This is the first in a two-part series of articles where Dave Waplington explores the basics of vehicle handling and performance.
|Cornering.||The Handbrake Turn.|
This article was prompted by some of our members at the recent Sprint Day. They made comments on the approach that many drivers had to the corner at the end of pit straight, and asked for the reason many drivers appeared to take the long way around. I thought that I might endeavour to delve into the mysteries of correct cornering technique and the factors that come into play when cornering hard. Let’s discuss the later first.
How Tyres Affect Handling.
Tyres are the most conspicuous component of a car’s suspension, though not necessarily the most important. From a police safety point of view, having good tread depth is all that is required. The tyre could be an ultra expensive, low profile, top of the range tyre or second hand or a retread. As long as it has tread, it’s OK. Tread enables the tyre to get rid of water, that would other wise lift the tyre off the road and cause aqua-planing. However, if we look at what happens to a tyre while cornering, tread becomes our enemy in dry weather.
It’s common knowledge that racing cars use slick tyres (no tread). The only contact that a car has with the road is the four little contact patches of tyre. The bigger the contact patch, the greater the grip. The gaps in the tread that allow the water to escape, reduce the actual amount of rubber that is in contact with the road. Another problem is that the tyre is elastic. A force applied to the tyre in any direction will cause the tyre to distort, having a deep tread allows this distortion to grow. Imagine your car cornering, the forces on the tyre cause it to distort. The tread block that is in contact with the road will bend. The next block to come around is straight, but when it too becomes subjected to the cornering forces, it will bend also. The combined effect is that the car moves at an angle to the direction that the wheels are pointing, as the bending tread blocks “walk” the car away from the curb. In a slick tyre, the only distortion you get is from the tyre’s sidewall, which exhibits far less distortion than the combination of sidewall and tread block. The angle that the car moves away from the direction of required motion is called Slip Angle. The higher the cornering force, the greater the slip angle.
Let’s drive the car around a theoretical bend that has an ever decreasing radius. The first thing we notice is that we need to apply more power to maintain speed. This is because slip angle produces friction. This is used to advantage by some race drivers, and I’ve used it myself in go karts as a way of scrubbing off speed without braking. Large slip angles produce more friction and slow the car, your lap times will be slower too, so a smooth, low slip angle cornering technique would be quicker overall. Finally, however, a point is reached where cornering forces are so great that the tyre begins to slide against the road surface. Now slip angle disappears and sliding friction takes over. The coefficient of friction is greater for a bigger contact patch, so slick tyres win again. However, it is impractical for us to carry two sets of tyres in our daily activities so that we can swap tyres if it rains, so I won’t mention slicks any more.
OK we have reached the point in the turn where slip angle disappears and sliding friction takes over. We know when this has happened because the steering becomes light, the only way that we can get back to a slip angle condition is to reduce speed, or turn at a greater radius, thus reducing the cornering forces. Now here is the bad news. If you are cornering on the limit, you cannot use the brakes to slow the car, but we can increase the radius of the turn. By all means slow down, by taking your foot off the accelerator, the large slip angle generated will slow the car. To increase the radius of the turn, turn the steering wheel away from the direction you are cornering, ie straighten the wheel. If you are cornering left, turn the wheel slightly right. This comparatively simple act will allow the tyres to grip the road surface again, returning us to a large slip angle situation, which will slow the car even more. This manoeuvre can be done many times in the same corner. Modern technology has enabled race fans to sit next to the driver of a V8 super car. Quite often we will see them working the wheel rapidly left and right. They are cornering right on the edge of adhesion and are straightening the wheel momentarily so as to maintain a slip angle condition. It takes much practice and experience to feel and anticipate the car’s handling to that degree.
By now you probably realise that correct cornering technique involves maximising the radius of each corner. In fact we effectively straighten the corner a little. To do this we use the full width of the road.
Let’s negotiate a typical right hand corner. Enter the corner on the extreme left hand side, and aim for the kerb on the right hand side of the road in the middle of the corner. This is called the apex of the turn. Now allow the car to drift back to the left hand side of the road as you exit the corner. If you are travelling at the right speed, the car will do the last bit for you. Cornering like this puts us on the wrong side of the road halfway through the corner, which is fine on the track, but don’t try it in the city.
