For the last decade Group N Rally Cars have often had the term “Dog-Box” used to refer to the type of internals used within their standard transmission cases.

Whilst many of us have some understanding of what this means, there are also many of us that don’t really appreciate what is going on with the “gears” inside of the “box”.


Holinger Engineering of Melbourne has been at the forefront of racing transmission technology for decades, making them well qualified to explain the pros and cons of the fabled Dog-Box.

Dog-Box or Synchro - What’s the difference?

The first thing to establish is a basic understanding of what is happening within a gearbox that enables a driver to select a gear.

For simplicities sake, imagine the gearbox has two shafts. One is an Input Shaft which is coupled to the engine via the clutch and the other is an Output Shaft which is attached to the wheels via the differential. Between these shafts are the different gears that can be selected by the driver.

Now imagine the gearbox with the clutch fully disengaged, disconnecting the engine and leaving the gears and shafts spinning at road speed. When a new gear is selected the Input Shaft must change speeds, because the Output Shaft remains at that constant road speed.

A regular synchromesh gearbox effectively has cone-clutches on the front of each gear pair that “synchronise” the speeds between them and the shaft they locate on.

As the driver applies pressure to the gear lever in the direction of the desired gear, the “synchro” acts as a friction clutch and matches the speed of the gears and shaft before engaging smoothly and with minimal fuss. Then the driver can re-engage the clutch to match the engine speed with the road speed that the gearbox is rotating at.

This whole process is quite refined in modern cars, and it takes time, something that we always try to minimise in racing.

Enter the Dog-Box. With this type of transmission, the synchromesh system is replaced by simple robust face “dogs”, also known as dog- clutches. The following picture shows the difference between the concepts:

In the upper portion you can see a standard gear, synchroniser and selector ring with many fine engagement teeth. In the lower part you can see an equivalent “dog” design, which has only 6 much larger and more robust engagement teeth, without the synchroniser between the gear and selector ring. The wider gaps between the dogs allow a greater amount of backlash: this is desirable as it provides a larger opportunity to engage the gear.  

In converting a gearbox over to a simple dog-change system, the driver can simply select the desired gear very quickly by not having to wait for the synchro to match the different speeds between the two.

The catch however, is that this speed change is virtually instantaneous. Even when the driver uses the clutch, the friction plate and input shaft both have to accelerate “instantaneously” to the new speed. All that energy and inertia must be absorbed elastically by the system. The gears themselves, the shafts, bearings, cases etc. all play a part in absorbing this shock.  Imagine the shock loading that occurs when the driver changes gears with the clutch fully engaged, or “flat-shifting”, which is generally the norm with dog-change gearboxes? Then the engine internals, flywheel and all the other associated parts have to change speeds “instantaneously” also. It can make your bones ache just thinking about it……

With this in mind, it’s easy to see a rather large drawback with the dog-change conversion kits that are on the market today. Whilst the internal components are strengthened substantially over their standard counterparts, they still transmit the previously mentioned “shock” loads into standard cases, differential gears and often other unmodified parts. The saying goes that “the machines only as good as the operator”, well it is equally true that the gears are only as good as what’s supporting them.

This has led to a trend in recent years, which has seen more and more rally cars fitted with stand-alone race transmissions, purposefully designed to take the loads associated with motorsport and dog-change designs. It would also be fair to say that 90% of them are also sequential change gearboxes, which further limits the damage a driver can do by avoiding a “missed” gear or incorrect selection at a critical moment common with standard H-pattern varieties.

Holinger Engineering, whilst best known for their V8-Supercar gearboxes, has been manufacturing both dog-change kits and stand-alone transmissions for decades, for everything from open-wheelers to rally cars. Design Manager, Leigh Nash, puts it simply: “Kits can often be more trouble than they are worth. If the rules allow only the internal components to be modified, like in FIA Group N for example, then that’s all you can do. But if you can change to a complete transmission, then do it! You’ll get increased performance, extra reliability and over time potentially save a whole lot of money. This I can promise.”

So whilst a complete race designed transmission might only seem a pipe dream for some, what is the reality? This can certainly be a “how long is a piece of string” type of question, but Holinger say that a proper transmission can typically cost around 40-50% more than an equivalent kit. So what does your money get you specifically?

“Well you start with a relatively clean sheet of paper,” says Leigh. “Of course there may be some space restraints in a certain type of car, but usually you can lay out the gearbox as a gearbox needs to be laid out. Motorsport places such high demands on the equipment, so incorporating oil pumps, oil spray bars, correctly sized bearings, better shaped castings made from a stronger material and more accurately made parts throughout the whole transmission is a must. Not to mention a dog-change dedicated shifting mechanism. This can only be achieved correctly with a complete design.”

“The cost is initially higher, there’s no denying that, but it’s worth the investment. Kits are often a case of being penny smart, pound stupid…. complete Holinger transmissions generally have very low running costs, due to far less costly failures and maintenance required. Over the course of a gearbox’s life, the price differential can more than pay for itself. After blowing up one gearbox with a kit installed, then needing to buy another to replace it, you’ve already spent more than what a complete gearbox costs and endured a DNF!”

The Australian Rally Championship is moving away from Group N and into a 2-wheel-drive formula. What does this mean for transmissions?

Whilst Holinger can manufacture kits for customers, they also offer a range of complete front-wheel-drive and rear-wheel-drive gearboxes that can be easily adapted to many types of cars. The FIA R2 and R3 classes that the new formula is likely to be based on allows the use of proper race-boxes, which means that competitors will be faced with some options on how to build their cars. With the variety of models bound to be running, it makes sense that a complete race gearbox will be a better way to go in the long term, as a competitor could also switch it between makes if required, something kits certainly don’t allow.  

So as a new dawn approaches rallying in Australia, so too may the way we change gears.    

For more information on Holinger Engineering, visit their website www.holin ger.com.au

- Article supplied by Holinger Engineering.



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