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Old 12-21-2016, 07:52 PM   #1
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Would you all please check my math

Ok, so I just received my EEZ RV T515 TPMS 6 sensor system and want to set it up. I went and had a 4 tire weigh & then front & rear axle weigh and have drawn out a diagram so I can see all this info in front of me and figure out the proper settings for this system. I don't need to list any other details here in this post because all the information is on the attached PDF drawing. I've also included the Michelin RV PDF tire guide for cross reference.

I know I am asking a lot here for any you to take a few minutes and look at my drawing with my numbers & then verify with the Michelin guide, but this is my first time doing this and I know monitoring tires is so important I want to make sure it's right.

NOTE:
• I calculated 20% for hi-alarm pressure and 10% low alarm.
• I was going to leave the temperature setting to default of 158 degrees. (Is that OK for now in winter & also in summer?)
• I was just a little concerned about my numbers because looking at what you all run I seemed so low on my PSI, but I really don't have much in my storage bays right now. As I load it up more for longer trips, I will re-weigh and set PSI accordingly Just wanted to make sure that the load I currently have I am in fact calculating correctly.
• We're not towing a toad at this time

thx
Attached Files
File Type: pdf TPMS-settings.pdf (65.4 KB, 90 views)
File Type: pdf pg.21-tire-pressure-guide.pdf (80.5 KB, 84 views)
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Old 12-21-2016, 08:39 PM   #2
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First, I'd want to verify that you have weighed with fuel and propane full, fresh a minimum of 1/2 tank, or full if you travel that way. I do black and grey empty, based on any fresh water reduction will ultimately end up in those tanks. Clothes, kitchen gear, firewood, tools, people, and everything else you travel with should be in the coach.

If the assumption is that is all true, then your calcs are close.
- Take the highest weight of the tire on each axle, for your calcs. I get 70 on fronts, and 75 on rears. However, you should always add at least 5 psi to those calcs. So I'd start with a minimum of 75 front and 80 rears.

Then I look closely at the excess weight carrying capacity for those pressures. If you look at the difference between 70 and 75 on fronts, for those 5 psi you only added 175 of weight carrying capacity x 2 = 350 lbs. That is not much excess carrying capacity at all, realizing that if you go shopping and buy some stuff or add a person, or load up the refrig, you can be over capacity. Rears are a little better.

I would up the pressures based on that further analysis, to a level of spare weight carrying capacity that you are comfortable you'll never exceed. Be sure you do it again if you change what you carry significantly.
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Old 12-21-2016, 09:04 PM   #3
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Originally Posted by Kiawah View Post
First, I'd want to verify that you have weighed with fuel and propane full, fresh a minimum of 1/2 tank, or full if you travel that way. I do black and grey empty, based on any fresh water reduction will ultimately end up in those tanks. Clothes, kitchen gear, firewood, tools, people, and everything else you travel with should be in the coach.
Yes on everything.
• Full fuel, 3/4 fresh water, grey/black empty, propane full, Clothes, kitchen gear, firewood, tools, people, and everything else you travel with should be in the coach - yes for this 10 day trip.

So yeah, as gear changes for future trips & weight increases I will for sure re-weigh and adjust PSI. I just wanted to make sure I was doing the math right.

Your point of bumping everything up 5 PSI for incidentals and weight creep is noted....will do.

We bought this coach 4 months ago, and I had been running 90 PSI all the way around as posted by the drivers door emblem until I got it properly weighed. Been working on it a lot the past 4 months to really get it up to snuff and the way we want it. So now that I know what it weighs and can set PSI correctly, it should not only allow the tires to wear better but give us a better ride and the TPMS will be set and work properly.

Oh, what the temperature setting to default of 158 degrees especially down here in Texas and the gulf states where we live & will travel a lot. (Is that OK for now in winter & also in summer?)

Thank you so much for taking the time to check me.
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Old 12-21-2016, 10:05 PM   #4
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Your calculations puzzle me. Adding up the weight for each front wheel doesn't equate the Front Axle Weight value. Same goes for the rear wheel data. You might want to check this again.

I also noticed a large discrepancy between the rear weights with the RH side being 900 pounds or so heavier than the left. The coach will handle much better if you can move 450 pounds to the left side somehow.

Your setting for the High Alarm at 15 psi over the cold weight may not be enough to compensate for pressure increase after warm up. I run my tires, Toyo's, at 90 psi all around. When warmed up I see values up to 108 -110 psi especially on the rears. I'm lighter than you running 19" wheels. I run 90 psi which is 5 psi over the sticker pressure because I found for my coach that is the handling sweet spot along with other mods I made.

