# Re: Truck tire explodes at 220 mph?

On May 13, 6:58?am, "Rocky" <wook...@xxxxxxx> wrote:

If you use a differential and keep one drive wheel from turning the other
drive wheel will spin twice as fast as the speedometer shows.

What was said above was, if you keep 3 drive wheels from spinning the last
wheel will spin 4 times faster than the speedometer shows.

That does NOT compute from my understanding of how an open
differential works. It's intended to allow the axles to turn at
different speeds, but it winds up dividing the available torque
between the two axles.

The ring and pinion gear have a fixed ratio, 4 to 1 (or whatever).

The final drive ratio NEVER changes, regardless of how many wheels are
off the ground, because the gears are made of steel.

The ring gear holds the bearings for two bevel gears that engage other
bevel gears that drive the two axles. The bevel gears also have a
fixed ratio that never changes because they are made of steel.

When the vehicle makes a turn, the outside wheel has to turn faster
than the inside wheel and the inside wheel has to turn slower, and the
differential allows the wheels to do that, while also dividing the
torque available at each wheel.

In vigorous sporty car cornering, all available torque is applied to
the outside wheel when body roll lifts the inside drive wheel off the
ground, the inside wheel may spin up a bit, but the outside wheel
stops driving momentarily if you have an open differential.

The arc that the vehicle is following suddenly increases radius
because the outside rear wheel isn't driving and the weight transfer
stops, and the inside rear wheel transfers whatever angular momentum
it has gained to the pavement and the car will yaw and roll a bit as
everything stabilizes.

Personally, I have never looked at a speedometer while my car was
acting like it was going to turn over and it was yawing and rolling
all over the road, but neither an analog nor a digital speedometer
would have time to react in this situation, the analog speedo has to
overcome internal inertia of the outer drum and the digital speedo
isn't designed to change the display that quickly.

But, back to the idea of the rear wheel turning so much faster. As I
said above, with the gears being made of steel, they cannot change
their ratio and allow the wheels to turn any faster than the engine
can turn them.

The engine cannot turn a wheel that is off the ground any faster than
it can turn the same wheel when it is on the ground, but it can
*accelerate* that wheel a whole lot faster.

Suppose you have a sporty car like like that little Honda roadster
that has a red line at 10,000 RPM and 4th or 5th gear is 1 to 1.

If the ring and pinion ratio is 4:1, the axles will be turning a
maximum of 2500 RPM in the 1 to 1 gear, and neither axle will ever
turn faster than 2500 RPM, no matter what happens, as long as the
engine never exceeds 10,000 RPM.

Suppose the rear wheel diameter is 24 inches. The wheel turns 840
times in a mile. So the rear wheel turns 840 RPM at 60 MPH, but it can
turn a maximum of 2500 RPM at the engine's redline in the 1 to 1 gear.

2500 divided by 840 = 2.975.

60 MPH X 2.975 = 178.5 MPH.

The little Honda roadster probably doesn't have the power to go that
fast because of aerodynamic drag and rolling resistance, but if you
jack up the rear end, the only resistance to the engine's power will
be the inertia of the drive train, and the wheels will accelerate
rapidly to 2500 RPM.

.

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