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…

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.

User-adjustment better than factory pre-sets

Posted on November 18th, 2008 in Driving Emotion,Engine Management,Handling,Opinion by Julian Edgar

I’ve been thinking about the way in which cars are heading. More and more these days you see driver-selectable modes. A sports mode – or even super sports mode – on a double clutch transmission. A button that sharpens throttle response, changes damping and alters auto trans shift points.

Two points.

Firstly, if the car drives badly when in standard mode, fitting a special button doesn’t fix the car. The ‘fix’ needs to be far more fundamental: at minimum, all modes need to drive well.

But the main point I want to make is this.

Why on earth are manufacturers giving only ‘digital’ control over this type of driver selection? Why an on/off switch when it would be far better to provide an analog knob that allows the driver to adjust the action of the system to their taste?

A knob for power steering weight.

The best and worst elements in new car design…

Posted on September 30th, 2008 in Handling,Honda,Opinion by Julian Edgar

It never rains but it pours.

After not testing any new cars for a while, this week is the fourth in a row in which I have had new Honda vehicles. The Hondas – Accord, Jazz and two Accord Euros of different specs – have all been interesting cars.

They’ve been interesting because each of the designs has had some major positives – and some major negatives.

The 3.5 litre V6 in the Accord is simply a magic engine – powerful, free-revving, fuel-efficient with its cylinder shut-down technology, and with a glorious sound as it heads for high revs.

But the steering of the car is amongst the worse I have ever experienced in a new car, and the dry road grip is simply terrible.

Why on earth do people object to making cars easier to drive?

Posted on September 18th, 2008 in Driving Emotion,Economy,Handling by Julian Edgar

I’ll let you into a secret.

I think it quite bizarre, but some people actually believe that the greater the driver skill needed to operate a car, the better the car must be.

The corollary of this is that is if a modification makes it easier to get more out of a car, the modification must be bad.

Now put this way you can see why I called the notion bizarre. But in the time I’ve been writing about car modifications, I’ve come across it quite a few times.

Here are just two examples.

eLabtronics Performance Modules

Posted on June 23rd, 2008 in Engine Management,Handling,Opinion,Turbocharging by Julian Edgar

Despite having in the past worked for an electronics hobbyist magazine, and having played with electronics for most of my life, I don’t consider myself to be any sort of electronics whiz.

In fact, I am painfully aware of how little I know and understand.

But that’s one reason I am so pleased that together with eLabtronics, we’ve been developing a whole range of off-the-shelf electronic performance modules. 

Why their need?

Well, I’ve seen it so often. Someone will ask on a discussion group or in a car club for some simple electronic device. Like, they want to automatically turn on something when a certain voltage is reached. Or they want to flash a light. Or they want a simple timer.

Always – absolutely always – there’ll be an electronics whiz that will come out of the woodwork.

Say it’s the flasher that’s desired. The ‘whiz’ will say as fast as he can:

“Oh yeah. Just use a triple-five and a few passives.”

The person making the original requests always says: “Pardon?”

Then expert says it all again, although this time faster and maybe with a URL for a circuit.

The beginner is then likely to say something like:

“OK I think I am getting it now.

“So how do I make the flash rate variable?

“And did I say, I want to pulse the car horn. Is that OK? Will the triple five do that?”

The answer of course is: no, a 555 IC won’t be able to handle the required power. And neither will it like working in a car without any protection circuitry on its power supply leads….

In fact, for every ‘simple’ circuit request, there are always – but always – complexities that are easy to overlook.

So when I say that I started working with eLabtronics over 10 months ago – and the first product is being written about in AutoSpeed only this week – you get some idea of what goes into apparently simple designs.

Of course, the eLabtronics Multi Purpose Module isn’t just a flasher. Or a voltage switch. Or a timer. The same hardware will be able to do all these functions – and plenty more – just by software changes made by the company. 

Which brings me back to the beginning. In the past we’ve covered a range of DIY modules in kit form. They were (and remain) very good designs – but the user had to build them. And many people aren’t confident or happy building electronic kits where just one, apparently trivial, wiring error can stop the whole thing from ever working.

The new eLabtronics modules are fully built and tested. Courtesy of their microcontroller design, they also have far more flexibility and options than those previous kits.

The ‘expert’ quoted above will be dismissive. But the electronics non-expert, who just wants to do all those apparently simple things, will love them….


