RcTek : Information & Resources for the Model Car Racer

Model Car Differentials - The Basics

This section of the RcTek site is devoted to differentials, what they are, what they do and the different types of that can be fitted to radio controlled model cars.

This particular article introduces the benefits and pitfalls of having a differential fitted to your model car.

What is a Differential?

This is more fully explained on the Differential Section Main Page, but a differential is an aptly named assembly that exists to allow the driven wheels at each end of an axle to rotate at different speeds.

Why do the Wheels Need to Rotate Differently?

The animation on the right depicts a car travelling around a corner (in this case, a continuous one!). The red lines represent the path that the wheels follow.

If you Play the animation you will notice that the inside wheels of the car are following a smaller diameter circle than the outside wheels.

This smaller diameter circle has an equally smaller circumference and to stop the driven rear tyres spinning and losing traction, the inside wheel needs to rotate at a slower speed to cover the reduced distance in the same time as the outside wheel. You may Stop the animation if required.

It may help to if you look at this mathematically, using the examples of the outer red circle being 2 metres diameter and the inner red circle being 1.6 metres.

The circumference of a circle is calculated by multiplying Pi by the diameter;

2 x 3.14 = 6.28 metres

1.6 x 3.14 = 5.02 metres

Hopefully you can now see that, with a difference between the two circumferences of 1.26 metres, the reason why you need a differential.

You can also deduce from this that the wider the car, the bigger the difference between the circles and the greater the need for a differential. Also any limited slip differential will also have to work harder to cope with the bigger difference.

Do I Need a Differential on my Car?

This depends on the car, it’s driver and the driving conditions. As you can see by Playing the animation on the left, when you go in a straight line you don’t get any benefit from a differential, the lines are the same length.

There are real benefits for not having a differential and not all model cars are fitted with them. You may Stop the animation if required.

The Pitfalls of Having a Differential

There are a few pitfalls in having a differential fitted to a model car.

Firstly, the weight of any differential assembly is weight where you don’t want it - high up and rotating. This weight is also power sapping weight that only adds to acceleration and deceleration times.

Also, as model car differentials generally tend to be larger than scale, you may wish to consider the gyroscopic effect of having this weight spinning at high speed. This gyroscopic effect will possibly have a stabilising effect on the car (it seems to work OK on satellites!).

Another point worth noting is the power losses associated with any type of differential. Some are worse than others, anyone who has tried to touch certain limited slip differentials on the large scale cars will tell you they get very hot. This heat is generated by energy that would be more effectively used to create speed.

Four Wheel Drive & the Differential

The front and rear wheels on a model car do not travel through the same arcs (parts of the circumference of a circle) when a corner is negotiated. This difference means that the the front wheels need to travel different distances than the rear ones.

Playing the animation on the right demonstrates the different paths taken by the front and rear wheels when a car negotiates a corner. This is part due to the narrower width commonly used on the front wheels, but is also due to the fact that the front ones turn and the rear ones don’t.

The front wheels, being steered at the correct angle, lead the car around the corner. The rear ones are not turned (we shall discount any Toe-In for the sake of simplicity) and trail behind. As they being forced in a direction that they would not ordinarily want to go a slip angle is generated.

A slip angle is as it suggests, the tyre travels at a slightly different angle than the wheel and this causes a slightly different path to be taken at the rear of the car. You may Stop the animation if required.

This slip angle effect is an important one that will be covered more fully in a separate article in the Model Car Handling section.

Although this difference is true for two wheel drive cars and should be taken into consideration, two of them are not being driven, so there are no speed difference conflicts between the front and the rear.

In a four wheel drive car however, this difference needs to be addressed or the car will be unpredictable.

Four Wheel Drive - Differential or Solid Axle?

As stated above, not all model cars are fitted with differentials. Since the advent of four wheel drive, many of the 1/8th & 1/10th scale circuit cars do not have them fitted as standard, they have a solid axle that drives both wheels equally.

The 1/8th scale rallycross cars, however, have three differentials fitted, and not uncommonly, they are of different types.

Four Wheel Drive with Solid Axle

The vast majority of competition 4 wheel drive model circuit racing cars are not fitted with differentials and gain performance from having a lightweight solid axle to power the rear wheels.

They get around the speed differences between the front and rear wheels (which is more important due to the foam tyres they use and the different diameter tyres they use front and rear) by the use of One-way Bearings in the front transmission axles.

These one-way bearings engage and disengage the drive to the front wheels automatically depending on the available traction.

These cars still suffer from having the inside rear wheel slipping, but the trade-off with loss of traction and increased weight of having a differential fitted is a worthwhile one.

The use of bearings in this way also allows the potential to drive the front wheels at a higher speed than the rear ones. This bonus, which is appropriately termed overdrive, has real benefits and may be covered in a future article.

Four Wheel Drive with Three Differentials

Although there are alternative configurations, the majority of the rest of the four wheel drive cars are fitted with three differentials.

Having read the above information about the different paths the front and rear wheels travel through when negotiating a corner it should be easy enough to understand why three differentials are needed for a full-time four wheel drive car to perform predictably.

The differential that is on the front axle caters for speed differences between the front two wheels, the differential on the rear axle caters for the speed differences between the two rear wheels and the centre differential copes with differences between the front and rear axle speeds.

As there are many different types of differential available that offer different benefits, the possibility exists to fit a different type at one end or in the middle of the car to take advantage of its characteristics. This is used to great effect on the 1/8th scale rallycross cars and as these articles are expanded you will hopefully be able to make an informed decision.

Related Information

Model Car Handling

One-way Bearings

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© 2001, 2009 by Darren Burnhill