Tire Pressue Puzzle

[manholt]

That's a new one, never heard of it.  Do you remember where you saw it? 

[wildebill308]

Carl this is like the one I saw.

http://www.amazon.com/gp/product/B000304102/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687562&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B0003040ZS&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1T3ZWNAZRVA43BK8MX01

If you could keep the hoses from rubbing it might be intersting.

Bill

[jleamont]

Bill, to answer your question, yes. Very common in the trucking industry. The concern is exactly what you mentioned, when one has a leak they both have a leak .

I would much rather have one flat than two flats, plus it takes longer to figure out which tire is the leaker also.

[manholt]

So much for another mouse trap.

[tireman9]

In real life, I see no advantage to balancing the air between duals with a hose. I do see some serious disadvantages.

The issue I have is that some consider this a substitute for TPMS, which it is not.

I also wonder about taking air out of a tire that is having its load increase as it's companion looses air. Those systems are suppose to limit the transfer of pressure if one tire gets a leak but you still are taking air out of a running tire that may have only a few psi to spare before it is in overload condition.

Some truck companies may feel that completing the delivery run, even at the expense of two failed tires is worth the effort.

[tireman9]

On 4/12/2016 at 0:44 PM, rpbuttery said:

Let me try this again.  I think I confused by mixing definition and terms.

Using the terms from a Tireman9 blog post, I am going to use 80 psi for the front tires minimum cold inflation pressure (MCIP), and 85 psi for the rear tires MCIP, as suggested by Tireman9 above.  Then, adding 10 psi to each, I will use 90 and 95 psi as my goal cold inflation pressure (GCIP), or set point (I.e., the pressures at which I will maintain the tires).

However, because my TireMinder system uses a 15% (which I cannot change) below set baseline for normal air loss alerts (rapid air loss alerts are,of course, immediate), I will set the TireMinder baselines at 95 psi for the front, and 100 psi for the rear.  Thus, normal air loss alerts (which in theory should never happen) would happen at 80.75 psi for the front, and 85 psi for the rear. 

Is this sufficiently muddy? ?

Yup you got it. As I said some TPM are easier to set the warning levels than others so we each need to stop and think about our own personal system rather than simply do something like what you read on a post.

[PARRISHNM]

22 hours ago, WILDEBILL308 said:

Well to keep this on track I checked my tire pressure and it was within a pound of when I last checked it. I saw a devise that ties the rear tires together (kind of a Y valve stem extension) so they run at exactly the same pressure. I was wondering if anyone had tried this. My concern would be if one gets a leak they both go down. This was a real and actual question.

Bill

You are right. If you sprung an air leak, both tires will go flat before you may be able to stop. There is a device that will permit you to accomplish balanced pressure in tandem tires. It is made by Crossfire. If the pressure drops to 90 pounds is automatically separates the two tires. You can mount your pressure monitor sending unit on the Crossfire unit to monitor the actual pressure in both tires. I have used these units through Alaska for 5000 miles, ( including the top of the world highway), and never had a problem with them. A total driving distance on the coach of 80,000 miles.

[halfmoa]

Just to clear something up, Wayne77590 is correct when describing the relationship of atmospheric pressure and altitude.  But his explanation may be misinterpreted by some when trying to figure out what their tire pressures are going to look like as the altitude changes.  Higher elevations will result in lower ambient pressures.  However, when checking our tires we are not measuring ambient pressure, we're measuring the relative pressure between the inside tire pressure compared to the outside tire pressure.  Wayne was correct when saying the difference in pressures at sea level vs. 10,000 feet elevation is about a negative 4.7 PSI.  But what you will see on your gauge is an increase of 4.7 PSI because the ambient pressure has decreased while the pressure in the tire (virtually a closed vessel) remains the same.  The relative pressure you see on your gauge will therefore read 4.7 PSI higher at the higher elevation.

[dickandlois]

4 hours ago, Halfmoa said:

Just to clear something up, Wayne77590 is correct when describing the relationship of atmospheric pressure and altitude.  But his explanation may be misinterpreted by some when trying to figure out what their tire pressures are going to look like as the altitude changes.  Higher elevations will result in lower ambient pressures.  However, when checking our tires we are not measuring ambient pressure, we're measuring the relative pressure between the inside tire pressure compared to the outside tire pressure.  Wayne was correct when saying the difference in pressures at sea level vs. 10,000 feet elevation is about a negative 4.7 PSI.  But what you will see on your gauge is an increase of 4.7 PSI because the ambient pressure has decreased while the pressure in the tire (virtually a closed vessel) remains the same.  The relative pressure you see on your gauge will therefore read 4.7 PSI higher at the higher elevation.

Halfmoa, The physic of pressure decreasing on the exterior surface should allow the tire to expand relative to the original interior space.(Temperature remaining relatively stable) This state should maintain a constant molecular activity that maintains a stable internal pressure volume.

The increased space due to reduced exterior pressure should cause a reduced pressure reading on the sensor that is reading pressure inside the closed, but larger space inside the tire. An over all lower pressure should result.

