Differentials Part–2
In part–1 of this two–part series we talked about how, mechanically speaking, a two–wheel drive car is really a one–wheel drive car, and how most four–wheel drive vehicles are two–wheel drive vehicles, on account of them being equipped with only a centre differential lock and no other means of wheel spin control. We also learned what a differential is and how, within an axle, it is a set of gears inside a casing that provides a ‘T’ –junction for the transmission of energy (drive) between the engine to the wheels. This is to allow each wheel of an axle to rotate independently of its partner.
The ability for two wheels of the same axle to rotate independently is entirely necessary to allow the vehicle upon which they are mounted to travel around corners. However, whilst a differential is a very useful bit of kit in one sense, there is a darker effect to having the ability to allow two ends of a shaft to rotate at their own individual pace.
 When any wheeled vehicle is being driven in a straight line, all wheels are travelling at the same speed. A very straight forward statement, you might say, but stay with it as it does get a little more complicated.
We also know that when an axle travels around a bend in the road, the wheel at each end will be travelling at different speeds. In fact there are three speeds being registered here. In a straight line, and at 30mph, the left hand (near–side) wheel, is travelling at 30mph, as will be the off–side wheel, and indeed the centre of the axle. If that vehicle is steered around a bend in the road, and let us say that bend is to the left, whilst the centre of the axle is still travelling at 30mph, the near–side wheel speed will drop below 30, and the off–side will accelerate above it.
So far, so good, but by how much difference will the speed of the individual wheels change during cornering, and do we really need to know? Well actually we do need to know, for the purpose of this article anyway, as by giving you this information we are building up to the main point.
The answer to the above question can be worked out by completing a simple mathematical equation.
 Still taking the one axle in isolation for the moment, and still travelling at 30 miles per hour, the method of working the speed loss from one side and the speed gain at the other is easy. As the vehicle speed, and therefore the speed at the centre of the axle, is 30mph, whatever speed is then gained by the wheel travelling the outer arc, as in the outside of the bend, is taken from the wheel travelling the shorter arc on the inside of the bend. Therefore, at 30 miles per hour, through our left hand bend, if the off–side wheel speeds up to 35mph, the near–side wheel has to slow to 25mph.
So, during cornering, whatever energy is lost by one wheel, that is picked up by the other, and as we know, this differential in speed has been allowed to happen because of the presence of the differential unit within the axle.
 To take this a stage further, if when you try and drive with one of your driven wheels on a slippery surface, if that wheel breaks traction and spins (a driven wheel that has lost traction is the least line of resistance in terms of use of energy), that wheel will assume the total speed of that axle. This is why, if you are climbing a hill in snow or ice, where you may have been doing 30 miles per hour, for example, you then end up with one driven wheel doing 60mph, the centre of the axle at zero and the other normally driven wheel doing zero — with the vehicle now stationary and stuck.
This is why wheel spin is such an aggressive experience, because as described within the above scenario, the spinning wheel suddenly doubles its speed, which is all down to the gearing in the differential unit.
The Differential Between Two Axles
If we are clear on the above information, we shall move on, because this is where it now gets really interesting.
Now that we can understand the effect of a differential within a drive axle, and how this transfers energy, we’re now going to look at a differential that allows a speed difference between two axles, as present within a 4x4 vehicle. In the same way that an axle differential will transmit the speed lost by one wheel across to the other upon that same axle, the centre differential of a 4–wheeled 4x4 vehicle will do the same between axles.
Thinking of an everyday driving scenario, under what circumstances may one axle be asked to rotate at a different speed to the other?
Acceleration on a slippery surface is one answer, and yes you would be right with that one, but power induced wheel spin tends to occur by only one wheel of a driven axle losing traction. What about braking?
How The Vehicle Accelerates When You’re Trying To Stop
Imagine you are driving a 4–wheel drive vehicle in the rain along a motorway, and the car has no ABS (we’ll talk more about ABS further on). Suddenly, and for whatever reason, you are in an emergency situation and you stamp on the brakes. If that application of brakes is aggressive enough to lock the front wheels into a skid, what do you think the effect will be upon the car?
Within a vehicle in motion, and through a differential unit within an axle, when one wheel of that axle loses speed, that energy is given to its partner. So, by the same rules, when there is a differential unit between two axles, when one axle loses speed relative to the speed of the vehicle, that energy is transferred from that axle to the other. This means that with our non ABS 4x4 in the motorway scenario described, if we lock both the front wheels under braking, whatever the speed of the car at that time, the free running axle will effectively try to double its speed. The overall effect will be that of the car initially accelerating instead of slowing. So, if you were driving at 70mph when this happened, the back of the vehicle will try to push the front at a speed of 140mph!
Anti–Lock Brakes Will Prevent Front Axle Locking
This is scary stuff having the rear axle trying to push you at double your initial speed, and at the point when you are frantically trying to stop, but before you go off into a state of blind terror of all 4x4 vehicles, we wish to assure you that if the 4x4 is fitted with ABS (anti–lock brakes), this effect will not happen. This is because, by definition, the anti–lock brakes will not allow the front wheels (front axle) to lock up. Also, the vehicle involved has to be one that is a permanent 4–wheel drive layout, as opposed to a part–time 4–wheel drive for the acceleration under braking effect to take place.
