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Saturday, July 30, 2011

Gage Accuracy, Rotation, Alignment, Fuel Economy

Went back and tried to review comments and questions. I know some have been answered in depth but some others maybe not enough to make the point clear so here are some short replies and thoughts.

May 15 Quampers asked about tire gauge accuracy.
Quampers, I currently travel with two digital gages that are accurate to 0.5 psi when compared to ISO certified pressure gauge in a tire testing lab. I can compare readings between these two gages and as long as they are within 1.0 psi or less of each other I can be pretty confident they are still giving me the correct pressure. I will have the gages checked on a two year cycle even if I find no change in their readings. These are the gages I will use to compare to others at my Tire Basics seminars. FMCA at Madison, WI Aug 11 and at Gypsy Journal Rally at Celina, Ohio Sept 27.

June 25 Tom asked about tire rotation and where to put the newer tires if you don't buy a complete set.
I have heard it said by others that if you don’t have a problem with irregular wear on your RV you don’t need to do an alignment or rotate tires. I guess that may be true as long as 1. The manufacturer of the coach bothered to do an alignment at the end of the assembly line and 2. You keep a close eye on the wear of your tires.
Now I can tell you that in my experience it appears that some manufacturers may be cutting corners when it comes to spending the time to set the alignment. After all your warranty is only good for a year and in that time most RV drivers will not get enough miles on their tires to see alignment problems. The second point is that even if you do complain the mfg will probably tell you it is your responsibility to ensure the coach or trailer is in align, even though there is no mention that as a new owner one of the first things you would need to do is to drive your new rig to a heavy duty alignment shop and do the work the manufacturer should have done in the first place.
When it comes to the question of where to place the new tires there are a couple of things you must consider. If your RV has dual tire position you need to be sure the Outside Circumference difference on a pair of tires in dual position is ¾” or less otherwise there will be more load transfer from one tire to the other than is advisable. Not Outside Diameter buy OC which is a more accurate measurement to get if you don’t have special tools. So for Tom it will depend on how many tires he needs to replace as to where they go.
When I recently rotated my tires because of miss alignment on fronts.
They looked like this in inner shoulder and this on outside shoulder

I measured all seven tires (including spare) then put the most worn tire as the new spare. It worked out that the still new original spare ended up on the front and I was able to match my duals to within ¼” for each pair.

Now if we are talking about a passenger car the two new tires should go on the rear to give you better wet traction and decrease the potential of a rotating skid and spin-out in an emergency. This is less of an issue for heavy RVs due to the weight cutting through the water better.
On multi axle trailers I would put the new tires on the front as this location is less likely to suffer a puncture and if you are going to lose a tire due to puncture I would rather lose the older tire. Hope this didn't confuse too many people. As you can see there are a number of things that need to be considered.

June 29 Bob asked about LRR or Low Rolling Resistance
LRR spec tires are designed to trade off a number of performance characteristics to improve fuel economy. Rolling Resistance is the force it takes to roll a tire under load. The more force the more energy needed to make the vehicle go down the road. I have not heard of LRR tires in RV sizes yet as most of this work is aimed at Hybrid cars getting 40 MPG or more.

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