Tire gauge obsessions

[Scotto336]

They used to say that a man with 2 watches never really knows what time it is, then came the internet, smart phones and smart watches.  I think the same can be said for tire pressure gauges, so I put together a test rig with my Fluke pressure calibrator to see where my 2 commonly used, shop pressure gauges stood against a $2000 calibrated Fluke meter.  I’ve been using the Motion Pro digital gauge for many years and then started using the Harbor Freight digital gauge recently since it’s in line with my compressor hose so easier to use.  I tested them both at 42 psi.  The Motion Pro came in 0.2 psi low and the Merlin was 0.4 psi high, so both were within 1% at that pressure.  The Merlin is advertised as + or – 1% up to 250psi and the Motion Pro was advertised as + or - .6 psi up to 60 psi, so both were within advertised accuracy.  I marked them accordingly.  This is not to say that they will all be this accurate, just the ones that I tested.  The Motion Pro is currently listed for around $120 though I saw one listed by Walmart for $39.  The Merlin was $60.  FWIW I did read a very bad review on the analog, oil filled gauge from Motion Pro so I would tend to avoid that.  While I was at it, I checked my made in USA, Milton pencil gauge that always comes with me and it was just a smidge over 1 psi High.  It goes for about $10 on Amazon.

[duckhawk64]

I have 6. I am obsessed. I won't go digital, except the r125rt, unmoving display. 

[Krid]

I own 3, one digital, a cheapo analog, and my favorite, a made in France Michelin made by Schrade  that I won as a door prize at a rally around 25 years ago. I have tested it many times against calibrated professional gauges and it always dead nots on or +/- 1lb which is close enough for me on the street. It stays on my bike!

[Lowndes]

Well....I used to be obsessive about it, too, full disclosure.

 

Until I got a TPMS.  It was an aftermarket brand with stem mounted external sensors.  FOBO to be exact.  A dandy system that shows the actual pressure and temperature, except that it used an app on your cell phone to display the values which was tiresome.  No puns here.

 

I got concerned that the temperatures displayed were not/could not be accurate with the temperature sensors whirling around in the breeze on top of the schrader valve.  Now I'm sure we all remember the Ideal Gas Law from our high school physics (P=nRVT), so if you know the pressure you can also know the temp, they are directly related, inexorably linked.  Mostly, 'cause nails and screws can cause variations in tire pressures, too.  But tire temperature is important, too, and I wanted to know the actual temperature of the tire.  So, I figured out a way to mount the sensor inside the tire.  Told you I was obsessive.

 

 

But I realized after riding and watching the TPMS readings while riding in various situations that the internal "tire pressure" varied.  And, the tire pressures varied continuously as I rode, by as much as 7-8 pisg. 

 

And, as it turns out, for several reasons, me hearties:

 

As you go up and down in elevation you gain about 0.5 psig for every 1,000 ft of altitude and vice versa.  

 

Also, as you gain altitude in the mountains, the ambient temperature usually drops, which can be 20-30 deg F.

 

Regular atmospheric fluctuations will only affect your psig by 0.12 psig.

 

As your tires warm up from riding (everything else staying the same) due to the flex of the tread and sidewall and slipping friction, you will see an increase in pressure by about 1 psig for every 10 deg F of the tire.

 

On a sunny day, your tires will warm even more in the sun (yes, even if they are rotating furiously).  If you change direction and the tire is in the shade of the bike, or if the sun goes behind a cloud, the tire will cool and the pressure will drop.  If you park the bike in the sun at lunch they can get really warm.

 

All the above effects are cumulative, also.

 

With all this going on as you ride, the tire pressures will be constantly changing.  So WHY get all persnickety about the exact pressure??  

 

 

 

 

 

 

[Rougarou]

I usually

 

 

If it's tight, it's got enough air, if there's give, it needs some air, if it's bulging, it's got too much air

[wbw6cos]

If you are on a track competing, then exact psi may make a difference.   I just wonder if it will be that noticeable whilst Sport Touring.  Can you tell the difference in handling at 41psi versus 42?   (I am being serious)

 

As long as the consistency is there every time you check, you should be good to go.  Am I right?

[Skywagon]

If I roll over a dime and can tell if it’s heads or tails, I put in some air. If I start floating I let some out. 

[RPG]

23 hours ago, Lowndes said:

Well....I used to be obsessive about it, too, full disclosure.

 

Until I got a TPMS.  It was an aftermarket brand with stem mounted external sensors.  FOBO to be exact.  A dandy system that shows the actual pressure and temperature, except that it used an app on your cell phone to display the values which was tiresome.  No puns here.

 

I got concerned that the temperatures displayed were not/could not be accurate with the temperature sensors whirling around in the breeze on top of the schrader valve.  Now I'm sure we all remember the Ideal Gas Law from our high school physics (P=nRVT), so if you know the pressure you can also know the temp, they are directly related, inexorably linked.  Mostly, 'cause nails and screws can cause variations in tire pressures, too.  But tire temperature is important, too, and I wanted to know the actual temperature of the tire.  So, I figured out a way to mount the sensor inside the tire.  Told you I was obsessive.

