By: James R. Davis
The other night I was discussing tires with a non-motorcyclist friend of mine. He is a bright attorney who is well schooled in physics and logic. Imagine my surprise when I learned that not only were a few tire facts not understood by him, but that most of my motorcyclist friends, whose lives depend in no small part on their knowledge of our sport, have the same blinders on.
When I told my lawyer friend that motorcycles can almost always stop more quickly than cars, I did not get the typical agreement based on the popular misconception that this is true because cars are heavier than motorcycles. Instead, he challenged that fact based on the ‘increasing popularity of ABS system on cars.’
- Stopping distance is not determined by weight! You will recall that Galileo (I originally said ‘Sir Isaac Newton’ in error) crushed the popularly held belief that heavier objects fall faster than lighter ones. Falling is an acceleration. Braking is simply a negative acceleration.
- ABS was not designed to enable you to stop more quickly. (See Anti-lock Braking Systems (ABS))
As we got on with our discussion of tires he happened to mention that he had just purchased a set of new tires for his car and that he expected to get about 60,000 miles from them before they wore out. This seemed to please him very much. I’m afraid I may have caused him to have second thoughts on the matter as our conversation continued.
I told him that I had a set of ‘high mileage’ (by motorcycle standards) touring tires on my motorcycle and that I would be happy to get anything in excess of 20,000 miles from them. He was amazed to learn that I paid nearly as much for my rear tire alone as he spent for all four of his car tires, particularly since I would have had to replace them three times before he replaced his once.
It must have sounded defensive on my part when I told my friend that if the manufacturer of my motorcycle tires was to announce a new tire with an expected life of 60,000 miles they would have trouble selling them.
I explained that the reason a motorcycle can stop faster than a car is that its tires provide better traction than automobile tires. Our traction is better because the rubber compounds our tires are made of are softer and thus ‘stickier’. Beyond their price in dollar terms, better traction tires cost mileage – they wear out faster than harder tires. [Obviously, stopping is a function of your brakes.
With the assumption that your vehicle was designed with brakes that are more than sufficient to lock (stop your wheels from turning), regardless of speed, then the stopping of your vehicle must be limited by available traction, not brakes.]
(It is a curiosity to me that many motorcyclists brag about the mileage their tires provide actively seeking to buy higher mileage tires without a thought to relative traction provided. I suppose that they believe the higher mileage is provided without traction cost – but is it?)
However, given a choice between buying tires that lasted three times as long or those that allowed faster stopping, it would be a no-brainer for most motorcyclists. If a motorcycle tire could be made that provided 60,000 miles of life, by virtue of being made of harder rubber compounds, the manufacturer could not give them away.
Well that concept got the attention of my lawyer friend! Can’t you just imagine the thoughts running through his head:
[blockquote]Your honor, my client cannot be held responsible for rear-ending the car in front of him because, as the skid marks clearly show, he tried his best to avoid the accident. The problem is not that he was following too closely at all. The problem is that he didn’t quite stop in time, which is obviously the fault of his tires.
Mind you, your honor, that my client spent premium dollars for what he believed were premium tires. They, after all, were advertised to last 60,000 miles. The tire manufacturer is clearly at fault for failing to tell my client that these ‘premium’ tires have less traction than do lower mileage, less expensive tires.[/blockquote]
The problem is that there is no way for a consumer to know what level of traction one tire has as compared to another. There is no indication on the tire sidewall, for example, that lists its traction, and there does not appear to be standards available to the consuming public that facilitates comparisons. It is entirely possible, though unlikely, that a rubber compound can be developed that provides longer life without sacrificing traction.
But there are other reasons that most motorcyclists should consider before buying a tire with a long wear life expectancy. For example, about every other tire that I have had to replace was the result of tire damage rather than normal wear and tear. (three nails in one tire cannot be safely plugged, severe sidewall cracking along with similar cracking in the tread grooves, cupping, etc.)
Maybe the discussion above explains why no manufacturer has announced a 60,000 mile motorcycle tire – yet. I wouldn’t buy one if they did.
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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)