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question in regard to tire pressure cold vs warm


SHOman

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I just had the dealer put on some new tires for me. New Avon Storms on my '04 1150rt , asked for 39f 41r.

 

At the same time i upped the preload a notch as I was dragging my center stand at times.

 

I rode about 120 miles to scrub the tires in. I noticed the ride was kinda rough. Attributed it to the preload change.

 

Got to a familiar road, ride it one way at a slower pace. Then turn around and commence at a brisk pace. Hit the first two corners and my front tire bounced around the corners. Not good.

 

I pull over and check the tires, the front is at 47.5 psi, back 48 psi. I have an "accugage" which is supposed to be as accurate as you can get. Instantly became pissed thinking the dealer screwed up again (dealer service has not been very good in the year i've owned my bike). I was up in the hills out of cell service atm.

 

I lower the tires to 39/41 (as an after thought, prob should have went a bit higher as the ambient temp was 90+ and the tires were at least warm if not hot).

 

Definate improvement, able to hustle down that road. Sticking to the road now. No more tire hop.

 

A few miles down the road, I call the dealer and let them know I am not happy, service writer says service manager is at lunch, she asks me to ride over so they can address it, so I decide to ride back to the dealer.

 

I talk to the service manager, ask him to bring his guage (it was a joke of a guage imo, straight off a gas station air compressor, just no hose attached). Back tire showed high 30's (after leaking a bunch of air out).

 

I ask for my money back or someone competent to redo their work. I dont trust that the work was done correctly if the mechanic cannot even fill the proper PSI into my tires. I am worried my wheels are gonna fall off.

 

Anyways, they redo the work. Afterwards we talk and his reasoning is that differences in guages and the tire getting hot is why I saw 47.5f and 48r.

 

Is it possible what he is saying is true? Is it possible 39/41 turned into 47.5/48? That's a pretty big difference.

 

I think I am getting BS'd. What say you?

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ShovelStrokeEd

OK, let's think about this a bit.

Tire gauges can and do vary from one gauge to another. Without a calibration certificate and reasonable care on the part of the user, you have no way of knowing if your gauge is any more accurate than his. There is nothing wrong with the pencil type gauges so long as they are cared for in a reasonable fashion and checked once in awhile against a standard. Ditto digital gauges, I have seen them vary quite a bit. I would call any gauge that comes within 5% of reading to be pretty good.

 

The temperature part is probably BS though and here is why.

 

Barring liquid water in the tire, the rate of change in pressure within the tire is directly proportional to the temperature. Problems arise with this until you get the units right though.

 

Temperature units for this equation to work are degrees Kelvin, a considerably broader scale than that of Fahrenheit or Centigrade. You said it was 90 deg F outside and let's assume it was somewhere around 75 degrees in the air conditioned service area, a difference of 15 degrees. So you would think your air pressure would go up by 90/75 or 20%. Not so bunky. The real difference is 305/297 or about 2.6%. Assuming you were at 40 PSI at the dealer, ambient temperature difference will raise tire pressure to 41.4 PSI. Allow for flex in the tire to raise the internal air temperature to say 120 degrees and you would have 322/297 or 8.4% giving a rise to 43.36 PSI.

 

Now to really be accurate, you would have to convert the pressure change into PSIA, not PSIG. Assuming sea level, you have to add 14.7 PSI to your gauge reading and do the percentage conversion from there. Suffice that you ain't getting a 10 PSI difference in tire pressure just from temperature change.

 

The relationship is ((P1*V1)/T1)=((P2*V2)/T2). Units are Pascals for pressure, Liters for Volume and deg K for temperature. The volume can be discounted giving (P1/T1)=(P2/T2).

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Well checked the tires just now. Should be cold by now. I measure and get 30f/34r Checked with two guages to make sure. eek.gif

 

 

Looks like I cant rely on the local BMW dealer to put the proper PSI in my tires. confused.gif

 

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gotta love my TPM system. I automatically corrects the pressure to 68F. I'm assuming there are temperature sensors inside hte tire because the pressure only varies by about 1psi in all conditions and temperatures.

 

tire pressures increase about 1psi for every 10 degrees F. IF the tire was 70F when filled, thne got ot 120F when riding aggressively, you'd see a 5psi increase. 44 or 45psi reading when it was filled ot 39psi cold, it not suprising.

