4x4 Vehicle safety
It’s been very wet round here recently with some areas experiencing their worst floods for over sixty years. As you might expect there are tales of selfless effort and other tales of total selfishness. On the one hand there were drivers with large 4wd vehicles who went out of their way to help others and then there were the few who ignored the needs of others and drove their vehicles with no regard for anyone or anything.
This got me thinking about the general safety of 4wd vehicles and I came to what I think is an interesting conclusion.
4wd vehicles are inherently less safe than 2wd vehicles.
Ignoring all the psychobabble about feeling safer in a larger vehicle which I don’t understand, this is my theory.
Stopping a vehicle from any speed requires that the energy stored by getting the vehicle up to that speed must be expended. The amount of stored energy is determined by the speed of the vehicle and its weight.
For any given speed, a heavier vehicle will have to get rid of more energy than a lighter vehicle. The energy goes into heating up the brakes, tyres and even the road surface and also in a certain amount of noise in some cases.
4wd and 2wd vehicles have the same number of brakes, so there is no difference there. The difference between the two types is the number of wheels that take power from the engine, either 2 or 4. To take power from the engine to an extras pair of wheel requires at least one, probably two, extra differentials and their drive shafts. These are heavy pieces of kit and add quite a lot of weight.
So we now have a heavier vehicle which is not as easy to stop, but the same number of brakes. On top of this the 4wd vehicle is capable of building up speed in conditions that would defeat 2wd. So we have the situation that a 4wd vehicle is easier to drive into trouble and then less able to stop.
2wd or 4wd, drive carefully out there.
This got me thinking about the general safety of 4wd vehicles and I came to what I think is an interesting conclusion.
4wd vehicles are inherently less safe than 2wd vehicles.
Ignoring all the psychobabble about feeling safer in a larger vehicle which I don’t understand, this is my theory.
Stopping a vehicle from any speed requires that the energy stored by getting the vehicle up to that speed must be expended. The amount of stored energy is determined by the speed of the vehicle and its weight.
For any given speed, a heavier vehicle will have to get rid of more energy than a lighter vehicle. The energy goes into heating up the brakes, tyres and even the road surface and also in a certain amount of noise in some cases.
4wd and 2wd vehicles have the same number of brakes, so there is no difference there. The difference between the two types is the number of wheels that take power from the engine, either 2 or 4. To take power from the engine to an extras pair of wheel requires at least one, probably two, extra differentials and their drive shafts. These are heavy pieces of kit and add quite a lot of weight.
So we now have a heavier vehicle which is not as easy to stop, but the same number of brakes. On top of this the 4wd vehicle is capable of building up speed in conditions that would defeat 2wd. So we have the situation that a 4wd vehicle is easier to drive into trouble and then less able to stop.
2wd or 4wd, drive carefully out there.
posted by Richard Galle at 5:13 pm
3 Comments:
I disagree with your conclusions. I'll work backwards just because the first is easier to explain.
The first part I disagree with is the idea that "the 4wd vehicle is capable of building up speed in conditions that would defeat 2wd." I live in Chicago and during winters it is very easy to gain speed with any car. I drive a FWD sedan, a RWD sedan, and a 4WD SUV. You can take any car and take it to dangerous levels. This is true because at any point above 10mph and you hit ice you will have no control. It’s not impossible (or uncommon really) to drive over 20-40ft stretches of ice. All of the cars obviously have the same ability to steer and stop. As I understand your comment here you’re trying to say that a 2WD car cannot gain enough speed here to experience a loss of control – this is completely wrong as you can easily gain 30-40mph in the worst of conditions. I would be confident that you can go faster, but with my will to live I have never tried. 40mph is obviously impossible. The other advantage for FWD cars (which is a form of 2WD) is that a significant amount of the weight is over the drive wheels. Where RWD or 4WD cars have a balance approaching 50% for each set of tires, a FWD car and easily approaches 70% of the weight on the front. While this allows the rear to slide more easily, it means that you can still control the direction of the car since the weight in the front produces traction. The other benefit is that the front wheels always allow more braking then the rear regardless of the drive train type: as stated already a FWD car improves the traction on the front wheels, which allows you to maintain more braking ability.
The second issue I want to front is the idea that “So we now have a heavier vehicle which is not as easy to stop, but the same number of brakes.” The problem here is that you have ignored the fundamental laws that govern our world. Let’s look at the physics: The ability of a car to stop is provided by the friction between the tires and the road. The formula is: Friction = coefficient of friction * normal force. The normal force is effectively the weight of the vehicle, as normal force = mass * acceleration due to gravity. Therefore Friction = coefficient of friction * mass * gravity. To stop the car you convert kinetic energy into heat energy (movement of molecules, light, and sound). The formula for kinetic energy is Kinetic Energy = .5 * Mass * Velocity^2. If we look at the equations then we can see that the Friction possible will be proportional to mass as the coefficient of friction and the gravity will be the same for both cars. In the same way the Kinetic energy that needs to be displaced will be proportional to mass since the constant and velocity will be the same. Therefore the mass of difference of the cars will not affect the steering or the stopping distance of the cars because the increase in inertia will be met with an increase of possible force, and thus equal possible accelerations.
The last issue is more general and is that 4WD cars allow the engine to be connected as much as possible to the road. This helps with control because it prevents the tires from spinning at different speeds. You always achieve more control when braking with the engine as compared to the disk brakes. The issue here is that the disk brake on one side might have traction while the other side may not, this means that one side of the car will begin to slow down while the other cannot – if this happens the car can be put into a spin which is obviously a horrible scenario and often ends in major damage to the car when it collides with a solid object, or even worse other cars. This could include oncoming traffic that cannot stop or avoid it due to the conditions. If the car is braking with the engine then the wheel that provides traction will slow the car while still having all wheels spin at the same speed, allowing the car to move all points forward at the same rate.
I hope my understanding and explanations have made sense to you, if you disagree then just mention it and I will do my best to explain it further. The other important detail is that I don’t disagree that many drivers abuse 4WD and put themselves into bad situations, I just disagree that 2WD vehicles don’t do the exact same. If anything the problem is that 4WD vehicles have too much clearance that allows them to enter dangerous conditions.
Good words.
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