What kind of wheel drive is better for everyday driving
Examining which one is better, I should note that the differences between the kinds are mostly manifested in the manner in which a car slides when the leading wheels slip or are about to slip.
The rear-wheel drive (or RWD) slips with rear tires — skidding and trying to veer across the road. It’s called the loss of stability, or oversteering. The front-wheel drive (FWD) slips with front tires respectively — drifting and mishandling the curve, which is called a loss of control, or understeering. Things are a lot trickier with the four-wheel drive (4WD) though: it can slip either with rear wheels, or with front ones, or with all four of them. Hence the difference in car behavior in sliding, and the difference in methods of handling that kind of situations.
Now imagine that we shifted to neutral on the go and are coasting. Which effectively turns the car with any kind of wheel drive into a trolley running-out. Does it make any difference then what kind of wheel drive the car has? Of course it doesn’t! For it’s just a trolley without a drive. Until we shift into gear and accelerate so hard that the leading wheels go slipping, that is.
There are some other differences between the kinds of wheel drive of course, they are not necessarily about the sliding, but that’s nuances.
Let’s now take a further step and recall that most modern cars are equipped with the Dynamic Stability Control (DSC) system, which may also be called the Electronic Stability Programme (ESP). What does this system do? Firstly, it selectively applies brakes to some of the wheels when the car is about to veer off the road or go into a skid or other similar situations. Secondly, it “stifles” the engine when the driver goes too far with the gas pedal and the leading wheels start slipping.
So the stability system makes it impossible to overdo the gas and does not allow the leading wheels to slip. Meaning the peculiarities of different kinds of drives lose their significance if the car is equipped with such system.
Which leads us to an important and uncompromising conclusion: the behavior of modern car with any kind of wheel drive is determined not by the drive type, but by how the stability system is calibrated.
However, there still exist the differences that show themselves even when the system is turned on, like the acceleration dynamics on slippery surface, flotation, comfort, steerability. But, first things first.
Acceleration on dry surface
RWD: the vehicle weight is redistributed to the rear axle during the start, increasing the rear wheels road grip. The leading wheels slip less, providing the efficient acceleration.
FWD: during the start, the weight is redistributed to the rear axle as well, the leading wheels load is decreased, they show tendency to slip, the acceleration efficiency may deteriorate as a result.
4WD: since all four wheels are leading, the weight redistribution does not affect the acceleration. What’s interesting, with hi-power engines (up to approximately 500 h.p.) which don’t allow slipping, 4WD has no advantages over the RWD. It may even be for the loss, as it has bigger weight.
Acceleration on slippery surface
RWD and FWD: on slippery surface, and on the ice in particular, the weight redistribution is insignificant, so the weight distribution is close to that of an idle car. In this case, the RWD has approximately 50% of its weight on its leading (rear) wheels, while the FWD has about 60% of the weight on the front ones. Which allows the FWD to accelerate faster on a slippery surface than the RWD.
4WD: weight redistribution does not affect the acceleration, as all four wheels are leading, including the rear ones that are more loaded and have increased road grip.
Besides that, with 4WD the engine torque is distributed in equal 25% proportion to each wheel (although other proportions are available as well), whereas with two-wheel drive each leading wheel gets 50% of torque. This means that the probability of wheel slip with 4WD is less than with two-wheeled monsters.
And finally, the main advantage of 4WD during slippery surface acceleration is due to the fact that the leading wheels carry the whole mass of the car. Whereas with two-wheel drives the leading wheels account for about half of the mass of the car, while the remaining half is basically a dead weight and only increases the inertia of the car. Therefore, the 4WD provides a more dynamic acceleration, particularly on slippery and loose surface.
But, on the asphalt surface, where the slip of the loaded rear leading wheels is unlikely, RWD usually is on par with the 4WD when it comes to acceleration, and the 4WD is meaningless.
There is a notion of stability as well – the ability of a vehicle to maintain the motion in a specified direction. This is defined by the torque on the rear axle. The bigger it is, the more the rear of the car bounces on the road. RWD is always the first nominee to veer off the slippery road! Number two is the 4WD, because although the rear axle torque is less than that of the RWD, yet it still is there. The FWD is the most stable during the acceleration, as it has no torque at the rear; the car’s rear obediently follows its front. Indeed, the 4WD does accelerate faster but the car’s rear still bounces around a bit. The FWD is slower, but more stable. Therefore, the easiest and safest winter option for the rookie driver is a FWD car.