Rich, 

[halfmoa]

Rich:

Once again the statements you are making are correct.  However, we are not measuring the pressure inside the tire with our tire gauges, we're reading the relative pressure of the tire compared to the ambient pressure.  Anyone with an air bed in the coach experiences the same issue, as we gain elevation the bed begins to increase in pressure, and gets as hard as a rock when going over passes, it's quite noticeable when you sit on it.  The pressure in the tire will remain fairly constant, but the pressure we see on the gauge will be higher with increased elevation, because the inside pressure remains the same, but the ambient pressure is decreasing, resulting in a higher relative reading on our gauges.  I've had this discussion with several people before, and the same argument comes up about the pressure in the tire remaining the same since it's in a closed container, but the gauges we use read relative pressure, not internal tire pressure.  www.tirerack.com/tires/tiretech/techpage.jsp?techid=167 gives a pretty good description of tire pressures "increasing" with increased elevation.

[jleamont]

If I had to choose I would have an auto inflation system installed, one like the attachment. Where it monitors your pressures and adjusts accordingly. We run them at work, my concerns in the RV industry would make me shy away. The main reason is price, the other reason you cannot adjust according to your weights across an axle.

http://www.meritor.com/products/brakes/trailer_mtis/default.aspx

Article from Trucking magazine with some new technology headed out soon for drive axles.

http://www.truckinginfo.com/article/story/2015/01/coming-soon-inflation-systems-for-drive-axles.aspx

 

[tireman9]

OK folks  Have to jump in.

 

Quick answer:  Don't worry about tire pressure change due to altitude change. Its Measurable but not Meaningful    as covered in my blog

 

Engineering analysis  in  my blog

 

Sorry for just pointing to my blog but I don't have time to write the two posts again. And you can just ignore the pressure change if you don't want to read the blog posts.

[tireman9]

On 4/14/2016 at 9:52 PM, DickandLois said:

Halfmoa, The physic of pressure decreasing on the exterior surface should allow the tire to expand relative to the original interior space.(Temperature remaining relatively stable) This state should maintain a constant molecular activity that maintains a stable internal pressure volume.

The increased space due to reduced exterior pressure should cause a reduced pressure reading on the sensor that is reading pressure inside the closed, but larger space inside the tire. An over all lower pressure should result.

Rich, 

Steel body tires do not change volume to a meaningful level so it doesn't really enter into this discussion.

[wayne77590]

I don't worry about altitude or temperature change.  I set my pressures so there is a fudge factor and I'll never be below or above weight load recommendations. Also, if going up in altitude increases the pressure and the temperature in higher altitudes is less than the lower altitudes, they just about balance out.  Even at sea level going from 70 degrees to 100 degrees is about 6% change in temperature so at 100 PSI it drops to 94 psi. If minimum pressure depending on specific weight is 90 PSI, well it's still within MFG's recommendations.  I never set my minimum PSI based on weight minimum I always give it a fudge factor just because there are subtle changes. It is science but it is not rocket science.

[jleamont]

10 hours ago, Tireman9 said:

OK folks  Have to jump in.

 

Quick answer:  Don't worry about tire pressure change due to altitude change. Its Measurable but not Meaningful    as covered in my blog

 

Engineering analysis  in  my blog

 

Sorry for just pointing to my blog but I don't have time to write the two posts again. And you can just ignore the pressure change if you don't want to read the blog posts.

Rodger, I am glad you are on here. I by no means am I a "tire expert". Have installed many along the side of the road, in the shop etc..etc.. Doesn't make me an expert, just means I know how to install them safely with the proper tools thanks to a tire manufactures training that we were required to go through for OSHA. Questions like the one above I had never given thought to, same with tire pressures increasing while driving and more so on hot road surfaces.

When I bought my first RV and TPMS system more information on the display became more puzzling and somewhat alarming, now I could see the pressures rise and began to panic. After a stop in route to CA from PA parked in the Mohave Desert in July I Googled it, pressures will increase while driving and the tires were made to do so. My panic was simple lack of knowledge. Since my old C was at its max GVWR on the steer axle and over on the rear (from factory) I had my tires set to the max PSI, when I saw them go over the max PSI on the TPMS I felt nausea, just waiting for one to explode, combined with the temp that day at 120 degrees, I was convinced we were not going to get out of there without a tire failure. Never had a problem. Thank you for clarifying what to look for and be concerned with and what not to be concerned with, to have an expert on here like yourself is very helpful, altitude is another one I would have never thought of until I saw something odd on the TPMS would I ever think of it.

[fltrvln]

Might I join in also?  I am confused when I look at the charts.  I have Cooper Roadmaster tires range F. They are 225/70R 19.5  Max for tire is 110

 My weights are

Ft driver  2750

Ft Pass    2700

Rear dual driver 5000

Rear dual pass 5000

The sticker in the coach reads 90 front, 80 back duals. ( Coach is a 30'A max weight of 18500. )

I have looked at the charts and just cannot seem to figure out what to run.  Please Tireman, can you help?

[wayne77590]

I would have the same concern Bill.  I think those systems have a cutoff point where the good tire will not lose any ore air then the set point. Not positive on that. Been a long time since  I read anything on it.

