Geometry

SOARER WHEELS AND TYRES.

There is only thing that is constant no matter what car you drive, you only have tiny points of contact with the road; your Tyres, and they are so crucial that Companies will spend 10's of millions of pounds on development. 9/10ths will see no end product, but the other 1/10th is what lets us drive with hardly a thought or care knowing full well that whichever manufacturer made our tyres they will do their job better than we can, as drivers, do ours.

The fact is most drivers [for 99.999% of their driving time] don't give them a second thought. Only after an incident do we suddenly not take them for granted. The most common incident is the breakaway around a favourite bend, we get out check that there is still some tread, reassure ourselves all is well, promise to check the pressures at the weekend and proceed with a little circumspection, 'til the next time. But being Soarer drivers you're a cut above the norm and you want to know why, and how, this happens.

The key words here are Slip Angles and Hysteresis. You will be familiar with the terms Toe-in and Toe-out [look at your own feet], and understand perfectly well that if all four wheels on your car pointed straight ahead any undulation in the road would send it off course and the first bend you took at even quite slow speeds would feel very awkward. You need to point the wheels to oppose one another on each axle, think of it as like an arrow without the pointed end, instead just butt ended, its flight would loose its accuracy, that accuracy comes from the opposing forces working the entire surface of the pointed head because the arrow travels in 3D, we need toe-in and toe-out to do the same for us on tarmac.

It's a tribute to modern Tyre and Suspension design that so much sure-footedness is delivered with such a tiny amount of opposition, this has come about because of the increasing accuracy of the Suspension manufacturers machinery which meant more design specific tyres could be made. This thanks largely to the efforts of Citroen and Michielin. Previous attempts at front wheel drive [e.g. Cord of USA] were vastly expensive due to this necessary accuracy, and were [usually] not intended for high production numbers.

So now we have a stable motorcar, able to travel in a straight line very nicely, then the road curves away. Round you go and it is at this point the Slip angle and Hysteresis come in to play, [they were there even when we were moving straight ahead, but to a much lesser degree]. If the tyre gripped the road perfectly then you would come to a dead stop, the tyre has to give up its grip for you to go somewhere so it must not stick to the road like glue! Now just as the tyre gives up its grip to go forward it has to give up some of its grip when we corner we call the difference between the perfect theoretical line and the actual line travelled the slip angle and how it lets go is the Hysteresis. The latter is the bit that can mean life or death for any driver.

Imagine if you will a tyre that had fantastic amounts of grip and in a particular corner could get you round at 90mph but at 91mph lets go with no warning. You just crash. I know that in that instance my speed would always be well below the 90mph limit. Now, if I used a tyre that could go round at 85mph but started telling me that it is losing grip from 70mph and did it in such a way that it inspired confidence I know I would be a better, safer driver with that tyre fitted to my car! I have to say that some tyres, namely the top grade Yokohama's, Goodyear's and Bridgestone's have such high levels of grip in the dry that it is very difficult to gain any experience of their slip angles until sometimes its too late and you are in a ditch wondering what the Hell happened.

For many drivers these tyres are too like the first of the above examples. By necessity these very grippy tyres have a subtle Hysteresis loop, which only shows itself at quite remarkable levels of adhesion in the dry, but when its wet or damp things happen much quicker than with an ordinary tyre. Having recently very nearly 'lost it' in a good friends R34 Skyline running the grippiest tyres a have ever experienced I can say from terrifying experience, they me no warning that they were at or even near their limit.

REMEMBER A LAW OF PHYSICS, THE HIGHER THE LATERAL LOAD, WHEN SHE GOES, THE QUICKER AND MORE FORCEFULLY IT WILL HAPPEN.

So, lets see if we can avoid these 'brown underpants' moments.

