What parameters can we change and think about how these parameters influence the performance of a vehicle in a balance of performance (BoP) regime? The physics discussion helps us link parameter changes to the primary modes of operation of a vehicle around a circuit, traveling in a straight line and cornering.

Different racing series have different options available to adjust performance of vehicles, but the general BoP variables typically include mass, total power output, minimum ride heights, aerodynamic elements, and fuel capacity, and to some extent tyres.

Mass

As implied by Newton’s Second Law, the mass of a vehicle directly, and inversely, impacts the ability of a vehicle to take advantage of the propulsive forces to accelerate. Whether we are talking about longitudinal or lateral accelerations, any increase in mass will reduce the acceleration capacity in those directions, while any reduction in mass will improve the acceleration capacity of a vehicle.

Because of the direct impact of mass on longitudinal and lateral accelerations, we can increase a vehicle’s mass to slow it down or reduce a vehicle’s mass to speed it up. A good rule of thumb is that a 10 kg increase or decrease in mass will result in a 0.15% increase or decrease in lap time, respectively. So, on a 100 second lap, 10 kg will have a 0.15 second impact.

Before we leave the topic of mass, think about how mass then influences the lap time of a vehicle as fuel is consumed. A vehicle with a 100 L fuel tank will be carrying approximately 72 kg of fuel at the beginning of a stint. By the end of the stint, and assuming no tyre degradation, this vehicle should be approximately 1.08% faster (1.08 seconds on a 100 second lap).

Total Power Output

As we discussed already, a vehicle’s power unit is responsible for generating the longitudinal propulsive force for the vehicle. This force, when all the resistive forces are overcome, is what drives the vehicle forwards though space, and defines how quickly the vehicle can accelerate longitudinally. A higher capacity for longitudinal acceleration leads to a reduction in lap time, while less acceleration capacity yields a slower lap time.

There are many configurations of power units encountered in racing, normally aspirated internal combustion engines, turbo/super-charged internal combustion engines, hybrid engines, and fully electric motors. I am going to focus here on normally aspirated and turbo-charged engines.

With normally aspirated engines, the total power output is primarily controlled by inlet air restrictors with a specified minimum diameter. The minimum diameter controls how much air flows into the engine, which in turn determines how much air is available to mix with fuel for combustion. Increasing the minimum diameter of a restrictor increases the volume of air that flows into each combustion chamber, which means a higher volume of fuel can be mixed with the air, and a bigger explosion can be created.

So, a larger restrictor diameter (more air) equals more power, while a smaller restrictor diameter (less air) equals less power. Engine restrictors come in two varieties, sonic and non-sonic. Sonic restrictors have a continuously curved profile along the length of the restrictor – much like the outlet of a trumpet – where the minimum diameter is found somewhere along the curved profile. Non-sonic restrictors typically have a conical inlet and outlet with straight walls and a flat cylindrical central section where the minimum diameter is found.

A small radius is applied where the straight walls meet the flat cylinder, and the length of the cylinder is prescribed by the sanctioning body. Non-sonic restrictors will influence the output power over the entire RPM range, while sonic restrictors only reduce power once the air flowing through the restrictor starts to choke at higher engine RPMs. The power output for turbo-charged engines is typically controlled by a boost limit, or a boost limit curve where increasing boost pressure results in a power increase and reducing boost pressure reduces output power.

A boost limit applies a single maximum boost level across the entire engine RPM band, while a boost limit curve assigns a maximum allowable boost as a function of engine RPM. A boost limit acts in a similar manner to a non-sonic restrictor in that the limit has an impact across the entire RPM range. A boost limit curve allows a sanctioning body to shape the power output across the RPM range. With boost limit curves it is possible to add or subtract power where it is needed, which is highly desirable from a BoP perspective.

In my personal experience, I have been able to successfully align the power outputs of normally aspirated and turbo-powered cars by first ensuring the power outputs of the normally aspirated cars are matched using inlet air restrictors, and then fine-tuning the output power of the turbo-charged cars by tuning the boost limit curves for those cars.

Engine power output is influenced by several other factors that may be used to balance vehicle performance. For example, sanctioning bodies may specify ignition angles to increase or reduce spark advance and impact the engine’s power output. Likewise, an air/fuel ratio (lambda) may be specified to control how much fuel can be delivered to the engine to add or reduce power. In cases where the engine ECU is locked or cannot be reprogrammed, it is possible to increase or reduce maximum RPM limits to control power output. If this cannot be programmed into the ECU, this would involve a team setting the shift lights higher or lower and the sanctioning body scrutinizing the shift RPMs through further data analysis following a session or event.