The high value isn't as important as the low value though. I would only allow 3 - 5 psi between the cold pressure and the low warning. You want to know ASAP if a tire is going down.

The temperature setting is good as default from the TPMS manufacturer works for most applications in any area of the country and time of year. You can set it lower if you wish but I haven't come close to this setting after warm up and temp rise doesn't bother me as much as pressure drop. I usually see a rise of 10 to 15 degrees over ambient temperature (OAT) 20 at the most in the desert.

Lastly, I would not go below the sticker setting as the low pressure cold. It will amaze you how much tires run at pressures set low heat up. The sidewall flex becomes an issue and the sidewalls may come in contact with each other on the rears which is not good.

I gave you a lot of food for thought. This comes from experience and trial and error. I had 3 blow outs on our first trip with the coach all within the first 500 miles. Old tires were the cause. The old Good(BAD)year tires were over 10 years old. Dig around on this forum for additional information. There's a lot of good ideas here.
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Old 12-22-2016, 05:37 AM   #5
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Originally Posted by c92vette View Post
Your calculations puzzle me. Adding up the weight for each front wheel doesn't equate the Front Axle Weight value. Same goes for the rear wheel data. You might want to check this again.

I also noticed a large discrepancy between the rear weights with the RH side being 900 pounds or so heavier than the left. The coach will handle much better if you can move 450 pounds to the left side somehow.
Yeah I knew someone was going to comment on that. I just didn't want to take the time to explain it and make the post to long.

1) So as for your first question, no it doesn't exactly add & match up but don't forget, I had to place the coach 3 different times on the scales and just my placement each time could be a little different time to account for this very small difference.

2) As for your second question, it also puzzled me. I told my wife wow 900 lbs difference, that seems like a lot. BUT...when you add up both sets of dual tires it comes within 80lbs of the total axle weight so again that could be because of my MH placement on the scales was just a little different each time. So 80lbs difference when you're talking 12,480 total lbs., I felt that wasn't a big deal. Now the 900lbs difference...I can't explain that. I'm not really carrying that much of a load. The only thing I can figure is that the heavy side is where my generator is and on the right side is the wet bay & I was only running 3/4 full fresh water (FW). But even if I would of been running full FW that would only of added maybe 100 t- 150 more lbs. so there would still be a big difference. I did this 4 wheel weigh to see what my weight distribution was so I don't know what to say about it other than, if you look at it from a percentage point of view from the overall weight or side to side weight, the difference isn't really that much. But that is why I took the heavy side measurement to calculate my TPMS settings. From what I've read over the past few months on this, the difference of left/right weights can be quite a bit and you either shift your load to try to even it out, and always take the heavy side side, add about 5% or round up and use that for calculations. I did think about going to another Flying J location and get a second weigh, but for $11 each time I get on them (which was $33 this time) I really didn't want to spend that money again. I guess I could just go weigh the back tires again for $22. These scales I assume should be regulated by weights & measures and should be within a certain +/- tolerance.

With your recommendation of low PSI setting of 5 lbs is a good one and I guess I could just keep all tires @ 90lbs right now like you said and just play it safe. If I'm wrong on everything I've written here, I'm open.
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Old 12-22-2016, 07:53 AM   #6
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I think I'm just going to set my fronts to 75lbs. and rears to 80lbs. This gives me a good fudge factor also for now. I'll set my low pressure alert on all tires to 5 PSI below that. That should be good for now don't you think?

But when I look at my drivers side weight load sticker it seems to say to don't go any lower than 90 PSI. But then again I may be misunderstanding this info. I always thought that sticker was to be used UNTIL you get an accurate weight. That's what is confusing to me. (see attached pic)

Also—in the near future—I'll have just my back left/right axles weighed again on different scales at a different location to see if those scales reflect the similar heavy passenger side.
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Old 12-22-2016, 08:47 AM   #7
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Low alarm threshold is good at 5 lbs under, because since you added 5 lbs originally, it's alarming at the psi which is the minimumpsi for the weight you are carrying.

That's how I have mine set.
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Old 12-22-2016, 09:17 AM   #8
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What do you all think about this 2nd drawing with new PSI numbers?

Or should I just go with 90 PSI all the way around and set low alarm to 85 & hi pressure alarm to 110 PSI? I just don't want to over inflate for the heck of it.
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Old 12-22-2016, 10:04 AM   #9
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You aren't calcing correctly.

You take the maximum weight tire (on the same axle), double it, and use that for your calcs. You do not want to take half of the sum of the actual axle weight, because that can create a situation where your heavy tire is overloaded.

