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Old 02-20-2016, 12:29 PM   #1
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Check my weight tire pressure math

Got my weights today and made a couple of item moves to redistribute some weight. Weight was overall pretty close side to side. I took highest weight on front then rounded up to next 100 pounds. I added an extra couple hundred pounds to front and also to the rear set. I also rounded up rear to next 100 pounds. Front came out needing 70 pounds and rear needing 75 pounds. I figured to add extra to each and use 85 pounds on all tires. I had all tanks full including grey and black water and white water. all propane and gas tanks full. I also had it packed with items for travel. This should be the heaviest I use it almost all the time. I was over the maximum weight for loaded unit by 10 that's ten pounds after adding a buffer. And I added several hundred pounds as a buffer. When towing my car I am under weight by 1,800 pounds. My toad only weighs 3200. Does this sound about right for the weight load. First time setting all this up. Weighting at CAT scales was a breeze. Weighted entire rig. Then pulled off got ticket and pulled back on and straddled the scale and weighted one side to get individual tires. Worked like a champ. Thanks for all your help. Hope I did all this right.
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Old 02-20-2016, 12:43 PM   #2
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Seems to me you have plenty of contingency via all of the rounding up you did. And over another 10% on top of that, using the 75PSI as the high value. 75+ 7.5 = 82.5PSI - so at 85 PSI you are top of that!

Glad CAT scales worked well of for you, and salute on running with PSI for your weights and your tire, with the added safety margins.

Travel safe, and have fun,
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Old 02-20-2016, 02:53 PM   #3
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Wow, pretty elaborate calculations, moving stuff, weight etc. I did it a bit different. I drove to the scales, weighed the front, back and total. Then, got home, consulted the Michlen tire chart for my particular tires and, lowered them down to the required amounts which, all of them were over 15 psi too much. Seriously better ride. I didn't bother with the side-to-side weights 'cause I couldn't move anything of substantial weight from one side to the other or, one end to the other anyways because things fit where they fit.

It's been running down the road like that now for almost 5 years. Tire wear is great and, no handling issues what so ever.
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Old 02-20-2016, 03:00 PM   #4
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Country Coach used to say to weigh the rig and use the charts, but the manual with our rig says to use the pressure on the weight sticker only and not reduce them.
Quote:
Tire Inflation
Country Coach recommends that the cold tire inflation pressures should at all times be maintained at the inflation pressure(s) recorded on the Federal Tire Label. There are no acceptable circumstances where tire inflation pressure(s) should be reduced below that pressure recorded on the Federal Tire Label.
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Old 02-21-2016, 04:00 AM   #5
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Keep in mind Goodyear says lowering air pressure below vehicle mfgrs. tire placard recommendation to obtain a softer ride is not correct:
"Tire pressure should never be reduced below the vehicle manufacturer's recommended levels to support load conditions in order to improve the ride quality of a vehicle. The difference in ride quality is not significant"
Michelin has a similar statement on their website.
There is a lot of confusion about those load/inflation charts and what is correct.
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Old 02-21-2016, 07:05 AM   #6
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Owned 4 motorhomes and have never had any of them weighed. Simply used the GAWR and the manufacturers weights which also matched the weights from the tire manufacturer. I've read probably hundreds of posts about RV tire care, weights, pressures and have come to the conclusion that unless you are carrying around a bunch of rocks, or half a dept store, my plan is sufficient. If you think you might be overweight then get it weighed and react accordingly. I will take note of the weights for my yet to arrive new BayStar Sport just "to know".
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Old 02-21-2016, 08:21 AM   #7
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Thanks for all the info guys. I was aware of the minimum air requirements and it is lower than I am setting mine. I was concerned about the weight though due to being new at all this and now knowing what weight would be once I loaded all our STUFF in it. Apparently we are both legal and in good shape. Hope to have many many miles of fun in the future and now that I know where weight is at I can move on and just watch tire pressure. Unless something changes with load no need to do this too often. Thanks for all your input.
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Old 02-22-2016, 02:21 PM   #8
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If you give here the weights on seperate wheels or Wheelpairs, I can put it in my calculator.

