FAKQ – Suspension
Frequently Asked Ka Questions
Introduction
– The Ka’s Suspension – What
Suspension Does – Jargon –-
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lthough
this FAKQ is mainly associated with lowering and stiffening the Ka, it is the
suspension, in conjunction with the wheels, that keeps your Ka in contact with
the road. As such, it is important to
detail a number of important principles and settings of the Ka.
The Ka’s front suspension features independent MacPherson
struts with an anti-roll bar up front, and semi-independent “twist beam” rear
axel arrangement at the back, as detailed here.
Although this may seem obvious, it is important to establish what, exactly, suspension does, how it does it, and why it does it.
First off, the idea behind a suspension system is to reduce the shock impact of an uneven road surface.
Without getting into too much technical detail, if a car
with no suspension travel encounters a bump on the road, it will first be
forced upwards (as the car rises over the bump) and then bought back down to earth. The forces involved can be severe and
certainly enough to jolt eyeballs and cause bruising. It is also possible that the wheels and tyres may lose contact
with the road surface for a moment, which is a very bad thing when trying to go
around a corner.
With suspension, when the car reaches the bump, some or
all of the force acceleration is absorbed by the suspension components and the
effective force is lessened and translated over a much longer period of
time. Some of the force of the bump is
temporarily stored within the suspension system, and subsequently released over
a period of time - in this case, the release is aided by the car coming off the
bump.
In the real world, the tyres and suspension work in
conjunction to iron out the bumps and undulations on every road surface, and so
smooth over the abrupt accelerations encountered by the car. A competent suspension system matches and
smoothes over the accelerations encountered over a number of bumps such that
the car body encounters little acceleration.
The above is a very simplistic explanation of how
suspension works, but it’s quite sufficient for this document!
It’s A Compromise
It would be relatively easy to set up a suspension system
to absorb all sorts of bumps and produce a smooth ride simply by making it
soft, such that the suspension readily “gives” and absorbs a lot of energy.
However, the disadvantage of this approach is that when
the driver wishes to change the velocity of the car - by accelerating, braking
or going around a corner - the suspension absorbs a certain amount of the
energy that would otherwise be used to change velocity.
As such,
you get body roll, or the nose drops / the tail rises when decelerating, or
vice-versa. A change in the car’s
attitude - and by this statement, I don’t mean that the lad has a temper on
- moves the centre of gravity of the vehicle, which influences the car’s
dynamics, specifically the grip that each tyre has. When accelerating hard, if the tail squats down and the nose rises,
the effective centre of gravity is pulled back towards the rear, and effective
grip at the front is reduced (which leads to wheelspin on a front wheel
drive car such as the Ka). Brake
heavily, and the nose dives, the tail lifts, and grip is reduced on the rear
wheels as illustrated here. When taking
a left hand corner, the lateral acceleration of the car means that the centre
of gravity moves to the right hand side of the car, so the car rolls with the
right hand side lower than the left, and the left hand tyres lose grip relative
to the right hand side, which gain grip.
If the suspension system can resist changing the attitude
of the car, it helps reduce the change in the centre of gravity, which improves
the dynamics of the car.
Furthermore, the more energy the suspension absorbs, the
longer it will take for the car to change direction. An extreme example is a mark three Fiesta. If you are tackling an S-bend at speed, when
you go to change direction in the middle, there is an initial bite of the wheels,
but then there is around a second delay and lurch until the suspension has
adjusted the car’s attitude, and then you can wallow in the other direction,
too. It’s not fun!
There are various ingenious methods of enhancing a
suspension system, such as the use of anti-roll bars to
prevent body roll but also maintaining a reasonably soft ride.
The Importance of A Stiff Chassis
The
stiffer the chassis, the better the suspension components can work - and it is
for this reason that modern manufacturers place much emphasis on the strength
of the chassis (as well as the safety aspect). Although not applicable to the Ka, since there is just one body
shape, generally the more openings to the car, the less stiff the chassis is, and
as such the more it will flex under extreme conditions. This is the reason why three door hatchbacks
are preferred over five door hatchbacks, four door saloons are preferred over
five door hatchbacks, and two door coupes (or saloons) are preferred over
four door saloons. And of course,
single seaters have no doors as such!
It is possible to bolster the chassis strength by the use
of a strut brace, or certain types of roll cage.
Because of the importance of chassis strength, this is one
reason why a bent chassis can make a car a write-off.
Maintaining Contact with the Road
In addition to providing a comfortable ride, the other key
aspect of a car’s suspension system is to ensure that the wheels maintain
contact with the road at all times. Of
course, it is possible to get the wheels to leave the road if you are being a
hooligan, and most suspension systems are not hooligan-proof. That said, the Ka does better than most
other small cars.
