Here we try and
bring together the two areas of weight transfer theory and suspension geometry, and
consider their relevance in achieving our single goal in suspension set up - optimising
the use of the tyres.
Nothing is more important than the
tyres. We must optimise the temperature and pressure range, and adjust dynamic
camber for reasonable tyre temperature gradient. We could get baseline numbers for
these using a "skid pad". But because such a facility is not
generally available in Australia, we collect tyre temperature and pressure data during
testing and practise at a race circuit. This data base will be the best window into
tyre performance available to us, short of high end data logging.
We will only be able to consistently use
all the grip potential of the tyres if we have a stable chassis platform and good tyre
compliance (tyres stay in contact with the road.) To this end, overall
roll and pitch resistance must be such that we keep suspension geometry within workable
limits, avoid suspension bottoming out, and hold the chassis platform to maximise the
desired effects of aero devices, if used.
Grip and Balance
"Wheel pair stiffness"
and "wedge" are our key concepts in weight transfer theory. Weight
transfer is distributed across the front and rear of the car in accordance with the
proportional stiffness of the wheel pairs that come into play for the particular movement
of the car we are looking at. The spring/anti-roll bar/shock combination are our
main tuning devices. We can also change roll stiffness by changing the proportional
height of front and rear roll centres. When we change the set up to balance the car,
we have also changed overall grip. If the car is now better balanced in the various
corner phases, then overall grip will have improved, and the car will be faster.
The set up combination needed to achieve
the amount of roll and pitch resistance the car needs to be balanced is unique to each car
set up (ie the identical car set up identically.) A multitude of factors can have an
influence, the short list including weight distribution, front and rear track, tyre sizes,
relative downforce, and dynamic settings for just about everything to do with the
spring/anti-roll bar/shock and the suspension geometry - including suspension jacking
forces and tyre scrub.*(see below)
Maintaining tyre compliance with the road
is a major consideration. In theory, we want to run the lowest rate springs,
consistent with a stable chassis platform, and reasonable response for the driver.
How much of the roll resistance will be allocated to anti-roll bars? How much to
shock absorbers? What are the downsides of too much anti-roll bar and/or too much
The driver constantly builds and lets go
the weight transfer in the car as he or she traverses the race track. As well as
roll, we have pitch under braking and acceleration. Diagonal weight transfer (or
warp) can be considered as weight transfer in a combination of roll and pitch, as we
discussed for corner entry and corner exit.
To allow the driver to use all the
potential of the tyres, we considered the importance of developing good response to all
driver inputs - good transient response. The driver should be able to
develop weight transfers smoothly. Transitions for understeer to oversteer should be
rogressive. Steering and suspension geometry is a factor, as are basic
considerations of track, wheel base and CG height. But tuning wise, we again look to
our spring/roll bar/shock package. Stiffer wheel pairs transfer weight
faster. So for faster response in either roll or pitch we stiffen the relevant wheel
pairs. For instance, we may choose to stiffen the front the front and
rear anti roll bars equally, for faster response in roll, without changing the balance of
It is clear that most suspension upgrades
for road cars are aimed at improved transient response - lower CG, stiffer springs bars,
and shocks can all dramatically improve driver feel and the reduction in roll
and pitch will reduce the affect of any suspension geometry deficiences. The car may
well be a lot quicker on the road, even though no attempt has been made to optimise the
balance of the car at the limit, as we have described in the weight transfer section.
For racing, we do need a baseline set up
for steady state balance of the car. But even this apparently simple aim has many
twists and turns. Can it be achieved straight off the drawing board? Not
always. A new sports car in the UK needed a spring change on the rear in it's very
first test session. In developing our own car, we may find ways of improving grip,
and then need furthur adjustments to re-balance the car. For most amateur racers,
this is about as good as it gets. We look for wheel stiffness that gives the driver
the response he likes and then adjust the steady state balance of the car by whatever
means we have available - springs, bars, aero adjustments etc.
We emphasised the importance of transients
because that is the majority of our lap time outside of straight line acceleration.
So we really need a greater insight than steady state understeer or oversteer. We
looked at combination pitch and roll during the corner phases. Certain
movements of the race car add greatly to understeer or oversteer. Transient
behaviour is more than just the response, or speed of weight transfer, that the driver
feels is OK.
There was a story sometime back about the
Swift people putting their Formula Ford on one of those "shaker" suspension
testing machines and finding a completely new set up for springs and shocks. The
car was a lot quicker. They said this set up would not have been
found in the normal course of track testing the car. You can bet the improvement was
in the transient behaviour. The steady state balance would have been much the same,
before and after the changes.
