New scale weights and tons of questions

[SteveSueMac]

New weights and questions regarding TPMS readings....this will be long - and for the tire engineers - that portion is closer to the end of this book I’m writing - really appreciate any insights you’d like to share!

Thought I’d start this season with some new scale baselines as I’ve added a topper, swapped out to GYEs and will eventually be carrying more in the bed for camping “shtuff”. So today, I went in mostly empty - other than a full tank of diesel in the truck and 2 full propane tanks on the AS - no full tank of water (312# I usually carry all season), most clothes and stuff out of trailer, maybe 200#? - DW and pooch not with me for the scales today. Will do all that fully loaded on our first trip in a couple weeks.

Scale tickets and GYE load chart attached.

First for the tickets. This is the first time I’ve used the new app for “Weigh My Truck” - couldn’t be easier! Love it! This is so fun and easy, to say nothing of informative - I highly recommend doing this if you haven’t yet! No matter the configuration, I’m well within each axle’s limits and the limits of the hitch receiver and total CGVWR.

OK, I did these in the order of hitting the scales as I drove it there with 6” of WD dialed in on the ProPride, then circled around and released the WD by lowering the jacks until the arms flopped freely, then dropped the trailer and went back with just the truck alone. Those are labeled on the attached tickets. Please check my logic on the following - all input welcome!

I’m seeing 1040 on the tongue (taking the total weight of the rig without WD (14620) minus the total weight of the truck alone (8000) minus the trailer axle weight seen with no WD (5580). That’s weird to me as it’s 60# higher than my last settings with a completely full trailer (maybe the full water tank at 300# included last time pulls up a bit on the tongue?).

The steer axle loses 460# when the trailer is attached (no WD). And at 6” on the PP jacks it’s lighter only by 180#. This suggests I’m restoring only 61% (280/460#) of the weight lifted off the front axle. While my truck manual calls for 50% FALR in this setup, I did have it much closer to 100% before at 6” so this again feels weird to me in that the bed of the truck was empty as was the trailer. I would have imagined the 6” on the jacks with nothing on board would have put me over 100% FALR. What am I missing or seeing wrong there? I also note that with 6” in this set of weights, the steer and drive axles of the truck are exactly the same (4480 each). That wasn’t an intentional target of mine, but interesting nonetheless. If I did push more to the front with say 7” on the jacks, what should I expect in terms of “feel” adding say 125 up front and removing 125 from the rear?

Last is the TPMS impact. I’ve had two different kinds of sensors with my Michelin p-rated 15” tires which had a max load (derated) of 1984/tire at max sidewall PSI of 50. Fully loaded for camping with WD engaged, the axles saw 6000#. If I add a 25% margin to that and divide by 4, that’s 1875#/tire so the 1984 was sufficient in my eyes. Here’s what I noticed with different sensors:

1. internal sensors which were the kind that link to a metal valve stem at that portion of the rim were very consistently raising pressure at 10% (from 50 cold to 55 after 30-45 minutes on the highway) and temps consistently increased by 30* above ambient. Those changes were consistent and predictable. Very little variation.
2. External sensors on the valve stems reacted differently but also predictably consistent at a rise of about 3 PSI over cold and temps rose about 10* above ambient.

These measures are drastically different (I think due to location of the sensors) but were consistent in trips with either setup. Remember - that’s at 50 PSI which was >25% margin on the average of the 4 tires based on scale tickets.

Today’s trip was with the new GYEs (ST 225 75 R15) loaded at 60 PSI. I chose 60 because if I maxed the GVWR of the trailer at 7600 and added 25% headroom to that, the load of 2380/tire at 60 PSI would be 5# above that figure - it would cover it.

Today, the TT axles saw 5700# with WD applied. Adding 25% to that and dividing by 4 gives me a weight that can be covered at 40 PSI (1880/tire). But I’m at 60 (TONS of headroom) and noticed the following with the TPMS:

1. PSI went up 8-9 pounds over cold - call it an average of 8.5 for a 14% rise in PSI, not 10%
2. Temps went up an average of about 38* above ambient (not 30)

This is again odd to me - I have more load capacity than before, but the impact on PSI and Temp rise are more significant.

The new internal sensors are DIFFERENT than the original internal sensors. These ones are mounted directly on the circumference of the inside of the rim with a band (not up at the valve stem). Could this be the reason the impact on PSI and Temp is more significant than before? In about 2 hours of driving, those new differences were also consistent. It seems like the difference between sensors being placed externally on the stems vs internally at the stem and internally on the rim may explain the variance in the range of PSI/Temp raises but what do I know??

