Calculating the correct tire pressure to use

[rideair]

Wow, I had no idea so much time and effort could be spent on tire pressue for something you are dragging around behind a truck. I have 5 Airstreams, all running 7.00x15 bias ply tires that always have the max pressure allowed (normally 60psi and they don't heat up). Finally after 12 years I had one of them (which was 11 years old) let go of about 2/3rds of the tread (but the tire did not blow out) going to Florida last year.

There's a reason why 18 wheelers normally keep their tires at max pressure. If they spent the amount of time it would take changing the tire pressure for each load, you would never get your fresh veggies from the store!

Some might say, "You will beat your trailer to death", if you're beating your trailer to death, I first would start looking at how old the axles are and then worry about the tires (get them balanced, with high pressure valve steams and if you're really worried get a set of Centramatics) and you should be fine running any tire at max pressure.

Oh well, I'll stick with the old bias plys 7.00x15's (you can still find them new, I have 12 of them waiting to be installed) at 60psi. Great thing about the old bias plys, if one blows out, I don't have a chainsaw going down the side of my trailer ;-)

Enjoy,

[ROBERT CROSS]

Well said Paul....

Bob

[jcanavera]

Here's some info from RVSEF on weighing your trailer. You should also investigate their site. They have a calendar (which will be updated as the year goes on) where their teams are scheduled for weighing services. You might find an event or location close to you. I have used them once when they were in the St. Louis area and was very pleased by the results and services they gave.

Frequently Asked Questions About RV Weighing

Jack

[MrJim]

Looks like RVSEF won't be near my trailer this year. (It's currently in Irvine, CA.) But thanks for suggesting them.

The main weight that I'd like to measure is hitch weight. I just bought a bath scale that claims to handle up to 400 pounds. Has anyone tried using such a scale to measure the hitch weight on an Airstream with an expected hitch weight within that range?

[Wazbro]

Originally Posted by MrJim

Looks like RVSEF won't be near my trailer this year. (It's currently in Irvine, CA.) But thanks for suggesting them.

The main weight that I'd like to measure is hitch weight. I just bought a bath scale that claims to handle up to 400 pounds. Has anyone tried using such a scale to measure the hitch weight on an Airstream with an expected hitch weight within that range?

This thread ha instructions on how to weight heavier trailer tongues then that range.

http://www.airforums.com/forums/f464...ale-24195.html

[Tireman9]

Originally Posted by MrJim

Looks like RVSEF won't be near my trailer this year. (It's currently in Irvine, CA.) But thanks for suggesting them.

The main weight that I'd like to measure is hitch weight. I just bought a bath scale that claims to handle up to 400 pounds. Has anyone tried using such a scale to measure the hitch weight on an Airstream with an expected hitch weight within that range?

Do you have a CAT scale or similar near you?. Simply get F & R axle weight with the RV attached. Then pull into parking area, drop the trailer and get another F& R.

You might discover you are not just adding hitch weight but transfering some load from front axle to rear also.

[nickmeloy]

Originally Posted by Tireman9

...Do you have a CAT scale or similar near you?. Simply get F & R axle weight with the RV attached...

A while back, we weighed our travel trailer on the CAT scales and got a total weight of 7,300 pounds, rigged for towing. Rigged for towing means with a full water tank, all of the misc. junk that we normally carry and weight distribution bars tensioned, but no clothing or food on board. On average, this translates to 1,825 pounds per tire.


Today, we were able to obtain individual wheel weights using portable scales. The results are (rigged for towing)


Left Front - 2,000 pounds Left Rear - 1,900 pounds


Right Front - 1,900 pounds Right Rear - 1,700 pounds


Perhaps one of the tire guys may wish to comment on these numbers.

[BlackAces]

Originally Posted by nickmeloy

A while back, we weighed our travel trailer on the CAT scales and got a total weight of 7,300 pounds, rigged for towing. Rigged for towing means with a full water tank, all of the misc. junk that we normally carry and weight distribution bars tensioned, but no clothing or food on board. On average, this translates to 1,825 pounds per tire.