 

I'd guess you gauge or their gauge is reading 1-2 psi off... or your is 1psi high, theirs is 1psi low. The fact that the 2psi stagger was correct, is a good indication that the dealer did their job correctly.

 

Expecting precision better than 5% for a $10-20 instrument is optimistic at best. If you want 1% accuracy... you'll need a $100+ instrument.

 

That would be pointless, because tire pressure will also vary due to altitude and even atmospheric pressure as well as temerature.

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Anyways, they redo the work. Afterwards we talk and his reasoning is that differences in guages and the tire getting hot is why I saw 47.5f and 48r.

 

Is it possible what he is saying is true? Is it possible 39/41 turned into 47.5/48? That's a pretty big difference.

 

I think I am getting BS'd. What say you?

 

First of all, you're waaaay too anal if you are looking at 47.5 psi. You can't get that close.

 

Second, I run a SmartTire device. I have seen nearly 8 psi increase on the rear tire and almost that on the front, particdularly in summer when it's hot.

 

Finally, you need to determine what the right tire pressure is for you, your bike and the way you ride. If the tires are under inflated to begin with, they will overheat due to the excessive sidewall flexing. Overheating means increased tire pressure. Try using the 10% rule before you start jumping in someone else's face. Measure your tires cold. Go out and get them hot riding the way you ride. Stop and IMMEDIATELY measure your tire pressures again. If they are more than 10% higher than the cold pressures, your tires are under inflated because the excessive sidewall flexing is creating heat. If the hot pressures are less than 10% higher than the cold pressures, your tires are over inflated. Even with this rule, do not exceed the maximum tire pressure stated by the manufacturer.

 

<rant>

Finally, you should NEVER let someone else adjust the tire pressure on your bike. Take responsibility for your own safety and stop blaming someone else.

</rant>

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The relationship is ((P1*V1)/T1)=((P2*V2)/T2). Units are Pascals for pressure, Liters for Volume and deg K for temperature. The volume can be discounted giving (P1/T1)=(P2/T2).

 

Yes, the temperature is absolute, but the units can be Imperial or metric; they divide out. While the volume can't be discounted, the change in volume can (O.K., this is semantics, sorry).

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Well checked the tires just now. Should be cold by now. I measure and get 30f/34r Checked with two guages to make sure. eek.gif

 

 

Looks like I cant rely on the local BMW dealer to put the proper PSI in my tires. confused.gif

 

You let air out when the tires were hot! What do you expect?

 

Your sig line tells me a lot. A&S is a good dealer.

 

BTW, it's "gauges".

 

Maybe you shouldn't be riding motorcycles.

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.5 psi increments are easy to see with an accugage. Goes to 1 psi increments. It supposed to be very accurate. That is why I bought it.

 

Well checked the tires just now. Should be cold by now. I measure and get 30f/34r Checked with two guages to make sure. eek.gif

 

 

Looks like I cant rely on the local BMW dealer to put the proper PSI in my tires. confused.gif

 

You let air out when the tires were hot, you idiot! What do you expect?

 

Your sig line tells me a lot. A&S is a good dealer.

 

Maybe you shouldn't be riding motorcycles.

 

Maybe you should learn to read?

 

This is the result after A&S told me to come over and they rechecked their work.

 

<rant>

Finally, you should NEVER let someone else adjust the tire pressure on your bike. Take responsibility for your own safety and stop blaming someone else.

</rant>

 

Yes god forbid I expect a place that charged me $118 to change my tires to get it right.

 

I got a lot of comments and excuses that day that I didnt bother to post. I found the one about the tires not being "BMW approved" was quite amusing.

 

As for A&S being a good dealer. I used to think that too. After a year of dealing with them I can no longer say that. It seems I am not alone either. Go search advrider or the club they sponser River City Beemers for that matter.

 

Maybe you should remember that what you experience may not be the same as others before you type?

 

No dealer is perfect and many will make mistakes. I dont know what it is about this bike and dealer but my experiences have been bad.

 

Other than the quality of service, I have no issues with them. They treat you very well.

 

BTW sig line was added last night. Why should you care what my sig line says anyways?

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First, I would like to say that I shouldn't have called you an idiot in my previous post. While you very well may be an idiot, this forum, one where an extreme amount of forum etiquette is demonstrated on a daily basis, is not the place to declare it. My apologies to the forum members for my lack of keystroke control.

