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# Launch Control: RPM Rise Rate

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## RPM Rise Rate

### 02.07

 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.