Friction is one of the essential requirements to enable a vehicle to remain on a roadway or stop.
During crash analysis it is often important to ascertain the friction value offered to a vehicles tyres by a road surface.
Knowing what this value is enables a crash analyst to determine such things as:
- Vehicles speed from emergency braking
- Distance it would take a vehicle to stop under given conditions
- Maximum speed a vehicle can negotiate a given corner.
- The speed a vehicle did negotiate a given corner.
- Vehicle tyre fault.
- Road surface fault.
- Determine whether a vehicle would roll over before the tyres slide
- And more.
To ascertain the friction value in a vehicle accident investigation, specialised equipment is used.
The most common device used internationally by crash investigators and analysts is an accelerometer or dynameter. The device is attached to the vehicle being tested or an exemplar vehicle. The test vehicle is then skid tested at the location a crash occurred. Normally 3 to 5 runs are conducted and a range of friction values used that are within 10%.
NZCI utilise a Vericom VC3000 accelerometer, which at the present time is the most advanced device available.
This device is imported from the USA and is an independent digital unit that actually calculates the deceleration factor on the vehicle, that is braking over a distance during a period of time.
Other devices used are:
Drag Sleds - these involve using a portion of a tyre, filling it with ballast,
weighing it, then attaching a line and dragging it across the test surface, while measuring the force exerted to pull it.
This method is not the most accurate as it is often difficult to closely simulate the same conditions a vehicle tyre is undergoing with respect to heat generation, tyre wall deflection, tyre type and momentum etc.
Friction Value Tables - These are located from research and feild tests conducted to asertain different friction values on different surfaces. These tables are used with caution when skid testing is unable to be conducted for various reasons.
Chalk Gun - Is a method of fitting a gun that paints a mark when initial braking is applied to the test vehicle. The distance from the paint to where the vehicle stops is measured and the speed of the vehicle prior to braking recorded on radar. A slide to stop calculation is then completed to find the friction value.
SCRIM - This is implemented by Transit NZ and some RCA's on New Zealand roads to evaluate the surfaces skid resistance. It involves using a modified water tanker which can complete approximately 60 km of non- stop self wetting skid testing. The tanker is fitted with two wheels, one for each tyre path per lane. The test wheels are treadless, pnumatic tyres with there own load and suspension. they are angled 20 degrees to the direction of the tankers travel. The tyres are lowered onto the wet surface and freely able to rotate as the vehicle moves forward causing it to scuff the road in a sideways manner. The ratio of the sideways force is measured and stored over 10 metre intervals. This is known as the sideways force co-efficient or SFC. The data is inserted into a road network database to assist with road maintenance and research. This method is supposed to closely simulate a wet roads skid resistance offered to a vehicles tyres.
Research has shown that different tyres, road surfaces and enviromental factors amongst others present different friction values.
Studies and field tests have revealed HMV tyres normally generate approximately 80% that of a normal car tyre. This is primarily due to the harder compounds used to manufacture truck tyres.
Feild tests and studies reveal that in most cases a cars brakes are set to provide 60% of the total braking on the front axle and 40% on the rear. From this information it can be seen that if one of the front brakes on a motor car are not functioning correctly the braking effeiency can be reduced up to 30%.