This is from another forum (it's talking about "do lighter cars go faster (top speed)" (start of quote)Originally Posted by georgiebest Top speed will not be affected what so ever by the mass of the car.(end of quote) If the car is not coming up against a gearing limit, yes it will. Not very much of one though. (start of quote)Originally Posted by Ecchi-BANZAII!! But it's the torque that makes it that fast.(end of quote) Nope. It's power. Since power is the rate of conversion of torque (force) to work - 1 horsepower being 550lbft per second - actual torque ratings are irrelevant to speed. Top speed is governed by two things. These are friction and the ability to overcome friction . For a car these things are surface friction (rolling resistance), aerodynamic friction (air resistance) and power. Surface friction is governed by just one thing - the car's weight. This is calculated using the car's mass and local gravity. Generally speaking, the rolling resistance is equivalent to around 1.35% of the car's mass regardless of speed - though be aware that both downforce and lift can alter this: Rolling Resistance (lbf) = Vehicle Mass (lb) x 0.0135 If this didn't exist, pushing a car would be child's play... Aerodynamic friction is governed by a whole host of things, but essentially describes the effort required for the car to push air out of the way ahead of it. This depends on local air density, on the cross section of the car (the shape it presents to the air in front of it), the speed it's going and the coefficient of drag (how efficently air moves over it): Air Resistance (lbf) = Vehicle Speed (mph) x Vehicle Speed (mph) x Coefficient of Drag x Frontal Area (Square Feet) x 0.00256 Cd x Frontal Area in square feet is also known as CdA so if you don't have the individual values, the CdA can be substituted. Add the two values to get total resistance: Total resistance (lbf) = Rolling Resistance + Air Resistance The amount of power required to overcome this resistance is, simply, the total resistance multiplied by the speed in mph, divided by 375: Power (hp) required at speed (mph) = Total Resistance x Speed (mph) / 375 But remember that this is the power at the wheels. Power at wheels is what actually moves you places and so it's what's required to overcome the resistance. Crank power is just a headline figure and the resistance of powertrain and drivetrain components will reduce it by up to 25% for a particularly sluggish and dull four wheel drive system, or as little as 12% for a very efficient drivetrain with the powered wheels nailed directly onto the differential (FWD, some MR), maybe even less. So, if we take the 253mph and 4150lb Bugatti Veyron with a CdA of 8.02 square feet... Rolling Resistance = 4150 * 0.0135 = 56.025lb Air Resistance = 253 * 253 * 8.02 * 0.00256 = 1314.182lb Total Resistance = 56.025 + 1314.182 = 1370.206lb Power (wheel) required = 1370.206 * 253/375 = 924.43hp Assuming the Veyron powertrain to be made of £5m worth of space-age materials and only losing 15% of the power on the way down, that's a required engine power of about 1035hp. This doesn't account for the change in CdA brought about by "top speed mode", but for the purposes of this comparison it doesn't matter. So, let's take 200lb of stuff out of the Veyron Rolling Resistance = 3950 * 0.0135 = 53.325lb Air Resistance = 253 * 253 * 8.02 * 0.00256 = 1314.182lb Total Resistance = 53.325 + 1314.182 = 1367.506lb Power (wheel) required = 1367.506 * 253/375 = 922.61hp So losing 200lb of stuff has reduced the power required to overcome 253mph-worth of resistance by a whole nearly 2hp (don't all gasp at once). This means that at 253mph the car still has nearly 2hp to play with and can overcome more resistance - it can go faster! How much faster? Well, resistance is a cube of speed, so the percentage "increase" in power results in a cube root of that percentage increase in speed: 924.43hp/922.61hp = 1.00197% Cube root of 1.00197% = 1.00066% 253mph * 1.00066% = 253.17mph Woo! 0.17mph (not by me y the way)
