February 9, 2008

How Fast Is Fast?

Dream on Dream on Dreamometer…

With the general use of GPS (Global Positioning System) technology widely spread these days, it is no wonder that such technology brings interesting questions.

A few years ago, we all knew that speedometers were unlikely to read exactly correct, but we didn’t know how far out they may be unless we had a radar reading to compare. Now, many riders have GPS navigation that gives lots of information, including a “calculated” speed.

It has brought some complaints from some rather irritated owners that drive along comparing the GPS speed to their bikes speedometer and find them differing. In most cases, the speed shown on the meter will be higher than that on the GPS, often by as much as 10% higher. Why?

• Actual measurement Vs calculated
• Wear factors
• Safety

Let’s look at these 3 things in more detail:

The meter needle is being moved (in most cases these days) by electric pulses coming from a speed sensor. As the wheel revolves faster, the pulses have less time between each and the meter registers these as a reading of vehicle speed. It may not take into account wheel-spin or slip that does not give forward motion. Nor does it account for tires replaced with an aftermarket brand that have a different outer diameter. Either of these factors will result in a difference between the actual measurement and the GPS calculation of where you were a couple of seconds ago compared to where you are now to give you a speed.

As parts wear, things change. In particular, as the tire wears down from full tread to wear bar, the distance it travels in 1 full revolution will lessen. Take, for instance, a 150/70 17 tire. If the overall diameter at the center of the treads is 520mm, then the circumference will be (r x π) 260 x 3.141 = 816.8mm. In 1 turn (with no slippage),  it travels almost 817mm (more than 32 inches).

If the tread is 8mm deep when new, at fully worn the same tire will only travel 804mm. You have now lost ½ inch of distance every time the wheel goes around. And this doesn’t even consider the effect of having the correct air pressure in the tire.

But the meter doesn’t know this change has occurred and only counts the pulses per wheel revolution, so the indicated speed doesn’t change when the actual speed has.

From a safety standpoint, it could be argued that in general, more people fall off or crash when they are traveling too fast than when they have slowed down. Therefore the last thing we (a manufacturer) want to do is make people go faster than they think they are going. So we err the meter to the optimistic (faster) side of the reading more than the pessimistic (slow) to take into account the above possible changes. Of course, these days, product liability dictates engineers must make every effort not to expose a company to a lawsuit, so that also has its effect.

To summarize, yes, the average speedometer will have a “built-in” error of up to 10% optimistic and will usually be much less than 5% pessimistic, but at least you can still read it on a cloudy day when your GPS  is searching……searching…….searching.

Have a technology question on your Yamaha that you would like explained? Fire me in a comment.

Cheers,

Dave

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One Response to “How Fast Is Fast?”

1. Jack says:
   

   Does the odometer also have a “built-in” error of up to 10% or can that be assumed to be as accurate as possible, aside from wear factors?

   Thanks for the question Jack. I had to do a little checking for the answer from my engineer friends. The short answer is yes, there is an included error, although it is less than the speedometer reading error. You can figure on between 4 and 6% in most cases, aside (as you mention) from the detraction factors. This means that under the worst case of tire wear, the reading gets closer to completely accurate.
   The mileages for maintenance schedules (found in the Owners Manual) reflect these figures.
   As an aside, these “errors” are completely normal for all manufacturers that I have had experience with.
   Cheers.

   February 17th, 2008 at 12:39 am

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