 

.5 psi increments are easy to see with an accugage. Goes to 1 psi increments. It supposed to be very accurate. That is why I bought it.

 

Just because your gauge displays 0.5 psi increments, doesn't mean the data is valid to that level. IMO, if you're within 1-2 psi, you're doing pretty good compared to 98.5% of the riders.

 

This is the result after A&S told me to come over and they rechecked their work.

That was after you ragged on them on the telephone call from roadside. What else would they do?

Yes god forbid I expect a place that charged me $118 to change my tires to get it right.

Don't tell me you paid $118 to have your tire pressure checked. If that's true, I have some swamp land in Arizona...

 

As for A&S being a good dealer. I used to think that too. After a year of dealing with them I can no longer say that. It seems I am not alone either. Go search advrider or the club they sponser River City Beemers for that matter.

 

Maybe you should remember that what you experience may not be the same as others before you type?

Sponsor? BMW clubs aren't H.O.G. chapters. Every dealer has it's critics, but to slander one is not good for US. BMW dealers are diminishing fast enough as it is. What would you do if you had to go 300 miles to the nearest BMW dealer? I believe it would have been much more effective to express your displeasure with the service manager rather than venting on this forum. I know I would not want to own a business where you and I are the typical customers. I can't imagine standing behind the counter all day long, smiling while listening to our whining.

 

BTW sig line was added last night. Why should you care what my sig line says anyways?

 

Because it tells me more about who you are than you realize.

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When my bike RT is in for svc and it's better to wait it out (timewise) than to drop the bike off and return tomorrow, I am fortunate enough to converse with the other customers and the staff at the local dlr.

I believe most dlrs are good in all aspects of the business.

What I have noticed during my 'waits' is the attitudes that some customers have the minute they enter.

Most dlrs try to accomodate, but the attitide of the customer refuses to flex.

I watched a guy yesterday come in with a used BMW battery that he thought was about to fail (from a bike too old to be in warranty), and he did not want to embark on a long trip with a failing battery. BUT HE DID NOT WANT A NEW BATTERY! he wanted it to be tested to see if it would hold a charge.

The dlr did evrything to please this guy, but you couldn't make him happy.

I believe it takes two, the customer and the dlr/svc rep, to make things good.

It's too easy to go dealer bashing.

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Initially you had some helpful things to offer and appreciate that. I don't understand why you decided to go with the personal attack route. I don't mind a dissenting opinion, that is sometimes very helpful in looking at things another way.

 

First, I would like to say that I shouldn't have called you an idiot in my previous post. While you very well may be an idiot, this forum, one where an extreme amount of forum etiquette is demonstrated on a daily basis, is not the place to declare it. My apologies to the forum members for my lack of keystroke control.

 

Same goes for you. thumbsup.gif

 

Now joking aside, I will try to address your reply the best I can.

 

"Just because your gauge displays 0.5 psi increments, doesn't mean the data is valid to that level. IMO, if you're within 1-2 psi, you're doing pretty good compared to 98.5% of the riders."

 

I only mentioned 47.5 because that is what they gauge showed regardless of how accurate the gauge truly is.

 

That was after you ragged on them on the telephone call from roadside. What else would they do?

 

No ragging on the phone. Was a pleasant conversation actually. I dont yell or belittle people in this manner. Doesnt help a bit. The service advisor asked me to come over to talk about it so I did.

 

Don't tell me you paid $118 to have your tire pressure checked. If that's true, I have some swamp land in Arizona..."

 

No that was what they charged for both tires to get changed.

 

"Sponsor? BMW clubs aren't H.O.G. chapters."

 

It is right on River City Beemers web page, maybe they sponser just the web page, I dont know the details of the relationship "site sponsered by A&S".

 

"Every dealer has it's critics, but to slander one is not good for US. BMW dealers are diminishing fast enough as it is. What would you do if you had to go 300 miles to the nearest BMW dealer?"

 

I totally agree. Needs to be more dealers, more alternatives.

 

"I believe it would have been much more effective to express your displeasure with the service manager rather than venting on this forum."

 

I did do that and was happy they double checked their work and thanked them multiple times. Wasn't happy with a lot of what I saw that day and what I heard or what was alluded to by the service manager.

 

The main purpose of my post was to get all the valuable opinions here if what I am being told is the truth or not.

