tire pressure

[HotRodZilla]

I have run my tires 41f and 43r. Loaded, unloaded, whatever. The difference between 43 and 42 is probably stupid. However, I had bad cupping on my first two sets of tires running just over the recommended pressure. I mentioned that and TWN suggested my current pressures. At the time, it transformed the bike.

Haven't had a cupped tire since...Also haven't put any Bridgestones on since. Lol...

 

[MCRIDER007]

Just about every motorcycle tire has a max allowable pressure printed on the tire of 42 PSI. This is not a recommended pressure for any bike; it is max safe pressure specified for the tire. The max pressure on the tire and the recommended pressures provided with or on the bike assume the pressure is measured at 68 degrees F. You can assume about a 1 to 1.5 PSI change for every 10 degree change in temperature. Putting 44 PSI in a tire is probably not a good idea, especially if it is done at a temperature significantly below 68 F.

The tire manufactures' print the pressure and load data because the DOT requires them to print something but the numbers you are referring to is the max LOAD at 42 psi cold and is a reference point more than anything else. If you are carrying more than the max load then you need to increase the psi to avoid overheating the tires. Motorcycle tires, unlike car tires, are actually designed to heat up to increase traction but the higher the psi, the cooler the tire will run. If the starting psi is too high the tire may not reach optimum operating temperature (and traction). Of course, the opposite is also true, too low of pressure can lead to excessive tire temperatures which can shorten tire life and may actually decrease traction. Tire pressures are a backdoor way to achieve the correct operating temperatures and the only way to know what the optimum pressure is for your load and speed is to measure the psi when the tire is hot and make adjustments to the cold psi to achieve the desired operating temperature (which is 140F for sport touring tires). A general rule of thumb is that the hot tire psi should be within 10-15 percent of the cold psi.

I'm always surprised how some riders think that 44 psi cold (rather than 42) is an unsafe pressure but they don't seem to be the least bit concerned when the hot pressures exceed 50 psi.

 

[Pterodactyl]

<blockquote class='ipsBlockquote'data-author="MCRIDER007" data-cid="1114801" data-time="1387908007"><p>

<blockquote class='ipsBlockquote'data-author="Pterodactyl" data-cid="1114783" data-time="1387903115"><p><strong class='bbc'>Just about every motorcycle tire has a max allowable pressure printed on the tire of 42 PSI. This is not a recommended pressure for any bike; it is max safe pressure specified for the tire</strong>. The max pressure on the tire and the recommended pressures provided with or on the bike assume the pressure is measured at 68 degrees F. You can assume about a 1 to 1.5 PSI change for every 10 degree change in temperature. Putting 44 PSI in a tire is probably not a good idea, especially if it is done at a temperature significantly below 68 F.</p></blockquote>

<br />

The tire manufactures' print the pressure and load data because the DOT requires them to print something but the numbers you are referring to is the max LOAD at 42 psi cold and is a reference point more than anything else. If you are carrying more than the max load then you need to increase the psi to avoid overheating the tires. Motorcycle tires, unlike car tires, are actually designed to heat up to increase traction but the higher the psi, the cooler the tire will run. If the starting psi is too high the tire may not reach optimum operating temperature (and traction). Of course, the opposite is also true, too low of pressure can lead to excessive tire temperatures which can shorten tire life and may actually decrease traction. Tire pressures are a backdoor way to achieve the correct operating temperatures and the only way to know what the optimum pressure is for your load and speed is to measure the psi when the tire is hot and make adjustments to the cold psi to achieve the desired operating temperature (which is 140F for sport touring tires). A general rule of thumb is that the hot tire psi should be within 10-15 percent of the cold psi.<br />

<br />

I'm always surprised how some riders think that 44 psi cold (rather than 42) is an unsafe pressure but they don't seem to be the least bit concerned when the hot pressures exceed 50 psi.</p></blockquote>

Spent several years in the tire business. I disagree with much of what was said here. The pressure printed on the tire is not just for yucks. But to each their own. Merry Christmas.

 

[Pterodactyl]

<blockquote class='ipsBlockquote'data-author="HotRodZilla" data-cid="1114793" data-time="1387906168"><p>

I have run my tires 41f and 43r. Loaded, unloaded, whatever. The difference between 43 and 42 is probably stupid. However, I had bad cupping on my first two sets of tires running just over the recommended pressure. I mentioned that and TWN suggested my current pressures. At the time, it transformed the bike.<br />

<br />

Haven't had a cupped tire since...Also haven't put any Bridgestones on since. Lol...</p></blockquote>

I really dislike Bridgestones. With as many cups as they produce they should be in the bra business.

