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Accelerometer Adventures - Part Two

Measuring your car's braking and cornering abilities.

By Michael Knowling

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At a glance...

  • Using low cost accelerometers
  • Measuring braking, cornering, aerodynamics
  • Part 2 of a 2-part series
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This article was first published in 2004.

In Part One of this series we introduced you to mechanical accelerometers and clinometers and demonstrated how to identify the ideal launch technique and gear-shift points for maximum performance. In this article we’ll look at braking and cornering performance...

Using a Clinometer...

Deceleration Measurement

A mechanical accelerometer (or clinometer) is a great way of assessing the braking performance of a vehicle.

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Braking performance is measured with the accelerometer mounted on the side window, as discussed in Accelerometer Adventures Part One. The accelerometer must be zeroed and mounted perpendicular to the road to ensure accurate results. Note that, unlike acceleration testing, it’s impractical to gather enough information to graph a braking g curve; all you can do is find the peak braking g.

Our brake tests were performed in a 2003/2004 Mitsubishi Verada GTVi. How well does a showroom-stock vehicle brake, we wondered?

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On a newly-laid bitumen road – with no other traffic nearby - we performed ten consecutive emergency stops from 100 km/h. Each stop was separated by less than a minute. This test showed that the Verada (with standard ABS and EBD) could develop up to 1g of braking deceleration. Interestingly, this peak was achieved at a road speed of approximately 10 km/h (ie just before coming to a stop).

What really impressed us, however, was the consistency – our first brake test peaked at 0.84g and the following nine manoeuvres varied between only 0.9 and 0.97g. Talk about consistent! Note that consistent braking figures are more difficult to achieve in a non-ABS vehicle – you’ll inevitably start locking a wheel after a few emergency stops.

To spice things up a little we then tried an emergency stop in the Verada on a dirt road. The importance of tyre to road surface adhesion was spectacularly highlighted with a peak braking deceleration of just 0.47g – less than half that on the bitumen road!

From an enthusiast point of view this is all pretty interesting – but what can you do with this info?

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Well, first, it’s important to have this sort of information available before making any modifications. If you fit a set of aftermarket brake pads it’s easy to compare peak deceleration and braking consistency to the pads you were running beforehand. This will reveal if your aftermarket pads need to be warmed up before they’re up to their optimal operating range (as is often the case). You will also be able to identify whether your newly-fitted brake cooling ducts are allowing your brakes to maintain consistent stopping power.

And don’t forget the effect of tyre-to-road adhesion. As the Verada clearly demonstrated on dirt, it’s absolutely critical to have a strong bond with the road. Expect a set of DOT-legal semi-slicks to give a significant improvement in stopping power on dry bitumen.

Lateral Acceleration Measurement

We must make it clear that an accelerometer will not give you a broad picture of vehicle handling. Handling encompasses stability over bumps, throttle lift-off characteristics, understeer/oversteer balance and more. An accelerometer will, however, give you a yardstick to the amount of lateral grip available.

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Lateral grip measurement is performed with the accelerometer mounted on the interior rear-view mirror - as discussed in Accelerometer Adventures Part One. The accelerometer must be zeroed and mounted vertically to ensure accurate results. This can be easily done by adjusting the mirror.

Also, note that you’ll need somebody positioned immediately in front of the accelerometer in order to obtain accurate results. Watching it from an angle will give false data.

The ideal venue for lateral grip tests is a skidpan but, without access to a racetrack, a deserted roundabout with a decent amount of run-off can suffice. Great care must be taken to ensure the safety of yourself and others.

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Our demo Verada GTVi was flung around a roundabout where it recorded a peak lateral acceleration of 0.97g and not falling below 0.90g at any stage. This is pretty good for a family sedan, but the limiting factor was front-end grip.

Note that the GTVi version of the Verada boasts Sports-spec suspension and relatively grippy Bridgestone 225/50 Grid II tyres mounted on 17 x 7 rims. The aforementioned lateral forces were achieved with front tyre pressures of 32 psi (measured when warm).

What would increasing tyre pressure achieve?

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After a quick stop at a servo we increased front tyre pressures to 39 psi – and the improvement was immediately noticeable. In addition to improved steering response and reduced steering effort at parking speeds, the extra front tyre pressure had obviously enhanced the Verada’s front-end grip. This was evident with much more consistent cornering load on the same roundabout – a minimum of 0.93g and a maximum of 1.0g proved it (up from 0.90 and 0.97g respectively).

Straight away – without any mechanical changes – we’ve demonstrated a zero-cost way to improve the Verada’s handling.

When you get to the stage of changing a car’s suspension components and tyres, the accelerometer continues as a very important tool. Does increasing front negative camber or castor improve lateral grip? What front-to-rear swaybar balance is best? Which tyres provide the greatest cornering grip?

These questions can all be answered.

Correction Factors

Longitudinal and lateral acceleration in a car will cause it to squat or roll on its suspension. This causes some problems for an accelerometer.

To compensate for this motion – which will otherwise cause optimistic readings – there are some rule-of-thumb corrections that should be applied.

For a typical streetcar, a 1.0g braking or acceleration measurement should be reduced by around 6 percent. Correction for body roll during cornering is more difficult to determine. A 1.0g cornering measurement in a stiffly suspended sportscar should be reduced by approximately 10 to 15 percent, depending on the amount of roll. In the case of a softly sprung commuter vehicle a 1.0g measurement should be considered optimistic by around 20 percent.

Don’t forget to apply these corrections or else you’ll be kidding yourself! (That's unless your're just comparing changes, rather than trying to get absolute figures.)

Other Measurements

We’ve covered accelerating, braking and cornering measurement – what else can a mechanical accelerometer be used for?

One of the most useful ‘alternative’ applications for an accelerometer is to compare aerodynamic drag. If you own a vehicle with an adjustable factory rear wing (such as a R33/R34 Skyline GT-R) it’s possible to find the angle of attack that creates the lowest aerodynamic drag. If you’re making your own aero add-ons (for a club racecar, for example) the accelerometer will also tell you how much extra drag you’re creating in the search for extra downforce.

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On a long, flat stretch of road you need to hold the car at a constant speed (say, 110km/h) and then put the gearbox in neutral. Call out increments of 10 km/h as the vehicle coasts down to a stop and have an assistant record data from the accelerometer. Next, repeat the procedure on the same stretch of road with the factory wing adjusted or your aero kit installed. Note there must be minimal wind present on both runs or else your figures won’t be directly comparable.

So what does this data tell you?

Well – so long as there is sizeable variance in aerodynamic drag – you’ll be able to find the most efficient angle of attack for the factory rear wing or get a feel for how much drag you’re add-on body kit has introduced. The higher the g readings while coasting down, the more aerodynamic drag.

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We tried coast-down tests in our demo Verada GTVi with its windows up and down (in an attempt to change aerodynamics) but our figures were inconclusive. This was despite using the extra-sensitive scale of the Whitworth’s Dual Scale Clinometer. Obviously, the change of aerodynamic drag needs to be pretty sizeable to be identified in coast-down tests.

Verdict

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For around AUD$30 a mechanical accelerometer – or, technically, a clinometer - is an excellent tool for assessing acceleration, braking and cornering abilities. It is also a very useful tool for making aerodynamic adjustments.

Only a stopwatch comes close for performance measurement per dollar.

Contact:

Whitworth’s Marine and Leisure

http://www.whitworths.com.au

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