If you’ve read along this far - bless your heart! Welcome your thoughts/insights/questions/comments. Thanks!

[Mollysdad]

1. What are the odds that your truck weighs exactly 8000#?
2. You gained 40# by connecting the WD? I knew the PP was magic, but???
3. Trailer axle with WD = 5700#
5700# divided by 4 = 1425#
According to GY's chart, 25 psi is just fine!
That's why I threw out the pressure charts and just did what I thought worked best.

[SteveSueMac]

Originally Posted by Mollysdad

1. What are the odds that your truck weighs exactly 8000#?
2. You gained 40# by connecting the WD? I knew the PP was magic, but???
3. Trailer axle with WD = 5700#
5700# divided by 4 = 1425#
According to GY's chart, 25 psi is just fine!
That's why I threw out the pressure charts and just did what I thought worked best.

[emoji3]

1) zero. Scales can be +/- 20 # so the truck could weigh 7981 to 8019....

2) it would be magic if I could lose 40# because of my hitch - oh, you mean the TRUCK! Yup - again, +/- 20 either way can explain that difference on 2 separate tix

3) yup - even with >25% margin 60 should be overkill but the changes in Temp and PSI are concerning - unless my theory about location of the sensors is correct....

[Mollysdad]

Originally Posted by SteveSueMac

[emoji3]

1) zero. Scales can be +/- 20 # so the truck could weigh 7981 to 8019....

2) it would be magic if I could lose 40# because of my hitch - oh, you mean the TRUCK! Yup - again, +/- 20 either way can explain that difference on 2 separate tix

3) yup - even with >25% margin 60 should be overkill but the changes in Temp and PSI are concerning - unless my theory about location of the sensors is correct....

I was attempting a little humor, obviously, very little.
The scales are intended to weigh 60,000 pound trailers, so, yes an error of even 100# is entirely possible.
Still a worthwhile part of the big picture!

[Al and Missy]

I think it is hard, and of limited value, to try to compare old scale numbers to new numbers, each with different loads. Also, when you are using two or three sets of numbers to arrive at a calculated result the tolerances can build up. What really matters is how the rig is set up when you go camping.

Also, I am confused by all the alternate ways to calculate tongue weight (theoretically, not just your figures), being using (1) the total weight without WD subtracting the trailer axles and the truck alone (your method), (2) using the total weight with WD subtracting the trailer axles and the truck alone or (3) the weight of the two truck axles with WD not applied subtracting the truck alone or (4) the weight of the truck axles with WD applied subtracting the truck alone. I lean toward (4). One should be able to measure the tongue weight at the wheels of the TV without introducing any errors from the trailer axle weights, and what matters to the truck would seem to be the effect of the trailer the way one uses it, i.e. with WD applied.

Regarding the TPMS data, in my opinion the pressures should be the same, except for tolerances of the measurement system. The pressure is the same regardless of where one measures it. There is no flow through the valve stem so there should be no pressure drop due to the restriction of the stem and valve core.

Temperatures are another thing. The external sensor is being cooled by air flow. The internal valve stem-mounted sensor is measuring mostly air temperature in the tire. The sensor banded to the rim may be getting a significant contribution from bearings and brakes. Once again measurement tolerances could be an issue. Further, in all things whether or not the sensors actually meet their specs could be questioned.



Al

[SteveSueMac]

Originally Posted by Al and Missy

...snip...

Also, I am confused by all the alternate ways to calculate tongue weight (theoretically, not just your figures), being using (1) the total weight without WD subtracting the trailer axles and the truck alone (your method), (2) using the total weight with WD subtracting the trailer axles and the truck alone or (3) the weight of the two truck axles with WD not applied subtracting the truck alone or (4) the weight of the truck axles with WD applied subtracting the truck alone. I lean toward (4). One should be able to measure the tongue weight at the wheels of the TV without introducing any errors from the trailer axle weights, and what matters to the truck would seem to be the effect of the trailer the way one uses it, i.e. with WD applied.

I may have this wrong but I think the first time I reported on scale numbers here I was testing 4, 5, and 6” on the scales and getting different numbers for TW each time (math). I recall getting the advice that TW is what it is - yes, it seems to drop lower with more WD applied as it get some pushed off the TV’s rear axle, but measuring without WD (still an approximation for some physics reason I don’t understand) is what the hitch receiver sees on loading which should be measured against its max capacity. Again - I could be wrong....