 

"Because it tells me more about who you are than you realize."

 

I do not agree. Some sig I placed at 1:00am when I was tired and mulling over all the issues i've encountered tells you little. I could say the same statement about someone making assumptions based on a sig line.

 

Anyways, I appreciate you mentioning the smarttire. Sounds like a good investment for a bike.

 

(sorry for the misquote's, guote feature didnt quite work out)

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ShovelStrokeEd

Expecting precision better than 5% for a $10-20 instrument is optimistic at best. If you want 1% accuracy... you'll need a $100+ instrument.

 

That would be pointless, because tire pressure will also vary due to altitude and even atmospheric pressure as well as temerature.

 

I fully agree with the first statement. I have tested a variety of pressure gauges once in my possession against the calibrated pressure tester we use at work. Only one survived this and did not wind up in the garbage. One of the gauges, a quite expensive one at that, was 8 PSI off at 40 PSI gauge.

 

Don't fully agree with the second statement. The temperature, yes, the altitude only partially. Yes, the tire pressure will change with altitude but only if you measure with an absolute pressure gauge. If the tire is at 30 PSIG at sea level, the same gauge will still show 30 PSIG at 10000 feet. The gauge is comparing the pressure with local atmospheric pressure. The real (absolute) pressure in the tire will have dropped considerably.

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Firefight911
As for A&S being a good dealer. I used to think that too. After a year of dealing with them I can no longer say that. It seems I am not alone either. Go search advrider or the club they sponser River City Beemers for that matter.

 

Maybe you should remember that what you experience may not be the same as others before you type?

 

No dealer is perfect and many will make mistakes. I dont know what it is about this bike and dealer but my experiences have been bad.

 

Other than the quality of service, I have no issues with them. They treat you very well.

 

As a member of RCB and a VERY satisfied customer of A&S, I would ask that you deal with A&S directly and not just shotgun blast and include other people or entities as you feel the need to have done to date.

 

If you did not receive satisfaction from Tim, did you go to Jeff? How about Randy?

 

As for RCB, A&S has been a staunch supporter of RCB and is merely a wonderful dealer supporting those who support them.

 

Do they have a 100% satisfacction rating? Absolutely not. You can please some of the people all the time, but not all the people all the time.

 

Go and TALK with A&S about your dissatisfaction. I can assure you, if you are reasonable (Keyword!!!!!!!), they will handle any concern you have.

 

Remember, you may be the customer, but the customer is NOT always right. But, you still are the customer.

 

A&S knows this!!!!

 

As for RCB, why don't you become a member and learn first hand how much support A&S throws our way before you start blasting RCB and its members!!!!

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Geez, this is getting ridiculous.

 

Never said a bad word about RCB. No need to defend them, I have met some members on occasion and they were great people, fun to ride with.

 

This post was solely intended to get answers to two questions (the two posed at the end of the OP), to get your valuable input as to what I experienced. A&S was not mentioned in the OP as it wasn't important. Somehow it has digressed into what it is now.

 

I knew nothing about R bikes and their nuances when I got the bike a year ago, and over the last year have learned a lot from the members here (thanks again). clap.gif

 

For the folks telling me to contact A&S. I did so and they did address this recent issue to the best of their ability.

 

I have had past service issues with A&S over the course of the last year. Once I got a clear understanding of what happened, recapped my experience and sent an email a week or so ago to A&S. I initially did not intend to email them until prompted by other BMW riders where I work. The main purpose of my contact was to make them aware of what happened so it wont happen to anyone else (like my coworkers or you).

 

I have received an initial reply from the GM that he'd have their service manager look into it. A week has gone by with no follow up so far. In reading his reply I am unsure if there is going to be any follow up or even a need for it.

 

How the latter ends up getting included in a post about tire pressure is beyond me.

 

For the folks that genuinely offered help and guidance, thank you.

 

If I personally offended anyone I do apologize, I never intended to do so and value everyone's opinions here.

 

I think enough has been said in this post. Time to move on.

 

 

As for RCB, why don't you become a member and learn first hand how much support A&S throws our way before you start blasting RCB and its members!!!!

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1) In reading the OP, I don't see any mention of A&S or anyone's names until it was brought up by Mr. Personal Attack. Seemed the OP just wanted an opinion about cold vs. hot tire pressures.