 

[Chuck35]

I have noticed a crappy looking wear pattern rapidly improve after fitting new suspension. AK-20's and a Penske.

 

[Donal]

...Putting 44 PSI in a tire is probably not a good idea, especially if it is done at a temperature significantly below 68 F.

Ol' RaY is a frost-back from Kanada, he dreams of days when it gets up to 68º.

OK, you are kneeled down putting 42 psi in your tires. Now you and your pillion get all geared up, stow your stuff in the bags and climb aboard. With all the extra weight, what happens to the tire pressure that was set on an unloaded bike? I rode a bike with a TPMS and I know what happened on this bike as well as what happens when the tires get hot too.

OK, looks like no one is willing to stick their necks out with this one, so here goes (it is the season to be silly after all). My guess is the pressure will stay the same but the size of the contract patch will increase. The size of the contact patch is a function of total load on the tyre (tire) divided by the pressure.

 

[MCRIDER007]

...Putting 44 PSI in a tire is probably not a good idea, especially if it is done at a temperature significantly below 68 F.

Ol' RaY is a frost-back from Kanada, he dreams of days when it gets up to 68º.

OK, you are kneeled down putting 42 psi in your tires. Now you and your pillion get all geared up, stow your stuff in the bags and climb aboard. With all the extra weight, what happens to the tire pressure that was set on an unloaded bike? I rode a bike with a TPMS and I know what happened on this bike as well as what happens when the tires get hot too.

OK, looks like no one is willing to stick their necks out with this one, so here goes (it is the season to be silly after all). My guess is the pressure will stay the same but the size of the contract patch will increase. The size of the contact patch is a function of total load on the tyre (tire) divided by the pressure.

I think you are right because the air pressure is not going to change unless the air volume in the tire changes although changing the weight that the tire supports can change the shape of the tire.

 

[Pterodactyl]

Air expands when heated, as do most gasses. Simple enough to test. Put 40 PSI in a tire, then go for an hour long high speed run on a warm, sunny day. Throw in some vigorous twisties as well and then remeasure the pressure. It will be up several pounds. Some run nitrogen in their tires because it is less susceptible to the expansion and contraction caused by changes in temperature;it is particularly popular amonst road racers. When we lived in Fairbanks I ran nitrogen in our cars with tire pressure monitoring systems. When you backed a car out of a 50 degree garage into -45 weather the pressure in a tire with normal air would plummet and set off the low pressure warning every time.

 

[Donal]

Air expands when heated, as do most gasses. Simple enough to test. Put 40 PSI in a tire, then go for an hour long high speed run on a warm, sunny day. Throw in some vigorous twisties as well and then remeasure the pressure. It will be up several pounds. Some run nitrogen in their tires because it is less susceptible to the expansion and contraction caused by changes in temperature;it is particularly popular amonst road racers. When we lived in Fairbanks I ran nitrogen in our cars with tire pressure monitoring systems. When you backed a car out of a 50 degree garage into -45 weather the pressure in a tire with normal air would plummet and set off the low pressure warning every time.

I believe you are referring to Boyle's law, which is absolutely clear about the relationship between pressure, temperature & volume. However, Ionbeam's question was about the relationship between load and pressure.

 

[bluesdog]

I have noticed a crappy looking wear pattern rapidly improve after fitting new suspension. AK-20's and a Penske.

+1

 

[RaYzerman19]

...Putting 44 PSI in a tire is probably not a good idea, especially if it is done at a temperature significantly below 68 F.

Ol' RaY is a frost-back from Kanada, he dreams of days when it gets up to 68º.

OK, you are kneeled down putting 42 psi in your tires. Now you and your pillion get all geared up, stow your stuff in the bags and climb aboard. With all the extra weight, what happens to the tire pressure that was set on an unloaded bike? I rode a bike with a TPMS and I know what happened on this bike as well as what happens when the tires get hot too.

Ha! In my neck of the woods, I'll almost bet my average winter temperature is warmer than yours! Talk to Bungie or somebody in Manitoba about cold........

My TPMS says no difference loaded vs. empty....... hot is another story.

 

[ionbeam]

...OK, you are kneeled down putting 42 psi in your tires. Now you and your pillion get all geared up, stow your stuff in the bags and climb aboard. With all the extra weight, what happens to the tire pressure that was set on an unloaded bike? I rode a bike with a TPMS and I know what happened on this bike as well as what happens when the tires get hot too.

....My TPMS says no difference loaded vs. empty....... hot is another story.

