Jans en ik hebben het de laatste tijd veel over het remvermogen van de Speedster.
Om de discussie iets breder te trekken:
There are a few basic facts that must always be kept in mind when discussing brake systems:
1) The brakes don't stop the vehicle - the tires do. The brakes slow the rotation of the wheels and tires. This means that braking distance measured on a single stop from a highway legal speed or higher is almost totally dependent upon the stopping ability of the tires in use - which, in the case of aftermarket advertising, may or may not be the ones originally fitted to the car by the OE manufacturer.
2) The brakes function by converting the kinetic energy of the car into thermal energy during deceleration - producing heat, lots of heat - which must then be transferred into the surroundings and into the air stream.
The amount of heat produced in context with a brake system needs to be considered with reference to time meaning rate of work done or power. Looking at only one side of a front brake assembly, the rate of work done by stopping a 3500-pound car traveling at 100 Mph in eight seconds is 30,600 calories/sec or 437,100 BTU/hr or is equivalent to 128 kW or 172 Hp. The disc dissipates approximately 80% of this energy. The ratio of heat transfer among the three mechanisms is dependent on the operating temperature of the system. The primary difference being the increasing contribution of radiation as the temperature of the disc rises. The contribution of the conductive mechanism is also dependent on the mass of the disc and the attachment designs, with disc used for racecars being typically lower in mass and fixed by mechanism that are restrictive to conduction. At 1000oF the ratios on a racing 2-piece annular disc design are 10% conductive, 45% convective, 45% radiation. Similarly on a high performance street one-piece design, the ratios are 25% conductive, 25% convective, 50% radiation.
3) Repeated hard stops require both effective heat transfer and adequate thermal storage capacity within the disc. The more disc surface area per unit mass and the greater and more efficient the mass flow of air over and through the disc, the faster the heat will be dissipated and the more efficient the entire system will be. At the same time, the brake discs must have enough thermal storage capacity to prevent distortion and/or cracking from thermal stress until the heat can be dissipated. This is not particularly important in a single stop but it is crucial in the case of repeated stops from high speed - whether racing, touring or towing.
4) Control and balance are at least as important as ultimate stopping power. The objective of the braking system is to utilize the tractive capacity of all of the tires to the maximum practical extent without locking a tire. In order to achieve this, the braking force between the front and rear tires must be nearly optimally proportioned even with ABS equipped vehicles. At the same time, the required pedal pressure, pedal travel and pedal firmness must allow efficient modulation by the driver.
5) Braking performance is about more than just brakes. In order for even the best braking systems to function effectively, tires, suspension and driving techniques must be optimized.
For maximum brake potential, vehicles benefit from proper corner weight balance, a lower CG, a longer wheelbase, more rear weight bias and increased aerodynamic down force at the rear.
To go further it is necessary to understand some of the physics involved, and that requires some definitions.
1) Mechanical pedal ratio: Because no one can push directly on the brake master cylinder(s) hard enough to stop the car, the brake pedal is designed to multiply the driver's effort. The mechanical pedal ratio is the distance from the pedal pivot point to the effective center of the footpad divided by the distance from the pivot point to the master cylinder push rod. Typical ratios range from 4:1 to 9:1. The larger the ratio, the greater the force multiplication (and the longer the pedal travel).
2) Brake line pressure: Brake line pressure is the hydraulic force that actuates the braking system when the pedal is pushed. Measured in English units as pounds per square inch (psi), it is the force applied to the brake pedal in pounds multiplied by the pedal ratio divided by the area of the master cylinder in square inches. For the same amount of force, the smaller the master cylinder, the greater the brake line pressure. Typical brake line pressures during a stop range from less than 800psi under "normal" conditions, to as much as 2000psi in a maximum effort.
3) Clamping force: The clamping force of a caliper is the force exerted on the disc by the caliper pistons. Measured in pounds clamping force, it is the product of brake line pressure, in psi, multiplied by the total piston area of the caliper in square inches. This is true whether the caliper is of fixed or floating design. Increasing the pad area will not increase the clamping force.
4) Braking torque: When we are talking about results in the braking department we are actually talking about braking torque - not line pressure, not clamping force and certainly not fluid displacement or fluid displacement ratio. Braking torque in pounds-feet on a single wheel is the effective disc radius in inches times clamping force times the coefficient of friction of the pad against the disc all divided by 12. The maximum braking torque on a single front wheel normally exceeds the entire torque output of a typical engine.
A few things are now obvious:
1) Line pressure can only be increased by either increasing the mechanical pedal ratio or by decreasing the master cylinder diameter. In either case the pedal travel will be increased.
2) Clamping force can only be increased either by increasing the line pressure or by increasing the diameter of the caliper piston(s). Increasing the size of the pads will not increase clamping force. Any increase in caliper piston area alone will be accompanied by an increase in pedal travel. The effectiveness of a caliper is also affected by the stiffness of the caliper body and its mountings. It is therefore possible to reduce piston size while increasing caliper stiffness and realize a net increase in clamping force applied. This would typically improve pedal feel.
3) Only increasing the effective radius of the disc, the caliper piston area, the line pressure, or the coefficient of friction can increase brake torque. Increasing the pad area will decrease pad wear and improve the fade characteristics of the pads but it will not increase the brake torque.
Schiet maar raak ... waar zit bij de Speedster het grootste probleem?
SteV6
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Het had maandag al moeten gebeuren maar door de drukte bij Lotus A'dan hebben we het verzet naar de tijd dat mijn auto er toch is voor de reparatie van de neus.... Kosten? Half uur werk en een schakelaar! Ja, je kan het ook zelf.. Maar ik doe het niet zelf!!