Another Human Powered Vehicle! Part 6 - A Unique Front Suspension

This article was first published in AutoSpeed.

Last week in Part 5 of this series I briefly discussed all the different types of suspension, listing their pros and cons for application in a lightweight Human Powered Vehicle. Rather surprisingly, the top three were: solid front axle, sliding pillar (or sliding kingpin) and swing axle.

However, each of these has substantial negatives. The first to be wiped from the list was the solid front axle: I wanted the suspension movements of the front wheels to be independent. Then there were sliding pillars, but the thought of trying to get a shaft to slide up and down at least 100mm without stiction, and with low wear, was rather off putting. (Furthermore, in such a system, how do you configure the steering to avoid bump-steer?)

So what’s left? – swing axles! Surely not swing axles?

No – Not Swing Axles!

There are some serious problems with swing axles.

As the vehicle rolls, the force acting along the line of the suspension arm tends to lift the vehicle body. This is, firstly, because the roll centre is high above the ground, and, secondly, the swing axle forms a short virtual swing arm. As a result, the outside, more heavily loaded wheel can ‘tuck in’, giving a sudden loss in cornering grip. (That's especially the case if there's a mid-corner hump.) The short virtual swing-arm also causes a lot of camber change of the wheel as the suspension moves through its deflection.

Swing axles have been used on a number of car designs, although usually on the rear rather than the front. The pictured Lightburn Zeta is one of the few cars that has run a front swing-axle design. Early Volkswagen Beetles, the Corvair and older Mercedes used swing-axle rears.

Leaving aside the lifting forces on the body (sometimes also called ‘jacking’), how much camber variation would actually occur with a swing-arm front HPV suspension? If we assume a track of about 800mm, a maximum swing-arm length (taking into account the space of the pivot point) is about 350mm. If the wheel travel is 120mm, the wheel camber will vary by 19 degrees. So if we have zero negative camber at full droop, we’ll have 19 degrees neg at full bounce.

But while that sounds like a crazy amount of camber variation, it’s not way out of the ballpark – my first HPV ran 0 degrees neg at full droop, 5 degrees neg at normal ride height and 10 degrees at full bump.

So how do you reduce the amount of camber variation? You can’t use longer arms because they’d then be longer than half the track. (Well, you actually can – see the breakout box at the end of this article.)

But actually there is a way of making the swing-arms longer!

Rather than having the swing-arms at right-angles to the vehicle’s longitudinal axis, why not have them angled forwards? If they’re angled forwards by (say) 45 degrees, the amount of camber variation is reduced. Furthermore, the roll centre is also lower!

Semi-Leading Arms

One suspension design that uses angled arms is the semi-trailing arm rear suspension used in 1960s and 1970s BMWs and Datsuns (and plenty of other cars as well). As this diagram shows via the red lines, the pivot axis for the arms is neither parallel with the long axis of the vehicle (which would make them swing-arms) and neither is it at right-angles to the long axis (which would make them trailing links).

Rotate the assembly through 180 degrees (or drive the car backwards if you like!) and instead of a semi-trailing arm rear suspension design, you have semi-leading arms. I have never seen or heard of any vehicle using this system but it seems to me that it answers all the HPV design criteria.

It has the potential to:

• Give appropriate negative camber increase on bump (and so of the outside wheel with body roll)

• Use only a single suspension arm per side, so be lighter than a double wishbone system

• Work with a spring motion ratio that can, depending where along the arm the spring is located, vary from a low motion ratio to a high motion ratio

• Provide a large wheel travel

Furthermore, by using semi-leading arms (rather than semi-trailing arms), the room forwards of the front wheels is left free for pedalling legs and steering castor will increase with suspension deflection (so under brakes, for example).

Steering?

I imagine one reason that semi-leading suspension has not been used previously on vehicles is because with conventional steering, massive amounts of bump steer would occur.

Bump steer occurs when the steering input is held steady but the wheels still steer as they move through their suspension travel. This movement tends to be opposite in direction for each wheel. In other words, the wheels might be parallel at normal ride height (ie toe of zero) but at full bump might point inwards (toe-in) or outwards (toe-out).

For a given suspension, the amount of bump-steer that occurs is dependent primarily on three things: the position of the inner steering ball-joint, the position of the outer steering ball-joint, and the length of the tie-rod. This diagram [taken from Fundamentals of Vehicle Dynamics (Gillespie)] shows the ideal length of the tie rod (here called a ‘relay linkage’) and the correct position of the inner and outer balljoints to avoid bump-steer in a double wishbone suspension.

If the steering geometry is incorrect, the amount of bump steer that can occur is massive – it’s not hard to get 10 degrees of steering change over full bump!

In a semi-leading arm suspension that uses conventional steering tie-rods (ie positioned at right-angles to the long axis of the vehicle), the tie rods will be much shorter than the suspension’s semi-leading arms. As a result, the outer tie-rod ball-joint and the wheel hub assembly will move through different arcs, causing the steering tie rod to pull on the steering arm and so create massive toe-out on bump.

However, and here’s where it gets even more interesting, some Greenspeed recumbent trikes use a steering system that angles the tie-rods forward in a way that’s very similar to the positions that would be adopted by semi-leading arms. So if the steering tie-rods and the suspension arms can be made of similar length and run parallel with one another, bump steer could possibly be avoided.

But hold on, there’s another problem! As many of you would know, semi-trailing arm suspension tends to toe-in on bump. (That works well on the rear suspension of a car, because the rear toe-in stabilises the cornering car by reducing the propensity for oversteer.) Turn this through 180 degrees and that becomes toe-out on bump. So if there are toe changes built into the design of semi-leading arm suspension, won’t bump steer inevitably occur?

Not necessarily.

As described above, bump steer depends on inner and out ball joint positions and the length of the tie-rods. But to put it another way, for a given steering system, the position of the inner and outer suspension arm mounts and the length of the suspension arm will determine bump steer outcomes.

So it should be possible to position semi-leading arms to work with the Greenspeed steering system, compensating for the bump steer that would otherwise occur with that type of suspension.

But at this stage that was all just theory – and, untried theory at that...

Next week: mocking up a system and testing for camber, castor and bump-steer changes

Long Swing Axles! As mentioned in the main text, swing axles normally can’t be longer than half the track of the car. That’s definitely the case with powered axles. But what about with non-powered axles? Pictured here is the rear suspension of a 1960s Honda 1300, one of the first cars made by Honda. As can be seen, the suspension system uses leaf springs and swing axles....and the swing axles pivoted from the opposite sides of the car! This is perhaps amongst the simplest car suspension systems I have ever seen. Not only does it have dynamic camber negative increase, you can see that the leaf springs are progressive in rate. Furthermore, with the lightweight leaf springs doing the wheel location, there’s no need for additional suspension links...