For a 500 HP vehicle, a good rule of thumb is that a 10 HP change in power output will result in a 0.31% change in lap time, i.e. increasing power by 10 HP will result in a 0.31 second reduction in lap time on a 100 second lap. Of course, this factor is highly dependent on the circuit layout, as there are circuits that are much more sensitive to power than others.

Minimum Ride Heights

We say “minimum” ride height because a sanctioning body will typically want to try and restrict a car from going any lower than the minimum prescribed ride height. These ride heights are typically static ride heights, so there is nothing stopping the vehicle from going lower dynamically while on track. Unfortunately, minimum ride height regulations can have unintended consequences on vehicle setups. Teams may start to introduce elaborate bump rubber, spring and damper settings as a way to pass the minimum ride height rules during technical inspection, but to still achieve a desired dynamic ride height while on track.

Ride heights have several impacts on vehicle performance. For all vehicles, increasing or decreasing the minimum ride height will impact the center of gravity height of the vehicle dynamically. An increase in CG height causes increases in lateral and longitudinal load transfer when accelerating laterally and longitudinally. Increased load transfer tends to degrade vehicle performance because of the influence it has on the vertical tyre loads when accelerating. For example, a higher CG in cornering causes a significant reduction in the vertical load acting on the inside tyres that acts to reduce the total lateral force the tyres can generate across the axle. As we have already seen, a reduction in lateral force on the tyres reduces the lateral acceleration capacity, which results in a slower cornering speed.

For aerodynamic cars, changes in ride height influence both the total downforce and the total drag. In most cases, increasing ride height causes a reduction in available downforce. This reduction in downforce then has an impact on the vertical loads on tyres acting to reduce the lateral or longitudinal force the tyres can generate. The opposite is true for reducing ride heights. So, increasing minimum ride heights can have the effect of increasing lap times due to reduced aerodynamic forces. The combined CG and aerodynamic effects of minimum ride heights make it very difficult to have any sort of rule of thumb for these changes.

Aerodynamic Elements

Aerodynamic devices are often used to control the downforce or drag of a vehicle. Downforce has an impact mostly on the cornering and combined acceleration components of a circuit, while drag mostly impacts the straight-line speed of a vehicle.

While we’ve already addressed the influence of ride heights, the aerodynamic properties of a vehicle may be changed with wing angles, wickers or gurneys, dive planes, splitters and the myriad of other potential aerodynamic elements that may be attached to or removed from the vehicle. There is usually no free lunch with aerodynamic devices, so you cannot add more downforce without also increasing drag or reduce drag without also reducing downforce. So, this needs to be taken into consideration when modifying the aerodynamic characteristics of a vehicle.

For properties such as wing angles, a sanctioning body may prescribe a range in permissible angles or define a minimum allowable wing angle. In general, increasing a wing angle acts to increase the downforce on a vehicle while also increasing the drag. Whether or not this change makes the car faster or slower depends on the sensitivity of the circuit to changes in downforce and drag. As there are circuits that favour higher engine power, there are circuits that favour higher downforce at the expense of increased drag.

Another simple element that can be changed to influence downforce and drag is a wing wicker or gurney. In most cases an increase in gurney height increases drag while increasing downforce. I have used gurney height as a tool to manage a vehicle’s top speed on several occasions. The impact of various aerodynamic elements on lap time is highly specific to each device, so it is also exceptionally difficult to have a general idea that may be applied to most situations.

Fuel Capacity

Fuel capacity does not fit very well with the discussions on Newton’s Second Law, but it does have a significant impact on the outcome of races. Fuel capacity defines how far a vehicle can go between pit stops. In many cases – especially where tyre warmers are not allowed – there are significant gains to be made by going one or two laps further on fuel stint. Likewise, in series where full course yellows can interrupt green flag running there is a definite advantage to being the first car to pit last. As such, teams, and manufacturers demand equality when it comes to how far they can travel on a full tank of fuel. Of course, the driver and fuel maps still come into play to ultimately determine how far one can go, but it is important that everyone is on a level playing field to begin with.

Tyres

Tyre dimensions and specifications are not something that change often in BoP Tables, but these changes may still occur. For example, I have experienced times when a new tyre for a car simply does not work with the vehicle, and a reversion to an older specification was required. In addition, I have seen changes to tire specifications where the tire dimensions are increased or reduced to influence the cornering capacity of a vehicle. Again, these changes are rare, but they do occur.

Scott Raymond, WeatherTech Championship Senior Technical Engineer, explains the physics of vehicle performance and the Balance of Performance.