Car Advice

Tech Talk – Jaguar F-Type

When he’s not lapping a new car around a race track, or driving over boulders, our Robert Pepper can be found delving into the technical specifications of the latest releases. Here’s what he’s gleaned about the Jaguar F-Type.

LIKE JUST ABOUT EVERY new car on the market, Jaguar’s F-Type is what it is because it uses a vast array of sensors and techno-wizardry to feed on-board computers that help alter, direct and generally keep the thing going in the right direction. With the vast array of technology and Electronic Control Units stuffed into modern vehicles it’d take a book to do everything justice, so here’s an overview of some of the more interesting tech beneath the skin of the Jaguar F-Type.

Dynamic Stability Control If only manufacturers could agree on the same name for exactly the same thing. Jaguar calls it DSC, or Dynamic Stability Control, but it’s the usual electronic stability control (ESC) which detects understeer (when the front of your car pushes wide in a corner) and oversteer (or when the back of the car tries to overtake the front). The on-board computers figure out what’s happening by monitoring individual wheel speeds, steering wheel angle, throttle position, yaw and pretty much anything else it can get a reading on, and it all happens in milliseconds. If the computer brain decides things are heading for a what in the trade is known as a “moment” then it then takes corrective action by braking individual wheels and if necessary, restricting throttle opening. That is standard ESC…

The Jaguar F-Type’s DSC is a little bit different because it has three modes, and it’s not alone here with BMW’s M3 also running various stability control modes, like most proper sportscars.  The Jaguar’s modes are DSC On, Dynamic Mode and Track Mode.

DSC On is what you’d use for normal driving, and as soon as the car steps out of line the computers are in like Flynn, sorting the car out for you. It is a well tuned system like most modern designs, subtly correcting as opposed to slapping the driver into next week – all ESCs are far from equal, and the real sportscar manufacturers put a lot more time into their systems those who make lesser vehicles.

Dynamic Mode is ‘kind of’ a racetrack mode. This allows a little bit more latitude before the computers get involved to keep you pointing in the right direction, and this is important. When a car is driven even sedately there is what is called “slip” – the drive wheels are always, microscopically, wheelspinning and every corner you take you are, almost imperceptibly, sliding the tyres, yes, even Grandpa in his Camry. I know it doesn’t feel like it, but you are. Now, as you drive faster, those slip angles increase, so there’s a fraction more wheelspin, a fraction more sliding around the corners. It’s not a full-on opposite lock powerslide or anything, it’s just creeping up to the limit of adhesion because losing tyre grip isn’t binary, it’s gradual, kind of like pulling Blu-Tack off a wall.

Dynamic Mode is about allowing a bit more slip than usual road driving. It means the driver has the freedom to get to that on or near limit grip level in a way which the DSC On mode doesn’t permit. In our family, it’d be like Dad watching the kids on the playground as opposed to Mum, different level of risk tolerance.

Now if the grip level is truly exceeded Dynamic Mode will come in and help, in the same way Dad would stop the kids from trying handstands on top of the swings. What you’re looking for in a good ESC mode of this nature is the ability to marginally overstep the limit with the driver able to make a quick correction before the computers get excited.

Finally, there is Track Mode. This is where ESC (sorry, DSC) is completely switched off. That is what you want for the ultimate thrills, and if you are going to try your hand at drifting – and why else did you buy a 400kW rear-wheel drive car (but only on a race track, of course). If you exceed the traction limits here the computers will just sit there, arms folded, laughing as you experience that horrible moment in time between the loss of control and the impact with the armco. However, ABS remains active even in Off mode. It is not clear whether torque vectoring stays on as that relies on the same basic system – I would hazard at guess at the answer being no as torque vectoring is a kind of stability control, but I suspect the EAD (active differential) remains working. Again, the fact the F-Type has a completely Off system is good, another mark of a true sportscar. In normal cars the Off switch is actually more akin to Dynamic Mode, not really off, just turned down a bit.

Tech Talk - Jaguar F-Type

Torque vectoring This is a system loved by marketing people because it combines two of their favourite words after “homage” and “heritage”. It is in effect a slew-steer system. Imagine a car under brakes, and then you could magically release the brakes on both left wheels. The car would turn (well, spin) to the right. This is how torque vectoring works, except that the brakes aren’t entirely released, just the pressure is varied slightly. The effect is to allow the car to turn in better. It is interesting that Subaru’s equivalent works only under power, whereas this one works under brakes. Jaguar’s system is both predictive and reactive – it predicts by tracking what’s happening with the car – steering wheel angle, braking input and speed etc –  and says you know what, this combination isn’t going to work, time for some vectoring before problems even happens.

Electronic Active Differential and Limited Slip Differential – all cars at present have something called a differential which allows the engine to drive both wheels on an axle, yet when cornering also allows the inside wheel to slow down relative to the outside wheel. This is excellent, but has a huge disadvantage for sportscars because when you corner hard there’s little weight on the inside wheel, and that means little grip, and that means you might spin up the inside wheel under power. And unfortunately that means there’s little drive going to the outside wheel, the one with weight on it and therefore grip.