You want to keep the tire pressures for all tires on an axle the same. So calculate your pressure based off the high weight side, and then use that psi also on the lighter tire.

The only reason you care about the total axle weights, is when you compare the actual weight against your axle rating, to be sure you are not overloading your axle components.

So in your case, you should be setting the rear tire pressures based on the 6620 PS weight. Figure out the psi on duals that will carry that weight, then increase it by 5 lbs. Use that pressure on both DS and PS rears.
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Old 12-22-2016, 10:49 AM   #10
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The placard numbers could be considered minimum or even maximum depending on how an owner looks at them. Listed pressures are what is required for the listed tire size if you loaded the coach to the maximum axle weight of 8,000 on the front and 15,000 on the rear. It's a good number to use if the coach ride and steering performance is acceptable and after check the coach is not overloaded.

Weight creep can be a problem. Call it a silent tire killer. Couple of pounds here, a new grill there, a new 300 piece tool kit, more shoes and cloths then you'll ever wear on a weekend and you're over weight for the pressure. Although I'm well under axle maximums by 1500 lbs. I still run placard pressure just to play it safe. The Navigator has started collecting rocks for a small flower garden she has.

For your TPMS, the high limit is a bit low and so is your intended low alarm point. I have my high at 25 PSI of normal and the low at 5 PSI below normal. Temp set to 140 degrees. Running at 85 PSI I have seen pressures pushing into 100 PSI a few times on a hot road during summer trips. An out of the blue drop in pressure rolling down the road doesn't happen very often if ever unless there's something wrong. A good number to use is 5 PSI under you inflation. Easy to recall rather than a percentage.
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Old 12-22-2016, 12:43 PM   #11
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As for right/left difference, I wouldn't sweat it too much. When I did my 4 corner weigh in my new-to-me Itasca, it was about 1300 lbs heavier on the left side than the right. Then I realized that both slides are on the left, along with stove, oven, & microwave. There is really very little that can be moved to try to even it out, so I set my tire pressure based on those heavier side weights. I'm still well under capacities on both axles (even if I use double the heavy side numbers), so I'm not too worried about the side to side difference, and my handling is much improved over the max pressure numbers on the door sticker.
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Old 12-22-2016, 02:09 PM   #12
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Ok thanks everyone for checking me.

I just got through setting them up and programing them. I decided to go with:
83 PSI all the way around cold pressure

Alarms:
108 hi-pressure
78 low pressure

We'll start here and we'll see how this works for now and make necessary adjustments in the future.

FYI...for anyone else new at this and was having a hard time following how I was coming to these conclusions, I've attached my final drawing w/weights, pressures & alarm settings based Michelins table which I've also attached again.

Merry Christmas everyone!
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File Type: pdf TPMS-settings-12-22-16.pdf (31.5 KB, 19 views)
File Type: pdf pg.21-tire-pressure-guide.pdf (80.5 KB, 13 views)
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Old 12-24-2016, 02:40 PM   #13
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Here's some further thoughts on this subject along with how I set my tire pressures and TPMS for optimal performance.

One thing to know about TPMS systems. They should warn of gradual tire deflation or rapid loss of pressure in the event of an issue during operation. The low point setting at a percentage of cold pressure is a good setting but understand that as the tire pressure rises due to operation this percentage increases. Also when setting the low point at a fixed setting, 5 psi below cold for example, the span between the actual and set pressure also increases during operation. Therefore the low setting is really a good way to warn of a low tire prior to traveling. That's why I feel it is still the more important setting versus the high setting but it doesn't provide for a slow deflation of the tire. Since the accuracy of the TPMS (TST claims 0.73 psi) is better than most stick tire gauges found in auto parts stores I use it as gospel during first check in the morning before we head out. If the monitor is beeping I know exactly which tire needs attention. During operation if a tire loses pressure on the rear duals, the adjacent tire begins to take on more load and this will happen long before the low pressure warning sounds. This should cause the pressure in that tire to rise due to extra heat generated from the additional load and possibly trigger the high pressure warning first. In this case I feel the upper limit should be set close as possible to the actual pressure seen during operation. Say it's actual operating pressure is 110 psi the setting might be 5 psi above or 115. Only trial and error can find the best setting at the top. To have a large range between these settings is of no value in the event of a slow loss of pressure unless the loss triggers the deflation warning. But as the adjacent tire pressure rises above the high setting that will provide an extra level of safety. Now this will only effect the dual tire arrangement as fronts or tag axle and towed tires are singular installations. For single tire arrangements you are at the mercy of the TPMS gradual warning working properly. We've primarily discussed the coach tire settings but don't forget the use of the TPMS on the towed vehicle. Way in the back of your set up a loss of pressure in a tire resulting in tire failure may not be known for many miles and after the real damage is possibly done. So if you tow be sure to set these limits as accurate as possible. Because of this the TPMS is a critical part of the warning system on any motorhome.

