Tire Inflation requests

[Tireman9]

Some general comments and requests to those posing questions about tires and specifically about how to set the inflation.

As reported in more than one thread on the topic of inflation, the answers and information can be confusing to many.
The specific numbers for one RV are seldom correct for another RV even of the same make, model and year, as the amount of "stuff" one person carries is never identical to the stuff someone else carries.

It will also help if when you post your initial question you provide the following facts. This will save everyone time and confusion.

FACTS that are needed include:
1. Full tire size including load range and tire company
2. GAWR for Front, Rear and Tag if so equipped. Also GVWR. This is on your "Certification label"
3. RV Co recommended tire inflation for each axle. Also on the Cert. Label
4. Actual "4 corner" weights for each axle when the RV is at its heaviest. If you don't have "4 corner weights" then individual scale weights for each axle. (If you have a TAG it would be "6 corner scale weights")

When we do not know the above information, we end up having to ask for the missing information which can add to the confusion with questions and quote answers sometimes generating more questions which makes it harder for those of us offering answers to keep the information of who said what and what question is being answered clear.

NOTE. If you discover that any of your scale weights exceed the max load rating of an axle or the vehicle you must do something to bring the coach in compliance at once. Doing calculations and setting pressures will simply need to be redone after you lower the RV weight.

Your assistance will be greatly appreciated.

[Dippytwo]

I’m sure this has been covered before, but I want to make sure I’m running the correct pressure in my tires. I have the Goodyear 245/70R 19.5 with max pressure 110 psi. My Thor sticker says to run at 82 psi (both axles), the RV shop that I got my CHF done at says to run at 110 psi since that’s the max load on the tires.
The GAWR: Front axle 7000 lb, Rear axle 12000 lb. GVWR: 18000 lb.
I took my MH to a CAT scale, the front axle is 6020lb, rear axle 9770lb, (total 15790lb) I have not been able to get a 4 corner weight. I looked at that Goodyear chart and not sure if I’m reading it right, the best I can figure out is they are telling me to run at 110 psi on all tires. So, 82 psi or 110 psi?

[4x4van]

The weights in the Goodyear chart are per tire. If you air them up to 110psi, the tires will support 9,080 lbs front and 17,640 lbs rear; WAAAY more than your coach should EVER weigh. You are good at 80psi, front and rear (your tires at 80psi will support 7,280 lbs front and 13,660 lbs rear). You can use the 82 on your placard if you are more comfortable with that.

[Gary RVRoamer]

the RV shop that I got my CHF done at says to run at 110 psi since that’s the max load on the tires.

Sad that ostensibly professional RV techs are giving out such bad advice. In this case, it is far enough off-base that it should be considered unsafe advice. 30 psi overinflation is enough to potentially cause steering & handling issues as well as premature tire wear and an extremely rough ride.

[Dippytwo]

OK, so I called Goodyear Tire and gave them my weights and she looked at the chart. She said both front and rear will be at 80 psi. Basically she said you divide the weight by 2 for the front axle and divide by 4 for the rear axle to get the numbers. I told her that Thor recommends 82 psi and she said that’s fine.

[Crasher]

I am amazed at how little some people know about proper tire inflation. There should be a Sticky that explains it to those who do not understand it. How many "Tire Pressure" posts have come up in the last year and many of them ask the same question. Then the same people come and explain "their version" of the "facts". Some are correct while others continue to be misinformed and pass that onto someone else. Tireman9 is the most qualified to offer advice and I would like to see him write the Sticky so that people can get the correct information when they ask the next time. I grew up with proper tire care and inflation requirements, but many are new to this and should be correctly informed instead of listening to "non qualified experts".

Just my opinion.

[Sky_Boss]

I've read many of your posts and find them greatly informative. I have a question about one of your thoughts on adding PSI above the minimum.

If I recall correctly you suggest adding 10% to the minimum to the PSI you should use in real life.

As an example, assuming I recall that correctly if the tire chart calls for 100 PSI then use 110 PSI. I also think you said never to exceed the wheel max.

Let me give you some specifics from my coach and get your thoughts.

