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. |
| 01:58 | As usual, testing and datalogging will be required to see exactly what values give you the best results. |
