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 model a vehicle is certainly not a quick or easy task;
while some information can be found out/measured in moments or with just a little research,
other information can require lots of effort and manual labour just to find out one number.
There are several websites dedicated to storing some of the numbers required by VHPA,
these can be accessed from the Links menu in the program.
Disclaimer
In order to measure some of the values required for accurate analysis in VHPA,
you may have to get underneath the vehicle with it raised off the floor.
I am not responsible for any injury of death that may be the result of this.
Likewise if a part needs removing to be measured, I am not to blame if you cannot fit the part again.
In brief, do not do anything unless you know what you are doing.
Seeking advice of a trained mechanic, where appropriate, is well worth considering.
| Vehicle Data | Setup Data |
Vehicle Data
Physical - back to top- Mass - Use a weighbridge. VHPA currently requires that the driver be seated at the time and the fuel tank empty. Can usually be looked up online.
- 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. As for CoG height, that is much more difficult, and I have no suggestions.
- 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 can be found by filling the vehicle with a known quantity (either volume of mass will suffice) of fuel and note the changed readings on the scales/weighbridge. The location can be obtained with a small bit of maths.
- 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 from the ground directly, then you can do that and 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. Again, this doesn't obtain the height values.
- 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, put the vehicle on axle stands, lightly accelerate in gear and look for which wheels rotate.
- Efficiency - Unless you have put the engine on a dyno, and then 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 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 - To be added...
- 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.
- Tyre dimensions - 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, weigh it, then double the figure (assuming the axle in question has the same components on each end, if not, weigh seperately and total).
- 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 - Not yet required...
- Unsprung Mass - To be added...
- Track Widths and Wheelbase - Measurable with a tape measure from the centre of each pair of tyres in question.
- 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 - back to top- Brake torque & balance - Some rolling roads can calculate this.
- Spring length - 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.
- 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.
- 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 sterring 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.
- 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...
- Tyre Compound - To be added...
- Tyre Pressures - These can be measured with an air pressure meter.
- Camber Adjust - To be added...
- 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 © 2004-2008 Ben Ponsford