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Friday, July 3, 2020

Valve Stems and TPMS Part 2

As a follow up to last week's post of why I recommend people NOT use rubber valve stems with their external TPMS sensors I decided to show some of the "guts" of valve stems.

Standard "rubber snap-in" stems like the TR413. If you look down the hole you can see the end of the brass part of the stem. More on this later.

These have been used for decades on hundreds of millions of tires. These can be installed by hand using a "puller" that stretches the rubber which makes the diameter of the stem small enough to "snap into place in the wheel hole.

Once installed the wheel "pinches" the rubber part of the stem to seal the air in. The arrows show the location of the wheel relative to the metal part of the stem.  Note the part of the valve stem that goes into the air chamber was cur off before I took this picture.

You can see that the brass stops before it gets past the edge of the wheel. This makes installation easier.

Next, we have the "High Pressure" stems such as the HP-500.

 Here you can see the brass part extends almost to the bottom of the valve and into the air chamber.

When the rubber is buffed off you can see that the brass part extends through the wheel hole (location shown with the arrows.)

Now when we look at a "bolt-in" stem, like this TR416s
we see the location of the wheel hole at the arrows. We can also see the much larger brass body (nickel-plated in this piece) that goes inside the air chamber and expands to a broad base. This type of stem needs to be installed through the wheel hole with the rubber grommet sealing the air. An external washer is used and the nut is to be tightened to specification to prevent air loss.

I am also including pages from the US Tire & Rim Association yearbook which publishes the "interchange and fitment" specs so all tire companies and valve manufacturers know what dimensions are required. This is the book where all the Load & inflation tables come from and might be considered the Tire Engineer's "Bible" and is used by tire engineers around the world when they are making tires that are intended to be used in the US.

Here we find the details of valve stem designs as specified by TRA.

Finally to show the attention to detail here is the spec for the little pin that sticks out of the valve stem. Not meeting this specification could be the reason your TPMS does not register your tire pressure. (Yes I have run into that problem)

 I just wanted to try and give you a little understanding of the attention to detail tire engineers go-to when designing tires and when trying to understand the "why" for a tire to lose air.


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