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Tire PSI
Just got fuel, filled water tanks, and went to the Templeton feed and Grain and we weighed the whole coach at 41,500 LBS Rear 27,800 LBS Front 13,700 LBS. I got a chart from Toyo and talked to a tec at Toyo but they weren’t sure if you took rear and devided by 6 or 4 ? Because of 6 tires? So if that’s the case it’s 4,600 per tire on duel on chart , I’m lost This label is in coach Should I go by the Label??
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13 Replies
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Close, but no cigar.
You need each axle weight. Then the inflation charts will list single or dual for inflation pressures and you use the single for tag and dual for the drive.
Obviously the weight on each tire is presumed to be equally spread side to side, but if you can weigh each individual tire position you can get very precise on pressures, but our coaches are usually pretty close to equal side to side.
FWIW, I use the inflation tables and then add 5 PSI just to compensate for gauge error, side to side differences, and how the planets line up.
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We live in a small town I don’t think anyone around here does that, weigh each tire? Do you mean a small scale to go under each tire?
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Just keep a 100 psi and don’t think you could go wrong. And you won’t need a calculator.
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Art, you do not need to weight each tire although we had that done once at a rally.
To get each axle weight it is just a matter of driving the steers on the scale, then drive so just the steers and drive are on, then the whole coach, then drive everything off until just the tag is on. You get the idea.
I have to strongly disagree with Glen because tire inflation pressures are all over the place because of different tire sizes, different coach and axle weights, and as a result different required pressure. Some of the coaches require as much as 120 PSI on steers and as low as 70 PSI on tags and everything in between.
100 PSI on a steer axle that needs 120 PSI is a recipe for disaster. 100 PSI on an axle that only needs 70 is going have the coach banging over potholes and expansion joints.
The correct pressures based on the charts provide the best tire footprint for braking and handling, provide the best ride, and will provide the margin for safety to prevent tire failure such as an under inflated tire is likely to do.
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Ditto on what Jon said, you need to run your tires based off the weight that each axle weights. While at the Prevost show in West Palm Beach the Michelin person said not to over inflate but even more important, you should never under inflate the tires. If you run them 5 pounds over as Jon said its not a big deal, but 15-30 pounds over will ride like a tank and wear the tire out faster, per the Michelin man.
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I will do that, So if the tags say coach weighs 47,000 lbs and yesterday it weighted 41,500 what’s up, is the 47,000 a max weight?? It also says 47,000 on Prevost tag on inner motor cover door.
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I don’t understand what you are saying Art. If you use your gross weight and the tag / drive axle weight it suggests your tag is likely to be in the 7000 or 8000 pound range. That means both of your rear axles are below the weight range on the inflation tables.
How you determine pressures is up to you because you can use the lowest pressure on the inflation table, or you can extropolate to arrive at what would be an acceptable pressure. There is also a formula I ran into once for calculating inflation pressures that if used would likely be an acceptable way to arrive at the proper inflation pressures.
I am not saying how I do it is correct, but on my coach my tag is below the listed weights so I decided to use the lowest pressure on the inflation table. I know the tag tires are over inflated to an extent because I extropolated and used the formula and instead of the 80 PSI I now use I probably could be safe at 70 PSI.
One important consideration all owners have to be aware of is there is no acceptable way of determining tire pressures without weighing each axle when the fuel is full, water tank is full, holding tank is empty and the coach is loaded with personal effects such as would be on the coach for a trip. Every conversion is different with some putting a lot of weight near the front, others being lightly loaded on the front, but heavy towards the rear and everything in between.
This is one formula for pressure calculations, but I am only showing it as an alternative for you to consider.
1984 96" with 8V92TA
(remodeled Liberty Coach)
Greenwood, MO -
Art: I believe 47,000# is the maximum weight (actually at the front and back as indicated) of your coach fully loaded. 41,500# seems light to me, but it looks like you have a 40′ coach and we have a 45′ coach. Maybe some 40′ Royale owners will check in and tell us if 41,500# is in the ball park.
When you weighted the coach, were you able to weigh the front and the back at the same time? Or did you have to move the coach to weight the front first and the back second. If you moved the coach during the weighing process that could have impacted the weight, because if the entire surface of where you’re driving the coach isn’t perfectly level as you move the coach to weigh the second axle the center of gravity of the coach could move which will shift weight impacting the weights.
The next time you see a truck stop with a CAT scale (like a Love’s where you can use the PC fuel discount card) swing in, top off fuel, and weigh. Those scales are designed to weigh each axle separately WITHOUT moving the coach.
