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Old 07-04-2013, 08:30 PM   #15
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My new 30' bunkhouse will be done by end of october - they use the 15's on it I presume - my dealer is inquiring to airstream about the upgrade - but it I unclear if they will do it per my dealer - he is still asking

Think my GVWR is 8800....
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Old 07-04-2013, 09:14 PM   #16
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Many run the 15" Michelin LTX tires on the original rims. Including those with the bunkhouse models (8400 GVWR).

No reports of issues so there is no evidence that I have seen that one must move to 16" wheels to gain peace of mind. This is backed as well by a favourite RV dealer Andy T who likes the 15" Michelins.

The peace of mind is moving away from the ST tires to something built frankly for a more demanding customer - automobile owners. RV owners (in general) seem to be more willing to live with crappy tires.

The fact is that the RV industry is all driven by lowest cost. There's rarely a spectrum of quality choices available. Unlike in the auto world where you can buy a better tire. It does not seem possible with ST tires. They are all dubious.

The other benefit of the 15" Michelins is a softer ride. Max pressure is 50 PSI while the 16" are higher (80?).
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Old 07-04-2013, 09:19 PM   #17
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Sounds better - hope 15 michellins work out for me then
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Old 08-26-2013, 08:49 PM   #18
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Originally Posted by afneill View Post
I think the reasoning is the increased load capacity you get with the 16" Michelin tires. The 235/75R15 Michelins have a load capacity rating of 2183# per tire for a total of 8732 for 4 tires. If that is sufficient to carry with your trailer after derating and allowing for the 20 per cent safety margin then you could stay with 15' tires. If you need more load capacity the 16" Michelins have a capacity of approximately 2650# per tire, or 10,600 for four tires. If you need greater capacity than the 8732 and want to go with the Michelin tires, you will need to go to the 16" tire. That is my understanding and an untechnical explanation.
How does one calculate the derating? This may be a dumb question, but is it simply the 20% safety margin or something in addition to it?
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Old 08-27-2013, 03:47 AM   #19
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Originally Posted by KYAirstream View Post

How does one calculate the derating? This may be a dumb question, but is it simply the 20% safety margin or something in addition to it?
I hope I have this right - you might want Capris or Tireman to check in here - but whether using 15s or 16s, I think you start by derating the LT tire by 10% (divide sidewall number by 1.1 - call that LTD1). That's a DOT standard (don't have the link handy but its been posted before). Then - I believe the recommended (but not required) "safety factor" of an additional 15% is taken off the tire's capacity (so divide LTD1 by 1.15 -call that LTD2). If LTD2 x number or tires is equal to or less than your fully loaded, hitched, WD applied trailer weight, you are good to go.

Caveat: That is an uneducated layman's understanding and requires accurate cat scale weights. Consult with a professional before doing something like this.
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Old 08-27-2013, 06:26 AM   #20
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Changing out a set of new Goodyear Marathons is senseless.

Truth is if you had the same number of people using Michelin tires on their trailers and not properly taking care of them you'd be seeing the same if not more complaints on them.

Unlike some I'm not smarter than the tire manufacturers I want a tire that is designed for a trailer....so I will continue to use Goodyears until they come out with a trailer tire that's better.
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Old 08-27-2013, 09:36 AM   #21
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My first impression of the swap to 16" was overkill, but some GYM research will indicate a genuine reliability problem, not something we want far from home. Airstream is silently endorsing this by selling them as an upgrade at their Service Center and and standard on the Eddie Bauer.

I think GYM's were originally installed to keep pricing down, an inexpensive beefy tire that will service most RV usage, such as weekend travel and short vacation jaunts. That's not how we use our Airstream.

The originals are worth something and that reduces the cost of upgrade. I bought the new set of four 16" Michelin LT and Sendel T02 wheels for $1400 and sold the originals for $500, a $900 upgrade.

There is some concern about 80 psi LT tires and it's effect on the trailer. Andrew Thomson at CanAm thinks it's too rough I believe, and he knows Airstreams. Airstream itself offers them as upgrade, or standard on the EB. Conflicting info. We went with the reliability of the 16's, knowing that we will be wise to avoid or drive slowly over unusually rough road surfaces, maybe even pull off and lower tire pressure for these roads.

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Old 08-27-2013, 10:10 AM   #22
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Quote:
Originally Posted by SteveSueMac View Post
I hope I have this right - you might want Capris or Tireman to check in here - but whether using 15s or 16s, I think you start by derating the LT tire by 10% (divide sidewall number by 1.1 - call that LTD1). That's a DOT standard (don't have the link handy but its been posted before). Then - I believe the recommended (but not required) "safety factor" of an additional 15% is taken off the tire's capacity (so divide LTD1 by 1.15 -call that LTD2). If LTD2 x number or tires is equal to or less than your fully loaded, hitched, WD applied trailer weight, you are good to go.

Caveat: That is an uneducated layman's understanding and requires accurate cat scale weights. Consult with a professional before doing something like this.
LT tires do not have to be derated. P tires do. Here is the federal regs on tire loading which has the detail on P tire derating. It also has some interesting reading on tire loads and reserve capacity.

