Setting suspension geometry using string.

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.



I like RWD, but I also like FWD, but which is best?

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.

Winter Driving

With all this travel chaos about you might be forgiven for thinking that driving in a British winter is simply impossible, not so.

There are two factors vital to safe driving on snowy or iced roads, the correct winter tyres and adequate driver skill. It is a sad reflection of our attitude to driving that for the majority of road users both of these factors are sadly lacking.

I have been arguing for decades that winter driving skills and skid training should be a compulsory element of the UK driving test, as it is in many other countries where similar conditions exist, I also strongly believe we should all retake our test every 5 years. But irrespective of any test conditions we should all do a refresher course every few years, I am fortunate that as part of my work I am frequently assessed and receive training, and even after a quarter century behind the wheel I am still learning. Most good driving schools offer refresher courses for experienced drivers, and many also offer specialist skid training, a sound investment in your own safety and that of other road users.

But all the skill in the world would be pointless if your car still has hard summer tyres on. Winter tyres use a different rubber compound that maintains better grip on cold, wet or icy roads, they also have a very different tread patter with many tiny groves to allow the tread to grip the rough surface of icy roads. If you have never tried them the difference is amazing, not only allowing you to pull away in slippery conditions but more importantly they dramatically reduce stopping distances and help maintain steering. In many countries using winter tyres at this time of year is compulsory, with very good reason. Some people use a spare set of wheels for their winter tyres and swap over when temperatures dip below 7C, but for normal driving good quality winter tyres can be used all year round saving all the fuss and bother. Winter tyres usually have a snow flake symbol on the side and the word ‘Winter’ just to make it clear what they are.

With these two steps, and driving slowly leaving plenty of stopping distance, you can enjoy winter motoring just as much as in the summer.

And if you don’t enjoy motoring then catch the bus and stop cluttering up our roads 😉

But the thing is that most people who drive have no interest in cars or driving, cars have become a necessity to the vast majority and are marketed as convenience goods, just like a microwave or dish washer. So they are not likely to read blogs like this or tweets from the AA, and to be fair most of the time they don’t need to.

I was watching cars struggling up a very slight incline to get out of a car park at a retail park recently, a BMW 3 series took several runs and needed the backed up traffic to reverse out of the way before managing to slither onto the road. This was quite entertaining, but the point is that although these cars were on summer tyres and the drivers hopeless, they all managed to get out eventually.

I suspect that as long as most people can get away with it, any calls for improved driving skills and winter tyres will fall on deaf ears.

How to make an old Jag fast.

The same basic principals apply to both XJ-S and XJ6/12 cars. First up which car to go for.

The V12 in 5.3 form is fantastic on the race track with a 6500rpm rev limit and over 300bhp readily available, although the standard cooling system is dire. The first V12s had flat cylinder heads and can be tuned up to over 600bhp (at great expense), later models had the High Efficiency HE heads which limit power but drastically improve fuel ‘economy’ and is still good for well over 400bhp. Early cars had a 4 speed manual gearbox as an option but these are hideously expensive, most have the immensely tough GM TH400 auto box which can also make a good race box when fitted with American drag racing parts. I raced an auto with manual override and it was superb. The last XJS cars had the 6 litre V12 with a 4 speed 4L80E auto which can be modified to work in manual mode either mechanically or electrically (paddle shift style), but these cars are rather expensive.

The 6 cylinder engines in the XJ-S were either the AJ6 in 3.6 or 4.0 forms or the four valve AJ16 version of the 4.0. All are powerful with over 300 bhp quite feasible. Available with either the 5 speed manual or the 4 speed ZF auto, again the auto can be modified for racing but the manual is a simpler option.

 

Basic track mods:

Brake pads, race brake fluid, jack the bonnet open an inch and fit good tyres. Give it a full service and off you go!

 

The more complicated version:

All cheap cars are rotten, so plan for welding. The front subframe which holds the engine up and holds the suspension on rusts from the middle out, good second hand ones are over £250 and quite a big job to change, so make sure you get a good one. The smaller cross member under the radiator also rots out but can easily be replaced with a strip of suitable metal and is not a deal breaker.

The front of the sills rots behind the ally splash guard but is reasonably simple to repair. The back of the sills is a very complex construction and includes the rear axle radius arm mount, this is a sod to rebuild but for racing it can be simpler to just cut the whole lot out and weld in a simple sill and convert the radius arms to a ‘cotton reel’ bushed rod that is mounted into a fabricated box intruding into the rear passenger foot well. This mod cuts out some weight too and also improves axle location.

