Your Ad here
Be sure to sign up for the weekly RV Travel Newsletter, published continuously every Saturday since 2001. Click here.
Huge RV parts & accessories store!
You have never seen so many RV parts and accessories in one place! And, Wow! Check out those low prices! Click to shop or browse!

Friday, March 16, 2018

How accurate is your TPMS?

I have seen a number of people express some concern about the accuracy of the pressure readings from their TPMS. You can read my previous responses HERE  and HERE

I have also pointed out that the primary purpose or "job" of a TPMS is to warn the driver of a pressure LOSS.

I am inclined to think that some are still concerned with their exact pressure reading. I also have to wonder how some people are comparing various pressure readings reported by their TPMS. If they have external sensors, the simple act of removing and replacing a sensor can allow some air to escape. While I doubt that the small loss of some air will result in a meaningful pressure differential it does add some uncertainty.

So the engineering DNA in me kicked in and I a devised a plan to test 12 sensors. These come from two different companies. One set of 6 external sensors is from Tire Traker and one set of 6 internal sensors is from Truck Systems Technology.

The question is: How do I make the test both fair and useful. I decided to eliminate as many variables as possible and get all 12 readings from the same air chamber at the same time and compare them all against my personal digital hand gauges that I have checked against an ISO certified laboratory gauge.
Note my hand gauge reads to 0.5 psi which is way more precise than anyone needs for checking tires in normal highway use.

Here is the test fixture I made.

It has 6 bolt in valves for mounting the external sensors plus a valve for me to use with my hand gauge. Also since safety is always of concern when dealing with a pressure vessel I added a pop-off valve.  One end of the fixture has a cap that can be removed so I can place the internal TPM sensors inside the 4" tube. It also has a regulated air supply to compensate for the very slow leak around the cap and an analog dial gauge that allows me to constantly, visually monitor the air pressure to ensure the use of my hand gauge does not result in a pressure change that isn't immediately compensated for by the regulated air supply.

Here are the results of my comparison test. The target pressure is 80.0 psi as reported by my handheld digital gauge.

Set A  1 reading of 78 psi,  5 readings of 79 psi
Set B  2 readings of 78, 2 readings of 79 and 2 readings of 80 psi

I also recorded the temperature.
Set A  4 readings of 66 F, one each of 64 and 68F
Set B  4 readings of 69 F  and 2 readings of 68

I do not consider any of the differences in the readings of pressure or temperature to be significant or meaningful for a TPMS.  I would consider the pressure readings from all 12 sensors to essentially be equivalent.
You can learn a bit more about what a "meaningful" measurement is HERE.

Tuesday, March 6, 2018

How would I set inflation on a smaller single axle trailer?

Got this question from a reader. 

Our Jayco Hummingbird came from the factory with P235/75R15SL tires. The TT GVWR is 3,750 lbs and the GAWR for the axle is 3,500 lbs. This is a single axle TT. The P-rated tires were like a pogo stick at max inflation.

We changed from the factory tires to Maxxis 8008's in ST225/75R15 size. The factory aluminum wheels are good for a max of 80 psig according to the stamp inside.

Also converted to metal valves stems for running our TPMS....because I'm an engineer who tends to overdo everything I touch .

The heaviest CAT scale weight has been 3,320 lbs on the axle and 3,780 lb GVWR. We've since removed a few items to stay within the 3,750 lb. GVWR.

I've always kept the tire pressure at the minimum sidewall stamp of 65 psig (Max load of 2,540 lbs at 65 psi cold). After reading some of your blogs and looking at the Maxxis load chart, if I assume an equal split weight on each wheel we would have a worse case of 1,660 lb load. Of course a perfectly balanced load isn't likely to ever happen. But even with adding 10% it would put us at 1,826 lbs per wheel. Maxxis says that for our particular tire 40 psig would give us 1,880 lbs capacity.

