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Old 04-21-2010, 05:40 PM   #1
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A couple of tire pressure questions

I have had my p32 chassis Winnebago Sightseer for a little over a month and finally got around to checking tire pressures etc. My intent is to weigh it next time out and make tire pressure adjustments according to the weight. However, when checking the pressures today I found the passenger front at 101 lbs and the driver front at 87 pounds. The guide at the drivers seat reccomends the pressure at 75 pounds. The max inflation stamped on the tire is 95 pounds. I have the chart for Michelin XRV ready to use after I weigh the coach and it only goes to 95 pounds.

Now, the questions....

Is it possible that there should be a difference of 14 pounds pressure between the front tires?

How many people inflate tires greater than the max rated? I think I would rather shift some weight around before pushing the max inflate limit. Is that sensible?

Could that much difference between the front tire pressures cause wandering? Actually the coach drove pretty good but some wandering was present.

Obviously I will answer some of these questions myself when making my adjustments. But your input would be of interest to me and might help me when setting my air pressures.

One other question, the TPMS that camping world has on sale in the latest flyer for $249 (system 43965). Any one have a review on it?
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Old 04-21-2010, 06:17 PM   #2
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Do not exceed the max cold inflation pressure on the sidewall of the tire. Also, all tires on the same axle should be at the same pressure, as determined by the highest individual wheel weight. The guide at the driver seat is based on the tires specified on the guide. Your tires on the rig now may not be what originally came on it.

I've got a TPMS from HawksHead Systems being shipped to me right now. I went with the replaceable batteries in the sensors.

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Old 04-21-2010, 06:25 PM   #3
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To get an accurate pressure you need to weigh each wheel. Then take the highest weight for the axle and add say 500 lbs. Now check the Michelin chart and that will be the pressure for both tires on that axle. The 500 lbs is the fudge factor as most rig gain weight just like the owner.
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Old 04-21-2010, 08:23 PM   #4
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How many people inflate tires greater than the max rated? I think I would rather shift some weight around before pushing the max inflate limit. Is that sensible?
Stubborn owners do it all the time. Is it right .. not at all. You need to weigh the axles at minimum or weigh the 4 corners and adjust the pressures to the highest rating for the weight in the inflation tables PLUS 5 psi. (safety)

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Could that much difference between the front tire pressures cause wandering? Actually the coach drove pretty good but some wandering was present.
It might but you would actually want to have the pressures within a very few pounds of one another. More important in your application as well would be the inflation pressure of the air cylinders in your front springs.

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One other question, the TPMS that camping world has on sale in the latest flyer for $249 (system 43965). Any one have a review on it?
TST is an iRV2 sponsor and I would take a look at their product. Their system gives pressure and temperature. I understand that follow-on systems are going to have user replaceable batteries. We have a number of satisfied TST owners in this forum and I'm sure they will be pleased to answer any questions you might have.
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Old 04-21-2010, 08:30 PM   #5
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yes, significant differences in front tire pressures can influence handling. This difference was due to improper inflation and should be corrected.

yes to what John said about max sidewall pressure and axles.

You can play games with tables and wheel weighings and such things but that is usually not worth the effort.

When you get on the road, check the tire temperatures with an IR thermometer after an hour or so. They should run 120F to 140F or so and be pretty much the same all around (except the side in the sun may run 10F or so higher than those on the shade side). If a tire is hotter that 140F it does not have enough tire pressure for its load. If your tires run below 120F, they are probably a bit overinflated.

The risk to being underinflated is that they get hot and that damages tires. This is a risk to avoid.

The risk to being overinflated is maybe a bit harsher ride and, if you put a lot of miles on the rig, some extra tread wear towards the center of the tire. This risk isn't such a big deal. This is why running at up to max sidewall cold pressure is the safe bet for a starting point.
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Old 04-21-2010, 09:32 PM   #6
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The risk to being overinflated is maybe a bit harsher ride and, if you put a lot of miles on the rig, some extra tread wear toward the center of the tire. This risk isn't such a big deal. This is why running at up to max sidewall cold pressure is the safe bet for a starting point.
What would be the ending point if maximum inflation pressure cold is the starting point?

