Master-Sport Engineers have drawn attention to the issue of stopping the car, depending on the technical condition of the car and the driver’s predisposition.
Theoretically, the braking distance (as referred to by car manufacturers refer and the journalists emulating them) is the path you will travel after braking the car, until your car stops Not wanting to bore you with the physics calculations, let us assume – as per the formula described in the scripts of high-school physics – the approximate braking path of a car, expressed in meters, in dry conditions, calculated from the formula h=0.0052v², where h is the path, and v is the speed in km/h (the product of the deceleration and the friction coefficient μ have been assumed at 7 m/s2)
This means that when driving 50 km/h, the braking distance is 13 m; at a speed of 70 km/h it is 25.48 m; at 90 km/h it is 42.12 meters; at 100 km/h it is 52 meters; and at 120 km/h it is 74.88 m. However, the interesting thing is the stopping distance of a vehicle, which means a section of the road the car will travel from the moment the driver sees danger until the vehicle stops. Thus, the previously calculated braking distance (50km/h-13m, 70km/h-25.48m, 90km/h-42.12, 100km/h-52m, 120 km/h-74.88m) is only one factor components. The actual stopping distance is in fact the sum of the braking distance, but also the way covered during the driver’s reaction and the distance crossed while removing clearance in the brake system (and in the suspension).
DRIVER’S REACTION
It is assumed that the time from when the driver sees a danger until they put their foot on the brake pedal is from 0.5 to 1 sec. During that time, the eye captures the image and transmits information to the brain, where the image is verified and the danger determined. A pulse is then transmitted from the nerves, forcing the movement of the legs and pressing the brake pedal.
The speed of the brain doing this reaction depends on two factors: the driver’s predisposition and conditioned reflexes
Predisposition is a complex of factors allowing for correct vision (speed of perceiving obstacles), correct and quick response of the brain (this is interfered by psychotropic drugs, alcohol, drugs, but also the weakness associated with an illness or elevated body temperature), and quick movements of the limbs (weakness after exercise, injuries, bad clothing etc.).
A conditioned reflex involves minimizing the brain’s work to a minimum on the zero-one principle: that is, the danger-response principle. This situation, however, requires the development of habits, and these can only be obtained during long hours of training (generally speaking, the number of kilometers traveled).
However, it should be noted that regardless of the speed and appropriate habits being learned, the car will travel on slack before the driver begins to react: 50 km/h : 6.95-13.9 meters, 70 km/h: 9.7-19.4 meters, 90 km/h: 12.5 – 25 meters, 100 km/h: 13.9-27.8 meters. 120 km/h: 16.6-33.3 meters
REMOVING CLEARANCE
From the moment the driver rests their foot on the braking pedal to pressing the friction linings, there is also a time gap of approximately 0.5-1 sec. Thus, the vehicle again moves freely for a few meters (50 km/h: 6.95-13.9 meters, 70 km/h: 9.7-19.4 meters, 90 km/h: 12.5 – 25 meters, 100 km/h: 13.9-27.8 meters. 120 km/h: 16.6-33.3 meters) This is due to two factors. Creation of an appropriate pressure in the system and pressing on the brake pedal.
How important it is to induce the appropriate pressure in a fully operational system, in order to remove the clearance: the brake pedal – pumps piston, terminal pistons – brake pads, brake pads-disc, is best evidenced by the fact that it is a priority in the PRE SAFE system, patented by Mercedes-Benz in 2002 (first used in S-Class).
In the case of worn parts of the braking system, in addition to the “normal” removal of clearance resulting from the design of the system, there is also a reduction of clearance resulting from greater path the lining needs to cross to the disc (especially in the case of a deformed disc), and a longer time to prepare the system for smooth braking. Additionally, clearance in the suspension system makes additional forces act on the pins, which may cause loss of directional stability
Pressure on the brake pedal
The pressing force is not immediately maximum, but is rising in a linear manner during the normal movement of the foot. Often, however, the driver presses the pedal quickly, then takes the leg back, reducing the pressure of the pads and the force. To eliminate this phenomenon, the BAS system (Brake Assist System) has been applied, which activates the power to prevent pressure drops in the system in case of sudden pressure on the brake pedal.
As you can see from this statement, a fully operational vehicle with a very good driver at the wheel at a speed of 100 km/h will cover over 27 meters before the actual deceleration occurs. If the vehicle is not fully operational, and in addition the driver does not have full predisposition, the distance rises to over 50 meters!!!!
BRAKING DISTANCE
Only afterwards does the proper braking start, resulting from the friction of the tire-road interface.
Assuming the formula for the friction force slowing down the vehicle’s movement T=m*μ*a (where m – mass of the vehicle, a – acceleration/deceleration, μ – coefficient of friction), it is worth noting that everything depends on the type of surface.
The safest is the concrete surface. It is smooth, slightly porous, and does not become too slippery in the rain.
Concrete: dry μ = 0.8-1.0; wet μ = 0.6-0.8. Asphalt is very pleasant to drive on, but only when it is dry and not too hot. In the rain, the asphalt becomes slippery and requires considerable speed limits, and on a hot day hot asphalt is soft and makes the tires hot. Asphalt: dry μ = 0.7-0.8; wet μ = 0.4-0.5.
The surface of basalt cobblestone becomes dangerously smooth with prolonged use, and is very slippery in the rain. Cobblestone: dry μ = 0.6-0.7; wet μ = 0.3-0.5. In the case of dirt road, it is primarily determined by the type of substrate. Dirt road: dry μ = 0.5-0.6; wet μ = 0.3-0.4.
When the road surface – no matter what kind – is covered with a tight layer of snow, traction to the road decreases, and the ratio is μ = 0.1-0.4 When the road surface is covered with a layer of ice (glaze), any adhesion almost disappears. Icy road: μ = 0.05-0.15
For example, it is worth comparing these two extremes: a concrete dry surface μ = 1.0 and icy concrete μ = 0.05, namely: on icy concrete, the traction force to the surface decreases almost twenty times
It follows that even the most perfect braking discs/pads, with no attention to the technical condition of the vehicle and the predisposition of the driver, does not always ensure impeccable braking. Hence the suggestion of Master-Sport engineers:
If so many factors affect the stopping distance, at least regarding the brake discs/brake pads, you should at least be certain of high quality. In this case it is preferable to trust laboratory testing and the ECE R-90 standard, whose symbol shall be permanently stamped on the brake pad/disc. This symbol guarantees quality confirmed by accredited laboratories which have carried out comprehensive product tests.
