Had a TPMS app installed on my phone the other day. Tire Minder TPMS APP-6. Now I have questions about correct tire pressures.
Tires were new 6 months ago, have 3,000 miles on them right now. They are Uniroyal LS24 275/80/R 22.5 LR G.
The tires themselves say Max Load at 110 psi.
The wall plate inside the coach (see pic) says "Cold single 97, dual 83 at maximum GAWR".
The chart referred to on the plate is in the other photo.
The coach weighs in at front axle 10,840, rear axle 17,020. Will be slightly more with fuller fuel and water tanks. I am flat-towing a toad but there is no tongue weight associated with that.
What would be the proper inflation of my front and rear tires based on this info?
https://www.rvsafety.com/images/pdf/michelinloadandinflationrvtruck.pdf
"The coach weighs in at front axle 10,840, rear axle 17,020."
Front 95 to 100
Rear 75 to 80
There must be a thousand posts on tire pressure and almost as many opinions. Even going online, everyone has a different opinion. I max all of them but others don't.
Pierce
I weigh the coach and go by the tire manufactures recommendations.
So always use actual manufacturer chart for their tires, not a different manufacturer
https://www.eastbaytire.com/wp-content/uploads/Uniroyal_Truck_Data_Book-2.pdf
Page sixteen
95 psi front
80 psi rear
Yes, that's the chart I need. I'll maybe go a little higher as we'll be heavier when we take off for a few months of cross-country adventuring this summer.
That would be very wise.
RVSEF suggests a 5 to 10 psi "safety buffer" above inflation for "as weighed for travel" to account for any variations.
You might watch to see if they (or Escapees) have a weighing opportunity while you're on the road if your initial numbers don't include people, pets, loaded pantry/bays/clothes, etc.
You can call Uniroyal's customer service with your weights and they will tell you the correct pressures, They were very helpful to me
Agree100%. Ratings are designed with many aspects. Summer conditions are certainly different than winter adverse conditions. Higher pressure runs cooler but reduces contact area. Your TPMS also will show temperature which is quite useful to me. I rely on mine in all my vehicles. I attempt to recheck TPMS entering every town just in case there is a new issue.
Scott
This is part of the reason I max out the tire pressure on our U300. Tires never gain air but pressure loss over a period of time is normal.
Our Foretravels are not that far from maximum weight plus the weight is always changing depending on the amount of water in fresh, gray, waste tanks. The amount of diesel and propane are also not constant. The load from canned goods, personal effects also may change depending on the type of trip the operator is taking. Since we are not full timing, our load tends to be a little less. That is one of the reasons, I have never weighed the coach. A full tank of diesel will not only change the weight on the right front tire but will also change the weight on the left rear set of tires.
Tires will run cooler at higher pressures. Important in summer as high tire temperatures are closely associated with tire failure. The same with high speed. Any spot on the tire will flex more often the faster the vehicle goes and will generate more heat. As pressures increase toward the maximum, the resistance to impact damage from chuck holes, etc also increases.
While tire contact area will be very slightly less as the pressure increases, the deformity of the tread and tire body decreases in turns giving bertter handling. Breaking distance will not change because the Meritor disc brakes can not lock up all six tires unless the coach is light, the discs are warm and probably not in any case. Our fire trucks had 10 inch drums and could easily lock up all tires. Not so with the Meritors. Since we cannot lock up the tires, they are always going to stop in about the same distance no matter what the pressure is.
Bus companies like the firm that I bought our 4107 from max their tires whether they have full passengers or are empty. Our FD did the same on all apparatus at the recommendation of the tire manufacturer.
Below is the tire inflation sticker from our German version MBZ 450SE. Note the approx. 12% increase from normal to full load and the 6 psi for 100 mph and over is about 18 percent above the maximum load pressure. Why? Because the increased pressure at high speeds means cooler running, better resistance to road irregularities/damage and better handling. Note the most important words on the sticker "COLD TIRES." and the words at the the bottom, add 4 psi for warm tires. The 4 psi must be converted to a percentage for our larger tires.
This is all from the company that invented crash testing cars way back in 1959 and what all manufacturer have to go through today.
Note that the axle will not distribute the weight evenly between the left and right wheels. The load charts assume even loading. That is one reason for adding some buffer. Foretravels do a pretty good job of evenly distributing the load but with floorplan and length variations along with your own load of stuff it's going to vary.
A four corner weight measurement is best so you know what the side to side variation there is.
I would state that just a little differently:
The correct/best method is to get all wheel position weights with ride height properly adjusted.
Take the heavier wheel position on each axle to go to the tire manufacturer's inflation table to determine the correct MINIMUM PSI for all tires on that axle. Add 5-10 PSI as a safety cushion over the minimum recommended PSI.
Absolutely, if all you have is axle weights, you will need to add an additional fudge factor to account for left/right imbalance.
If you really want to tech it up. Run balance hoses on the duals starting at chart pressure. Now find a flat surface and measure from the hub to the ground on each pair. Add pressure to the shorter side until the hub to ground value is equal.
Repeat on the fronts. Recheck the rears.
Have fun.
Mike,
Am I correctly reading that you would recommend potentially running different PSI in different tires on the same axle?
I have never seen that advice from a tire manufacturer.
If you get an RVSEF weighing, they use the heavier axle end as the guiding number for inflation, then suggest 5-10 psi as the buffer on top of that for both axle ends.
e.g. one end's weight corresponds to 80 psi, the other to 85 psi. RVSEF will recommend 85 psi minimum for both, and between 90 and 95 psi preferable for margin.
I've never heard of that in the rear and in the front, unequal air pressure would also change the tire compression on bumps, etc resulting in an annoying slight change of direction all the time on rough roads.
On the race track, I would always measure the tire height, not by measuring the diameter but by marking the tire with chalk at the bottom of the tire with a corresponding mark on the road. The tire was then rolled forward one revolution until the tire mark was again on the bottom and the distance measured. On a circle/oval track with left turns (don't they all) the tire with the greatest circumference would be on the right rear with around 5 to 10 psi more than the left rear tire. I also had weight jacks at all four corners so there are millions of possible tire pressure and weight combinations plus how is the track bar set? Preloaded for best corner entry, exit? This is why some cars are faster than others.
As far as braking on our Foretravels goes, 90 psi on the gauge does not stop as well as 110 psi and while the pressure at the governor can be increased to 130 psi, the brake cans are a set size. You can increase the size of the cans to make lock up possible all the time but the coach braking system was designed to protect RV drivers from themselves while providing safe stopping in most road conditions. Even with training, the typical RV driver is never going to achieve the level of competence an OTR or emergency vehicle operator has. Sorry, but that's the way it is. Can you imagine the rear end of a coach coming loose on a downhill turn in the rain?
The pressure you feel in the pedal is only because there is a spring behind the pedal to give a "feel" to the amount of air you send to the cans.
Pierce
If the tires are the same diameter, it will drive the best. A few pounds should get them very close , if you are super anal, less than 10 delta..
The rim to ground is the best REF point. plus or minus 5mm should be fine. More than 10mm means that the diff is rolling and slowing you down .