Although I have an IR gun that data is a rough estimate at best. A scale house ticket is the best data that an owner can have. Inflating tires to meet a specific need is where the average owner is going to benefit the most from ride comfort, longevity and safety. Inflating a tire to the maximum pressure cold is not where tire manufacturers want you to benchmark their tires. Tires are only a part of a system and as such need to properly set to the task they are performing as assigned by the complete vehicle manufacturer. It yields no benefit to inflate a tire to sustain an 8,000 pound load when there's only 6,000 pounds on the tire.

This concept is repetitively taught by qualified instructors from RVSEF, Michelin, Camp Freightliner and others.
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Old 04-21-2010, 10:50 PM   #7
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What would be the ending point if maximum inflation pressure cold is the starting point?
the minimum air pressure needed to support the weight on the tires which would be indicated by the tires not getting too hot on the road.

Quote:
Although I have an IR gun that data is a rough estimate at best.
IMHO it is good enough and doesn't mislead with a precision that greatly exceeds the accuracy of measure (like weight on scales for tire pressure reasons tend to do). Its primary benefits are that it is easy to do, can be done at any fuel or rest stop to detect problems early, and measures the primary indicator of improper tire pressure that is worthy of most concern.

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This concept is repetitively taught by qualified instructors from RVSEF, Michelin, Camp Freightliner and others.
yes, indeed. It is formulaic and can be taught without ambiguity and that makes training easy. Finding scales to continually update your current running weight so you can adjust tire pressure (which you can only properly measure when cold and in consideration of other conditions) is nice in theory but not so much in practice.

What I think most folks need, in contrast, is something that is more likely to actually be done while traveling and that can be maintained and monitored easily enough to assure proper safety by early detection of potential problems including, but not limited to, the morning tire pressure theoretical point.

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the average owner is going to benefit the most from ride comfort, longevity and safety.
I'd put safety first and primary. Longevity in terms of miles is not normally a concern for most RVers as they tend to run out of years before they run out of tread. The issue of comfort is important and impacts safety but other factors are often much more significant in this area than tire pressure, especially since RV's tend to have tires being run fairly close to maximum loads, anyway.

Another area that folks can investigate is why performance drivers often overinflate their tires for races and such. There are some good lessons there as well.
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Old 04-22-2010, 06:10 AM   #8
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Another area that folks can investigate is why performance drivers often overinflate their tires for races and such. There are some good lessons there as well.
I always run my tires in an overinflated profile however they are not inflated to maximum pressure cold. I have an 8.5K front and a 15K rear GAWR. I inflate to my GAWRs although I rarely achieve that weight because I would be overloaded. To meet those ratings I need to run 95F and 90R in inflation pressure however I won't achieve the maximum load on the tire for those pressures.

Personally, I won't start at 110 PSI and run at that pressure because of max inflation pressure cold is posted on the sidewall of the tire.

I am on my 2nd set at 85,000 miles.

Tires on race cars are adjusted during a race upward or downward in increments of 1/2 a pound at times to improve handling. Over inflating a race tire at 200 MPH in my opinion will reduce the size of the footprint of the tire and that's what the drivers are going to want to achieve is a car that's got grip and doesn't leave too much rubber on the track. When race tires get hot they start falling apart and getting the pressure right gives a driver the racing edge yet allows the driver to at least get few dozen laps on a set of stickers.

Tires are a system and both load and pressure have to be matched as closely as possible. Air is the only tool that allows a driver to insure that a tire meets all 3 operating needs for load carrying, ride comfort and safety. RVs don't shed loads like dump trucks they carry an average load 75% of the time. Hitting that number is possible and achieving that with a rational and learned appreciation for weights, pressures and loading is the better approach.
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Old 04-22-2010, 09:55 AM   #9
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I recommend getting it weighed and adjusting tire pressure per the folks at Aweigh-we-go RV Safety, Merritt Island, Florida

NOTE: at least one poster said that all tire pressures on an axle should be the same.