When viewing a car pull up the rear seat base, rain water leaks in from the quarter light seals and pool in the seat base/ inner sill area. The race car solution to a rusty bottom is to cut out the set base and weld a simple plate over it. Also check the front foot wells where water from leaking screen seals can pool and rot the floor. Many cars have been undersealed which is unhelpful as the rot starts from the inside of the car and the underseal can hide it from inspection.

Mechanically the cars are strong, but bushes, bearings and ball joints wear. For racing I replace bushes with polyurethane and budget for new bearings and ball joints. Brakes get extremely hot so we use BNS grease in the wheel bearings which copes with the heat. The gearbox mounting is a cunning and complex unit which will be worn and make clonking noises when driving hard but can be replaced with a simpler rubber mount. The steering rack has very soft bushing so fitting pollybushes sharpens up the steering considerably, some cheapskate racers just limit movement on the bushes by simply putting tie wraps round the bush edges!

The cooling system on V12s is dire, I flushed the accumulate rubbish out then fitted coolant made of about 1% water wetter, 10% anti-freeze and 89% water which has better heat transfer ability. Then I removed the visco fan and associated heavy bracketry, the fan cowling and the original electric fan. I fitted a large electric fan instead as the fan is only needed in the pits. Jacking the bonnet open an inch lets the hot air out which is just as important as letting cold air in.

The brakes fade horribly on track, I used EBC Yellow Stuff race pads and Motul RBF600 brake fluid. The fluid is vital, it has a higher boiling point but has to be changed more regularly. To get a bit more cooling air round the rear inboard brakes I took the access plates out of the boot and removed the boot seal to let the hot air out, another approach is to cur the boot floor out completely which further improves cooling and makes access much easier to the rear axle as well as saving weight.

Weight loss is key, the sound deadening is everywhere and it is a good days work ripping it out. A tar based substance is glued onto the floor and has to be chiselled off. The interior heater system is very heavy and can be largely thrown away, although leaving the drivers side screen fan helps demisting. I used RainX anti-fog on all the glass to prevent misting, much lighter solution than a heater. The standard XJ-S seats only weigh 7kg so make a good cheapskate racers choice. Door lock solenoids can be junked to save 2kg, but the door cards weigh naff all so leave them in. I would also leave the electric mirrors on as they are less than 1kg each and work very well.

The centre exhaust silencer can be replaced with a straight through tube for a few more bhp and less weight. The standard intake airbox is a little restrictive and the trumpet can be cut off and a larger hole formed with a radiused edge, it is vital to get cold air to this and running ducting from the headlight surround works well. The middle headlights on quad light models can be removed to make an excellent cold air intake point, although some bodywork has to be cut out to get into the engine bay. The standard paper air filters work well when new, no need for expensive sponge filters.

The engine oil needs to be able to cope with high speeds and temperatures, I used Castrol RS which is now superseded by Castrol Edge. The other fluids have to be changed for quality higher performance versions too, including power steering, gearbox and differential.

All XJ-Ss had and LSD as standard, the 6 pot models had the lower 3.54:1 ratio needed for racing and is a straight swap to replace the overly high V12 item.

With the car lighter it will sit stupidly heigh on its springs. Eibarch make a suitable race springs but they are pricey. I cheated slightly by using the Jaguar ‘Sports Pack’ springs from a 3.6 model on my V12 which worked out just right. Adjustable front dampers are a handy mod and can be tweaked to suit different circuits – hard for flat ones like Silverstone and softer for less even ones such as Croft.

Fitting 50 profile tyres on standard wheels drops the gearing a tad more and lowers the car a bit too. Buffing the tread down to 4mm will stop them going off due to heat build up, this maintains grip levels and actually improves wear rate. I used Toyo Proxes T1R tyres as they were mandated by the race series, and they seemed to work well even in heavy summer rain.

You may wonder about roll cages, but if the car is ever used on the road I would avoid them because of the injury risk from hitting a steel bar next to your head in a collision. Cages work well when the driver is secured in a race seat with a race harness and wearing a crash helmet. The Jags are strong cars anyway so a heavy cage is of questionable benefit unless racing.

All that remains is to fit a race harness to hold you steady and a race steering wheel to speed up response and it’s time to head for the track to have more fun than is decent.

 

Check out these web sites which I have found very useful:

http://www.jagracing.co.uk/index.html

http://www.xjs6000.cc/

http://www.jag-lovers.org/

http://www.mig-welding.co.uk/

 



How much power does a sports car need?