I'm running Michelin 305/70R 22.5. My wheels are Accuride with a maximum 120 PSI cold. My tires are also share the same max 120 PSI cold and have a minimum 75 PSI for weights below 5375# for a single and 9530# for dual use. I have 6 corner weights so I will provide the heaviest tire weight.

Tag - 3407# which is far below the 5375# calling for the minimum of 75 PSI.

Drive (Duals) - 8053# which is significantly below the 9530# calling for the minimum of 75 PSI.

Steerer - 7520# - The chart calls for minimum of 115 PSI.

In the case of the steerer I can see that I'm kind of "stuck" in a very narrow band and have to watch the pressure to avoid both over and under inflation. I've been running 115 PSI in them and I'm loath to run 120 PSI but I suppose that is best. I'll touch on that in a moment.

(BTW...the weights I provided were done with full water and fuel but since then I have routinely traveled with less than 50% water so some of the weights I gave are a tad heavier compared to what is normal now.)

So the question is whether or not I should increase the PSI on the tag and drive wheels by 10%. What would be the advantage to adding 10% for those tires?

OK...this might be a side bar and maybe your answers will address my current situation.

My tires are about 5 years old with 35,500 miles on them.

In a few days I am having the tag and steerer wheels replaced. During that process I am having all my tires removed and the drive wheels inspected to make sure there isn't a problem we can't see from external inspection. I am having the DUALLYVALVE stem grommets replaced on all 8 wheels in the process.

My tag and steerer tires need to be replaced because the ribs on the side of the tire are almost entirely worn down. The central tread isn't too bad.

My drive tires show even wear across the center treads for all 4 tires. The outer side ribs of all 4 tires also appear to be proportionately worn down with the center tread. Based on tread wear I expect I still have 1 maybe 2 years left on them.

I've had a full wheel alignment (drive/thrust angle & steerers) at the factory. I've had a couple alignment experts also recently look at tread wear and after inspecting tire tread wear both said that based on tread wear patterns my alignment is not an issue.

The opinions I received were that the tag and steerer wheels are subject to more sideway wear issues when compared to the drive wheels. Clearly we all know about tag wheel tires scrubbing so it makes just as much sense that the steerer tires are also similarly affected during sharp turns and maneuvers. Meanwhile, the drive wheels basically run along and are not as seriously affected by sideway forces.

Do you think adding 10% to the tag wheels would help them in the wear of the side tread area?

[Tireman9]

Originally Posted by Sky_Boss

I've read many of your posts and find them greatly informative. I have a question about one of your thoughts on adding PSI above the minimum.

If I recall correctly you suggest adding 10% to the minimum to the PSI you should use in real life.

As an example, assuming I recall that correctly if the tire chart calls for 100 PSI then use 110 PSI. I also think you said never to exceed the wheel max.

Let me give you some specifics from my coach and get your thoughts.

I'm running Michelin 305/70R 22.5. My wheels are Accuride with a maximum 120 PSI cold. My tires are also share the same max 120 PSI cold and have a minimum 75 PSI for weights below 5375# for a single and 9530# for dual use. I have 6 corner weights so I will provide the heaviest tire weight.

Tag - 3407# which is far below the 5375# calling for the minimum of 75 PSI.

Drive (Duals) - 8053# which is significantly below the 9530# calling for the minimum of 75 PSI.

Steerer - 7520# - The chart calls for minimum of 115 PSI.

In the case of the steerer I can see that I'm kind of "stuck" in a very narrow band and have to watch the pressure to avoid both over and under inflation. I've been running 115 PSI in them and I'm loath to run 120 PSI but I suppose that is best. I'll touch on that in a moment.

(BTW...the weights I provided were done with full water and fuel but since then I have routinely traveled with less than 50% water so some of the weights I gave are a tad heavier compared to what is normal now.)

So the question is whether or not I should increase the PSI on the tag and drive wheels by 10%. What would be the advantage to adding 10% for those tires?

OK...this might be a side bar and maybe your answers will address my current situation.

My tires are about 5 years old with 35,500 miles on them.

