Tires, E rate Load vs. tire pressure - Diesel Forum - TheDieselStop.com
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post #1 of 7 (permalink) Old 05-21-2014, 11:55 AM Thread Starter
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Tires, E rate Load vs. tire pressure

On my 2011 SD, the placard at the doors says front tire pressure 65 psi while rear 80 psi.

In discussing tire pressure vs. load with tire mfrs, they suggest to pump them up to what the cold pressure rating is stamped on the side of the tires. In this case, the E loaded tires stamp is 80 psig. When asked tire mfr if can use lower pressure, they state the tires could run "hotter".

Now I understand the tire companies pressure vs. load charts well enough to know if lighter loads, prob can use less air pressure. And some SD owners do let air out when not loaded up.

Yet Ford placard has 65 and 80 psi. Guess Ford not concern with "hotter" tires on front, as going against the tire mfr suggestion to inflate to max cold pressure marked on the tires. But then maybe not knowing how the truck/tires are used, the tire mfr offer a blanket statement to cover their butts.

I do tow a fiver with this SD, and careful to not overload the truck over its max allowable carry capacity - includes the fiver hitch point wt, passengers, fuel, other misc items in the truck. I do run the rear tire at 80 psi cold pressure due to the fiver hitch weight and other weights. But the front at 65 psi has me baffled.

Any comments? Any truck or tire specialist out there as why Ford placards 65 for front tire?
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post #2 of 7 (permalink) Old 05-21-2014, 03:08 PM
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Have you checked your axle weights at a scale with the fiver hooked up? I would think the front axle weight is less than the rear axle weight hence the lower tire pressure required.

This doesn't really answer your question but I thought others might find it interesting. I obtained this chart from Michelin a few years back.


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post #3 of 7 (permalink) Old 05-21-2014, 03:57 PM
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First, everyone here probably should read my article, "Tires for Towable RVs & Tow Vehicles". It's published on www.thisoldcampsite.com .
Second, when I had my SRW F-350 I towed with 75# in the front & 80# in the rear. None of the tires ran hot or showed unusual tread wear. When not loaded, I ran 65# all around. Once again, no unusual tread wear.
Third, RVing friends of mine with dually trucks & big 5th wheels air up their E rated tires to 80# because that's what's on the sidewall. All of their tires are showing accelerated tread wear in the center of the tread indicating "crowning" due to over-inflation. Some of these folks are full timers so they drive their trucks solo quite a bit without adjusting tire pressures.
What I do with my '13 model dually is pretty much what what the tire placard in the door jamb says: 75# front & 65# rears & I never adjust tire pressures for towing or solo; it stays the same all the time. The truck is used mostly for towing our big 5th wheel with some solo miles thrown in & my tires do not look overloaded when hitched up, they don't run hot when towing & they are not showing any unusual tread wear.
I hope this helps. Please let me know what you think of the article.

2013 F-350 Lariat 6.7 L PSD Crew Cab Dually, FX4, white platinum metalic tri-coat with black interior, N-Fab step bars, factory spray-in bedliner and 5th wheel hitch, trailer tow package, integrated brake controller, upfitter switches, Sync with My Ford Touch, memory group, remote start, reverse sensing with rear view camera, nav, satillite radio, Weather Tech floor mats, limited slip diff, moon roof, climate controlled seats, engine block heater and tailgate step.

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Last edited by wpetko; 05-21-2014 at 04:02 PM.
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post #4 of 7 (permalink) Old 05-21-2014, 04:12 PM
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If the tires are inflated at less pressure than the placard when not loaded to the max rating, there's no reason they should run hot. Heat comes from flexing in the tire and the lower pressure at lower loads is sufficient to keep the tires as round as they would be at placard pressure and rated load. Thus the amount of flex would be the same. As doczenith1 posted, a load/pressure chart for your tire size should be used, along with weighing each axle at the typical lightly loaded and fully loaded conditions. Then use the corresponding tire pressure for the load (and maybe a couple psi more for safety).

Also, running on tires that are inflated to the max with light loads makes for a bone-jarring ride, and stresses suspension components more than they need to be.

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post #5 of 7 (permalink) Old 05-21-2014, 06:09 PM
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Quote:
Originally Posted by klhansen View Post
Also, running on tires that are inflated to the max with light loads makes for a bone-jarring ride, and stresses suspension components more than they need to be.
I'll add that it can also be less safe. With the max inflation and light loads the tire can hop over bumps and if done so under moderate to heavy braking this can activate the ABS which could lengthen the stopping distance.

Al Bedford, P.E.

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post #6 of 7 (permalink) Old 05-21-2014, 09:48 PM
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Quote:
Originally Posted by wpetko View Post
First, everyone here probably should read my article, "Tires for Towable RVs & Tow Vehicles". It's published on www.thisoldcampsite.com .
Second, when I had my SRW F-350 I towed with 75# in the front & 80# in the rear. None of the tires ran hot or showed unusual tread wear. When not loaded, I ran 65# all around. Once again, no unusual tread wear.
Third, RVing friends of mine with dually trucks & big 5th wheels air up their E rated tires to 80# because that's what's on the sidewall. All of their tires are showing accelerated tread wear in the center of the tread indicating "crowning" due to over-inflation. Some of these folks are full timers so they drive their trucks solo quite a bit without adjusting tire pressures.
What I do with my '13 model dually is pretty much what what the tire placard in the door jamb says: 75# front & 65# rears & I never adjust tire pressures for towing or solo; it stays the same all the time. The truck is used mostly for towing our big 5th wheel with some solo miles thrown in & my tires do not look overloaded when hitched up, they don't run hot when towing & they are not showing any unusual tread wear.
I hope this helps. Please let me know what you think of the article.
I read your article. Good write up. I don't think you mentioned speed can cause a blowout. Did you just miss it or was it intentional?

Matthew

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post #7 of 7 (permalink) Old 05-22-2014, 08:21 AM
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You're right. I didn't mention that speed can cause a blowout. Truthfully, I didn't think of it because it seems so obvious to me. ST (special trailer) tires are almost all rated for 65 mph & many tow faster than that at times. One reason I forgot to mention speed related blowouts is that I run LT tires on my rig & they have a much higher speed rating. Let me quickly add that excessive speed when towing is dangerous for more reasons than blowouts. I'm probably "preaching to the choir" here, but we should stay at least close to the posted speed limit when towing. I really see no practical reason to exceed 65 mph. 'Just my 2 cents.

2013 F-350 Lariat 6.7 L PSD Crew Cab Dually, FX4, white platinum metalic tri-coat with black interior, N-Fab step bars, factory spray-in bedliner and 5th wheel hitch, trailer tow package, integrated brake controller, upfitter switches, Sync with My Ford Touch, memory group, remote start, reverse sensing with rear view camera, nav, satillite radio, Weather Tech floor mats, limited slip diff, moon roof, climate controlled seats, engine block heater and tailgate step.

2014 Lifestyle (formerly Carriage) 5th wheel 38' long with 3 slides & all the goodies.

SOLD: 2005 F-350 Lariat, 6.0 PSD, Torqueshift auto trans, crew cab, SRW, short box, FX4, ESOF, limited slip dif., tow command, upfitter switches, 6 CD changer, Lariat Lux. package, camper package, engine block heater, triple A-pillar gauge set (Autometer) with pyrometer, trans temp & voltmeter, MBRP 4" cat back exhaust w/5" dual wall chrome tip, dark shadow gray/ med. flint interior.
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