Today, we were able to obtain individual wheel weights using portable scales. The results are (rigged for towing)


Left Front - 2,000 pounds Left Rear - 1,900 pounds


Right Front - 1,900 pounds Right Rear - 1,700 pounds


Perhaps one of the tire guys may wish to comment on these numbers.

I think you should balance your load.

BA

[dkottum]

Originally Posted by BlackAces

I think you should balance your load.

BA

Seriously?

[CapriRacer]

Originally Posted by nickmeloy

A while back, we weighed our travel trailer on the CAT scales and got a total weight of 7,300 pounds, rigged for towing. Rigged for towing means with a full water tank, all of the misc. junk that we normally carry and weight distribution bars tensioned, but no clothing or food on board. On average, this translates to 1,825 pounds per tire.


Today, we were able to obtain individual wheel weights using portable scales. The results are (rigged for towing)


Left Front - 2,000 pounds Left Rear - 1,900 pounds


Right Front - 1,900 pounds Right Rear - 1,700 pounds


Perhaps one of the tire guys may wish to comment on these numbers.

First, be aware that you didn't weigh the trailer in the worst case condition. At the very least, you missed the clothes and the food - and while that may seem trivial, those things add weight and we need to compensate for their absence.

Second, there is a discrepancy between the weights you posted. The average of those 4 reading is 1875 #, not 1825# you previously quoted.

Also, the round numbers leads me to believe that the weights are rounded to the nearest 100#. I think it is important to be aware of that.

So doing the math, the worst wheel position is the Left Front. That is about 7% above the average and 10% about the 1825# reading you quoted. Obviously there is a side to side variation that would be helped by moving stuff around.

I think it is a good idea to have tires operate at 85% of their rated capacity - so that means you need tires rated at 2353#.

I'm trying to find what tires you have on your trailer, but I went back 6 months....... So what are they? I would want to adjust ST tires so you could operate at 85 mph - as opposed to the normal 65 mph - but I don't know if you have ST tires.

[Tireman9]

Capri may have a typo re Max tire speed. ST tires are normally limited to 65 MAX. This is not an average speed but instantaneous MAX. Think of it as you would an engine rev limit. While it is possible to exceed the engine "red line" if you do, good things do not always happen and engine life is cut significantly.

Having said that, there is a document from Goodyear saying that if you increase the max inflation on their Marathons by 10 psi you can increase the max speed limit to 75. They also say that the load capacity DOES NOT increase above the capacity for 65 psi. They also say you cannot further increase the speed limit with additional inflation. There is apparently a similar one from Powerking based on the new Nylon cap ply construction on 15" & 16" tires.

This document applies to Goodyear tires only. You cannot use the Goodyear document to adjust a different brand or design tire anymore than you could use a GM shop manual for engine specs on a Ford.

[Tireman9]

Originally Posted by CapriRacer

Jim,

I hope your realize that lowering the pressure also lowered the load carrying capacity of the tire - and in turn, that increases the risk of a tire failure - and I hope I don't have to tell you about tire failures on trailers.

Also, there has been a lot of changes since 1971. While tires have gotten better, more scrutiny has been place on the inflation pressure recommendations made by vehicle manufacturers. I would be very hesitant to state that what they recommended in 1971 is applicable today. To be sure, you need to weigh the trailer.

I support what Capri said.

[slowmover]

Originally Posted by CapriRacer

First, be aware that you didn't weigh the trailer in the worst case condition. At the very least, you missed the clothes and the food - and while that may seem trivial, those things add weight and we need to compensate for their absence.

Second, there is a discrepancy between the weights you posted. The average of those 4 reading is 1875 #, not 1825# you previously quoted.

Also, the round numbers leads me to believe that the weights are rounded to the nearest 100#. I think it is important to be aware of that.

So doing the math, the worst wheel position is the Left Front. That is about 7% above the average and 10% about the 1825# reading you quoted. Obviously there is a side to side variation that would be helped by moving stuff around.