Originally Posted by Al and Missy

Regarding the TPMS data, in my opinion the pressures should be the same, except for tolerances of the measurement system. The pressure is the same regardless of where one measures it. There is no flow through the valve stem so there should be no pressure drop due to the restriction of the stem and valve core.

Temperatures are another thing. The external sensor is being cooled by air flow. The internal valve stem-mounted sensor is measuring mostly air temperature in the tire. The sensor banded to the rim may be getting a significant contribution from bearings and brakes. Once again measurement tolerances could be an issue. Further, in all things whether or not the sensors actually meet their specs could be questioned.

I totally see your point about temps. External sensors logically would run colder in my mind. But the pressure readings on the externals were always lower (in terms of rise over time) than those from the internal sensors at the valve stem. And I too thought having the sensor on the rim would possibly increase temps but the pressures rises about 4 points more than it did at the other location of the internal sensors. Which makes no sense to me.

So I wondered whether my cold tire pressures were too low but as noted, I could (by the charts) still have headroom at 40 PSI and I’m at 60! And I was running P-rated Michelins at 50 PSI before the GYEs at 60. In others words - I find it hard to believe I’m getting higher temps and pressures because I’m under inflated!

Weird. I will call TST this week and report back on that...

Thanks for the insight, Al!

[ROBERT CROSS]

We fret too much, it ain't rocket seance...

My weights...HaHa with 1000lb bars
1200lb TW
560 to the FA
160 to the AS
720 moved

I returned 100% and backed off 100lb with no noticeable effect on stability.
I wonder about the 50% rule, could it be a design restriction on the TV and not based towing stability?

TPMS
Towing TV tire pressures...80psi sidewall, 70psi cold.
Airstream...80psi sidewall, 68 cold.
I pay as much attention to tire temps as I do to pressure.

POI...tongue weight is best done on a scale un-hitched trailer dead level. That way you know for sure what you are dealing with.

Bob
🇺🇸

[dznf0g]

Originally Posted by ROBERT CROSS

We fret too much, it ain't rocket seance...

My weights...HaHa with 1000lb bars
1200lb TW
560 to the FA
160 to the AS
720 moved

I returned 100% and backed off 100lb with no noticeable effect on stability.
I wonder about the 50% rule, could it be a design restriction on the TV and not based towing stability?

TPMS
Towing TV tire pressures...80psi sidewall, 70psi cold.
Airstream...80psi sidewall, 68 cold.
I pay as much attention to tire temps as I do to pressure.

POI...tongue weight is best done on a scale un-hitched trailer dead level. That way you know for sure what you are dealing with.

Bob
🇺🇸

I think it is the opposite. They determined 50% provides front end stability while keeping the rears firmly planted for better handling....like braking into corners where the trailer may push the rear of the truck laterally. Can't assume everyone has and uses a quality controller correctly. When I restore 100%, I get wheel spin, especially on wet surfaces when pulling away from a stop. At 50%, not so much. That tells me something.

[Bill M.]

What I read in one place is that the newer 50% restoration figure for pickup trucks was to keep more weight on the rear wheels to prevent "jackknifing" from a severe braking maneuver.

[ROBERT CROSS]

Originally Posted by dznf0g

I think it is the opposite. They determined 50% provides front end stability while keeping the rears firmly planted for better handling....like braking into corners where the trailer may push the rear of the truck laterally. Can't assume everyone has and uses a quality controller correctly. When I restore 100%, I get wheel spin, especially on wet surfaces when pulling away from a stop. At 50%, not so much. That tells me something.

Ahh yes P/U trucks... my mistake they are designed that way.

I'm not used to dealing with that...

Bob
🇺🇸

[dznf0g]

Originally Posted by ROBERT CROSS

Ahh yes P/U trucks... my mistake they are designed that way.

I'm not used to dealing with that...

Bob
🇺🇸

Of course, it depends on what is in the bed. When all my toys are loaded, I am near payload capacity, and adjust accordingly. This is where my elaborate load spreadsheet comes in handy.

[Al and Missy]

Originally Posted by SteveSueMac

... but measuring without WD (still an approximation for some physics reason I don’t understand) is what the hitch receiver sees on loading which should be measured against its max capacity. Again - I could be wrong....

...!

