The Physics of Force - Tire Review Magazine

The Physics of Force

Without the braking force generated by tires, vehicles can’t stop. Well, actually

they can, but it’s hell on wheels – and oil pans.

Without cornering forces generated by the tire, a vehicle can’t handle. Well, actually it can if you consider a guardrail as part of a steering system.

Practically speaking, without the essential physical forces generated by tires, a vehicle can’t properly stop, corner or accelerate.

Remember this thought by a GM engineer a few years back? "The tire is the most complex, scientific instrument on a vehicle," he said. To bad OEMs only want to pay bottom dollar for them, but that’s another story.

Something has to push or pull against the road surface to get a vehicle to move. In every case the answer is the same: tires.

While carmakers like to brag about the crisp handling, snappy acceleration and effective braking of their steeds, a vehicle can’t create cornering, braking or acceleration forces. It can only take advantage of the ability of the tires to do those jobs.

That’s why vehicle engineers can’t develop a suspension system, for example, until they factor in tire performance characteristics.

The physics of generating traction on dry pavement are different than those for traction in mud or sand. And the forces needed to drive smartly through a turn on wet pavement are generated differently than those needed to generate acceleration (straight-line traction) on dry pavement.

Applying basic physical laws as they relate to tires and vehicles turns up some interesting points with your customers. A little E=MC2 salesmanship, if you like.

Tires, after all, are the only part of a vehicle in contact with the ground. For that very reason, tires are the only vehicle component that can generate the forces necessary to get the full measure of performance designed into a vehicle.

Auto engineers have been using tires to improve vehicle performance (and occasionally hide a few flaws) for years.

Anti-lock brake systems are a prime example. ABS allows a vehicle to approach the maximum traction (braking) forces of the tires in all situations with any driver. The advantage offered by ABS is based on the principle that a rolling tire almost always generates more braking force than a sliding tire.

Showing Their Slip

The key point is that tires do all of the work. But just to make things clearer, tires generate those forces by slipping.

Up to a point, the more a tire slips the more force it generates. However, when pushed beyond this limit, a tire quickly reaches a point where the forces actually drop off.

Put another way, when a tire is not sliding or slipping on the pavement it is said to be "free rolling" and exhibiting "0% slip." There is no braking force at 0% slip. At the other end of the spectrum, a locked-up tire that has stopped rolling is at 100% slip.

What happens next? Stopping distance increases and steering is nearly impossible. At this point, the driver can turn the steering wheel but nothing happens. The vehicle goes straight even if it’s in the middle of a turn.

Again for clarity, there is an optimum amount of slip for each situation. It is the job of vehicle and tire engineers to balance all these physical forces. Given the broad range of vehicle and tire offerings, plus road and weather conditions, the task can be daunting.

The idea is to remain true to the laws of physics while accommodating variables in vehicle characteristics and driving habits. Ultimately, it falls to the driver to understand what’s happening and react appropriately. And that’s where things can go bad.

Engineers to the Rescue

Since no everyday driver can keep a tire slipping at its peak braking force, automakers developed ABS and traction control systems to maximize handling and reduce tire wear. And keep drivers off the guardrails.

Typically, tire squeal is the first signal a driver notices that slip is even taking place. In every instance, knowing the limits of a tire’s cornering force is vital for safe driving. The good news is that tires working with ABS and traction control systems help keep drivers out of trouble.

The biggest benefit of ABS is its ability to keep tires rolling even during panic braking. ABS doesn’t allow the brakes to lock, thus providing a higher level of braking forces for the average driver.

More importantly, this allows the driver to steer and maintain control of the vehicle, even during all-out panic braking. ABS doesn’t change the laws of physics. Ice is still ice. But the beauty of ABS is that it allows the tires to operate near their peak efficiency. Of course, this peak declines as surfaces get progressively slipperier.

No Trade Outs

Basic physics says that a tire can only generate a certain amount of force in any direction. If a driver uses up all of a tire’s available force in braking, there’s none left for cornering. That’s why sliding tires can’t generate any cornering force.

Because ABS cycles the brakes to prevent lock-up, motorists get some of both – braking and cornering force ®“ but at reduced levels.

Tires allowed to operate near their peak braking force by ABS will yield the shortest stopping distance for average drivers in most situations. This is why most of your customers probably believe ABS is only designed to shorten stopping distances. While this is true, there is more to the story.

ABS also compensates for vehicle load changes, a condition common to pickup trucks as their beds go from fully loaded to party loaded to empty.

An empty pickup truck has less load on the rear tires, which means less braking capability in the back. To compensate, the vehicle sets the brake bias so the front brakes receive more pressure than the rears.

If the bed is loaded with sand, the rear tires can generate more braking because they are under more load. But, remember, the brake system is set up to send more braking to the front tires. This can cause the front tires to lock up under heavy braking since the rears are now braking much closer to their full potential.

However, if the vehicle maker had set the brake bias for the loaded condition, the rear tires would have locked up under heavy braking. Result: A potential spinout.

ABS to the rescue. Because ABS modulates each wheel individually it can compensate for changes in front/rear braking capability, according to load.

Spinning the Wheels

In one sense, traction control is ABS in reverse. The sensors required by ABS to detect wheel lock-up under braking are used in a traction control system to detect wheel spin under acceleration.

Just as a locked wheel generates no braking force, a freely spinning wheel generates little acceleration or cornering force and can cause loss of control.

Excessive wheel spin activates the traction control system, which applies the brakes or reduces engine power (or both) to reduce wheel spin and allow the tires to generate a higher level of traction (or acceleration) force.

ABS and traction control systems are major reasons why customers enjoy a vastly improved driving experience as they move across snowy, rainy and icy surfaces.

Tire technology has had to stay ahead to permit ABS and traction control systems to perform.

Today’s tread compounds are flexible enough to offer cornering, braking and acceleration forces at low winter temperatures, and durable enough to withstand the rigors of high summer temperatures.

Further, tread compounds are strong enough to withstand the forces that act on open, highly kerfed tread designs during cornering, braking and acceleration.

Thanks to computer-assisted design, complete tire structures have been redesigned to produce even higher levels of forces required by today’s vehicles.

With the emphasis on better handling, shorter stopping distances and enhanced control regardless of weather conditions, it’s a good time to be a driver. Just make sure your customers know that without tires they really aren’t going anywhere.

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