Tires: Tire load ratings, dual axles, google search

Question
At a recent truck show, I saw tandem axle trucks being lifted for maintenance by different brands of mobile lifts.  Typically, the lifts fit under one tire (two lifts for the rear of a truck for example).  My question is this.  For certain types of trucks, the lifting equipment is exceeding the load rating of the tire - is this a concern?  Typically the lifting mechanism contacts the tire at two points.  What exactly does the load rating mean?  For example, I saw a straight truck that when loaded, weighted about 25,000lbs. with about 18,000 of the 25,000 distributed over the rear of the truck (dual axles, 8 tires total).  I think the tires had a load rating of 6,000 lbs. each.  When on the ground, that is 18,000 lbs. distributed over 8 tires (2,250lbs. per tire) and is well below the tire load rating.  However, when the vehicle was lifted in the air using mobile column lifts, four columns were used, one under each tire.  Now there was 18,000lbs. distributed over two tires (in the rear)which clearly exceeded the rating of the tire - 18,000/2 = 9,000lbs. per tire.  Not being a tire expert, I wonder if this is practice that should be avoided?  Does the tire load rating take into account some dynamic loading as it goes down the road?  I hope you understand my question.  I have not been able to find an answer as of yet.

Answer
Michael,

I am having trouble understanding what you are trying to say.  The only way this makes sense to me is if the rear axle is only lifted by  2 of the outside tires, instead of all 8.  Put another way, one of the axles is not used in lifting the vehicle off the ground, and only the outermost pair of tires is used.

If this is true, there is another issue that also would impact this.  Are the tires resting on a flat plate - or are the tires cradled between 2 bars - round or square?

I just did a Google search and discovered that there are indeed lifts that do exactly as I described:  Lift using a cradle that contacts only 2 of the outer rear tires.

From my perspective, this is not a good idea.

Tires are designed to operate on flat(ish) roads with the loads and speeds as specified on the sidewall - and even then there are some circumstances where the load and/or the inflation pressure has to be adjusted.  I think lifting a truck using a pair of bars - even angled bars - is somewhat problematic.  If only the outer tires are used, this becomes and even worse situation, and if only one of the the tandem axles is used, plus the truck is loaded - well, this gets worse and worse doesn't it.

But I wonder if the surface area that is actually in contact with the tires might be large enough that the effect on the tire is within the bounds of reason.

Yes, I think this practice should be avoided, but I think more study is needed.  Put another way:  The way tires are rated for load carrying capacity is quite different than the way these lifts work and the load rating is NOT applicable, so this needs to be looked at differently.  Of course, this would be different for each lift and the way the tire is cradled.