UNDERSTANDING GRIP
Grip is key! Grip is what keeps your car on the road or track, allows you to accelerate, turn corners and stop. There is a limit to the available grip off your car based upon many factors and forces, and there are ways in which to increase grip. In order to understand what your car’s handling characteristics are it is essential to understand the factors behind grip. This will allow you to drive you car to it’s limits, understand what is going on and sensibly plan changes to your setup to increase grip and drive your car faster.
Tyre contact patch
The area of rubber that is in contact with the road is called the contact patch. It provides a finite amount of grip to do a number of things. The available grip, in simple terms, is divided longitudinally and laterally. When you are accelerating or braking, the grip used is front to back of the tyre – longitudinal grip. When you are turning, the grip used here is side to side of the tyre – lateral grip.
It must be noted that if you are using all the available grip of the tyre longitudinally, for example when you are braking, there will be no lateral grip available for turning. It is best therefore, especially when starting out on your track-day career, to utilise grip in a single direction at any one time. For example, try to brake hard on the straight before a corner (use all the longitudinal grip), release the brakes, then turn into the corner (maximum lateral grip will now be available).
We can increase the grip available at the tyre contact patch by fitting stickier track-day tyres. These tyres will be made from a softer material and will generally increase tyre adhesion on racing circuits. Due to the softer compounds used however, they will probably wear out quicker than your standard road tyres. It may be wise to use tyres that suit the overall usage of your car, which will undoubtedly be a compromise, or have a set for road and a set for the track if your budget allows.
Road Surface
The tyre contact patch equates to half of the grip story, never forget that the road surface is providing the other half! Dry road surfaces provide much higher levels of grip that when wet, different surfaces and even different tarmac types all provide varying levels of grip.. It is essential to adopt a driving style that suits the condition and surface of the road ahead.
Weight transfer
We have stated that there is a limit to available grip and discussed how grip is made up of directional forces acting on the tyre contact patch. We will now look at how weight transfer can increase or decrease the grip available at the contact patch.
It is important to remember that if you transfer weight away from the tyre contact patch, you lower the adhesion and hence grip available, if we transfer weight to the tyre contact patch, we increase the levels of grip available. Weight transfers occur as a result of the chassis twisting around the car's roll centre, (we will not go into roll centres in this article). In simple terms, whilst accelerating, braking or steering is occurring, the car’s body rotates in the opposite direction. Often, weight transfer can be seen as it compresses the suspension on one side of the car, while releasing the weight on the other side (seen as body roll). Weight is transferred longitudinally (front to back) and laterally (side to side). The causes and effects of weight transfer are as follows:
Acceleration
When you accelerate, the weight of the car is transferred towards the rear of the car longitudinally (front to back in this case), the rear suspension compresses and available grip at the rear increases whilst decreasing at the front.
In a rear wheel drive car this increase in grip at the rear can help to reduce wheelspin, whilst in a front wheel drive car, as the front wheels loose grip due to the rearward weight transfer, wheelspin becomes more likely.
Deceleration & Braking
When you decelerate or brake, the weight of the car is transferred towards the front of the car longitudinally (back to front in this case), the front suspension compresses and available grip at the front increases whilst decreasing at the rear.
It must be noted that cars can brake far harder than they can accelerate and so the weight transfer is greater, for this reason weight transfers under braking conditions are more likely to affect the balance of the car. Braking too hard into a corner is likely to cause the rear brakes of the car to lock up and there is great potential for the car to a spin (hence why we suggest braking be done before turning!)
Steering
When you turn a corner, the weight of the car is transferred sideways laterally. If you are turning right, weight is transferred towards the left side of the car, the left side suspension compresses and available grip on the left side of the car increases whilst decreasing on the right hand side.
It is worth noting that weight transferred whilst cornering can increase grip (and get you through the corner quicker) if the steering is used progressively and weight is transferred smoothly. However, if you turn into the corner too quickly, the sudden transference of weight can unsettle the car and cause the tyres to break traction suddenly often resulting in a spin.
REDUCING UNDERSTEER
We assume you have reached the limits of your available grip whilst cornering and that one of two things is happening, your car is either understeering or oversteering.
Here we discus understeer, which is where grip is lost at the front wheels during a corner. Recognisable as a tendency for the car to want to go straight on, instead of round the corner, despite applying more steering input. Understeer is neither spectacular nor a fast way around a corner so let’s understand why it is happening and get rid of it!
For the purpose of this article, we will assume that you have not induced understeer yourself by simply driving too fast into the corner, accelerated too quickly around the corner or used up your entire grip reserve in braking whilst trying to steer and that the road surface is dry and free from oil and other slippery stuff. You have good tyres and the tyre pressures are correct.
