To the OP double check the factory sticker and make sure it reflects the tire wheel combo you have. National ordered upgraded chassis with 22.5 wheels and the same 235/80 tires but the sticker still shows a stock chassis with 19.5 wheels. So pressures are shown as 100 PSI. When using Michelin's chart I should run 85/80 front rear. For simplicity and some margin of error I run 90 all around.
For the naysayers that say you have to run what the sidewall lists, which is MAX cold pressure to carry the max TIRE rated weight. If you are no where near that weight you will get poor ride, poor handling and poor wear out of the tires. All of the above caused by the contact patch being shrunk by the over inflation.
As far as questioning the 5 psi fudge factor, unlike a regular vehicle our loads change often, think passenger sitting in a different location, few extra cans of soup or bottles of wine , my favorite choice, on the heavy side and now the load has changed. 5 psi won't effect the ride or footprint measurably but will keep the carrying capacity a little above what you know you carry "usually"!
Ray, Cheryl, Cory & of course Miss Molly the four-legged child
The tire inflation tables give you the correct cold inflation pressure for the weight they are carrying. The load limit for the tire at a particular pressure. The charts are not out of date.
Your tires are load range G. I have no idea why they put load range G tires on the coach but that is the reason for the pressure being lower than you expected. It would have been just fine with load range F tires.
I have a 2000 34 foot F53 with load range F Goodyear's and carry 90 in front and 80 in the rear.
The tires will seem very stiff at high pressures because they are designed to be inflated for the weight they are carrying.
2000 Sea Breeze F53 V10 - CR-V Toad
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I'm not running a MH, but have used the tire tables plus 5 psi for luck on our TT's for over 200,000 miles, with only tire issues being a few repairable nail punctures. Treads seem to wear flat accross, as they should if properly inflated.
Might be coincidence, but I never use Chinese tires on road going vehicles.
Chuck
Wonderful Wife
Australian Shepherd
2010 Ford Expedition TV
2010 Outback 230RS Toybox, 5390# UVW, 6800# Loaded Not yet camped in Hawaii, 2 Canada Provinces, & 2 Territories I can't be lost because I don't care where this lovely road is going
Here's why you see differences: Your motorhome manufacturer lists tire pressure for a fully loaded coach using the OEM tires. Their lawyers will insist on these numbers. Your actual PSI will be different for several reasons:
1. Your coach is not fully loaded. After weighing your coach, tire manufacturer charts trump the numbers posted on your coach. Your tires do NOT care if they are carrying freight, lead weights or a motorhome. They care about weight per axle AND speed (more about speed below)
2. You have replaced your tires with a different brand. There are minor weight carrying / PSI differences between tire brands.
3. You start out on a cold morning and drive into hot weather. Large changes in ambient temperature affect tire pressure. To avoid confusion, many tire manufacturers list only cold PSI (FWIW: Race tires are different - they show hot performance curves too). Tires don't care about temperature. They care about PSI. Your tire pressure will increase as the temperature rises. If the change is excessive, you tires will have more pressure than necessary.
4. You drive slower or faster than average. Speed is a squared term in the weight carrying equation. Your tire tables assume a nominal speed like 65 MPH. If you drive at 70+ MPH you need more PSI than normal. Drive at 55 and less, and you will be able to use less. Again the tire manufacture performance curves are lacking detail in this area.
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Happy Ford F-53 Class A Owner (2008 Gulf Stream)
2010 Ford Fusion Toad (with 6 speed manual transmission - the only way to tow)
Brake Buddy Vantage, Blue Ox Aladdin Tow Bar,
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1. Tire performance follows a fairly flat curve around the normal PSI range. Being 1 PSI low does NOT mean you are suddenly in grave danger. And adding 15% on a flat part of the curve doesn't change much except maybe ride characteristics.
2. Tires warm up. That moves you to the middle - and more horizontal - part of the performance curve where a 15% PSI change makes no real difference!