 

2) "Paid $118 for checking tire pressure": what if you had them change the oil and they overfilled it so blue smoke was belching out the muffler? Would that too be OK? Where do you draw a line with expecting things to be done correctly? I guess we draw it at proper tire pressure?

 

This forum is a good place to relate experiences which could benefit all, and that is all I see. Just don't understand why OG is taking such a personal offense at the posting and the poster.

 

confused.gif

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Geez as I am sure can speak for himself.

 

IMO he took instant offense at my not so complimentary sig line I was mucking around with last night that referred to my dissatisfaction in regards to A&S (since removed to keep the peace).

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Geez as I am sure can speak for himself.

 

IMO he took instant offense at my not so complimentary sig line I was mucking around with last night that referred to my dissatisfaction in regards to A&S (since removed to keep the peace).

 

OIC--that changes things...a little. Still no excuse for a personal attack, which btw is forbidden on this forum.

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I still stick with my original idea. Check your tires with your guage and have him check them with his. Compare the two readings. If your guage agrees with his, then see what you readings are when you get home and heated. If your guage doesn't agree with his, then one of you has a problem. Go for a third or fourth guage tie breaker.

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Yes, the tire pressure will change with altitude but only if you measure with an absolute pressure gauge. If the tire is at 30 PSIG at sea level, the same gauge will still show 30 PSIG at 10000 feet. The gauge is comparing the pressure with local atmospheric pressure. The real (absolute) pressure in the tire will have dropped considerably.

 

It's Sunday. Probably my dumb day of the week.....

 

But

 

I'm having trouble with the above statement by Ed. Given that absolute pressure is referenced to a vacumn, I would have thought that it would be a constant as you changed altitude, ignoring any change in volume of the tire.

 

Did Ed say it backwards, or does the tire expand that much, or am I just confused?......<sigh>

 

Stan

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Joe Frickin' Friday
Don't fully agree with the second statement. The temperature, yes, the altitude only partially. Yes, the tire pressure will change with altitude but only if you measure with an absolute pressure gauge. If the tire is at 30 PSIG at sea level, the same gauge will still show 30 PSIG at 10000 feet. The gauge is comparing the pressure with local atmospheric pressure. The real (absolute) pressure in the tire will have dropped considerably.

 

Ed, ya got that backwards. Take a non-leaky tire from sea-level to 10K feet, and its absolute pressure (a function of tire volume and the # of molecules of air contained therein) will not change. But the tire's gauge pressure (which is what your hand-held tire pressure gauge reads) will increase by about 4.5 psi, owing to the drop in local ambient pressure.

 

Suddenly makes me wonder how the SmartTire system handles changes in atmospheric pressure. The transducers inside the tires must be absolute gauges (since they do not have access to ambient pressure); is there an additional transducer inside the display unit that provides an absolute measure of local atmospheric pressure, so that the tire's true gauge pressure can be calculated?

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<"Ed, ya got that backwards. Take a non-leaky tire from sea-level to 10K feet, and its absolute pressure (a function of tire volume and the # of molecules of air contained therein) will not change. But the tire's gauge pressure (which is what your hand-held tire pressure gauge reads) will increase by about 4.5 psi, owing to the drop in local ambient pressure.>"

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Maybe it's my slow day too but I don't understand why a gauge would read more OR less at different altitudes. When you press the gauge to the valve stem, air rushes into a sealed chamber and pushes a piston against a spring, which pushes a plunger with pressures engraved on it. How far it can compress (or stretch) the spring indicates the tire's pressure. I don't see that the piston is working against the ambient atmospheric pressure at all. For an analog gauge or electronic gauge with a pressure sensor, same story. Or am I just all wet? blush.gif

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Joe Frickin' Friday
<"Ed, ya got that backwards. Take a non-leaky tire from sea-level to 10K feet, and its absolute pressure (a function of tire volume and the # of molecules of air contained therein) will not change. But the tire's gauge pressure (which is what your hand-held tire pressure gauge reads) will increase by about 4.5 psi, owing to the drop in local ambient pressure.>"

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Maybe it's my slow day too but I don't understand why a gauge would read more OR less at different altitudes. When you press the gauge to the valve stem, air rushes into a sealed chamber and pushes a piston against a spring, which pushes a plunger with pressures engraved on it. How far it can compress (or stretch) the spring indicates the tire's pressure. I don't see that the piston is working against the ambient atmospheric pressure at all.

