Glen,
You are right on the money. Height placement of the sensing equipment can
be THE most critical variables contributing to inaccurate times for exactly
the reasons in your example. We generally suggest placement of the sensors
as low as possible, catching the front tire which will not vary much unless
the tire hops over the beam. Otherwise placement high enough to clearly
catch the body of the vehicle, but must catch every vehicle.
A note of interest on how NHRA handles sensor placement, which is governed
by the NHRA sanctioning body and not the local track. Sensors at the
starting line must be below 3" above ground to catch the front tire. At
the 60 foot point on the track, the sensors are 10" above ground level to
allow for the lift of the front end which varies on every venicle so the 60
foot time is never an official time, just a gauge for each driver for
comparison of runs with the same vehicle. The finish line and speed traps
at the end of the 1/4 mile have the sensors at 6" above ground level to
allow for normal vehicle lift under acceleration.
Given these rules dictated by the governing body, part of the 'game' is to
take advantage of the rules as best as possible for your vehicle. Everyone
knows where the beams are, so everyone has an equal opportunity to 'adjust
accordingly', the equivalent to selecting the right tire for the track or
the vehicle.
I'm not making a point for the SCCA to govern sensor placement. If
Autocross paid big bucks to the winner, there would be compelling reasons
for more rules.
With all the vatiables that exist, timing to 0.01 is probably reality in
order to remove the effect of all other variables, but the 0.001 gives a
deciding factor to minimize the ties from occurring. If a driver was paid
based on how many 0.001's they cut off their time, we'd need 0.0001 ...
Dennis Laczny
RACEAMERICA Timing Systems
http://www.raceamerica.com
>Where it could make a difference is in height variations between the start
>and finish lights. As Dennis Laczny of Raceamerica points out, it
>requires at least 1/2 inch of solid object to trip the sensors he uses.
>Assuming that he's got about the best you can get, let's do some
>calculations.
>
>As I mentioned, at 30 mph, a car is traveling 44 ft/sec. That's 528
>inches/sec, or 0.528 inches per millisecond. Lets assume that the start
>and finish lights vary in height by 3/4 of an inch. Car one crosses the
>start line and his front bumper blocks the beam, triggering the timer.
>Going through the course he crosses the finish line but that beam is 3/4"
>lower. That lower beam just misses his bumper and hits the front of the
>body work which is 2 inches behind the bumper. His time is now increased
>by 4 milliseconds. Car 2 does the same thing but his bumper cuts the beam
>at both the start and finish. His time is 0.002 seconds better than car
>1. In this example, car 1 crossed the finish line in a faster time but
>minor variations in equipment robbed him of the win.
>
>My proposal is to recognize that we can't be that accurate and round times
>to 0.01 seconds which represents 5 1/4" at our typical speeds. The rules
>already have a tiebreaker procedure - you take the next best runs and
>compare them. That would reward the consistently fast driver.
>
>Just some food for thought. Only once would it have affected me. When I
>lose it's usually by a much wider margin like this past Sunday. The #1
>guy drove well, I drove poorly in places and my times prove it.
>
>glen
>================================
>Glen E. Thompson
>glen.thompson@worldnet.att.net
>'89 Mazda RX-7 GTUs
|