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This article was first published in 2006.
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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.
Toe
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.
Camber
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.
Castor
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
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.
Warning!
Car
manufacturers develop suspension systems through massive engineering development
– from design to construction to durability testing. It’s fair to say that no
aftermarket suspension manufacturer does even a fraction of the OEM testing
before selling products (like suspension kits) that can radically alter
suspension angles. Normally, a warning of this sort would then go on to expound
on how manufacturers know best about handling for enthusiasts, and that you
shouldn’t tinker with this stuff as the car might become an evil monster. Which,
if you go overboard, is true enough.
But
this warning is not about that. Instead, we’re concerned that some aftermarket
suspension components may simply not be sufficiently durable or well designed to
be long-term safe.
In
one of the earliest AutoSpeed articles we covered the fitting of replacement
aftermarket upper A arms on a Nissan Skyline GTR – see
Camber Corrections. In that story we
highlighted what we considered to be poor workmanship: a thread on a securing
nut was burred; the plating was badly done (and one nut had been welded into
place after the assembly had been plated!); washers were only mild steel and
bowed as the adjusting bolts were done up; and the bolts were misaligned with
their holes.
In
Suspension Intervention
we told you how a Nissan
Pulsar GTiR driver experienced massive toe changes after driving hard. The
culprits? Front aftermarket camber bolts that were bending.
More
recently in
Fitting Front Camber Adjusters
I expressed
concern about the undersizing of camber adjustment bolts incorporating
eccentrics...
...and
how aftermarket manufacturers who used a different design of camber adjustment
that claimed not to be using undersize bolts were often in fact doing just that.
(Note also the response from Whiteline Suspension’s Jim Gurief in a letter to
AutoSpeed –
Response.)
In
that story I fitted camber adjustment bolts to a front-wheel drive Toyota Prius
– hardly a powerful car. To make absolutely sure the slotted washers (which were
inserted into oversized holes drilled in the strut) could not move I had them
tack-welded into place. The altered camber was then maintained by the tension of
the bolts acting on lock washers. In retrospect, I should have been more
critical of this design approach – if the bolts lost tension by stretching or
the nuts came loose, the camber could radically change in an instant.
Which
is exactly what happened.
One
day, without warning, the camber changed so much (and therefore so did the toe)
that the steering wheel needed to be turned 90 degrees to keep the car straight.
As a result, one front tyre wore to the metal in just the 15 or so kilometres it
took to nurse the car home.
I
had watched the camber adjustment bolts done up and they were certainly
tightened sufficiently; it’s my gut feeling that they stretched. Either way,
when I thought about the fact that just the friction of the serrated washers
acting against the welded-on tabs stopped the camber changing, I wasn’t happy
about the safety of the car. As it happens I had a second pair of factory front
struts available and I installed these in place of the modified units. Yes,
camber went back to being close to zero, but I fitted stickier tyres and the
overall handling – if anything – actually improved.
This
is not to dissuade you from having aftermarket suspension components fitted.
Aftermarket springs, for example, seem to be generally well made and seldom - if
ever - fail in service. But I think it pays to be very careful when considering
suspension kits that alter castor, camber or other major angles. After all, your
life is dependent on the integrity of these components....
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