I was testing a compact SUV the other day, it’s spec sheet fits in nicely with the current competition; about 240bhp in a 1400kg car, 4WD and 0-60 in the region of 7.9s. Although this may not be earth shattering performance by modern standards it set me thinking, about two decades ago I was working on a car that some of you may of heard of; the Escort Cosworth.

Now the Cossy set the world alight with its ‘blistering performance’, even that bloke Clarkson had one of his own and spent far too long telling everyone. Part of the Cossy legend comes fro the fact that in full rally cross tune it could hit the dark side of 600bhp, but even the standard version was in reality rather quick.

But now the Focus five pot has more than 300bhp and luxury saloons must have 500bhp to be a serious contender. Things have clearly moved on, but surely an old quick car is still a quick car? If I hopped into a Cossy today would I be bored rigid with it’s hum drum performance?

So I did a highly un-scientific poll on Twitter, asking people what felt quick to them, and the results were remarkably consistent, although that might just be an indication of the type of people who follow me!

Allegro fan extraordinare @OneCarefulOwner commented “the goalposts have moved in a major way; my Maxi 1750HL had a blistering 96bhp, nowadays small diesels have more grunt.”

@racing_waiting pointed out that defining quick was a well trodden path “tricky question, drivers republic struggled, imotor struggled, road my previous mag stuggled.”

On the subject of the old XR3i @HairyCalahan observed “times have changed. expectations too. xr3i fine for it’s day”

But getting down to numbers @vHenryk considdered that “pretty much. A ‘sports car’ doesn’t have to be a ridiculously expensive 0-60 in less than 5 secs thing to deserve the name.”. Whilst @torquespeak said “Puma convinces me anything above 120 has a decent shot. 8 secs to 60 not first degree rapid but a hoot on the twisty bits!”

Of course power and weight are only part of the storey as student and car nut @MrPA sugests “On a decent road anything can be fun. I have a few corner-filled favourites which are brilliant in my mum’s 1.2 Clio (75bhp!).” a fair point and one echoed by @cotswoldracer “Indeed , my old AX GT 700Kg & 85bhp , and going by memory about the same as my 145 in terms of acceleration (8ish secs to 60), my Alfa 145/950kg/150bhp quick-ish , another 35 bhp would make it even more fun :)”

@carpunk observes the importance of weight in its own right “Guess 100hp in a 1000kg car will always feel much quicker than 250hp in car 2x the weight because of inertia, braking mass etc “ and @jonbradbury agrees “I think modern expectations have increased, & so has most weights. Though 160hp in 1T 944 shifted lot better 115hp Gti & the 115hp GTi shifted better than the 115hp XR3i.”

The consensus seemed to be that a ‘sports car’ of between 1 and 1.5 tons should have between 180 and 250bhp.

Which brings up some interesting points, firstly that a lot of ordinary family cars are actually high performance sports cars, and it may well be that the only reason that they are not regularly parked in hedges and Armco is the astonishing amount of technology dedicated to combating incompetence behind the wheel.

But going back the that point about the way cars feel, this fantastic driver assistance seems to have come hand in hand with a duller edge to the driving experience. Back in the day a sports car would engage and entertain the driver, not only with its performance but also with its ability to snap back and slap the unwary in the startled face. Putting your foot down in a high powered car not only thrilled but also surprised many a driver when glancing at the speedo to see the needle significantly further round than expected. Over enthusiastic cornering could result in the car suddenly swapping ends or having less wheels on the ground than is healthy. Motoring enthusiasts call this sort of thing ‘fun’, but unfortunately normal people call it dangerous, and so because there are more of ‘them’ buying cars than there are ‘us’ cars have become less dangerous, and sometimes less fun.

But from an engineering point of view fun and safe can co-exist. Some manufacturers have cottoned on to the fact that whilst safe and dull is best for the mass market there is still a significant market for thrilling cars, and having driver aids set to only come in when disaster is otherwise inevitable yet allowing a reasonable degree of sideways progress makes good sense.

For instance if you turn off the traction control on a Jaguar XKR you can light up the tyres and do doughnuts, but you will still struggle to accidentally oversteer backwards into the vicars rose garden because the system is still active and helping the driver achieve their intended trajectory. I have driven one with the traction control completely removed, and to say one needs ones wits honed and ready for extreme service is an understatement, it’s not fun on a wet B road – it’s simply scary.

Maybe 500bhp is fine when controlled by modern electro-wizardry, but has the same thrill factor as a raw 200bhp in a car with no aids at all. So how much power does a sports car need? Well it would seem the definitive answer is ‘it depends’.

