I went to the Autometer web sight to see if I could find out the
accuracy of their tach. Not a mention anywhere. So I pulled out their
'04 catalog and scoured it for any mention of accuracy. Nada!! My
guess, they are not proud of their accuracy or else they would have it
plastered all over the place.
Usually electronic instruments have their greatest accuracy mid-scale.
I'm assuming your needle is nudging the top end so accuracy is further
degraded. But lets give Autometer the benefit of the doubt and say your
Tach is accurate to 0.5%. (Step in here Dave if you know it is better
than that). So at 7,500 rpm your tach (best case) is going to have an
error of +38 or -38 rpm. Using your "336" formula with 76 rpm, 28"
tires, 3:1 (before the pinion broke!) I get about a 2 mph error. On
top of that your Tach has a sample rate of 50 (page 97 of catalog) which
adds a 0.02 second recording delay. Your record speed of 4 miles per
minute is an "average" taken over the 15 second mile ET.
So you are telling me you held it balls-to-the-wall for 15 seconds, with
tires a perfect circle and zero tire slippage, tracking
straight-as-an-arrow, using a tach with a best case error of 2 mph, that
records data every 7 ft (at 4 miles/min), and using all of the above,
you can calculate your speed to within 1 mph of clocks that are accurate
to 0.001 mph.
I have the greatest respect for anyone that can control a machine with
wings going around in circle! Plus, your data reduction is better than
mil-spec! You are right. . .keep using the "336" formula. :-) -Elon
-----Original Message-----
From: Keith Turk, Subject: Re: Gear Ratio Formula (Part 1)
(Snip. . . )This concept works well both for planning and has given me
the exact run numbers when compared to the SCTA,USFRA and the ECTA
timing systems. it's not off by more then 1 mph ... Sooo if you need to
be closer then that maybe we are working on the wrong thing....
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