Grossly misleading technical articles

Posted on April 30th, 2009 in AutoSpeed,Opinion by Julian Edgar

Long ago, even before I was a Geography teacher, I studied how to teach it. The head of the Geography department at college was a very smart person, and a brilliant teacher.

One day we were talking about teaching analogies and models, and the difficulty in simplification of knowledge without introducing straight-out erroneous ideas.

His example of the latter was: Clouds bumping into each other make thunder.

Much better, he pointed out, to say even to the youngest child: Thunder happens because of lightning.

In fact, clouds are a good example of these ideas. My little boy, who is 4 years old, asks what clouds are made of.

Tiny, tiny water droplets, I say.

So, how does rain happen, he asks?

I say: The tiny droplets run into each other and join together. When they are big enough, they fall to the ground.

While I am saying this, sometimes I think of a much more sophisticated model: water vapour, latent heat of evaporation and condensation, relative humidity, dew-point, hygroscopic nuclei – and other concepts.

A meteorologist would probably think of vapour pressure, a chemist might think at a molecular level, a physicist might consider terminal velocities, a climatologist might consider climate change, a minister of religion might think of God, an agnostic might think of the magnificence of nature.

In the description of clouds and rainfall that I say to my son, I am conscious of the gross simplifications I am making.

But that’s OK: every single thing I know about the world is a gross simplification of reality.

The intellectual models I use to make sense of what occurs around me are just reducible approximations of what really happens.

When I write technical articles in AutoSpeed, I am conscious that all the time I am presenting fundamentally simplistic models. I hope that they’re not of the ‘clouds bumping into each other make thunder’ type: but they may be.

Recently, I wrote an article on suspension roll centres, virtual pivot points and other ways of analysing suspension designs. In doing so, I consulted five different suspension design textbooks, and also considered very carefully the experience I have in developing human-powered vehicle suspensions, and modifying car suspensions.

As always, I was quite conscious during the writing of the article that the model I was presenting of reality was likely to be flawed: as I have already implied, every model we have of reality is, to a greater or lesser degree, flawed. However, I hoped that the information would benefit people’s understandings, especially in practical outcomes.

The day after finishing the article, I looked through a complex SAE paper on suspension roll centres. This paper immediately debunked several suspension ‘myths’, most of which I had implicitly or explicitly promulgated in the article I had written.

However, the paper was working at a level analogous to the ‘vapour pressure and hygroscopic nuclei’ theory of why rain falls: if I based my article on the SAE paper in question, perhaps less than half of one percent of AutoSpeed readers would understand anything I wrote. (If in fact I could understand it myself!)

So I could easily decide not to write anything at all: if it’s not ‘right’ and ‘correct’, surely it shouldn’t be written?

But that would be like saying to my son: I cannot tell you why rain falls; it’s too hard to understand.

I cannot tell you what a roll centre is; it’s too hard to understand.

Or I cannot tell you what a voltage is; it’s too hard to understand.

I cannot tell you what engine detonation is; it’s too hard to understand.

I cannot tell you how a tyre behaves when cornering, it’s too hard to understand.

And so on.

And these things – and all things – really are too hard to understand… if you want as ‘correct’ an understanding as it is currently possible to have.

Are my articles full of errors? So by definition, very likely.

Anyone who suggests that the technical articles they present for general readers are perfectly correct – or do not mislead in the slightest – just do not understand the nature of knowledge – and how all our descriptions of what goes on around us are just relatively simplistic models.

Me? I try to use the simplest model that’s consistent with not being grossly misleading…

How utterly stupid they are

Posted on April 28th, 2009 in Opinion,pedal power by Julian Edgar

As with any recreational pursuit, cyclists come in sorts of shapes, sizes and special interests.

I’m interested in heavy recumbent touring pedal machines; my neighbour – a man in his sixties – likes traditionally shaped ultra-lightweight racing machines.

Each morning his car heads out, bike on the rear rack, to allow him to get in some cycling before work.

He rides with a like-minded group who sprint (well, in my terms it’s sprinting!) at 35 km/h or more on the flat roads of the Gold Coast.

Then, a few days ago, he abruptly stopped his morning rides. A broken shoulder blade, multiple abrasions and concussion will tend to do that.

