Tire load limit question

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.

Karl

Moderator Emeritus
Joined
Mar 3, 2005
Posts
5,154
Location
Elkhart Lake, WI for the summer. Work at Road Amer
The maximum load stamped on my tires is 3042 lbs. for a single tire and 2778 lbs. for dual tires @80psi. Am I missing something really basic here? Why are they derated when mounting in tandem? Does it have something to do with interference between them when the sidewall flexes under extreme conditions or low inflation pressures?
 

Ned

Moderator Emeritus
Joined
Feb 1, 2005
Posts
25,107
Location
USA
That's common to rate a tire for less load in a dual configuration.  Possibly due to the heat of 2 tires in close proximity?  I really don't know, but that seems like a possibility.  The wheel spacing is set by the wheel configuration, not the tire, so I don't think that would have anything to do with it.
 

Gary RV_Wizard

Site Team
Joined
Feb 2, 2005
Posts
73,525
Location
At our Silver Springs FL home
The wheel spacing is set by the wheel configuration, not the tire, so I don't think that would have anything to do with it.

Spacing is controlled by the rim but is definitely affected by the tire, since the sidewall bulges beyond the rim, especially on the bottom.  Different tires may require different rim spacing and a tire that is OK at high inflation pressures may have a problem at lower psi becasue it bulges more (and gets hotter too).

The only explanation I have seen for derating duals is that they run hotter than singles because  there is less airflow on one side of the tire.  That makes sense, but I don't know if that is the whole story or not, even though it was in an professionally written article on tires.  I've seen too many incomplete or flat out erroneous explanations in glib web articles to be 100% confident.
 

Ned

Moderator Emeritus
Joined
Feb 1, 2005
Posts
25,107
Location
USA
The tire mfrs set the minimum spacing requirements for dual configurations and I expect it would accomodate any valid inflation pressure and load.  The temperature variable was the only other item that I could think of that is affected by the load and inflation pressure.  Perhaps one of the mfr web sites has an explanation.
 

BernieD

Well-known member
Joined
Mar 1, 2005
Posts
5,873
Location
Goodyear, AZ
Karl said:
The maximum load stamped on my tires is 3042 lbs. for a single tire and 2778 lbs. for dual tires @80psi. Am I missing something really basic here? Why are they derated when mounting in tandem? Does it have something to do with interference between them when the sidewall flexes under extreme conditions or low inflation pressures?

Interesting question Karl, never really thought about it before. One thing could be the fact that so many roads are crowned that the 2 tires in tandem will have different weights on them due to the different angle of the road to the alignment and this may provide some cushion for that. Just a SWAG.
 

Gary RV_Wizard

Site Team
Joined
Feb 2, 2005
Posts
73,525
Location
At our Silver Springs FL home
The tire mfrs set the minimum spacing requirements for dual configurations and I expect it would accomodate any valid inflation pressure and load.

The point is that if you change brand or even model of tire, the tire spacing requirement may differ. This could necessitate a change of rims to provide adequate spacing.
 

Ned

Moderator Emeritus
Joined
Feb 1, 2005
Posts
25,107
Location
USA
RV Roamer said:
The point is that if you change brand or even model of tire, the tire spacing requirement may differ. This could necessitate a change of rims to provide adequate spacing.

I think that is what I said, only different :)
 

Karl

Moderator Emeritus
Joined
Mar 3, 2005
Posts
5,154
Location
Elkhart Lake, WI for the summer. Work at Road Amer
Bernie,

Not such a WAG as you might expect. The Michelin site had a video that talked about it for all of about 5 seconds. Showed a rear view of tandem wheels with the rightmost one slightly off camber (due to the road sloping off) and stating that the uneven loading is the reason for the derating. Next year, when the tires need replacing, I'm seriously thinking about upgrading to 245/75R19.5's. I know the chassis will handle the extra capacity - in fact, the newer F53's come with the 19.5's. Thanks to all.
 
Top Bottom