WHEELS & BRAKES
Power is nothing without control
Wheels
Rims
For safety reasons, the FIA is regulating the minimum weight of the rims in order to provide safety factors into the structural design. For Le Mans Hypercars, minimum weight is 9 kg (for 13'' rims). Material used is magnesium.
Uprights
Uprights used to be fabricated from steel plates and turned hub and bushes (welded together). Nowadays, most uprights are machined from one piece of aluminum (or matrix aluminium MMC). This manufacturing technique is much more precise and light than fabricated uprights.
Special high precision bearing are required to limit friction losses when diameters are high (in upright) or rotational speeds are high (in gearboxes) - they also help with packaging (small sections).
Tires
Tires are the only contact between the car and the track. Understanding tires behaviour is therefore essential.
Modelling tire sliding / forces is the most difficult part of modelling an entire racing car and a major limiting factor to build realistic driving simulators. The main reason is that racing tires are thermal and chemical components providing high performance grip (up to 2.0 grip coefficient, as opposed to 1.0 for road tire) by using compounds that are partially cured. On the race track, bulk temperature inside the compound reaches >100 deg C activating some chemical reactions that continue to cure the tire. The softer the compound, the ‘less cured’ it is. Therefore a racing tire has a shelf life after which it has become fully cured and is too hard. F1 and Formula E teams are starting to develop sophisticated thermal tire models to improve driving simulators and make better suspension design decisions early on.
Building a robust tire model for 424 driving simulator is one of the challenges before us...
F1 tire construction
Braking system
Fundamentally, brake torque needs to be as consistent as possible throughout the braking zone, corner to corner and throughout the race. And the brake balance (front to rear) needs to follow what the driver is setting up.
Braking Efficiency is all about keeping all parameter in the right window
Brake pad / disc friction coefficients vary greatly with temperature (especially with carbon brakes).
Brake pad / disc temperatures vary throughout braking and corner to corner (200 degC start of braking, 800 degC end of braking).
Brake pad / disc temperatures are always a bit different left to right and front to rear.
With pad and disc wear, the pressure of the pad on the disc can become uneven
Carbon Brake Discs are very sensitive to temperature
Nowadays, premier class race cars have carbon brake discs which are more efficient than steel ones, for example. But to reach the highest level of braking, accurate temperature management is necessary. Therefore, both core cooling and face cooling are required to reduce the temperature of the disc efficiently - the design detail of air paths allows to tune the mass flow. Brake discs have got more and more holes for core cooling to increase exchange surface area for a given section (drill diameter going from 10mm down to 1mm in F1 over the years).
Nevertheless, a carbon disc must not get too cold either. Indeed, If a carbon disc gets to cold (wet conditions or safety car) it can glaze: its surface chemical property is altered and its friction coefficient drops. It is very hard to ‘de-glaze’ a disc on track.
Brake by Wire, the future of braking
On electric racing cars, the use of regenerative braking torque applied by the MGU through the transmission and driveshaft is an additional parameter adding braking torque to ‘hydraulic torque’ (torque from the discs/pads). F1 and Formula E are using brake by wire systems at the rear to manage brake torque properly in conjunction with the regenerative braking torque:
Traditional master cylinders are connected to the driver brake pedal
The front fluid pressure is activating the front brakes directly like a traditional system (fail safe system without additional electronics involved)
The rear fluid pressure is measured as a requested brake pressure for the rear. The BBW (brake by wire) system calculates the hydraulic pressure required to achieve this final torque request. Because the regenerative torque is added to the hydraulic braking torque, this calculated torque differs from the target torque. In some cases, no hydraulic torque is applied and only regenerative torque is slowing down the rear wheels.
BBW system allows a smooth and consistent braking torque balance front to rear.