DOT 49 CFR Part 571 Docket No. NHTSA-03-15400 RIN 2127-AI54
Section IV, Paragraph D

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Old 08-27-2013, 10:54 AM   #23
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Why 16" tires? Because folks are looking for a tire that is more reliable and robust than the OEM 15 ST tires.
I just finished going through this exercise because our 3 year old 15" ST tires started to separate. I don't think this is coincidental. ST tires have proven to be a design manufactured with sub par grade of materials that doesn't hold up to the rigors of UV, ozone, heat and highway speeds.

The tire company that I am working with must have known they had a bad product since they decided on a good will gesture and sent me 4 new tires so I would feel good about their company. The tires were ~1.5 years beyond the warranty period. Why else would they have done this.... I am grateful that they sent me the tires and I will use them for now. But the next go around I will upgrading to 16" tires if nothing has changed on the ST tire scene.
http://www.airforums.com/forums/f438...is-108403.html
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Old 08-27-2013, 11:15 AM   #24
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Here is the reason for derating the Michelin LTX (P) 235/75R15 XL sidewall rating of 2,183 pounds @ 50 psi to 1,985 pounds:

*************************************
49 CFR 571.110

Tire selection and rims and motor home/recreation vehicle trailer load carrying capacity information for motor vehicles with a GVWR of 4,536 kilograms (10,000 pounds) or less.

S4.2.2.1
Except as provided in S4.2.2.2, the sum of the maximum load ratings of the tires fitted to an axle shall not be less than the GAWR of the axle system as specified on the vehicle's certification label required by 49 CFR part 567. If the certification label shows more than one GAWR for the axle system, the sum shall be not less than the GAWR corresponding to the size designation of the tires fitted to the axle.

S4.2.2.2
When passenger car tires are installed on an MPV, truck, bus, or trailer, each tire's load rating is reduced by dividing it by 1.10 before determining, under S4.2.2.1, the sum of the maximum load ratings of the tires fitted to an axle.

S4.2.2.3
(a) For vehicles, except trailers with no designated seating positions, equipped with passenger car tires, the vehicle normal load on the tire shall be no greater than 94 percent of the derated load rating at the vehicle manufacturer's recommended cold inflation pressure for that tire.
(b) For vehicles, except trailers with no designated seating positions, equipped with LT tires, the vehicle normal load on the tire shall be no greater than 94 percent of the load rating at the vehicle manufacturer's recommended cold inflation pressure for that tire.
********************************

As mentioned above in a previous post, LT tires are not derated.

Thus the bunkhouse trailer with a GVW of about 8,800 pounds exceeds the capacity of the 15" Michelins (4 x 1985 = 7,940 pounds).
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Old 08-27-2013, 11:24 AM   #25
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"Thus the bunkhouse trailer with a GVW of about 8,800 pounds exceeds the capacity of the 15" Michelins (4 x 1985 = 7,940 pounds)."

Maybe not. If 860# tongue weight or more of the GVW is carried by the tow truck, it is within capacity as described. And that is based on loading the trailer to GVW which is not always done.

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Old 08-27-2013, 12:14 PM   #26
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Quote:
Originally Posted by dkottum View Post
"Thus the bunkhouse trailer with a GVW of about 8,800 pounds exceeds the capacity of the 15" Michelins (4 x 1985 = 7,940 pounds)."

Maybe not. If 860# tongue weight or more of the GVW is carried by the tow truck, it is within capacity as described. And that is based on loading the trailer to GVW which is not always done.

doug
Doug,
If you studied a bit of what Capriracer went over in my Carlisle thread he is using a safety margins when figuring weight ratings (Post #19, 20 and 25). My trailer had axle weights of 7360 which exceeded the safety margin of the "P" rated LTX Michelin tire. A "D" rated ST tire would do but and E would be better and best case is the 16" LT tire. The "P" rated tire was derated once for being placed on a trailer and again so as not to carry more than 85% of the derated load (6800#).
My 1994 30' Classic was past the bubble (safety margin) for using the 15" Michelin. If the trailer was lighter I would definitely use it.
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Old 08-27-2013, 12:32 PM   #27
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I stated conservative and got the EB wheel upgrade....

I read enough to become reasonably concerned and was convinced really - and the rims and tires look great too so that's nice
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Old 08-27-2013, 01:49 PM   #28
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I stated conservative and got the EB wheel upgrade....

I read enough to become reasonably concerned and was convinced really - and the rims and tires look great too so that's nice
After failures with the original D rated tires and failure of 2 E rated ST tires with both failures occurring late in season 3 and early season four of use, it really becomes an economical decision. Not even considering the cost of the damage from a blown tire (I caught mine prior to blow out on the road), it's obvious that 3 years is the limit of an ST on my Classic. Hence the higher cost of the LT tires and wheels is recovered in the first 10 years of use assuming that I'm on my 3rd set of ST tires and (making assumptions based on others experience), I'll be on my second set of LT's.

I think you made a good choice. That's the great side of this forum. Prior to its inception, we all pretty much lived in a vacuum on these issues.

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