The solution is generally a Limited-Slip Differential (LSD). This is a mechanical device that restricts the ability of the differential to let the wheel turn at different speeds so you don’t get this inside-wheel-wheelspin problem. It also makes drifting (powersliding) easier for the same reason, and if you try a fast getaway with one driven wheel on a slippery surface and one wheel on a grippy surface then again, the wheel on the slippy surface just spins and you go nowhere. Whether or not a sportscar has some form of LSD fitted is another indication of whether it’s a sportscar or a posecar.

The F-TYPE has an LSD in lower-spec models, but it has an EAD in the F-Type R, which is an Electronic Active Differential. That as you may expect is pretty much just an electronically controlled LSD, and thus better. Most of the time having an LSD is a pain because it restricts the differential’s action, so you get increased tyre wear, greater turning circle, scrunchy noises on tarmac when you turn tight and so on. Wouldn’t it be nice to turn the LSD on when you need it, and off when you don’t? That is the EAD. It can leave the differential entirely open, and then start to lock it up more and more as and when the situation requires, for example trying to initiate a drift, or a dragline start. It can open the differential up too, for example high-speed understeer under power, or understeer under brakes. With a mechanical LSD you’ve just got pretty much the one setting, all the time.

Tech Talk - Jaguar F-Type

Spoiler At the back of the F-Type there’s a spoiler which lifts up into the airflow over the car, breaking up the smooth airstream. Why would you do this? The answer is downforce, or to be precise, reducing upforce. Once you’ve over about 50-70km/h the dominant force acting on a car is air, not mechanical friction. The F-Type can muster the sort of speeds well above those that light aircraft cruise at, and you may notice that side-on most cars kind of resemble an aircraft wing and that means they generate lift at speed. This is because as the airflow over the top of the car speeds up, creating a low-pressure area. The odd thing about lift, and its horrible counterpart, drag, is that it obeys the speed-squared law so double the speed, four times the lift and drag, triple the speed, nine times and so on.

This means that at high speed powerful cars produce quite a lot of lift. This is good in a way because there’s less weight and thus less rolling resistance on the tyres, but bad for exactly the same reason because the less weight on the tyres, the less grip they have, and at high speeds grip is by general consensus regarded as a Good Thing.

So what Jaguar have done is put a spoiler up which partially destroys that nice clean airflow and reduces the lift force by up to 120kg. It doesn’t deploy until a speed of 113km/h (70mph, oddly enough the British freeway speed limit) has been reached, but you can manually deploy it earlier should you wish.

Now this is all very well in theory, but in the absence of hard, comparative evidence to the contrary I think it’s a gimmick. Firstly, the car weighs around 1700kg, losing 120kg isn’t going to make much difference. It would be different were it say a 500kg racecar. Secondly, the only figure we have is 120kg, which presumably is the figure at the top speed of 300km/h, not a speed you’ll have a chance to do very often. And finally, the spoiler also increases drag, which will reduce acceleration. Then there’s the deployment. You’d really want it on for braking and around corners, then off as you straighten the car, but not too early as you wouldn’t want a sudden loss of grip caused by the spoiler folding away.

So anyway, it’s a nice little toy but I very much doubt there is any practical application whatsoever, even on racetracks; happy to be proved otherwise by Jaguar boffins…

Tech Talk - Jaguar F-Type

Unsprung weight and brakes The F-Type R has an option for ceramic brake discs which are big enough to be re-used as a helipad. They are also, all up, 20kg lighter than the normal brakes and this is important because it is unsprung weight.

Cars have suspension, which is essentially a spring which absorbs the undulations in the road, and a damper (or shock) which damps the spring’s resulting oscillation so the car doesn’t kangaroo down the road.  The weight which is supported by the springs is called sprung weight, and the weight which isn’t is called unsprung weight. There is not much unsprung weight – the wheels, the brakes and about half of the suspension system. The rest of the car is supported by the springs.

Reducing any weight in a car is good for performance in every way, but unsprung weight is particularly beneficial because then the wheel can be more easily controlled by the spring and damper. As an example, reach your arm out straight and move it up and down. Easy to do. Now do the same, but with say a bottle of wine in your hand. It’s harder to start and stop your arm now, and that’s just like adding (or reducing) unsprung weight. So the benefit to cars of less unsprung weight is more precise control over the suspension’s ability to handle undulations and load, which means better, sharper handling and ride. And a 20kg improvement is pretty decent. Whether or not you’d actually notice without a very careful back-to-back drive is another question, but nevertheless it is an impressive achievement.

While on the subject of brakes it’s important to understand what they do, which is convert kinetic energy (the car’s movement) into heat, and then dissipate that heat to the atmosphere. On the road, there’s not much work to do because there’s relatively little energy to convert as the car slows from say 100km/h to 50 through gentle braking, and there’s many seconds if not minutes before the brakes need do any work so there’s plenty of time for what little heat has built up to dissipate.

