Date:         Mon, 30 Nov 1998 23:44:57 -0800
Reply-To:     Per Lindgren <lindgre@ONLINE.NO>
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Per Lindgren <lindgre@ONLINE.NO>
Subject:      Re: Vanagon total drag and mechanical losses
Comments: To: Blue Eyes <lvlearn@MCI2000.COM>, vanagon <vanagon@vanagon.com>
Content-Type: text/plain; charset=us-ascii

Blue Eyes wrote:


> <snip>My second concern is known as speedometer lag.  One of my long time pals
> has
> worked for the automotive press for decades and was the technical editor for
> one of the most popular US auto magazines.  I asked about the use of their
> expensive "fifth wheel" calibrated output device that they clamped to the back
> of test vehicles.  It was a lot like a motorcycle tire with a hall effect
> transistor counting pulses caused by moving magnets which showed a certain
> angle of rotation by the tire.  By marking a spot on the tire and starting
> spot on the ground, you can count off 100 rotations and measure that distance,
> so you know how far one tire rotation is with good accuracy.  Now all you need
> is a digital timer and you can start recording accelerations, decelerations
> and absolute speeds with a high degree of confidence.  You can smoke a test
> vehicles tires and show 40 mph on it's speedometer, but that 5th wheel just
> records pulses and time increments, so it's unaffected by those inaccuracies.
> I was told they had learned that speedometers typically lag toward their
> previous value on both acceleration and deceleration readings.  So you can't
> trust them until speed has stabilized.  But we want to measure speed in flux,
> so we can't trust the speedometer readings.  Grrrr. <snip>

Why not make your own fifth wheel? As you wrote, you can roll it 100 rotations and
get the accurate distance. Here is how I would do it: first, make a frame that
attaches to the car, then a fork with a pivot and a bicycle wheel, a hatch strut
to give it an even pressure to the ground, and a bicycle speedometer. These
speedos can be programmed to fit the circumference of the wheel, by the exact
millimeter. The only drawback is that you would have to mount it close to the
driver or passenger seat, because the bike speedos don't have too much wire. With
a known distance, this system can be made to be 100% accurate.