Race Driving Fundamentals: Weight Transfer
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Weight Transfer
11.39
| 00:00 | - Understanding and manipulating weight transfer is one of the key elements to being fast and maintaining control on the racetrack. |
| 00:07 | While the term weight transfer is almost universally used, if we want to be technically accurate, it's load that we're transferring, not weight. |
| 00:16 | This is a subtle difference but I'll mention it before we get inundated with emails telling us that we're using the wrong terminology. |
| 00:24 | Since rightly or wrongly weight transfer is the term most people use, we'll continue to use that terminology throughout this course. |
| 00:33 | Weight transfer is essentially how the vehicle's weight is being distributed between the 4 corners of the car as the car experiences the lateral and longitudinal forces that result from cornering, accelerating and braking. |
| 00:47 | Let's start by considering a racecar moving at a constant speed on the track. |
| 00:51 | We'll assume for simplicity that it has a theoretically perfect weight distribution with 25% of the vehicle's weight being applied through each tyre to the racetrack. |
| 01:02 | Now let's see what happens when the driver approaches a corner and begins to brake. |
| 01:07 | As the driver applies the brake, weight from the rear of the car is transferred towards the front tyres. |
| 01:13 | If we had a set of scales under each tyre, we'd now see the weight measured under the front tyres increases and the rear decreases. |
| 01:21 | This also makes itself apparent because the front suspension will compress under the additional load which results in the front of the car diving. |
| 01:29 | Once enough speed has been washed off, the driver will release the brakes which transfers some weight back onto the rear tyres, getting us back close to our equal weight distribution. |
| 01:39 | This is short lived though since the driver will also begin turning into the corner which will result in weight being transferred to the outside of the car which in turn compresses the suspension and results in the car rolling away from the corner. |
| 01:53 | Lastly as the car exits the corner and the steering is released, the driver will get back on the throttle, transferring weight towards the rear and causing the car to squat. |
| 02:03 | So these are the basics of weight transfer and how it's affected by our inputs on the brake, steering and throttle. |
| 02:10 | If we did have a set of scales under each wheel, we'd see the individual corner weights change due to weight transfer but of course the total of all 4 weights would still stay constant. |
| 02:20 | Obviously the total weight of the car doesn't change. |
| 02:24 | What's more important though is the effect weight transfer has on the available grip and this is where things get a little bit more complex. |
| 02:31 | One of the fundamental aspects we need to understand is that within reason, the more vertical load a tyre has on it, the more grip it can provide. |
| 02:40 | This however isn't a linear relationship and as vertical load on the tyre increases, the tyre's coefficient of friction actually reduces as we can see in this graph. |
| 02:51 | This means in simple terms, that if we double the load on the tyre, we'll increase the grip but we won't be doubling it. |
| 02:58 | With this in mind, when we've transferred more weight to one area of the car via driver input, this area will have more grip. |
| 03:06 | Of course there's no free lunch here and while one area will gain grip, the opposite area will lose grip. |
| 03:13 | But the amount of grip gained at the loaded region of the car isn't the same as the amount of grip lost at the opposite region of the car. |
| 03:20 | To illustrate this, let's consider our racecar with equal weight distribution and therefore equal grip. |
| 03:27 | For simplicity let's assume that we have a total of 100 units of grip available and hence each tyre has 25 units of grip. |
| 03:35 | Now we'll go back through the previous scenarios, starting with the driver applying the brakes. |
| 03:40 | The weight transfers to the front wheels and this increases the grip level on each front wheel from 25 to 33 so each tyre has gained 8 units of grip. |
| 03:49 | At the same time though the grip on the rear wheels is reduced from 25 to 15. |
| 03:55 | This means that the total amount of grip is actually reduced from 100 to 96. |
| 04:00 | Essentially the overall grip is reduced under braking now by 4%. |
| 04:06 | Let's pause and just explain what's happened. |
| 04:09 | We've seen the front tyre grip increase by 8 units, however the rear grip has reduced by 10 units. |
| 04:16 | This comes back to that coefficient of friction offered by the tyre and how this changes as the vertical load increases or decreases. |
| 04:24 | Essentially we gain less grip on the tyres that become more heavily loaded, compared to what we lose on the tyres that become more lightly loaded. |
| 04:33 | This is one of the key concepts of weight transfer because when we transfer weight around the car, the overall grip level is always reduced compared to what we had under steady state conditions. |
| 04:46 | Hopefully it should also be obvious that if we turned into a corner with this grip distribution, the available grip on the rear tyres is relatively low and it's likely that the rear will slide, resulting in oversteer. |
| 04:58 | If instead the driver releases the brakes smoothly before turning into the corner, some of the grip is restored to the rear and the car overall will have more total grip which can potentially allow it to turn into the corner at a higher speed. |
| 05:13 | Next the driver turns the steering wheel causing the weight and therefore grip to move to the tyres on the outside of the corner. |
| 05:19 | In this case, the right hand side tyres for a left handed corner. |
| 05:23 | Hence the grip on each right tyre increases to 33 but consequently the grip on the left tyres is reduced to 15, again meaning that our overall grip level is reduced from 100 to 96. |
| 05:36 | There's a limit to what we can do in a corner since weight transfer is unavoidable. |
| 05:41 | We need to deal with the fact that our overall grip level is reduced while cornering however this also explains why we see racecars run a curb and end up with the inside wheels in the air without having a dramatic effect on the car's cornering ability. |