If the suspension cannot ensure that all wheels maintain
contact with the road, then it should ensure that the most important ones do
during any given manoeuvre. In the case
of many front wheel drive cars, the inside rear wheel can lift from the road if
you try hard enough. Personally, I’ve
yet to get the Ka to do this, although the Puma will do
it quite well, as will the classic Mini and most Golfs. A good
chassis and suspension set up only lets you know that the car is cocking an
inside rear wheel if you ease the steering wheel back quickly, and there’s a
jolt as the wheel comes down to earth.
If the
suspension is not set up properly, the wheels may well lose contact with a
rough road surface, or the effective pressure may be reduced enough to cause a
significant reduction in grip. Combine
this with any intended acceleration (in this case, either when cornering, or
changing speed) and you can have a problem. In a straight line, this can lead to either wheelspin if you’re
accelerating, or the wheels prematurely locking up if you’re braking (in
both cases, electronic systems such as traction control and ABS
can prevent this). When cornering,
this can lead to greater problems by starting a slide.
All modern cars with factory specification suspension
should be capable of handling most ordinary conditions. It’s when you stiffen and lower the car, and
change the bound and rebound rates, that you can encounter problems.
Toe
The toe measurement indicates by
how much the front wheels are either pointed in towards one another (toe in) or
pointing away from one another (tow out).
This can determine the steering “feel” of the car on the move - too much
toe in or toe out, and you can upset the handling, braking and acceleration
thus making the Ka feel unpredictable, not to mention causing uneven tyre wear.
The Ka’s front wheel toe setting
should be set to a quoted 0°10’ ± 0°10’, with an allowable tolerance of 0°35’
toe in to -0°15’ toe out.
The Ka’s rear wheel toe should
be set to 0°24’ toe-in, but the allowable tolerance is from 0°43’ to 0°03’.
However, these figures are
likely of academic interest and if you need to get the toe adjusted, I strongly
suggest you take the Ka to a suitable equipped and trained garage.
Camber
The camber measurement
illustrates the angle of the wheel relative to the road in the horizontal
plane. A camber setting of 0° simply
means that the wheel (and tyre) are perfectly flat with the road. Different cars have different settings to
accommodate the design, the load, and the cornering forces. The camber is an important setting, since it
determines the size of the contact patch between the tyre and the road.
The Ka’s front wheel Camber
setting depends if the model has power steering or not, as this table
illustrates:
Manual steering: nominal of
0°00’, with a tolerance of 1°24’ to -1°24’.
Powered steering: nominal of
0°03’, with a tolerance of 1°27’ to -1°21’.
Both models have a maximum
variation of a quoted 1°15’.
As above, these figures are
likely of academic interest and if you need to get the toe adjusted, I strongly
suggest you take the Ka to a suitable equipped and trained garage.
Ride Height
The ride height simply represents
how high off the ground the vehicle sits, and is usually measured when
unladen. When you see an advertisement
claiming that a suspension kit lowers the car by 35mm, it is this height that
is 
reduced.
Bound and Rebound Rates
The bound and rebound (or
bounce and re-bounce) rates simply show the rate at which the suspension
components bounce off an impact (the compression or extension of the
suspension as you go over a bump) and then the rebound (the rate at which
the suspension components return to their normal setting). These rates are carefully matched to the
car, since too high or too low a setting can result in a jiggery ride, loss of
contact with the road when going over a bumpy bit of tarmac, and a twitchy
cornering attitude.
What You Can Do
Anti-Roll Bars
One of the compromises associated with suspension systems
is that, in order to maintain a reasonably smooth ride, the suspension needs to
be reasonably soft. However, this leads
to floppy, soggy body control during cornering and lots of roll. One solution is to use anti-roll bars. As their name suggests, these are horizontal
bars that are designed to resist body roll.
Ten years ago, anti-roll bars were not usually fitted to supermini sized
cars, but these days they are far more commonplace. Many modern cars have front anti-roll bars, but rear bars are not
quite so common place. The Ka features
a front anti-roll bar.
Strut Braces
A Strut Brace is designed to prevent chassis flex - in other
words, when cornering, the chassis does tend to flex, and the strut brace will
help prevent this. Strut braces are
usually mounted in the engine bay between the suspension turrets, but some cars
also have them between the rear suspension turrets.
To the Klub: there’s a serious amount to be done on
this one, to be completed as and when I’ve finished a certain interim JOGLE project, so ask me again in late May if it’s not
posted up just yet!