Match Duals... Does anyone really do this?

I believe that sometimes people understand that there are suggestions and recommendations concerning the proper care and maintenance of tires but all too often they just don't bother to follow the published guidelines as they feel it may be too much work or some guidelines are simply put out to get people to buy more tires.

Let's look at one example. I have previously covered the importance of "matching" dual tires by measuring the Outside Circumference (OC) and ensuring that each pair are within 3/4" for this measurement. You can read THIS post and THIS one for more details. But is this practice followed by truckers who may put 100,000 miles on their rigs in just a year or so? I think this might help you believe that the practice is followed.

Here are a couple of pictures I was able to capture at a stop-light.

As you can see from these pictures, this driver has matched the level of wear on the front pair of tires. Now this isn't absolute proof that the OC was matched but given the relatively even wear rate of truck drive tires it's reasonable to assume the change in tread depth results in a similar change in OC.

You want to match dual tire dimensions as if one tire is significantly larger than the other it will be forced to carry more load than the smaller tire. This will result in higher operating temperature which is not good for long-term tire life.

While measuring tread depth and doing some calculations may seem reasonable this practice ignores the reality that all tires "grow" in use so a new tire and old used tire will actually measure different if you do an actual OC measurement than if you just do calculations based on tread depth measurement done to the 0.01" accuracy.


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How to match dual tires when replacing one new tire.

Had a question on my video on matching Dual tires.
"I get the importance of size-matching, but how are you going to get an adequate & accurate measurement of the remaining dual while it's still on the truck or coach? It'd be expanded under pressure, somewhat distorted where it presses against the ground, and would have some wear on it. Is one supposed to compare a tire under those conditions to a new, unworn, un-inflated tire & expect them to be within 3/4?"

Well I can understand the bit of confusion as "Dual Matching" can be a challenge when trying to replace one tire on an RV. Lets work through the process.
First off we need to be sure we are doing the correct thing in replacing just one tire from a dual pair. Normally people have one dual tire fail due to some form of damage and subsequent air loss. In the past, before TPMS, the driver would continue down the road till someone waved him over pointing out thr tire failure. This meant that for some unknown number of miles the remaining dual had been operated at 100% overload. The general guideline would be that you need to consider the remaining tire to have had its structure damaged and it should be replaced. There are many documented cases where only one tire was replaced and the damaged tire fails a few miles to a few weeks later. At that point the driver acts surprised there was a failure.

Today we have TPMS available so that in the case of a puncture or other slow leak the driver may get adequate warning so he can stop before the tire with low inflation suffers a catastrophic blowout because it was run with 20% or greater air loss. In this case the fully inflated tire may not have been overloaded for more than a mile or two as the RV moved to a safe place to pull over, so after internal and external inspection it may be safely retained and put back in service.

Now the RV owner is confronted with the challenge of how to properly match tires for dual application. The 3/4" circumference is the correct maximum difference for tires and tire growth and wear can make it a challenge to meet the goal.

When measuring a tire for dual matching it needs to be off the vehicle and fully inflated. If the tire is brand new you can expect even radial tires to see some size increase after a couple hundred miles so it is difficult to match new and used and get correct numbers.

The solution if you are replacing just one tire would be to confirm your front tires meet the 3/4" difference and then place a new tire on one front and the old tire on the other front position.

Meeting the 3/4" max difference is critical as ignoring this figure can result is a failure down the road even if the tire has been properly inflated.

How to replace a tire in "Dual" application

First off if you have a towable RV and your tow vehicle only has four tires you can go get a cup of coffee as this post doesn't apply to you. However, the information here can still be important to know and pass on if you ever happen to be sitting around a campfire and hear someone talking about replacing a failed dual tire.

Now lets be sure everyone understands what is meant by "Dual" application vs "Tandem".

Tandem means one following or behind the other. Most trailers have either two or three axles in "tandem" and each axle has just two tires on it. This utility trailer has tandem axles.

Dual means two tires side by side on the same end of an axle. Most Class-C and Class-A RVs have Dual tires in the rear drive position. A drive axle with dual tires has four tires on it. This version of "Bigfoot" has dual tires on each axle.

 Ok we have definitions behind us. This post will address the proper way to match a pair of tires for dual application. This is very important, as mismatched duals can result in overloading and in extreme cases could lead to tire failure.

If you are buying new tires you should find that all the tires are almost identical in OD when first inflated. If you were to accurately measure the tires after a few hundred miles use you would find that the tires have "grown" or stretched out a bit as they were heat cycled, but again the tires would be very similar in dimensions.

If you were going to try and measure the OD of a tire you would also learn that it takes special tools to get a direct measurement. The best way to measure how big a tire is is to measure the circumference. You can do this with a standard tape measure. You don't even need to calculate OD from the measured circumference as I will give you the guidelines based on circumference. You just need to be sure you keep the tape at the center of the tire.

Here I have affixed one end of a standard tape measure to the center of an inflated tire using masking tape.

I next carefully roll the tire around, keeping the tape taught and in the center.


When the tape comes around I can get a fairly accurate measurement of the circumference. In this example I didn't use the end of the tape as the tang doesn't lay flat, so using the 3" mark I see 95" for an OC of 92.0 inches.

Now you should match tires +/- 3/4" of circumference in dual application. 


If placing new tires in dual application you do not need to do this measurement but if trying to match used tires or a new tire with a used tire this matching is important.


BOTTOM LINE:
If you do not match the tires the larger tire will be forced to carry an increased load and in extreme cases can result in failure.