How brakes work

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 big car to do an emergency stop from high speed might put the equivalent of a thousand bhp through the brakes make the discs glow 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 by the drum wanting to explode, plus 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 high temperature 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 standard procedure is to crash. 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.

Most 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.

Larger brakes with a greater surface area to dissipate the heat into the air can cope with harder use but very large brakes need large wheels in order to fit. But all the force generated by the brakes has to be transmitted into the road by the tyres, so if the brakes are already capable of braking traction then there is little point upgrading them before upgrading the tyres. As ever the best solution depends on how the car is to be used.

The beginners guide to exhausts

The pipe the takes the exhaust gas away from the engine and lets them loose at the back of the car so the occupants don’t breath it in. Normally it has mufflers (silencers) to reduce the very high sound levels that the engine produces, without some sound reduction the cars occupants would end up deaf very quickly.

Usually the exhaust comes in several parts, the bit attached to the engine is the ‘Manifold’, this is connected to the ‘System’ which goes all the way under the car to the back. The system starts with the ‘Down pipe’ coming from the manifold down under the front bulkhead, then there may be a front section with catalysts, a mid section with a larger silencer and possibly a separate rear section with a smaller silencer and finishing with a ‘Tail pipe’ showing at the back, although there are many other arrangements too.

As well as transporting the waste gasses safely away and muffling the noise down to acceptable levels, the exhaust also effects the engine performance, its has to be big enough so the flow is not restricted. But also the gas speed needs to be preserved for high speed power, so making the exhaust to big can actually reduce power. As with all tuning its a fine balance to get the best performance, and there is no one perfect solution.

The bit that bolts to the engine is the Manifold, it has a tube for each one of the cylinders which join together. The exact way they join together and the length of the tubes makes a big difference to the tune of the engine, they can improve low end torque or sacrifice that for peak power. Its important to get the shape and size of the manifold ports to match up with the exhaust ports on the engine, any mismatch can restrict area or leave a step which causes turbulence and reduces flow.

There are two main types of muffler, one uses absorptive rock wool matting and the other type sends the exhaust gases through a sort of maze which breaks up the sound pulses. Generally the absorptive type removes high frequencies and the labyrinth type removes the basey boomy noises.

Most standard systems have a mixture of both, but for a more sporty sound they can be replaced with simpler ones that have less noise reduction and slightly more flow.

Twin pipes are still popular, factory fitted to most V engines which have two exhaust manifolds, they run an exhaust pipe on each side of the car floor pan and finish with two tail pipes.

On V6 and V12 engines these can be two totally separates systems, but on V8 engines they often have a balance pipe between the two systems close to the engine in order to run smoothly because of the way the firing order overlaps, giving an uneven sequence of exhaust pulses on each bank and that distinctive burble.

The least important part for performance is the tail pipe, usually finished of with a decorative trim.

Many systems run twin tail pipes running from the back muffler, although some systems try to get the twin pipe look by fitting a Y piece close to the back.

Catalysts (cats) convert partially burnt fuel and fumes into carbon dioxide, water and nitrogen. They do this by passing the exhaust gas over an immense area coated with an incredibly small layer of precious metals such as platinum which do the actual catalysing bit.

And it really does need a huge surface area to work, this is archived by folding the surface into a honey comb and by giving it a microscopically rough surface. In fact a typical catalyst can have the same surface area as a football pitch, all folded up into something the size of a 3 litre pop bottle, amazing.

It only works when its hot, at least 300C and preferably 600C, so it is usually put as close to the engine as possible so as not to loose any heat. In order for it to heat up quickly cats are usually made of ceramic which makes them fragile, so the catalyst brick is supported in the can by a soft fibre mat.

So if the cats hit a bump in the road there is a fair chance they will shatter. Also if the engine is tuned badly then un-burnt fuel will burn on the cat face and melt it.

When cats were first fitted back in the ’70s they were too small for the job and would restrict flow, modern cats are usually very good at flowing and can even cope with mild tuning, but for big power gains usually a bigger sports cat is needed. Racing cats use a metal brick instead of fragile ceramic, it takes longer to warm up but can take more abuse.

Exhaust systems can be either mild steel that has been coated in an aluminium based protective layer making it look dull silver, or made of stainless steel which lasts much longer and looks shinier. Stainless is a harder metal and so when it vibrates it makes a higher pitched noise, some people claim stainless exhausts sound ‘tinnier’ than mild steel ones.