My take is that the tongue weight (actually a trailer parameter as pointed out by Robert Cross) isn't really appropriate for the TV alone as the "tongue weight" is applied to the ball which, on hitches like the ProPride, Hensley, Equalizer and others, is not applied directly to the truck. 750# on a 6" drawbar has a very different effect on the truck than on a 18" drawbar.

I also think that if you can't measure it on the truck axles, it didn't happen. The "tongue weight" when sitting still without WD applied doesn't seem relevant to the performance of the truck to me. As others have stated and you have considered, weight on the TV axles and balance between them are probably the most important parameters. I think that the measurement without WD applied is to determine how much weight was lifted off the steer axle so one can transfer 50%-100% back depending on their preference.

We engineers do tend to over-analyze things, but it's what we do, right?

Al

[Wanna EB]

I did a similar weigh recently but failed to do a truck only.
2017 f250 6.7
2014 Eddie Bauer 25
Not loaded for camping


My results:
No WD
SA 4120
DA 5060
TA 5200

6”
SA 4320
DA 4780
TA 5280

7”
4320
4760
5300

I found it interesting how little the impact of an additional inch was.

Regarding TPMS, I just installed the Ford monitors and observed a rise of about 8 PSI.

Thanks for your writeup

[SteveSueMac]

So TST’s response is suggesting the change is most likely due to the differences in the rubber compounds of the tires between Michelin and Goodyear, AND the location of the sensors. They seem to think the internal banded sensors are more accurate due to their location in the center of the tire.

The person I talked with there explained that on their coach they had Michelins with 105 PSI cold that went to 125 and now with Yokahama tires that 105 jumps to 135 which they attribute to the different rubber compounds in the tires.

I’ve got an email in to Goodyear to probe further - will keep you posted....

[gypsydad]

Sometimes we get ourselves confused, least I do, when I read all these inputs...not sure this deserves the excruciating detailed focus on TPMS inside the tire or valve stem sensors outside, the total accuracy of PSI and exact PSI for your loaded AS? For me, I am happy with the valve stem sensors from TST; I believe they are fairly accurate when traveling, and at a minimum give me piece of mind that the tire pressures and temperatures are all fairly consistent. As for TT weights; I weighed my 28' last year with the TV attached and then with TT tires on one scale, while tongue (level) was on the other. I know my TT and TV can fluctuate by 100-300lbs on any given trip based on what I take along, water level, etc. For the AS, I feel pretty confident dividing the scale weight by 4 and running the chart recommended PSI (45-50) in my GY E's. I would much rather be taking a trip right now, then exploring this topic with anymore detail...so think I will...take a trip.

[SteveSueMac]

Believe me - all I want to do is take trips gypsydad.... [emoji3]

Al - I’m no engineer - I’m not even a good pretend one. That’s why I’m asking all these questions. At the end of the day, it may mean nothing - I’d just like to know....

Bob - your stress free attitude probably adds years to your life [emoji3]

dznf0g - are you talking about the 50% FALR or the fact that with 6” on the ProPride I’ve got 50/50 balance on the truck? I think the FALR. I’ve had 50 before and got porpoising. Closer to 80%, that levels out much better. In this case, current tickets, empty (not loaded for camping) the whole rig seemed a little more sensitive to bumps - not porpoising so much but not a lot of weight to hold it all down...

And I think the biggest concern I have is a recollection (maybe failed?) that one of our tire engineer members posted once that a rise of more than 10% PSI or 30* above ambient was a problem. I was so psyched that my prior internal TPMS sensors read exactly that. Now with a higher increase in both - which makes no sense seeing as I have even MORE load capacity than before - I’m worried. Maybe for nothing. Would love for them to chime in too....

But yeah - I just want to hit the road [emoji3]

[dznf0g]

Originally Posted by Al and Missy

My take is that the tongue weight (actually a trailer parameter as pointed out by Robert Cross) isn't really appropriate for the TV alone as the "tongue weight" is applied to the ball which, on hitches like the ProPride, Hensley, Equalizer and others, is not applied directly to the truck. 750# on a 6" drawbar has a very different effect on the truck than on a 18" drawbar.

I also think that if you can't measure it on the truck axles, it didn't happen. The "tongue weight" when sitting still without WD applied doesn't seem relevant to the performance of the truck to me. As others have stated and you have considered, weight on the TV axles and balance between them are probably the most important parameters. I think that the measurement without WD applied is to determine how much weight was lifted off the steer axle so one can transfer 50%-100% back depending on their preference.

