Another Human Powered Vehicle! Part 7 - Developing the New Front Suspension

This article was first published in AutoSpeed.

Last week I covered the development of a new independent front suspension idea for a Human Powered Vehicle – semi-leading arms. On paper, this approach seems to have the following benefits:

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

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

• Works 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

• Provides a large wheel travel

• Increases castor with bump (but gives less castor with droop!)

• Leaves plenty of room for pedalling legs

And finally, to minimise bump-steer, the approach looked like it could work with the pictured Greenspeed non-crossover steering system.

However, all those advantages were yet to be tested in any way.

Front-End Refresher Camber Camber is the angle that the wheel leans away from vertical when the vehicle is viewed front-on. If the top of the wheels lean inwards towards the centreline of the vehicle, the wheels are said to have negative camber. If the top of the wheels lean outwards, the camber is said to be positive. Camber is measured as an angle expressed in degrees. Toe Toe refers to how parallel the wheels are when viewed from above. If the leading edges of the tyres are closer together than the trailing edges, the car is said to have toe-in. If the leading edges of the tyres are further apart than the trailing edges, the car has toe-out. With toe-in present, each wheel is steering a little towards the centreline of the car. Bump steer is where toe changes occur as the suspension is moved through its travel with the steering input held stationary. Castor Castor refers to the angle of the steering axis away from vertical when the car is viewed from the side. Cars use positive castor. That is, when viewed from the side, the steering axis is further forward at the bottom than the top. The affect of this is that when the steering axis is extended right down to the road, it touches the road ahead of the contact patch of the tyre. In other words, the contact patch of the tyre is behind the steering axis. This gives self-centring of the steering.

Semi-Trailing Arm Systems

While I’ve never seen any design information on semi-leading arm front suspension, there’s some available on semi-trailing arm rear suspension. So that’s where we’ll start...

Semi-trailing arm designs vary from car to car, with one critical difference being the angle of the pivots (note: pivots, not the arms themselves) when compared with a line across the car. A pure trailing arm has a pivot angle of 0 degrees, whereas a swing axle has a pivot angle of 90 degrees.

Semi-trailing arm suspension designs in cars use very small angles – their pivot axis is much closed to being across the car than along it. For example, one BMW used a 15 degree trail angle, while some other references quote 25 degrees as being used in some manufacturers’ rear suspension designs.

So what changes occur with different angles? The first two factors we’re interested in are camber change and toe change.

The greater the trailing arm angle, the greater the camber change. So maximum camber change is at 90 degrees (ie swing axle) but no camber change occurs at 0 degrees (pure trailing arm).

The closer the trailing arm angle is to 45 degrees, the greater the toe change. At 90 degrees (swing axle), no toe change occurs. At 0 degrees (pure trailing arm) again no toe change occurs. It’s at the mid-angle where the max is.

If you want to do the maths, http://e30m3performance.com/tech_articles/susp-tech/rear_curves/index.htm is the best resource I have found to mathematically explain what occurs (although note I couldn’t make the toe equation work). But to really see what’s happening with camber and toe, I suggest making a simple model of a suspension arm from a bent piece of wire and then moving it through its arcs.

Semi-Leading Arm Suspension

If the leading arm suspension was to work with the Greenspeed steering, what angles would the suspension arms adopt? The answer is about 55 degrees – far more than car systems but because of the requirements for more dramatic changes in suspension angles, not necessarily a problem in itself.

Using both a plumb bob-style angle finder and the mathematical equation from the site above, a trailing arm angle of 55 degrees and a suspension arm length of about 45cm theoretically resulted in a 5 degree increase in negative camber per 50mm of bump from standard ride height (and of course, a 5 degree per 50mm loss in neg camber in droop). So if the static neg camber was 5 degrees, 50mm of bump would result in 10 degrees neg, and 50mm of droop would result in 5 degrees of positive camber.

But what about toe changes? The model showed that while there was some toe-out in bump and toe-in in droop, it was actually all pretty small – small enough, I thought, to be able to be compensated for by anti-bump-steer positioning.

And castor? This also increased about 5 degrees in bump and lost 5 degrees in droop. (And that is significant – no-one could tell me the precise affects this would have in a straight-line when one wheel passed over a bump...)

But the semi-leading arm design was giving changes in camber that were in the right direction (camber loss in droop was more than I’d like, but you can’t make the system response asymmetric as you can with double unequal length wishbones) while the castor change, although again dramatic in car terms, wasn’t a major dilemma – not when static castor is to be about 10-15 degrees.