Understeer is a common trait of vehicle suspensions. If you’ve ever gone into a turn, only to find the car is taking a wider line than you had imagined, causing you to turn the wheel further in order to get around, then you’ve experienced understeer. It is built into cars because it is considered safe. Apparently. You realise that you can’t corner quickly, so you slow down. Having the engine at the front puts weight over the front wheels, which adds to the vehicle’s tendency to understeer. By far the best handling car is one where all the weight is at the centre of the chassis, but where would the passengers sit? Having said that, Lotus have been making mid-engined cars for decades, cars which are renowned for their handling.
However, we drive normal cars and having your “normal” car show understeer tendencies may be fine for dry weather driving, but in the wet, or during a competition, a hill climb or sprint perhaps, it’s something you need to be able to control. And you can.
The simplest thing that you can do to your car to reduce understeer, is to inflate your front tyres by a small amount over the comfort setting suggested by the owner’s handbook. That extra bit of pressure supports the tyre, reducing sidewall distortion and therefore the slip angle of the front wheels. When my GT6 is readied for a competition, tyre pressures are the first thing to get attention. The GT6 handbook suggests 26 psi (180kpa), which allows a comfortable ride, even in a car with such stiff springing as a GT6. For competition, and autokhana work, this needs to be increased to around 34psi (235kpa). The increased tyre stiffness has the added advantage of supporting the car during rapid braking as well.
If you have the need, and the money, take your car to a suspension alignment company and tell them that you need less understeer. They can adjust the suspension to reduce it. If the adjustments don’t deliver the handling needed the suspension on most cars can be modified. All TR and Spitfire based cars, which hang their suspension off a full chassis, can be adjusted to extremes with thick packing pieces. Obviously, the wheel alignment machinery is essential, don’t do it at home. Have you ever seen my car parked at a club do, and noticed that my front wheels appear to be falling in at the top? It’s true that they are not perpendicular, it’s called negative camber and it was done on purpose to help counteract understeer.
When turning a corner a little faster than normal, and finding that the car is exhibiting pronounced understeer, and waiting in your path is a power pole or large tree, what do you do – besides panic? The response is both simple and at the same time extremely difficult to do. The simplicity part is that you straighten the wheel slightly and momentarily, to make a greater radius turn. The slip angle of the front wheels will reduce giving you control once again. You will find that you can now turn back into the corner and get around safely. The difficulty comes from the fact that your mind is telling you (screaming actually) that you are steering the wrong way! If your car is sliding towards an immovable object, why would you steer towards it on purpose? The answer is because your front tyres are at the limit of adhesion, or beyond, any further turning of the wheel into the corner is going to make the situation worse. You can’t use the brakes for the same reason. Either of the two options mentioned require that the tyres have a friction against the road surface. If the wheel were turned further or the brakes were to be applied, the front wheels would lock, even with gentle application, and the car would go straight. The steering wheel could be twirled this way and that with absolutely no reaction from the car what so ever – until it hits something. The third and correct option is to release pressure from the accelerator, straighten the steering wheel momentarily, then turn into the corner again. What you have done is to increase the radius of the turn, and by so doing, reduced the cornering forces on your tyres allowing them to grip the road surface. Overcome the desire to turn the wheel harder into the turn. The limit is nearly reached, and the car will go straight on.
The best way to notice understeer and learn to correct it is by finding a gravel track or driveway and making turns at ever increasing speed. On gravel, your car will exhibit the symptoms at a greatly reduced speed. Another safe alternative is to join in one of your club’s autokhanas.
Oversteer is exhibited when the rear of the car breaks away and attempts to overtake the front. Having the engine in the back makes it more pronounced, since the extra weight causes a tendency for the car to develop a pendulum effect. This fact does not appear to have lessened Porsche sales however.
Oversteer can be brought on by excessive speed in a turn, which allows the rear wheels to lose traction, or break away. In a normal car, body roll will take the pressure off the inside tyre, allowing it to lose grip, this puts more cornering force on the outside tyre, which will exhibit an increased slip angle which could ultimately lead to a slide if it isn’t noticed and corrected in time.