I think it is a good idea to have tires operate at 85% of their rated capacity - so that means you need tires rated at 2353#.

I'm trying to find what tires you have on your trailer, but I went back 6 months....... So what are they? I would want to adjust ST tires so you could operate at 85 mph - as opposed to the normal 65 mph - but I don't know if you have ST tires.

No such thing as too many scale tickets. Knowing the likely range of weights, whether for tires or WDH, is beneficial.

Agreed with the need for more depth.

[Tireman9]

Originally Posted by nickmeloy

Snip

On average, this translates to 1,825 pounds per tire.

Left Front - 2,000 pounds Left Rear - 1,900 pounds

Right Front - 1,900 pounds Right Rear - 1,700 pounds

Perhaps one of the tire guys may wish to comment on these numbers.

Ok With your example we see the fallacy of assuming equal load distribution and that getting actual loads when fully loaded (all your stuff) can compound the error.

Lets assume you add 400# of clothes, food and ahop for a few items during your travels. You also manage to accurately distribute this additional load on your 4 tires. So now you have 2100, 2,000, 2000, 1800 # load on each tire. But you selected your inflation based on your original total load 7300# simply dividing by 4.

If you had ST225/75R15 LR-C but incorrectly lowered your inflation to 40 psi (1880# capacity) to smooth out the ride, you would in fact be overloading one of your tires by 12% another by 6% and one by 1%. You may have compounded the error by driving at 65 with some stretches at 68 to 70 mph as your truck has the power.

Would you be surprised if you had a tire failure? When you had the failure would you simply claim it was the fault of the Chinese made tires?

If 225/75R15 is your size you should be using a minimum cold inflation of 50 psi in the LR-C tires. Better would be to go to LR-D and run 65 psi and if you insist on exceeding 65 mph max you should be running 75 psi in your tires.

75 psi is a long way from 40 psi.

Hope this clears up any confusion.

I do thank you for posting your actual numbers. I think I will use these facts in a future post on my Blog as some refuse to accept that trailer tires & axles are not always split 50/50 & 50/50 for load distribution.

[HowieE]

Tireman9

I have frequently posted that all tires of a given size are required by Federal law to meet the same load standards. With that in mind I have also posted that one could use the Goodyear inflation chart, the only one currently available online, when determining tire pressure.

Your comment that one can't not use a given chart with another manufactures tire leaves a void when deciding on tire pressure as a function of load.

All too many RVers rely on the Max. pressure printed on the tire side wall and thus frequently over inflate their tires. What option would you suggest to determine the proper inflation.

I run my Michelins at 45 psi. That is 10 psi over the charts but I do that so I could run in on 5 tires if necessary. 45 being the pressure for 4 tires with my load. Were I to inflate to the Max. side wall notation I would shake the cabinets off the wall.

[slowmover]

TT tires, as with all trailers, do not conform to the standards needed/given for the tow vehicle. The forces exerted on them are different (and can be extremely high) thus the load/pressure charts do not apply.

One might "differ" about this, so show us the TT manufacturer that recommends other than full pressure (sidewall marking).

.

[HowieE]

I am sorry. The tire pressure posted by RV manufactures are written by Lawyers and have nothing what so ever to do with tire usage or design. It is clearly a transfer of legal responsibility. Another consideration TV manufactures generally use the cheapest tire they can find to meet their need. If you question this you might look at the history of Goodyear tires on new Airstreams.

Now back to watching the GM COEs testimony before Congress to get out from under the stupidity of her predecessors.

[nickmeloy]

Originally Posted by CapriRacer

First, be aware that you didn't weigh the trailer in the worst case condition. At the very least, you missed the clothes and the food - and while that may seem trivial, those things add weight and we need to compensate for their absence.


I agree, however it is what it is. We had access to the portable scales on short notice and took advantage of the opportunity.

Second, there is a discrepancy between the weights you posted. The average of those 4 reading is 1875 #, not 1825# you previously quoted.