He’d been out with his mates, riding fast to catch up with a breakaway group ahead. He reached the rearmost person and leaned over to pat him on the back. He doesn’t know what happened next – perhaps he startled the other rider who swerved, or perhaps at just the moment he took one hand off the handlebars the very narrow front racing tyre fell into a groove in the road.

But whatever the cause, when he regained consciousness he was lying on the road, in pain and with the greatest of desires to get the hell out of there and to safety.

The cycling group helped him, and it wasn’t long before he received medical help and then, subsequently, was home.

His injuries are certainly not trivial, but it could have been much worse: he could have been dead.

The short loss of consciousness and the concussion indicate that his head hit the road. So does the state of his helmet….

The helmet is destroyed.

A piece of the foam has broken right away…

…but what’s even more interesting is that the foam is cracked in multiple places. In fact, there’s barely an area of the helmet that doesn’t have large or small cracks in it.

To look at it makes me feel slightly ill: without a helmet, those cracks would probably be in my neighbour’s head.

The helmet did its job in just the way it was designed to.

I look at riders – often young – who don’t bother wearing a helmet and think of how utterly stupid they are…

It’s not in the texbooks…

Posted on April 23rd, 2009 in Handling,Opinion,pedal power,Suspension by Julian Edgar

I am not certain it will happen: I hope so.

As time has passed, the development of ultra light-weight vehicles has become a more important theme for AutoSpeed (and this blog). It’s rather like our longstanding acceptance and enthusiasm for hybrid vehicles: it’s a change in transport architecture that simply makes sense.

(Of course, ultra light-weight vehicles have existed previously – especially just post-World War II with the German and British three wheelers. But over the last 50-odd years, there have been almost none produced.)

So what do I hope will happen? The development of an increasing number of such machines.

If that occurs, especially on an individual constructor level, then people will face some unique and very difficult problems.

We tend to take for granted developed automotive technology, and to see engineering solutions only within that paradigm. But when it comes to ultra light-weight vehicles, that’s simply wrong.

For example, take Ackermann steering – that’s where when cornering, the inner wheel turns at a sharper angle than the outer wheel, resulting in no tyre scrubbing. If I was to say that I have spent the last three days struggling with Ackermann steering, some people would laugh.

“It’s all in the books,” they might say, “just angle to the steering levers inwards like this diagram shows. Been done a million times. Next problem?”

But you see, that solution largely applies only if steering like a car is used – with a steering box or steering rack.  And, for ultra lightweight vehicles, both steering boxes and steering racks (or, any currently available, anyway) are way too heavy. 

So, how do you achieve Ackermann steering without a steering box or steering rack?

Australian recumbent pedal trike manufacturers Greenspeed have some brilliant solutions. (Disclaimer: my wife sells Greenspeed trikes.)

One of their approaches looks like this (the drawing is not to scale.) The system uses wheels that turn on kingpins, two steering tie-rods, and one central linking member turning on an offset pivot. The steering is by handlebars; these connect at the points marked ‘H’ and have a motion that is a combination of both sideways and fore-aft.

This steering system achieves full Ackermann compensation, and requires only four rod-ends and one pivot point. (These are in addition to the two kingpin pivots.)

That is simply an incredibly light and effective steering system.

I recently spent day after day coming up with alternative steering systems for my recumbent pedal trrike – and then building them. It’s quite easy to end up with steering with two kingpins, six rod-ends and two pivot points – typically, about 50 per cent heavier than the Greenspeed system!

Making things more difficult for me was that, unlike the Greenspeed trikes with the above steering system, my design uses long-travel suspension. And, getting rid of bump steer (ie toe changes with suspension movement) is another nightmare.

Again, people will be thinking only in an automotive paradigm.

“Bump steer? That’s easy – just set the length of the tie-rod so that it’s the same as the distance between lines drawn through the upper and lower ball-joints….” (and so on).

Trouble is, my suspension system doesn’t even have upper and lower ball-joints… Instead, it’s a leading arm, torsion beam, dead axle with a Watts link.

Shown here is (another) rough diagram. In fact, this is pretty well how my system is with Ackerman compensation and zero bump steer. (The really knowledgeable amongst you will have picked a slight error in the drawing.)

The point is that none of this design can take lessons straight out of textbooks – especially automotive textbooks. Of course, the fundamental elements (like the Watts Link, the concept of Akermann steering correction and so on) are all well documented, but in unique applications, actually applying those ideas is another thing entirely.

