RV Tire Safety
with RV tire expert Roger Marble
Question: What effect does speed have on tire failure?
“I have an 8000 lb. trailer with four Goodyear Marathons. The truck is a Dodge 2500 diesel so I can cruise at 75 mph subject to wind and hills. How much, if any, does speed factor into tire failure? I replace tires every 4-6 years regardless of what they look like, and keep them inflated at the PSI marked on the tires.”
Answer:
Basically, increased speed means increased temperature in the critical areas of a tire. Here is a graphic showing the relative temperature in different areas of a tire, with red being the hottest and dark blue being the coolest.
When you increase the temperature of rubber you are effectively increasing the rate the rubber is aging. I believe we all understand that old rubber is not as flexible as new rubber. If it isn’t as flexible you end up with new cracks or cracks that grow every time the rubber is flexed. More cracks and larger cracks are what result in tire failure.
For the above reasons plus others, the major tire companies suggest that in RV operation tires never be driven any faster than 75 mph, and you can see this max speed stated in their literature for many of their RV tires.
But, wait – you say your tires have a “Speed Symbol” that relates to 87 or 99 or maybe even 130. Here is some information on speed ratings. Just because you have a tire with a fast symbol doesn’t mean it can be driven at those speeds for 20,000 or 40,000 miles. About the only thing you can count on is that tires with a “higher” symbol will provide better steering response than a tire with a lower rating.
Now back to the question of how fast can you drive or tow your RV. I previously mentioned 75 mph as the upper limit, but for some tires there are other things to consider. With ST type tires, as found on many trailers, we need to remember that the load capacity is based on a formula that originally specified a max speed of 65 mph. In the past, there were Technical Bulletins that advised that inflation needed to be increased (but not above the sidewall max) by 10 psi to go 70 mph. In addition, load needed to be decreased by 10% if you want to go 75. Still, there was that 75 max.
Many trailer owners know that they need to replace their tires before they wear out. If they don’t they are probably going to have some type of failure. But those same people will not replace the tires on their pickup till they are significantly worn. Why? The answer can be seen if you simply compare the load and speed ratings of an LT tire and an identical size ST type tire:
LT235/75R15 101Q LRC1985# 50 psi 99 mph
ST235/75R15 110R LRC2340# 50 psi 106 mph
So exactly what type of magical engineering has tire company X used to achieve both increased load capacity plus increased speed capacity when it is the air pressure that supports the load? If they have the ability to make that ST tire really perform at those speed and load conditions what is wrong with their LT type tires? IMO, what we are looking at is a marketing plan taking over the engineering reality.
Bottom line:
You may be able to pull the trailer at speeds above 65 mph or drive the Class-A diesel pusher faster than 75, but you will be consuming the finite life of the tire and can expect a failure before you wear it out no matter how you maintain your tires.
Read more from Roger Marble on his blog at RVtiresafety.net.
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My personal opinion, based on owning both a diesel pusher motorhome and a travel trailer, is that driving an RV any speed higher than 60 mph is stupid. I calculate fuel mileage on every tank, and I use an infrared heat gun on every tire and wheel assembly when I stop at rest areas. Temps are lower, fuel mileage is higher, tires and vehicles last longer. The End.
If you’re an RVer, what’s the big hurry?
The R in RV stands for recreational. What’s the big hurry. Speed doesn’t kill it’s just the sudden stop. Slow down, smell the roses, save a life which might possibly be your own or mine.
And a hearty “Amen” to that. Remember when, as kids, a lot of folks went out on a “Sunday drive”? Just for the fun of it. No hurries. There may be some things I’d like to change about those days, but that isn’t one of them.
You can go that fast in an emergency as we have had to do but normal travel is 60 mph. It is much more relaxing and saves tons of money in gas. Trying to control a normal auto at 75 is dicey and an RV is just plain crazy!! Slow down and it will make everything perform correctly and you will be alive longer to enjoy your RV!
Are there any real studies on the effects of speed vs load on these tires or are we just going on some assumptions? Speeds have increased over the years across the board, why should I have to go slow? And if the tire companies aren’t keeping pace then shame on them. If I have a tire rated at 130 at max load and I’m only doing 75 I’m supposed to believe they are not up to the task?
Disclaimer: If you are happy cruising along at 60 or 65 then good for you but like many things in life one size does not fit all.
Make sure to let everyone know were your driving so we can leave you lots of room.
I just know that I ran a semi for years and I went at a pace that was comfy for me and the machine that I was driving at the time.
If you are in that big a hurry…you are not stopping to smell the roses…and you are a threat to others on the road by dragging your trailer faster than the tires are rated for. I have seen too many wrecked RV’s along the roads because some fool was driving too fast for road conditions or failed to maintain their rigs. Todays society along with their fast paced lifestyle and RV’s are a bad mix.
A few years ago I remember cutting out an article that said that, for every five miles you go over 55 mph, you lose something like 3 miles per gallon. So when you’re going anything like 65 or 70 and upwards, I guess you have money to burn and don’t mind wasting gas.