You misread the first paragraph in my original post. 7,300 divided by 4 is 1,825. Before obtaining the individual tire loading results, this was all I had to go on to estimate my average individual tire loading.

Also, the round numbers leads me to believe that the weights are rounded to the nearest 100#. I think it is important to be aware of that.


Good catch. The officer operating the portable scales would only read them to the nearest 100 pounds.

So doing the math, the worst wheel position is the Left Front. That is about 7% above the average and 10% about the 1825# reading you quoted. Obviously there is a side to side variation that would be helped by moving stuff around.


I agree that there is a load bias, with the left side being (on average) 150 pounds heavier than the right side. What I am trying to ascertain is whether or not this 150 pound bias is significant for a trailer that weighs between seven and eight thousand pounds.

I think it is a good idea to have tires operate at 85% of their rated capacity - so that means you need tires rated at 2353#.


This is the type of information that I seek. The trailer had GYM ST225/75R15 LRD tires on it when we purchased it used. We ran the GYM tires at 65 psig cold, and our towing speed was never above 55 mph. After reading this forum for a while, I began to wonder if the LRD tires had the reserve capacity needed for this trailer and changed to a LRE tire. The current tires (ST225/75R15 LRE) are a brand called Gladiator, which is a China import.

I'm trying to find what tires you have on your trailer, but I went back 6 months....... So what are they? I would want to adjust ST tires so you could operate at 85 mph - as opposed to the normal 65 mph - but I don't know if you have ST tires.


The current trailer tires are ST225/75R15 LRE run at a cold inflation pressure of 75 psig. We tow at 55 mph. While I am driving this trailer, it will never see 85 mph. I was unable to locate a load vs inflation pressure chart for the Gladiator brand. The sidewall information on the tire put the maximum cold inflation at 80 psig.

Now comes the questions. What should we do? Is our load distribution close enough? I don't view a load distribution as being static. Since it can change during our trips, and we don't have access to portable scales, are we OK?

[nickmeloy]

Originally Posted by Tireman9

Ok With your example we see the fallacy of assuming equal load distribution and that getting actual loads when fully loaded (all your stuff) can compound the error.

...
I do thank you for posting your actual numbers. I think I will use these facts in a future post on my Blog as some refuse to accept that trailer tires & axles are not always split 50/50 & 50/50 for load distribution.

You may use this post in your blog. Please see post 78 in this thread for additional information before you do so. Also, please feel free to contact me if you should want additional information.

[CapriRacer]

OK, a couple of thoughts:

First: Mr Meloy, those numbers all seem to work for me. Even if you add in the weight of food and clothes, I think you still have a good combination. I would suggest that if the opportunity presents itself, weigh the trailer again to confirm things.

Second, Load tables. While there are some exceptions, hardly ever are the load tables specific to a given tire manufacturer. Yes, you have to be careful to get the right table. The tables for ST tires is different than it is for LT tires, and those are different than for P type tires. But it just isn't the case where EACH tire manufacturer has its own load table, but you have to be careful to get the right one.

The exceptions? Every so often we encounter a tire manufacturer who is not following the tire standards as published by the major tire standardizing organizations. There's no law that says he has to, but it is such a good idea to stay with the herd on these subjects. In trailer tires, there is someone quoting 3500# load carrying capacity for an ST235/80R16 Load Range E - and that is non-standard. They are doing this to accommodate 7,000# axles. This one makes me especially nervous.

Third, vehicle inflation pressure specifications - the ones on the vehicle tire placard - are chosen by engineers. The good ones, of course, have listened to what the lawyers have said and applied those words to the values. I have publicly taken issue with those values - and I do so when I don't think the engineers have consulted with their attorneys. The values specified are based on the load tables and the weights of the trailers, but the engineers don't always do a good job. But they are always based on the published load tables. In the case of trailers, it would be wise to weigh the trailer and do your own calculations. In the case of cars, vans and pickup trucks, I think you can trust recently produced vehicle placards (say 2008 and newer.)