I am not setting out to suggest I am some kind of hero – all the designers of solar race cars and pedal-powered tadpole trikes have tackled the same ground. But what I am saying is that the challenge is massive, that achieving a good outcome in terms of suspension and steering dynamics – all at a weight that is less than just the steering wheel of a normal car – is difficult beyond belief.

Ackermann and bump steer? If it’s in a typical car textbook, in this class of vehicle it’s usually not the solution…

Do we need so many traffic lights?

Posted on April 21st, 2009 in Driving Emotion,Opinion by Julian Edgar

Back when I was a kid, growing up in the northern suburbs of Adelaide, every six months or so there’d be some local excitement.

Normally it was presaged by a squealing of tyres, followed by a loud bang. On one occasion I can remember that after the bang there was the sound of an engine revving hard.

What had happened was a car crash at one of the local road intersections.

All the roads were grid-like; all used the ‘give way to the right’ rule – I don’t remember any ‘stop’ or ‘give way’ signs at those junctions. 

The frequency of crashes was so high that when the right noises occurred in sequence, no-one stood around wondering what was going on: instead, everyone started running towards the scene.

Then, when I was about 13 or 14, the crashes suddenly stopped. What had happened was that small roundabouts had been placed in the intersections where crashes had most frequently occurred.

I don’t remember hearing a single crash from that point onwards. There might have been some minor bingles, but as the intersections had become quite tight, they would have been only at low speeds.

At the roundabouts drivers were travelling less quickly and were also required to be observant and participatory. Well, certainly less quickly, and more observant and participatory, than they had been when barrelling through a junction with just a cursory glance to the right.

I was reminded of my childhood because I have been increasingly hearing the idea that many traffic lights should be removed. That’s especially the case on secondary and tertiary (feeder) roads.

The arguments go like this:

• When traffic lights are green, people assume an absolute right of way. They don’t check for other cars and in fact, pay little attention to anything other than the colour of the light. So when crashes occur at traffic light controlled intersections, the impact speeds are high and so the crashes are likely to cause death or major injury.

• Even traffic lights controlled by smart systems (variable length time periods, sequencing of green lights on successive intersections) can, through prolonged idling times, cause increases in fuel consumption and emissions.

• The cost (both in installation and in running) of traffic lights is much higher than many other traffic control approaches.

It is being said by some that rather than giving driver apparent certainty, it is better in many situations to create uncertainty – to make the driver unsure of their surroundings. Perhaps the best example of that are those (rare in Australia) precincts that mix both pedestrian and vehicular traffic – cars just creep along at a walking pace.

Of course, in many applications traffic lights are just fine.

But the next time you’re out on the road in city conditions, note the dozens – perhaps hundreds – of traffic lights you pass, and start to consider: are these really necessary… or could this intersection have been handled in a different, simpler, cheaper and more effective way?

I’m no natural designer…

Posted on April 16th, 2009 in Opinion by Julian Edgar

I am not a natural designer; never have been and never will be.

The designs I produce are therefore the work of many hours of tortuous thought: of sketching and thinking; of looking up reference books and thinking some more.

Even a simple bracket can take me a long time; a more complex system can take me so many hours that, to be honest, you’d be frankly disbelieving if I told you the total.

How I envy those who combine natural artistry with engineering skills; those who have the ability to create beautifully elegant structures and shapes that also perform with engineering integrity. I think of those attributes – and the people who spring to mind are designers of bridges, of buildings and of dams: civil engineers and architects, rather than mechanical engineers.

Isambard Brunel and Thomas Bouch, certainly.

In the automotive field, it is hard to think off-hand of revered designers who could combine artistry with engineering elegance.

Colin Chapman?


Ferdinand Porsche?

More so, I think – but ‘Porsche’ normally also includes son Ferry… so perhaps you’re getting two for the price of one.

Gordon Murray?

Yes, probably so.

Alex Issigonis?

Engineering? Without a doubt! Aesthetics? He had no bloody idea.

Anyway, as I was saying, automotive designers tend not to be able to combine innovative ideas with artistry.

Me? I can’t do innovation or aesthetics!

But I can try.

One of the outlets for my attempts has become, over the last few years, human-powered vehicles. Pedal bikes and trikes, if you like.

(A few years ago I had steel coil springs custom-wound for the suspension of a recumbent pedal trike I’d designed.