I hear that often.. I am not, however, convinced.. I won't argue it, but I will say to take it up with the experts at aweigh-we-go..

The idea is this.. your tire if properly inflated will lie flat on the road

If over inflated it will be "U" shaped with the center pressing down hard and very little wear on the outside, this results in loss of control and shorter tire life

If under inflated invert the U (Top hat ^) with the edges pressing down harder and the center not pressing down, Also you get softer sidewalls

This results in shorter tire life and loss of control

Proper inflation gives longest tire life and best control

It's that simple.
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Old 04-22-2010, 11:30 AM   #10
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The idea behind tires on an axle being inflated the same is related to proper vehicle loading. There should not be a significant weight difference in a vehicle from side to side which means the weight on tires on one side of an axle should be reasonably close to that on the other side. That, in turn, coupled with the idea that you shouldn't mix tires on an axle, means that the inflation requirements on both sides should be the same.

As for the "lie flat on the road" -- this issue should not be overblown or taken out of context. The differences are minor between 'fine tuned' inflation and 'max sidewall' inflation in this context and most RV's don't put on enough miles fast enough to make wear the big age issue. If you do see excess center wear, by all means reduce the pressure a bit. But, to be safe, always err on the side of higher pressure. If your tires run hot, add air, as it is heat that kills tires.

As for control, see the discussion about performance drivers. The friction between tire and road is the product of pressure and footprint (which is one way to measure the weight on the tire) and not on pressure alone.

As for "take it up with the experts" - I suggest accepting responsibility for one's own decisions and actions and not depending upon any other supposed authority without knowing the context, conditions, and reasons.

Yes, proper inflation is good but no, it is not that simple. Thinking things like this are simple is asking for trouble.

This thread is going as many technical ones seem to do and that is too bad. The logical fallacies are accumulating. We have the reduce to the absurd, the appeal to authority, ...

the ad hominem almost always follows so I think it time to step out with the suggestion that anyone looking for answers and learning look behind the advocacy, find the underlying reasons, and realize the margins that exist in the phenomena and the measure.
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Old 04-22-2010, 03:23 PM   #11
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When you checked your pressures, were both sides in or out of the sun?

With my TST system I can confirm that sunshine alone can attribute to a significant rise or drop in pressures. Sitting still or running down the road.

As DriVer suggested, you may want to take a look at the TST system. Their customer service is second to none and their product aint to shabby either!!
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Old 04-24-2010, 02:59 PM   #12
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I have set both tires while in the shade to 90 lbs (max reccomended is 95) as well as setting the airbags to 90 (max reccomended). This gives me a baseline to start from and make adjustments to. The ride is not my top priority and I will sacrifice some ride for handling and safety, but the ride seems really good right now. I have not been on the freeway with the new settings but on a local 4 lane at 55 the thing actually seems like it drives better.

However, when I get on the freeway with the truck ruts etc and the trucks it will be interesting to see what happens. I do intend to weight the RV and make adjustments if I am way off the mark with weights vs pressures, or if I get blown sideways by the trucks.

Thanks to all for the tremendous feedback. I can not imagine how difficult RV ownership would be without the internet and willing contributors to this great website.
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Old 04-27-2010, 06:41 PM   #13
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In case someone is interested in following this thread...... I took it on the interstate today. I was very impressed with the improvement gained by simply changing the air pressures i mentioned. Trucks had very little or no impact on me when they passed. A very windy day today and it drove better than ever. I am not saying I didn't feel the wind, but it was very easy to control, it was fingertip driving.

I have not weighed it yet for fine tuning and have made no adjustments to back tire pressures. I certainly hope I can get as much improvement with my next changes.
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Old 04-27-2010, 08:16 PM   #14
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The tires on my RV were all set at 100# when I took possession. After weighing the RV and checking the Michelin tables, I set the rears to 95# and the fronts to 90#. The ride and handling both improved, I wouldn't run the tires at the max of 110# for any reason.

I use the Tire-SafeGuard system which monitors pressure and temperature both.
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