Setting tire pressures at the recommended chart setting will give you no buffer in the event of weight addition though. In other words if the coach axle weight is "x" and the chart setting is "y" adding weight of 200 pounds to that wheel set can change the recommended setting by 5 psi according to some of the tire charts I have looked at. As one poster said the DW is collecting rocks for the garden at home. Storing those rocks in the basement bin right in front of the rear wheel could increase the weight on that side 300 pounds. Plus the transfer of weight under normal use, ie: fresh water transferred to the gray tank may have an effect. Now you will have under inflated tires for the new load on that axle. That makes the sidewall work harder and increases the chance for tire failure. Always give yourself a buffer for setting pressures. I used the plus 5 psi rule. You can't hurt a tire by inflating to the max pressure stamped on the sidewall but underinflation will increase your chances of having a bad day.

Finally handling and ride come into play with tire pressures. I have done the CHF (Cheap Handling Fix) on our coach. Then I set tire pressures to 5 psi below the maximum pressure stamped on the sidewall. I drove the coach two or three hundred miles and noted the ride. Then I lowered the pressures in 5 psi increments down to the recommended chart setting and repeated the process noting the ride and handling each time. I got good input from the DW as far as ride goes. After the experiment was complete I crunched the data to find the Best Handling Pressure (BHP) and set the tire pressures using two parameters. First, 5 psi above the chart setting for weight (the plus 5 rule) and second at the BHP. Note: the BHP must be equal to or higher than the plus 5 psi chart setting. My BHP setting was 5 psi higher than the chart setting or equal to the plus 5 rule. If it is higher than the plus 5 rule setting it is okay as long as you don't go over the maximum pressure as stamped on the tire.

I then set my TPMS, a TST 507 Flow Through system at 87 psi low warning and 113 psi high setting. This is 3 psi below the cold pressure of 90 psi that is my sweet spot pressure and 3 psi above the highest operating pressure I have seen. Of course I weighed the coach prior to all of this with full fuel, fresh water, empty gray and black tanks, passengers on board, the essentials of food, equipment, and clothing, and our Edge on the dolly hooked up. This represents our basic traveling weight. My calculations were based on the highest weight of the wheel set across the axle and not on each individual wheel weight.

So now I know I have the correct tire pressures set at the best handling I can get with a weight buffer and the TPMS set for optimal operation.
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Old 12-29-2016, 08:17 AM   #14
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Good job for all that are making the effort to know the facts on what the RV weighs and how it is distributed on the tires. Too often people just get the axle weight and divide by 2 and assume a nice 50/50 side to side split which is almost never reality.

As a retired tire design engineer and RV owner I suggest:

The set pressure is the Cold Inflation Pressure or CIP which is when the tire has not been warmed up from running or in direct sunlight in previous 2 - 3 hours.

RE warning levels.
Temp. Most TPMS have the temp set to 158F (70C). The reality is that the external sensors are reading temperatures that are lower than actual internal air chamber temperatures which are themselves lower that the temperature internal to the rubber structure. Basically I would not worry about that setting.

High Pressure. Tires simply do not fail due to high pressure as most highway tires (passenger, LT or Truck) are capable of withstanding at least two times the pressure molded on the sidewall unless the tire has been damaged from impact, cut or running low. If you want a setting I would suggest a +25% over your CIP number.

My TPM low warning is the minimum inflation needed to support the heavier end load on each axle. MY CIP is the Min infl + 10% (this works for all types of tires better than a fixed number i.e. 5psi as some folks have a minimum of 65 and some need 120.

My TPM has both a low pressure warning level and a "Rapid loss" or "Quick leak" that will sound when a few psi has been lost from the higher hot pressure reading in a short time. This is much better than only having a single low pressure warning point which means you would not get warned till after you lost all the air from the temperature build up (2% per 10F increase) plus loosing more till you get down to the low pressure level which is 25% below what is needed to support the load in some cases. This really means you may have damaged the tire and probably shortened its life by running low.

I also use the "Morning Reset" feature to check the CIP while I sit inside the coach with my coffee watching others out, down on their knees in wet grass, checking tire pressure.

Safe travels to all in 2017
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