First, do you need all that grip? Simply put, it all depends on your driving style and ability and how deep your pockets are! Also are you getting the best out of what you already have? Go out to your car with a pen and pad and get some info off the Tyre sidewalls and the Wheelrims, the size [225/55×R16], the Uniform Tyre Quality Grade [Treadwear XX[X], Traction X[X], Temperature X], Plies [Sidewall [X], Tread [X],[X],[X] , rims 16×7JJ (50) with the X's filled in by you. If you have different makes all round your car then you may need a bigger pad you may also be running different rim sizes and tyres make a note of the lot. Oh, and remember to look for the words 'ROTATION' with a little half arrow That was fun wasn't it!, some info isn't there is it!, did you look on the inner wall?

OK, lets start, tyre size the '225' bit means the overall width of the tyre is 225 mm at its maximum and you should, at least, have the same size on the same axle! The '/55' is the Aspect Ratio and is represented as a percentage of the width as the height of the sidewall in this instance, 55% of 225=123.75 mm, again, you should have the same A/R on the same axle!

The 'R' bit means it's a Radial Ply tyre (more of that later) next up the '16' means the tyre fits a 16" rim, by the way, ever wondered why we have mixed units? I mean mm & Inches. When all tyres were Crossplies they were measured in the Sopwith decimal inch system i.e. 5.90×13 etc., I've not a damn clue why the Sopwith system apart from the fact that Tommy Sopwith was a fine Engineer, but so it goes. Then there was the collaboration between Citroen and Michelin and you guessed it, they're unit of measurement was, and still is, Metric and the Michelin was the first proper Radial and it was so superior that it took the World by storm. So all radials are measured Metric.

The UTQG is very useful, don't listen to those who say 'it means nothing' they are, in almost every way, incorrect. The first number relates to how hard wearing the Tyre is, 100 means 30,000 miles of normal use by a 100 HP, 22 Cwt motorcar, this translates to about 9,000 miles in our Lexii. (d'yer fink dat's the plurus fer Lexus.) So when you see '95' as the first number on a £250 tyre you know they're gonna cost you 3p. per mile per tyre!!!! My own tyres are rated 320 (29,000 miles) and set me back less than £60 ea. 'all-in' less than 0.25p. per mile per tyre. That's a 12 fold difference so what do we each get for our money, are those 'gumballs' 12 times better? Well actually, er, No! For all the reasons mentioned above. Next is traction, the best (for road use) available is AA, followed by A, B and C the minimum for us must be A, but bear in mind that the AA rating was introduced in '97 and some of the tyres at present rated A would be re-rated as AA. This is a WET BRAKING traction value NOT lateral or radial acceleration. There is a corollary between these but it is difficult to explain without the use of formulae. The next part of the UTQG is temperature and this relates to the your tyres ability to run at sustained high speed and the A rating means that they have passed the 145mph for ½hour in near desert conditions without sustaining any failure. But the real trick is the interpretation of this raw data with the next bit.

Plies, if you thought it was clever getting the right mix of rubber compound then the contradictions in designing the belts that give strength and suppleness to the tyre carcass has reduced grown men to gibbering, dishevelled and tearful individuals sitting in padded cells wearing a 'cuddle-me-self' coat. It is so complex that I know of no Computer Program that can do it! Even Formula One tyres are human engineered, I think it is the last part of a F1 car that is! It has always been this way at the top and with each new generation of tyre design the older design gets used by a less well-known producer usually (now) in Taiwan or Korea. So those budget tyres were 'state of the art' 3 or 4 years ago and trust me there have only been tiny incremental improvements since then, so I think the premium is not really worth paying. Just get the highest Treadware number with 2 A's and make sure you have 5 plies on the tread, 2 outer polyester, the middle 2 of steel and the innermost Nylon, and at least 2 plies in the sidewall, I think now all side walls are synthetic and Rayon seems to give the best flexure control with polyester not far behind.