Donal/Ray gots it right. The TPMS on the BMW showed no pressure change. I was also surprised at how little the pressure changed from straight cold roads (~40ºF) to warmer roads (mid 80s) that were mostly curves and tight hair-pin corners when loaded with every pound the bike could carry. The average temperature rise was 2º; by the ambient temperature numbers I would have expected a degree to two more. Guess I wasn't pushing the tires as hard as I thought and had the tire pressure set correctly for the load.

Air expands when heated, as do most gasses...Some run nitrogen in their tires because it is less susceptible to the expansion and contraction caused by changes in temperature;it is particularly popular amonst road racers...

It has been shown that if you fill your tires with air using an air dryer on the air line that the pressure change with heat will behave the same as N2. Empirical evidence indicates that the culprit in pressure increase is moisture in the air. Pure N2 is inherently 'dry' so over the relatively small temperature change in tires there is no perceptible change in pressure using a tire gauge. An airplane flying from NY to Phoenix can have the tires at -65ºF while flying and then touch down on a runway that is >150º degrees so using N2 in this situation is a simple way to ensure no moisture is in the tires giving a stable tire pressure.

 

[RaYzerman19]

Ambient temperature has a big effect......... I participate in a cold ride contest on another forum, where we compete to see who can record the lowest temperature ridden (at least 10 mile ride from home). At the freezing mark, I had set my front tire at 40 psi. Two days later, it was -10C (14F) and when I left home the front tire was 31 psi (obviously underinflated, Doran TPMS flashing a low pressure warning). I knew I didn't have a leak so I proceeded on my ride. 10 miles later it was only 34 psi on mostly straight roads. Total ride was ~35 miles with a few stops for photos to document the ride, but the tire pressure did not reach more than 36 psi. In the summer, with it set at 40-42 psi, the pressure will rise at least 6 psi.

Oh yeah, I'm currently sitting in 5th place in that contest, the #1 guy so far is at -21C/-7F (Iowa), #2 Colorado, #3 Missouri, #4 Massachussets..... we're expecting our S. Dakota buddy to attempt to beat his last year's record -18F..... my temperatures in the so-called Great White North won't get that low.......

 

[Pterodactyl]

It has been shown that if you fill your tires with air using an air dryer on the air line that the pressure change with heat will behave the same as N2. Empirical evidence indicates that the culprit in pressure increase is moisture in the air. Pure N2 is inherently 'dry' so over the relatively small temperature change in tires there is no perceptible change in pressure using a tire gauge. An airplane flying from NY to Phoenix can have the tires at -65ºF while flying and then touch down on a runway that is >150º degrees so using N2 in this situation is a simple way to ensure no moisture is in the tires giving a stable tire pressure.

So much for Mr. Boyle. "pressure and volume is a constant for a given mass of confined gas as long as the temperature is constant." Moisture is not a factor in the formula except as it may affect the volume of the gas. Do you have a link to the air dryer info?

 

[ionbeam]

...Do you have a link to the air dryer info?

I did not include links to most companies that make N2 generation equipment for tire inflation. While most do mention that N2 is dry they avoid using water or moisture terms, almost like they have something to sell they don't mention that any gas or gas combination will be pressure stable as long as moisture/water is not present.

Aircraft tires are usually inflated with nitrogen or helium to minimize expansion and contraction from extreme changes in ambient temperature and pressure experienced during flight. Dry nitrogen expands at the same rate as other dry atmospheric gases, but common compressed air sources may contain moisture, which increases the expansion rate with temperature.

Nitrogen assures more consistent pressure increases due to increases in operating temperatures in a racing environment because of the absence of moisture. This is especially good for participants in track days, high-performance drivers education schools and road racing.

Cooler running temperatures: When air is pressurized, the humidity in it condenses to a liquid and collects in the air storage tank you use at the local gas station. When you add compressed air to the tire, the water comes along for the ride. As the tire heats up during driving, that water changes to a gas, which then expands, increasing tire pressure. Because nitrogen is dry, there is no water in the tire to contribute to pressure fluctuations.

As for moisture, changes in humidity affect tire performance two ways. First, the density of humid air fluctuates more with temperature than that of dry air, so removing humidity can keep your tire pressure more consistent, especially when the temperature climbs over 100 degrees Fahrenheit. That may be a legitimate concern in Formula One racing, but it's not much of an issue if you're just tooling around town.

Water vapor in compressed air acts as a catalyst, accelerating rust and corrosion. Water vapor also absorbs and holds heat. And, when it changes from liquid to vapor, water expands tremendously in volume. So, tires inflated with wet air tend to run hotter and fluctuate in pressure more. That’s why racing tires, where fractions of a psi can radically change handling, are inflated with dry nitrogen.

