As 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.

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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.

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I have never been for a ride in a stage coach but it’s something I’d very much like to do. And preferably at full speed, the team of horses at a gallop. Why? Well, primarily because I wonder how well the coaches ride.

I have a book on Cobb & Co, the best known and largest of the stagecoach companies in Australia’s history, and the map showing the routes that the coaches took is stunning. Especially in Queensland, they penetrated way into the inland – true Outback territory. The roads – always dirt and often largely unmade – were terrible and yet the point to point times were actually quite quick. (The coaches ran to timetables like buses do today.)

The coaches used long-travel (and large!) elliptical leaf springs – sometimes transverse as well as longitudinal – and had huge wheels. AFAIK, damping was provided only by the inter-leaf friction of the springs – no dampers were fitted.  In short, the suspension design was as far away from contemporary small wheel, short travel, highly damped suspensions as possible.

But I have a suspicion that these vehicles might have had a very good ride indeed. The large wheels simply wouldn’t have noticed the bumps that a modern car’s wheel would crash into; the very long suspension travel and low natural frequency (at a guess the static deflection would give a resonant frequency near to 1Hz) is close to ideal for human comfort.

A horse-drawn stage coach riding better than a current car? I wonder…

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When you were taught to drive I’d wager 10:1 that no-one ever said anything about left-foot braking. The left foot was for the clutch, or in an auto car, for bracing yourself when cornering. (The driving instructor never said anything about that either? Oh well.)

I first started left-foot braking about 15 years ago. After reading a story on RWD handling that described left-foot braking, I decided to have a go. The first thing that I found was that after years of accelerator operation, my right foot had developed a super sensitivity – but my left foot was used to only operating the clutch. Left foot braking therefore resulted in a crick in the neck, until I learnt some sensitivity with that foot as well!

The worth was proved when I found myself pedalling a loan car, one that handled like it was shod with 75 series rubber pumped up to 20 psi.  The auto car had chronic understeer, but – much to my surprise – I found that it could be largely cancelled-out with a dab or two of left foot braking.

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The email was short and simple: Julian – From all your driving experience can you describe which (one) characteristic makes driving a pleasure?

I assume that the writer means which one characteristic of the car – and that’s a bloody good question.  

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As I write I’m getting over a cold. I am well enough to be mobile but not well enough to work. Well, that’s what I tell myself anyway.  

As many of you will know, I am becoming more and more interested in lightweight vehicles. One of my cars is a Honda Insight – amongst the lightest of all production cars on the road – and I find the downsides of its design usually quite minor. (If I need to carry more than two people, I take Frank the Falcon.)  

Now the Honda might be light, but it still has four wheels when surely three would be enough. Using a tadpole configuration (two front wheels and one rear) would also allow the car to be nicely streamlined, something that would be helped by a front mount engine and front wheel drive. That way, the classic teardrop shape for low aero drag would be much easier to implement.  

The starting point for such a car would be a FWD half-cut, say a Mira or Suzuki 660cc 3 cylinder turbo. Use the complete driveline, subframe, steering and front suspension and brakes, add on a tube frame chassis and then run the single rear wheel and suspension from a motorbike.  

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I’d no sooner finished writing A Disappearing Suspension Technology than I came across something that goes a long way to explaining the reason that swing-arm suspension was used by such hugely respected engineers like Porsche and by companies like Mercedes.

The magazine article is on a very interesting car produced by one of the all-time greats in suspension theory. The designer of the car was Bill Milliken and the premise was that by using narrow tyres running a huge amount of negative camber, very good cornering grip would be able to be obtained.

Now there’s a lot more to his car than just that (by clicking on the magnifying glass you can enlarge the article scans enough to print/read them) but the narrow tyres/huge neg camber is a very short summary.

The tyres being used by Auto Union and Mercedes pre-WWII race cars were similar in width to current big motorcycle tyres and so would have been far less susceptible to loss of grip through lifting of the flat tread of the tyre that would otherwise occur through negative camber. In fact, the lateral thrust from the camber achieved by the swing-arms would, as the Milliken car shows, have made a major contribution to cornering grip.

It makes me think that a lightweight car running low pivot point swing-arm (or semi-trailing or leading) suspension and motorcycle tyres could develop a lot of grip while maintaining an ultra lightweight suspension, in turn giving a very low unsprung weight and low total vehicle mass. And the narrow wheels and tyres would also give far lower rotating inertia, improving acceleration and braking still further…

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The other month I found myself commuting 160 kilometres each day, most of that on two, three and four lane freeways. When everyone’s travelling at basically the same speed, it’s an ideal opportunity to look at the suspension behaviour of other cars. For several kilometres of bumps, you can literally eyeball from close quarters the front or rear wheels of a car travelling at 100 km/h.

One of the interesting things is watching the front dynamic camber variations. Theory says that you want a neg camber increase in bump, primarily to keep the outside, loaded tyre closer to vertical as the car rolls. But theory also says that this dynamic camber increase is pretty well impossible to achieve with MacPherson strut suspension, unless the steering axis inclination is radical (which in turn brings other problems).

And can’t you just see it in action when you watch adjoining cars!

On my local roads, the (pre VE) Commodores and nearly all Japanese and European small cars have front wheels that just move up and down. But watch a Falcon, or any of the European cars with double wishbones, and you can see clear dynamic camber variations.

And the same thing applies at the back, except this time the wheels just moving up and down are those connecting to torsion beam rear axles (FWD cars) or solid rear axles (RWD cars). On cars with multi-link or wishbone suspensions, the camber change is quite obvious to the eye. Of course I’m not talking about much variation – perhaps a few degrees. But you can still see it.

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Now forgive me if this seems pretty esoteric: it probably is. In fact, I’d never really even thought about it until a year or so ago; I’d never actually experienced it until today.

Most of you would be familiar with the idea of ‘toe’. Toe-in is where the wheels point inwards – when viewed from above, they’re constantly steering towards the centreline of the car. Toe-out, as you’d soon guess, is where the wheels are constantly steering outwards from the centreline. Zero toe means the wheels are parallel to the centre line.

Most cars these days run zero toe or just a very small amount of toe-in. Toe, usually measured in millimetres (although degrees would make far more sense), is at most only 1 or 2mm: the amount the wheels steer inwards or outwards is very small indeed.

OK – so that’s static toe. But what about when the suspension moves up and down?

If, during suspension travel, the wheels stay steering exactly in the directions they were originally steering in, the suspension is said to have zero bump steer. If the wheels steer inwards on bump, they’re said to have toe-in on bump. Toe-out on bump is defined as you’d expect it to be. (Note that in all these quoted cases, the steering wheel is held still – it’s the suspension itself that’s doing [or not doing] the steering.)

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