On May 28, 2004, at 12:03, Richard Urschel wrote:
> How accurate are our timers?
>
> Having made no changes to my car for the past 18 months, and having
> concentrated exclusively on driving during that period, I have finally
> developed a consistent ability to duplicate runs within a tenth.
> However, it still seems that in at least 1 out of every 25 runs the
> time is inexplicably faster or slower than expected by 3 tenths or
> more.
>
> Unless our timing software confirms a trip on both the front and rear
> wheels the perceived time discrepancy may be due to intermittent
> failures of the timers to consistently trip. There would be no other
> indication of this unless the timer failed to trip on both the front
> and rear wheels at either the start or finish. A 1 in 50 intermittent
> failure rate would only cause this once in every 1250 runs, but would
> fail to trip on the front wheels only at either the start or finish in
> 1 out of 26 runs.
I frequently misunderstand you, Rich, so let me check my understanding:
- Your premise is that in about 1 of 25 runs, either the start sensor
or finish sensor fails to pick up the front wheel and gets the rear
wheel instead, causing a time variance of "3 tenths or more"
- We're not talking about precision here, we're talking accuracy - in
other words, how the run time compares to reality, not whether the
timer system is inherently precise to +/- .001 or not.
If that is all correct, I have the following observations, if not,
please disregard and clarify:
> So, does anyone know if our timing systems confirm a double trip at
> the start and finish or do they just accept what they get while
> rejecting any additional trips for a half second or so afterwards? Are
> the lights even capable of resetting and retripping in two tenths of a
> second?
Dunno, ask Boris to check the manual. I note an assumption here,
though, that the lights are triggering on the wheels. I don't know
that to be the case for low-riding cars, are you sure it's true for
yours? I would think it more likely for it to generally trigger on the
nose of your car, and sometimes miss that and get the front wheel, than
for it to miss one wheel but get the other.
I have seen/heard of problems with reflectors missing highly-polished
chrome wheels on occasion, BTW. I avoid that by letting mine get all
gooked up with brake dust.
Let's do a little math:
I'm using 8' for your wheelbase, that's 96", adjust as necessary.
x = 8' / .3 sec = 26.67 ft/sec, which converts to 18 mi/hr
It's my impression that most finishes run somewhat faster than that,
but I could be wrong. It might be more characteristic of the start
light. Note that if your wheelbase is shorter, or the time difference
is greater, that speed would be lower.
Finally, it seems very odd to me that for all runs you typically
maintain times within .1. Even with perfectly consistent driving, I'd
expect greater changes than that from tire heating. Do you have a
breakdown of how many of these anomalies are faster vs slower, and on
which run of the event they occur?
KeS
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