Regular compressed air can fluctuate considerably when water vapor is present.

There is no significant difference in expansion and contraction characteristics of nitrogen compared to air when moisture is absent.

 

[RossKean]

The trouble with Boyle is that the equation works for an "ideal" gas. Water vapor does not behave as such. At a given temperature and pressure, there may be a certain amount of water in the gaseous state. If the temperature drops, the water vapor condenses and becomes a liquid and the pressure will drop far more than the amount predicted by the math. Conversely, an increase in temperature will increase the pressure by far more than predicted because of the evaporation of free water. As it has been said, the advantage of pure nitrogen is almost entirely due to the fact that it is (usually) dry from the tanked source. This is why it is especially important to have a dry source of gas and to minimize the amount of water-based tire lube when installing new rubber. Air vs nitrogen behaves almost identically in terms of pressure vs temperature. Size of molecules (N2 vs O2) makes very little difference for long term pressure stability as well.

Edit: I see Alan is faster at the keyboard than I am!

 

[RaYzerman19]

The trouble with water vapour is that as air is compressed in your compressor, it heats up and has a greater capacity to absorb more humidity. On a very humid day when you start up your compressor, that is bound to be a bit worse. So, really two things at play..... the hot air your compressor provides will contract some when it cools down (in your tires) and your pressures may be off a bit at ambient (say 68F). The water vapour also cools/condenses with a potential of lower pressures at ambient........

You can put a dessicant drier on your outgoing compressor hose, you should drain your compressor tank of water periodically (usually a valve on the bottom of the tank), and if you have a good compressor with a large enough tank, fill 'er up the night before and let it cool off before you inflate your tires......... Alternatively, and likely the easiest, overinflate your tires a couple of PSI and adjust after a few hours (or next morning) of cooldown to ambient.

The other problem is front tires don't hold much air and are subject to greater variation by slight air loss or gains. It only takes a little bit to change your pressures by a couple of PSI. If you've noticed, same occurs on a rear tire but to a much lesser extent...... darksiders probably noticed that the rear doesn't change much at all due to the relatively huge difference in air volume.

I personally like the TPMS which allows me to check pressures without fooling around, and no chance of losing air due to using a pressure gauge. My TPMS is spot on with my other pressure gauges....... no worries.

 

[ionbeam]

I try to keep my large compressor tank empty of water. Nearly 80% of what I put in my tires is N2 and argon which limits the amount of water and water vapor that can get pumped into my tires. Unfortunately, that rogue Oxygen shamelessly and with very little discretion hooks up with almost everything but O has a real thing going for Hydrogen.

 

[roger dodger]

I have always been interested in the 'perfect' pressure for my tires. I even did a test in the parking lot of a COG rally in Colorado.

After swapping my rear tire (with many more educated helpers than I) I asked what pressure the newly installed rear (Avon) should be inflated ? The answers ranged from OEM manufacturer of 36 to the sidewall max of 42. This is nothing new, like ********, everybody is entitled to their own opinion.

What was surprising was that I pulled out my digital (BMW style) gauge and set the tire to 42. Then some distrusting fellow suggested to check it w/ his pencil style gauge. We got 40. Then the COG director offered his expensive 'gauge on a rope' and we on got a pressure of 38. I know some of you are thinking some air was lost w/ each check. But we went back to my digital gauge and still got 41! (ok a little pressure was lost, but not much). So the rest of the day we discussed the comparable accuracy of various tire pressure gauges.

I now carry 2 digital gauges, and usually a pencil gauge also. But no matter what pressure you aim for, how accurate is YOUR gauge? I think some definative tests should be run on our various tools, to determine if we are really running the pressure we THINK we are running. (TPMS are also notoriously questioned for accuracy also)

Still, I set my PR3s to 40 / 42 everytime. Only on a major slab run will I raise it to 42 / 44 for better mileage. YMMV

 

[ionbeam]

I was lucky enough to be able to check my Bourdon pressure gauge against a calibrated gauge and found it to be about perfect. Bourdon gauges typically offer ± 2% accuracy, but are also sensitive to being dropped and abused. I went home had checked my other gauges against my known good Bourdon gauge and found two stick types to be within a pound but all the others were off not less than 4 lbs. I'll volunteer my gauge as a reference gauge if anyone wants to compare.

When I had my FJR safety inspected last summer the tech presumptuously set my tire pressures to the pressure label on the swing arm; I was surprised to find that his gauge and mine read exactly the same. I talked to the tech about changing my tire pressure without asking me and strangely his ear burst into flames during the one way discussion.

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