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Old 08-11-2015, 03:25 AM   #1
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tire pressure

Getting ready for a big coast to coast trip and I'm wondering what's the best tire pressure for long highway driving on my trailer. The Max recommended tire pressure on the tire or something different. I know the pressure increases when driving and the tires heat up. I want to do all I can to avoid any blowouts on our Columbus 320RS fifth wheel.

Thanks,
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Old 08-11-2015, 03:29 AM   #2
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The Max recommended tire pressure...
Five words nailed it.
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Old 08-11-2015, 03:33 AM   #3
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Thanks.
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Old 08-11-2015, 10:23 AM   #4
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From trailer I need to know next.
Howmany axles and howmany total of tires on the road.
for instance 3 axles dual load has 12 tires on the road, as exagerated example.
Then need to know GVWR and GAWR's , so what maximum allowed totalweight of trailer ( GVWR) and maximum allowed on the axles per axle.
then need to know if you use a weight distribution Hich wich devides the load different over the wheels of combination.

From tires I need to know 3 things.
1. maximum load per tire or Loadindex
2. Loadrange or kind of tire to determine the AT-pressure wich is not the maximum pressure of the tire.
3 speedcode of tire , for instance N is maxspeed of 140km/86m/h and ST is speed for wich maxload is calculated 65m/104km/h.

Give all that and I will put it in my made calculator and give a picture of it in my answering post.
I can calculate for you a highest pressure for on the road with still acceptable gripp and comfort (so no bumping) , and give a lowest pressure at wich the tires wont damage at the speed you ride at max( so also give that maximum speed you use and wont go over for even a minute).
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Old 08-11-2015, 10:34 AM   #5
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Five words nailed it.
Agreed. Running anything less is for the comfort of occupants and a trailer has none (or shouldn't! ).
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Old 08-11-2015, 10:40 AM   #6
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Originally Posted by paluka44 View Post
Getting ready for a big coast to coast trip and I'm wondering what's the best tire pressure for long highway driving on my trailer. The Max recommended tire pressure on the tire or something different. I know the pressure increases when driving and the tires heat up. I want to do all I can to avoid any blowouts on our Columbus 320RS fifth wheel.

Thanks,
I am sitting at the FROG rally today and this question was asked at a seminar on suspension and ride components. The answer given by the MORryde spokeperson was weight the trailer and find each tire load on the trailer. Used the tire inflation guide from the tire manufacture to provide the proper air pressure. If you do not know the individual tire loads than inflate to the max inflation pressure on the side wall of your tire, cold.

He even stated and I agree that each trailer will have different tire loads do to the items that you carry in the trailer, so you either weight the trailer or use the max inflation pressure.
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Old 08-11-2015, 11:33 AM   #7
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If you have tires of the original load range then I agree 'max sidewall pressure' or as near to it as makes no difference. This is because, in almost every case, the load capacity of each of the components of the suspension is sized to be no more than required to support (GVWR-(tongue or pin))/(axles*2) and the max tire pressure is usually required to meet that.

In my case, I was uncomfortable with my LRC tires being ~97% loaded and so changed to LRD in the same size which now are about 82%. The LRD tire's manufacturer's inflation/load chart shows that the original LRC spec pressure (50psi) is still sufficient to run the load but the max inflation pressure is 65psi. So I strike a sort of happy medium... 62psi cold and they usually get up to about 67 or 68 (according to my TPMS) when running 65mph on the highway in the heat of the day. A warm tire being a bit over max pressure is fine - that's planned for.

You probably won't have the problem of adjusting the pressures on a summer E-W trip like I had on my winter N-S trip. Leaving Canada in mid February at about -5F, by the time I got to Miami that original 62psi would have blown the tires off the rims if I hadn't aired-down. Conversely, what I had aired-down to in the deep south would have had them running on the rims (or underinflation blowouts) if I hadn't aired-up each day on the return trip. Moral of the story: a good tire pressure guage used every morning and a portable compressor should be in your mandatory carry-items and a TPMS is strongly recommended.
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Old 08-11-2015, 12:25 PM   #8
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I suggest a review of what the tires actually say.
I think you will read "xx Psi at Max Load of yyy pounds" and you will not see "Max xx psi at yyy pounds load" (ya I'm not mentioning the kpa or Kg but you should understand)

If you have a Motorhome then doing the weight then look up the load to learn the minimum CIP then that's the normal approach but if you have a multi axle trailer you might ask yourselt why trailers seem to have so many more tire failures earlier in life than seen on Motorhomes. This even when LT type tires are used on the TT.

The reason is the significantly higher 'Interply Shear" force placed on the tires whenever you turn. The trailer tires are dragged around every corner or curve as they can't have their centers rotate about a centerline that points to the center of the turn radius.

This results in tire structural forces that try and tear the belts apart.

You can Google 'Tire Interply Sherar" if you want to see the engineering analysis.

The best approach for TT is to weigh the unit to be sure you are not overloading any individual tire.
THEN set the CIP at the pressure molded on the tire sidewall associated with the tire max load capability.
It would also be a good idea to do what Dave S. did and ensure you have at least 10% with 15% better "reserve Load" i.e. tire capacity vs measured individual tire loading.