When I contacted the spring company – and in following discussion – I called the vehicle a ‘human-powered vehicle’.

When I picked up the springs, the salesman loudly proclaimed: “So these are for the bike, are they?”, by emphasis and with a touch of derision, clearly making his thoughts felt.

I replied: “Well, it’s got three wheels, so that makes it a trike doesn’t it?”

With visions of a child’s tricycle filling his mind, he was speechless.)

One of the beauties of building an HPV is that you can try pretty well anything you want – both legally and physically.

In my first really successful HPV, I used a front suspension design comprising semi-leading arms, with an anti-roll bar and a high geometric roll centre. Damping was by track change and Firestone rolling lip airbag springs were the springing medium.

(And if you think: “Yeah, whatever – WTF… that’s how I was too a few years ago. Boy, does building our own vehicle make you think hard about fundamental concepts of vehicular design!)

With its massively high roll centre and changing track, varying castor and camber, this suspension design went against almost everything I have ever read about suspension (for any vehicles!) – but on-road, it proved to be very good indeed.

And to be quite honest, that makes me feel warm inside: I did something that, AFAIK, no-one else has done on a vehicle – and it worked!

(Of course, someone, somewhere must have done it – but it’s certainly not a recognised front suspension design.)

Today, I got to the stage in my latest HPV build that I could statically assess the new front suspension – in terms of travel and roll stiffness, at least. (As I write this, the machine is at least another week away from road testing.)

And, at this early point in the design, it looks really good.

How so?

Well, it resists dive more than a single wheel bump. In fact, it has a much lower spring rate for a single wheel bump than a two-wheel bump. And, since most bumps are one-wheel, that’s good.

It incorporates the function of anti-roll bar into a suspension member. That saves potentially a lot of weight – not just for the anti-roll bar, but also for the links and rod-ends and anti-roll bar bearings.

And the suspension design has only four ball bearings (at a total of two pivot points).

…and it weighs – including suspension arms, pivot point bearings and two springs – just 1.4kg.

To carry a max dynamic load of about 150kg.

Its design? It’s a leading arm, torsion tube, dead axle, with a Watts link.

Weird? Very!

Unheard of? Again, I am not aware of any vehicle of any description using this approach on the front, steering, suspension.

Effective? I’ll find out when plunging down my local 20 per cent grades at 60 or 70 or 80 km/h, clad only in shorts, shoes, a t-shirt and bike helmet…

Beware the expert…

Posted on April 14th, 2009 in Driving Emotion,Opinion by Julian Edgar

Experts? I wonder…

A long time ago I came up with an electronic way of overcoming a boost cut on a turbo car. For less than a dollar, you could prevent a MAP sensor output from rising above a certain voltage. And for that single dollar, the voltage level was also adjustable.

At that time (and remember, it’s long ago – when I was certainly a lot more innocent), I told a turbo workshop proprietor about the technique, and how I was writing a story on it for a national magazine.

As this guy sold commercially produced boost cut units for lots of money, I thought he’d be fascinated by the approach.

But no, not a bit of it.

“You shouldn’t put that sort of technology into the hands of people who don’t know what they’re doing,” he said.

“Today’s cars are complex and people will blow up engines.”

When I pointed out that  someone could blow-up their engine if you gave them even a screwdriver, he was taken aback.

Of course, the reason he didn’t like the idea of a DIY $1 boost cut unit was that his market for $200 ones was likely to evaporate.

So one reason that experts hate amateurs working in their patch is that often it costs them money.

Another reason is more subtle: it implicitly belittles their expertise and training.

I remember once when I was getting a lot of arc welding done. I was building a supercharger bracket and the 10mm steel plate was being welded by a local welding specialist. He was doing a very good job, too – and at a high cost.

Watching him in action, I thought I should invest in an arc welder – and the next week I bought a cheap secondhand welder. When he was doing my next lot of welding I told him of my purchase, expecting that he would be enthusiastic.

But as with the boost cut man, again not a bit of it.

“It takes years of experience,” he said. “You can’t pick up the skill of welding just by buying one.”

I hadn’t even implied that I could, but again his negative vehemence was off-putting and surprising.

Given that I’d been pouring money into his pocket, I thought he might have said: “Great! If you need any tips or get stuck, just ask.”