Now to rim size I know of 2 standard fitments for a Soarer 15×6½ and 16×7 and the standard tyres are 215/60R15 VR and 225/55R16 VR resp'y. What we get from this is the height of the Sidewall and their Rolling Radii, 129mm & 124mm resp'y and 639mm & 654mm resp'y. The former set tells us that that the 16" rim is the sportier, the lower the A/R the lower the amount of potential flexure there is in the side wall and as the A/R lowers, higher grades of sidewall material have to be used. The latter is necessary to maintain a reasonably accurate speedo although as a rule ±1% will not be too bad.

Another factor to bear in mind is the ratio of the rim width and the tyre width. There are certain ways to affect how a tyre will behave after you have selected a suitable candidate, one very interesting way is with the rim width. It used to be a guideline that if the rim was approximately 75% the tread width the tyre would be at its best, then the figure went up to 80% then 85% and now even 90% is used to provide very sharp responses as the sidewalls are nearer to the theoretical ideal. For example I run 16×8 all round but use 225/55 (90%) on the front and 245/50 (83%) on the rear with additional 10mm spacers on the rear only. This set up gives me great turn-in from the front and the additional rubber coupled to the greater flexure given by the more forgiving rear rim/tyre ratio means I have a near faultless set-up (for me). Due the stance of the car being wider, thanks to the rims, tyres and spacers, 25mm front, 65mm rear, my car is so much more stable than the original and deliciously chuckable on those 'cheap' tyres. We will all acknowledge that the Soarer is a big car, and that it should not really be able to outrun an enthusiastically driven modern GTI Hatchback on 'B' roads, but with this set-up I did this day, catch, overtake and leave behind such an enthusiastically driven machine. I admit the overtaking was just sheer power but the rest was set-up. By the way the rims are 2 pairs of fronts from some Supra's, just find a local Supra specialist and he will have replaced at least half a dozen standard wheel sets for expensive showy rims for some unthinking rich kids, (a little bit of cynicism crept in there, sorry). Slap £100 down on the table and drive off with a set.

Now, I don't own a V8 so this part is theory only (I have driven a couple) but I would guess that the set-up I just described would not suit either the V8 or its Driver, so I offer this set-up as a starting point. Get your self a set of 16×7½ they will fill out your wheel arches and make the car look more 'chunky' keep your 225/55R16's giving you an 85% rim/tyre ratio. Then when they are worn out go for some Avon RTT's they are simply the best all round tyre for comfort, they wear really well, are quiet, have very good turn-in and give a really positive feedback when approaching their limit. They cost about £90 ea but seem to be the ones for progressive and relaxed driving.

Offset, if you change the size of your rims then you must look at this so as to avoid fouling of the wheel arches. So what does it mean? When we only had crossplys the interface between the rim and the hub was central to the rim width, this was a hand-me-down from how wire wheels and even cartwheels had been made and it seemed 'right'. Ah, but those guys at Citroen had a problem, we know it now as 'torque steer' but back then it seemed quite insurmountable, the lighter the car the more prone it was to this phenomenon. The wheel became a huge dumb-bell which was OK when the wheels movement was confined either to just steering or power transmission but not both. Worse still, at the start, the radials were more wayward than the crossplys! Michelin got round that by changing the internal belting sequence. The problem has persisted even to this day but the simple solution found at the time by Michelin was to set that interface closer to the outside edge of the tyre. This seems to 'equalise' some of the more high level inertial forces acting on the wheel. So that is why we have offsets and, again, they used mm to describe it, so we have on our rims a number like 40,50 or 60. As our motorcars bodywork became enclosed and more aerodynamic and speeds increased it helped again with high speed steering so you mess about with it only with sufficient knowledge. This you are well on the way to possessing. With my 16×8's I have a 50mm O/S with no fouling, so how much wheel is there each side of the interface? 2" on the outside, 6" inside, I chose to do this in inches, as it's easier to 'see' the relationship between them, you see it don't you, so those 7½'s would be at their best with a 40 or 45mm O/S. At the rear, the wheels are still very 'set in' to the bodywork so put a pair of 10mm spacers on the back axle, it may seem like just a 'macho' thing but there are real advantages to be gained. It also makes the car look great!! Do you remember I asked if you would note if you saw the word 'ROTATION' with a little arrow pointing (hopefully) the right way to describe a forward motion? Well, if you did I hope they were on the same axle and always in pairs! People who really ought to know better very often tell me that this does not matter, but on every car I have driven when this has been tested, the wet braking performance is degraded to a point where it is simply damned dangerous.

