There’s probably only one thing that all cars have in common, four road wheels. But exactly which wheels move the car along the road vary widely. ‘RWD’, ‘FWD’, ‘4WD’, and ‘AWD’ are all drive systems that make life confusing! Over the next couple of issues of The Private Fleet Newsletter we’ll explain the differences. This month we’ll look at front wheel drive (FWD) versus rear wheel drive (RWD) and next month we’ll look at four wheel drive and all wheel drive.
|Rear Wheel Drive|
By far the most predominant way that cars got traction was by driving the rear wheels. For decades it was considered the only way to go – let the front wheels steer the car and let the back wheels drive it, and ne’er the twain shall meet.
This became even more emphatic as cars became more powerful, as it was beyond the technical skills of the engineers to put power down through the front wheels and still steer the vehicle effectively.
Handling was also an important factor as modern aids were not available and rear wheel drive certainly gave the best- and the safest, handling at the time.
|Front Wheel Drive|
Whilst front wheel drive has been around since the early1920’s, and it was, to some degree, popularized by Citroen with its ‘traction avant’ in the 1930’s, it didn’t really catch on until the 1960’s with the advent of small FWD cars like the Mini. This innovative design, with its ‘east/west’ front engine configuration, coupled with front wheel drive provided much more interior space and saved a lot of weight by discarding the long, cumbersome and weighty driveshaft. Also by having most of its weight over the front wheels it had considerable traction advantages in slippery conditions.
But there were, and to some degree, still are drawbacks, the most notable of which is called ‘torque steer’. This is where, due to the engine driving the front wheels it distorts the steering, particularly with more powerful engines. As technology has advanced, however, this has become much more manageable and much less noticeable.
There is one other significant difference, and that’s in the handling department. Front wheel drive cars are more susceptible to ‘understeer’, whereas rear wheel drive cars are more likely to suffer from ‘oversteer’.
|What is ‘ Understeer’?|
Understeer is when the car doesn’t want to turn as quickly or as tightly as you want it to. You have either approached the corner too fast or there is a sudden lack of surface adhesion on the curve and the car wants to go straight on rather than turn in to the curve.
If you want to turn the steering wheel further to get in to the corner, the tyres will just slip more. The same will happen if you still have your foot on the accelerator.
Understeer is pretty easy to get under control as your natural reaction- to lift off the pedal – is exactly what you should do. Even if you panic a bit and apply the footbrake it will assist in getting front wheel grip back again and will get you out of trouble – providing you don’t panic too much and lock up the brakes.
|What Is ‘Oversteer’?|
Oversteer is more tricky to control than understeer as your ‘natural reaction’ won’t work!
Again, you’ve approached a corner too fast, or if conditions have changed and there is a sudden lack of adhesion, your car will protest. But with oversteer the front wheels retain their grip, but NOT the back ones! They will want to slide out sideways, attempting to spin the car around. That’s oversteer.
So how do you correct it?
Yes, you’ve been told to turn into the skid, but there’s more to it than that. Don’t apply the brakes (your natural reaction) as that will just make matters worse and bring the back around more quickly. Keep light pressure on the accelerator, turn your steering wheel into ‘opposite lock’ (which really means to turn it the opposite way to where you were wanting to go).
This action will start to correct the skid and the back of the car will stop wanting to hang out and bring itself back in line.
But you’re not finished yet!
As the back of the car starts to swing back in you are in danger of letting the momentum take over so the back simply continues to swing out the other way and spins you in the opposite direction to the first attempt!
So you’re almost back to square one, unless you have predicted this, and this is how to do it.
As the back of the car starts to swing back in, you turn your steering wheel back to the way you wanted to go in the first place. This will stop the momentum and allow you to straighten up the car and get it back under control.
Sounds complicated, doesn’t it? Well, it certainly takes some skill or practice and that’s why advanced driving schools spend a lot of student time practising how to control oversteer.. Because it is more difficult to control it is also the reason why engineers try to ‘dial out’ oversteer in favour of mild understeer.
So what about four wheel drive and all wheel drive systems (and there are differences)?
We’ll look at their attributes and some shortcomings next month.