Thread: Measuring stuff
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Old 05-15-2020, 05:59 PM   #1 (permalink)
JulianEdgar
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Measuring stuff

Rather like when air/fuel ratio meters and knock sensing equipment became widely available and so transformed DIY engine tuning, the use of on-road aero measurement means you no longer have to guess - instead you can see for yourself what is happening. And the measurements are all made in an absolutely real world environment.

Wool tufts - see flow behaviour.



Example uses: Find areas of separated and attached flow, ascertain flow directions, see changed flow behaviour caused by modifications.

Equipment: yarn, scissors, tape, camera.

Complexity: easy to do, but interpretation of results requires some knowledge

Comment: still the easiest and best

Cost: $5 (if you have camera)


Coarse pressure measurement - measure large aerodynamic pressures.



Example uses: find best location of hood vents, measure pressure differentials across heat exchangers.

Equipment: Magnehelic gauge, hose, tape.

Complexity: easy to do and interpretation of results also fairly easy

Comment: good entry into world of aero pressure testing

Cost: $50 (secondhand gauge)


Surface pressure measurement - measure pressures on panels above and below the car, both negative and positive.



Example uses: directly measure pressures causing lift/downforce, see changed pressures resulting from modifications like spoilers and air dams, measure wake pressures.

Equipment: Magnehelic gauge, surface pressure puck, sealed vessel, ball valve, hose.

Complexity: needs far more care than when doing coarse pressure testing. Interpretation of results fairly easy

Comment: brilliant window into seeing what is actually causing thrust/drag, lift/downforce

Cost: $100-200.


Throttle stop testing - directly measure changes in total drag over about 2 per cent.



Example uses: measure drag change results of modifications

Equipment: digital speed measurement, display of actual throttle blade opening.

Complexity: needs care in testing procedure, interpretation of results easy

Comment: much better than coastdown testing but probably won't work on cars with CVT or electronic throttles

Cost: depends on what needs to be purchased - $0 - 100


Lift / downforce measurement - directly measure amount of lift or downforce



Example uses: see how much lift is occurring, measure effectiveness of modifications, calculate coefficient of lift (front, rear)

Equipment: ride height sensors (eg from an air suspension car), regulated 5V supply, electronic means of averaging output value

Complexity: fitting sensors can be tricky, some basic knowledge of electronics needed, results easy to interpret

Comment: once installed, can be accessed whenever data is needed. Accuracy will diminish with very stiff suspension cars.

Cost: $50-100


Eroding clay technique - see fine details of airflow



Example use: see attached / separated flow

Equipment: fine clay (eg fire clay), water spray bottle

Complexity: needs patience and care in application and testing, analysing results requires some knowledge

Comment: allows you to see very tiny details of flow

Cost: $20

__________________
Modifying the Aerodynamics of Your Road Car

"Your book is unique! It was high time that someone covered vehicle aerodynamics through the practical eyes of someone like you." - Dr Wolf-Heinrich Hucho, the founder of modern vehicle aerodynamics

Last edited by JulianEdgar; 05-15-2020 at 10:20 PM..
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