Gathering Data
Any simulation is only as accurate as the data it is fed!
This page describes how to obtain usefully accurate data for a vehicle.
Gathering the data necessary to accurately simulate a vehicle is certainly not a quick, nor easy task;
while some figures can be easily researched or measured in minutes,
some of the methods described require substantial effort for the sake of a single figure.
There are several websites dedicated to storing some of the figures required by VHPA for production cars.
These sites, which can be accessed from the Links menu in the program, coupled with some quality time with your trusty search engine, can go some way to helping you collect the bulk of the numbers.
Disclaimer
Some measurement procedures for values required for analysis in VHPA may require the vehicle to be supported off the floor or partially disassembled.
I am not responsible for any injuries that may be the result of such activities, nor your inability to put your car back together correctly.
To summarise; do not do anything unless you are sure you know what you are doing.
Seeking advice of a trained mechanic, where appropriate, is well worth considering.
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Vehicle Data
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Setup Data
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Vehicle Data
Physical -
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- Mass
- Use a weighbridge. VHPA currently requires that the driver be seated at the time and the fuel tank empty.
- Body Centre of Gravity
- A set of heavy duty scales under each wheel will let you measure the front/rear and left/right distributions.
A single axle weighbridge will also allow for front/rear distribution readings to be obtained.
The value of the height of the centre of gravity is more difficult to obtain.
Here is a guide to measuring the CoG height, by James R. Davis.
Although written for motorcyles it is equally applicable to cars, although not as feasible in practise.
Alternatively, it can be estimated from the roof height and mass of the vehicle. This estimator should feature in VHPA from v3.1.4.
- Fuel Tank Capacity and Centre of Gravity
- Tank capacity is easy, just run the tank to empty (or better, drain it), then fill up at petrol station to obtain your reading.
CoG for the fuel tank can be found by filling the vehicle with a known quantity (either volume or mass will suffice) of fuel and note the changed readings on the scales/weighbridge.
The lateral and longitudinal location can be obtained with a little maths.
The vertical location is more difficult, requiring either a repeat procedure of the guide linked to above, or estimation with tape measure.
Note the figure required is the centre of the tank, not the bottom or top.
- Reference Height
- The height above ground from which all other height measurements are taken.
It is completely arbitrary and can be anything and serves to make measuring the other height values easier.
If you wish to measure everything directly from the ground, just leave this figure at zero.
- Number of Passenger Seats
- Looking and counting should suffice here.
- Passenger CoG positions
- Same method as fuel tank location for lateral and longitudinal readings.
Vertical is more difficult again, if you can measure the height of the bottom of the seat, you could estimate how much higher the CoG of the passenger will be.
Drive-train -
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- Peak torque/power
- These can usually be found online; if not, talk to your engine manufacturer.
- Idle point & Limit
- Assuming your vehicle has a rev counter, the idle point is the rpm value when the engine is idling in neutral.
Likewise the limit is the maximum rotational velocity your engine will spin to.
Note if this is unknown it is NOT recommended to test for this value in neutral, particularly if your engine does not have a rev limiter.
There won't be a need to analyse your vehicle if you blow the engine up.
If you do not want to test for yourself, either look the value up, or just guess.
- Fuel Type
- If this is unknown then handling is by no means your first concern. The filler cap may come in handy.
- Type
- If unknown, place the vehicle on axle stands, lightly accelerate in gear and look for which wheels rotate.
- Efficiency
- Unless you have put the engine on a dynamometer and measured the wheel torque on an accurate rolling road
(in which case you can just compare the vertical scale of the plots), this number will have to be estimated.
Typical values seem to be around 80% for a 4WD vehicle and 85% for a 2WD vehicle.
- Num. Gears & Reverse
- The number of forward gears, and the ability to go in reverse, should be clearly labelled on the gearstick (if the vehicle has a manual gearbox, at least).
- Shift Time
- For manual transmission, this is obviously very driver dependent in actual usage.
The figured entered into VHPA should be the minimum consistently achievable time.
Running a video camera inside the car while driving (focussed on the gearstick) should let this value be timed upon video playback.
For an automatic or paddle shift gearbox, a microphone placed inside the car and some audio editing software should let you arrive at a reasonable estimate.
- Engine torque curve parameters
- Although some important aspects of the torque shape are defined by the peak torque and power locations, there is still a large degree of flexibility within those constraints.
A dyno or rolling road plot is required here.
