Both Feet Out - - Rallying 102, Class B and C Navigation

by Ev Newton

When Both Feet Out rolls around on November 9, (2003 -ed.) it will be the navigator’s job to keep the team on time.   First-timers should just drive close to the right speeds and stay on course.  Ready for some real TSD navigation?  Here’s how.

BFO will have 3 equipment classes: Class A is any car using a non-stock odometer or a computer linked to the odometer.   (This includes BMW’s on-board computer, folks.   Tape it over if you want to run Class B or C.)   Class A people won’t need my help.   Class B is your bedrock calculate-time-from-your-stock-odometer-as-you-go class.   Class C uses any freestanding calculating equipment but must tape over odometers and on-board computers.   First, we’ll look at Class B navigation, then Class C.

About timing.   At BFO, we will time cars in 1/100’s of a minute, not seconds.   Calculate in seconds, if you wish.   (Quickly now, how much is 2 minutes, 29 seconds plus 1 minute, 46 seconds?)   Or calculate in 1/100’s and convert to seconds to compare with your watch.   Or work entirely in 1/100’s of a minute, as I propose in this article.   If you don’t have a watch reading in 1/100’s of a minute (who does?), you can make one ( á la Martha Stewart) out of an ordinary analog watch with a sweep second hand. Cut a round paper bezel to fit around your watch so the watch face shows through.     Divide the bezel into 100 equal tick-marks, Scotch-tape it over your watch face, et voil á! – a 1/100’s-reading watch!   I’ll have one at the start of BFO.

Here’s the hard part; take it slow and easy, and write down the formulas.  Class B navigation is a system of tracking perfect rally time for comparison with actual time of day.   If you know how your odometer reading (ODO) compares to Official Mileage (OM), you can correct the given CAS speed factors to calculate perfect rally time mile by mile.   BFO will give you lots of  Official Mileages ( OM’s) during the 10-mile odometer check, so you can compare your ODO to OM.   At the end of the odo check, divide your elapsed ODO miles by OM to get a correction factor (CF).   For instance, if you measure 9.95 ODO miles (interpolated) over the odo check and the OM for the odo check is 10.25, your CF is 9.95/10.25 = 0.971.   Write this down.   At any time, you can multiply OM by your CF to see what your ODO should read.   ( OM x CF = ODO).   CF also tells you how many minutes to take to cover each ODO mile at a given CAS speed.   At CAS 35, for instance, you should travel 1.714 Official Miles every minute.   That’s 60 ÷ 35 = 1.714 minutes per mile (MM).   To figure out how many ODO miles to travel every minute, multiply the official MM by your correction factor (CF). In our example, CAS 35 translates to 1.714 x 0.971 = 1.664 minutes per mile according to your ODO  (MMODO).  

CF = ODO ÷ OM    (Correction Factor = ODOmeter miles divided by Official Miles)

MMODO = MM x CF   (ODO Minutes per Mile = Official Minutes per Mile x CF)

Here’s a head start:   BFO will use only five CAS speeds:   25, 30, 35, 40 and 45 mph.   Somewhere, write down the MM factors for these five speeds:

    2.400 (25 mph)    2.000 (30 mph)   1.714 (35 mph )  1.500 (40 mph)    1.333 (45 mph)

When you finish the odo check, multiply each of the five MM’s by your CF and keep the five resulting MMODO’s handy.   For instance, if your CF is 0.971, your MMODO’s are:

    2.330 (25 mph)     1.942 (30 mph)    1.664 (35 mph)    1.457 (40 mph)    1.294 (45 mph)    

Now the easy, repetitive part:   Make a chart, as shown below.   Every time your ODO clicks over a mile, add an amount of time equal to the MMODO for the CAS speed.   Or add 1/10 of your MMODO for each 1/10 mile traveled.   For instance, if you started Leg 1 after the odo check at 10:35 a.m., your chart looks something like this:

 

CAS	MMODO (factor)	ODO (miles)	Add Minutes	Elapsed Time	Correct Time	If your watch reads: this, then - - -
30	1.942	0.0	0	0.000	10:35.00	10:35.00 (on time)
-	-	1.0	1.942	1.942	10:36.942	10:37.00 (.06 early)
-	-	2.0	1.942	3.884	10:38.884	10:38.88 (on time)
-	-	2.6	1.164	5.049	10:40.049	10:40.10 (.05 late)
35	1.664	2.6	0	5.049	10:40.049	(same)
-	-	3.6	1.164	6.713	10:41.713	10:41.73 (.02 late)
Checkpoint	-	3.8	0.194	6.907	10:41.901	10:41.90 (perfect!)

See how you handled the speed change at 2.6 miles on your odometer?   You ran the elapsed time up to the ODO mileage at the speed change, switched to your new MMODO factor and continued with the new factor.   To Pause 0.20 minute at 3.6 miles, just add the 0.200 to your calculated time:

-	-	3.6	1.664	6.713	10:41.713	10:41.73 (on time)
Pause	-	3.6	0.200	6.913	10:41.913	10:41.01 (on time)

Practice at the kitchen table with a real clock and an imaginary odometer.   With practice you can give your driver a time reading every half-mile or better.   (Experts using this system can give readings every 1/10 mile or as fast as they can read the odometer.)   By the way, if the navigator cannot see the odometer, ask the driver for readings or just calculate ahead a few 1/10’s of a mile and tell the driver what the ODO should read when the calculated time comes up, then let your driver decide whether he or she is early or late.   Practice.

A few words on Class C navigation.   I’ve heard pretty good Class C rallyists say that you can’t calculate in this class.   Au contraire!   In Class C, you have no odometer, so you calibrate your speedometer.   Class C cars don’t need no steenking correction factor.   Run a few segments of the odo check at exactly 30 mph.   0.40 Official Miles should take 0.80 minutes; 1.61 OM = 3.22 minutes, etc.   Repeat at 35 mph, noting how your speedometer compares to perfect speeds over the known OM’s.   Once the driver “understands” the speedometer, the navigator should trust the driver to travel at correct rally speed and should calculate mileage as if the car were always exactly on time.   Do this until the rallymaster gives an OM with which to compare your calculations.   When an OM comes up, the navigator will know whether the driver is early or late and by how much.   Construct a chart as follows.   (The “Add Miles” amount is “Add Minutes” divided by the MM factor.):

CAS	MM (factor)	Add Minutes	Elapsed Minutes	Add Miles	Expected OM	If Actual OM is this, then - - -
30	2.000	0.00	0.00	0.00	0.00	0.00
-	-	1.00	1.00	0.50	0.50	unknown
-	-	1.00	2.00	0.50	1.00	unknown
-	-	0.56	2.56	0.28	1.28	1.25 (.03 mile early)
25	2.400	1.00	3.56	0.417	1.697	unknown
-	-	1.00	4.56	0.417	2.114	2.13 (.02 mile late)
Checkpoint	-	0.48	5.04	0.200	2.314	2.31 (perfect!!)

If the car is early or late at the Official Mileages, adjust speed accordingly.  For instance, the car in the example was early by 0.03 mile when it reached the given Official Mileage of  1.25 miles.   At 30 mph, that’s 0.06 minute.   Knowing this, the driver slows briefly to shed the extra time, then returns to rally speed.   With practice, a good Class C team can “calculate” correct leg times very closely using this system.

Or you can just wing it.   Whether you calculate like a mad mathematician on Both Feet Out or relax and enjoy the ride, plan on a day of great roads and friendly competition.   See you there.