Or yust give what you weighed and the weights after you added reserves.

But also need configuration of motorhome ( front 1 axle 2 tires is clear but rear 1 or 2 axles and single or dual load ( 2 or 4 tires on the road per axle).

then maximum speed you wont go over for even a minute in your use.

From tires I need 3 things:
1 maximum load or loadindex
2 Loadrange or plyrating or beter AT-pressure ( wich is not the maximum allowed cold pressure of tire).
3 maximum speed of tire or speedcode ( Q and above is calculated in maximum load for max speed of 160km/h.
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Old 02-23-2016, 10:41 PM   #9
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Over the last 12-15 years of reading different forums, I've lost count of the times that two major reasons for getting 'Four Corner' weights have come been reported:

1) Some manufactures shipped coaches from the factory with weights right at max levels of axle capacity. This is before the owner adds any of their possessions, and or say needs to travel with a 3/4 Grey and 1/2 black tank of extra weight.

2) With others, while the 'total axle weight' may have been within limit. Due to poor designs, one side was 'over limit'.

These are conditions that warrant understanding, and when possible loading of coach in such away as to minimize further impact/risk.

Another observation, is the contradictory in nature info you can find even on a tire manufactures website. Some saying always run with he coach manufactures recommended PSI values. And others that say a four corner weight and application of PSI as called out for on the manufactures tire charts, is the way to go.

My opinion, after much research and talking with two different tire manufactures tech support is to:

> If you do not have actual 'ready to travel' four corner weights for your rig. Run with the Manufactures recommended PSI setting.

> If you do have actual 'ready to travel' four corner weights. Run with the actual tire manufacturers recommend PSI setting for that tire model. Add plus 5 PSI, or 10%, whichever is greater to that value. This is for both contingency for coach 'weight creep', and also for the times you might travel without being able to check and maintain the PSI as much as you would like.

And finally. I do suggest taking the time to research and making our own decisions on what you feel comfortable doing. Ignore other posters recommendations, including my own, when they contradict what you feel is right for you to do!

Best to all,
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Old 02-24-2016, 03:34 AM   #10
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Added my opinion in the quote in blue.

Quote:
Originally Posted by Smitty77 View Post
Over the last 12-15 years of reading different forums, I've lost count of the times that two major reasons for getting 'Four Corner' weights have come been reported:

1) Some manufactures shipped coaches from the factory with weights right at max levels of axle capacity. This is before the owner adds any of their possessions, and or say needs to travel with a 3/4 Grey and 1/2 black tank of extra weight.

2) With others, while the 'total axle weight' may have been within limit. Due to poor designs, one side was 'over limit'.
To my opinion a little weightdifference R/L is inavoidable so you always have to take as much as possible reserve in the pressure to cover that.

These are conditions that warrant understanding, and when possible loading of coach in such away as to minimize further impact/risk.

Another observation, is the contradictory in nature info you can find even on a tire manufactures website. Some saying always run with he coach manufactures recommended PSI values. And others that say a four corner weight and application of PSI as called out for on the manufactures tire charts, is the way to go.
If you go from manufactures tire chart, keep in mind that American lists are calculated with an other formula then European lists.
To laws of Nature a tire with same specification and construction should be calculated in Pr/Loadcapacity with the same formula.
American TRA stepped over to the European ETRTO formula as late as 2006 , but only for Standard load and XL// tires. and left the LT and TR calculation to the old formula that leads to higher loadcapacity's in the lower pressures.

My opinion, after much research and talking with two different tire manufactures tech support is to:

> If you do not have actual 'ready to travel' four corner weights for your rig. Run with the Manufactures recommended PSI setting.
In my made spreadsheets , besides using my own formula, When having no 4 corner weight , I use the GAWR of vehicle, and add front 5% to that and rear 18% , then calculate the pressure for that.
18% rear is for expected overloading ( thoug not allowed by law) and unequall loading R/L, but also for Pressure loss in time, misreadings of pressure schales. So another extra means to determine the pressure when you dont have enaugh information.


