Tire Pressure question

[Rex R]

I hope this won't start a firestorm of BMWST controversy... like an oil thread... or a real tire thread.

 

Background:

I am one of those dedicated souls who checks tire pressure religiously--every time I ride. If the pressure is more than 1/2 psi over or under, I'll adjust it before riding. I know the need for that level of precision is debatable. However, that is not my question.

 

As we (probably) all know, tire pressure should be measured before riding, when the tire is cold. i.e. not heated by road friction. I'm sure we also all know that ambient air temperature affects tire pressure. i.e. fill your tires correctly on a cool morning and if you don't ride until after lunch and the air temperature has risen 15 degrees, you'll probably need to adjust your tires again.

 

For the first time I am storing my bike in a heated garage. So when I check the tire pressure before my ride, the tires are at a temperature 15 - 20 degrees higher than the environment in which they will be ridden.

 

Inquiry

Other than moving the bike outside for 2 or 3 hours before checking the pressure, how do you deal with the temp. differential? Does anyone have a chart or graph that indicates how to adjust tire pressure for different ambient temperatures?

 

I eagerly await your input.

[Firenailer]

I check it before I ride, and keep my current RT at 36/40 out of a heated garage. Never had any problems or concerns.

 

I've been doing it the same way for the last 20 years with a few different bikes and so far it's all good!

 

 

[szurszewski]

What fraction of a pound do you lose on each check? I'm all for checking and all that, and I have my prefered psi, but 1/2lb threshold? I feel like I'd lose that much checking and adjusting...

[Rex R]

What fraction of a pound do you lose on each check? I'm all for checking and all that, and I have my prefered psi, but 1/2lb threshold? I feel like I'd lose that much checking and adjusting...

 

Rarely enough to change the reading, unless I fumble it. It takes a good 1/2 second to a full second to drop the pressure 1/2lb. Longer for the rear tire than the front due to larger volume.

 

But more to the point, I figure once a day, before I head out (when I have access to an air pump) is usually sufficient. It's not like I test the pressure after every fuel stop, bathroom stop, and food break. Even if I wanted to check that often, I know I wouldn't get an accurate reading, at least not without making my bathroom breaks 3 hours long to let the tires cool down. And I haven't spent that much time in the bathroom since junior high school...

 

 

TMI??

[ESokoloff]

Several months back I purchased and installed a Doran tire pressure monitoring system.

 

With it I've seen the rear tire pressure fluctuate as much as 10PSI.

After a while you learn what the normal operation range is and what to expect.

 

I now check my tire pressure before every ride at a push of a button and have piece of mind that if I have a leak that exceeds the preprogrammed limit I will get an alarm notification.

[12R12RT]

Add or subtract 1 pound for every 10° F.

 

Figure that the reference point for setting tire pressure is 70°F, so if it is 50°F, subtract 2 psi from the desired pressure.

 

In my case I run 42/42 so, if I were checking them and thought they needed adjustment, I'd set them at 40/40 on a 50°F morning.

 

Tom

 

 

 

 

[mneblett]

Add or subtract 1 pound for every 10° F.

 

Figure that the reference point for setting tire pressure is 70°F, so if it is 50°F, subtract 2 psi from the desired pressure.

 

In my case I run 42/42 so, if I were checking them and thought they needed adjustment, I'd set them at 40/40 on a 50°F morning.

 

With due respect, I submit this is not a good approach because it leads to your tires being under-inflated at colder temps and over-inflated at high temps.

 

This is the approach BMW uses with its tire pressure monitoring system -- reporting the pressure that the tires *would* be at if the ambient temperature was 68F (20C) (e.g., if it is actually 50F and the tire had 40 psi, display 42 psi). It's one of my pet peeves about the BMW system, as "corrected" pressure is useless information to me.

 

Regardless of what the outside temperature is, a tire that requires 42 psi to be at its optimum profile (optimum shape relative to the rim) needs to be at 42 psi -- i.e., its needs to be at 42 psi when ambient temp is 30F, when it is at 70F, and when it is at 110F. Cold tire inflation pressure is the pressure that should be present when the tire has not been in use (i.e., not at normal operating temperature) -- regardless of what is the current ambient temperature.

 

A tire that reads 40 psi at 50F would be at 42 psi when the ambient temp is 70F -- but at 50F it is still 2 psi under-inflated.

[John Ferris]

Get all of your gauges together and test them on a large tire (car or truck).

I have 5 gauges and none of them are exactly the same.