 

But it is. Pushing on one side of the piston: tire pressure. Pushing on the other side of the piston: spring + atmospheric pressure.

 

Note that when your gauge reads zero psi, the tire is still filled with atmospheric (i.e. non-zero) absolute pressure.

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<"Ed, ya got that backwards. Take a non-leaky tire from sea-level to 10K feet, and its absolute pressure (a function of tire volume and the # of molecules of air contained therein) will not change. But the tire's gauge pressure (which is what your hand-held tire pressure gauge reads) will increase by about 4.5 psi, owing to the drop in local ambient pressure.>"

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Maybe it's my slow day too but I don't understand why a gauge would read more OR less at different altitudes. When you press the gauge to the valve stem, air rushes into a sealed chamber and pushes a piston against a spring, which pushes a plunger with pressures engraved on it. How far it can compress (or stretch) the spring indicates the tire's pressure. I don't see that the piston is working against the ambient atmospheric pressure at all.

 

But it is. Pushing on one side of the piston: tire pressure. Pushing on the other side of the piston: spring + atmospheric pressure.

 

Note that when your gauge reads zero psi, the tire is still filled with atmospheric (i.e. non-zero) absolute pressure.

 

I disagree--the ambient air pressure is already pressing against both the outer and inner surfaces of the "piston" (of the gauge) prior to letting the tire's pressure into it, and the gauge is also reading zero prior to doing this, so pressures are equalized. The only thing offering any resistance to the tire's pressure is the spring.

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I have an "accugage" which is supposed to be as accurate as you can get.

 

I have two of them and they consistently read 4" apart. The lower-reading one is consistently 2" higher than either digital gauge I have. confused.gif

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Joe Frickin' Friday
I disagree--the ambient air pressure is already pressing against both the outer and inner surfaces of the "piston" (of the gauge) prior to letting the tire's pressure into it, and the gauge is also reading zero prior to doing this, so pressures are equalized. The only thing offering any resistance to the tire's pressure is the spring.

 

the pressures on the inner surface of the piston are not additive in the manner you describe; if they were, then the "test and hold" gauges (the ones that hold a reading until you press a button to release the air) would read higher and higher pressures as you checked more and more tires. The tire pressure gauge measures the difference between absolute pressures on either side of that piston, and it is the tire's absolute pressure (not the gauge pressure) that pushes against the inner surface of the piston. If local atmospheric is 14.7 psi, and your tire gauge says "40 psi," then your tire's absolute pressure is 54.7 psi. Your gauge's reading will change if the local ambient pressure changes.

 

You needn't take my word for it; you can actually check this yourself. Drive/ride from a low-altitude location to a high-altitude location, let your tires cool off thoroughly, and check the tires with whatever tire gauge you want; you'll find that the gauge pressure reading will have increased measurably, about a half a psi per 1000 feet of altitude gain.

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Well, I took my new Storms out yesterday. The front measured 34.5 psi cold in the garage at 4400' amsl. At a little over 8000' amsl, hot and nicely scrubbed from a 30 mile run with about 15 miles of twisties to finish, the front read 36.0. I am using a digital gauge. I don't think it is referencing to ambient air pressure, or the difference just from altitude change alone would be quite a bit more than that. Anyway, for whatever it's worth, that's my little experiment.

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Well, I took my new Storms out yesterday. The front measured 34.5 psi cold in the garage at 4400' amsl. At a little over 8000' amsl, hot and nicely scrubbed from a 30 mile run with about 15 miles of twisties to finish, the front read 36.0. I am using a digital gauge. I don't think it is referencing to ambient air pressure, or the difference just from altitude change alone would be quite a bit more than that. Anyway, for whatever it's worth, that's my little experiment.

 

Twisties, I believe you just demonstrated exactly what I was saying. Especially when using a digital gauge, which has a sealed chamber with an electronic pressure sensor, which is NOT relating anything to the ambient pressure--just reading the pressure in the tire as it pressurizes that sealed chamber. It went up a typical amount that I have noticed mine rise after riding a while (I have TPM on my R1200RT).

