|The gearbox is the part of the car that
transmits all that rotational energy from the engine into something
useful to turn the wheels, which by happy coincidence moves your car
either forwards or backwards!
Gearboxes are one of the areas in a kit build that causes a tremendous
amount of confusion. A correctly chosen gearbox matches the engine
output and allows your engine to generate forward motion with maximum
efficiency. Choose the wrong type and you will have at best, a car
that accelerates badly, at worst a car that will only do sixty miles
an hour at 7000rpm in top gear.
But first a word about torque and BHP. This is an area of confusion
that can lead to many false assertions.
Put as simply as possible, torque equals acceleration, horsepower
equates to top speed. But in itself this does not explain anything
really. Its much too simple to be useful.
Torque is the amount of weight a machine can lift. Explained in another
way, its the amount that an engine can pull, it has nothing to do
with engine speed. Horsepower on the other hand, is the amount of
work required to lift 33,000 lbs one foot in one minute. Horsepower
therefore is related to torque and rotational distance over a period
of time. Easy huh?
Fortunately for us, there is a simple formula to convert torque to
Power (HP) = RPM * Torque (lbs) / 5252
The 5252 figure is a constant. We could go into a lengthy explanation
as to how this figure is derived, but take it from me it works.
What has any of this to do with gearboxes. Well a gearbox is a torque
converter. If a gearbox has a ratio of 2:1, it means that for every
two revolutions of the input, there is one output turn. In our example
the output speed is halved, but the output torque is doubled. A reduction
gearbox increases torque and reduces rotational speed. A car gearbox
both reduces and increases the engines torque depending on the gear
|Here is the gearbox
type I intend on using for my kit build. Its an Audi type 012
5-speed manual designed for small to medium size saloon cars.
This particular gearbox came from an 1995/1996 Audi A6 2.5 TDi.
This gearbox type can also be found in VW Passat's as well as
Audi A4's. These cars are notorious for quick acceleration and
high top end speed. A newer version is also available in 6-speed
and has a compatible bolt pattern making upgrades much simpler.
The ratios are difficult to obtain for this box. All the published
data for this type of gearbox does not refer to this particular
type (Gearbox code CUS). So in order to figure out the ratios
you can either take the gearbox apart or you can make a simple
measurement using the procedure outlined below, which will at
least give you the final drive ratio. These ratios were kindly
supplied by Neil, thanks mate!
My ratios are as follows:
| Final Drive
Procedure for estimating gearbox ratio's
This procedure allows rough estimates to be made as to the ratios
and final drive present in your gearbox. This works to an extent for
any gearbox where you know the 1:1 gear. On older cars this is usually
the highest gear, but on this type of Audi gearbox, the gear that
provides the closest 1:1 ratio is actually fourth. In reality its
not 1:1, but is 1:0.943. This is close enough to estimate ratio's.
1) Remove the gearbox mounts and roll the gearbox on to its side so
that one of the drive shafts is not able to turn. You can leave the
gearbox upright and simply lock one of the drive shafts so it is unable
2) Make sure the gearbox is in fourth gear, and turn the gearbox input
shaft so that all the slack in the drive train is taken up. A pair
of mole grips lightly applied to this shaft can be used to turn this
shaft. Protect the shaft by using cardboard on the jaws of the mole
3) Mark the free drive shaft with some chalk to mark the 12'o'clock
position. Mark the gearbox in a similar fashion as a datum.
4) Turn the input shaft exactly twice, this will give you almost one
turn of the drive shaft. Take this value and divide the number 1 by
it, ie find the reciprocal. Formula is 1/n, where n is the number
of turns of the drive shaft.
5) Multiply this value by four and hey presto you have your final
|Calculating gearbox shift points
So I know the gearbox ratios with a fair degree of certainty,
the final drive ratio and I know the output power of the engine. I
should be able to calculate the points at where shifting would be
required and the top speed of the car. To determine the speed of the
car you also need to know the rolling radius of the wheels.
