XM-3150 Throttle Response
I thought I’d start a new topic for this issue. In my previous review of this scooter I noted an issue with the throttle response - I thought it was abrupt especially at mid throttle cruising speeds. I did some investigating and found that when you plot signal voltage over twist grip rotation, the curve has an “S” shape and is indeed steep in the middle.
To make my measurements I fastened a protractor on the end of the twist grip and made a pointer. Then I removed the left side cover to gain access to the ‘economy’ switch, where I was pleased to find that the wiring was very easy to manipulate thanks to a jumper harness that the factory added just to accommodate the economy switch. The circuit only involves 3 wires - 5 volt power supply, on one end (usually red), a ground on the other end (usually black), and the signal return in the middle (white or gray). You can simply unplug the harness from each end and plug the remaining connectors together to bypass the switch entirely. All the jumper harness does is allow the economy switch to add a 330 ohm resistor in series to the 5 volt power supply, dropping it to 4 volts. But then you can take that unused jumper harness, add a tap to each of the three wires, and make your very own ‘break out box’. Just get a voltmeter and a bundle of those multi-colored jumper test leads and measuring is fun and easy.
First I measured the stock twist grip in the ‘power’ mode, taking voltage readings every 10 degrees. Then I entered the data into a “works” spreadsheet and let it make a graph. (By the way, make sure the bike is on the center stand with the rear wheel off the ground and that the side stand is down - the motor will run if the side stand is up.) First observation - the grip has 70 degrees available, but the first 25 degrees are not being utilized. The sensor returns .8 volts at zero drgrees and does not reach the motor start thresh hold until right around 1.4 volts at just over 25 degrees,. At the other end, the voltage maxes out at 4.2 volts at 60 degrees - leaving another 10 degrees unused. That means that you have only 25 degrees to work with which is about 1 / 16 of a turn - which is not nearly enough if you ask me. In the middle of the range, the slope is steep, gaining a volt for every ten degrees., twice as much as a straight line would be from beginning to end.
I hooked up the econo switch and plotted it - it looks much the same as the ‘power’ graph although not quite as steep and signing off at 60 degrees with 3.2 volts which equals 30 mph on the road.
Next I rigged the protractor to a 5000 ohm potentiometer. These are cheap and easy to come by at an electronics store. As expected, the graph was a nice straight line but the only problem is the pot uses 270 degrees of rotation. To hook one up one would probably need an old fashioned throttle cable leading to a remote Rube Goldberg device. I also tried a 1000 ohm pot thinking it might have a different slope but it plotted almost exactly the same as the 5000 ohm one did. I guess they return a percentage and it doesn’t matter if its 1000 or 5000.
That’s where it stands at the moment. I need to plot some other hall effect sensors and see if they’re all the same. On the potentiometer side I’d like to try adding a resistor to the ground side and see if that affects the slope. If I can adjust the slope with resistors then I could tune the start and end points mechanically with my yet to be built “device”. If anyone has any questions, suggestions, or advice - please don’t hesitate to chime in.