Kevin,
The only thing you missed is the "within a tenth", so of the three tenths I was
saying two tenths might be caused by the cell triggering on the rear wheels.
Wheelbase is 84". At 25 mph that is .19 seconds. 48 mph is required to get it
just under a tenth.
The nose of my car is 15" high at rest, more under acceleration. Prior to the
front tire there is 8" of clearance, and in between the tires there is 4".
Given that the entire relection must be blocked and the height of the beam
across the course can easily be affected by elevation changes between the
boxes, I'm not so sure that the beam can't pass below my car on asphalt lots.
That's an interesting question that you and John raise, and should be easy
enough to test. On reflection, it IS asphalt lots where I remember being
perplexed by the displayed time.
If the beam is set high enough to be broken by the body work then confirmation
of a second trip would make no sense, so I suspect no timing software does that.
As for tire temps, one run with bias ply Autocrossers does it. Keeping them
cool becomes the objective after the second run. I averaged six runs at the
last three events, and lately I've been getting within a tenth of my best by
the second or third run unless I'm having trouble figuring out part of the
course.
> 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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