Putting wider tyres on your car will give the wheel a bigger contact patch with the road, and slip angle will be reduced. If body roll is the culprit, then reducing it will reduce the tendency to oversteer. Stiffer springs will support the car better, but will make the ride harsh. However, the roll stiffness can be increased without sacrificing comfort, up to a point, by fitting a device which counteracts the vehicle’s tendency to roll. The simplest device is an anti roll bar fitted to the front of the car. Since the car is a stiff structure, any reduction in roll is effective on all points of the car regardless of where it is fitted. Ok I will concede that cars are not totally stiff, but from a practical viewpoint they are stiff enough. After all, we don’t race formula 1 cars do we?
There are as many anti roll devices as there are suspension types, so I won’t dwell on the subject here. Needless to say that anyone contemplating a handling package for their car can get heaps of advice from our technical people and from the advertisers in the mag. Many suspension centres sell, and will install, after market chassis tuning kits.
A word of caution is necessary, making changes to the car in the manner described does not eliminate oversteer entirely. What it does achieve though is to increase the speed at which oversteer occurs, allowing faster cornering, but if the car does “let go” the driver has less time to “catch it” by the physical means explained below.
The control of oversteer by the driver is easier to do than to control understeer. This is because the actions you take “feel right”. If turning left and the rear of the car starts to slide threatening an all out skid, which will ram the car into the tree on our left, it feels right to steer in the opposite direction, away from the tree. Because of this, many drivers correct oversteer instinctively. A typical correction scenario would go like this… Feel the car starting to break away, take pressure off the accelerator, turn away from the corner (into the skid), experience will allow you to feel when the skid has stopped then turn back into the corner and complete your turn. All of that would have taken perhaps half a second – longer if the road was greasy. If the vehicle speed is excessive, or the rear tyre has lost pressure due to a puncture and you’re caught out, there are two things that can be done, but you have to be aware of what is around you. On a race track, corners have run off roads and soft sand or gravel areas to arrest the car, so turn the wheel to full opposite lock and wait for the car to stop. You may find it will slow sufficiently to allow you to regain control. If the corner has been negotiated already and you’ve been caught out by moisture or an oil patch, simply apply the brakes fully. Locking up the wheels will allow the car to spin to a stop in a straight line. Because you are not attempting to control the vehicle and the tyres have lost their grip, the vehicle will go straight. It may be heart in mouth time, but it works. It takes a bit of practice for this procedure to become automatic, and it’s not something you can practice readily around the streets, although some do, especially in winter. However, a gravel track or a disused gravel pit is ideal. The loose surface causes the vehicle’s handling deficiencies to exert themselves at low speed. I don’t wish to get political, but many years ago, the state government of the day reduced funding for road safety and shut down the Road Safety Council premises at Mt Lawley. The facilities there included a smooth concrete figure eight roadway that was covered in oil and kept wet. You then drove a car with bald tyres around the course. Both understeer and oversteer could be demonstrated and practice had at controlling them, at very low speed.
The Handbrake Turn.
There ARE times however, when oversteer is your friend (notice that I never said that in the Understeer article?), this is usually when you are competing in an event of some sort. It may be a special stage on a rally or an autokhana. For instance, when negotiating a tight turn like a 180 degree change of direction around a witches hat, we can put on full lock and wait interminably for the vehicle to go around, or, we could use oversteer to increase the vehicle’s turning capacity. We can do this on purpose, by using excessive speed and sharp steering manoeuvrers. This will cause the rear of the vehicle to slide, this means that it will turn in a much smaller area and therefore in less time than usual. Of course, if you’ve just fitted a handling pack to reduce oversteer, you will have to do something really drastic to get oversteer to happen.
Enter the handbrake. We already know that oversteer happens when the rear wheels have lost their grip, jamming on the hand brake while turning fast into a corner will cause the rear wheels to lock up. This breaks traction and allows the laws of physics to take over. The front wheels are still in contact with the road surface, so the rear brakes away in rapid oversteer. During the turn, keep the front wheels pointing in the direction that you want to go, regardless of the angle and position of the car, once you let go of the handbrake (ie take it off) the skid will stop and you can drive out of the skid in the right direction.
Before I sign off lets have a word about handbrakes. Triumph sports cars have been fitted with “flyoff” handbrakes exactly for the purpose just mentioned. On these cars (and Austin Healey and others), the handbrake ratchet has to be cocked by the driver. If the handbrake is pulled up and then let go, the handbrake will simply fall down and release. To keep the handbrake applied, as when parked, the button has to be pushed. To release the handbrake, simply pull it up a little more, the ratchet clicks off and the handbrake will then fall as you let it go. If you want to see this demonstrated, simply talk to any TR, Spitty or GT6 owner, and ask them to show you.
