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What Is G-Force and How Is It Related to Harsh Driving?

What Is G-Force and How Is It Related to Harsh Driving?
Author: Matt Broughall, Technical Support Engineer

Frequent questions we often hear at Geotab are: “What is g-force?” and “How do g-force values relate to real world driving habits?” In MyGeotab, harsh driving is defined in terms of g-force.

G-Force and Acceleration

Before we get to the software, let’s talk about g-force. A g-force is a measure of acceleration. 1G is the acceleration we feel due to the force of gravity. It’s what keeps our feet firmly planted on the ground. Gravity is measured in metres per second squared, or m/s2. On Earth, the acceleration of gravity generally has a value of 9.806 m/s2 or 32.1740 f/s2. Since this is just a different scale for measuring acceleration, it not only applies to gravity, but can be used to quantify any acceleration.

How G-Force Relates to Harsh Driving

You can configure the Geotab telematics device to produce immediate audible feedback in response to excessive driving behaviors such as harsh acceleration, harsh braking, and harsh cornering. You can access the driver feedback options from Vehicles on the main menu in MyGeotab. Select a vehicle from the list. On the Vehicle Edit page, select the Driver Feedback tab.

Configuring Accelerometer Driver Feedback Settings

When setting up harsh driving rules in MyGeotab, changing the level of sensitivity will affect when feedback is given for a harsh driving event — either a beep or a spoken alert if you have GO TALK installed. To adjust the sensitivity of a rule, go to Rules & Groups > Rules. Move the slider and click Save.

A larger number (e.g. -0.61 G) is less sensitive, meaning that only the most aggressive driving events will break the rule, triggering driver feedback. Moving the slider to the right increases the sensitivity of monitoring.

mygeotab driver feedback settings lower sensitivity setting option
Lower sensitivity setting for accelerometer driver feedback (Passenger Car).

A smaller number (e.g. -0.47 G) is more sensitive and means the rule is easier to break.

mygeotab driver feedback settings higher sensitivity setting option
Higher sensitivity setting for accelerometer driver feedback (Heavy-Duty)

It’s also important to mention that since this is an acceleration and not a force, the vehicle’s mass or size doesn’t matter at all. In the harsh driving rules, we specify the vehicle size (passenger car, truck/cube van, and heavy-duty truck) as a suggestion. A sports car can safely take a corner faster than a truck.

For a table of the average G-force exertions for various vehicle classes, see the Geotab Product Guide.

sports car and transport truck

Recommendations for Setting up Driver Feedback

We recommend starting in the middle of the specific vehicle class and adjusting from there based on your fleet objectives and the unique aspects of your fleet. For example, if you are seeing too many harsh driving events and you want to manage that, you might select a more sensitive setting. Similarly, to closely monitor an ambulance carrying an EMT and patients in the back, you would increase the sensitivity. On the other hand, for a box truck carrying paper towels, a lower sensitivity might be sufficient.  

MyGeotab’s Three Harsh Driving Rules

If we look at the three harsh driving rules, we can do some simple calculations to find out how fast a driver would need to take a corner or accelerate from a stop sign to break these rules.

Harsh Acceleration and Harsh Braking

The Harsh Acceleration and Harsh Braking rules are really the same thing and measure how quickly you are speeding up or slowing down. The least sensitive setting in the default rule for acceleration is 0.43 Gs which is what a driver (or Geotab GO device) would experience if you accelerated from standstill to 60 km/h in 3.95 seconds. The braking rule is even more lenient and would require that you come to a complete stop from 60 km/h in 2.78 seconds, deccelerating at 0.61 Gs.

To get a better understanding of linear acceleration and g-forces, check out this handy Acceleration Calculator.

what is g-force cornering and up down

Harsh Cornering

Harsh Cornering is a little trickier to both calculate and grasp intuitively. Any time you change velocity you are undergoing an acceleration. When you are turning a corner, this acceleration is 90 degrees from the direction you are traveling, regardless of how tight a turn you are making. Think about spinning a weight on the end of a string. You are always pulling the weight towards your hand but it never gets any closer.

So how do we measure this? To determine the acceleration you experience in a corner, all we need is the radius of the corner and your speed. I measured the radius of a normal right turn at an intersection of two four-lane roads and found it to be about 17 metres. This means that if went through this corner at 32 km/h you would break the most lenient harsh cornering rule which is set at 0.47 Gs.

If you want to play with the number yourself, you can find a centrifugal force calculator here for accelerations in a corner. Note: You do not need to input an angular speed or mass to calculate g-force.

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  • Posted June 28, 2019 at 3:34 pm | Permalink

    My workers continue to get alarms for speeding because Geotab’s system has NOT updated the speed limits over the years. When will Geotab update the speed limits to align with changes in Northern IL and surrounding towns?

    • Posted August 14, 2019 at 3:30 pm | Permalink

      You can update speed in your database by clicking on the road and selecting Update speed limit. We use a collection of commercial and community sources for speed limits and are at the mercy of their updates.

  • Posted January 18, 2019 at 4:09 am | Permalink

    i want to calculate harsh acceleration,harsh breaking and rash turning.
    at which acceleration we can measure this?
    for example i am taking threshold point as 0.4 g.

    • Posted January 25, 2019 at 2:36 pm | Permalink

      Hello, 0.4G is the acceleration force threshold. Where G=9.81m/s^2

      Centripetal acceleration formula A=V^2/R where V=velocity and R=Radius
      Deceleration (braking) and acceleration formulas A= (vf – vi) /t where vf= final velocity, vi=initial velocity and t=time

      In any of these cases, acceleration must be greater then 0.4G (or in the case of braking less then -0.4G).

  • Posted January 3, 2019 at 4:10 am | Permalink

    Thanks for article.
    If you like, you can also add calculation of Acc of X, Y, Z from raw value of X,Y,Z. It will be helpful for user like us.

  • Posted May 6, 2018 at 1:52 pm | Permalink

    Sir, at what frequency band width, g- force is available? When sampling at high frequency, how to distinguish noise and actual g- force?

    • Posted July 24, 2018 at 11:32 am | Permalink

      The device makes use of filters which filter noise from normal operation, while allowing for accident events to be detectable. We use the G-force to determine the axis of orientation, and can distinguish it from noise with the use of time averaging.

  • Posted January 4, 2018 at 5:46 am | Permalink

    sir, in this way, whether you see acceleration for harsh driving by only seeing longitudinal acceleration (i.e) in axis of car moving or resultant acceleration (i.e) sqrt(x^2+y^2+z^2)? thanks in advance.

    • Posted January 18, 2018 at 3:20 pm | Permalink

      Our harsh driving rules measure each axis independently, we measure harsh acceleration separately from harsh cornering. Furthermore, we do not consider the z-axis in any of our stock harsh driving rules but a custom rule can easily be created to measure this.

  • Posted March 16, 2017 at 12:11 pm | Permalink

    Vibratory g’s in a truck are higher than the limits you mention above. So how do you filter or otherwise discriminate between steady g’s from maneuvers and those from vibrations?

    • Posted March 27, 2017 at 5:27 pm | Permalink

      Thank you for your question. There are two main ways we handle vibration noise. The first is in our firmware where we sample at a higher frequency than we report, which has the effect of filtering high frequency noise. The second is how the device is installed. We have harnesses which not only work as adapters but also as an extension so the device can be affixed to a location in the car that experiences less vibration, like a bundle of wires, while still accurately recording longer period accelerations like cornering or braking. If you have specific examples of devices you have that are experiencing excessive noise, please contact your Reseller for support.

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