RcTek : Information & Resources for the Model Car Racer

Model Car Differentials - Advanced

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

This particular article explains the theory behind improvements that can be made to the open differential.

Improvements to the Open Differential

This article deals with the issues behind the improvements that can be made upon the ability of the open differential to create traction during cornering and assumes you have both read through, and understood, the Differential Basics, Differential Parts and How Differentials Work articles.

This points made in this article do not take into consideration an open diff that is fitted with seals and is filled with an oil, this we would interpret as a limited slip differential.

Beyond the Open Differential

There is nothing wrong with the open type differential when it is used in low powered cars at reasonable speeds. Model cars, however, are usually relatively powerful vehicles that are driven at the extremes of speed and available traction.

Due to the tendency of the open differential to transfer the driving force to the wrong wheel (You may Play and Stop the animation on the left) there is a need for an alternative that doesn’t exhibit these traits.

As stated in previous articles, there are many alternatives to the open differential and each have their own benefits and disadvantages, but they all have one thing in common, they are designed to close the ‘open’ nature of the open differential.

The simplest way of doing this is to use a solid axle, but this does not allow for any speed differences between the inside and outside wheels and compromises the handling of the car to a certain extent.

What is required is a happy medium that allows the car to corner without all drive being transferred to a wheel with little (or no) traction.

It’s all about Traction Control

Anything other than an open differential could, in reality, be thought of as a traction control device for corners, as the open differential adequately solves the problem of rotating the wheels at different speeds when the car is turning.

Improving an Open Differential

As stated at the top of this article, we would not consider an open differential is fitted with seals and is filled with oil to be an open diff in the true sense, as the oil acts to limit the open nature.

An open differential can most easily be improved as far as traction in the corners is concerned by using a high viscosity ‘sticky’ grease inside the casings. This increases the resistance of the sun and planet gears to freely rotate relative to each other and thus reduces the tendency to ‘diff out’.

Greases are available in different grades and a little experimenting with them should pay dividends. This testing should be done before race day as cleaning out the differential casings on race day isn’t recommended.

What You Loose

Any differential other than an unrestricted open differential has one disadvantage associated with it, a loss of steering.

This loss of steering is due to drive being restored to one or both of the rear wheels. This driving force tries to propel the car in the direction that the wheels are facing and therefore understeer occurs as the rear wheels push in a different direction than the front wheels.

Although it it stated by some that you do not lose steering with some differentials, this is only partly true, as under power there will be steering loss, what happens off power is dependant on the differentials speed and ability to disengage the drive.


That’s about it for now, more may be added at a later date, but don’t hold your breath!

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© 2001, 2009 by Darren Burnhill