| 05:55 | This is because the outside tyres are providing the majority of the grip and doing most of the work. |
| 06:01 | Finally after passing the apex, the driver will now begin accelerating which transfers weight rearward, increasing grip on the rear tyres at the expense of grip at the front. |
| 06:11 | In this case we'll assume that each rear tyre now has a grip level of 33 while the fronts are again reduced to 15, meaning our overall grip level is still 96. |
| 06:22 | The reduction in front grip is why understeer at corner exit is so common since the front tyres may not have sufficient grip to allow the car to maintain its line. |
| 06:32 | As we've discussed, the overall grip available when weight is being transferred around the car is less than we have under steady state conditions and this is something the driver can manipulate with control inputs. |
| 06:45 | The key here is to be as smooth as possible and limit the amount of weight transfer occurring so that our overall grip level remains as high as possible. |
| 06:55 | To highlight an extreme example of purposefully being aggressive rather than smooth, let's consider a rallying technique called the Scandinavian flick. |
| 07:04 | This is something you often see employed by rally drivers in order to utilise a lateral transfer of weight from one side of the car to the other in order to purposefully overcome the available traction and break the rear tyres loose which can then overcome the car's natural tendency to understeer. |
| 07:22 | This is done by initially turning the car away from the corner. |
| 07:25 | Let's say the car is approaching a right hand corner. |
| 07:28 | The driver will turn the car to the left, transferring weight and grip to the right hand side of the car. |
| 07:35 | The driver will then brake which transfers grip to the front of the car before aggressively turning back to the right. |
| 07:41 | Since the majority of grip is now on the front of the car, the rear will tend to slide into oversteer. |
| 07:48 | There are less extreme examples where driver input can be used to manipulate weight transfer in order to correct a handling or balance issue. |
| 07:56 | If we have a car for example that tends to oversteer at corner entry, then this suggests we have a lack of rear grip at this point in the cornering sequence. |
| 08:05 | We can manipulate this by braking slightly earlier which in turn will allow us to release the brake more before we begin to turn the car into the corner. |
| 08:14 | Since we're now using less brake pressure, this will have the effect of transferring more weight and hence grip back to the rear wheels which is where we need it. |
| 08:23 | Another example where it's very easy for a novice driver to experience the effect of weight transfer is in a long corner that we can take at the limit of the available grip. |
| 08:33 | Here we'll find that we're at part throttle as we negotiate the corner. |
| 08:37 | If we apply too much throttle, we'll transfer weight towards the rear and as the speed increases we'll find that the car begins to understeer and run wide. |
| 08:46 | By lifting slightly on the throttle, this achieves two things. |
| 08:50 | Firstly we're transferring weight back to the front wheels which has the effect of increasing grip and tightening our line. |
| 08:57 | Since we're off the throttle, the car will also slow down, meaning that we don't require as much lateral acceleration to make it through the corner. |
| 09:04 | Making gentle adjustments to the throttle through this sort of corner gives us the ability to drive on the limit of the available grip as well as manipulate the balance continually to suit. |
| 09:16 | While this course is predominantly focused on your driving, it's important to understand that a key element to weight transfer is the setup of your vehicle. |
| 09:24 | In basic terms, the setup of a vehicle can be adjusted to assist a driver in controlling the weight moving to each corner of the vehicle. |
| 09:32 | The total amount of weight that's transferred is a function of the wheel base, track width, centre of gravity, height and acceleration. |
| 09:40 | Making changes to the springs and anti roll bars changes how the weight is distributed between the tyres while making damping adjustments changes the rate at which the weight is transfered. |
| 09:50 | None of these adjustments affects the total amount of weight transfer, only how it's shared between the tyres and at what rate. |
| 09:57 | All of these adjustments come back to one thing, aiding the tyres of the vehicle and having the optimal load in order to provide maximum grip. |
| 10:07 | Surface conditions on the racetrack also have a significant impact on how much weight will be transferred which is simply because the lower grip level offered by a wet track, means less acceleration is possible, lateral or longitudinal. |
| 10:21 | In wet conditions we often end up running a softer more compliant setup as we're generally more focused on maximising the mechanical grip from the track which has been degraded by the wet track surface. |
| 10:34 | Of course the amount of body roll is going to vary from car to car. |
| 10:37 | If it's a standard road car, you're going to see it move around a lot. |
| 10:41 | Look at a single seater on the other hand and you'll next to no body roll in comparison. |
| 10:46 | This difference doesn't change the concept of how a car should be driven though. |
| 10:51 | The end goal is still the same. |
| 10:53 | Loading the tyres to their optimum via driver inputs, aided by vehicle setup. |
| 10:58 | To summarise this module, the key concept you need to keep in mind is that as we drive the car around the track, we're transferring weight or more correctly load around. |
| 11:09 | This affects the available grip at each tyre which in turn will impact the handling balance of the car which we can use to our advantage. |
| 11:16 | Remember that the amount of grip we gain at the more heavily weighted region of the car, is less than the grip we lose at the non weighted region due to the relationship between vertical load and the tyre's coefficient of friction. |
| 11:29 | This in turn means that any time we have weight transfer in the car, our overall grip level will be reduced. |
