THE RIGHT TRACK
One of the most often overlooked,
yet highly important, aspects of chassis construction is steering
geometry. Whether you are building a chassis, or simply updating
an existing vehicle, there is more to this subject than meets
you drive an everyday production car, you tend not to pay
much attention to the steering. Other than the heaviness (or
otherwise), there is usually little reason to comment.
However, jump behind the wheel of a car that has been radically
lowered or, worse still, built from scratch by someone who
doesn’t have a full grasp of vehicle geometry, and you’ll
almost certainly have good reason to comment.
Bump steer, lack of castor, unacceptable tyre wear - or even
steering that locks up solid on full lock - are all consequences
of a lack of forethought when the steering and front suspension
systems were installed.
I took a trip down to JMW Performance Construction in Southend
where Jon Webster ran through the art of keeping your car
on the straight and narrow.
Assuming for our purposes here that you are either building
a car from scratch with a new chassis, or are fitting a strut
front suspension system to a street rod that perhaps previously
came with a drop tube axle, the first thing to do is to construct
while a professional chassis shop will use a full-size jig on
which to build a complete chassis, it is very unlikely that
you will have either need or space for such a thing at home.
All that is required in this instance is a very simple jig that
will be used to hold the struts in the correct location while
you measure and then construct the remainder of the suspension/
steering to suit.
The jig need only be a piece of 3in x 2in box section about
the same length as the car is wide. Two uprights should be welded
onto this in an exactly vertical position (i.e. 90 degrees to
the box section) at a width ‘X’ apart. The distance
‘X’ (see illustration) is found by first establishing
the maximum width apart that the front wheels (with tyres mounted)
can be within the limitations imposed by the bodywork. Next
you need to measure the thickness of the front hubs from the
wheel bolt face to the flat surface on the strut where the spindle
joins the strut body (again, this is clearer in the illustration).
Finally, measure the tyre width and the offset of the wheel.
Now, some basic arithmetic gives you the solution to the following:
‘X’= TK -2 (TW + 0S - H)
TK = Track Width
TW = Tyre Width
OS = Wheel Offset
H = Hub Thickness
As an example, TK = 60ins
TW = 8ins
OS = 4ins
‘X’ = 60” -2(8 + 4-5)
Thus in this example, the two upright brackets of our jig
would need to be welded on so that the inner faces were 46ins
The two uprights will now need to be drilled very accurately
(in fact, it may be easier for you to do this before you weld
them in place) with a hole large enough to take the strut
spindle snugly. These holes should have their centres at a
height above ground level that exactly equates to the radius
of your wheel and tyre combination.
Now, the next step is to measure accurately and find the exact
centreline of the jig, using the inner face of the uprights
again as the datum points. Mark the jig accordingly.
Similarly, mark a line along the jig which theoretically passes
through the centres of the spindles when viewed from above.
You will need this to establish the wheelbase of your car.
Place the jig under the car and measure accurately (sorry
to keep using that word, but it is very important to get things
as precise as possible) from certain fixed datum points, such
as the rear axle location and the centreline of the chassis,
so that the jig can be located in exactly the right position
to achieve correct alignment.
you are happy (double check), the jig should then be fixed in
a semi-permanent way to the chassis by bolting or tack welding
using a couple of metal braces. The chassis will need to be
set at your chosen static ride height when the jig is fixed
in position and consideration must be given to the matters of
suspension travel and tyre to bodywork clearance.
Although this isn’t so vital on a drag race car where
upward suspension travel (i.e. compression) is usually fairly
modest, on a road car it is important to make sure that your
tyres won’t end up rubbing holes in the tops of the front
wings under braking or over rough roads.
Happy? Fine, then bolt the struts up to the brackets using hub
nuts and large washers.
camber angle will be zero (i.e., the spindles will be parallel
to the ground) but note that the struts themselves will incline
towards the centre of the vehicle at the upper end. This will
have been designed in to gain, amongst other things, tyre clearance.
Now you can set the castor angle. This is the angle from vertical
(when viewed from the side of the vehicle) of the line drawn
through the centreline of the strut, or through the upper and
lower ball-joints if your car runs some other type of suspension,
on older cars, we are talking about the inclination of the kingpin
Using an incline spirit level, or a castor gauge, move the strut
so that it is inclined back at an angle of either between 2
and 4 degrees for a road car, or four
and six degrees if you have an A arm
bottom set-up, or eight to ten for a drag racer.
The castor angle is what gives you self-centering steering on
a road car, or arrow-straight tracking on a race car. The greater
the castor, the heavier the steering, but the straighter the
car will run, when you have set the angle precisely on both
sides, lock the hub nuts off tightly to make sure the struts,
consideration is the matter of the steering rack - that is,
assuming you’re not going to stay in the dark ages with
a weighty steering box.
Choosing a suitable steering rack is important, especially where
packaging is tight such as on a narrow vehicle like a Pop. What
you are after is a rack that has the smallest width between
track rod arm pivot points as possible.
