SEPTEMBER 2019 ISSUE
How many times has the driver been heard to say “we missed the set-up today” when trying to explain away a poor performance? Sometimes you will have heard him say “we are chasing the set-up”, or perhaps even “we got the right set-up on the car today.”
What is this mysterious ‘set-up’ that the driver is referring to? And how is it so influential on a racing car’s performance? In this three-part tech talk series, we are going to try and de-mystify and explain this famous phrase ‘set-up.’ Following the first installment on how alignment affects the set-up of the race car, in this second of the three-part series we will focus on ride height and how that plays into the set-up of the RC F GT3.
Ride height is the clearance between the road surface and the lowest most points on the chassis at the front and rear of the RC F GT3. On a race car, the ride height is an important part of the set up for a number of reasons. The ride height affects the performance of the car by altering the aerodynamic performance and balance as well as the mechanical performance and balance.
First, let’s look at why it is important from the mechanical aspect. Having a ride height that is too low will cause the car to hit the road surface and as a result will be difficult and uncomfortable to drive. In the case of the IMSA WeatherTech Sportscar Championship, running the car too low at the front will infringe a minimum ride height rule of 50mm under the leading edge of the splitter. By running the car lower than this, the car may be disqualified – not a good idea!
The stiffness of the front and rear springs is a major factor in determining the ride height of the race car. Soft springs require the car to be set higher off the ground, whereas stiffer springs allow the car to run closer to the ground.
Generally, the front of the car is set to run lower than the rear of the car. This is quite typical with racing cars and this is often referred to as ‘rake’. If the driver comments that he needs more rear grip, then the engineer may elect to reduce the ‘rake’ in the car by making an adjustment to the rear ride height - lowering the rear of the car.
The Lexus RC F GT3 car is fitted with a full length underbody, which means that all ‘wetted surfaces’ of the car have an aerodynamic effect on the cars performance [‘wetted surfaces’ are those body surfaces that come into contact with airflow and then exert an aerodynamic influence on the car]. The ride height is influential in controlling the underbody ‘wetted surfaces’ – and this is where the aerodynamic effect occurs. The rear wing, fitted to the car is adjustable and by setting at a high angle of incidence, it generates more downforce at the rear of the car.
Sometimes the driver will comment that he has too much grip at the rear of the car and needs more grip at the front to make the car more responsive in the corners. Because there is no front wing to adjust on a GT3 car, the engineers will make adjustments to the ride heights of the car to give the driver the balance they are looking for.
In next month’s tech talk, the third of this three-part series on set-up of the Lexus RC F GT3, we will examine the final three factors, including the dampers, corner weights and anti-roll bars and how they affect the set-up of a race car.