Help me understand my tire pressures

[pacevette]

We have a 2008 38' damon challenger 376. I have not yet had it weighed. But according to the label inside the cabinet it states 14,500 on the rear and 7500 on the front. I looked on Goodyear's website and looked up psi vs weight. If I am reading this right my pressures right off the bat should be set at 90 psi. I see others running lower pressure For a better ride. I apologize for my ignorance. I just want to make sure I am reading these correctly.

[pacevette]

After my post Im guessing that weight is the maximum allowed per axel.

[FIRE UP]

Originally Posted by pacevette

After my post Im guessing that weight is the maximum allowed per axel.

pacevette,
You're not "ignorant", you're asking a valid question. And, you're on the right track to answering it yourself. But, the simple and smart thing to do is, load that coach, just like you would for any normal trip, complete with all the equipment, supplies, water, fuel, animals, ANYTHING that you're going carry normally and, that would include you and the Commander In Chief, (aka WIFE) and, head off to the local scales. Scales can be sought after at, Moving-Van and storage facilities, Refuse disposal facilities, truck scales and more.

DO NOT WORRY ABOUT THE FOUR CORNER WEIGHT MEASUREMENT!!!

Just get the coach weighed. Get the front axle, the rear axle and, the total weight. THEN, go the tire pressure chart of the maker of YOUR tires and, be guided by what they print. The coach builders put specs on there based on certain guide lines but, are not specific to ACTUAL loading of your particular coach. Those specs are within reason to operated it safely. But, for the best ride, the best tire mileage, the best handling and more, go by the specs of the tire chart, AFTER COMPLETE LOADING.

Oh, by the way, do it with a full WATER TANK too.
Scott

[pacevette]

Thank you. There is a CAT scale about 15 miles from my house. Hoping to get this done in the next few days. As far as the rear axle goes, do I dive the weight by 2 or 4?

[Mr_D]

Originally Posted by FIRE UP

pacevette,
You're not "ignorant", you're asking a valid question. And, you're on the right track to answering it yourself. But, the simple and smart thing to do is, load that coach, just like you would for any normal trip, complete with all the equipment, supplies, water, fuel, animals, ANYTHING that you're going carry normally and, that would include you and the Commander In Chief, (aka WIFE) and, head off to the local scales. Scales can be sought after at, Moving-Van and storage facilities, Refuse disposal facilities, truck scales and more.

DO NOT WORRY ABOUT THE FOUR CORNER WEIGHT MEASUREMENT!!!

Just get the coach weighed. Get the front axle, the rear axle and, the total weight. THEN, go the tire pressure chart of the maker of YOUR tires and, be guided by what they print. The coach builders put specs on there based on certain guide lines but, are not specific to ACTUAL loading of your particular coach. Those specs are within reason to operated it safely. But, for the best ride, the best tire mileage, the best handling and more, go by the specs of the tire chart, AFTER COMPLETE LOADING.

Oh, by the way, do it with a full WATER TANK too.
Scott

Umm... not according to the tire manufacturers! You need to know the heaviest end of an axle and set all the tires on that axle to that pressure.

[Smitty77]

Scott is a gent I respect and usually agree with... But, if possible, I do encourage you to also weight one side of each axle too. If the scale is wide enough, just run down the edge, with one side of the axles about 1/3 of the way chassis width on the scale. (Avoid just running down the edge of the weight area with just tires, slide over a bit to get more onto the scale platform.)

Then do the math:

Full axle weight - one side of the axle weight. This gets you both sides weight.

Take the higher side weight, and use this when checking the tire manufacturer's chart for PSI. I personally, will bump up to the next level of weight, if I'm within 20% or so towards the top weight listed on a line. Always make all the tires on the same axle, at the same PSI. I also then add 5psi to the charts recommended PSI, for three reasons:

1) Traveling and packaging weights can vary over the years.

2) Consistent cross winds, can 'load' one side with more weight.

3) Some fudge factor to allow for some loss of PSI while traveling, and still remaining within the safe range.

Don't get me wrong, having axle weights are better then not having any weight at all. But, I've seen some large differences on axle weights from side to side. Our older T-28 Bounder, had the 75 Gal water tank, in the pack right corner, and it loaded that rear axle enough higher in weight, to move it up the chart for PSI.

My best to you, travel safe, and have fun,
Smitty

[pacevette]

Ok, I have the RV weighed today.
6720 lbs on the front axle.
13020 on the rear.
total weight was 19740

The fuel tank,water,tank and propane were all full. I'm guessing 600 lbs would be sufficient weight to add for full travel mode. Now I referred to Goodyear's tire chart and if I am reading it correctly it states 80 psi. I feel that may be a bit low. Am I wrong to assume this? I was thinking 85 psi would be a safe number.