When the suspension is designed, the width between the track
control arm pivots on the chassis must coincide exactly with
the width of the track rod arm pivots of the rack.(see picture
below) If there is a variation, the car will suffer from terrible
bumpsteer (i.e. self-steering as the suspension rises or falls).
The rack should be located so that, when viewed
from the front, the track rod arms and the track control arms
are parallel at all times. However, note that, when viewed from
above, there is no need to have the track control and track
rod arms parallel as there will be no fore-aft suspension movement.
If there is, you’ve got big troubles!
The length and angle (again, when viewed from above) of the
steering arms themselves will be the determining factor in setting
what is known as the Ackermann geometry.
When the front wheels of a vehicle turn into a corner, the radius
of the curve travelled by the inner and outer wheels will be
different (the difference being the width of the track).
With a difference in radius comes a difference in circumference
-the distance covered by the outer wheel will be greater than
that covered by the inner one. Unless something is designed
in to take this into account, excessive tyre scrub will occur
on cornering making for premature tyre wear.
Enter the Ackermann principle. What you have
to do is ensure that the angle of the steering arms as viewed
from above is such that when two straight lines are drawn through
the lower pivot point of the struts and the track rod ends on
each side, if extended they would intersect on the cent reline
of the rear axle.
Another important thing to note when you do look at things in
plan form is that you must ensure that on full lock the track
rod arm and the steering arm (attached to the strut) must not
be able to form a straight line, or worse still, go ‘over-centre’
which would cause the steering to lock up totally.
Using a narrow steering rack, and therefore allowing the inner
pivot points of the track control arms to be closer together,
will mean that for a given track width the control arms can
be made longer. This is a desirable thing as the longer the
arms, the smaller the change in track width will be as the suspension
moves through its full range of travel. Keeping the maximum
track width change to two inches or less will help reduce bumpsteer
to a minimum.
In a perfect world, the track control and track rod arms will
be as long as possible, but by using a very narrow rack and
chassis you will almost certainly run into packaging problems.
For example, the width of the engine will largely dictate the
minimum distance between the chassis rails and this distance
will have a direct effect on the width the track control arm
pivots are apart (and hence the width of the rack).
thing to bear in mind when considering the installation of a
rack is that the narrower the rack is, the closer to the centre
of the car the steering column will have to run. OK, it can
be dog-legged with a UJ or two, but you still can’t move
the actual point at which the column enters the rack.
In terms of height, when viewed from the front of the vehicle,
the track rod and track control arms should be mounted so that
they slope upwards at the outer end by one to two degrees on
a race car, or droop by one to two degrees on a road car. This
will allow for changes in attitude of the suspension during
normal usage (a drag car rides with its nose up a lot of the
time under acceleration, while a road car will tend to see more
regular downward movements under braking, cornering etc).
With drag cars, the matter of suspension travel is of paramount
importance with regard to turnability. A heavy automatic transmission
with a relatively low power output, such as a regular street-driven
bracket racer, will generally need more suspension travel to
get them to hook on the line than a lightweight, high-powered
manual car which will conversely require fairly minimal suspension
When you have satisfied yourself that you have covered all aspects
of the geometry, you can proceed with making the necessary brackets
and mountings to carry the rack, track control arms and struts.
Keeping everything still mounted on the jig, tack weld all the
necessary parts in position on the chassis, connect up the track
rod and suspension arms and then remove the jig.
The pivot points of the track rod arms (arrowed) should coincide
with the inner mounting points of the suspension-track control-arms.
In the photo sequence below, notice how the relationship between
the track rod arm (top) and track control arm (bottom) remains
the same throughout suspension travel.
springs removed from the struts, manually move the suspension
through its whole range of movement, checking for any binding
or excessive track changes (if you did your sums right, none
of this should be a problem). Now fit the wheels and tyres and
check that you have adequate clearance under the wings, that
the tyres do not foul the chassis, or the track rod arms don’t
go over-centre with the steering arms.
you are totally happy, weld the mounting in place and, in theory
at least, you should now have a suspension and steering combination
that will be a pleasure to use.
A final word about choosing wheels and tyres. To keep wheel
bearing loads and tyre scrub to a minimum, you should choose
wheels that have an offset such that a line drawn down the centre
of the strut will meet the ground at the centre of the contact
patch of the tyre.
Generally, aftermarket struts such as those from
Strange, Lamb or Koni etc will have been designed in such a
way as to allow the fitting of fairly wide wheels without having
to resort to heavy insets to gain tyre suspension clearance.
So, to recap briefly, when designing your steering system you
need to consider eliminating bumpsteer by keeping track control
and track rod arms parallel when viewed from the front; you
need to make sure that the steering arms and track rod arms
cannot go over-centre on full lock; remember to design in Ackermann
geometry so as to reduce tyre scrub and steering effort; add
sufficient castor to keep the car running straight and have
adequate self-centering; keep track control and track rod arms
as long as possible to keep track changes to a minimum. Finally,
choose wheels that don’t place excessive loads on wheel
If you keep that lot in mind,you won’t go far wrong.