Tyre types. That 'ROTATION' sign tells you this is a 'directional' tyre. This is a fairly recent innovation for Road going tyres, if you look at the tyre you will see that the blocks of rubber that make up the tread pattern form an arrow shape it may be broad and 'broken' or very pointed and look like a series of 'V's. It's the introduction of a special sort of silica with a high valence polymer that has made this and other newer designs viable. The other newish tyre type is the Asymmetric and generally these seem to be best on the rear axle of a rear wheel drive motor. They perform well on the front only when its dry, in the rain especially summer deluges they can aquaplane badly, i.e. lift off the Tarmac! They simply cannot bail out the standing water if they are too wide and for any given width the Directional will be better at clearing the water. For our Soarers, because of their weight distribution Directional is by far the best for us.

DON'T EVER BUY REMOULDS OR PART-WORN TYRES, PLEASE, DON'T, EVER.

So, we've done it? Not quite, if you go the V8 route; same rims, tyres, widths etc. then you have 1 more task. When the Tyres are mounted on the rims don't let the fitter attach them to the car as they are readied, assemble them together as pairs and juggle them, keeping the axle pairing. Try to pair the least added weights on the front axle, this may seem strange but the steering response is improved!

Tyre pressures Iit may seem obvious but it is not the tyre that keeps you rolling and supports the car ,it's the air inside and pressures are really important. In the testers world there is only one way to get the perfect pressure. Take your car, a 'trident' tyre temperature gauge (it records the temperature in the centre and both shoulders of the tyre), a foot-pump with a very accurate pressure gauge, a Dictaphone, some sandwiches, don some overalls and hit the A44 between Oxford and Mid-Wales. Throw in a couple twisty roads in the marches of Wales and after a day (providing it rains) you emerge with the correct result. This was the case up to about the mid-seventies, (except the 'trident' was the testers own hand) but as the roads filled up most of the testing was done behind gates on 'fake' roads. Although even now real road experience is still used, but really as confirmation of the closed results. I fervently believe if one of the 'old boys' could be persuaded to get hold an XK8 and went through the old routine it would be a vastly better car but to be fair I think the questions being asked are different now, from then. Still, nostalgia's not as good as it used to be.

So, having had 3 thoroughly joy filled weekends (it took me so long because of the PWR/ECT/NORM switch) driving my favourite roads, I finally found the 'right' pressure for my set-up. 34 PSI Front, 33 PSI Rear, measured cold for everything except the very fastest of motorway (autobahn) runs when the stability improves (marginally) with 2 more PSI in the rear set. For our V8 owner no such problems with 7½" rims just add 1 PSI on the standard setting would seem about right.

So in 3500 words we have covered the little bit of rubber that touches the road. Now lets deal with our Suspension.

SPRINGING TO ATTENTION

The Soarer has 3 different types of suspension Coil & Damper, Air and Active. All 3 use steel components and the black art of smithying is intrinsic to their performance. It used to a very voodoo rich subject dealing with Spring Rates, Damping Valves, Bushing, Lever Arms and on and on and on. To short-circuit all of this, the complexity eventually boiled down to 25 formulae and any competent Engineer can access them; some are even on the net! Briefly, what does each part do?