Then when you know the weights, go to the tire pressure chart for your tire manufacturer (probably/should be online) and you can see the recommended pressures.
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When I weighed coach I pulled the front on and weighed it at 13,700 then whole coach at 41,500 then front off scale so rear was 27,800 add front and rear it does come up to 41,500
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Total coach weights are a little predictable. Art’s weight is well within the range of what might be inspected for a 40 footer, with 45,000 to 46,000 not unreasonable for a 45 foot XL coach, and just under 55,000 for a 45 footer with 2 slides. Less if it is a non-slide XLII coach.
What makes for the wide range of tire pressures is how the converter arranged the bays. If the generator and batteries are up front on a 40 footer the front axle has a lot of weight and obviously pressures can be expected to be high. If the weight was concentrated toward the rear, the steer axle gets lighter and it is the drive that gets the weight.
When the 45 foot coaches are considered (motorhome chassis) with the greater rear overhang, weight shifts rearward and if the front bays do not have heavy weight from items like batteries or the generator, despite the heavier chassis weight, the steer axle is potentially quite light. Putting weight in the front bays will add to the steer axle weight, but with the engine hanging quite a ways back from the rear axles weight in the front also is carried in part by the rear axles. It would not be unreasonable to see some drive axles at their weight limits.
All of the locations of weight on a coach decide tire pressures and what might be weights, and therefore tire pressures for a Royale, might not be close to what could be the correct ones for a Marathon and neither would likely be the same as those of a Liberty for example.
As slides were added and the extra weight of slides impacted the various axle weights and gross weight Prevost shifted to 365 tires and beefed up suspension limits. But before the switch to 365 tires some conversions had 315 tires at the limit for pressures as a result of the weight on the steer axle. I can recall a number of owners running as much as 130 PSI in the steer axle and switching over to 365 tires to get the pressures down and the ride softer.
Heat is the enemy in all things relating to our coaches, whether it is engine temps, differential temps, transmission temps and heat from a dragging brake. But tires that are under inflated are heat generators and in the case of a tire running at a pressure 20 PSI lower than required, the tire industry treats those as tires that should be removed from service until the tire can be inspected and okayed to return to service. The heat from an under inflated tire shows up inside the tire as a dark band going around the tire on the inner liner. That dark ring is the sign of heat damage. We tend to be very concerned with tires and their pressures because a tire failure can greatly affect our safety. That is one reason why products like Pressure Pro, and good pre trip tire inspections are very important.
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According to inflation charts my front is 13,700 divided by 2 equals 6,850 LBS so the chart says the front should have 95psi in front tires so for now I set front tires from 110 PSI to 105 PSI and until I can weigh coach so I can tell the difference from drives axle weight and tag axle weight I dropped the 4 drives from 110 PSI to 100 PSI and tags from 110 PSI to 95 PSI , I’m sure I’m still a little high but better then low.
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Higher than required pressures are the best way to go if there is any doubt. Every coach when purchased should be weighed for purposes of determining tire pressures, and the coach should be loaded as it would when in use.
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GVWR = Gross Vehicle Weight *Rating*. It’s the heaviest weight at which the vehicle should be driven, according to the manufacturer.
GAWR = Gross Axle Weight Rating, similar to GVWR but for individual axles.
The European Tire and Rim Technical Organization lists an equation to calculate allowable load on a tire at pressures lower than the reference pressure:
Q_r = Q_max*(P_r/P_ref)^0.8
Q_r = Measured tire load
Q_max = Maximum load carrying capacity (printed on tire)
P_r = Inflation Pressure
P_ref = Reference pressure, compatible to Q_max
i.e. Tire sidewalls that say 8270 lb at 120 psi, 8270 lb is Q_max and 120 psi is P_ref
Rearranging the equation to solve for inflation pressure:
P_r = P_ref*(Q_r/Q_max)^1.25
So if you measure 6,000 lb on a single tire, using the above load rating and reference pressure:
P_r = 120 * (6000/8270)^1.25 = 80 psi
The difference between this and the mopar link above is raising the ratio of Q_r/Q_max to the power of 1.25 (instead of leaving it at 1, resulting in a pressure of 87 psi). This makes the mopar link a little more conservative (and easier to calculate)… at least until he gets to dual weights. The dual weight ratings are always less than simply doubling the single weight rating. Simply use the dual weight rating printed on the tire instead of doubling the single rating.
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