think.

Aside from keeping road grime from getting into the Schrader valve underneath and mucking things up, properly installed caps (!?!) help keep air from getting out of your tires.

That's not a big deal when the bike is stopped or moving slowly, assuming you check pressure regularly.

But there's enough centrifugal force acting on the valve at speed to compress the valve's internal spring, allowing the air inside to escape. If the cap is in place, there's nothing to worry about.

Otherwise, the faster you go, the more air you're likely to lose. If the valve fails without a cap, you're in a world of hurt.

So spend a couple of bucks on some nice steel caps with slotted tops that will let you keep the valve internals tight. You'll have one less thing to worry about.

Not sure whether I'm believing this.

Edited by jblosser
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I've had a long stem core on my 750 tear. It left me stranded on a freeway.

The stems on my R6 are the short version. I don't think they would tilt or fold over.

The tire guy was talking about the core of the stem compressing and leaking out air, and not about the stem folding over.

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Tire guy, at a trackday, told me that centrifugal force draws the valve stem core down and air escapes from the tire. Is it true?

He replaced my plain plastic valve stem caps with metal ones. Indicated that the rubber seal inside keeps the air in.

Seemed kind of odd.

In theory it is possible, but valve stem covers would seem to negate the ability for the air to escape if there was enough centrifugal force present to effect the valve stem.

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I'm kind of bummed. The guy took my drilled and safety wired caps. Now I got shiny Harley Davidson chromish looking bling caps on the bike.

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i'm gonna call bullisht...

i've been running with no caps on my valve stems for quite a while, and i've been well into triple digits on more than one occasion... never had one leak.

I've had one break off, but that was because of a big heavy valve stem cap that someone bought me with a blinking light in it.

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Word of this gets out and valve stem caps are sure to hit $20 a pair by season's end.

We'll just end up stealing them off of semi-truck tires like when we were kids! (maybe that was just me)

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call whatever you want, it happens. I'm not sure what you mean by will into tripple digits but in the one I stated you are looking at a very long front straight on a R1 that is probably doing 170ish. When he went out his tire was fine after a few laps the bike felt odd. He came in and the tire was down 5-10 psi. He filled it up, put caps on and it never happened the rest of the weekend. You have to have caps on at the track for a reason.

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