> If you do have actual 'ready to travel' four corner weights. Run with the actual tire manufacturers recommend PSI setting for that tire model. Add plus 5 PSI, or 10%, whichever is greater to that value. This is for both contingency for coach 'weight creep', and also for the times you might travel without being able to check and maintain the PSI as much as you would like.
When you have axles weighed I add in my spreadsheets front and back 10% for misbalance R/L and again for pressure loss in time and misreadings,etc.
When 4 corner weights , I make corner with lowest load per axle, 85% of the load I calculate the pressure for, Highest load I take care to not stay lower then 95% of that load you calculate the pressure for. All to give as much possible reserve with still acceptable comfort and gripp. That 85% is the border I determined ( so discussable) to give yust no bumping, and 95% to be needed for misreadings of weight and pressure-scales and extra weight at moments.

Looking that loads up in list would give lower pressure because of the calculation done in America for LT.

My calculation is even more pessimistic then the European calculation, and to my conclusion in time, comes closest to the ever to be constructed ideal formula to laws of nature.

And finally. I do suggest taking the time to research and making our own decisions on what you feel comfortable doing. Ignore other posters recommendations, including my own, when they contradict what you feel is right for you to do!
But also dont even trust the tiremakers to give the right information.
The chanche for SL and XL, to EUR calculation was not for no reason, and company's dont always give the real reason as explanation for their chanche. And In many topics I call myself "Dutch Pigheaded Self-Declared Tirepressure-specialist" to show that what I write is my opinion and conclusion , so decide by checking and own reasoning the value of it.

Best to all,
Smitty
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Old 03-04-2016, 03:56 PM   #11
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On our RV (see signature), the GVWR for the rear axle is rated 12,000lbs, or 3,000lbs/tire for 4 tires. We do not exceed GVWR, and all 4 rear tires are matched and purchased 18 months ago. On the rear tires, the L.C.C. x2 is 3,750lbs at max 110psi cold. Our door tag recommends 85psi cold per tire on the rear axle. 25psi under the max psi seems to me to be a bit low. Would 90psi (vs. 85psi) be reasonable number for better mileage; I do not want to over-inflate?

Likewise, the GVWR for the front axle is rated 6,000lbs, or 3,000lbs/tire for 2 tires. Our door tag recommends 75psi cold per tire on the front axle. We have different and matched 4 year old front tires in good condition with very low mileage. On the front tires the L.C.C. x1 on them is 3,640lbs at max 95psi cold. I'm thinking 80psi (vs. 75psi) might give better mileage. While the RV does ride well overall at the recommended psi's, again the psi's seem a little low when looking at the tires. Even with a 10% increase in psi due to heat, both sets would still be under the max psi for either set of tires. Any thoughts?
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Old 03-05-2016, 04:27 AM   #12
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@peggwn

First have to have it clear about the tires before I can calculate for you .
Is the maximum load of rear tires 3750 lbs AT 110 psi a tire for single or Dual load?

Front is clear 3650lbs AT 95 psi ( F-load/12PR) .
But maximum speed or Speedcode of both Rear and front would be nice to know.

And configuration of motorhome is front 1 axle 2 tires on the road GAWR -6000lbs, rear 1 axle 4 tires on the road so dual load or twinwheel-axle GAWR = 2x 3750=7500 lbs ?????.
GVWR 12000 lbs.

Probably you can do with higher pressure with still acceptable comfort and gripp. This is because Factory advice is bassed on GAWR's equally devided over the sides, wich it never is. A 48/52% division per axle is pretty common for Motorhomes, and 45/55 has happened. So advice should be with that much reserve so heavyest side is still below the loadcapacity the pressure is calculated for .
Even this loaddivision is often crossed between the axles, but not important for the pressure advice.

The pressure rising by temperature rising in the tire by driving, is included in advice, what is given is cold pressure= when inside tire temp = outside tire temp, that simple, and to be complete at AmbiŽnt temp( = outside tiretemp) of 18/20degr C/ 65/68 degr F.

This is when not driven and no sunshine on the black heat-absorbing tire, and other factors I forgot.