In this case, it may be that your car is set up with a natural tendency to understeer (manufacturers often set cars up to understeer once the limit of grip has been overcome as it is seen as a more stable state than oversteer). The plan now must be to make the handling of the car more neutral so the front does not loose grip more readily than the rear. Some of the factors at work here will be:
· The drive layout of the car
· Weight distribution within the car
· Suspension and chassis setup
· Weight transfer during cornering
By “drive layout of the car” we mean, does the drive layout of the car naturally induce understeer such as a front wheel drive car where a single set of tyres shares grip for acceleration, braking and cornering duties. Whilst changing the drive layout is extremely complex, you can consider other ways such as suspension setup to overcome inherent understeer.
Weight distribution within the car is another complex problem to influence, engines, gearboxes and axles are generally seen as fairly fixed in position although you can consider relocating some components such as batteries, fuel tanks and making panels and glass lighter with carbon and Perspex.
However, there are some simple modifications you can make to your car’s suspension and chassis setup to dial out understeer and influence the effects of weight transfer whilst cornering. You need to induce greater weight transfer to the front wheels to increase grip. You can also set your cars suspension geometry to give you the handling characteristics you want.
Modifications to Reduce Understeer
· If you have adjustable dampers, try softening the front dampers or stiffening the rear dampers to encourage weight over the front wheels.
· Use softer front springs or harder rear springs. Most standard cars and uprated suspension kits use front springs that are stiffer than the rear as they have to hold up more weight (engine, box, steering bits etc) and manufacturers often set cars this way to induce understeer on the limit as it is the more stable state. By fitting stiffer rear springs you can reduce understeer and even induce oversteer in front wheel drive cars.
· Fitting after market ant-rollbars can help. Use a softer front anti-rollbar or a stiffer rear anti-rollbar, this has a similar effect as altering the spring rates, as previously described, whilst still allowing wheel movement and a degree of ride comfort.
· Increase toe-in at the front, set toe-out at the back encourages the rear of the car to break traction more readily. You may need additional parts to do this at the rear depending upon the setup of your particular car.
· Increase front camber, reduce rear camber. Many coilover kits have adjustable camber top plates, if not, camber adjustable top mounts can be purchased separately or, if not available for your model, you may find camber adjustable bottom arms are available.
· Use softer compound front tyres to increase grip.
· You could try reducing the front tyre pressure (exercise caution with tyre pressures, always use the tyre manufacturers recommended pressure limits).
REDUCING OVERSTEER
We assume you have reached the limits of your available grip whilst cornering and that one of two things is happening, your car is either understeering or oversteering.
Here we discus oversteer, which is where grip is lost at the rear wheels before the front wheels during a corner. Recognisable as a tendency for the back of the car to slide as it wants to overtake the front! Oversteer is spectacular but it is not the fastest way around a corner so let’s understand why it is happening and how we can reduce overseteer! Oversteer is caused by a number of factors such as how the car is being driven and current setup of the car.
For the purpose of this article, we will assume that you have not induced oversteer yourself by simply driving too fast into the corner, accelerated too quickly around the corner or braked to harshly into a corner (unsettling the rear tyres and allowing them to break traction) or lifted the throttle mid-corner (the result is called lift off oversteer). You have good tyres and the tyre pressures are correct.
It is worth noting that most performance or “driver’s cars”, whether they are front wheel drive, rear wheel drive or four wheel drive, are setup by manufacturers to tend towards oversteer. This is because they are generally setup to generate lots of front end grip for good initial turn-in characteristics.
The plan now must be to make the handling of the car more neutral so the rear does not loose grip more readily than the front. Some of the factors at work here will be:
· The drive layout of the car
· Weight distribution within the car
· Suspension and chassis setup
· Weight transfer during cornering
By “drive layout of the car” we mean, does the drive layout of the car naturally induce oversteer such as a powerful rear wheel drive car with a heavy engine and gearbox up front. Whilst changing the drive layout is extremely complex, you can consider other ways such as suspension setup to overcome inherent oversteer.
Weight distribution within the car is another complex problem to influence, engines, gearboxes and axles are generally seen as fairly fixed in position although you can consider relocating some components such as batteries, fuel tanks and making panels and glass lighter with carbon and Perspex.
However, there are some simple modifications you can make to your car’s suspension and chassis setup to reduce oversteer and influence the effects of weight transfer whilst cornering. You need to induce less weight transfer to the front wheels to maintain grip at the rear. You can also set your cars suspension geometry to give you the handling characteristics you want.
Modifications to Reduce Oversteer
· If you have adjustable dampers, try softening the rear dampers or stiffening the front dampers to reduce weight transfer to the front wheels.
· Use softer rear springs or softer rear springs to reduce weight transfer to the front wheels.
· Using a stiffer front anti-rollbar or a softer rear anti-rollbar has a similar effect as changing spring rates whilst still allowing wheel movement and a degree of ride comfort.
· Reduce toe-in at the front, increase toe-out at the back encourages the rear of the car to break traction less readily.
· Reduce front camber, increase rear camber. Many coilover kits have adjustable camber top plates, if not, camber adjustable top mounts can be purchased separately or if not available for your model, you may find camber adjustable bottom arms are available.
· Use softer compound rear tyres to increase grip.
