Lesson 7 - Weight Transfer - Brakes and Throttle
Welcome to Lesson 7 - Weight Transfer - Brakes and Throttle! In this lesson, we will be discussing the physics of weight transfer in a race car and how it affects braking and throttle inputs. We will also explore techniques used by professional drivers to optimize weight transfer for faster lap times. Understanding weight transfer is crucial to becoming a better driver, and this lesson will provide valuable insights into this important aspect of race car driving.
Weight Transfer under Braking
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Weight transfer under braking refers to the distribution of a vehicle's weight from the front to the rear during the braking process. When a car's brakes are applied, the weight of the vehicle shifts forward, causing the front suspension to compress and the rear suspension to extend. The physics behind weight transfer under braking is based on the laws of motion and can be explained by the concept of the center of gravity.
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The center of gravity is the point in a vehicle where its weight is balanced in all directions. When a car brakes, the weight of the vehicle shifts forward, causing the center of gravity to move forward as well. As a result, the front wheels have more weight on them, which increases the traction available for braking, while the rear wheels have less weight on them, reducing the amount of available traction.
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The amount of weight transfer depends on several factors, including the vehicle's weight, speed, and braking force. The faster the car is moving and the harder the brakes are applied, the greater the weight transfer will be. Additionally, a vehicle's weight distribution and suspension geometry can also affect weight transfer.
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The effect of weight transfer under braking can be calculated using the formula:
ΔF = W x (h/r) x a
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where ΔF is the change in vertical force on the front wheels, W is the weight of the vehicle, h is the height of the center of gravity above the ground, r is the wheelbase, and a is the deceleration.
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Weight transfer can have both positive and negative effects on a car's braking performance. The positive effect is that weight transfer can increase the amount of traction available to the front wheels, allowing the car to stop more quickly. However, the negative effect is that weight transfer can cause the rear wheels to lose traction and lock up, leading to skidding or even spinouts.
To manage weight transfer under braking, drivers can use a technique called trail braking. This involves gradually reducing the pressure on the brakes as the car approaches a corner, allowing the weight to transfer gradually and keeping the car stable. The driver can then use the throttle to balance the car and maintain traction through the corner.
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In conclusion, weight transfer under braking is a crucial factor in a car's braking performance and can be understood using the principles of physics. By managing weight transfer through techniques such as trail braking, drivers can optimize their braking performance and maintain control of their vehicle.
Weight Transfer with Throttle
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When a car accelerates, weight is transferred from the front to the rear of the car. This is due to the force generated by the rear wheels pushing against the ground, which causes the rear of the car to rise and the front to dip. This is known as weight transfer under acceleration.
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The amount of weight transferred depends on the acceleration force, the weight of the car, and the distance between the front and rear axles. The higher the acceleration force, the greater the weight transfer. Similarly, a lighter car will experience more weight transfer than a heavier car. Weight transfer can also be affected by the location of the car's center of gravity.
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Weight transfer under acceleration affects the car's handling characteristics, as the weight transfer affects the tire's grip on the road. When weight is transferred from the front to the rear, the front wheels have less weight on them, reducing their grip on the road. This can result in oversteer, where the rear of the car starts to slide out. To counteract this, some racing cars have a rear weight bias, which helps to increase the weight on the rear wheels.
Weight Transfer and Grip - Advanced
Why do we need to know how a car’s weight moves around when we’re driving on track? Well, it’s crucial because if you remember our article on grip, the more vertical load a tyre has on it, the more grip it has.
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So, to be clear the more weight that we have over one area of the car – as it dives, rolls and squats – the more grip that region will have. As you can imagine, as the car’s weight moves around during different points of the corner, the grip is also moving. This sounds like it could be an issue, but fast drivers use it to their advantage.
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One thing to note is that the transfer of weight (and grip) to one area of the car (e.g. the front tyres) will take away grip from the opposite area (e.g. the rear tyres).
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To better understand this, let’s take a look at a few diagrams that will show the amount of grip each tyre has at each point in the corner as weight transfers.
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For the sake of keeping things simple, I’m going to give each tyre a maximum grip level of 25, therefore giving the car a total grip level of 100.
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In the diagram below, you can see that the car is travelling towards a corner at terminal velocity (no acceleration, deceleration or turning), with a balanced platform. Each tyre has an equal vertical load on it, with a value of 25.
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Next up, we’ll be getting on the brakes to decelerate the car to the correct entry speed for this corner. The diagram below shows the car in dive. Notice that the front tyres now have a grip level of 30 each, but the rears only have 15.
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If we were to turn into the corner with the car in this pitch, we would be limited by rear grip. That is to say; the rear would break traction long before the front and so the grip at the rear of the car limits the speed we can take into the corner.
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Therefore, it’s a better technique to smoothly release the brakes so that the front of the car rises and balances the platform (more of a 25, 25, 25, 25 split) before we turn into the corner. Now the car has 10 more units of grip at the rear and we can enter the corner faster.
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The next diagram shows the point at which the car is coming into the apex – the platform is even front to rear, but much of the weight (and grip) has rolled over to the left side of the car as we’re turning right.
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You can notice that the inside of the car is mostly unloaded and isn’t giving us much grip. This is why it’s possible to run over curbs (sometimes more) and get away with it – because it’s mainly the outside tyres that are helping us around the corner.
After we’ve reached the apex, we’re beginning to open up the car’s steering and get back on the throttle. As we do this, the car’s weight will move from one side to the rear of the car, as you can see in the diagram below.
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Notice that now the rear tyres have the majority of the grip, as the car’s squatting. A common handling problem is what we call ‘throttle-on understeer’ which is where the front slides as you exit a corner.
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In the diagram, the rear tyres have a grip level of 30, but the fronts only have 15. The balance is off, and the front would be grip limited – the end of the car that would break traction first.
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This type of understeer is pretty much impossible to resolve with driving technique, but could be resolved with a set-up change – something that we’re going to go into to towards the end of this series.
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The End ->
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