Month: December 2010

There are some deluded factions of the motoring press that have loudly and ignorantly derided a very useful and miss-understood service that helps motorists in need and saves police time.

The motorway Traffic Officers are referred to by one loud mouthed buffoon as ‘wombles’, but they answer a long standing complaint made by hoards of motorists since the early days of motorways; why are police used to do non-police type work such as directing traffic round a crash site, staying with crash victims until help arrives, sweeping up debris off the motorway, putting out emergency signs and cones etc. The list goes on, there are a huge number of time consuming jobs that although important don’t really need an officer with the powers of arrest and a highly equipped patrol car.

For years drivers argued, mostly in pubs, that it would be far better to have someone else to do these tasks and free up the police to chase criminals. And that is exactly what the Traffic Officers were invented for, yes my friends this is a case of the government actually listening to the motoring public.

And it works too; a friend of mine was involved in an unfortunate incident when they managed to spin on a motorway slip road and ended up bouncing off both crash barriers before coming to a halt at 90 degrees to lane 1. Which is bad. The Traffic Officers arrived and immediately coned off the lane whist they pushed the wreck to a safe position, then after assessing the situation for safety they cleared up the debris and re-opened the lane very swiftly. The police arrived and took details, did a breath test and the usual checks but then were immediately free to move on to the next drama, meanwhile the TO stayed with my chum until the recovery truck arrived. They also notified the roads agency that the barrier needed mending. All in all a very efficient, helpful and potentially life saving service. What’s not to like?

Sometimes where people have been hurt they have to close the road to recover and record all the evidence which can seem like an unnecessary delay to the uninformed driver, and sometimes being better to be safe than sorry means that some delays do turn out to be unnecessary, but I for one would far rather they were reasonably cautious than risk my life.

Every time I see a TO parked up apparently doing nothing I breathe a sigh of relief that this indicates that this particular stretch of road has no problems, no one has crashed, no one is having an utterly miserable day, no one is dying, it’s damn good to see a dormant patrol car. And when I slowly crawl through a conned off incident I give a cheery waive to the TO directing the traffic and restoring order to the chaos behind them.

So why are they being lambasted in certain media circles? Probably due to a fine and heady mixture of making up a good storey, being controversial to sell more copy but mostly pig ignorance and stupidity. Just the usual then.

Racers claim that better brakes make faster lap times, but how can something that slows you down make you faster?

Simple; the better the brakes the longer you can stay on the power before braking for the next corner.

Brakes are all about heat, and ditching as much of it as quickly as possible, they work by converting the cars speed energy into heat energy which is then taken swiftly away in the air streaming through them, in theory. But a big car at high speed has an awful lot of energy; for instance getting a Bugatti Veyron to do an emergency stop from high speed might put the equivalent of several thousand bhp through the brakes make the discs glow bright red.

There are two basic types of brake, drum brakes get their name from the drum of steel with curved shoes inside that are pushed outwards against the inside of the drum when the brake pedal is pushed. These can be found on the back axles of cheaper cars and are quite frankly a bit pants; the braking force is limited, the pistons are small and the pressure pushing the shoes out into the drum is similarly small.

By comparison disc brakes can exert a much higher force onto the disc without risk of it failing.

It uses a disc with a set of pads held in a calliper that are forced against both sides of the disc when the pedal is pressed, generating much more force.

In both cases the disc or drum part is attached to the wheel hub so it rotates with the wheel and the pads or shoes are held stationary on the axle, or strut, or what ever dangly bits are attached to the suspension.
All brakes work by friction, pressing a pretty darn tough pad of friction material against the spinning metal, the harder the friction material is pressed against the metal the more friction is produced and the greater the braking force.
Brake systems use a special type of hydraulic oil to drive the pistons which push the friction material into the disc or drum. At the pedal end there is another piston in the master cylinder which is connected by hydraulic brake pipe to the slave cylinders at each wheel. In some cars the brake force is artificially increased by a servo directly connected between the brake pedal and the master cylinder, this uses vacuum from the intake manifold to move a large diaphragm when the brake pedal was pressed, as the pedal moved down small holes in the servo control section are progressively uncovered which applies more vacuum to the diaphragm which in turn applies a greater force to the master piston of up to four times the force at the pedal.

