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Professional Motorsport Data Analysis: Gear Changes

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Gear Changes

06.26

00:00 - When we have a manually shifting transmission, we have control over when to change between each gear which if you've made it this far, you're hopefully already well aware of.
00:10 I mention this only because optimising gear shift points is an important part of getting speed out of a vehicle and we can use data analysis to make that happen.
00:19 There are plenty of different scenarios where being in a different gear for the same road speed has merit.
00:25 But when it comes to deciding when to shift to maximise forward acceleration, there's a simple method to calculate this.
00:32 The goal is simple, we want to maximise the forward force that a powertrain can give us at any point in time.
00:38 The torque curve, gear ratios, final drive and tyre size we have will ultimately determine the optimum point to shift gears in order to maximise the propulsive force.
00:51 Looking at this plot which shows us the propulsive force available to the driven tyres for each gear, plotting against vehicle speed, we can see how the force drops away as the vehicle speed increases.
01:03 At any point in time, we want to make sure we're as high up on this plot as possible.
01:09 The higher we are are, the more force we have to accelerate the car.
01:13 The point of intersection of each of these propulsive force curves for each gear is the ideal shift point.
01:20 As this is the maximum forward force for our powertrain configuration that we have.
01:24 If there is no intersection between 2 consecutive gears on this plot, it means the shift point should be as late as possible while still being safe or the engine.
01:33 All we need to do is look up the engine speed for each of these intersection points which can be easily done by plotting the same information but on an engine speed axis.
01:44 In this example, we can see here that the optimum shift point engine speed and also the engine speed that we'll see once the next gear is engaged.
01:52 As long as we have the engine torque information, gear and final drive ratios, this is a simple calculation to make.
02:00 We've made a simple spreadsheet tool to help you calculate these points for yourself that you can download from the related resources section below this video.
02:07 This tool is based on some simplified assumptions but will still prove to be an effective resource to get you very close to your optimum shift points.
02:16 To help visualise this in real terms we can add in some extra information to give us a fuller picture of what's going on.
02:23 Let's take the case of a car with zero downforce.
02:26 Here the maximum tractive force is going to be limited by the vertical load on the drive wheels and their effective coefficient of friction.
02:34 That means if we plot it, we'll be looking at a straight line and if we neglect the weight transfer acceleration which is a simplification then the vertical load doesn't change.
02:44 If we then super impose the same powertrain force we showed before and only trace over the top of the force for each gear by ignoring anything under this intersection, this is the maximum force the powertrain can provide if we shift at the optimum points.
03:00 The intersection between these two lines is the point where the car will stop being traction limited.
03:06 Any force above this line, the car is traction limited and that force is unusable.
03:11 So in this situation, this black shaded area is unusable and the green shaded area is usable.
03:18 Notice that high road speeds is where the car is engine power limited.
03:23 If we then add some downforce the traction limit line has some shape to it.
03:28 This is because of the downforce being proportional to the square of the vehicle speed.
03:33 With some downforce, we stop being traction limited at a slightly lower vehicle speed due to adding vertical load to the tyres.
03:40 As an extra side note, if we know the drag coefficient of our car, we can also plot where the car will become drag limited by also plotting the drag force.
03:49 The point of intersection between this line and the maximum propulsive force is where we become drag limited.
03:56 You can see how these calculations are not only useful for showing the optimum shift point for each gear, but also for choosing gear and final drive ratios for your application.
04:06 With this information we have enough to start programming our shift lights by taking the optimum shift points we calculated earlier.
04:12 There will always be some tweaking required based on the driver's reaction times to the lights and the time taken to shift.
04:19 And this is where analysing the data comes in.
04:21 One way to analyse how close we're shifting to our optimum shift points is by creating some math channels that show the optimum shift point automatically for each gear.
04:31 The ones I have here are simply programmed for the optimum shift point for that gear but gated by positive longitudinal acceleration and engine speed above a certain value just to reduce the clutter.
04:44 Over the top of these channels, we can also plot engine speed.
04:47 Now looking at the amount of under or overshoot for each gear change can give us some help in tuning the required offsets in our shift lights.
04:55 Each acceleration zone around the track will require different offsets because of slightly different rates of acceleration thanks to track gradient, traction level, or just how busy things are for the driver at that part of the track.
05:08 We should focus tuning our shift lights for the most important straights which usually means the longest straights.
05:15 For this data, let's say this is the area of track we want to optimise the shift lights for.
05:21 We can see in this area of the acceleration zone that the driver is intentionally short shifting to help suppress wheel spin.
05:28 But zooming in on the shifts from 4th to 5th and then 5th to 6th, we can see that the shift is happening a little too early.
05:36 In this case, we want to offset the shift lights by an additional 150 RPM in order to maximise the propulsive force by getting the actual shift in the right place.
05:46 For this method to work, the driver must always focus on driving to the shift lights as repeatably as possible which will allow the changes in shift light timing to help.
05:57 You'll also need to take a look over a few different laps and do a bit of averaging to look for a trend for how much to change the shift light offset rather than basing everything off a single lap.
06:08 It's rare for the same shift light settings to work while at different circuits.
06:11 So if this is something you're paying attention to, you can expect to be tweaking them for every track you go to.