Also a comment on Max Speed.
If you have ST type tires and there is no speed symbol on it the max speed rating is 65 mph. This is clearly stated in industry standards books Think of this like engine redline on a tach
If you have a speed symbol letter on ST or LT type tire you are still limited to 75 max in RV use as this is a limit that is in published literature from many major tire companies.

If you choose to ignore this advice, I suggest you not bother to complain when you have a tread separation.
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Old 08-11-2015, 02:53 PM   #9
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My trailer tire sidewalls say 'Max load xxx at yyy cold' (with xxx in both kg and lbs and yyy in kPa and psi). They don't actually say a max inflation pressure (which I was surprised to not find). I guess that's to be implied by the yyy - you can't get more xxx by going higher than yyy and consistently live to tell the tale.

My passenger car tires say 'Max load xxx/Max press yyy' on separate lines (essentially as independent terms) and nothing about that being a cold pressure. Both these are Goodyear tires so no standard even within the same company. My TV LT tires say 'Max load xxx at yyy Max press' (again, nothing about 'cold') and my motorcycle radials read essentially the same as the trailer tires (they are radials too). So no real standardization in that statement across manufacturers let alone within a single manufacturer.

But I think we're saying the same thing - there is always a maximum pressure never to be exceeded and its OK to inflate to somewhat less than that (car manufacturers recommend that all the time). Its just that the load capacity is going to be less than the max load as the pressure goes down from the max but the ride may improve to a point (not that ride matters much to a trailer).

If the manufacturer provides a load/inflation chart, that is going to be calculated for the optimum contact patch size/deformation and weight distribution (psi of the rubber on the road) over the contact patch and is generally calibrated for best tire life and wear.

But without that chart and without a scale its hard to guess your optimal psi... so setting to max psi and possibly compromising wear and ride is always going to be better for safety than erring on the underinflation side. And safety is always the overriding factor.

P.S. An interesting side effect of this sidewall-reading exercise is that I was surprised to learn that both my Chinese trailer LRD tires and TV LRE tires (Bridgestone made in USA) have the same body construction: tread 2 poly/2 steel/1 nylon, sidewall 2 poly. Somehow that makes me feel better about the trailer tires.
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Old 08-11-2015, 05:21 PM   #10
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Quote:
Originally Posted by DaveSchwartz View Post
My trailer tire sidewalls say 'Max load xxx at yyy cold' (with xxx in both kg and lbs and yyy in kPa and psi). They don't actually say a max inflation pressure (which I was surprised to not find). I guess that's to be implied by the yyy - you can't get more xxx by going higher than yyy and consistently live to tell the tale.

My passenger car tires say 'Max load xxx/Max press yyy' on separate lines (essentially as independent terms) and nothing about that being a cold pressure. Both these are Goodyear tires so no standard even within the same company. My TV LT tires say 'Max load xxx at yyy Max press' (again, nothing about 'cold') and my motorcycle radials read essentially the same as the trailer tires (they are radials too). So no real standardization in that statement across manufacturers let alone within a single manufacturer.

But I think we're saying the same thing - there is always a maximum pressure never to be exceeded and its OK to inflate to somewhat less than that (car manufacturers recommend that all the time). Its just that the load capacity is going to be less than the max load as the pressure goes down from the max but the ride may improve to a point (not that ride matters much to a trailer).

If the manufacturer provides a load/inflation chart, that is going to be calculated for the optimum contact patch size/deformation and weight distribution (psi of the rubber on the road) over the contact patch and is generally calibrated for best tire life and wear.

But without that chart and without a scale its hard to guess your optimal psi... so setting to max psi and possibly compromising wear and ride is always going to be better for safety than erring on the underinflation side. And safety is always the overriding factor.

P.S. An interesting side effect of this sidewall-reading exercise is that I was surprised to learn that both my Chinese trailer LRD tires and TV LRE tires (Bridgestone made in USA) have the same body construction: tread 2 poly/2 steel/1 nylon, sidewall 2 poly. Somehow that makes me feel better about the trailer tires.

OK a number of points.
Different type of tires (PSR, LTR, STR, TBR) each have different regulations. I covered these in detail on my blog post "When is Minimum inflation the Maximum inflation?" of July 3 2015.
Sorry but not allowed to post the direct link. These regulations are standardized by tire type and I covered that in the post.

CIP vs hot pressure. Tire engineers always mean "cold" i.e. when tires are at ambient temperature, not warmed by the sun and not being warmed by being driven more than a mile or so in the last couple of hours.

Tire construction. Unless you look at "Commercial" tires or Truck Bus Radials you will see sidewall of 1 or 2 ply of Polyester and tread with an additional 2 ply of steel. Some have an additional ply in the tread of Nylon, some don't. It's also possible that there is a partial ply of Nylon over the belt edges but this would not be listed on the tire sidewall. It may or may not be identified in sales literature.

There is also sometimes a Safety Warning with a statement about tire seating pressure of 40psi max which is not the same as the CIP once a tire is seated.

Yes this can be confusing but I cover this and many more variations on questions about tire inflation in my blog.
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