Some cars with Anti-lock Brake Systems (ABS) use a powerful electric pump to do this instead. The ABS system measures wheel speeds and if it detects that a wheel is slowing down faster than a safe limit then it knows that that wheel is about to start locking up, so it lets the brake pressure off the individual wheel by opening a solenoid valve in the ABS valve block, just for a tiny fraction of a second until the wheel frees up just enough to know it wont lock. You can feel this when it happens as a sort of buzzing or vibration under the brake pedal. ABS allows maximum braking force without the risk of skidding. But if you are going to fast then you are still going to crash no matter what the brakes do.
The fierce heat generated from heavy braking has to be dissipated into the air which is why race cars have ducts taking fresh air from the front of the car to the disc centre, the hot air then has to go somewhere and the design of the wheel should allow it to escape readily. To get more heat into the air some discs are vented with radial channels cast into the disc to draw air from the centre outwards, some discs also have small holes drilled through for even more ventilation but these can lead to cracks starting unless they are made very well. Groves on performance discs can help remove the tiny gas layer that build up between the pad and disc sometimes and increase pad bite, the down side is that they can increase pad wear when used aggressively.
The brake size needed on a car depends on its weight and how fast it is likely to go, more powerful cars can more readily get up to higher speeds they need bigger brakes. Bigger pistons and a larger diameter disc make better brakes. Also if the brakes are going to be used for long durations, such as when racing, there is less time between brake applications for them to cool down adequately, this is where vented disks can be a real benefit.
All that heat soaks through the system into the brake fluid and although it is engineered to work at these very high temperatures in extreme cases the temperature can get high enough for the oil to boil, this generates gasses which compress easily and make the brake pedal feel very soft. This is brake fade and in really bad cases the brake pedal can sink to the floor with very little braking force generated, pumping the pedal up and down a few times can sometimes help but basically if the brakes fade on a race track then the car generally crashes. That is why on roads with long descents the car’s speed should be controlled by using a low gear and engine braking rather than holding the brakes on for extended periods.

Old brake fluid absorbs water which boils and fades much more easily which is why it must be changed every few years to stay safe. Silicon based fluid is different and doesn’t absorb water but moisture still pools inside the system and needs flushing through every few years, it’s also a bit more squashy than mineral fluid making it unsuitable for fast acting ABS.

Brakes are often overlooked and any wear only becomes apparent at the mot or in an emergency stop. The trouble is that they have a hard life and can disintegrate with the friction material splitting off the steel backing or wearing down to nothing unnoticed, and they usually seem to work fine right up to the point were they don’t work at all and you crash. Maintenance and regular inspection is vital.

It’s well known that suspension alignment is crucial for handling and safety, but how much can be checked at home? The main one that we can usually adjust is the front toe angle, but even though there might not be a simple adjustment for rear toe, camber, castor etc. it is worth checking them soon after buying a vehicle, just to see if it is straight.

What’s needed is some sort of datum point from which to take measurements to the wheels. The simplest way of doing this is to use two pieces of string, the thinner the better, one each side of the car at hub height and parallel to each other. With the car sat centrally between them we can measure from the string to the front and rear edges of each wheel and find out at what angle they are pointing in or out.

This method takes practice and initially may take time to set up and get consistent results, especially when you mess thing s up by tripping over the string. But with perseverance it becomes a very quick and surprisingly accurate operation.

ACCURACY VS PRECISION

It’s very easy to spend ages taking detailed readings, but beware. It’s easy to have accidentally moved the string, thus highlighting the difference between accuracy and precision. For example if you measure a block of metal that is in fact 50mm wide with a cheap digital vernier that reads 47.9836mm, it is very precise but not very accurate. By contrast if you use an old tape measure it may read 5cm, not very precise but completely accurate.

With the string method we are looking for accuracy, and consistent results.

Park the car on flat and even ground, this is very important; you can’t do it on grass.

Attach a piece of string to a suitable stand that won’t move when you pull the string tight, such as a breezeblock, at each end. The string must be at the same height, give or take a couple of millimetres, as the wheel hub centres and extend beyond each end of the car by a foot or so. The string should be as close to the car as possible without snagging on any bodywork etc. Ideally we would get this parallel to the centre line of the car, but as that can be difficult to define, just make it parallel to the sills to start with. It can be adjusted in a moment. Next put the other string on the other side in the same manner.

Now measure the distance between the strings at the front and the rear of the car to check they are parallel and adjust the string stands evenly until the same measurement is achieved at both ends.

Bear in mind that most cars have different track widths front and rear, so don’t expect the same hub to string distance front and rear.

When taking a measurement hold the ruler just underneath the string and take a reading from the edge of the string closest the car. With practice, and fine string, it is quite possible to get precision to 0.5mm, more than enough for our purposes.

You should now have two parallel lines at hub height, so to measure toe angle you can measure at a right angle from the string to the front and rear edges of the wheel or tyre, but beware, there are pitfalls here. Toe angle is very rarely quoted as an angle, that would be far too simple, so instead we work with a measurement which is the difference in the distances between the front and rear edges of the wheel/tyre. Now, to make it more complicated there are three systems in use, usual European method is to measure to the edge of the wheel rim (which is often bent and suffering kerb damage making the measurement difficult), the Japanese method is to use the step before the rim (more sensible as less prone to damage but depends on wheel design as how easy it is to get ruler on), the Americans go for the mid point of the tyre side wall (how you judge where the middle is can be tricky). So it is important to check the manufacturers’ specification before measuring your car.

Now, as we won’t be measuring between the wheels, but instead from the string to the wheel, we are effectively looking at half the car. So what we need to do is combine the reading from both sides.

For example on the car pictured here, the measurements from the string to the front left wheel are 47mm at the front of the wheel rim and 46mm at the rear edge. So the front edge is 1mm further inboard than the rear.

Now onto the right front wheel and I have 51mm at the front edge and 49mm at the back. So it is 2mm inboard at the front.

