Another look at braking.
Lee Watson. Honda UK.
Lee started with an image of a Ford Anglia, taking many of us back to our early driving years. The Anglia was used in the official Highway Code table of braking distances.
The comparison was made against a fully laden HGV vehicle, which takes much longer to stop. Certainly a factor that must be considered when a driver decides to cut in early, particularly when expecting to stop. Question to ask is, can the HGV stop too?
Tests done by the MOD show one of the best vehicles for stopping quickly is a Land Rover. This can stop in a very short distance.
Which Magazine conducted tests for cars' stopping distance at 60mph. They found the best method to measure was using a GPS system.
On small cars the following results were achieved:
|VW Polo 34.2m
|Ford Fiesta 34.7m
|Skoda Fabia 34.9m
|Mini Cooper 35.7m
|VW Up 36.1m
Make of tyre and performance can make a big difference.
Braking distances as quoted by the Highway Code are simply an average, and HGV will take longer to stop. Some exotic cars can beat the stopping distance by a considerable margin. Super cars can stop very quickly, some at 65mph can stop in 85 feet. So if following such a car, assume a much greater distance to ensure you can stop in time.
4 factors make up thinking distance:
- Observe - Is eyesight up to standard, driver could have eyestrain , could be colour blind, etc. Visual overload, there are areas where too many signs are sited. Most signed area is in Beverley in North Yorkshire. This makes it impossible for the driver to take in all the information offered, and can be distracting. Also if sunglasses are worn this can reduce effectiveness of spotting brake lights on other vehicles.
- Orientate - This phase requires you to understand the situation you are seeing. A more experienced driver will quicken up the procedure. Inexperience can take seconds longer.
- Decide - Driver decides what action he will take.
- Act. - Then puts into action what is decided to deal with the hazard.
All manufacturers will invest in measuring stopping distances. A GPS system is used. Rather than a human driver, a robot is used. This is the most reliable way to ensure that repeatability is achieved. To get a true comparison, exact situations must be repeated.
Output is shown in a graph form. It cannot differentiate between thinking and actual braking time, though there is some time lag evident before the vehicle starts to slow due to brake pressure being applied.
A very small part of the tyre is ever in contact with the road. Data was shown from Michelin. To avoid skidding, the tyre needs to keep moving on the road. This is called slipping. If it stops slipping, that is when skidding occurs.
Very small movements happen all the time, these are micro movements, sub mm in length. The tyre performs like memory foam, returning back to normal position once it leaves the road surface as the wheel revolves. This is essentially how the tyre grips. If you imagine how a caterpillar moves this is how a tyre moves on the road surface. It forms a temporary bond with the road surface at the point of contact.
Rough Road Surface
When the tyre is static it will sink into the varying surface levels. When moving, the grip will be loosened. If wet, then volume of water gathered will depend on how much grip is lost.
Be aware of different road make up. Shale, grip is good. Tar and chippings do not provide good grip surprisingly, and cobblestones are very bad.
The compound that the tyre is made from affects grip dramatically. HGV tyres are made from a very cheap compound. Such high mileage means cost savings are needed. Be aware again that HGV’s stopping distances are much longer than expected.
Wet Grip Performance
When the road surface is wet, it adds considerably to stopping distances and grip. When buying new tyres, they will be rated Wet Grip Performance on a scale A through to G. The difference in stopping distance from an A rated tyre to G rated tyre is 18 metres, so it is worth buying as high a rating as possible.
Tyres are designed to move the water away to improve adhesion. The tread squeezes the water out. Corners of the tread make contact with road and move the water through the tread, dispersing it.
Most new tyres will have 7mm of tread depth on supply, and legal requirement is 1.6mm minimum. The difference in stopping distance between the two can be 7 meters. It is possible to buy winter tyres. These will increase grip in icy and snow conditions. They will also stop you quicker than summer tyres in the colder conditions. However in temperatures in excess of 28 degrees, the winter tyres stopping distances increase dramatically.
Always check tyre pressures, at least once a week. If wrongly inflated, they will lose grip with the road.
It is best to check pressures when cold. (Hot is still considered a problem even if the car has been stationary for 2 hours). If you must check when tyres are hot, then add 5 psi (0.3 bar) to the recommended tyre pressure. In Lee’s findings, filling tyres with Nitrogen does not maintain pressure any better than ordinary air.
Cars manufactured since 2012 have tyre pressure monitoring systems fitted. These are very useful additions to cars and generally work very well. Some cars have Indirect Monitoring systems fitted. These are very complicated, and can be problematic if not reset properly when new tyres are fitted.
In 2016 Autobild Magazine tested different tyres on a VW Golf. Cheaper tyres took as much as 5 meters extra stopping distance. Always fit quality tyres to your vehicle. When changing tyres, advice is to put new on back, and old on the front.
Brake pads made from a bonding agent, friction agents, lubricants, fillers and dampening agents, and binders. They are constructed on a steel backing plate, then shims added, then an under layer and finally friction material / scorching.
In modern pads, asbestos and lead are now not used. You should not run brake pads down to the backing plate. The final layer of friction / scorching material contains a higher level of rubber, so be aware that when the pad is nearing the end of its usefulness, wear rate will increase.
Obviously better quality pads will reduce stopping distances. When braking in the wet, the pads will reduce from .4g braking efficiency in the dry to just 0.15g in the wet. This is another reason you must allow more distance to stop when wet conditions prevail. Pads will heat up to 300 degrees when used a lot. The hotter the pads, the less efficient they will be.
Hand Brake Button
Contrary to Roadcraft advice, Honda recommend that when applying the hand brake, the button is not depressed. If you press it and release it and there is any danger of slippage, there is a danger the hand brake will not be applied properly, thus allowing the car to slip. The recommendation is to read owner’s manual to see what the manufacturer recommends. Electronic parking brake will not be a problem. But, always leave the car in gear when parking up.
Suspension / Dampers
Worn shock absorbers can increase stopping distance by up to 50%, but there is very little data available.
Different systems are fitted by manufacturers depending on the type of vehicle. You should ensure you are aware of what systems are fitted and know what they do and how they assist you. ESP (Electronic Stability Program) and Stability Modulator are different types. It is important you check which type your car has fitted.
They all vary, and include features such as traction control, hill start assist, emergency brake assist, dynamic handling, tyre pressure monitoring, trailer stability assist, and hill control.