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This article was first published in 2004.
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The ability to measure a car’s accelerative, braking and cornering forces has
obvious benefits for the enthusiast. If you have plenty of money you might want
to invest several hundred dollars in an electronic accelerometer (such as a
G-Tech). But if you can’t justify this expenditure you should be interested in
this – mechanical accelerometers for around AUD$30...
For thirty bucks you can identify your car’s best launch technique, pick the
best up-shift points, assess braking performance, cornering forces and more. But
let’s cover some basic theory before we get carried away...
Background of Mechanical Accelerometers
Most mechanical accelerometers incorporate a clear semi-circular tube
containing a ball bearing that slides under acceleration. When fastened to the
side window of a vehicle, the ball slides rearward as you accelerate and forward
as you decelerate. The further the ball slides the greater the acceleration or
deceleration.
Acceleration (or deceleration) is a measurement of how quickly a change in
speed occurs. Acceleration can be measured in metres per second but the other
approach is to measure g. One g is the downward acceleration caused by the
Earth’s gravity and is equivalent to 9.8 metres per second. At an acceleration
of 1g – or 9.8 metres per second per second.
G measurements can be read from a device called G-Curve, produced by
Analytical Performance. As far as we can determine, however, this company no
longer exists and there is no equivalent product on the market.
This is where we turn to clinometers.
Clinometers – which work on exactly the same principle as a ‘proper’
mechanical accelerometer - are widely used in yachting. The only important
difference is the scale is marked in degrees rather than in g. With the aid of a
scientific calculator, however, you can quickly convert a measurement in degrees
into g. Simply enter the number of degrees measured and hit the tan button. Here
are some conversions...
20 degrees = 0.36g
25 degrees = 0.47g
30 degrees = 0.58g
35 degrees = 0.70g
40 degrees = 0.84g
45 degrees = 1.00g
50 degrees = 1.19g
Another type of clinometer exists where a vertical pendulum is swung across a
marked scale under accelerative force. These products are widely used in
woodworking and are known as angle finders – they are not as suitable for in-car
applications.
Selecting an Accelerometer
Two marine-type clinometers that are suitable as in-car accelerometers are
the Lev-O-Gage and Dual Scale Inclinometer. Both can be ordered on-line through
Australia’s
Whitworth’s Marine and Leisure.
Retailing for AUD$29.95, the
Lev-O-Gage (stock number 52381) is sturdy, well finished and uses fluid inside
the tube to dampen the movement of the ball. This helps prevent overshooting. We
found this gauge very easy to read thanks to its contrasting colours and clear
markings. A range up to 50 degrees (1.19g) is also ample for most accelerating,
braking and handlings tests.
At just AUD$14.95, the Dual Scale Inclinometer (stock number 52377) is a
bargain. In testing we found it more difficult to read than the Lev-O-Gage but
it held one clear advantage – the top scale is extremely sensitive. With only a
slight curve in the tube, the top scale reads only 5 degrees at full scale and
offers excellent resolution. The lower scale is marked to a maximum of 45
degrees (1.0g), which is slightly less than the Lev-O-Gage. This gauge is also
fluid filled.
Tested side-by-side, the Lev-O-Gage and Dual Scale Inclinometer gave
virtually identical readings. Both are well suited to in-car applications.
Testing with an Accelerometer
If you want to measure longitudinal (fore-aft) acceleration the clinometer
should be mounted on one of the vehicle’s side windows. It’s important to ensure
the gauge is level (ie the ball should sit at zero) when parked on your area of
test bitumen. It’s also important that the gauge is mounted perpendicular to the
ground, not angled with the curvature of the side glass.
To measure lateral (left to right) acceleration you should mount the
clinometer to the interior rear-view mirror. Again, the gauge must be zeroed and
mounted perpendicular to the ground to ensure accuracy. Note that, in either
case, your stretch of test bitumen must be flat – or as close to flat as
possible.
There are a number of ways to mount the clinometer to the vehicle.
If you’re performing only a quick test there are no problems attaching the
clinometer to the window using Blu-Tak (or similar). Blu-Tak forms a strong bond
for short-term use and can be moulded to give a perfectly vertical mounting
angle.
If you plan to regularly use the accelerometer as a tuning tool, you might
want to fabricate a mount to suit. The Lev-O-Gage seen in this photo has a
custom mount comprising a couple of strips of aluminium, Perspex and a pair of
rubber suction cups. The rubber suction cups allow the gauge to be quickly
swapped from car to car.
Once the clinometer is mounted in the vehicle you can use it to identify peak
gs while accelerating, braking and cornering. You can also plot a detailed g
curve under acceleration – this can be obtained by sampling the clinometer
readings at specific rpm or road speed intervals. This data can be later graphed
on Microsoft Excel (or similar) to give you a graphic representation of your
car’s rate of acceleration. It’s virtually impossible to obtain a g curve under
heavy braking and hard cornering – it all happens too fast!