The wheel is mounted by 5 chamfered studs to (at the front) the Kingpin, (at the rear) The Stub axle (except the Active which has another Kingpin) these items are connected to the upper wishbone and the lower wishbone and is nearly vertical. It is tilted over slightly to the front of the car and this is the Castor Angle and it is also tilted slightly at the top either in toward the body or the other way and this is the Camber Angle. The castor angle is what used to bring the steering back to 'centre' after taking a corner. Citroen used this as another way of controlling the dreaded 'torque steer' on FWD cars, the Kingpin was tilted quite far forward at the top, making the weight of the car bear down on the centring. That is why some quite small cars have Power Steering. If you think about it you are 'jacking' the car up with the steering. Soarers have a very slight castor angle (more of a 'failsafe' really) and self-centring power steering, this is good as the further forward this component is tilted the more it affects (adversely) the handling. We are off on a tangent, there are certain words bandied about by 'TV pundits' about 'handling' and 'roadholding' and for the most part they talk out of their -, this is what the terms really mean.

Handling is Hysteresis. Seat of the pants stuff (subjective).

Roadholding is Quotient. Measurable (Lateral 'G' etc.).

The 2 pairs of wishbones are attached to a partially cast, exceptionally rigid cross-member with very hard bushing. The locating points are carefully judged to allow for anti-dive & anti-squat effects, this is done by tilting their axis lines, analogous to the Castor, Camber and Toe-in mentioned earlier, the shocker is fixed to the lower wishbone (bottom) and the monocoque (top). The sprung force is achieved by; 1, A coil (helix) of steel, effectively a torsion bar wound up into a confined space. 2, Air or Gas filled units.

The sub-frame assemblies mean you can isolate unwanted reactions, one of the most difficult items in this respect is the anti-roll bar, always been the perpetrator of many knotty problems, setting it to react within a sub-frame whilst still working effectively on the monocoque was the solution on the Soarer. This requires great accuracy, go out to your car get on the floor and look up at the mounting points of the wishbones, and marvel at the vernier discs fitted to each point of attachment, this is truly bloody marvellous. Not even the legendary Mercedes 'S' class build quality could take this scrutiny.

The truth is, no modern car should deal with the road badly. The only thing is cost cutting before the production starts deals the deadly blow. Take a simple thing like the bushes, which keep the metal components of the suspension from touching. Good Manufacturers spend a lot of time and money getting them made well, taking care to specify the right rubber and tolerances and it pays off, I am not an admirer of BMW but in the 70's they upped the quality on these components just as Jaguar lowered theirs. This simple thing meant that when Tony Dron drove the XJ6 and the 528 the latter had a consistency of handling, which made it the better car, and I think he said so, and many other Motoring Writers agreed with him.

The fact that the Jaguar had better intrinsic designs in most parts of the car shows just how important detail is. That's how to fall down on the small scale, how about on the grand scale. A little known (to the general public) piece of suspension creed is that the front and rear suspension assemblies have a base resonance. It's the low frequency actuation of all the components as apiece, and a good Engineer would work out the frequencies and ensure that the front and rear are not reciprocal. Sounds simple, but low frequencies interact and the ideal of the front base resonance being at a higher frequency than the rear is hard to achieve. The only recent cars that have failed have been engineered in America, where the desire for tight control of suspension has only recently taken hold. I know, I know, the Soarer was designed in America, but it was ENGINEERED in Japan, had it been the other way round, I doubt if I would own one.

We have precision made all round independent, dual wishbone suspension with progressive rate coil springs, a front and rear anti-roll bar and top and bottom wishbone angle attachments to stop diving and dipping at the front. This is the best form of road car suspension you can get and provides the most effective control of the wheels. The best companions to this set-up are gas-valved shockers. The reasons are simple the older, oil filled, units needed to be set very stiff when cold, so as the car warmed to its task the warm shockers were 'just right'. On high performance cars this used to mean a bone-jarring ride for the first few miles. For a Lexus this would be no good, so gas-valved it was. But why stop there? While we are at it, give the car 2 separate settings, and why not have rapid-reaction adjustable valving (TEMS) so as to allow the steering inputs (via an accelerometer) to stiffen the side under the most load and soften the lighter side? Don't believe me? On my car, if you are in PWR/ECT mode and turn late and very sharply in a bend the car hardly seems to roll at all, it leans of course but it does not roll!