But no, I’d hurt his pride by apparently suggesting than anyone could buy a welder and so be an expert.

The third category of expert to be wary of is the person who ridicules any simpler approach to solving problems at which they’re specialists.

I’ve seen this in action most recently, with my copper wire modelling approach to designing space-frame structures. The experts look at the approach and nearly choke on their cornflakes.

“You can’t work out any stress levels,” they say. “You cannot accurately model anything by that approach.”

Of course, when asked how else to design space-frames (without using engineering mathematics or complex and hard to access software), there are no answers. Apparently, it is better to simply not attempt to design and build a space-frame, rather than to do so with an approach that has huge benefits over using no aids at all.

The corollary of all of this is that experts believe you should work only in your professional field of expertise, that – in other words – no hobbies should be embraced, no amateur past-times partaken in.

When it’s put like this, you can see the mixture of vested interests, pride, and the inability to understand that expertise can be reducible, invariably results in expert denigration of non-experts attempting anything.

I’ve seen it so often over the years: not only the aforementioned boost cut over-ride, welding equipment and space-frame modelling; but also aerodynamic testing by wool-tufts and pressure measurement, intake flow testing by pressure measurement, turbo boost control using injector duty cycle as the sole input, turbo boost control using a pressure regulator, single dimensional voltage interceptors working on the airflow meter signal, making trial aero undertrays out of cheap plastic sheet or cardboard, making plastic intake ducts from stormwater pipe and fittings, forming door pods for speakers using expanded polystyrene, swapping-in springs from other cars, DIY detonation detection systems, fitting a rear anti-roll bar, using suspension on human-powered vehicles, fitting your own subwoofer, using a pot to shift a voltage signal, altering regen braking on a hybrid car – the list goes on and on.

In fact, I can state that for every innovative, DIY technique we have ever covered in AutoSpeed, at least some experts have suggested the approach would not work. But in every case the techniques have worked extremely well!

I reckon that often you can get further ahead by actively ignoring experts. That’s especially the case if the technique is one that you have devised yourself and have found to be effective…

Building (another) failure

Posted on April 9th, 2009 in Opinion,pedal power by Julian Edgar

I think it was after I crashed for the third time that I started losing confidence in my new machine.

All were low-speed crashes, but still, they were hitting-the-ground crashes. Just as well they were from a pedal bike.

After the saga of the pictured first Chalky (front-wheel drive, delta, leaning, recumbent, suspension design) that reached the stage of being about two-thirds finished before I decided that the build was not going the in the direction I had hoped, I was very excited about the second Chalky.

This one was much more conventional – in terms of weird human-powered machines, anyway.

A long-wheel base, recumbent, rear-wheel drive, suspension bike. I had plans for rider-operated ‘trainer wheels’ to provide low speed stability, but I secretly hope that it would be stable enough to be easily ridden without them.

I used the same static front end geometry as the Greenspeed Anura and ran 130mm of suspension travel front and rear, using my favourite Firestone airbags. The rear had a chain path positioned for anti-squat suspension behaviour, and I investigated very thoroughly different types of anti-dive front suspension designs.

And, after many hours of work brazing the (very expensive) chrome moly tubing, I had a machine I could ride.

Ride – and fall off.

I don’t want to over-emphasise the falling off bit, but still, it wasn’t good.

Because a recumbent like this has more weight on the back than the front, and because it is steering of the front wheel that provides the balance (ie puts the centre of gravity over the line joining the front and rear tyre contact patches), on this sort of bike a fair bit of steering is needed to stay upright. I experimented with different steering ratios until I had quick – but not nervous – steering. I also dialed-out all bump-steer.

I experimented with different positions of the front suspension’s upper leading link, and while I could reduce brake dive, it also increased (to an unacceptable level) suspension harshness. 

Talking about the suspension, I also think the spring motion ratios were not right: the machine bottomed-out excessively. To prevent simultaneous nose-dive and bumps bottoming the front end, I added a long bump rubber – but the main spring rate was clearly still too low.

The high centre of gravity and soft front spring rate meant that, with vigorous pedalling, full front extension occurred – the rear anti-squat worked fine but the front suspension extended each time.

In short, it was simply nowhere near as good as my existing recumbent trike – nowhere near as good.

Yes, the design of Chalky #2 potentially allows for folding into a small package, but if the stability, ride, and pedalling suspension behaviour are way inferior, it’s hard to justify this approach as the way to go….