Flotation is a separate matter, especially important for residents of areas with snowy winters and suburban areas. The principle here is quite simple and well-known: pulling is easier than pushing, and four leading wheels are always better than two. Hence the 4WD is the flotation champion, FWD comes second and the RWD comes in a good third.
Going into corner begins with the front wheels turning at the arc, which triggers the risk of them sliding (drifting). The lower the drift potential is, the faster and safer going into corner is.
RWD: the engine torque is absent on the front axle, so there is no risk of drifting due to an overtorque, the drift may happen only in case of going into a very sharp corner with too much speed.
4WD: part of the engine torque goes to the front axle, so the drifting may happen both as a result of going into a corner at great speed and as a result of pushing the gas pedal too much. That means the chance of drifting is higher than with the RWD.
FWD: the torque is fully transmitted to the front axle, making the front wheels the most susceptible to gas overdose, and the chance of drifting is the highest among all kinds of wheel drives.
Going into a corner, RWD is the fastest and safest; 4WD is a bit less safe, while the FWD is the most unsafe. This works both for asphalt and slippery surfaces.
Drifting means the loss of control, while skidding – the loss of stability only, whereas the car remains under control. So, on the one hand, the drift is more dangerous than the skid, ’cause the car is going totally the wrong way. But to stop skidding you must possess at least some driving skills, in particular being proficient in speed steering technique. Getting out of the drift is much easier and does not require special driving techniques (but of course, you have to have enough space on the road to maneuver). Regardless, a drift is considered a much more dangerous instance.
Due to their features, RWD is more prone to skidding while the FWD tends to drift, should the driver push the gas too much. In simple terms, the RWD is safer, but requires higher driving skills. Contrary to popular opinion, the FWD is not safer than the RWD, but easier to handle for a rookie.
With too much gas, the FWD is equally prone to skidding (if the torque distribution between the axles is 50:50), and drifting, and can behave both as 4WD, as FWD and as RWD during the slide. The good thing is, it’s all predictable – with FWD the car’s front wheels skid, with RWD – the rear ones do. And with the 4WD, any of them might. That is why this kind of wheel drive requires truly advanced extreme driving skills – with all kinds of wheel drives. Moreover, when it comes to 4WD, the driver has to be skilled in exactly the 4WD he’s driving. During the slide the engine torque can be distributed axle-to-axle using differentials, to change the kind of wheel drive for a short time. You thought it was a front axle sliding and revved up a bit, and dang! you’re already crashing sidewise into a barrier cuz your rear axle started skidding too after you toyed with gas. The situation is getting worse with the part-time 4WD.
Turning a corner by drifting is a different kettle of fish. The 4WD makes sense then, after all it is utilized in rally for a reason. It’s kind of its skid is coming from the rear torque but doesn’t turn the car around back to front – thanks to the front axle torque. And it’s kind of accelerating sidewise. Again, it’s a car slide we’re talking about. But the real question is – why the 4WD vehicle for the in-city driving?
All in all, the RWD is the fastest and most comfortable for asphalt surface. It gets stuck in the loose surface, and is unsteady during slippery surface acceleration in the absence of ESP/DSC. It’s difficult to handle on a slippery surface, making it quite dangerous for inexperienced drivers.
The FWD is the most stable during slippery surface acceleration and has decent flotation. It’s the most suitable kind of wheel drive for most unsophisticated drivers for city operation, and it’s the least dangerous one.
The 4WD without ESP/DSC is the least predictable of three, requiring the defensive driving skills for all three kinds of wheel drives and flawless steering and pedaling. It’s ideal for off-road, rally driving and slippery winter roads. And it’s meaningless for asphalt. Never the safest one, especially in hands of unqualified driver – this way, in fact, it becomes the most dangerous.
The contemporary cars with different kinds of wheel drives and stability systems will differ only a little — in slippery surface acceleration and flotation. In terms of active safety and loss of stability or control all drives are equal. Although 4WD has a clear advantage during the slippery surface acceleration. Therefore, if the car is equipped with a stability system and the driver has a proper perception of a car (no expectations too high and a sense of false superiority of 4WD) the 4WD can be considered the safest of all.