Knowing the relationship between tire performance, temperature and speed, I actually run about 15% under the nominal Michelin cold PSI numbers. By driving slow for the first 10 minutes, I stay within the actual performance limits (as speed is MUCH more important than PSI in the performance curve equation!!!). After the tires warm up, I am in the flatter part of the performance curve and can easily use my chosen PSI with its inherent smoother ride and better control.
The tyres are an integral part of the suspension so it makes sense to make use of them in that role too. Just as you wouldn't fit shock absorbers that were too harsh for the weight/suspension, why overinflate the tyres.
In Australia - where we have more than a few bad roads - if I have a long stretch of washboard or similar roads to drive on, I lower the pressure and reduce the speed to suit. The rig rides better and the tyres run less risk of being damaged by sharp stones.
1. Tire performance follows a fairly flat curve around the normal PSI range. Being 1 PSI low does NOT mean you are suddenly in grave danger. And adding 15% on a flat part of the curve doesn't change much except maybe ride characteristics.........
Sorry, but tire performance is definitely not FLAT - even around
normal pressures. ANYTHING 15% over-loaded is much more prone to fail - and the same is true of tires.
ClassAGeek wrote: .....2. Tires warm up. That moves you to the middle - and more horizontal - part of the performance curve where a 15% PSI change makes no real difference!........
Again, tire preformance is NOT flat and if you use more pressure, you get less pressure build up.
To try to illustrate the overall point of using a larger value, go to your local new car dealer. Look at the vehicle tire placards and compare them to the GAWR's listed elsewhere on the vehicle. Care to guess what you will find? If it is good enough for the major car manufacturers of the world .........
Sorry, but tire performance is definitely not FLAT - even around
normal pressures. ANYTHING 15% over-loaded is much more prone to fail - and the same is true of tires.
If you are trying to be pedantic, why not point out that curves are never straight, no line can be absolutely horizontal or the quadratic equations used to model tire performance are only approximations subject to continuous probability distribution errors?
Fair enough - I'll restate: " the tangent to the performance curve is more horizontal near the centre region of the nominal performance range and tends to steepen toward the ends the nominal range - particularly at the high end ". Does that work for you?
(for those who care, a tire performance curve, plotting Y: CCC versus X: PSI, looks like a fat Gaussian distribution with a negative skew)
CapriRacer wrote:
Again, tire preformance is NOT flat and if you use more pressure, you get less pressure build up.
To try to illustrate the overall point of using a larger value, go to your local new car dealer. Look at the vehicle tire placards and compare them to the GAWR's listed elsewhere on the vehicle. Care to guess what you will find? If it is good enough for the major car manufacturers of the world .........
Actually it is not good enough for vehicle manufacturers. They will have access to raw performance data. We don't - so we must interpolate based on the data available.
Adding 15% pressure to any tire below the maximum for the tire and rim will generally increase its CCC. But that is NOT what I am looking for. I want the best compromise between CCC and ride comfort. These are typically conflicting attributes. When the 'tangent to the performance curve is more horizontal", there is little to be gained in terms of CCC by adding more air.
I am now intrigued: Why did you choose a 15% and not 25% or 50% 'safety factor'? Do you dispute that there is a diminishing benefit from higher pressure near the middle of the normal operating range?
ClassAGeek wrote: ..........I am now intrigued: Why did you choose a 15% and not 25% or 50% 'safety factor'? Do you dispute that there is a diminishing benefit from higher pressure near the middle of the normal operating range?
Many, many years ago, car manufacturers used to size tires such that they were minimally able to carry the load of the vehicle (there were some exceptions) - and over the years, they have increased the size of the tires. I can detect 3 such incresaes. The net effect is that today the tires have about a 15% over-capacity.
Manufacturers of light trucks (pickup, suv's, etc.) did do that until the Ford / Firestone thing some 11 years ago. They have since increased the tire sizing such that today, they also have about 15% over-capacity - which is where the value comes from.
And - No - I am not disputing that the effect diminishes - but getting further away from the peak is beneficial in reducing the risk of a tire failure.
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, my favorite choice, on the heavy side and now the load has changed. 5 psi won't effect the ride or footprint measurably but will keep the carrying capacity a little above what you know you carry "usually"!