We engineers do tend to over-analyze things, but it's what we do, right?

Al

You are right. TW is the amount of trailer weight on the ball. When we discuss total effect on the truck, we should use the term "receiver weight". The readings are surprisingly (unless you really understand the physics) different, for several reasons.

Robert shows the correct way to determine receiver weight. I would only suggest a couple minor changes to be even more accurate. I have a HAHA too and I insert the actual stinger into the hitch head (along with the attached Rock Tamers. The Sherline scale is placed on a floor jack and its head is centered right below the stinger/receiver pin location. The trailer is, of course perfectly level. I raise the jack just enough to begin to compress the scale plunger (needle movement). I then raise the tongue jack so the weight is now in the scale.

This methodology not only accommodates the additional stinger/RT weight, but the scale reads at the actual center of axle-to-receiver load distance.

Doing this will surprise those who read by putting the scale under the tongue jack. Your receiver weight will be quite a bit less than you think it is, even with the stinger and accessories added to the mix.

[kpm]

Originally Posted by SteveSueMac

New weights and questions regarding TPMS readings....this will be long - and for the tire engineers - that portion is closer to the end of this book I’m writing - really appreciate any insights you’d like to share!

Thought I’d start this season with some new scale baselines as I’ve added a topper, swapped out to GYEs and will eventually be carrying more in the bed for camping “shtuff”. [...]

Today’s trip was with the new GYEs (ST 225 75 R15) loaded at 60 PSI. I chose 60 because if I maxed the GVWR of the trailer at 7600 and added 25% headroom to that, the load of 2380/tire at 60 PSI would be 5# above that figure - it would cover it. [...]

I always run my GYE tires at the pressure on the trailer placard - 80psi cold. Tire experts on this forum (tireman9?) have explained that it is a mistake to use inflation tables for trailers because on travel trailers the tire sidewalls have to withstand additional lateral forces. This is especially so for those trailers with multiple axles.

[Tireman9]

Looks like you are trying to do solid engineering with collecting lota of data.


Don't have the time right now to did deep but can offer one quick comment. I did an internal vs external TPMS last year from March in the Cold (25 to 40F) till later in Summer when ambient hit 90. I also built a test fixture to expose all 12 sensors to the same pressure at the same time. I posted the numbers and results on my blog (I'm not allowed to post the link here)


The bottom line on pressure reading vs my ISO calibrated gauge
"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 would consider these numbers to indicate essentially equal pressure readings with +/- 1.25 psi range. I would not be concerned with TPM pressure reading varying by a couple from a calibrated digital hand gauge.


Temperature comparisons are a different story. I have heard of some data that indicates the air in a tire can have a range of a few degrees in a running tire depending on the location of the thermocouple.


My testing yielded a 30F to 40F temperature difference between internal and external sensors but this changes as the ambient changes.


The bottom line on TPMS I don't consider the temp difference to be meaningful once you understand what is happening. Just watch for one tire suddenly seeing a bigger temp rise than the others.

[SteveSueMac]

Originally Posted by Tireman9

Looks like you are trying to do solid engineering with collecting lota of data.


Don't have the time right now to did deep but can offer one quick comment. I did an internal vs external TPMS last year from March in the Cold (25 to 40F) till later in Summer when ambient hit 90. I also built a test fixture to expose all 12 sensors to the same pressure at the same time. I posted the numbers and results on my blog (I'm not allowed to post the link here)


The bottom line on pressure reading vs my ISO calibrated gauge
"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 would consider these numbers to indicate essentially equal pressure readings with +/- 1.25 psi range. I would not be concerned with TPM pressure reading varying by a couple from a calibrated digital hand gauge.


Temperature comparisons are a different story. I have heard of some data that indicates the air in a tire can have a range of a few degrees in a running tire depending on the location of the thermocouple.


My testing yielded a 30F to 40F temperature difference between internal and external sensors but this changes as the ambient changes.


The bottom line on TPMS I don't consider the temp difference to be meaningful once you understand what is happening. Just watch for one tire suddenly seeing a bigger temp rise than the others.

Thanks tireman9 - I’ve read several of your posts on rvtiresafety.net and your reply here (thanks for that). So are you saying the fact that the 4 tires are consistently showing the same increase in pressure and temps suggests we’re in good shape? No concerns that the temps rise more than 30* above ambient or that the pressures are rising more than 10% above cold - so long as they’re all rising at the same levels? Just want to be sure I’m understanding you...

Thanks!