| | | | | | | | |  | | | | Why there are so many tire problems for some and none for others?
When they simply fall off, you have to consider if it’s due to an obvious reason: Was the tire not on tight enough? Sometimes it’s just that simple. On the other hand, there could be a ton of factors to consider. Here are a couple.
First and foremost, the wheel mating surface – which is the back side of the wheel opposite the lug nuts -- usually is going up against a thin metal spacer we call a shim. The shim slides over the wheel studs in the wheel hub and is between what is called the "brake hat" and the wheel itself. It’s there because the brake rotor slides over the rear hub, and something has to hold it in place.
Now picture a soup bowl lying upside down on top of a dinner plate. There is a ring of bolts around the outer edge of the bowl holding it to the plate. A brake hat, which is made of aluminum, is like this inverted bowl. The brake hat has a brake rotor attached to it that is made out of steel.
The steel brake rotor is subject to thousands of heat cycles. Every time you hit that brake pedal, the brake pads hit the rotor and cause friction, which creates heat. The brake pads make immense heat. As soon as you let off, the fresh outside air cools that brake rotor down a little.
All of these heat cycles, as we call them, cause expansion and contraction. The weak link is the aluminum brake hat that sits over the rear hub on the rear housing. It isn't recommended that you bolt the wheel directly to the brake hat, so this shim we mentioned is in between.
So there are a handful of dynamics at play here: A hot brake rotor that has heat transmitted from it. A brake hat that it bolts to is sandwiched between a wheel spacer, a wheel and a rear end hub. The hub has heat from the axle inside it, which heats up the brake hat from the inside, and the brake heat from the outside. The wheel has tire pressure and side flex twisting it on corner entry. Put this all together and frankly, it is amazing that more wheels aren't falling off! Tells you what a good job these guys do on pit road.
The side load on the wheel and the force against it is immense. The impact guns on pit road, driven by tanks of nitrogen, spin the lug nuts on at tremendous speed. In pit stop practice we check the lug nuts after each stop to see how tight they are.
Of course, everyone has a different idea of how tight they need to be. I have heard everything from 50 to 90 foot pounds with a torque wrench (a tool to check tightness). With that kind of range, maybe they just weren’t tight enough?
The next thing that can happen is fairly simple and it happens to everyone. Lug nut glue. The lug nuts are pre-glued to each wheel so the nuts are already in place during quick tire changes. The problem that can stem from this is fairly simple. When you glue the nuts on, excess glue drips through the wheel and squeezes all around the hole in the wheel. When the nut gets spun onto the lug stud, some of the glue residue gets built up on the stud itself. Do this a couple times and it packs down in the threads on the stud. Although the lug is tight, it’s gripping onto a soft surface.
When everything goes back through the heat cycles and side load, all this motion loosens the glue up. Next thing you know, the lug nut is loose.
So there are a couple things we deal with on the wheel situation. In my next blog, I’ll let you know about those seemingly minor issues that can cause blown tires.
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