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Old 04-02-2014, 10:24 AM   #71
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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.
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Old 04-02-2014, 10:26 AM   #72
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Originally Posted by CapriRacer View Post
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.
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Old 04-02-2014, 10:46 AM   #73
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Originally Posted by CapriRacer View Post
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.
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Old 04-02-2014, 10:57 AM   #74
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Originally Posted by nickmeloy View Post
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.
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Old 04-02-2014, 11:14 AM   #75
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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.
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Old 04-02-2014, 11:26 AM   #76
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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).

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Old 04-02-2014, 11:34 AM   #77
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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.
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Old 04-02-2014, 12:21 PM   #78
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Individual Tire Loads

Quote:
Originally Posted by CapriRacer View Post
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?
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Old 04-02-2014, 12:34 PM   #79
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Additional Information in Post 78

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Originally Posted by Tireman9 View Post
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.
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Old 04-03-2014, 07:34 AM   #80
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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.)
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Old 04-03-2014, 08:14 AM   #81
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Originally Posted by CapriRacer View Post

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.)
Thanks, CapriRacer - In another thread, there was a specific question about this. I assumed the placard is based on original equipment. For example, my 27FB FC had GYMs and the placard says 65 lbs. But in switching to the 15" Michelins (whose sidewall says 50 lbs max) I am assuming the tire's sidewall takes precedence. And - that go get the full load bearing capacity (2183 derated by 10% for 1984/tire for trailer application) they are to be loaded at the 50 lb max - but definitely not overloaded to the 65 lbs on the placard (which assumes GYMs). That is also why I assume I should update the placard.

Are those assumptions accurate, faulty, or something in between?
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Old 04-03-2014, 09:35 AM   #82
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In your post # 75 you said "all tires of a given size are required by Federal law to meet the same load standards." This is partially correct but incomplete statement. DOT regulations require all tires of each type (P or LT or ST) must meet the same regulations but the regulations identify the test loads based on industry published tables or tables published by the tire MFG if different. While it is true that well over 90% of all tires made in a given type & size follow published industry standards, there are a number of exceptions. Some of these exceptions show up in the tables you can find on the tire company web site or possibly in literature at a store but there are a small number of tires made where the importer has issued an exception letter directly to DOT identifying a specific tire as having a non-standard load. So I stand by my statement that you should consult the information found for RV application of your tires on the web site for the manufacturer of your tires. I assume you already know that some have different specs for RV application than when the same tires are used in non-RV application but not everyone realizes this.

RE "What option would you suggest to determine the proper inflation?" I have covered that extensively in my blog. I can't publish links here but the summary is that the multi axle design of trailers places higher structural forces on tires than tires experience, even with identical inflation and normal load, when placed on the "corners" of a motorhome. Therefore, to decrease these destructive forces you should always run the max inflation pressure as shown on your tire sidewall. This inflation is also probably the inflation shown on your RV tire placard.

RE your +10 over the chart. Are you running Passenger type tires that have a 35 psi pressure on the sidewall? Since you didn't provide the size tire info or your individual tire loads I can only speculate.


RE your post #77
While lawyers may be involved it is the tire company engineers that do the load calculations and it is engineer representatives to the US Tire & Rim Association that approve the numbers in the standards book.

RE the RV MFG selection of tires. I agree that they many times seem to choose the lowest cost option that allows them to meet the regulatory requirements.
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Old 04-03-2014, 09:50 AM   #83
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Thanks, CapriRacer - In another thread, there was a specific question about this. I assumed the placard is based on original equipment. For example, my 27FB FC had GYMs and the placard says 65 lbs. But in switching to the 15" Michelins (whose sidewall says 50 lbs max) I am assuming the tire's sidewall takes precedence. And - that go get the full load bearing capacity (2183 derated by 10% for 1984/tire for trailer application) they are to be loaded at the 50 lb max - but definitely not overloaded to the 65 lbs on the placard (which assumes GYMs). That is also why I assume I should update the placard.

Are those assumptions accurate, faulty, or something in between?
Your switch from tires that said 65psi on the sidewall to tires that say 50psi sounds like you went down in Load Range and you also lost load carry capacity and the level of reserve load. NOT a good move when contemplating tire failure.

Can you provide the full size information shown on your placard and as shown on the sidewall of your current tires? Also what are your individual tire loads when your TT is fully loaded? Lacking that what are the individual axle loads?
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Old 04-03-2014, 10:41 AM   #84
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When I walked past the Eddie Bauer trailers on the line at the factory with the 16" SenDel wheel and 16" Michelin LT225/75R16/E LTX M/S2 tire option, the trailer tire pressure placard said inflate to 80 psi, the same maximum number seen on the tire sidewall. Those models with the GYM tires has a maximum pressure of 65 psi, the same as on the sidewall.

When I bring the trailer back from the solar and disc brake conversion at the end of next week, we will put the twin bed mattresses back into the coach, load all of our "stuff" and put the rig up on my two sets of four wheel scales. These wheel scales are rated 5,000 pound per scale and are claimed to be accurate to five pounds.

The visual design of the Classic model 30 makes one think the unit would be heavier street side since the bathroom and refrigerator and cabinetry appear more massive on that side. The street side has the storage area for kitchen pots/pans food as well as an additional external access area. The scales will tell the tale.
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