 

 

But I realized after riding and watching the TPMS readings while riding in various situations that the internal "tire pressure" varied.  And, the tire pressures varied continuously as I rode, by as much as 7-8 pisg. 

 

And, as it turns out, for several reasons, me hearties:

 

As you go up and down in elevation you gain about 0.5 psig for every 1,000 ft of altitude and vice versa.  

 

Also, as you gain altitude in the mountains, the ambient temperature usually drops, which can be 20-30 deg F.

 

Regular atmospheric fluctuations will only affect your psig by 0.12 psig.

 

As your tires warm up from riding (everything else staying the same) due to the flex of the tread and sidewall and slipping friction, you will see an increase in pressure by about 1 psig for every 10 deg F of the tire.

On a sunny day, your tires will warm even more in the sun (yes, even if they are rotating furiously).  If you change direction and the tire is in the shade of the bike, or if the sun goes behind a cloud, the tire will cool and the pressure will drop.  If you park the bike in the sun at lunch they can get really warm.

 

All the above effects are cumulative, also.

 

With all this going on as you ride, the tire pressures will be constantly changing.  So WHY get all persnickety about the exact pressure??  

 

 

 

 

 

 

I thought the FOBO units mount on the outside of the rim??

[9Mary7]

21 hours ago, wbw6cos said:

If you are on a track competing, then exact psi may make a difference.   I just wonder if it will be that noticeable whilst Sport Touring.  Can you tell the difference in handling at 41psi versus 42?   (I am being serious)

 

As long as the consistency is there every time you check, you should be good to go.  Am I right?

Sport-touring will not get you to the rarified "zone" where 1-2 lbs will be a difference you can feel while cornering. You'll feel 5 lbs difference, usually low from a slow leak.

Consistency is what provides confidence while street riding. My .02  

[Lowndes]

3 minutes ago, RPG said:

I thought the FOBO units mount on the outside of the rim??

You are correct, and not just FOBO but for all "reasonably priced" aftermarket systems that I know of.  

 

I was trying to get accurate tire temperature by putting the sensor inside the tire, not outside where it would be cooled by air flow.

 

FOBO makes a "T" valve stem so that you can mount the sensor and still have a port for adding/removing pressure.

 

 

FOBO said it wouldn't work because the miniscule radio signal from the sensor (to the display) would not penetrate the tire.  It did penetrate, no problem.

 

The biggest 2 problems I saw were, 1) mounting the tire with the sensor in the way, and 2) if you have a flat that sensor would get demolished.

 

I found the temperatures interesting but the MAIN thing is the pressure.  If you don't have any pressure in the tire, who cares what the temp is, unless it's on fire.

 

Also found the ALARMS on the tpms to be VERY helpful; saved me several times, bike and car.  I cannot see either wheel or tire when sitting on the bike, too much plastic in the way.  One time I stopped for lunch, when I came out and got on the bike something was beeping very persistently.  Had a flat front tire.  Would have found out the hard way in the first turn.  Another time I was riding, picked up a nail in the rear tire and the beeping got me to stop before it got dangerously low.  For $35 that's a deal.

[RPG]

7 hours ago, Lowndes said:

You are correct, and not just FOBO but for all "reasonably priced" aftermarket systems that I know of.  

 

I was trying to get accurate tire temperature by putting the sensor inside the tire, not outside where it would be cooled by air flow.

 

FOBO makes a "T" valve stem so that you can mount the sensor and still have a port for adding/removing pressure.

 

 

FOBO said it wouldn't work because the miniscule radio signal from the sensor (to the display) would not penetrate the tire.  It did penetrate, no problem.

 

The biggest 2 problems I saw were, 1) mounting the tire with the sensor in the way, and 2) if you have a flat that sensor would get demolished.

 

I found the temperatures interesting but the MAIN thing is the pressure.  If you don't have any pressure in the tire, who cares what the temp is, unless it's on fire.

 

Also found the ALARMS on the tpms to be VERY helpful; saved me several times, bike and car.  I cannot see either wheel or tire when sitting on the bike, too much plastic in the way.  One time I stopped for lunch, when I came out and got on the bike something was beeping very persistently.  Had a flat front tire.  Would have found out the hard way in the first turn.  Another time I was riding, picked up a nail in the rear tire and the beeping got me to stop before it got dangerously low.  For $35 that's a deal.

Very interesting. I've had the FOBO's on my 1150RT for a long, long time (mounted on the outside of course) and loved them. Never a flat or sudden pressure loss but the app allows for customizing all those parameters. And it bluetooths any warnings to the Sena on my helmet. Just never saw them mounted that way so I wanted to ask. Sold my RT last month (for a new RT) and moved the FOBO's to my R90s, which gives me the same piece of mind when riding.