Up to 3 lbs apart from the highest to lowest.

How do you know which is right. ?

OCD

 

[Selden]

...I have 5 gauges and none of them are exactly the same.

Up to 3 lbs apart from the highest to lowest.

How do you know which is right. ?

OCD

 

OCD is probably the most appropriate comment to date. Except in race conditions, tire pressure is just not that critical most of the time. The only times I would consider a mid-day adjustment are a) in the desert, where it might be in the 30's at dawn, and near 100° in the afternoon, or b) large, sustained change in elevation.

[12R12RT]

Add or subtract 1 pound for every 10° F.

 

Figure that the reference point for setting tire pressure is 70°F, so if it is 50°F, subtract 2 psi from the desired pressure.

 

In my case I run 42/42 so, if I were checking them and thought they needed adjustment, I'd set them at 40/40 on a 50°F morning.

 

With due respect, I submit this is not a good approach because it leads to your tires being under-inflated at colder temps and over-inflated at high temps....

 

What do you do when you leave home on a February morning at 50° and ride up into the mountains into ambient temps of 30°, drop off the backside back into a desert valley with temps of 70°, then ride into a coastal range with temps of 50°, and back home where the temperature has risen to 70°? This would be riding from Redondo Beach CA up over CA2 to the Angeles Crest Highway, down into Palmdale and out to Mojave, then back over towards Ojai, along Pacific Coast Highway.

 

I've also ridden out of the California Central Valley where it was 80° and up into Sequoia and Yosemite along the General's Highway, where I ran into sleet and snow and an ambient temperature of 25°.

 

A couple of months ago I left Cody, Wyoming where it was a balmy 65° to run up over Beartooth Pass. At 11,000' feet it was hailing and the temperature was 35°. Riding down into Red Lodge, after the hail let up, the temperature was about 45°.

 

It's a real world out there. Even on a ride that never changes in altitude, but spans dawn to dusk, the ambient temperature can change by 20°.

 

A tire that has a desired inflation pressure of 42 psi (determined by personal preference, experience, and riding style) is not going to fail to perform if the pressure drops by 5psi due to temperature. For that matter, you will find personal preference in inflation varies by more than 5 psi.

 

I will grant you that your approach may be desired for a track environment, but I can't agree that every time the temperature drops by 10 degrees we should stop and adjust our tire pressures.

 

I will maintain my approach - if your tires need attending to, inflate them to the pressure you like to see at 70°, adding or subtracting 1 psi for every 10° the ambient varies from 70°, and ride.

 

Tom

[mneblett]

What do I do? I check them once a week. That generally follows the seasons. If I am on a trip and go from 30F to 90F, I might make an adjustment to keep the hot pressure in check. It's not _that_ critical.

 

My point (which may not be all that far from yours): If you are setting your tire pressure, set it for the environment you expect to be riding in most of the time that day. For 99.99% of folks, that will be (using the 42 psi example), 42 psi at whatever is the current ambient temp.

 

The reason for my original comment: I didn't want folks to misinterpreting your statement as suggesting that if the ambient temp is 30F and they are planning on a normal ride (like commuting to/from work), they should be lowering their tire pressure to 38 psi (i.e., deliberately under-inflating the tire).

[Tourmaster]

Just curious...does a tire heat the same amount on a near freezing day, say 35 deg F as it would on a hot day, like around 90? My gut tells me it heats more on a hot day but...

And by the 'same amount' I don't mean to the same temperature.

[RandyBailey]

Man, I thought I was anal...this takes it to a new level. Instead of stressing so much, spend more time riding!

[Larsman]

38/42 "cold" at any starting ambient temperature works for me and has for decades. I used to check them every 10 days or so until I bought the RTW with TPMS. I have used an external gauge once in the past month.

[mneblett]

38/42 "cold" at any starting ambient temperature works for me and has for decades. I used to check them every 10 days or so until I bought the RTW with TPMS. I have used an external gauge once in the past month.

As long as you remember that 38/42 displayed on the RT's TPM is not the actual pressure in the tires (unless the ambient temp happens to be 68F), do what works for you.

 

For example, today it was ~40F ambient. If my K16 or R12 TPM would have shown 42/42, I'd know the actual air pressure was more like 39/39; this is exactly what I've observed when testing with gauges. If the tires are actually at 42/42 (as they usually are) at 40F, I see 44-45 on the "corrected" TPM display.