Safety Warning
Note that you should never perform accelerometer testing without the aid of
an assistant.
Ideally, your assistant can use a compact movie camera to record movement of
the accelerometer ball while you call out relevant rpm or road speed. If a
compact movie camera is unavailable, your assistant will need to take down notes
from the gauge at a fierce rate and you’ll inevitably need to go back over some
figures.
Of course, testing should be done only in a safe environment. A racetrack is
ideal.
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Using a Clinometer...
Acceleration Measurement
Using a current model Mitsubishi Verada GTVi as a demo car we began using our
Lev-O-Gage to identify the ideal launch technique. The 163kW GTVi automatic can
sprint to 100 km/h in less than 9 seconds but – without a LSD and
front-wheel-drive – it can be sensitive to wheelspin off the line.
Here’s the acceleration curve we recorded by simply planting the accelerator
from a standing start. As you can see, the car storms off the line pulling up to
0.49g but it soon tapers off to below 0.3g. Acceleration is near constant until
about 40 km/h and then the car changes into second gear. At this point, acceleration
declines steadily to a road speed of approximately 55 km/h where acceleration
levels out at around 0.3g. The change into third gear then occurs at around 80
km/h and – like before – acceleration steadily declines until we reach our 100
km/h maximum.
Next we tried stalling it up off the line but, unfortunately, 2000 rpm was
all the GTVi’s brakes could hold before the car started to creep forward. Perhaps as
a result, the GTVi showed no measurable improvement with this approach.
So there’s Lesson One – this particular car launches just as hard whether you
stall it up or just tromp it.
Next, we decided to switch off the traction control (TCL) and try the same.
Interestingly, we did find a gain here. Without TCL and with a bit more
wheelspin, the GTVi pulled a peak of 0.52g off the line but, from then on, it
accelerated at exactly the same rate as previously. Sure, it’s a small total
improvement - but a very repeatable one.
Okay, now we have found the ideal launch technique – let’s find the optimal
gear change points...
With the GTVi’s 5-speed auto transmission left in Drive it makes wide-open
throttle up-shifts at around 5600 rpm. It makes these 5600 rpm up-shifts
regardless of road speed and the aerodynamic drag associated with it. Would
holding onto a gear for longer or short-shifting in certain gears improve
performance? Let’s find out!
This graph shows acceleration in 1000 rpm increments with the transmission
held in first, second and third gear. Note that we were unable to obtain
high-gear/low-rpm acceleration due to the torque converter, but we were able to
spin the engine to the limiter in each gear.
As you can see, acceleration drops away steeply at high rpm. Note that the
shape of these acceleration curves is indicative of the Verada’s GTVi’s tractive
effort at different rpm. (Tractive effort is the force being applied at the
tyres and is commonly measured in Newton Metres.)
In the case of the GTVi it is worth taking the engine to the rev limiter in
each gear. This is because acceleration prior to the rev limiter remains greater
than at any rpm in the next higher gear. This is especially the case when
shifting up from first gear where - even when taken to the rev limiter - there’s
still a big step down to the acceleration in second gear.
In short, the Verada GTVi gives its best performance when wrung out to max
revs.
Note that, in many other cars, the acceleration curve attained in two
consecutive gears may intersect at some point on the graph. If they do, this is
the ideal point to make an up-shift. Holding onto a gear beyond that rpm will
not achieve maximum performance.
Turbo Cars
Care must be taken when using an accelerometer to determine the ideal shift
rpm in a turbocharged car.
It’s common for a turbo car to see a boost overshoot after each gear change.
As a result, an early up-change can lead to higher boost pressure at lower rpm
than would otherwise occur with a later gear change. Note, however, the
magnitude of the boost overshoot varies depending on engine load and the rate of
rpm increase (which is different in each gear).
Make sure you take these dynamic characteristics into account when searching
for the best shift points.
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A Modification Perspective
In addition to finding ways to extract the optimal performance from a
specific vehicle, an accelerometer is also a great way to find directions for
modification.
For example, the above test of the Verada GTVi showed that peak performance
comes by revving the engine to the limiter before making an up-shift. This tells
us the gear ratios are too wide and a close-ratio gearset would benefit
acceleration. The acceleration drop-off immediately before the rev limiter is
also indicative of a lack of breathing. A more aggressive set of cams would
probably do the trick.
Of course, the accelerometer is also useful for before-and-after assessment
of your new go-fast mods.
This graph shows the acceleration curve of a Daihatsu Mira turbo before and
after fitment of a big exhaust and a boost controller. In third gear, peak
acceleration rocketed from 0.21g to 0.34g and top-end performance – as a
percentage – remained equally strong. This is a tremendous improvement but,
equally, an accelerometer will show you when your mods aren’t making an
improvement...
Stay tuned – in Part Two we’ll be using our clinometer to test braking and
cornering!
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