We’ve had enough of mundane motoring, we want the ultimate – without the price tag of a Ferrari or Aston Martin. What have we learned from this small series of articles on vehicle handling that will allow our car to out gun the big guns?
1. The vehicle weight (engine etc) should be as close to the centre of the vehicle as possible. We’ve already mentioned Lotus. However, all we really need is to place the engine as far back in the chassis as possible. At the next All Triumph Day, have a look at the relative engine positions between the GT6 and the Spitfire. You will find that the Spitfire engine is a long way back in comparison and is much lighter. In fact the Spitfire is one of the best handling cars ever made. Back in the seventies, I read an article in the American magazine, Road and Track, where a university had developed a method of measuring a vehicle’s handling characteristics without all the maths. It included a ramp that could be tilted sideways. As the vehicle was rolled over the surface, the amount that it deviated from centre was a measure of it’s handling characteristic. The winner was a student’s Spitfire, which recorded better characteristics than the professor’s Ferrari, much to the chagrin of the later.
2. Other heavy components such as gearbox and differential should also be placed near the centre. If you ever get to inspect an Alfa Romeo GTV, you will see that the diff and gearbox are all together in the same housing – as are Porsche and Volkswagen. Many manufacturers try to concentrate the heavy items in a bid to imbue their cars with better handling.
3. We’ve learnt that big thick anti roll bars are essential and stiffer springs will reduce roll. Lowering the car will lower the centre of gravity. Sports cars are built lower than sedans for this purpose. Have you noticed how many 4 wheel drive truck owners think that they are driving sports cars? The suspensions may be stiffer, but their centre of gravity is way up in the air!
4. Size is important. Tyre size that is. The bigger the contact patch, the better the grip, all things being equal. However, things seldom are equal and some manufacturers claim a softer tread compound will grip the surface better than a long wearing, hard compound. Something to think about next time you order a long life tyre. And if more air in the tyres helps support the sidewall, how about reducing the sidewall dimensions. Many dressed up cars today have large diameter wheels with very low profile tyres. That is, tyres that have a much smaller sidewall. These tyres exhibit less distortion in the sidewall as a result. The bigger wheel size is needed to bring the rolling diameter back to what it would have been. Otherwise speedo accuracy and fuel economy are compromised. Their owners may have bought them for their looks, but they have the added advantage of lower slip angles.
5. A word about slick tyres. Are you aware that Dunlop sell a tyre that they call their R rubber tyre. R stands for race, and it is a soft compound tyre with the bare minimum of tread to make it legal. The tyre is only slightly grooved and not very deeply either. The tread blocks are huge as a result, but the longevity is seriously reduced. If you want to go racing and keep it legal then this is the tyre for you, just be prepared to replace them more often.
6. There are many other handling options that are accessible to us and those of you that are interested can find many books on the subject at places like Pitstop. However, before I close have you ever wondered why cars, and in particular, Triumphs, have independent rear suspension? One of the bigger problems facing car manufacturers is how the car handles bumps. When a car runs over a bump at high speed it can cause the car to bounce about in the extreme. The reason being that the car is connected to the road through the suspension. When the wheel bounces, so does some of the car. The term for describing the effect is “unsprung weight” The more unsprung weight a suspension system exhibits then the more the car will bounce. The method that has been found to reduce this effect is independent suspension. Independent suspensions decouple the wheel from the car, the only unsprung weight being that of the wheel, tyre, brake and part of the axle. In live axle cars the diff, heavy axle casings and part of the prop shaft and gearbox are unsprung. The independent suspension allows the wheel to bounce with less being transmitted to the car. The car is therefore easier to control. The use of a lighter wheel material helps too, so “mag” or alloy wheels do more than look good after all.
7. Right then, I need a lower car, the lower the better, it has to be mid engined with the diff and gearbox in as compact a configurations as possible. A stiff suspension with anti roll bars. It needs low profile, wide tyres with little or no tread sitting on wide alloy wheels. Sounds like an open wheel race car to me.