 

Bottom line for me, until BMW starts showing actual (uncorrected) tire pressures, the TPM is primarily only good for sudden deflation warning, because you certainly can't tell what the actual pressure is unless you know the actual ambient temperature and do the mental calculations to "uncorrect" the TPM's display.

[Hank R1200RT]

With regard to 12R12RT's post...50 degrees F in Redondo to 30 degrees on Angeles Crest would definitely be a consideration...but you will have to ride 30 miles or so before you get to that temperature differential. I am certain that the normal 80 MPH on the 2 freeway will heat up the tires to a degree that the ambient temperature doesn't matter.

 

OTOH, if you spend an hour in Newcomb's bench racing, obviously the tires will cool down.

 

Try this experiment the next time you make that ride. But record ambient temperature and tire pressure after a ride with this temperature differential, and see what the effect is. Then do the same after an hour at Newcomb's. If you have an IR thermometer to measure the tire temp, even better.

 

The best part is that you get an excuse for a ride if you need one

 

[12R12RT]

With regard to 12R12RT's post...50 degrees F in Redondo to 30 degrees on Angeles Crest would definitely be a consideration...but you will have to ride 30 miles or so before you get to that temperature differential. I am certain that the normal 80 MPH on the 2 freeway will heat up the tires to a degree that the ambient temperature doesn't matter....

 

^This!

 

And to Tourmaster's question, I don't know, but I always figured it was pretty much linear. If the tires heat up to 20° over ambient running in 70° weather, they probably heat up by 20° running in 30° weather. Never tested it. Don't much care.

 

When I check my tires I account for the temperature. If it is 50° and they are at 40psi, good to go. 90° and they are at 44, good to go.

 

Once rode into Arizona in 115° freak heat wave in May. Absolutely did not give two shits about what the temperature or pressure was. Everything was too hot.

 

Tom

[Paul In Australia]

Your argument makes sense, except the fixed view on psi no matter what the ambient temp is. My understanding is the tyre manufacturer pick a fixed ambient to measure what a tyre pressure should be not a fixed tyre pressure at any ambient? Am I correct here?

Regards

Paul

[Tourmaster]

Man, I thought I was anal...this takes it to a new level. Instead of stressing so much, spend more time riding!

 

The RT is all but away for the winter, so I will have 4 or 5 months to WORRY about stuff like this!

[tallman]

Effect of temperature and altitude on tire pressure

 

Check 'em cold.

Change in altitude extremely small (2%).

 

[David Langford]

on my bikes I look at tires every time I ride and doubt if I check pressure more than 3-4 times a year. Ive had my 14 rt for 3 months and checked when I got it, set pressure at 38/42. The tpm on bike matched my gauge and I haven't lost one pound in 3 months-just checked. Been riding street since 81 and get great tire life and never had a flat yet, knock on wood.

In my experience a good tire will lose about a pound of pressure every 3-4 months or so

[12R12RT]

In the two months that my bike was in the shop the tires went down by three pounds each. It pretty much sat the whole time.

 

These are Michelin PR3s.

 

Tom

[szurszewski]

In the two months that my bike was in the shop the tires went down by three pounds each. It pretty much sat the whole time.

 

These are Michelin PR3s.

 

Tom

 

I've seen your other posts about that two month stay-cation your bike took - maybe the techs were stealing your air when they weren't out riding your bike...or maybe they tried adjusting tire pressure to see if that would affect your -seemingly- electric issue

[12R12RT]

They better not be stealing my air. I imported that air from my ancestral home in Utscheid. That is rare German air with just a faint bouquet of French countryside.

 

Ich wäre nicht überrascht, dass sie meine Luft gestohlen, sie haben gestohlen Sauerstoff für zwei Monate.

 

Tom

[elkroeger]

If it was important that your tire pressure remains perfectly constant all the time, we would not be travelling on pneumatic tires.

[Joe Frickin' Friday]

Effect of temperature and altitude on tire pressure

 

Check 'em cold.

Change in altitude extremely small (2%).

 

Their claim was 2 psi for a 5000-foot elevation change, which happens to be 2% only if we're talking about a truck tire running at around 100 psi.

 

From atmospheric charts, I see a psi change from sea level to 5000 feet of 2.4 psi. For a motorcycle tire running at around 40 psi, that's more like 6 percent. I think most of us, if we checked our tires in the morning and found them to be low by 2.6 psi, would add some air.

 

 

[tallman]

Most likely.

 

Depends on ambient and ride plan.

Is that amount similar to the BMW monitoring system

readouts and the built in projection?