Posted on May 1st, 2008 in Handling,pedal power,Suspension by Julian Edgar

georgina-on-laden-trike.jpgAs many of you will know, on my recumbent pedal trike I use a Firestone airbag for rear springing. This air spring has major advantages over other springing approaches but as it has little intrinsic damping, external damping is needed.

The rear damper is an ex-R1 Yamaha motorcycle steering damper. This is an unusual design for a motorbike steering damper in that it runs an external passage connecting the sides of the piston. The piston is a loose fit in the bore. The damping action in standard form is provided by the oil passing through the bypass passage, and also making its way past the loose piston. (I assume that the steering damper can be tuned in its action by placing restrictors in the bypass passage.)

To make the steering damper suitable for use as a suspension damper, I modify a plug in the external passage and insert in this passage a one-way valve. This allows free-er flow of oil on bump and more restriction to flow on rebound. Bump damping is therefore provided by the oil flowing in the bypass passage around the open valve and also around the piston, and rebound damping by the oil flow past the piston only.

This gives the desired asymmetric bump/rebound damping.

Expensive tyres?

Posted on February 5th, 2008 in Driving Emotion,Handling,Tyres by Julian Edgar

kh18.jpgI am starting to wonder how much people should spend on tyres.

Years ago, when I owned a Subaru Liberty RS, I bought a set of sticky track tyres of the type that were only just road legal. They gripped phenomenally well, even in the wet. Given the minimal tread depth, the latter was a real surprise to me.

And at other times I have also bought other very expensive tyres, largely being guided by brand name and word of mouth.

But now I am not sure that on cars of less than stratospheric performance, it’s worth spending a lot of money on tyres. Instead, I am starting to think that if there are problems with handling, the money should be spent on the suspension instead.

Three wheel cars

Posted on November 13th, 2007 in Driving Emotion,Handling,Suspension,testing by Julian Edgar

I’ve written here previously about three wheel cars but recently I came across a very interesting article on the topic.

It was first published in US magazine Road and Track in May 1982 and is based around a report prepared for the National Highway Safety Administration by race car engineer Paul Van Valkenburgh. Van Valkenburgh, working for Systems Technology, Inc, was tasked with seeing whether or not 3-wheel cars had intrinsic dynamic deficiencies over 4-wheel cars. As in, do 3-wheleers really have a propensity for overturning, have uniquely (bad!) handling, and so on.

Excerpts from the magazine article:

He tested eight 3-wheelers, four with a single front wheel and four with a single rear wheel, and he compared them to four roughly equivalent 4-wheelers.

The first concern of those who insure, if not those who drive, 3-wheelers is the possibility of turning over. A theoretical dissection of the problem turns out to be less important than simple observation that one test 3-wheeler had better overturn resistance than the best 4-wheeler, a Fiat X1/9. Sound ridiculous? It isn’t if you look at the numbers. If you get the center of gravity (cg) low enough and close enough to the 2-wheel end, and have a wide enough track, you can build a 3-wheeler that won’t overturn in the most extreme maneuvers on flat pavement. Naturally, if you retain the identical cg location and track width of a given 4-wheeler, three wheels will provide a lower overturn safety margin. But these design parameters are never cast in stone and, in limited production, you can put the cg and track just about anywhere you choose.

To be honest, the 3-wheeler with the best overturn resistance was sans body, and therefore had an unrealistically low cg. But another 3-wheeler, fully equipped, was still almost as overturn resistant as the Fiat. Also, I should note that one of the 3-wheelers could have been overturned in testing and in fact was by its owner. It had a high, rearward cg and narrow track, and at 0.6g it would take up on two wheels, precisely as predicted from static tests. To a skilled driver, this was no problem, and it could be balanced there like a motorcycle as long as you had enough time, space and presence of mind.

But now I come to a philosophical question: Is a vehicle that will overturn during hard cornering “unsafe”? Perhaps it is even more of a legal question, as there are some current lawsuits trying to determine if Jeeps and other recreational vehicles are “unsafe” in overturn. If overturn in cornering is unacceptably dangerous, what about tall, narrow, commercial vehicles, such as loaded tractor-trailers, which can go into oversteer at about 0.3g and overturn at about 0.6g?

Even if I assume that engineers of 3-wheel automobiles would optimize the overturn design limits, I still need to know how their stability and handling compare. I will ignore the myriad ways of analyzing stability (static, dynamic, transient, steady-state, oscillatory, divergent, covergent) and just consider oversteer/understeer. Most car enthusiasts have an intuitive feel for these terms and respect for the potential dangers of unexpected oversteer at the limit.