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<"the pressures on the inner surface of the piston are not additive in the manner you describe; if they were, then the "test and hold" gauges (the ones that hold a reading until you press a button to release the air) would read higher and higher pressures as you checked more and more tires. The tire pressure gauge measures the difference between absolute pressures on either side of that piston, and it is the tire's absolute pressure (not the gauge pressure) that pushes against the inner surface of the piston. If local atmospheric is 14.7 psi, and your tire gauge says "40 psi," then your tire's absolute pressure is 54.7 psi. Your gauge's reading will change if the local ambient pressure changes.

 

You needn't take my word for it; you can actually check this yourself. Drive/ride from a low-altitude location to a high-altitude location, let your tires cool off thoroughly, and check the tires with whatever tire gauge you want; you'll find that the gauge pressure reading will have increased measurably, about a half a psi per 1000 feet of altitude gain.

 

Joe, think about how an electronic digital gauge works. When you press the "test" button, it looks at whatever ambient pressure exists and then resets the display to zero--regardless of altitude. When that 36psi air entrapped inside that tire (yes it's still 36psi even at a higher altitude) enters the sealed chamber and presses on that pressure sensor, the gauge will read 36psi--same way it works at sea level. Now the only way I see a different reading could occur at higher altitude is if the side walls of the tire expand some due to the lower atmos. pressure at altitude, causing a slightly lower reading. I am sure there is probably some small factor involved here but it would be minor.

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Joe Frickin' Friday
Joe, think about how an electronic digital gauge works. When you press the "test" button, it looks at whatever ambient pressure exists and then resets the display to zero--regardless of altitude.

 

Yes.

 

An electronic digital tire pressure gauge like this (i.e. one that requires you to "wake it up" before measuring tire pressure) has an absolute pressure transducer. The wake-up/test action gives it an opportunity to measure/remember local ambient pressure; when it measures your tire's absolute pressure a few seconds later, it uses that stored ambient-pressure value to calculate the tire's gauge pressure, which is what ultimately gets displayed on the screen.

 

The fact that the display reads zero psi after you turn it on - despite the fact that local ambient pressure is well above zero - is a clear indication that the display shows gauge pressure and not absolute pressure.

 

When that 36psi air entrapped inside that tire (yes it's still 36psi even at a higher altitude) enters the sealed chamber and presses on that pressure sensor, the gauge will read 36psi--same way it works at sea level.

 

No.

 

If you have 36 psi absolute in your tire, yes, you will have 36 psi absolute in your tire regardless of altitude. But your gauge (whether electronic-digital, piston/slide, or Bourdon-tube) will read 21.3 psi when you're at sea level, and 25.9 psi at 10,000 feet (assuming no leakage and no temperature change).

 

If you have 36 psi gauge in your tire at sea level, you will have 40.6 psi gauge in that tire at 10,000 feet (assuming no leakage and no temperature change).

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I think we're forgetting about the function of the tire and the 2 purposes the air serves inside the tire.

 

1) The most improtant function of air in a tire is to push against the inside wall of the tire and support the weight of the motorcycle. This force is RELATIVE to the atmospheric pressure. The tire gauge is also relative to atmospheric pressure.... unless you have a very expensive gauge which reads absolute.

 

2) THe second function of the air in the tire is to help disipate heat generated. Although the effect of reducing tire flex has a much greater effect, regardless, more air = more molecules to aborb and transfer heat to the rim.

 

The first, which effect performance the most, IS affected by altitude changes. If the abosolute pressure is 15psi outside and 50psi inside... the net force supporting the weight of the tire is 35psi. If the bike has 400 lbs on the wheel the contact patch is roughly (if you ignote tire wall stiffness and assume weight is distributed evenly)11.4 in^2.

 

If you go up in altitude, and the outside pressure is 13psi, the tire will now have 37psi pushing on the inner wall, the contact patch is now 10.8 in^2.

 

IF you ride and the air temperature in the tire goes up 20 degrees, you may now have 39psi in the tire.

 

However, as altitude increases, the temperature normally decreses, so the effect is less noticeable.... but when I traveled to Colorado, I set my pressures cold at 5500 ft in Colorado Springs my first night and decided that was a adequate compormise for my trip. I let my car tires just stay a little high... as it was still well under the sidewall maximum.

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Joe, think about how an electronic digital gauge works. When you press the "test" button, it looks at whatever ambient pressure exists and then resets the display to zero--regardless of altitude.

 

Yes.