The difference between quality brands and budget options is often in the grade of metal, cheap stainless will start to rot nearly as fast as quality mild steel. Also cheaper systems can end up with rusty welds, mild steel systems should have been coated after welding and stainless systems should be welded with stainless wire, not the cheaper mild wire. If the welds on a new system look rusty then it was a cheap one.

Sound affects our mood and generates strong feelings, so the exhaust sets the tone for the whole car. Get it right and the car sounds strong and purposeful, get it wrong and it sounds like a fart in a tin can.



Things you might not know about tyres.

As ever rubber-ware is critical, the rubber compounds are carefully engineered with a range of other substances such as carbon powder and silicon and then heat treated to give it just the right properties. There is a hell of a lot of technology in that black stuff.

The tread pastern is designed with channels that pump water out of the contact patch area at an amazing rate, for instance an F1 rain tyre can pump out 80 liters per second, but the tread does a lot more than that. The flexibility of the tread blocks allows them to move and adapt to the road surface to find grip, winter tyres have lots of small deep blocks of soft rubber with extra tiny groves in them so they can even get some grip on ice. A lot of people in the UK don’t realise there are different tyres for summer and winter use, but in many countries swapping to winter tyres when the cold weather starts is compulsory.

Winter tyres don’t work so well in summer, at speed the narrow tread blocks wobble about and overheat which looses grip, so summer tyres have wider tread blocks with a shallower tread depth. Track day tyres go a step further and have fewer grooves and some of the tread blocks go right the way round the tyre.

The tread compound actually wraps round the microscopic lumps and bumps in the road surface to give grip. At speed the rubber molecules have to grab hold of the road then let go very quickly, softer rubber reacts faster and flows deeper into the road irregularities giving more grip but gets ripped apart more easily when it has to let go, so soft tyres wear faster.

Full on racing slicks have no grooves at all to maximise the contact area and reduce overheating. It still has a tread layer because the rubber compound that contacts the road is much softer than the rubber compound used the make the structure of the tyre.

The side walls have to be stiff enough to keep the tread section under control and the base layer under the tread layer needs to be strong enough to hold the tread securely and resist punctures. They are reinforced with cords of steel or Kevlar, the precise weave effects how the tyre deforms on the road and so effects handling. Generally track tyres are more supple but wear out faster and with only one or two plies are more prone to damage, by contrast tyres built for vans and trucks are harder with many more plies making them last much longer and resist damage at the expense of ultimate grip.

Stiffer or lower profile sidewalls give a quicker change of direction, but can’t follow rougher roads so easily and may skitter a bit, that’s why race cars don’t often use ultra low profile tyres. A taller and more flexible sidewall is better on poor quality back roads, but it also introduces a small delay making it feel slow to turn in and a bit vague.

A wide wheel holding a narrow tyre holds it very rigidly, which is great for flat smooth race tracks but stops the tyre adapting to rougher road surfaces. By contrast a narrow wheel on a wide tyre allows the tyre to move side to side and curling up at the side when cornering hard making the handling a bit sloppy. Excessively wide wheels in narrow tyres may allow the bead to be pulled off the rim, which is bad.

Changing the tyre pressure can transform a car’s handling. Lower pressures allow more flexibility but too low and the tyre looses control which is very dangerous. Higher pressures hold the tyre more rigidly, to high and it can’t react well and the handling becomes a bit wooden. The best grip level is somewhere in the middle, and it varies depending on the intended use of the car, a little lower for a comfy ride in a road car and a little higher if the same car is on a race track.

Tyres age, the first visible signs are tiny hair line cracks in the base of the tread blocks which means its past its best and in no use for performance driving, but it also perishes from the inside so old tyres should be avoided, 3 years for a track tyre and 6 years max for a road tyre is the norm. The tread rubber gets harder over time as it ages and also because it gets hot in use which reverses the heat treating process it was made with.

On a race car when the tread overheats the grip disappears very suddenly, this is called ‘going off’. If road tyres are required by the regulations the tread is cut down to about 3mm depth to minimise the heat generated by the tread blocks wobbling about. New race tyres are run through a gentle warm up and cool down first to settle the compound molecular structure, going straight out at full tilt on new tyres ruins them.

The tyre is the only thing that connects the car to the road, everything that the engine and suspension does ends up as a single simple force on each tyre’s tiny contact patch. Tyres effect the cars performance and handling more than any other single component, and its not just a case of bad tyres vs good ones, but its about choosing the right type for your car’s purpose.