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Launch Control: RPM Rise Rate

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RPM Rise Rate


00:00 When we looked at the ground speed based launch control technique, I mentioned there's no way of effectively implementing this on a four wheel drive vehicle.
00:08 Since all four wheels will spin at the same time and we have no valid reference for ground speed.
00:15 While this is true, we do still have one possible option that can be used to implement what I'd like to call passive traction control.
00:23 This technique can't be implemented on every ECU and it requires a function that can limit, or clamp, the right of change of RPM.
00:33 In plain English, this means that we can control how quickly the ECU will allow the RPM to increase.
00:39 Let's consider what this means in the context of a four wheel drive car.
00:44 If we can get a data log of a good launch with limited wheel spin we can base our RPM rise rate function off this.
00:53 Let's say we launch at 5,000 RPM, and after the clutch is released the RPM increases to the rev limit of 8,000 RPM in 1.2 seconds.
01:03 This would give us a rate of change of RPM of 2,500 RPM per second.
01:10 The working here is simply 8,000 RPM minus our launch RPM of 5,000 and then divided by the time of 1.2 seconds.
01:19 What we know now is that a good launch without wheel spin produces a rate of change of RPM of 2,500 RPM per second and we can enter this in our ECU under the launch control function.
01:33 If we now launch the car and the car wheel spins it'll result in a higher rate of change of RPM than 2,500 RPM per second and the ECU will produce an RPM limit to control the wheel spin.
01:47 In general, you actually want to sit the rate of RPM change slightly above your measured rate of RPM change to allow a little margin for wheel spin.