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Measuring Tire Pressure


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Since we getting all sorts of eye-opening info on this new section of the forum, I wanted to mention something else that was at one point news to me. I had read somewhere that using those pencil-shaped tire gages like they give you free when you buy tires for your car aren't very accurate. So I bought one of those round-gage dial pressure gages, which appears to be better.

 

I just got to thinking about Dunlop Steve's comments about tire pressure and thought it's important to be reading accurately. Are there any certain makes/models of gages that are recommended to be used (or avoided)?

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Since we getting all sorts of eye-opening info on this new section of the forum, I wanted to mention something else that was at one point news to me. I had read somewhere that using those pencil-shaped tire gages like they give you free when you buy tires for your car aren't very accurate. So I bought one of those round-gage dial pressure gages, which appears to be better.

 

I just got to thinking about Dunlop Steve's comments about tire pressure and thought it's important to be reading accurately. Are there any certain makes/models of gages that are recommended to be used (or avoided)?

 

Tire Gauges.

 

The measuring device in your hand is only as good as it can be CALIBRATED.

 

I have observed very expensive tire gauges be 5 lbs off, and give away cheap ones being spot on. There is no sure fire bet on gauges.

 

2 factors that are important: 1) will the tire gauge repeat and give the same pressure over and over. 2) getting that gauge calibrated?

 

How do you get your tire gauge calibrated? We have calibrator that is available for use at all the AMA National road races at our service truck. This gauge is calibrated to a master gauge from the Dunlop Factory in Buffalo NY. All our tire machine and gauges we use get calibrated by this standard several times throughout each season. If we have a gauge that is always changing, we get rid of it. It must constantly repeat every time we calibrate it.

 

How would someone get their gauge calibrated when they do not have access to our standard? I honestly don't have an answer to that. Lets see who chimes in and give a good way to calibrate a gauge in the field. Ideas?

 

Keep in mind, if a gauge is 2lbs off, it not bad. It's just 2 lbs off. If your gauge reads 30 when its really 32, just note "+2" on the face and keep using it. As long as its always 2lbs off you still have a very usable tool, as long as you know the offset. Some gauges have the ability to adjust the calibration, if so do it, otherwise just note with a marker on the face the offset. (-1, +3, -2)

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I would think the simple pencil type ones would basically be the most reliable. I've never calibrated them, but they've lasted longer and been more reliable than any $20 gauge I've invested in. I've got 6 of them (up to 5-6 years old) and, if I remember, will put them on the same tire and see if they're all still the same or close.

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If your looking for an accurate pressure reading, you need to take into account the temperature of the tyre (tire).

 

Ideally, you'd measure the pressure when the tyre is at its working temperature. ie: After you have ridden for sometime. Check the pressure again once the tyre has cooled, to get an ambient reading. This does give you a general base line value to start/check from before you ride. But, this would change (+/-) based on the ambient temperature.

 

I have a base ambient value, and a tyre warmer check value (rims are heated) this is constant and accurate. These values are usually tweaked based on track conditions and anticipated track temperature. Having a chat to the tyre tech/rep will give you an indication of what the track is doing).

 

Unless, you're using an inert gas, the pressure of the tyre will change depending on the temperature of the tyre.

 

Cheers

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If your looking for an accurate pressure reading, you need to take into account the temperature of the tyre (tire).

 

Ideally, you'd measure the pressure when the tyre is at its working temperature. ie: After you have ridden for sometime. Check the pressure again once the tyre has cooled, to get an ambient reading. This does give you a general base line value to start/check from before you ride. But, this would change (+/-) based on the ambient temperature.

 

I have a base ambient value, and a tyre warmer check value (rims are heated) this is constant and accurate. These values are usually tweaked based on track conditions and anticipated track temperature. Having a chat to the tyre tech/rep will give you an indication of what the track is doing).

 

Unless, you're using an inert gas, the pressure of the tyre will change depending on the temperature of the tyre.

 

Cheers

 

Yes mostly true, But this thread is about the accuracy of the GAUGE itself.

 

Your point is well taken, but for another thread.

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How would someone get their gauge calibrated when they do not have access to our standard? I honestly don't have an answer to that. Lets see who chimes in and give a good way to calibrate a gauge in the field. Ideas?

Excuse me while I geek out...

 

Creating your own calibrated gauge shouldn't be that difficult - all it takes is a bit of ingenuity and knowledge of the ideal gas law: Pressure * Volume = n * R * Temperature.

Take a tubular tank where you can connect your gauge in one end and be able to compress the air using a piston. First, vent the tank to atmospheric pressure. Then, Compress the air volume to 1/2 using the piston - and you have 2 bar (+/- the difference in atmospheric pressure, which isn't that much). Sure the compression will heat up the air a bit, but that's really marginal (Temperature is in Kelvin, so we're at ~300K to start out). There are 14.5 psi to 1 Bar. You should be able to make smaller volume compressions to have 2-3-4 points where you can calibrate your gauge to.

 

So 1/2 volume = 2 Bar = 29 psi, 1/3 volume = 3 Bar = 43.5 psi. 14.5/20 volume = 20 psi.

 

The rest is left as an exercise for the gentle reader ;)

 

 

Kai

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  • 3 weeks later...

I feel well enough to get back to it and check my gauges. One of my 6 pencil gauges is 7 years old, and one as new as a couple months. Some have been kept in glove boxes, some under the bikes seat, and one in a tool box. I checked both cold car and motorcycle tire pressure with all of them. I checked one car tire pressure and two different motorcycle tire pressures (street and track). I checked the pressures on the bikes hot also.

 

What did I find? You can buy those expensive digital gauges and pumps all day, but in the end you can give me a bicycle pump and pen gauge to manage my tire pressures. They were all spot on and all matched my higher "quality" gauges. When I get to the point 1/2 pound makes a difference, I'll change gauges.

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