Resident tech-head Rod Dawson delves into the mysteries of the gearbox

If you’ve read our last technical guide, you’ll see we were talking about the importance of links within the workings of a car in particular the engine and the transmission. We’ve already had a closer look at the clutch assembly and the flywheel, so this time around we’re going to cover the most important link in the chain: the gearbox. The gearbox is the crux of the transmission in any car, and by transmission I mean every component in between the flywheel and the road wheels, including the gearbox, the transfer case (if four-wheel drive), the propellor shaft, the transaxle (if front-wheel drive), the differential and the half shafts.

A clutch is obviously vital for the reasons described in the last issue, but as far as the transmission is concerned, the characteristics of the gearbox are really what determines how a car will perform. It’s also a complex and sometimes fragile piece of machinery that has to be matched to the engine to properly carry out its function.

In this article we’re going to look at the main components of a standard manual gearbox. If you want to know what’s inside an automatic, go down to your local public library and read a copy of Popular Mechanics. Then you can compare notes with the other trainspotters sitting around you. Sorry people, automatics might be good for V8s and funny cars pulling off six-second quarter miles, but we’re interested in actually having some element of control over our cars here.

So what would you find if you were to remove your gearbox from your car and strip it down on the dining room table? Inside the casing of a regular manual transmission there are three shafts. If we work our way backwards from the engine, we have the input shaft, which you may remember is connected to the engine and flywheel by the clutch assembly. The input shaft is what carries the power from the engine into the gearbox. It does this via a toothed gear that is permanently fixed to the end, which is located inside the front end of the gearbox. The second shaft within the gearbox is called the layshaft. This is not a direct linear continuation of the input shaft; it is a separate component and is connected to the input shaft by a toothed gear on its front end. It is also located on a different plane to the input shaft sometimes below, sometimes beside and sometimes above the end of the input shaft. The last shaft within the gearbox is called the output shaft. This is what transfers the power from the engine to the propellor shaft and on to the differential.

It’s important to understand the layout of the three shafts to gain a proper understanding of how they all work together. Where the input shaft ends, the layshaft begins but the layshaft is on a different plane, as already explained. The output shaft, however, sits parallel to the layshaft and for a very good reason. Running along the length of the layshaft are toothed gears, which are fixed to the shaft. That is, they do not freewheel on bearings separately from the layshaft. For every turn of the layshaft, all toothed gears turn as well.

Here’s where it gets tricky. These toothed gears mesh permanently with the corresponding gears on the output shaft. The toothed gears on the output shaft are what determine the rotational speed of the gearbox’s output shaft as a ratio of the number of turns of the engine input shaft. A gear with a ratio of 2.0:1 means that at an engine speed of 2000rpm the input shaft will be spinning at 2000rpm but the gearbox output shaft will only be spinning at 1000rpm. In simple terms, the lower the gear ratio the easier it is for an engine to turn the wheels and move the car from a standstill. A gear ratio of 1:1, which is sometimes the ratio of the top gear on a 5-speed manual, means that for every rotation of the input shaft there is one rotation of the output shaft. This means that the engine can efficiently transmit 100 per cent of its torque (assuming the clutch is in good working order) without having to work too hard. Unless an engine has exceptionally high torque figures, most gearboxes will have lower ratios for first, second and third gears, with fourth and fifth gears approaching a ratio of 1:1. Sometimes in a 6-speed manual gearbox there is an overdrive gear, such as 0.75:1, which means that only three-quarters of a rotation of the input shaft is required to turn the output shaft one full revolution.

So, back on to these shafts and how they’re connected. As I explained, the gears on the layshaft are fixed to the shaft. However, the gears on the output shaft are not fixed directly to the shaft. They freewheel on bearings, which are instead themselves fixed to the output shaft. They’re of a similar kind to regular wheel bearings or journal bearings on a crankshaft. You’ll understand why these gears are mounted this way when you read on further. Here’s where you need to put that imagination into play: I’m going to give you two scenarios, and you will need to picture in your mind what is happening within the gearbox to understand why the gears on the output shaft are not fixed and instead run on roller bearings.