As it turns out, there was a strong distinction between 4-wheel cars and single-front-wheel cars — but not single-rear-wheel cars. Although the sample was admittedly small, the inescapable conclusion is that all single-front wheel 3-wheelers will oversteer at their limit of adhesion. Conversely, all single-rear-wheel cars had strong understeer at the limit. And in neither case could the opposite effect be created, in spite of all the chassis tuning.

Conventional 4-wheelers have a constantly increasing steer angle as speed or g-forces increase. The same is true (to a potentially greater degree) with single-rear 3-wheelers. But the steering on single-front 3-wheelers levels off and then decreases with increasing g’s, requiring counter-steering to avoid a spin.

If these results were difficult to predict, they are easy to explain after the fact. Oversteer/understeer is a result of  many vehicle dynamics factors, such as tire size, type and pressure, suspension characteristics, steering compliance, weight distribution and roll resistance distribution. With all other factors being roughly equal, the end of the car with the greatest weight and greatest roll resistance (springs or anti-roll bar) will have the lower limit of adhesion. Put another way, a nose-heavy car or one with lots of its roll resistance up front will understeer, and vice versa for rear/oversteer. The implications are obvious. With a single front wheel, most of the weight is at the rear, not to mention all of the roll resistance.

On some of these 3-wheelers, extremes of state-of-the-art chassis tuning tricks were attempted, with negligible effect. Regardless of large tire and pressure differences, camber changes and changes in weight distribution that were reasonable from an overturn standpoint, they still oversteered. But again, is oversteer unacceptable? There are a lot of naive folks driving around out there in oversteering production sedans (because of low tire pressures) who will never encounter that limit even in an emergency. The fact of oversteer is easy to obtain; the implications are more than a little speculative.

So overturn can be avoided, and single-rear 3-wheelers have a comfortable degree of understeer. But what about handling — how do they feel? Professional researchers resist being quoted on subjective impressions, but at least here they report a numerical value for handling response. These yaw response times represent the time required for a car to reach a steady cornering condition after a quick steering wheel input. Ordinary 4-wheelers range form perhaps 0.30 seconds for a large car with soft tires, to 0.15 sec for sports cars. All of the 3-wheelers were below 0.20 sec, to as low as 0.10 sec. and that is quick.

The answer is not in the number of wheels, or their location, but in mass, tires and polar movement. The effect of polar movement has been considered for years, but this is the first report I have seen with actual figures. The 3-wheelers had, on the average, about 30 percent less polar movement (normalized for weight) than 4-wheelers, because of centralized masses and less overhang. And the ones with the lowest figures and best tires had the quickest response. Van Valkenburgh says that some of the 3-wheelers had yaw characteristics akin to those of formula cars.

All of the rest of the tests showed no measurable difference between 3- and 4-wheelers. The testers subjected the cars to crosswinds, bumps in turns, braking in turns, free steering return, lane changes and off-camber turns, and although there were many vehicle-specific problems (as you would expect with one-off prototypes) the number of wheels was unimportant.

The technical problems involved in producing a practical 3-wheel car do not appear to be overwhelming. And the potential benefits of cost and fuel conservation would seem to make it worthwhile. As Van Valkenburgh succinctly put it, “a properly engineered 3-wheel car can be made as stable as a properly-engineered 4-wheel car.” But recall your initial reaction to the questions of stability and handling. The big problem is psychological — market acceptance of a radical change. Even if the 3-wheel layout were twice as good, I wouldn’t speculate about its future. One of the most powerful forces on earth is the inertia of an existing idea. But if 3-wheelers ever have a chance to make it, their time is now.

One reason I don’t think much of the Type R Civic…

Posted on September 24th, 2007 in Handling,Honda,Opinion,Power,Turbocharging by Julian Edgar

type-r-on-dyno.jpgToday I returned Honda’s Type R Civic to the Queensland office. I am quite happy to see it go: I think the Civic Type R is a pretty weak car – something I make clear in our road test that will appear in AutoSpeed in due course.

With a 2 litre naturally aspirated engine that revs to 8000 rpm and develops 148kW, it might look the goods on paper – but the reality is very different.

To go further, I think the idea that small, naturally aspirated engines can compete with turbo cars is the stuff of fairytales.

The Peugeot 206 GTi 180  and Ford Focus ST170 were similar cars in concept to the Type R Honda – all based around the idea that naturally aspirated, high revving engines have some intrinsic advantage over their forced induction competitors. That’s a purported advantage over turbo competitors that have more peak power – and vastly more average power through the rev range.