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Accelerometer Adventures - Part One

We introduce you to mechanical accelerometers and go in search of optimal straight-line go!

By Michael Knowling

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This article was first published in 2004.

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.

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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.

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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.

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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

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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.

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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.

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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.

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Of course, testing should be done only in a safe environment. A racetrack is ideal.

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.

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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!

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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.

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.

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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.

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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!


Whitworth’s Marine and Leisure

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