Contrast this to a racetrack where you’re doing full-on emergency stops maybe three or four times a minute, for many minutes on end, and from much higher speeds that you’d reach on a road. Now you have vastly more heat to do with, but also very little time for it to dissipate. This is why brakes on roadcars overheat on racetracks and then do something called “fade” which is a rather gentle term considering how scary it is when you try to stop and find you cannot.

So if you’re going to take a roadcar to the track the first thing you need to do is upgrade not the engine, not the tyres, but the brakes. Happily, Jaguar can do that for you, and I am reliably informed the ceramic brakes do not fade even under heavy track use, and given they are sufficiently large to be reused later as a BBQ hotplate for your extended family, I can well believe that claim. Again, this is an important measure of a true sportscar.

There’s also something called Pre-fill, which is an unusually bland and descriptive name for a clever system, maybe it was named by an engineer not a marketer. When you press the brake pedal there is a bit of slack to take up in the system, and that slack is taken by up Pre-fill in anticipation of the stop. Most of these systems work by moving the brakepads closer to the disc and pressurising the fluid, kind of like depressing the brake pedal a little way. The result is a quicker stop, and better brake pedal feel which Jaguar say they have calibrated according to whether the car is on a racetrack or in a carpark.

Configurability The DSC Dynamic Mode already mentioned does more than just change the ESC limits. It also changes the gearshift mode, so gears are held longer before upshifts, downshifts made earlier and the car detects the best gear for rocketing out of the corner. Throttle response is also sharpened, so smaller throttle movements produce bigger changes in power, and that requires a little bit of getting used to, it feels rorty and untamed which is what you want in your sportscar, so huge fun.   Steering is also quickened, and the suspension is hardened up for less body roll. This is done by electronically changing a valve in the damper. Imagine a coffee plunger – you push the plunger down and coffee flows through the holes, and now imagine if the plunger’s holes were larger or smaller, it’d be easier or harder to push the plunger down. That’s essentially what’s happening with the dampers. This affects both bump and rebound – bump is when the suspension compresses, and rebound is when it extends after having been compressed.

The F-Type is also clever enough to stiffen suspension in other situations, for example under heavy braking there’s a weight transfer to the front in any car. This means that the front suspension compresses, so if you hit a bump you may find there’s little travel left due to nosedive. Stiffening the front under braking fixes that problem. Same deal for cornering, it’ll stiffen the outside dampers and, presumably, loosen the inside ones to allow for the weight shift to the corner, and for hard acceleration it’ll harden up the rear suspension to avoid excessive compression.

Tech Talk - Jaguar F-Type

The Noise Ah, the noise. A sportscar has to delight all the senses, and the F-Type R has a real Roar, which I imagine is modeled on the noise made by lions when fed Porsche engineer body parts. Like many cars these days that is done via a symposer which channels specific parts of the engine noise to the cabin. Jaguar haven’t gone as far as BMW and started playing artificial noises through the loudspeaker, which I for one think is the right move. There’s also a dial to allow more or less exhaust noise. At low-decibel racetracks you have to keep it down.

Smart gears All modern automatic gearboxes are smart, or at least not as dumb as they used to be. Back in the day the decision to choose a given gear was made by things mechanical which measured engine load. That is why automatics were famous for being in the wrong gear at the wrong time. Today’s autos are different, because they have lots and lots of sensors to figure out what is going on. They know how fast the car is going, whether it is cornering, how hard it’s cornering, how sportingly the driver is driving and pretty much everything needed to figure out the right gear for the right time. And with eight ratios to choose from there’s a gear for every occasion. Within the adaptive gearbox there’s a few interesting features – Fast Throttle Off notices when you come off the throttle quickly and prevents an upshift, and under heavy braking the car downshifts more quickly than usual. Essentially, you need never worry about gearchanges again, but you can take manual control if you wish, and in slippery conditions you may well want to choose higher gears than the computers suggest.

Gearing & power There are two engines in two configuration, a V6 and a V8, both are supercharged. Supercharged means that the engine drives a compressor which stuffs more air into the cylinders, which means more petrol can be ignited, producing a bigger bang and more power. It’s the same as turbocharging except that turbos have the compressor driven by the exhaust gases of the engine. There’s two ways to measure engine output – torque and power. Torque is a turning force, and power is how quickly that turning force is delivered. Imagine pedalling a bike – if you can push the pedals around you have enough torque, but how quickly you can turn them is power. A stronger cyclist might use a higher gear which means it’s harder to push the pedals around but they can do that because they’re stronger (can produce more torque), so they go faster. Or they may just use the same gear and pedal quicker (more power).

The differences between the variations in each set of engine appear to be in the tune. The V6 S produces almost exactly the same power at a given RPM than the V6 until you’re above about 5000rpm. The V8 S seems to be limited in torque to 625Nm which it produces between about 2000 and 5800rpm, whereas the V8 R is allowed to rise gently to a natural peak of 680Nm, which translates into a bit more power but especially so past 5800rpm.

 

So, there you have it a little more of a look into the magic underneath the skin of the Jaguar F-Type.  Seems like it’s got brains as well as beauty, and it drives well on the track, too!


Robert Pepper

Robert Pepper