In a few days I am having the tag and steerer wheels replaced. During that process I am having all my tires removed and the drive wheels inspected to make sure there isn't a problem we can't see from external inspection. I am having the DUALLYVALVE stem grommets replaced on all 8 wheels in the process.

My tag and steerer tires need to be replaced because the ribs on the side of the tire are almost entirely worn down. The central tread isn't too bad.

My drive tires show even wear across the center treads for all 4 tires. The outer side ribs of all 4 tires also appear to be proportionately worn down with the center tread. Based on tread wear I expect I still have 1 maybe 2 years left on them.

I've had a full wheel alignment (drive/thrust angle & steerers) at the factory. I've had a couple alignment experts also recently look at tread wear and after inspecting tire tread wear both said that based on tread wear patterns my alignment is not an issue.

The opinions I received were that the tag and steerer wheels are subject to more sideway wear issues when compared to the drive wheels. Clearly we all know about tag wheel tires scrubbing so it makes just as much sense that the steerer tires are also similarly affected during sharp turns and maneuvers. Meanwhile, the drive wheels basically run along and are not as seriously affected by sideway forces.

Do you think adding 10% to the tag wheels would help them in the wear of the side tread area?

Don, Before we get to tire inflation we need to be sure the TAG loading is set correctly. Most RVs with TAG have some method of adjusting the total tag load. This shifts the load on or off the front axle and off or on the drive axle.

Hopefully either your RV dealer or RV mfg or even others on this forum with similar coach can provide instructions on how to shift load and the approximate percentage each axle should be carrying. By decreasing the axle load on the tag axle you would shift load off the fronts and onto the drive axle

Re steering or side load on tag vs front.
Front tires rotate around a center line that points to the center of a turn. This is accomplished with suspension design called Ackermann. Yes they have side force but since they are turned toward the center of the radius the side slip & force has some limits.

Tag axles do not steer except for some special city buses or if the high cost feature is added and available from the mfg when the chassis is constructed. Tag axle tires are forced to slip and slide around corners and essentially do most of their pivoting around the center of the drive axle. This sliding is the cause of strange wear on tag axle tires.

RE tire inflation. Your coach appears very lightly loaded based on the capacity of the drive tires tires. You are correct to never go below the lowest inflation in the tables for your tire even when the tire is very lightly loaded. The "max" inflation for tires and wheels is always the cold pressure and not the running pressure.

After you adjust your TAG (I am guessing here) let us know the new tire loads and I would be happy to review your calculations for learning the CIP.

[Sky_Boss]

Originally Posted by Tireman9

Don, Before we get to tire inflation we need to be sure the TAG loading is set correctly. Most RVs with TAG have some method of adjusting the total tag load. This shifts the load on or off the front axle and off or on the drive axle.

...


RE tire inflation. Your coach appears very lightly loaded based on the capacity of the drive tires tires. You are correct to never go below the lowest inflation in the tables for your tire even when the tire is very lightly loaded. The "max" inflation for tires and wheels is always the cold pressure and not the running pressure.

After you adjust your TAG (I am guessing here) let us know the new tire loads and I would be happy to review your calculations for learning the CIP.

The "ride height" for my coach was recently set to Spartan specification. That is what establishes the tag loading and thus the front axle loading also. I do run the drive wheels at 85 PSI. My theory is that by doing so the those tires remain firmer/rounder and thus remove weight from both the tag and steerer wheels. For what it is worth, I did multiple full axle weighings on a CAT scale and they appear to confirm my theory after weighing the coach with the minimum 75 PSI vs the 85 PSI in the drive wheels. With 85 PSI my steerer axle runs about 150-200# lighter and that helps.

Newmar derated the front axle from the Spartan specs of 14,600# AWR to 14,200# AWR. Why? Well, I firmly believe the did this because they run the 305/70R tires at all positions. The problem is that they use the same chassis for 4 and 3 slide configurations with various left to right balance numbers. The reduction in AWR would cover a lot of left-right imbalance variances and thus reduce the chance either front tire is overloaded. However, since I have a very good understanding of my corner weights and that my coach is well balanced L-R I know that I can run the front axle at almost 14,600 and not need more than 115 PSI per the tire charts. FWIW, I'm running the front axle at 14,500# if I am carrying full fuel, propane and water along with all my "goodies" in the basement, refrigerator and such. I seldom carry full water and, of course, I loose 8# in fuel weight every 6-7 miles I drive. LOL

I know some may disagree but that is how I run it...moving on. LOL

So, I think my tag is properly adjusted at this time. Also, the ride height was set with the tag at 75 PSI, Drive 85 PSI and steerer 115 PSI. It is important to make sure tires are at proper inflation when setting ride height.

