Tubeless tires?

My first time on a Dino my 3c produced a tiny bit of clutch slip, the operator, ex Gp Mechanic sugested what produced it was what he called spring flutter. Suggested as he was guessing. Overcoming spring tension on anything is a guess; how often, are Schrader valves are replaced.
 
martymoose said:
Yes, you are correct Cam, but how does the Centripetal force effect the valve stem?
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Centrifugal force is pretty much universally misunderstood. It's commonly thought to be what pushes things outward when they're in a rotating system, like clothes in a spin dryer. I've been guilty of using it that way myself because that's how most people conceptualise it, and I don't wanna confuse them. However, that general belief about as wrong as it can be. It's actually the reverse of what's going on.

Here comes a basic physics lesson. skip ahead if you know this stuff ...

Imagine for example that you're swinging a weight around over your head on the end of a piece of string. The commonly held belief is that there's a force pushing the weight outward, which keeps the string taut. But that's completely wrong. The only force acting on the weight is inwards, towards the centre of rotation. That force is provided by the string and it keeps the weight travelling in a circle. The proof that it's acting inward is elementary. A floppy piece of string can only act in tension, so it must be pulling the weight inward towards the centre of rotation. The weight wants to travel in a straight line. It's the inward force of the string that keeps deflecting it from the straight path and makes it travel in a circle. That inward force is called Centripetal force. It comes from Latin vis centripeta, which means centre seeking.

Centrifugal force (from Latin vis centrifuga meaning centrer-fleeing) is simply the equal and opposite force to centripetal, as demanded by Newton's 3rd law of motion. In the weight/string example it's the outward force exerted by the weight on the string. If that force is strong enough to break the string, the weight will no longer be under the influence of centripetal force, and it'll take off in a straight line in whatever direction it was going when the string broke (Newton's 1st law).

That's pretty much the gist of the physics.

So in the tyre valve situation, to be pedantically correct, there is actually no force pushing the valve open (or outward) when the wheel is rotating. Like the weight on the string, the only force acting on the valve plunger is centripetal force towards the centre of the wheel. That force is exerted on the valve plunger by the little valve spring, which in turn is supported by the valve stem (to answer your question about the role of the valve stem). The best way to get it to sink into your brain is to imagine what would happen if the rest of the wheel magically disappeared all of a sudden, so all that remains is the little valve plunger whizzing about in thin air. There would be nothing forcing the valve guts inward to travel in a circle and it would simply fly off in a straight line into the distance.

Centrifugal force in the tyre valve example is the outward force that the valve plunger exerts on the valve spring. If that force is sufficient to compress the spring, the valve will open and the tyre will deflate.
 
I really like the concept of Desmo Tyre Valves; shimming should be fun. You pay a premium if there Red.
 
Thanks to Cam for the explanation, still trying to fully absorb it.
Thought I'd look up to see how Bugatti solved this issue and came across some interesting figures and explanations.
At high speeds 261 mph centrifugal force multiplies the weight of components dramatically, we know that right! , each wheel experiences up to 3,000 g of force. A standard 2.5 gram valve cap effects a force up to 7.5 kg (16 + lbs) causing the valves to bend and leak air.
Bugatti use custom light weight BF1 systems tyre valves.
Given the forces involved I now appreciate how the spring can easily compress and if there is any dirt or corrosion under the seal how the seal will fail.
Coming back to the main point it confirms the need for a diligent inspection of the Schrader valve and body for dirt, scoring and seal damage. New valves with every tyre change.
 
Andy J said:
So we all need to switch to presta valves? 🤔
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Not being very involved in the bicycle scene, I'd never heard of presta valves before, so I just learned something. Thanks Andy. I just googled them to see what the story is. They're a bicycle tyre valve that has a locknut rather than a spring to hold the valve closed, which makes them a bit of a fiddle to use.

I'm not sure whether yours was a serious question, but the short answer is no. They offer no advantage over a Schrader valve and cap, and they're way less convenient to use. It seems the only reason they're used on bicycles is that they're skinnier than Schrader valves (5.84mm vs 8mm diameter), so the hole through the rim is smaller. I guess when your rim is only an inch wide, the hole size becomes pretty important. My bicycle (mountain bike with pretty chunky rims) has Schrader valves though.

Not that it's an option to use them on a motorcycle anyway since they only come fitted to bicycle inner tubes. They don't make them for motor vehicle applications.
 
I’m glad you learned something new Cam, I’ve also learned quite a bit reading this thread.
My comment was indeed intended to just widen the discussion a bit more.
 
Schrader valves are held closed by the air pressure inside the tyre/ tube, little spring only helps it close at low pressure. For a centrifugal force to overcome the pressure behind the valve you must have a low working tyre pressure to start with. They use Schrader valves on loads of things as wheel as millions of pneumatic tyre, they are super reliable. Valves caps are meant to work in conjunction with the valve but again, there are millions of vehicles going around without them perfectly happily. I reckon if you’ve experienced a sudden valve failure you must have had low tyre pressures to start with. I’ve only ever seen it once on the Dyno which is not a real world situation.
I fitted fire escape windows for a lady once, she asked how quickly could she get out from it. I told her the smoke would get her first…😂
 
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