We extend the concept of transient
behaviour to include "handling balance variation". There is the
potential to influence and tune understeer and oversteer characteristics during transient
movements of the car. We look for improvements that will help the corner phase we
are looking at, without causing too much of a problem somewhere else. Say you want
the car to turn in better. Increasing the rate of the rear roll bar would probably
be the worst decision - you would reduce corner exit traction. Increasing rear
spring rates may be a better way, or raising the rear roll centre better again. If you had
an adjustable 4-link rear suspension, the best way might be to set it for slight roll
oversteer in forward pitch to help turn in, and roll understeer in rearward pitch
(acceleration out of the corner). You improve two areas of handling balance with the one
Never let your thinking revolve around the
idea that springs and anti-roll bars only control steady state balance (and in some way
that shocks look after the transient stuff by themselves). I hope I have been able
to show this is definitely not the case.
Before shock tuning for balance came into
the picture, possibilities for influencing handling balance in one area without affecting
another were limited for road racers. For instance, if you increased front roll
stiffness (say with the anti-roll bar) to improve power down on corner exit, the driver
may have to deal with increased understeer in mid corner. Speedway has never been so
restricted because of the multiplicity of available asymetrical set ups. But shock
absorbers give us the potential to influence wheel pair stiffness differently in bump as
opposed to rebound, just as the 4 -link can give us different axle steer in bump as
opposed to rebound, opening the way for asymetrical like effects, even though the car is
still square! Speedway tuners can go one better again - they might only have
to change the rebound or bump on one shock to get the change they want. All is
revealed in the shock section of this web site.
Transients and Shocks
Shocks can add a lot of stiffness in a
wheel movement. They have to, because of the loading they must absorb to fullfill
their basic task of controlling oscillations in the springs. As soon as wheel
movement stops, the shock loading goes away. So they have no affect on steady state
Could we use these quite considerable
loadings in the shocks to tune for understeer/oversteer balance during
transients? You bet we can. And in our shock section, we show you how, with
a generalised approach which will work with any movement of the race car.
Every book I have read on suspension has missed this point. Only Mark Oritz, in his
excellent articles in Race Car Engineering Magazine showed the way. The concept is
fascinating, because for the first time road racing cars can use a set up that
performs like an asymetrical one, such as a speedway car might use, yet the car is still
square (performs the same in RH and LH corners).
So it turns out that there is no mystery
at all, about what we are trying to do with shock tuning, only in the
execution. We have to be absolutely clear about what wheel movements are happening
and we need a very accurate, highly consistent and adjustable device for our shock (read
also expensive.) Paul Haney says that when monotube shocks were first used in Indy
car racing in the early 80's, these cars ran away by 1/2 a second a lap towards the end of
the race. Using the old twin tube shocks, the teams thought their loss of speed was
the tyres going off. But it was the shocks overheating and loosing efficiency.
The monotubes stayed consistent longer.
Evidently, we do not need expensive racing
shocks on the road. A high quality precisely valved non adjustable unit
will do just fine. But what about your road registered performance car,
used in club motor sport? We hope to do some track tests soon with Pedders Extreme
shocks on a road car, to see if we can tune for transient behaviour. We'll report
what happens on this web site.
Transients and Dynamic Wedge
To assist in our understanding of
transients we defined the term dynamic wedge. Speedway teams have used the wedge
concept for decades because of their need to run asymetrical set ups. Road racers
need it too, to gain a better insight into transient behaviour. See the weight
transfer section of this web site.
*A note on suspension
You should know and understand the pros and cons of the suspension geometry you
have. We can help with this by analysing your suspension in the Susprog 3D program.
If we can't get to see your car, email us, and we can send you the sheets to
measure up your car your self. You should know what tuneable devices you've got e.g.
4 link rear suspension, and what potential you have for tuning with roll centre heights
and static wheel alignment angles.
The situation will be different in every
class of racing. For instance, they say the new Pilbeam Sports Prototype has got
very good pitch control so they can run softer springs - anti dive geometry in the front,
and a combination of anti squat and anti lift in the rear. But if your running a
50's or 60's historic production sports or sedan, considerations like these go out the
window. You might have one of those front suspensions with the pick up points all
over the place. (Toyota were still doing this up to the late seventies.) The result
is very strange happenings with wheel bump or droop - track change (scrub), wheel
base change, caster variation, and bad bump steer. If everybody is in the same boat,
no worries. You can still tune the set up and get a nice handling car
that's quick and fun to drive. Only problem is, if one of the makes in the
class has more straight up and down geometry (as per later model production cars), they
are potentially faster.
The reason for wanting to hold consistent
dynamic geometry in a race car is firstly not to scrub the tyre over the road surface or
build jacking forces in the suspension, and secondly to give the driver reliable linear
response to his inputs of steering, braking and acceleration. For instance, roll
centres should move up amd down in the same direction and by similar amounts. Linear
response is also the reason why excessive rising rate suspension and progressive springs
are now well out of favour on race cars. You'd think that the
same would hold for anti dive and anti squat geometry - they also cause rising rate and/or
wheel base variation. The reason the're still used (in moderation) must be that it
is the least hurtfull way of achieving a stable chassis platform in
pitch. There is no effect in roll, only in pitch.