Now that I have talked this out loud with you, there doesn't appear much I should do differently. But...I'm listening in case you see something I'm missing. Two heads are better than one.

I've attached a screenshot of part of a spreadsheet I use to monitor my weights.

THANKS!!!

[Crasher]

Originally Posted by Sky_Boss

The "ride height" for my coach was recently set to Spartan specification. That is what establishes the tag loading and thus the front axle loading also. I do run the drive wheels at 85 PSI. My theory is that by doing so the those tires remain firmer/rounder and thus remove weight from both the tag and steerer wheels. For what it is worth, I did multiple full axle weighings on a CAT scale and they appear to confirm my theory after weighing the coach with the minimum 75 PSI vs the 85 PSI in the drive wheels. With 85 PSI my steerer axle runs about 150-200# lighter and that helps.

Newmar derated the front axle from the Spartan specs of 14,600# AWR to 14,200# AWR. Why? Well, I firmly believe the did this because they run the 305/70R tires at all positions. The problem is that they use the same chassis for 4 and 3 slide configurations with various left to right balance numbers. The reduction in AWR would cover a lot of left-right imbalance variances and thus reduce the chance either front tire is overloaded. However, since I have a very good understanding of my corner weights and that my coach is well balanced L-R I know that I can run the front axle at almost 14,600 and not need more than 115 PSI per the tire charts. FWIW, I'm running the front axle at 14,500# if I am carrying full fuel, propane and water along with all my "goodies" in the basement, refrigerator and such. I seldom carry full water and, of course, I loose 8# in fuel weight every 6-7 miles I drive. LOL

I know some may disagree but that is how I run it...moving on. LOL

So, I think my tag is properly adjusted at this time. Also, the ride height was set with the tag at 75 PSI, Drive 85 PSI and steerer 115 PSI. It is important to make sure tires are at proper inflation when setting ride height.

Now that I have talked this out loud with you, there doesn't appear much I should do differently. But...I'm listening in case you see something I'm missing. Two heads are better than one.

I've attached a screenshot of part of a spreadsheet I use to monitor my weights.

THANKS!!!

I'll add what I learned from my coach. Your drive axle is not carrying what it could carry. This is caused by too much pressure in the tag suspension air bags. If you were to adjust the tag suspension so that it was carrying 5000# , the steer axle would decrease from 14,500 to 14,250 and the drive would increase from 15,980 to 17,980. The result is having the steer axle within it's limits and the drive axle closer to it's rating. The tag would be just fine. I seriously question being able to lighten the front axle by going from 75 psi to 85 psi in the drive axle tires. The reason being that the proportioning valves will cause the tag axle to carry the same weight regardless of what pressure the drive axle is at. I suspect the difference you saw was a variation in the scale and position of the tires when on the platform. I noticed too, that your front left is 540# heavier than the front right. I've read that 500# is the maximum difference between front tires, so you're at that limit. If you adjusted the LR ride height valve up 1/8", it could transfer 250# from the FL tire to the FR tire making them very close to equal. The result would be a 400 to 500# difference between the rear duals, but it would be spread over two tires instead of one. The acceptable difference for the drive axle is 1000#.


I learned the relationship between ride height, tag suspension psi, proportioning valves and tire pressures through many weighs and conversation with the chassis builder. They helped me make the improvements and balance I wanted and then applied what I had learned to their new tag coaches beginning in 2014.