[Lowndes]

On 6/11/2024 at 11:33 AM, wbw6cos said:

...  Can you tell the difference in handling at 41psi versus 42?   (I am being serious)

 

As long as the consistency is there every time you check, you should be good to go.  Am I right?

 

I seriously doubt that 99% of us here could tell the difference in "41psi versus 42".  (It may be closer to 100% but you gotta gotta leave a little wiggle room so no one gets their britches in a bunch). 

 

The way I see it is the difference in size of the contact patch (41 vs 42 psig) would be easy to determine, but the difference would only have an effect when the contact patch is at the very limits of traction, i.e. hard cornering, hard braking, hard acceleration (wheel spin), or combination of #1 with #'s 2 or 3.  We very seldom even get close to those limits in our normal riding.  For one thing, we have to be concerned with the other side of this equation which is the road surface.  On a track the surface is very consistent and clean and you know it is cause you just came by here a few minutes ago.  On our roads and around that next bend it could be smooth sailing or a dead cat in our lane.  Sorry, I don't have any empirical data on the coefficient of friction of a dead cat but live cats can have a very high - greater than 1.0 - coefficient, i.e. when climbing trees and shin bones.

 

OK, so the contact patch area calculation (in rough terms for our purposes here) would be to combined weight of the bike, rider, and all the accoutrements hung on the first two, and let's just be very complimentary to any surviving readers at this point and say the bike is 500 lbs, and the rider is 200 (YRMV).  Because the tires are pneumatic the average force they can "put on the ground" is the pressure in the tire, let's say 40 psig.  We spend most of or time with both wheels on the ground so the equation looks something like this:  total weight of bike and rider divided by 2 wheels (50/50 weight dispersion for simplicity) divided by the air pressure in the tires.

 

     700 lbs/2 tires/40lb/in² = 8.75 in²/tire (contact patch area)

 

So let's change the tire pressure to 41lb/in²:

 

     700 lbs/2 tires/41lb/in² = 8.54 in²/tire (contact patch area)

 

The difference is only ~ 0.2 in².  That would be a square that is ~0.45 inches on each side, or, not much.  But...

 

The higher the pressure the smaller the contact patch and the less flex the tire is having in the sidewall and the tread while riding.  This flex is causing the tire to heat up and as this "rubber" gets hotter it also gets softer and begins to feel squirrelier.  Also, as the contact patch gets smaller the loads get higher on the tread rubber (it's the same load(s) but supported by a smaller area of rubber (and pavement).  This higher specific load on the rubber will cause it to flex more and heat up more, so there is an equilibrium point somewhere on the pressure and that's what we want and hopefully what's in the owner's manual..  Throw in ambient temps from zero to 110° F, wet to dry conditions, every kind of road surface all while trying to keep a bunch of anal and skinflint BMW drivers happy and it's a tire designer's party.

 

 

The coefficient of friction, rule of thumb, for a street tire on dry pavement is somewhere between .7 and .8 , so let's use .7 in this:   

 

.7 x 350 lbs/tire = 245 lbs (of force required to get one tire slipping).  This is the resistance to slipping from combined side loads in cornering and longitudinal loads from acceleration or braking.

 

This is all for "static" friction.  Once it starts sliding in relationship to the pavement surface it will have a "dynamic" coefficient of friction which will usually be much less.  Sometimes this will be a smooth slide and sometimes a high frequency "grab and release" like the old favorite fingernails across the chalkboard, or a tire "squealing" (spinning or sliding sideways). 

 

What did I miss??

 

 

 

[Scotto336]

Interesting analysis.  At the risk of sounding pedantic, my only comment would be let's not confuse force with pressure.  The  force exerted on the ground should be half the weight in a 50/50 weight distribution.  This will be expressed in pounds or newtons, not psi or pascals.  The pressure exerted on the ground would be in psi.  To the question re: what difference can you feel,  I would never be able to feel a 1 psi difference on the track.  Perhaps someone could, but for me it was about 3 psi before I had a perceived difference.  On the street I think some bike/tire combinations are more sensitive than others.  I had a ZX14R that would feel very disconnected at the front if it was down 3  psi.  On my old airhead BMWs I wouldn't detect that so readily. 

[Stiggy]

So today (and every other Sunday actually,) I pumped an indicated 41 lbs (on my digital pump,) into 3 tires knowing that tomorrow when I take one of those bikes to work, the TPMS will read 42 lbs.

(I feel like maybe I'm not doing it right now after reading Tire Gauge Obsessions   )

[Lowndes]

2 hours ago, Stiggy said:

So today (and every other Sunday actually,) I pumped an indicated 41 lbs (on my digital pump,) into 3 tires knowing that tomorrow when I take one of those bikes to work, the TPMS will read 42 lbs.

(I feel like maybe I'm not doing it right now after reading Tire Gauge Obsessions   )

Confucius say, "Man with two watches never know what time it is."