[Pusherman]

Originally Posted by FIRE UP

pacevette,
You're not "ignorant", you're asking a valid question. And, you're on the right track to answering it yourself. But, the simple and smart thing to do is, load that coach, just like you would for any normal trip, complete with all the equipment, supplies, water, fuel, animals, ANYTHING that you're going carry normally and, that would include you and the Commander In Chief, (aka WIFE) and, head off to the local scales. Scales can be sought after at, Moving-Van and storage facilities, Refuse disposal facilities, truck scales and more.

DO NOT WORRY ABOUT THE FOUR CORNER WEIGHT MEASUREMENT!!!

Just get the coach weighed. Get the front axle, the rear axle and, the total weight. THEN, go the tire pressure chart of the maker of YOUR tires and, be guided by what they print. The coach builders put specs on there based on certain guide lines but, are not specific to ACTUAL loading of your particular coach. Those specs are within reason to operated it safely. But, for the best ride, the best tire mileage, the best handling and more, go by the specs of the tire chart, AFTER COMPLETE LOADING.

Oh, by the way, do it with a full WATER TANK too.
Scott

I agree with Scott. Yes, 4-corner weights are more desirable, but hard to come by.

So use the process which Scott outlined above, and then add +5psi to each tire over what the manufacturer's website states when you take the axle weight and divide by 2. This should help compensate for the weight differences per side. Make sure all pressure settings are done COLD.

Remember - same psi on all tires of the same axle.

You will be close enough and plenty safe.

[Gary RVRoamer]

Divide the rear axle weight by 4 and then add 250 lbs to allow for an unequal distribution of the weight. Then look up the required psi in the tables.

Divide the front axle by 2 and add 500 lbs, for the same reason.

Both of these allow for one end of the axle to be 500 lbs heavier than the other. That's not an unusual amount.

After looking up the psi values. I would go up one entry in weight, which is usually 5 psi. That gives you some extra margin to make sure you never run under-inflated.

[Pusherman]

Originally Posted by pacevette

Ok, I have the RV weighed today.
6720 lbs on the front axle.
13020 on the rear.
total weight was 19740

The fuel tank,water,tank and propane were all full. I'm guessing 600 lbs would be sufficient weight to add for full travel mode. Now I referred to Goodyear's tire chart and if I am reading it correctly it states 80 psi. I feel that may be a bit low. Am I wrong to assume this? I was thinking 85 psi would be a safe number.

I always advocate for +5psi over what the tire manufacturer's website recommends based on uneven weight across the axle plus any accuracy issues between tire gauges, temperature, etc.

[Pusherman]

Originally Posted by Gary RVRoamer

Divide the rear axle weight by 4 and then add 250 lbs to allow for an unequal distribution of the weight. Then look up the required psi in the tables.

Divide the front axle by 2 and add 500 lbs, for the same reason.

Both of these allow for one end of the axle to be 500 lbs heavier than the other. That's not an unusual amount.

After looking up the psi values. I would go up one entry in weight, which is usually 5 psi. That gives you some extra margin to make sure you never run under-inflated.

Gary, one comment on your recommendation above. The inflation tables take into consideration if it's on a single axle (S) or a dual axle (D). Divide axle weight by 2 in both cases.

[Ray,IN]

The load/inflation charts reflect the minimum air pressure to support that load, not the optimum for safety. The Rubber Manufacturers Association-RMA, states to never run less than the mfgrs. placard-stated air pressure. Read page 55 of this RMA pdf.
Running less air pressure to obtain a soft, coushy ride at the expense of a tire failure is not valid IMO.

[Gary RVRoamer]

Gary, one comment on your recommendation above. The inflation tables take into consideration if it's on a single axle (S) or a dual axle (D). Divide axle weight by 2 in both cases.

That depends on whose inflation table you are looking at. Most companies show individual tires but a few use axle end values, i.e. a pair for duals. Just be careful when you look at the table so you know which value to use. Michelin, for example, recently changed their RV Tire Guide to show dual pairs. Goodyear, Continental, etc. still show individual tires.

Regardless, you have to make an allowance for an unbalanced axle if you only have axle weights. 500 lbs/axle end is a reasonable amount, in my opinion.

[Pusherman]

Goodyear RV, Michelin, Toyo, Firestone, Bridgestone all use the (S) and (D) notation on their tire inflation charts for RV-size tires.