Launch Control: Tuning
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Tuning
06.04
| 00:00 | The principle behind tuning a ground speed-based launch control system is that under normal conditions, with no wheelspin, there will be a direct correlation between engine RPM in first gear and ground speed. |
| 00:13 | If we know what this correlation is, and for argument we set the launch RPM limit in our launch control table to match, the engine speed will be controlled to prevent any wheelspin. |
| 00:25 | While this might sound great in theory, in practice, it won't work at all. |
| 00:30 | Firstly, you'll find that we actually achieve maximum grip from our tires if we have a small amount of wheelspin. |
| 00:37 | Generally, a slip value of around 10% is accepted as the optimum value, although some testing is worthwhile to find out what actually works best. |
| 00:48 | Secondly, if we look at the theoretical RPM required to achieve, let's say, five or maybe 10 kilometers an hour of ground speed, it will probably be only slightly higher than the idle RPM. |
| 01:00 | Obviously, if we clamp the engine RPM this low, it would mean the engine would effectively stall as soon as the clutch is released and the car begins moving, because the RPM would be dragged down so low. |
| 01:13 | With this in mind, we need to modify our strategy a little. |
| 01:17 | What we need is a high initial launch RPM that will allow the car to wheelspin initially. |
| 01:23 | Then, after the car is moving, we can then reduce the launch RPM a little until the ground speed catches up and the RPM almost matches that required for our current ground speed. |
| 01:34 | At this point, we can start increasing the RPM limit again while maintaining a desired slip rate. |
| 01:41 | Data logging is the first place to start with this process, and I begin by gently accelerating through first gear until I reach the rev limit. |
| 01:50 | This will give me the theoretical RPM versus ground speed correlation that we want to use for the launch limit table. |
| 01:58 | This would represent the lowest engine RPM limit we would want to use if you're going to use a multi-position switch, although, as discussed, we will normally we'll want to allow a little wheelspin. |
| 02:10 | If we want to target a certain slip percent between the front and rear wheel speed, we can do this by increasing the launch rev limit from the theoretical correlation by the same percent. |
| 02:22 | For example, if the RPM required to achieve 30 kilometers an hour is 4000 RPM, we can add 10% to the RPM limit to achieve a target slip of 10%. |
| 02:34 | In this case, we would enter a rev limit of 4400 RPM in the launch table at 30 kilometers an hour, which would allow the speed of the driven wheels to reach 33 kilometers an hour. |
| 02:47 | While we can easily calculate a rev limit table that will result in a certain amount of slip, getting the transition from the actual launch RPM to the table values is a little more difficult, and again, we can use data logging to help us. |
| 03:03 | My technique is to start by finding what launch RPM will work for a particular car before I worry about the rest of the table. |
| 03:11 | In this respect, I'm really testing the system and treating it like a basic two step type of launch control and we can test until we find a launch RPM that results in the car launching cleanly without either bogging or excessive wheelspin. |
| 03:27 | Once we've found a launch RPM that will let the car launch cleanly, we can data log some manual launches until we get a result that is as close to ideal as we can. |
| 03:38 | Once you have this data, you can then look at the correlation between engine RPM and ground speed and replicate this in the launch control tables. |
| 03:48 | I find it easiest to break the launch up into three parts and deal with them separately. |
| 03:54 | The first part is the actual launch RPM, which we've already covered. |
| 03:59 | The next part is the transition from launching to about half of the maximum speed you can achieve in first gear. |
| 04:07 | And the last part is where we expect the drive speed and ground speed to be relatively matched. |
| 04:14 | The last part is reasonably easy to tune, as we can apply our desired slip right to the table and fix the engine speed limit relative to ground speed as discussed. |
| 04:24 | It's the middle part or transition that needs the most amount of time and the most work. |
| 04:30 | I find that we can normally start by reducing the RPM slightly below the launch RPM once the car begins moving to help reduce wheelspin and allow the tires to gain traction. |
| 04:41 | It's a balancing act, though, as if we reduce the RPM too far, the engine may have insufficient power to sustain wheelspin and hence bog. |
| 04:51 | The correct results can only be found by testing and it will depend on the engine power, the torque curve of the engine, the car's weight, and the grip levels available. |
| 05:03 | I will start with the values found from logging a good manual launch, and then test by data logging to check what happens if we either increase or decrease the RPM target. |
| 05:15 | You'll quickly find which works best for your application. |
| 05:20 | Likewise, you can test to see at what point in first gear you can start clamping the RPM to your theoretical target to achieve your desired slip. |
| 05:31 | Again, this will depend on the factors I've just mentioned, and my starting point of 50% through first gear is just that a good place to start. |
| 05:40 | If this point is too low, the engine will drop off its power band and bog, while if it's too high, you will end up with excessive wheelspin, which will ultimately hurt your acceleration. |
| 05:51 | It all sounds complicated, but when we move into the worked examples, you'll see how these techniques can be applied quickly and easily to achieve great results. |