However, there are many permanent 4–wheel drive vehicles out there that do not have ABS. Take Land Rover, for example. This manufacturer has been producing its 4x4 vehicles since 1948 and in that time has sold them by the millions. In fact, up until the year 2000, two–thirds of all Land Rovers ever sold were still in use. They are indeed the most common 4x4 vehicle to be found on our roads, but that doesn’t mean to say they are any better or worse than those offered by other car makers, as their frequency is perhaps due to the sheer length of time of production.
Forgetting the more upmarket variants, such as the Range Rover, Discovery and Free Lander, the traditional Land Rover, as in those in the style of the Defender, 110 and 90, tend to be less technically sophisticated and it has only been in very recent times these vehicles have been equipped with ABS. You will of course know if your vehicle has ABS if you own one, but if you see one in your rear view mirror, just be mindful that in an emergency it could be joining you in your car — as could other makes of off–road vehicles and for the same reason.
You Can Use Your Diff–Lock On a Wet Road
If you drive a permanent 4–wheel drive vehicle, and it does not have ABS fitted, you can protect yourself from this nasty experience through driving technique.
Most of these vehicles have what is called a differential lock, which as discussed in part–1, is a device fitted to the centre differential unit to enable to driver to lock it up so that the two propeller shafts, which drive each axle, cannot rotate independently. If you are driving in wet weather conditions on a motorway, or similar, where the curves in the road are fairly gentle, Operate your differential lock control to the locked position. Under sudden braking this will prevent the front axle from locking up without the rear doing the same and therefore prevent the acceleration under braking effect from taking place.
In fact, if you are driving with the centre differential lock of your 4x4 in the engaged position, and you do have to stop in a frantic hurry, this set up will help you to stop very much more effectively than a family car, as you will gain maximim stopping power from all four wheels.
What About Transmission Wind–Up?
Transmission wind up is a term used to describe a situation that can arise where the transmission shafts of a vehicle become stressed by experiencing opposite forces, and this would typically happen across an axle if each wheel was not allowed the freedom to rotate at a different rate to its partner, such as when the vehicle is cornering.
The easiest way to understand this would be to grab hold of a broom by the end of the handle and then have someone else to hold on to the other end whilst you both try to twist it in opposite directions, or one twist whilst the other holds still. Although you will not be strong enough to actually achieve it, if you could move the ends in opposite ways you would create a huge amount of sprung–loaded tension in the shaft. If it were the case that two axles of a 4x4 could not rotate at independent speeds, the propeller shafts between would become stressed, and this can be damaging to the transmission.
However, on a slippery surface, such as gravel, mud, grass, or even a wet motorway, there is enough slippage from lost grip of the tyres to prevent the tension building up. Some 4x4’s will not allow you to engage the diff–lock without dropping into the low–ratio gearing — as facilitated by what is called the transfer gearbox. If this is the case then obviously driving at road–going speeds with the diff–lock engaged is not an option.
With Skilled Braking Method There Should Never be a Problem
As stated within our pages that cover braking, a skilled driver will be able to sense when the front wheels are about to lock and can hold the pressure on the brake pedal at a point just prior to where lock–up takes place. Referred to as Threshold Braking, this is another way in which you can bring your 4x4 vehicle to a well controlled stop, even in an emergency.
When Top Gear Did an Emergency Stop Test, They Got it Badly Wrong
Quite an number of years ago, the BBC TV motoring programme, Top Gear, filmed a comparison test to show that high–performance cars and sports cars were actually safer because they had better brakes and could therefore stop quicker than other vehicles. The three vehicles chosen for this test were a Porsche 911, a Ford Mondeo class family type car and a Land Rover Discovery 200–Series.
All three vehicles were driven along a section of runway, each at the same initial speed, and with the driver performing an emergency stop as he reached a traffic cone marker. The winner of the contest was the Porsche, presumably as intended, and whilst the family saloon came second, there was much crowing done as to how the Discovery had almost doubled the Porsche for stopping distance. In fact, there were suggestions as to how dangerous 4x4 vehicles were for this reason.
There are two issues here. One is that the brakes of a vehicle cause a braking effect upon the wheels, but it is the wheels (tyres) the give the braking effect upon the vehicle, and as the brakes on production cars are matched to the vehicle to which they are fitted, the argument between performance car and family car under braking was invalid.
Secondly, as the 200–Series Discovery was not equipped with ABS, and as the driver locked the front tyres during the experiment, when you consider the information given within this article above, of course the vehicle nearly doubled the distance taken to stop, as compared to the Porsche. Had the Discovery, during the test, been used with its differential lock in operation, it would not only have stopped in a much shorter distance, but would have most likely outbraked the Porsche!
The sad thing is Top Gear probably didn’t know the real reason as to why they got the result they did.
Julian Smith
Ride Drive Limited
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