That means that I have a total 3mm toe in. And the car is slightly closer to the string on the left and I also haven’t got the steering set quite as straight as I thought, but the latter doesn’t affect the results.

Measurements at the rear are 47mm on the left and 50mm on the right. In both cases it is the same at the front and rear edges. So the rear wheels are completely parallel.

All this makes the assumption that the wheels are not bent or dented, so it is useful to check the run out first.

Jack and properly support the axle up until the tyre is just off the ground, then place a suitable datum, a breeze block perhaps, next to the wheel and clamp a ruler onto it so that its edge is just touching the wheel rim. Next, spin the wheel and see if the gap opens up or the ruler gets pushed back, anything more than a fraction of a mm is less than ideal, although its amazing how much wobbly wheel factor some people live with. If the wheel is bent then it is still possible to check the geometry, but the wheel has to be set so that the front and rear edges where we take measurements from have the same offset, but its obviously best to get it fixed first.

Another potential problem is that as the car moves forward or backwards, the suspension moves on the bushes. This means that you get a different toe angle reading depending on whether you reversed the car or went in forwards before measuring it. The best bet is to take a set of measurements then drive the car in the other direction for a few feet and take another set. This usefully highlights any potential bush or bearing troubles too.

Normally it’s the readings taken when the car has moved forward that are more important, after all that’s the direction you normally drive in, but the driven axle has to cope with being pushed forward when accelerating and being pushed backwards when braking. So both readings must be within specification.

Next up we can check to see if the front and rear axles are centred properly. The first step is to check the wheelbase on both sides and compare this to the manufacturers’ specification, bearing in mind that some cars, such as the Renault 4, have a different wheelbase on each side.

Having set the strings up to be parallel measure to the hub centres, and also to suitable datum points on the body/chassis such as the seam on the sill. If the car has been badly repaired after a shunt then one axle could be offset causing the car to crab. Should the results show the car is out of alignment, it might be a case of rebuilding the suspension or possibly the body may need straightening. However, before embarking on any drastic action based on the string and bricks method its worth double-checking at a professional alignment facility.

The next step is to use another piece of string and a bolt to test camber, most cars have the tyre inboard of the wheel arch, or at least they should. So by attaching a simple plumb bob to the top centre of the wheel arch with some sticky tape we can measure from the string to the top and bottom edges of the wheel and see how far it leans in or out. Now, this is absolutely dependent on the car being on flat ground, so check the ground with a long sprit level first. Also the tyres need to be inflated correctly and be matched so the wheels on each side are the same height off the ground, check this by measuring from the ground to the wheel.

Now, camber measurements are usually quoted as angles, so we need to convert our mm measurements in to degrees. This involves a small amount of school trigonometry, what we have is a triangle; if the top measurement (O) was 31mm and the bottom one was 21mm then there is 10mm difference, if the top and bottom measuring points on the wheel are 300mm apart then the triangle has a long side (H) of 300mm and a short side of 1mm, so the camber angle (a) is the inverse sine of 10/300 = 1.9 degrees.

Checking your car’s steering and suspension geometry can be quite simple, once you get the hang of it, and although the accuracy isn’t as fine as with laser alignment kit it does give a very useful idea of which way your wheels are pointing, can help highlight suspension faults and it’s a technique that is used by some of the top motor racing teams to this day.

Practice this method until you become proficient and you’ll have a simple and easy to set up system ready for use almost anywhere.

Another great pub debate question, of course 4WD is the best solution, but some purists would call that cheating.

Technically its a very complex subject, not least because RWD or FWD is only a small part of the whole picture. Suspension geometry and weight distribution are critical, but also tyres have a dramatic effect, and a cunning change of rubber can change the car’s handling characteristics utterly.

Best traction is usually found when the most weight is bearing down on the driven wheels, favouring FWD or mid engine RWD, but of course the engine is usually less than 15% of a cars weight, and at speed the aerodynamics take over, so even that rule is not set in stone.

There are, of course, rules of thumb. Most dynamics engineers reckon that FWD works best for up to approximately 300 bhp, above that and the weight shift rearwards when accelerating favours RWD.

When accelerating out of corners, FWD will tend to accelerate the car in the direction the front wheels are pointing in, more or less, where as RWD will tend to accelerate the car along its centre line, which on a corner where the front wheels are pulling the front away from that line so the driving force pushes the back end out, so FWD cars can get on the power sooner. But as ever, either case can be engineered around.

Going fast down the road also depends to a surprisingly high degree on how well the car suits your driving style; if your car does exactly what you are expecting, under or over steering, then you will get the best from it. It’s that predictability and familiarity that allows you to place the car accurately and easily just where it needs to be. That’s why two team mate’s F1 cars from the same stable are often set up to handle very differently. Also visibility is important, if the corners apex is masked by a massive A pillar then you cant judge your position properly on your mountain road. Confidence is key.

This may go some way to explain why some people swear by one or other set up, there will always be die-hard RWD fans who just cant get to grips with FWD, and equally there are hoards of FWD evangelists who can’t understand why anyone would want a car that spins off the road when you accelerate round a corner.