I can't say that I'm comfortable going all the way down to 40 psig, but I feel ok with 50 psig, even though this is grossly over-pressurized for the given load. I know that at 65 psig the TT rides like a log wagon and we recently bent a spindle on the axle without even knowing it, I wonder it the limited travel of the torsion axle combined with the tire pressure came into play because we were under the GAWR of the axle and never even felt anything out of the ordinary during the trip, of course we are pulling a 3,750 lb trailer with a '17 GMC 2500HD w/ Duramax so we don't feel much anyways.

So if it were yours what pressure would you choose? I've been running at 65 psig and I think that's too much, 50 psig sounds good to me, but it's still too much pressure according to the weight charts.....


Here is the answer I gave him.

My approach
OE tires P235/75R15 are  rated for 2,280#@35 psi but on a trailer we need to De-Rate the load capacity so 2028/1.1 = 1844# load capacity.

Your measured axle load was 3,320#
If we assums a nominal 53/47 side to side split we get 1,760# for heavy end  and a 60/40 split gives 1,982# for the possible heavy end of the axle.

An ST225/75R15 LR-C is rated 2,150# at 50 psi. Since we are looking at a single axle trailer we can check the tables and find 40 psi is rated to support 1,880# and 45  psi can support 2,020#.

Since we always select the pressure needed that can support the heaviest end of an axle and we always inflate all tires on any one axle to the same inflation we could select  40  to 45 psi for our CIP.
I would set my TPMS Low Pressure warning level to 40 psi and my CIP to 45psi.

If this was a multi-axle trailer we would want to lower the special belt shear forces and run a higher inflation. Maybe 50 psi minimum

Thursday, March 1, 2018

Can I run my LR-E at 65 psi? or is this overloading the tire?

As trailer owners start applying the new Goodyear Endurance ST tire, many are discovering that for some sizes the Endurance tire is only available in a Load Range that is higher than their OE tires. Some are concerned about what inflation to run. I have even seen some claim that running a LR-E at LR-D inflation i.e not 80 but at 65 that the "tire will be overloaded, heat up and fail".

While I understand some of the confusion I do not agree with some of the concern or replies.

Tire load capacity is a function of the tire size and inflation level as long as you stay in the same "type" tire.  By "type" I mean P type or LT type of ST type or for large RVs "truck" type.

If you stay with the same type and use the same numeric "size" then the only thing left to change is the Load Range or "Ply Rating". While I do not like using Ply Rating as it is an old and discontinued nomenclature it may help for better understanding in this post for you to think of the old term.

Important Point. "It is the air pressure that supports the load NOT the Ply Rating." This statement is supported for every tire made by every tire company in the world through the use of Load & Inflation tables. These tables show a size and then for different levels of inflation the load capacity of that tire when inflated to that level. You will never see a tire shown where a LR-D at say 65psi can support 1,500# and for the same size the same tire when having a LR-E rating shown a higher load capacity at 65 psi. Not even just 1 pound more.

So a LR-E can support the same load at 50 psi as a LR-C or the same load at 65 psi as a LR-D at 65.

You will not be overloading the LR-E if you load it to the 65 psi rating shown for thet type & size tire and inflate it to 65 psi as you would for a LR-D. Since you are not overloading the LR-E tire it is not going to overheat at 65psi with the 65 psi load so the LR-E tire is not going to "overheat" at 65 psi any more than the LR-D will "overheat" if it is loaded to the 50 psi load rating and inflated to 50 psi.

When going to a higher "Ply Rating" you can then increase the CIP which increases the tire Load Capacity which means it will actually be running cooler because of the greater "Margin". The higher inflation will also lower the Interply Shear which may lead to longer tire life.

When making the change you do need to confirm the upper inflation level for the rim. The wheel manufacturer should provide that information. As an alternative the wheel will have a max load capacity stated. Looking at the OE tire size that comes on that wheel look for the inflation that corresponded to that load and I would consider that to be the wheel inflation rating.