I'm filled with admiration for the Toyota Engineers who did this, in my experience no one has ever done it better or even equalled this magnificent feat of suspension magic, its that damn good!I think some of you are saying what's the difference between the car 'leaning over' in a corner and rolling? Look at it this way, after a couple of drinks I may lean on the Bar of my favourite Pub (Alcohol is a muscle relaxant) but after a lot more drink I become rolling drunk, miss the Bar, and fall flat on my face, that's the difference, its all about control. All-in-all an impressive list but to crown it all, put the front and rear assemblies on their own sub-frames so as to ensure different resonance frequencies and a near vibrationless ride. So is the suspension faultless? No, not for me at least. The springs could be just a little bit more progressive. I want the PWR/ECT shocker settings all of the time. The original toe-in settings were more suited to the asphalt of Japan more than the undulating English Country lanes. I posted a more performance-orientated set in the Club Forum earlier this year. I have changed them again this new set are really only suited to my style of driving and not published for general use.

Iain Wiltshire - Newbury, Berkshire, England. 20 March 2001
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Front toe-in Left	MIN. 0.0mm	OPTIMUM 0.3mm	MAX 0.6mm
Front toe-in Right	MIN. 0.0mm	OPTIMUM 0.3mm	MAX 0.6mm
Front toe-in Total	MIN. 0.0mm	OPTIMUM 0.6mm	MAX 1.1mm
Rear toe-in Left	MIN. 0.9mm	OPTIMUM 1.2mm	MAX 1.5mm
Rear toe-in Right	MIN. 0.9mm	OPTIMUM 1.2mm	MAX 1.5mm
Rear toe-in Total	MIN. 1.8mm	OPTIMUM 2.3mm	MAX 2.9mm

Front toe-in Left	MIN. -0.3mm	OPTIMUM 0.1mm	MAX 0.6mm
Front toe-in Right	MIN. -0.3mm	OPTIMUM 0.1mm	MAX 0.6mm
Front toe-in Total	MIN. -0.6mm	OPTIMUM 0.2mm	MAX 1.1mm
Rear toe-in Left	MIN. 0.9mm	OPTIMUM 1.3mm	MAX 1.7mm
Rear toe-in Right	MIN. 0.9mm	OPTIMUM 1.3mm	MAX 1.7mm
Rear toe-in Total	MIN. 1.8mm	OPTIMUM 2.6mm	MAX 3.3mm

Front toe-in Left	MIN. 0.0mm	OPTIMUM 0.3mm	MAX 0.6mm
Front toe-in Right	MIN. 0.0mm	OPTIMUM 0.3mm	MAX 0.6mm
Front toe-in Total	MIN. 0.0mm	OPTIMUM 0.6mm	MAX 1.1mm
Rear toe-in Left	MIN. 1.9mm	OPTIMUM 2.2mm	MAX 2.5mm
Rear toe-in Right	MIN. 1.9mm	OPTIMUM 2.2mm	MAX 2.5mm
Rear toe-in Total	MIN. 3.8mm	OPTIMUM 4.3mm	MAX 4.9mm

Front toe-in Left	MIN. 0.0mm	OPTIMUM 0.3mm	MAX 0.6mm
Front toe-in Right	MIN. 0.0mm	OPTIMUM 0.3mm	MAX 0.6mm
Front toe-in Total	MIN. 0.0mm	OPTIMUM 0.6mm	MAX 1.1mm
Rear toe-in Left	MIN. 1.9mm	OPTIMUM 2.2mm	MAX 2.5mm
Rear toe-in Right	MIN. 1.9mm	OPTIMUM 2.2mm	MAX 2.5mm
Rear toe-in Total	MIN. 3.8mm	OPTIMUM 4.3mm	MAX 4.9mm