But that was just a model – what would the real system do?

Bump Steer

The next step was to make certain the bump steer could be controlled - and not only that caused by the toe changes inherent in the semi-leading arm suspension design but also ‘traditional’ bump steer caused by the suspension arms and steering tie-rods being of different lengths and/or using different pivot point placements. The best way to do this (and to check on the other angle changes) is to build the suspension and steering – or at least, have enough there that the results can be seen.

I built the suspension arm, tack-welding the ends in place. I also sourced the Greenspeed steering and mounted the pivot in the central backbone tube. Using a Greenspeed kingpin assembly (the company is happy to sell any component parts of their HPVs), I could then set the system up with the correct chassis height.

So what were the results?

Bump steer is best assessed by attaching a long lever (arrowed) to the wheel, extending forwards and parallel to the ground. (Incidentally, in this view the front of the machine is closest to the camera.)

The lever is then lined up with a straight edge (I used a wooden box), the steering held fixed, and the wheel and suspension moved up and down through its travel. Any non-parallelness between the lever and the box (when viewed looking down from above) indicates bump-steer.

Initially, some bump-steer was occurring but by adjusting the height of the inner suspension pivot, this was soon dialled out. In order that extremes could be seen, I used a wheel travel of plus/minus 75mm - at a total travel of 150mm, more than I expect the wheel travel to actually be.

Here is the wheel at full droop. Note the gap between the blue box and the lever attached to the wheel is even – that is, the lever is parallel to the box edge.

The wheel at normal ride height – and the gap is still even. Note the negative camber that has occurred in the change from full droop to normal ride height.

Now the wheel’s at maximum bump – and you can see it hasn’t steered much at all. And yep, that is now a fairly radical camber! (See the “How Much Camber?!” breakout box below.)

Note that while this technique for judging bump steer looks pretty primitive, it works extremely well. It’s also easy (by means of blocks and clamps) to change the height of suspension pivot points or, by washers, steering tie-rod end heights. If I ever build a full-size car, I’ll use exactly the same approach.

Other Angles

With the suspension set up to test bump steer, it was also an ideal opportunity to measure actual camber and castor changes. One thing I learned from my first trike design is that it’s easy to get hung up on taking these sorts of measurements. Clearly, as indicated by the space used here and the time taken in the workshop, you must have a good handle on the angles – but by the same token, at this stage of the build, a degree or two here or there is nothing to worry about. So I was happy to use a simple hardware store ‘angle finder’ to see what was going on.

Once again measuring the changes over a greater wheel travel than will be used, camber at full droop was 4 degrees positive, at normal ride height 5 degrees negative, and at full bump 15 degrees negative.

Measuring the castor showed 3 degrees at full droop, 10 degrees at normal ride height and 15 degrees at maximum bump. This degree of castor change is, AFAIK, completely unknown in front (steering) suspensions. Given that uneven left/right castor can cause a car to pull to one side, it’s a concern. However, I can’t see it causing massive steering inputs on one-wheel bumps; in constant radius cornering (where the left/right castor will be more uneven for a longer period of time), it may have an effect.

Knowing that the idea of semi-trailing arms is quite radical, especially in terms of the castor change that results from suspension movement, I asked Whiteline Suspension guru Wojtek Rogulski this question: What will be the on-road affect of a front suspension that varies in castor with bump (more castor with higher bump, less castor with droop)? (I purposely didn’t say it was on an HPV!) His reply added some further positives that I hadn’t thought of: Such variable caster will have equally variable dynamic camber change with steering lock, in a favourable direction. That is, the amount of additional negative camber on the outside wheel will be higher then and amount of reduction of negative camber on the inside wheel. This should then allow for less static negative camber for the same outcome - reduced compromise between straight line and cornering set-up. I would suggest that some on-road effects would be increased steering weight in off-centre position, increased self-steering, improved cornering and turn-in post initial corner entry, and increased cornering speed. A this is dependant on suspension travel not steering lock, its effects will depend on the amount of suspension displacement, which may or may not be significant. But, I would still see this as a good thing none-the-less. Other factors to consider would be possible wheelbase change, and effects on Ackermann angle and scrub radius. With regard to his last points, scrub radius won’t change because the steering axis inclination doesn’t change (ie the relationship between the kingpin angle and the wheel is fixed); Ackermann may change a small amount, and wheelbase should also change only minimally.

Conclusion

Without actually riding the vehicle on the road, no-one knows whether a semi-leading arm suspension will work: there’s only so much that can be gained from measuring angles and talking to people... Riding the design is sure to be interesting!