Speedo accuracy?

I am just curious... How accurate are our speedos? Especially towards the top end. I know sport bikes have an error percent to them. If it indicates say 80 or 90 what am I actually doing? How about Top end like 120+ still accurate? 4-5mph off? More?


Hope you guys had a good Christmas and new year. Ride safe!

 

Not very. If I want accuracy, I put my GPS on there. Mine reads 5-7 miles off at low speeds, closer to correct at higher speeds.

 

An economical way to get Digital Accuracy is to employ a CycloComputer.

The most difficult aspect is mounting the Magnet and Magnetic Pick-up on the Front Wheel.

You have the option of running a wire or getting a Radar-wireless rig.
You get to program the Diameter of the Wheel to set the display to show accurate speed in a constant digital readout.

 

use your smartphone i'm sure you
can find a speed app I have a few on mine
mount it right beside your speedo

 

Yamaha mechanical speedometers TEND to be "optimistic" (reading faster than you're actually going.)

If yours is reading "slow" then it's probably gummed up and could use some attention; plus it wouldn't hurt to clean/lube the cable as well.

Mechanical Instrument tech: http://xjbikes.com/Forums/viewtopic/t=15303.html

 

Mine was just cleaned and lubed I don't think it's slow by any means. Just curious to know how fast I'm actually moving.

 

You won't find that out accurately with a stock Yammie speedo. They were engineered to read a bit "fast" so when it says you're doing 120, you're only going about 108~112.

 

This explains how the Speedo attempts to indicate the velocity of the bikes forward motion.

From Wikipedia:

"When the car or motorcycle is in motion, a speedometer gear assembly will turn a speedometer cable which then turns the speedometer mechanism itself. A small permanent magnet affixed to the speedometer cable interacts (rotates within) a small aluminum cup (called a speedcup,... looks like a bell) attached to the shaft of the pointer on the analogue speedometer instrument.
As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field.

The effect is that the magnet exerts a torque on the cup, "dragging" it, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.[1]

The pointer shaft is held toward zero by a fine torsion spring. The torque on the cup increases with the speed of rotation of the magnet (which is driven by the car's transmission/ bikes speedo drive). Thus an increase in the speed of the vehicle will twist the cup and speedometer pointer against the spring. The cup and pointer will turn until the torque of the eddy currents on the cup is balanced by the opposing torque of the spring, and then stop.

Since the torque on the cup is exactly proportional to the car's speed, and the spring's deflection is proportional to the torque, the angle of the pointer is also proportional to the speed. At a given speed the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.

The return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the gears that drive the flexible cable, or the final drive ratio in the differential, and the diameter of the driven tires."

 

BINGO!!! Thank you. A picture is worth a thousand words; together-- priceless...



The cup (C) isn't aluminum (aluminum doesn't respond to magnetism, it's not a ferrous metal.) But hey, it came off Wikipedia.

 

Wow! Thank you very much guys that answers my question and my curiosity as to how it works!