On most bikes the throttle position sensor is on the throttle body, coaxial with the main throttle valve. Tap into the lead that connects to the wiper of the pot - this typically shows a voltage ranging from just over one volt at closed throttle to 3.5 volts at full throttle. On newer Yamahas with YCC-T, connect to the sensor detecting twistgrip rotation, not the one on the throttle's butterflies.
In Our Last Two Installments Of Art & Science we covered the basics of data acquisition (Jan. '08) and analyzing engine rpm data (Mar. '08). Speed and rpm, generally considered the two most important channels, are almost always given dedicated inputs on data acquisition systems and provide plenty of information on their own. As always, however, more data from other sources can allow you to better analyze that speed and rpm info, as well as look at other aspects of setup and riding that speed and rpm traces on their own can't reveal.
To record additional channels usually requires tapping into a system's auxiliary inputs, which may call for some expansion. Systems range from basic setups that have no allowance for additional inputs at all to others-like our AiM Sports Drack box-that have multiple auxiliary inputs. Adding extra channels is a simple matter of buying the sensors and plugging them into the box. Our Racepak G2X system has a single auxiliary input, into which must be plugged one of the company's expansion modules. Each new channel requires an additional module, with the modules piggybacking on each other as more inputs are added. Decide before buying a system how many channels you are likely to need, and make sure the unit you purchase either has that capability or can be expanded to meet it.
A typical throttle postion trace (blue), overlaid with speed (black) and rpm (red) data from a lap of Buttonwillow's west loop, recorded with our Racepak G2X GPS-based unit. The graph shows zero percent (fully closed) at the bottom and 100 percent (fully open) toward the top. Note the use of scales here to separate TPS data from the speed and rpm traces. Stacking the graphs directly on top of each other can result in a too-busy display, but separating them slightly allows you to still relate the trace to speed and rpm while making it much easier to see important details.
Now that you've got the extra channels to play with, the question becomes a matter of what additional data to record. The manufacturers of most systems offer add-on sensors for recording a variety of information, and as bikes become increasingly loaded down with electronics and OEM sensors it's an easy matter to tap into existing sensors for data-the possibilities are practically endless. Our first choice, given a single addition channel, is usually throttle position because it is easily connected and tells more about rider inputs than any other one channel.
Practically every new motorcycle has a throttle position sensor (TPS) as part of its fuel injection and emissions package, and this can be tapped into either at the sensor or at the ECU-just as we did last issue for the rpm signal. Use your service manual to find the active wire of the sensor, being sure to connect to the potentiometer that detects twistgrip opening. Be careful on bikes with dual throttle valves and on newer Yamahas with YCC-T-we want to measure rider input, not what the ECU controls.
A throttle position trace can be broken down into three sections: opening, wide-open and closing. Each part shows certain characteristics that can help the rider be more aware of how he uses the throttle over the course of a lap; in our experience throttle position is the easiest data for a rider to relate to, partly because it is one input he is directly responsible for and partly because even small changes can be easily seen on the graphs. The biggest benefit of a TPS trace is that it can be used as visual proof that the rider is not at wide-open throttle when or as much as he thinks-and this is true for even expert-level club racers. Many times we've surprised riders by showing them whole straights where they are not at full throttle but think they are.
Perhaps the most useful aspect of using TPS data is that it forces the rider to concentrate more on throttle application. For example, when we've pointed out to riders an anomaly in throttle application in one corner or section, they typically address the issue for that specific case and go on to make similar improvements in other areas of the track-simply because they are applying the same technique over the whole course and are making a conscious effort to do so.
A throttle position trace can be the most humbling-nothing deflates a rider's ego more than telling him he is at full throttle for only 10 seconds over the course of a two-minute lap, for instance-and also the most useful. If you are serious about using data acquisition to improve your riding, be sure that whatever system you choose can be expanded to accommodate the additional input.
This is a typical braking scenario at the end of a straight. Rider A's throttle trace (blue, with speed in black) has a crisp, straight trailing edge, indicating a smooth, quick transition from throttle to brakes. Rider B (yellow, with speed in red) shows some hesitation in getting off the throttle, with a gradual reduction from 100 percent over time. Note the effect on the speed traces and how much distance is involved - rider A would certainly have an advantage in a late-braking duel with rider B. | 
The bump in the leading edges here - with throttle decreasing before increasing to fully open - shows the rider regripping a long-turn throttle tube. This particular rider didn't realizethis was the case, but a quick-turn throttle eliminated the bump and made an improvement. | 
The leading edge of the throttle trace should be a smooth increase to full throttle, with few dips or hesitations. It may seem counterintuitive, but a steep slope can sometimes indicate the rider could get on the gas sooner (but less aggressively) in the corner - it helps here to overlay data from two riders and look at segment times. |