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GPS'ing for Speed
Speed Tuning your RC with GPS
by Tony Arnold StampedeProject.com
as published in RC Magazine.

$80-150 Street Price

With R/C's getting faster and faster, such as the recently reviewed 60+ MPH Jato 3.3, there are more off track races going on than ever before. For bragging rights a GPS satellite receiver, has become the standard speedometer accessory of choice.  These celestial driven speedometers have even spawned virtual online racing to see who is fastest.  Don't believe me?  Look on any RC forum where hundreds of posts appear of "my RC is faster than yours" with GPS'ed verified speeds posted as undeniable proof.  Now with the addictive speed of the superpower Mamba Max 5700 brushless motor system locked into my R/C, it seemed only natural to find out how fast the system really is and a GPS receiver is the obvious tool for the job.  Starting at around $80, GPS units are not cheap and the question rattling around in my head was whether GPS units are simply a one function top speed gadgets or could they be useful long-term tuning tools.

I secured a selection of the most light and diminutively sized WASS enabled GPS units I could find.  These units were Lowrance's small but full featured iFinder GO and GO2 units and Garmin's tiny 1.5"X3" wrist mountable Foretrex 101. These inexpensive "don't cry when you crash" units, do not feature PC interfacing, a color display, or updateable street maps, however they still provide all the basic features you need to tune your R/C.  The ideally suited "trip calculator" feature on the units provided current, maximum, and average speeds, distance, and trip time.  "Trip time" is an intriguing feature in that it only counts up while you are moving, so it can in essence function as a make shift lap timer during your practice track days or as a moving run-time meter for bashing.

There has been much debate over GPS receiver accuracy versus professional radar guns.   Top of the market radar guns such as the benchmark standard Stalker II still show a +/- accuracy of 1-3 MPH.  In the hands of the highly trained, radar guns can be highly accurate, however as explained to me by a friend and Police Academy instructor (who would like to remain nameless), "significant reading errors can be obtained if the unit is used incorrectly by an un-trained user... along with other variables, moving the radar gun with or against the direction of the target even slightly while obtaining a reading could also result in highly inaccurate data."  Newer laser speed guns promise the best and most accurate option for reading speed, but are hardly an affordable option for us R/C'ers and still require a second  person to man the gun during the testing process. To add to the accuracy argument, inexpensive hobby or sporting grade radar guns lack the law enforcement provided training that maintains accuracy, are typically less than 40% accurate even when used correctly, and are not required to be either initially or regularly calibrated - hey but they are cheap. 

The big confusion of GPS accuracy was regarding older GPS units. With WAAS (Wide Area Augmentation System), the location accuracy of a GPS unit shrank to <3 meters from <100 meters.  Many people mistakenly thought this change greatly increased the speed accuracy of WAAS enabled GPS receivers, it did but unfortunately that equated to about .05 of a MPH improvement.  What really increased the speed measuring accuracy on the then newer WAAS enabled units was faster and more powerful internal processors and receivers.  These newer processors could read satellite positioning faster, more accurately, and triangulate speed faster per second and provide more accurate speed and location data.  Hand held GPS units now provide greater than +/- .05 MPH accuracy 95% of the time, even on inexpensive units. This translates to about +/- 1-3 MPH at walking speeds, but precision increases to over that of even radar at 20+ MPH due to a higher number of moving average calculations.  All three GPS units I tested displayed speeds within .5 MPH of each other with my real truck's cruise set at 35, 45, and 60 MPH.  The newer generation of WAAS enabled GPS units available today, such as the ones I tested, are relatively inexpensive, very accurate, easy-to-use, can be easily justified as a multitask'er (outside of the RC hobby), and can be operated without help.  The verdict is that GPS receivers are the best option for us untrained civilians to accurately measure position, speed, time and distance while outdoors for around $80-$150.  On to testing.

The RC
My highly modified Traxxas Rustler stadium truck, AKA the Ultra-Rusty, was selected as the test vehicle to be flogged.  The Ultra-Rusty features a long list of modifications as well as the all powerful Mamba Max 5700 brushless system.  I wanted some scientific data as to which gearing was the best for bashing, the track, or maximum speed runs.  Now well armed with a three similar GPS units and the durable Ultra-Rusty, I was determined to find out how useful a GPS receiver could be to an R/C'er.

Tuning
Testing proved interesting as most GPS measurements conflicted with my perception.  If your goal is to spend time playing with gearing and cell count to determine absolute top speed of your R/C, there is no better device than a GPS unit for the job.  For example I was positive that my 12 cell equipped Ultra-Rusty was faster during it's high speed runs geared at 26/87, but in reality it was about 9 MPH slower than it was at it's consistently fastest 60+MPH gearing of 22/87. Those same surprising realizations occurred again when attempting to identify my longest run-times for bashing. Using the "trip time" feature I discovered that the lowest 12/87 gearing did not net the longest actual moving run times and disengaging the drake brake in the Mamba Max profile did increase runtimes.

As you were tuning at the track for your next race, did you really know which setup was actually faster?  This is where a GPS morphs from a toy to a useful R/C tool.  The trip time feature becomes your own personal lap timer and the average and maximum speed readings provide real data as to which setting works and which didn't.  Maybe you had a high maximum speed but a slow run because your suspension or tires are out of sync with the track or perhaps you are just geared a little too high and can't get good acceleration in the turns.  Lap times still a little on the high side from lower max speeds on the straights? Time to tune the gearing.  Winning races is about knowing which tire, suspension tuning, and gearing setup runs faster on particular tracks.   At the track my favorite suspension, tire, and Mamba Max controller profile was again not my fastest setup, even though I could have sworn it was, until I looked at the data.  My old setup was very good but a simple gear and controller profile change made my rig even faster and I would have never known without the GPS data.

These tough and highly recommended little units survived a multitude of 40+MPH wrecks throughout testing with little more than some road rash and through it all, netted big speed and tuning dividends.  The information I gained will most likely put me in a higher position on the next race, allow me to bash longer, and allow me to burn someone in a street race. No longer is a GPS receiver just a toy, in my mind it is the tuning tool of the future for R/C'ing and may even assure my spot on the podium or at least bragging rights.

What to look for on a GPS receiver
At least a 12 parallel channel, WAAS compliant, with 3D capability, A trip calculator feature that provides current, maximum, and average speeds, distance, and trip time is indispensable. For a more practical all purpose GPS unit for navigating or even to help you find the location of the new track, a good base map with major streets loaded is a big plus. The basic functionality Garmin Foretrex was a perfect unit to keep weight to a minimum and it's handy wrist strap made securing the unit to a variety of R/C's quick and easy.  Although not as light as the Foretrex, the inexpensive base map loaded iFinder GO units are full featured, lighter than most units, and small enough to still fit under most bodywork. 

Garmin
Foretrex 101 Street Price $135
www.Garmin.com

Lowrance
iFinder GO & GO2 $80-$130
www.Lowrance.com