Edited: What I just said was covered in some previous posts but for some reason I could not see the previous posts. Now i can't see how to delete this post.  Oh well! 

[wildebill308]

13 hours ago, Wayne77590 said:

I would have the same concern Bill.  I think those systems have a cutoff point where the good tire will not lose any ore air then the set point. Not positive on that. Been a long time since  I read anything on it.

Edited: What I just said was covered in some previous posts but for some reason I could not see the previous posts. Now i can't see how to delete this post.  Oh well! 

Yes I know,they say it is good to keep learning. But now we have to learn how to use this web site every day.  There is even a difference depending on what browser you are using. Had Internet Explorer as the default on one and Google Chrome on another. There is quite a bit of difference in how this site is displayed.

Bill 

[tireman9]

On 4/16/2016 at 8:12 PM, fltrvln said:

Might I join in also?  I am confused when I look at the charts.  I have Cooper Roadmaster tires range F. They are 225/70R 19.5  Max for tire is 110

 My weights are

Ft driver  2750

Ft Pass    2700

Rear dual driver 5000

Rear dual pass 5000

The sticker in the coach reads 90 front, 80 back duals. ( Coach is a 30'A max weight of 18500. )

I have looked at the charts and just cannot seem to figure out what to run.  Please Tireman, can you help?

To start off I have to say you have done a good job of learning the facts by getting the "4-corner" weights. You are also lucky in having a coach so well balanced.

If you review some of the posts on my RV Tire Blog you will see a number of examples where we did a step by step through the calculation and setting of pressure. Since you have the tables, I think it better to have you (student) do the homework rather than me (teacher) do it for you. I am a firm believer that one of the best ways to learn a subject is by doing. So your "homework" assignment is as follows.

What do the tables show as the minimum inflation needed for your front tires to support 2750# ?

Min infl for rear in dual to support 2500#?

What inflation do you get if you increase these inflations by 10%?

and what is the final answer if you round up to the next full 5psi increment?

 

Extra credit question.  What should you set your TPMS warning pressure for F and for R tires?

 

[fltrvln]

Ok Tireman, I looked at the tables and the best I can figure is 70psi for both front and back plus the 10% = 77 plus 5 = 82.

Now, my label says 90 front , 80 back. 

   70         75      80      85     90      95

S 2895 3040 3195 3315 3450 3640(F)

D 2720 2860 3000 3115 3245 3415(F)

Just a double check please, am I right - I should be running around 82 all the way around?  The tires look a bit "poochy" at that

I do have the Tireminder, set right now at 90 front, 80 back.  So I guess I would need to "up" that to 95 all the way around? 

Ft driver  2750

Ft Pass    2700

Rear dual driver 5000

Rear dual pass 5000

I had asked a Michalin dealer for help, he couldn't figure out his own tables.  The tires are Roadmaster range F.  Not my choice to have run but I was out in the boonies when I had to get tires, right then (large crack in the sidewall).

 

[tireman9]

33 minutes ago, fltrvln said:

Ok Tireman, I looked at the tables and the best I can figure is 70psi for both front and back plus the 10% = 77 plus 5 = 82.

Now, my label says 90 front , 80 back. 

   70         75      80      85     90      95

S 2895 3040 3195 3315 3450 3640(F)

D 2720 2860 3000 3115 3245 3415(F)

Just a double check please, am I right - I should be running around 82 all the way around?  The tires look a bit "poochy" at that

I do have the Tireminder, set right now at 90 front, 80 back.  So I guess I would need to "up" that to 95 all the way around? 

Ft driver  2750

Ft Pass    2700

Rear dual driver 5000

Rear dual pass 5000

I had asked a Michalin dealer for help, he couldn't figure out his own tables.  The tires are Roadmaster range F.  Not my choice to have run but I was out in the boonies when I had to get tires, right then (large crack in the sidewall).

 

First I need to clarify my +105 and go up 5.  The +10% is correct  My intent of rounding to 5 is to end in a pressure that is easier to remember and easier to read on many non-digital tire gauges.

In your example that means 70psi is min per table  +10% = 77  and then round up to 80psi.

 

If your +10% had given you 82psi I would have suggested round up to 85. With these numbers it looks like Load Range F is sufficient so not much reason to spend the $ on LR-G tires unless your GAWR numbers would require greater inflation.

Your placard numbers 90 & 80 were based on the RV company estimate on how much "stuff" you would be carrying. It looks like you carry less stuff than they expected.

If you think your tires look better by increasing the pressure to 90 & 80 I see no problem with that since we have confirmed the inflation needs with actual data.  I do need to say that I do not trust or approve inflating tires based on appearance and usually the person doing the visual inspection is off by 20 to 40 psi.

 

You didn't say what your GAWR is for F & R but I am guessing it is higher than 5500 F and 10,000 Rear. 

Hope this clears stuff up.

[fltrvln]

thank you for the double check. I just kept looking at those tables that said I should inflate to 70 and it just didn't seem possible to be correct.  So I will run it at 80 all the way around.

My GAWR is 7000 F and 12000 R so that is ok. 

Trish.