[Tireman9]

Originally Posted by Dippytwo

I’m sure this has been covered before, but I want to make sure I’m running the correct pressure in my tires. I have the Goodyear 245/70R 19.5 with max pressure 110 psi. My Thor sticker says to run at 82 psi (both axles), the RV shop that I got my CHF done at says to run at 110 psi since that’s the max load on the tires.
The GAWR: Front axle 7000 lb, Rear axle 12000 lb. GVWR: 18000 lb.
I took my MH to a CAT scale, the front axle is 6020lb, rear axle 9770lb, (total 15790lb) I have not been able to get a 4 corner weight. I looked at that Goodyear chart and not sure if I’m reading it right, the best I can figure out is they are telling me to run at 110 psi on all tires. So, 82 psi or 110 psi?

Lacking the actual load on each end of each axle I suggest you assume a reasonable un-balance of 53/47% as almost no RVs have side to side load split to perfect 50/50 which is what RV companies use 9because it's simple?)
So using your numbers:
53% gives 3,191 Front and 2,591 Rear per tire
Gy single at 80 psi is rated for 3,640#
Dual @ 80 is rated 3,415
The GY chart shows 80 psi as the minimum so you should never go below that inflation.
Since you already have a load margin because you do not load your RV a lot I would suggest you do not need my normal +10% but would suggest a nice round 85 psi.
I would set your TPMS warning level at 80 psi.

NOTE my 53/47 is just an approximation as some RVs have considerable more unbalance. I would still suggest looking for 4 corner weights.
THIS web site has a list of places to check.

[Tireman9]

Originally Posted by Crasher

I am amazed at how little some people know about proper tire inflation. There should be a Sticky that explains it to those who do not understand it. How many "Tire Pressure" posts have come up in the last year and many of them ask the same question. Then the same people come and explain "their version" of the "facts". Some are correct while others continue to be misinformed and pass that onto someone else. Tireman9 is the most qualified to offer advice and I would like to see him write the Sticky so that people can get the correct information when they ask the next time. I grew up with proper tire care and inflation requirements, but many are new to this and should be correctly informed instead of listening to "non qualified experts".

Just my opinion.

Thanks for the compliments. I started this thread in the hopes that people could at least provide the basic needed information when they post a question. On my tire plog I have a number of posts where i go step by step.

[Tireman9]

Sky-Boss & Crasher

My background is as a tire design and quality engineer. I have worked on Truck Tires, Passenger & Light truck, Mini "Donut" spare and Indy race tires and at least one Agriculture tire project. BUT I never was involved in multi axle bus chassis design so that area is out of my expertise. As such my information is aimed at what I believe will be the best for the tires but may not be what the chassis engineers would like to see.

I would like to see tires loaded to no more than 85% of their rated load. This gives a margin to account for road crown, side wind loads and side loads due to cornering.
I suggest a +10% on inflation so people are not chasing the day to day pressure variations due to changes in ambient temperature and elevation. If you have to mess with your tires too often it will become "work" so people with not want to do it and proper time maintenance can suffer.
And of course I would like everyone to run a TPMS with the warning level set no lower than the inflation needed to support the actual load on the tires.

If you have set your suspension the way you want then I would suggest you use your actual weights ("4 corner" or 6 corner) and published load tables and my guidelines above. You both seem capable of doing the basic math your selves.
If you still have questions and have reviewed the posts on my tire blog and still have questions you can always PM me.

Roger

[Crasher]

Sky-Boss. This statement is incorrect: (The "ride height" for my coach was recently set to Spartan specification. That is what establishes the tag loading and thus the front axle loading also.) The ride height valves adjust the height of each corner, but do not set the Tag loading. The tag loading is determined by the ratio of the proportioning valves or manual regulator valves if so equipped. A proportioning valve sets the tag load regardless of where the ride height is set. A 65/35 ratio valve will distribute 35% of the rear weight to the tag and 65% to the drive axle. The tag loading "Will" determine the steering axle load. On a 42-45' coach, the amount added to the steer or subtracted from the steer is approximately 7 to 1. For every 7# you reduce the tag weight, you will reduce the steer 1#. That's what the ratio is on my 43' tag coach, but others can vary slightly from that depending on wheelbase. There are many tag coaches on the road that force the steer axle to carry too much while underloading the drive axle because the tag is carrying more than needed. Most of them are within the axle and tire ratings, but the weight is poorly distributed.