Where the Rubber Meets the Road (part I)

November 14th, 2014 by Team OAM NOW / Athletic Mentors

By Terry Ritter, Team OAM Cyclist, Road Cat 2, Master’s 45+

I recently read in an automotive magazine article that a given high end racing division has 14 engineers on a team. Two of the engineers are dedicated to just tires: their selection, maintenance, and pressure settings. This isn’t a surprise as small changes in tire pressure, say 38 to 36 psi, can make a car handle noticeably different. Obviously, tires are important as they provide the actual surface contact with the ground, be it road or dirt, for the vehicle.

Just as paramount is the relationship between cyclist and his/her tires. The tire/tube system uses trapped air as a spring that supports the weight of both bicycle and rider attached. Though tire compound is important in cornering and braking adhesion, how the spring is set up likely has a bigger impact in bicycle dynamics. But how do these pressure changes play out in regards to how a bike corners and handles?

Terry tiresA spring is defined by its rate, which is another way of saying how stiff it is. The higher the pressure in the tube/tire, the higher the spring rate and greater the stiffness.  Also, the less it’ll deflect for any given force. It’ll also return from a deflection faster for any given force. If we revisit our tube/tire system as a spring, the trapped air is the actually part that provides the support for the rider.

Now, let’s come back to the car and use it as an example. If we had a little subcompact, say a Honda Civic, and we fit it with springs from a F-150 pickup truck, we’d expect it to ride very, very rough because the spring rate would be too high. More specifically, because the weight of the car is a downward force on the springs, and because they were designed to handle the greater force of the truck, the springs wouldn’t deflect as much force over any given bump, and would bounce back too quickly. We have all driven over a series of braking bumps on a gravel road that leaves the car feeling like it’s floating and we losing control. This would happen frequently in our Civic with its too heavy springs. In other words, you want this rate to match the vehicle….and the rider, which brings us back to the bike.

If a 175lbs rider were to inflate his or her tires to the proper pressure, the pressure determines the spring rate. For this heavier rider, the spring rate (pressure) needs to be higher to compensate for the greater downward force delivered by the higher weight. Like the lighter car (Honda), a lighter rider would want a lower spring rate, or less pressure. If using the same pressure as the larger rider, he or she could expect the spring to deflect faster resulting in a rough ride. This quick deflection, due to a higher than needed spring rate, also causes the tire to lose contact with the ground surface and float over irregularities. Loss of contact is rarely good. We would sense this loss as sliding or less secure grip while taking a corner, and as a result, be more cautious and slow down. One of the goals of great cornering is to maintain speed, so slowing down here is counterproductive.

And yet riders, especially smaller ones (<150 lbs), constantly run tire pretty too high. They are Civics on F-150 chassis. Instead of the tire deforming at a slower rate due to ideal pressure, it encounters surface irregularities and quickly bounces back, taking the tire off the road at the tread/surface interface and leaving the rider less confident. An additional undesirable consequence of over inflation is the change in shape and surface area of the contact patch. As the tire is inflated more and more compared to the weight pressing down on it, the tire patch becomes smaller and smaller. There is actually less rubber on the road. Again, not a good thing.

Both of these things produce a tire that gives less confidence to a rider when cornering. It’s important to find a pressure that works well for our weight, road surface, and riding style. So, riders looking to improve riding skills should also be investigating bike mechanics and specifications in addition to physical training.

In part II, I’ll go into a bit more on tire pressure and volume as it relates to bike handling.


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