 

An electronic digital tire pressure gauge like this (i.e. one that requires you to "wake it up" before measuring tire pressure) has an absolute pressure transducer. The wake-up/test action gives it an opportunity to measure/remember local ambient pressure; when it measures your tire's absolute pressure a few seconds later, it uses that stored ambient-pressure value to calculate the tire's gauge pressure, which is what ultimately gets displayed on the screen.

 

The fact that the display reads zero psi after you turn it on - despite the fact that local ambient pressure is well above zero - is a clear indication that the display shows gauge pressure and not absolute pressure.

 

When that 36psi air entrapped inside that tire (yes it's still 36psi even at a higher altitude) enters the sealed chamber and presses on that pressure sensor, the gauge will read 36psi--same way it works at sea level.

 

No.

 

If you have 36 psi absolute in your tire, yes, you will have 36 psi absolute in your tire regardless of altitude. But your gauge (whether electronic-digital, piston/slide, or Bourdon-tube) will read 21.3 psi when you're at sea level, and 25.9 psi at 10,000 feet (assuming no leakage and no temperature change).

 

If you have 36 psi gauge in your tire at sea level, you will have 40.6 psi gauge in that tire at 10,000 feet (assuming no leakage and no temperature change).

 

Mitch--help me out here--please define the term "absolute pressure" without using the word "tire".

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Joe Frickin' Friday
Mitch--help me out here--please define the term "absolute pressure" without using the word "tire".

 

Absolutely. lmao.gif

 

From the EIT Reference Manual, 8th Edition:

 

Fluid pressures are measured with respect to two pressure references: zero pressure and atmospheric pressure. Pressures measured with respect to a true zero pressure reference are known as absolute pressures. Pressures measured with respect to atmospheric pressures are known as gage pressures. Most pressure gauges read the excess of the test pressure over atmospheric pressure (i.e., the gage pressure).

...

Absolute and gage pressures are related by [the equation below]. It should be mentioned that P_atmospheric in [the equation below] is the actual atmospheric pressure existing when the gage measurement is taken. It is not standard atmospheric pressure, unless that pressure is implicitly or explicitly applicable.

 

P_absolute = P_gage + P_atmospheric

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<"Pressures measured with respect to atmospheric pressures are known as gage pressures. Most pressure gauges read the excess of the test pressure over atmospheric pressure (i.e., the gage pressure).'>

--------------------------------------------------------

 

OK--makes perfect sense. So explain why, if the gauge resets itself to ZERO before testing at 14,000 ft. (thus eliminating the ambient pressure from the equation) , why would it give a lower reading than if the gauge was at sea level?

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Joe Frickin' Friday
<"Pressures measured with respect to atmospheric pressures are known as gage pressures. Most pressure gauges read the excess of the test pressure over atmospheric pressure (i.e., the gage pressure).'>

--------------------------------------------------------

 

OK--makes perfect sense. So explain why, if the gauge resets itself to ZERO before testing at 14,000 ft. (thus eliminating the ambient pressure from the equation) , why would it give a lower reading than if the gauge was at sea level?

 

Because it's broken? grin.gif

 

It should give a higher reading at 14K feet, since the tire's gage pressure is higher at 14K feet than at sea level. Refer back to that equation:

 

P_absolute = P_atmospheric + P_gage

 

Let's rearrange this a bit:

 

P_gage = P_absolute - P_atmospheric

 

If P_absolute is constant (no leakage, no temp change), and you reduce the local P_atmospheric, then P_gage increases.

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What a bunch of nerds, all of you. It's like a National Cliff Claven convention. tongue.gif

 

Ain't it the truth. Mitch, I don't think either of us will adopt the other's point of view, but if you're coming to the "UN", maybe we can further debate this burning issue over a warm scotch while sitting around the camp fire....or not. grin.gif

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So explain why, if the gauge resets itself to ZERO before testing at 14,000 ft. (thus eliminating the ambient pressure from the equation) , why would it give a lower reading than if the gauge was at sea level?

 

Most tires gauges that I'm aware of don't rezero themselves nor are manually zeroed. Normally they measure the absolute differential pressure between the atmosphere and the tire valve. Most use a metallic membrane that under pressure causes a change in voltage that when scaled, correlates to a pressure value. The really cheap gauges use a plastic piston and a compressed spring. The greater the pressure exerted, the further the spring is extended. The spring pushes a long platic rod with graduated markes on it. Very simple, but not terrible accurate... but good enough for general automotive use.

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