Scenario one: With the engine in your car running, the clutch engaged and the gearbox in neutral, the engine input shaft is spinning (obviously), as is the layshaft (remember it is always connected to the input shaft) and all of its gears (they are fixed to the layshaft), as are all the gears on the output shaft (remember the gears on the layshaft are constantly meshed with the gears on the output shaft). But wait the output shaft is not spinning at all. Why? Because the gears on the output shaft are mounted on bearings, so although the gears are spinning around, they are only doing so on the bearings and the shaft does not move at all.

Scenario two: With the engine off and the clutch engaged, you are having your car towed with the driving wheels in contact with the ground and the gear selector in neutral. The rotation of the road wheels, which are connected to the differential and propellor shaft, which is connected to the output shaft in the gearbox, causes the output shaft to rotate. But wait none of the other shafts or gears are moving at all. Why? Because the output shaft is this time rotating on the bearings without affecting the gears running along its length.

And here is the question: how do we connect the power from the input shaft to the output shaft if the gears are running on bearings and don’t seem to be doing a whole lot otherwise? This is where the selector forks and collars enter the equation. Running along the length of the output shaft, in between the gears themselves, are the selector collars and synchromesh rings. It is the activation of these units, which are controlled by the motion of the gear lever inside the car, that connects the inner workings of the gearbox together and allows the transmission of torque from the input shaft to be transferred to the output shaft via the layshaft.

So how is a gear on the output shaft engaged to enable this transfer of torque through to the wheels? Let’s say you are driving along and want to shift up a gear. When you depress the clutch pedal the engine is temporarily disconnected from the gearbox. This has the effect of removing power from the input shaft, which in turn means the rotational speed of the laygears (and correspondingly the gears on the output shaft) slow down. As they slow down, the movement of the gear shifter inside the car activates one of the shift levers on the outside of the gearbox casing. This activates the shifter fork, which slides the collar along the output shaft either forwards or backwards, depending on which gear you are selecting. The collar itself is permanently connected to the output shaft and does not freewheel, but can slide along the shaft as it is mounted on splines. As the collar nears the rotating face of the gear, a cone-shaped section on central axis of the gear slides into a coupling inside the collar. As it does so the speed of the gear on the output shaft begins to match the speed of the output shaft (the speed of the output shaft is determined by the rotational speed of the road wheels, whereas the speed of the gear itself was being determined by the input shaft). When the friction has caused the speed of the output shaft gear to match the speed of the collar, the collar then fully slides over and engages dog teeth on the face of the gear. So while this newly selected gear was previously running on its bearings at a speed determined by the input shaft (which is at this point disconnected), the gear is now connected directly to the output shaft via the dog teeth within the collar which is connected to the output shaft via the splines. When the clutch pedal is released the engine is connected to the gearbox again and the torque travels through the input shaft, then the layshaft, and then directly through the selected gear on the output shaft, which has been engaged by the dog teeth on the collar. So now the torque from the engine has a connection with the road wheels, and all through that sliding collar which is locked against the face of the gear.

This explains why when you miscalculate a gear shift, that nice graunching sound is not generated by the clashing of gear teeth like most people imagine. The gears on the layshaft and the output shaft are permanently meshed. It is the sound of the dog teeth on the face of the output gears missing their engagement points on the collar. If synchromesh is worn out and is not slowing down the output gear sufficiently, the dog teeth will not be able to slot into position and engage the gear smoothly.

In terms of the materials used within a gearbox, the gears themselves are generally of cast iron construction while the shafts are usually steel. It does mean that over time the gear teeth and faces can wear down with extended use, but like anything mechanical, longevity is heavily dependent on the quality of lubrication and the frequency of servicing. It’s important that you service your gearbox on a regular basis with the correct viscosity oil and to make sure all shifter connections are checked over and tightened.

Modifying a gearbox is no less complex than modifying an engine and can have an equally dramatic effect on the performance of the car. Gear ratios can be changed without too many issues, but remember that the size, power band and torque delivery of your engine were all factored into the equation when the manufacturer designed the gearbox to suit. Unless you plan to compete on long race circuits or short rally courses, there’s not a lot of point altering gear ratios. You’re better off developing the robustness of the gearbox as a unit. Steel selector forks can replace the factory cast iron units for increased strength. This is a common modification that can improve shift quality and resist breakage. But in terms of internals, the best option is to have your gearbox stripped and ensure the synchros are replaced. These are the components that suffer the most wear and tear if they’re in good order the rest of your gearbox should continue to function well for its working life.

Words: Rob Dawson

This article is from Performance Car issue 142. Click here to check it out.