In short, I think it’s another failure.

So I’ve started designing Chalky #3…

My driving life is now changed forever…

Posted on April 3rd, 2009 in AutoSpeed,Driving Emotion,Economy,electric,Electric vehicles,Global Warming,Opinion by Julian Edgar

I feel like one of the first pilots of jet-powered aircraft. They immediately knew that they were flying the future: there could be no going back to pistons and propellers.

Today I drove the car that, for me, spells the end of the piston engine for performance cars.

The car was the all-electric Tesla, and its performance – and the way it achieved that performance – was just so extraordinary that I am almost lost for words. That a start-up car company has created such a vehicle is simply unprecedented in the last century of automotive development.

For the Tesla is not just a sports car with incredible performance (0-100 km/h in the fours) but also a car that redefines driveability. Simply, it has the best throttle control of any car I have ever driven.

Trickle around a carpark at 1000 (electric) revs and the car drives like it has a maximum of just a few kilowatts available. It’s the pussy cat to end all pussy cats: Grandma could drive it with nary a concern in the world. Put your foot down a little and the car seamlessly accelerates: heavy urban traffic, just perfect.

But select an empty stretch of bitumen and mash your foot to the floor and expletives just stream from your mouth as the car launches forward with an unbelievable, seamless and simply immensely strong thrust.

There are no slipping clutches, no flaring torque converters, no revving engines, no gear-changes – just a swishing vacuum-cleaner-on-steroids noise that sweeps you towards the horizon. The acceleration off the line and up to 100 km/h or so is just mind-boggling – especially as it’s accompanied by such undemonstrative effort. The car will do it again and again and again, all with the same phenomenal ease that makes this the winner of any traffic lights grand prix you’re ever likely to meet.

And it’s not just off the line. Want to quickly swap lanes? Just think about it and it’s accomplished. 

In fact drive the car hard and you start assuming that this is the only mode – outright performance. But then enter that carpark, or keep station with other traffic, and you’re back to driving an utterly tractable car – in fact, one for whom the word ‘tractable’ is irrelevant. Combustion engines are tractable or intractable; this car’s electric motor controller just apportions its electron flow as required, in an endlessly seamless and subtle variation from zero to full power.

It’s not just the acceleration that is revolutionary. The braking – achieved primarily through regen – has the same brilliant throttle mapping, an approach that immediately allows even a newcomer to progressively brake to a near-standstill at exactly the chosen point.

A seamless, elastic and fluid power delivery that no conventional car can come remotely close to matching; a symphony on wheels to be played solely with the right foot; an utterly smooth and progressive performance than can be explosive or docile, urgent or somnambulant – literally, a driveline that completely redefines sports cars.

There’s no going back – my driving life is now changed forever.

Footnote: the Tesla drive was courtesy of Simon Hackett of the ISP, Internode.

When wheels steer themselves

Posted on April 2nd, 2009 in Driving Emotion,Handling,Suspension,testing by Julian Edgar

I thought that the idea that car wheels just went up and down over bumps, and were steered only when the driver turned the steering wheel, was pretty passé.

Passive ‘steer’ systems have been in production cars for many years, normally of the rear end.

In broad brush strokes, the systems work like this: The rear bushes are set with differing stiffnesses in different planes, such that when the wheel is subjected to a lateral force (as it is in cornering), it no longer remains parallel with the car’s long axis – that is, it steers.

For example, rear wheel compliance steer is often set to give toe-in, thus settling the cornering car.

The original Mazda MX5 / Miata had such a system. (It’s worth pointing out that the MX5 is generally regarded as one of the best handling, relatively cheap, cars ever released.) In their 1989 book MX-5 – the rebirth of the sports car in the new Mazda MX5, Jack Yamaguchi and Jonathon Thompson write:

No Mazda rear suspension is complete without some form of self-correcting geometry, as have been seen in the fwd 323 and 626’s TTL (Twin Trapezoidal Links), the 929’s E-links and the RX-7’s complex DTSS. The MX-5 double wishbones are no exception, though to a lesser degree. The designers need not worry about camber changes; a recognized virtue of the unequal length A-arm suspension is the admirable ability to maintain the tires’ contact area true to the road surface, attaining a near-zero camber change.

