loquin
Well-Known Member
If an auto weighing 3200 pounds, loaded, has 32 psi tire pressure, the area of rubber in contact with the road is 3200 pounds/32 pounds per square inch, or 100 square inches. That is therefore, an average of 25 square inches per tire. If the load weight goes up or down, the tire surface area goes up or down, accordingly.
An auto tire (assuming a 6 inch wide tire face) with the above assumptions, traveling at 60 miles per hour, is moving at over 1000 inches per second. This means that a point on the tire face is only in contact with the road surface for about 0.004 (4 one-thousands ) second. A slick auto tire would have to push water on the road, up to three inches to get it out of the way. However, if you manufacture groves in the tire, this distance is shortened considerably, which is why this is done in auto tires.
Now, lets talk about a bicycle. Road bikes are run at high pressure - 100 psi (or more.) For a 170 pound rider, and a 30 pound bike (heavy for a road bike, I know,) that's 200 pounds of weight, divided by 100 pounds per square inch, means that only 2 square inches of rubber in contact with the road, or 1 square inch per tire. Cruisers & mountain bikes, with their fatter tires, are often run at about 50 psi. For the same weight of bike/rider, we're looking at just 2 square inches per tire, in contact with the road surface in this case. Since the bike tire is rounded, you end up with (as one site mentions) a canoe-shaped surface. (it's actually an flattened circle, or ellipse.)
However, a bicycle tire has a MUCH narrower 'face' in contact with the road than does an auto. A road tire might only have a half inch 'face' in contact with the road, whereas a balloon tire would have an inch or so. And, at 25 miles per hour, while a point on the tire would be in contact with the road for about .005 seconds, the average distance to move water is only a quarter inch for a road bike, and half an inch for a balloon tire. But since the tire surface in contact with the road isn't rectangular (like a car) but 'pointed,' similar to the bow of a ship, the bike tire is able to squeeze the water out from under the tire MUCH more easily. Kind of like an Olympic diver going cleanly into the water, versus me and my belly-flops!
Sheldon did present the results of the testing. The speed needed to hydroplane is approximately 9 times the square root of the tire pressure (speed in knots, pressure in psi.)
An auto tire (assuming a 6 inch wide tire face) with the above assumptions, traveling at 60 miles per hour, is moving at over 1000 inches per second. This means that a point on the tire face is only in contact with the road surface for about 0.004 (4 one-thousands ) second. A slick auto tire would have to push water on the road, up to three inches to get it out of the way. However, if you manufacture groves in the tire, this distance is shortened considerably, which is why this is done in auto tires.
Now, lets talk about a bicycle. Road bikes are run at high pressure - 100 psi (or more.) For a 170 pound rider, and a 30 pound bike (heavy for a road bike, I know,) that's 200 pounds of weight, divided by 100 pounds per square inch, means that only 2 square inches of rubber in contact with the road, or 1 square inch per tire. Cruisers & mountain bikes, with their fatter tires, are often run at about 50 psi. For the same weight of bike/rider, we're looking at just 2 square inches per tire, in contact with the road surface in this case. Since the bike tire is rounded, you end up with (as one site mentions) a canoe-shaped surface. (it's actually an flattened circle, or ellipse.)
However, a bicycle tire has a MUCH narrower 'face' in contact with the road than does an auto. A road tire might only have a half inch 'face' in contact with the road, whereas a balloon tire would have an inch or so. And, at 25 miles per hour, while a point on the tire would be in contact with the road for about .005 seconds, the average distance to move water is only a quarter inch for a road bike, and half an inch for a balloon tire. But since the tire surface in contact with the road isn't rectangular (like a car) but 'pointed,' similar to the bow of a ship, the bike tire is able to squeeze the water out from under the tire MUCH more easily. Kind of like an Olympic diver going cleanly into the water, versus me and my belly-flops!
Sheldon did present the results of the testing. The speed needed to hydroplane is approximately 9 times the square root of the tire pressure (speed in knots, pressure in psi.)
For miles per hour, multiply knots by 1.151, or, change the constant from 9 to 10.35Calc Knots said:
Calc MPH said:
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