Last week we defined all the different front end angles – camber, toe, castor, castor trail, steering axis inclination and Ackermann. This time we take a look at the relevance of all these angles to the road.
But first, a major qualification. It is almost impossible to get universal agreement amongst ‘experts’ with regard to front-end geometry. Not only do various people have a diversity of views but even the most widely agreed upon ideas change over time. Sometimes that’s because technology has changed (eg the advent of near universal power steering means that more castor is now accepted than when manual steering was widely used) but in other cases (eg toe-in, neutral toe or toe-out) the story varies even more.
For example, I once had a small turbo front-wheel drive road car that I occasionally ran at the track. I thought a change in wheel alignment angles could make a big difference so on the Friday I requested a ‘track’ alignment. But by Saturday morning I realised the car was almost undriveable – it darted all over the road whenever I accelerated hard. I’d made a commitment to be at the racetrack by 9am and at eight I was desperately searching for someone who could do a wheel alignment – anyone! I found a backyard garage that was happy to remove the toe-out that had been set. In this car, static toe-out probably combined with power toe-out to make the car twitchy in the extreme. But in other cars, just the same amount of toe-out would have given the quicker turn-in that this setting often results in...
Therefore, no-one can be absolutely certain of how well a wheel alignment will work until the set-up has been tried. Not only is there suspension bush movement that dynamically results in often quite different angles to the static angles set during a wheel alignment, but it’s the combination of the angles (not to mention how those angles work with tyres, damper rates, spring rates, steering ratio and the driver’s preferences!) which will dictate what is optimal.
That’s not meant to be a wimp-out (in fact it’s easy to be dogmatically authoritative: (“camber should be set to minus 2 degrees”), but a realistic appraisal of the situation.
As with all front-end angles, the first looks should be at the manufacturer’s recommended value and then at what the car is currently running. Broadly speaking, zero toe or toe-in is the norm. Toe-out reduces lateral stability and increases tyre wear on the outside shoulder. Toe-out will usually improve turn-in response (but may make it twitchy) while toe-in will dampen this change in yaw response. Toe is adjustable on all cars and so it costs little to change – you won’t need to shell out for a special suspension ‘kit’. For this reason alone, it’s worth experimenting with it.
Negative camber reduces longitudinal (accelerative and braking) grip and aggravates any existing tramlining. (Tramlining is where the wheels follow every longitudinal imperfection in the road). In addition, if a car isn't driven hard enough, tyres with plenty of negative camber will suffer dramatic wear on the inner tyre shoulders. However, negative camber is often used as a pre-emptive approach to improving handling. This is because as a car goes through a corner, its alignment angles change due to chassis and suspension component movement, and bush deflection. These changes see the outside wheel move away from its usual upright position towards positive camber. However, by dialing-in some negative camber when the car is stationary, the camber position of the outside (ie loaded) wheel under full cornering loads becomes closer to upright. This delivers the maximum available cornering grip.
Camber is generally not factory adjustable (or is adjustable by only a tiny amount) and so changing the camber can be quite expensive as – for example – upper or lower strut mounts or bolts may need to be changed.
Not often mentioned but an important point to know is that an increase in negative camber requires an increase in toe-in, and the combination of the two can change steering feel quite a lot. As with all front-end angles, being initially conservative is wise – start with the factory angles and change them only a little, even if you’ve added a suspension kit that gives you a lot of power over wheel alignment.
Positive castor increases the amount of negative camber that occurs during cornering, which means that the outside tyre ends up being closer to vertical than it otherwise would have been. This means better cornering grip. However, unlike simply dialing-in negative camber, positive castor brings no problems in relation to longitudinal grip, tramlining or tyre wear. The only ill-effect is slightly increased steering effort - and this shouldn't be a problem on cars with power steering.
Changing castor invariably requires modification to the suspension – for example, commonly the tension rod is swapped for an adjustable design that allows the wheel to be pulled further forward. Another approach is to use an eccentric or firmer bush.
Steering Axis Inclination
Steering Axis Inclination (or kingpin angle) is normally only able to be adjusted if you’re building a vehicle from scratch. Steering axis inclination should be maintained at an angle as close to vertical as possible but still resulting in a steering axis inclination that gives slightly positive or slightly negative scrub radius.
When building a vehicle, the important aspects to look at are the mounting positions of the upper and lower balljoints (or on a McPherson strut car, the lower balljoint and the upper strut mount) and the wheel offset. The combination of these factors will dictate steering axis inclination. For example, the lower balljoint will often need to be well within the dish of the wheel if the steering axis inclination isn’t to become too great and the wheels with available offsets are still able to be used.
Ackermann angles are only able to be adjusted if you’re either building a car from scratch or making major modifications to the steering. Ackermann is of most relevance for slow speed manoeuvres, where most references suggest steering as per traditional Ackermann angles is best. However, if you’re setting-up a high-speed car and want to look at the effects of Ackermann, http://www.smithees-racetech.com.au/ackerman.html will give you plenty to read.
Note that the actual wheel angles achieved in cornering are very hard to calculate from just simple drawings, as not only will bush deflection (etc) alter them, but in cars with steering boxes, as the steering tie rods move laterally, the inner ball-joints move through fore-aft arcs which affect the actual steering angles actually achieved.