Topic starter has not given more info , but you almost gave it all, so I will do what I promissed him , for you.
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Old 03-06-2016, 08:47 PM   #13
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Thanks, JADATIS, for taking so much time to answer a fairly newbie's question. I seemed to have misused my V's and A's. Maybe this explanation is more correct and less confusing, and give you the info you need.

The GVWR for the entire coach is 18,000lbs. We have a total of 6 tires: 2 on the front and a pair of dualies on the rear.

The GAWR for the front axle is rated 6,000lbs, or 3,000lbs PER tire for 2 tires on the road. The front tires are rated at 3,640lbs at max 95psi cold on a single axle. Our last weigh-in showed our front axle weight is 5,480lbs.

The GAWR for the rear axle is rated 12,000lbs, or 3,000lbs PER tire for 4 tires on the road. Would that not be 6,000lbs per side (a pair of tires)?

If that is correct, then in pairs the 2 tires rated at 3,750lbs each (@ 110psi max cold in pairs) = 7,500lbs; this should be more than enough to support each side of the drive axle.

Our last weigh-in showed our rear axle weight is 10,760lbs. Assuming an equal weight on each side that would be 5,380lbs per side, or 2,690lbs per tire.

We have not been able to do a 4-corner weight, but we have yet to exceed our GVWR or the respective GAVRs, and we try our best to balance side-to-side; not precise, I'll grant you. We learned this much from this wonderful iRV2 website.

As to the tire temp, I park the old gal (no, not the DW, although...never mind) in an enclosed garage and the last temp inside was 49. When traveling we almost always run between 60 and 65 mph on major roads, but sometimes lower if on a non-major road. I can check later, but I'm confident that the tires are rated for that speed range or higher.
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Old 03-07-2016, 04:17 AM   #14
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Assumed your tires to be N speedrated, so maxspeed of tires 140km/86m/h.
Then first filled in the GAWR's and Loadindexes as given ( for that speed ).
And made picture of that wich I include.
Filled in a what if weight on front is 520 lbs less, to get yours weighed axle load once.
Then a poor 112 lbs weightdifference possible before bumping. But this is going from 86m/h pressure . Advice given F82psi R 98 psi.
Also lowered the loadindex to get the 160km/99m/h deflection , at wich I based my bumping factor on, and gave advice pressure of F88psi and R 98 psi.
You must see this as highest pressure with still acceptable comfort and gripp, to my conclusions, and maximum reserve.
This bumping border I determined is discussable , probably you can go further.


More important is the damage border, and for that I highened up the loadindex by 3 to get the maximum load that would be given for 110 km/68m?/h, yours given maximum speed.
Advice F 75 psi R 90 psi.
This must be seen as absolute minimum for savety of tires at your speed.
Still a combined overload and unequal load R/L on the axles possible of F 315 lbs and R 1405 lbs before damage begins to the "heavyest"tire on the axle ,when realy driving that 68m/h .
When you measure a pressure cold ( at 65/68 degr F ) of this lowest 75 and 90 , fill up 88/98 when possible, but you most likely did not damage the tires by to much heat by driving the speeds you do. If you are not able to fill up you can also decide to use a lower speed , fi 55m/h, so your tires wont get to hot. All for maximum savety.

then a remark: Mostly the GAWR's together are a bit more then the GVWR.
For instance this would mean that your GVWR is lower then you gave fi 17000 lbs, and you yust added the GAWR's because you did not find GVWR.

EDIT: reading back I saw you also gave rear weighed of 10760lbs and filled in in the spreadsheet -1240lbs to get that and it gave direct bumping as message, but again this is not such an important border for savety, better a bit discomfort then damage to tires.

filling in these loads in part2 gives maximum to fill advice of F83 / R86 psi.
And minimum for 100km/62m/h gave F69 R76 ( also saw speed of 60/65 and used 68m/h for GAWR's used advice. So wiht your used 70/80 you probably did not damage the tires yet.
But then how was it weighed , all in it incl persons, and what is the weight now?
Again this with reserve added.
And try to find the speedcode or maximum speed of tire, to comfirm, if fi Q speedrated the pressure can be lower, and my given advice already gives bumping.
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