[GRRiders] GRR tech-inspection

[Joe Gross]

On Thu, Jul 07, 2005 at 10:17:23AM -0400, David Pyle wrote:
> A question about the rear lights being vertical, or perpendicular to 
> the ground. I use three lights, two on my seatstays and one on my 
> seatpack. The ones on the stays are not exactly 90 degrees, probably 
> more like 80 degrees, and it is not really practical to get these 
> perfectly vertical. Is there any range of acceptability, or will you 
> only accept 90 degrees to the ground.

I had that same problem with some seat stay mounted tail lights. The
problem was that my stays were at too sharp and angle and the light
bracket didn't allow a sharp enough angle.

The solution was to take a pair of needle nose pliers and extend the
slot for the mounting bracket to allow the light to face directly
backward. Your modification technique may vary according to your
bracket design.

> I have always thought that since the lights are below the line of
> sight of a car, and considerably below the line if sight of SUV and
> trucks, that a slight angle up was beneficial, but I am willing to
> admit that I have never tested this hypothesis.

Let's do the math.

Assuming your light is mounted only 10 degrees off from horizontal.
I'll also give your light a very generous 4 degree angle of effective
focus. That means the light is best effective between 8 and 12 degrees
from horizontal.

The ratio of height to distance is sin(8) or 0.14. That means for
every 1 foot of distance behind you the light is effectively aimed at
a point 0.14 ft above the light.

If a truck driver's eye level is 10 feet above your light that means
the driver will enter the focus of your light when the truck is
10/sin(8)=71 ft away and will exit the focus of your light when it's
10/sin(12)=48 ft away. That leaves only a distance of 71ft-48ft=23ft
where your light is focused on the driver.

If you're moving at about 15mph and the truck is going about 65mph
the relative speed difference is 50mph. 50mph is 50*5280/60/60=73
ft/second. The truck will cross the 23 ft of focus for your light in
23/73=0.3 seconds.

About a half second after seeing your poorly aimed light the driver
will have turned your sleep-deprived self into road pancake.

The bottom line is that your high school trigonometry teacher was
right when she told you that math may some day save your life.

Joe