So here are the figures we are going to need:
| Estimated engine output power (BHP)
|| 300 @ 5200 rpm
| Engine torque
|| 324 lb/ft
| Tyre Diameter
|| 26 inches
Based upon these values and assuming we have not fiddled with the
rev limiter in any way (set at 6000 rpm), the terminal speed will be 170mph.
Time to various speeds starting from 1000 rpm is:
|| Time to reach speed
|| 1.8 seconds
|| 2.5 seconds
|| 3.7 seconds
|| 4.6 seconds
|| 5.4 seconds
|| 6.1 seconds
|| 8.0 seconds
|| 9.3 seconds
Standing 1/4 mile with engine beginning at 800 rpm is 12.6 seconds.
These figures were generated using a piece
of shareware called CarTest. Its a little difficult to use, but it
seems to be reasonably accurate.
Make sure you enter the details very carefully. I had used the incorrect size of driven tyre and it makes a big difference to the figures you get.
|Here is the gearbox after
extensive cleaning. First step is to remove all the grease and accumulated
debris. I used a power washer for this after soaking everthing with
The next step involves a hand help shot blaster. I used glass beads
to do the blasting as they do not pit the surface as sand would. You
need to spend much longer than with sand but the results speak for
The gearbox has gone from looking exactly like it has come off a donor
car to one that looks brand new. All of the corrosion has been removed
and the rust that had formed on the bolts has gone completely.
The hole visible in the lower part of the gearbox is where the clutch
cylinder is installed. I have removed it so it can be cleaned on its
The other side of the box is visible
here. The shaft exiting the top is the gear shift rod. It can slide
in and out as well as rotate.
The electrical connection visible is the reverse switch. The speed
transducer is installed on the other driveshaft. It generates a
pulse for every 69 inches the rear wheel travels.
The bolts installed in the top are not likely to be used but have
been installed. This is so when the shot blasting was done, the
beads dont get stuck in the threads. I dont know if I will need
to use every tapped hole provided, but it makes sense to protect
them all just in case.
|The gearbox adapter plate and clutch are
shown installed in this picture. The flywheel is secured using bolts
supplied with it, and must be assembled with a light coating of oil.
You should not use thread locker. The clutch cover plate is installed
using three supplied steel pins, and six steel bolts.
The starter motor in my instance has a longer nose than those normally
used by parallel designs, so a significant amount of material needed
to be removed to make it fit correctly.
The bell housing on the gearbox also needs material removed as the
clutch cover has greater protrusions than the original.
You will require a clutch friction plate alignment tool. Halfords
sell one if you get really stuck.
|The only way to check the amount of material
from the inside of the gearbox, is to install it and then attempt
to turn the engine over with the starter motor. Chances are you wont
even get the bellhousing to bolt snug against the adapter initially.
Remove the gearbox and check to locate polished areas where the clutch
cover has fouled the bellhousing. I used an angle grinder to remove
a small amount of material at a time, re-installed the gearbox and
tried to turn it over again. I repeated this process about six times,
and ended up with an engine that turns over properly with no unpleasant
noises from the bellhousing.
Audi have areas clearly visible in the bellhousing where they have
used a rotating tool to perform this exact procedure. These are good
places to check for rubbing, as if they have felt the need to relieve
metal here for a standard sized clutch, the bigger parallel clutch
will definetly require work in these areas.
As you can see here, an engine hoist is very useful in the repeated
removal and installation of the gearbox. Try not to turn the engine
over too much at this stage unless you have some oil in it.
||This is the rear view of the gearbox in
position. The two shift rods operate in the reverse pattern to what
is required at the gearbox.So in order to make a viable shift pattern
at the gearstick, a lever is required to reverse the push/pull shift
rod, and the rotational movement to effect the side to side motion.
The image shows two prototype brackets and also a simple bearing used
to shift the rod backwards and forwards.
