Explainer: Your bike’s IMU

A motorcycle, being a single-track vehicle, has the inherent flaw that it can fall over.

Lose significant traction mid-corner in a car, and assuming you don’t hit anything, it will usually remain upright on its four tires. Do the same on a motorcycle, and unless your name is Marc Marquez, there’s a decent chance some of the shiny bits of your bike are going to scrape across the pavement very soon. This is where you need an IMU.

Inertial Measurement Units have found their way into most high-end bikes on the market today. It’s an electronic device that uses gyroscope and accelerometer sensors to measure G-force and angular rate, and it can be found in aircraft, spacecraft, drones, military vehicles, and virtually every smartphone on the market today. It may be small in size, but it’s a significant component in making modern motorcycles safer and more fun to ride. It works with the other electronics integrated on the bike, such as the fly-by-wire throttle, engine management, and ABS, to help prevent a significant loss of traction.

Before such electronics, a rider on the limit had to use his or her own senses to detect lean angle, steering inputs, brake and throttle pressure, and tire traction (or lack thereof) to control the bike. As one racer put it, “traction control is my right wrist”.

What does it do?

Humans detect G-forces via fluid sloshing around the inner ear. This, in conjunction with visual inputs and tactile feedback from the hands, feet and butt, helps let the rider know what the motorcycle is doing underneath them. The modern IMU replaces those human senses with electronics that can detect the same movements more accurately and faster, and react faster, as well.

Thus, the motorcycle’s computer has the advantage of knowing the actual speed of each individual wheel, the precise horsepower output of the motor at any given millisecond, and the exact lean angle of the bike in fractions of a degree, not just human educated guesses. Even Costa’s butt can’t compete with that.

The IMU is what makes cornering traction control and cornering ABS possible, since a motorcycle reacts quite differently to throttle and brake inputs in a corner compared to a straight line. The information from the IMU is filtered through different algorithms to arrive at the proper reactions for a given situation. For example, the amount of throttle input at 100 km/h and 45 degrees of lean angle with the riding mode set to “Sport” is different than the throttle input at 30 km/h and 25 degrees of lean angle with the riding mode set to “Rain,” even when the rider twists the throttle the exact same amount in both situations.

Performance riders and drivers will sometimes refer to the “traction circle” when discussing grip. On the traction circle, north and south indicate forward acceleration and braking, while east and west indicate cornering in either direction. At maximum acceleration (the very top of the circle), 100 per cent of traction goes to getting power to the ground. At maximum cornering (the very left or right of the circle), 100 per cent of the traction goes to cornering forces. In between max acceleration and max cornering, a percentage of power input can be combined with a percentage of cornering forces, and as long as the combined amount of both doesn’t exceed the available traction, grip should be maintained.

The same applies to braking and cornering – if you stay inside the traction circle, you should stay rubber side down. What the IMU and computer does is to make the traction circle calculations for the rider, figuring out how much throttle or braking inputs can be applied with a given amount of cornering forces, before too much grip is lost.

What’s inside this box of wizardry?

But what exactly is inside an IMU? There are microelectromechanical systems, or MEMS, which are essentially tiny physical components, as small as one micrometer in size. That’s one-thousandth of a millimetre, so pretty small. In the case of an IMU, the MEMS are a microscopic mass captured by similarly tiny springs, where the movement of the mass due to G-forces is measured via changes in the electrical charge within the structure. An array of these miniature assemblies can produce a gyroscope or an accelerometer that feed their information to a chip, and together these components comprise an IMU.

The other crucial piece of the puzzle – at least for motorcycles – is the widespread implementation of fly-by-wire throttles, pioneered by Yamaha with the 2006 R6. The physical throttle cable is replaced with wires and electronic signals that allow the bike’s computers to intercept and change the signal before it reaches the throttle bodies. Without this capability, the control of engine output would not be accurate enough for the IMU information to be utilized effectively.

Ultimately, what all this means is that Skynet will rule the world very soon. Or rather, motorcycles are just as fun to ride, but much safer than they have ever been. Safety features like cornering ABS and cornering traction control, as well as convenience features such as cruise control and quick-shifters, have made motorcycles faster, more fun, and harder to crash, and the price of these technologies is coming down every year.

Just as cruise control, ABS, and traction control are standard on almost every new automobile (with the latter two mandated by law), it will not be long before these features are standard on most bikes, as well. I, for one, welcome our cyborg overlords.