Wheels & Tyres -
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- Tyre dimensions
- These should be embedded into the wall of the tyre. Measuring is an easy alternative.
- Rotating mass
- Remove the wheel, tyre and brake disc from an axle, then weigh all these parts together. Note the figure is for a single wheel.
- Wheel:Spring Motion Ratio
- Remove the springs and move the wheel from full droop to full bump in small increments, measuring damper lengths at each point.
Plot this data on a graph and the gradient of the line is the motion ratio.
Note this may vary through travel, giving a progressive wheel rate.
In this case, take an average around the point where the wheel is most likely to be.
- Wheel:Anti-Rall Bar Motion Ratio
- To be added...
- Unsprung Mass
- Similar to the obtaining rotating mass, you will also need to remove and weigh any part that moves when the wheel deflects.
For suspension parts that are also fixed to the sprung mass, the common rule of thumb is to use 50% of their mass is being unsprung.
Note this figure is "per end", so for four wheeled vehicles, will be at least double the rotating mass.
- Track Widths and Wheelbase
- Measurable with a tape measure from the centre of each pair of tyres in question.
Aerodynamics -
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- Frontal area
- Can be estimated by taking the absolute width and height and multiplying by a percentage.
Adjust the percentage to what looks right for the shape of the vehicle. 85% is good for most passenger vehicles.
- Coefficients of drag
- Read up on high speed coast down tests for coefficient of drag (actually CdA will be obtained, so then divide by your frontal area).
- Coefficient of lift
- If the vehicle has aerodynamic components (undertray or wings), then logging the spring deflections at various constant speeds will give the change in force so long as the spring stiffness is known.
- Aerodynamic component centre of effects
- To be added...
Setup Data
Brakes -
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- Brake torque & balance
- Some rolling roads can calculate this.
Some passenger vehicles use knee points to continuously vary the brake balance with pedal pressure, although I do not know how widespread this practise is.
VHPA does not currently simulate kneepoints.
Suspension -
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- Spring travel
- For a coil spring, measuring the length of the coil when the vehicle is jacked up or on axle stands will not give an accurate reading as spring pre-load will cause some deflection.
The spring needs to be removed from the vehicle and then measured, which will obtain the length.
By measuring the diameter of the coil, and counting how many turns theh coil makes, the length of the spring when coilbound can be obtained.
The difference between these figures is the spring travel. Note that when installed, the vehicle may likely hit the bump stops before coilbind is achieved.
- Spring stiffness
- Either apply a known load to the spring and measure the delfection, or use calculator utility that will be in the next version of VHPA
(more measurements but the spring does not have to be removed, so long as you know the free length).
- Bump/Rebound Damping
- Requires a damper dyno.
- Anti-Roll Bar Stiffness
- Use the calculator utility that will be in the next version of VHPA.
Steering -
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- Toe-In
- Measure with string or a tracking gauge.
- Caster
- Measure using a caster gauge. Turn the wheels 20° (or some other arbitrary value) one way and zero gauge. Turn 20° the other way and read off gauge.
- Ackermann
- Measure using turnplates with a protractor on them, then use VHPA to adjust the Ackermann percentage at your steering lock until results match.
For accuracy, it is best to use the largest steering angle possible.
- Maximum Lock
- Measure using turnplates with a protractor on them. Average left and right readings to account for toe and Ackermann.
Gearing -
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- Individual Gear Ratios
- These cannot be conveniently measured unless you already have the gearbox in pieces are thus are able to count the teeth on the gear.
The gear ratios can often be obtained from car specification websites, else from the gearbox manufacturer.
- Final Drive Ratio
- As above, but this figure is the ratio at the differential.
- Differential Type/Settings
- To be added...
Tyres -
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- Tyre Compound
- To be added...
- Tyre Pressures
- These can be measured with an air pressure meter.
- Camber Adjust
- To be added...
Downforce/Misc -
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- Wing Angles
- This is the angle, relative to the pitch of the vehicle body, of the wing.
A rough angle measurement could be obtained with a protractor, or by measuring the gradient and then performing a little maths (take the atangent of the percentage gradient).
- Passengers
- This controls how many passengers are in your car and where they are sitting, so no measuring required.
- Handicaps
- This is how much mass you have to add to your vehicle to meet either minimum mass regulations for your series,
or control ballast you have had to add due to regulations (usually due to a high place finish).
If this would apply to your vehicle, then the figure should be known.
Copyright © 2004-2024 Ben Ponsford