SUPPLEMENTAL INFORMATION
OPTION 9
ii
TIMEWISE 798A
Selecting an
Appropriate
Offset Bias
When you arrive at a checkpoint at what you know to be the perfect time, there is still a
chance your arrival will be recorded as one count early or late. This may not be the fault of
the timing crew. Rather, it can be the result of imperfect correlation between the 798A clock
and the checkpoint clock. Limited display resolution and basic uncertainties affect all timing
systems. The electronic response time of checkpoint triggering hardware can also be at fault.
For example, even when everyone’s clocks are supposedly in “perfect synchronization”,
arriving at a checkpoint exactly when a second ticks might be recorded as one count early.
This is due to timing uncertainties within the clocks. To counter this problem, rallyists often
run “a little late” by inserting a positive offset (a slight “pause”) into the driver’s error. The
theory is that even though the driver’s display is showing
0:00.0
as you cross the checkpoint
in-line, the slight positive offset will actually make you arrive a fraction of a count late. This
helps you arrive “within the count” intended by the rallymaster.
As mentioned, a standard 798A can insert a fixed positive bias of 0.45 seconds (hundredths).
Many rallyists feel this “pause” offset improves their chances of zeroing the checkpoint.
On the other hand, rallyists also know that checkpoint workers typically exhibit a small
delayed reaction as they split a checkpoint clock when a car crosses a timing line. A slight
negative offset (a “gain”) can be used to account for such a “late” split.
And if the rallymaster hadn’t located the timing line exactly at the truncated mileage used in
the official course calculations, an additional level of a negative offset would be needed.
Further, when a pneumatic hose is used at a timing line, there is generally a small delay as air
compresses within the hose. Here again, a slightly negative offset is needed.
Finally, even a checkpoint mirror system that dramatically darkens as a rally car passes (as
the vehicle interrupts a reflection of the bright sky) can produce a late split. No checkpoint
worker can react instantly as the mirror changes from bright sky to dark car silhouette. Plus,
the fact that such systems often locate workers a slight distance from the timing line
aggravates the problem. Anticipating when a distant oblique angled mirror will darken can
be difficult. To compensate for this delay, a slight negative offset may be needed.
OK…What’s the
Bottom Line?
So how much offset bias should be used, and in which direction? It’s up to you. Option 9
allows you to adjust for the uncertainties within all checkpoint timing equipment. And your
knowledge of human response time and the experience level of regional checkpoint workers
may dictate whether or not an offset is needed during a specific rally.
Confirming
Operation of
the Offset Bias
OK, so you’ve decided to use the error offset, and you want to be sure the system works as
you’d expect. Here’s a simple test which you can use to see the offset bias in operation.
First, select the timing mode for both the main and driver’s module as “tenths of seconds”.
This is so that the time displays do not count as fast as they would in “thousandths of
minutes”. This makes it easier to read the display values.
Then adjust the calculate time to display an exact even minute (
xx:xx.000
). This step is not
absolutely necessary, but it does make subtraction easier should you wish to compute the
error yourself. Also, select a + 0.5 or – 0.5 offset bias. This, too, will make it easier to
compute “on-the-fly” comparisons between the driver’s
ERROR
and the split error in the
main computer’s
AUXILIARY
display.
Then set the
CLOCK
to a value that is a minute or so behind the calculated time.