So the chassis designers’ efforts were directed at obtaining a desired amount of toe-in attitude that improves vehicle stability in such maneuvers as spirited cornering and rapid lane changes. Toe-in was to be introduced when the suspension is subjected to lateral force, not to accelerative or braking force. They considered that the MX-5 with its configuration, weight and suspension, would have sound basic handling characteristics, and the lateral reaction would be all it would require to further enhance its vehicle dynamics.

The lower H arm’s wheel-side pivots, which carry the suspension upright, have rubber bushings of different elasticity rates. The rear pivot is on a firmer rubber bushing than the front. The front rubber bushing deforms more under load induced by lateral force, and introduces an appropriate amount of wheel toe-in, which is in the final production tune a fraction of a degree.

Pretty well all current front-wheel drive cars have some form of passive rear wheel steering. The Honda Jazz uses a tricky torsion beam rear axle in which, according to Honda, “the amount of roll steer and roll camber has been optimised to deliver steady handling”.

But even better, the company has released graphs showing the toe variation over suspension travel (note: travel, rather than lateral force), with the current model compared with the previous design. As can be clearly seen, in bump (as would occur to the outer wheel when cornering) the Jazz (especially the new model) has an increasing amount of toe-in. Also note the differing shape of the curves in rebound (droop).

And it’s not just the ostensibly non-steered end that uses toe variations built into the suspension design.

Several suspension textbooks that I have suggest that setting up the front, steering wheels for non-zero bump steer can be advantageous. Chassis Engineering by Herb Adams (incidentally, a very simple book much under-rated) states:

Exactly how much bump steer you need on your car is like most suspension settings—a compromise. It is common to set the bump steer so that the front wheels toe-out on a bump. This will make the car feel more stable, because the car will not turn any more than the driver asks.

To understand this effect, picture what would happen if your car had toe-in on bump. As the driver would start a turn, he would feed in a certain amount of steering angle. As the car built up g-forces, the chassis would roll and the outside suspension would compress in the bump direction. If the car had toe-in on bump, the front wheels would start to turn more than the driver asked and his turn radius would get tighter. This would require the driver to make a correction and upset the car’s smooth approach into the turn. The outside tire is considered in this analysis because it carries most of the weight in a turn.

Assuming that your car has the bump steer set so that there is toe-out in the bump direction, the next consideration is how much toe-out. If the car has too much toe-out in bump, the steering can become imprecise, because the suspension will tend to negate what the driver is doing with the steering wheel. Also, if there is too much bump steer, the car will dart around going down the straightaway. A reasonable amount of bump steer would be in the range of .010 to .020 per inch of suspension travel.

Fundamentals of Vehicle Dynamics by Thomas D Gillespie positively describes using roll steer, where the toe variation of the left-hand and right-hand wheel is in the same direction, to alter understeer and oversteer effects.

Even that most exotic of road cars, the McLaren F1, had designed-in passive steer.

Writing in an engineering paper released in 1993, SJ Randle wrote of the front suspension: “Lateral force steer…. was 0.15 degrees/g toe out under a load pushing the contact patch in towards the vehicle centreline. This is a mild understeer characteristic – precisely what we wanted.”

In the case of the rear suspension, “the net result being a mild oversteer characteristic (ie toe out under a force towards the car’s centreline) or around 0.2 degrees/g. We had hoped for an understeer of 0.1 degrees/g.”

Such passive steer suspension behaviour would become especially important in vehicles that, in order to achieve other design aims, have dynamic deficiencies. So for example, a very light car that is aerodynamically neutral in lift, and has a low aero drag, is likely to be susceptible to cross-winds. On bump the passive toe-in of the rear wheels, and toe-out of the front wheels, would help correct this yaw.

Note that adopting these techniques doesn’t require the actual mechanical complexity – or weight – of the suspension systems to change.

But of course it’s quite possible to over-do these effects. As indicated in the quotes above, we’re talking very small steer angle changes. You can’t even transfer the ideas from car to car: the current Honda Jazz steers fine; the Honda City (that apparently uses the Jazz rear suspension) has an unmistakeable, unhappy, ‘rear steer’ feel that is disconcerting on quick lane changes. 

But it seems to me that if you are building any bespoke vehicle and simply state point-blank that there should be no bump steer at the front, and no lateral compliance leading to toe changes at the back, you’re taking away a pretty important string from your bow.