The gears are selectable with this configuration and the car would
certainly be driveable, but the action is somewhat sloppy at present.
The rotational movement of the shaft needs to be higher, so the sweep
of the shaft needs to be made shorter. Also the longevity of this
arrangment is unlikely to be high. These components will be remanufactured
to overcome these problems.
|Here is the same arrangement viewed from
above. All the components are made from either stainless steel, aluminium
or leaded steel. I can fabricate components easily using these materials
at work, and attach any of them with the exception of aluminium with
my tig welder. I only have a DC tig welder so welding aluminium is
out of the question.
These components are not the final versions and will be used as patterns
to manufacture more tidy looking parts.
|This is the longer clutch slave push rod.
The top one is the original which is too short. The bottom one is
the one I have made. It measures 100mm in length, the six-speed Audi
gearbox will require a longer shaft at 110mm.
The rubber boot fits over the shaft and prevents clutch dust from
entering the slave cylinder. The recess cut on the shaft, the one
closest the centre is where the boot fits. The other end slips over
a recess on the slave itself.
|This is the final version of my gear shift
assembley. It is fabricated from stainless steel. I painted it black
because I like the look better than the plain material. It kind of
looks hlaf finished to me otherwise.
The shift rod is located on the rear of the gearbox, and its motion
is opposite to the required shift pattern. If I just hooked the cables
up without this mechanism, first gear would be over to the right and
down, ie upside down and back to front.
The main lever which moves the shift rod inside and back is mounted
on a bearing pillow block. This device is usually used in conveyer
belts and the like. It has spherical bearings to allow misalignment
of drive shafts and is ideal in this application as the lever needs
to rock slightly. The shift rod needs more movement than the shift
cables allow, so the lever is also used to increase the range of motion.
|This is the above view of the shifter.
You can see the gear shift to the right of the gearbox. An adapter
is bolted to the end of the shift rod to couple it to the two actuating
At the moment, there are a number of ball and cups which will be replaced
with rose joints just as soon as they arrive.
The cables used in this Audi installation are actually those normally
used in the Renault 25 gearbox kit that parallel designs supply. The
kit that they supply for the Audi six speed gearbox are too short
as the shift rod is on the side of that gearbox.
|This is the side view of the gearbox.
The ball and cup at the top are due to be replaced with rose joints
to provide a more durable solution. The shaft extension is made from
leaded steel that I turned at work on a lathe. The ball joint is coupled
to an aluminium collar which rides on the the steel shaft. The bearing
on this surface is plain, dissimilair metals used here automatically
lubricate themselves although the stud holding the cup can be removed
to inject new grease should it be neccessary.
My gearbox selector had a problem,
so some lateral thinking was required to solve it. I could select
3rd and fourth without any problem, but any of the gears either
side was extremely difficult and clearly not acceptable. Two reasons
caused this problem;
1) The side to side motion of the gearstick was insufficient for
the required rotation of the selector shaft.
2) When the gearstick was shifted left or right of centre, pushing
the gearstick forward or back would cause the shaft to twist making
gear selection very hard, especially reverse which requires slightly
I knew the selector was basically sound as I could select gears
easily by manually turning the selector. So I needed to rethink
the whole selector to cable interface.
The top version is my first attempt, it works to a fashion but has
problems as above. The bottom one is my new version. As you can
see its a little bit more engineered than the previous version.
Adam at work helped me machine the steel (More out of concern for
his milling machine, I go through quite a few cutting tools when
I use it alone!)
The rotational part of the shaft is now coupled to a linear bearing.
This lets the shaft rotation remain at the same degree regardless
of if the selector is in, out or in the centre. The linear bearing
does not have the ball fitted to it yet (its still fitted to the
selector shaft above), and a phosphor bronze bearing to support
the end of the shaft is not shown either. This stops the 'wobble'
of the shaft as gears are selected and should allow for a much more
positive change with minimal sloppiness.