THE SUSPENSION PAGE
I've been lucky enough to be involved
in motorsport since 1984. I have only owned the one race car, the first racing
car car I've bought, but through a great
deal of research & development I have learned quite a bit about making a
car go, stop, and go around corners. To give you an idea of what a good suspension
is capable of, when I first ran my racing car I could only do a 73 second lap
around my local track. Whilst, yes, I've learned to drive a lot faster, a fair
chunk of the extra speed has come from developing the suspension in the car.
It's now capable of doing sub-56 second laps around the same track. I've run in a few Sports 1300 Nationals
over the years, and it's never gotten worse than 3rd place so it does go quite
Improving your car's suspension
has a multitude of benefits -
1. You don't have to rebuild it every season along with your engine. It lasts a lot longer in most applications.
2. You don't have to show off your great skill as a racing driver if the car is easier to drive than all the others at the same speed.
3. You will be able to show off your great skill as a racing driver as you pass the other cars with apparent ease.
4. The tyres will last longer.
5. The car will stay in one piece longer, as it is less likely to test the strength of the fence!
There are a number of different suspension types, but the main ones are McPherson strut, non parallel unequal length double wishbone, and live axle. (The live axle itself having many different ways of locating it)
No matter what type of suspension a car has, it will react when turning a corner. If pushed hard enough, a car may 'understeer', or it may 'oversteer'. Understeer is when the front of the car wants to keep going straight ahead and oversteer is when the rear of the car moves out and the car starts travelling sideways through the corner. This is what people mean when they say how the car 'handles' - It isn't to be confused with the car's ultimate ability to stick to the road. That is called road holding, or more accurately cornering power. (As an example, you might picture a Formula One racing car with bicycle tyre on it. It would still feel very nice to drive, but would not be able to go through corners as fast as a conventional road car, which would probably be suffering howling understeer but never the less go through the corner faster. Another example is a train; Picture a train going around a corner on it's tracks - The train would definitely be described as having 'neutral' handling, of course, and the actual cornering power would be pretty good. But if you moved the tracks & wheels closer together then even though the train would still have neutral handling it's cornering power would be less & less.)
So, while the two areas aren't
actually related, they are certainly on a first name basis! Obviously, to
get to the point where you can reach the car's best cornering power, you
must be able to drive the car hard through the corners and so the
car's handling becomes very important very early on when trying to get better
lap times, making the car more pleasant to drive, etc. The number of excellent
books on the subject is rather large and I couldn't improve much on what
they say, but I will try to describe how the basics work so you can work
it out for yourself.
I can really only skim over the basics without too much description, so the best way to find out is to buy a copy of the excellent series of books by Carrol Smith, "Prepare to Win", "Tune to Win", "Engineer to Win", and "Drive to Win". Also worth getting is the earlier version of "Race and Rally Car Source Book", by Alan Staniforth. They are probably the best books available on how to set up cars and you should go out and buy them all right now! Yes, seriously - They all well worth getting and will pay for themselves very quickly. See my Suggested reading page for more details.
Anyway, lets get
our hands dirty!
On this page you will find information on-
Centre of Gravity
Centres- The car has two roll centres about which it rolls when
cornering. There is a roll centre generated by the suspension at each
end of the car, and they will move around as the car corners, brakes,
accelerates, and any combination of the former. To find out where your
roll centre is for each end of the
car, it's probably pretty easy.
At rest, if the car has McPherson struts then the front roll centre is about 1/3 of the distance between the lower control arm's pivot point and the ground.
If it has a live axle, then the roll centre is often right in the middle of the diff housing but can be moved by means of a Watts link, Wobb link, Mumford link, Panhard rod, Jacob's Ladder, etc.
If it has a double wishbone set up, then it's very likely to be about level or slightly below the inner pivot for the lower arms.
Something like this -
It's very important to try to keep
both the roll centres at the correct height (in relation to each other) and
also to move in similar directions to each other as
the car goes up & down and rolls. In the average road car, this is predetermined
by the manufacturer and can't be easily changed. If you do want to make changes
though, then it's possible to buy kits that'll let you move the lower control
arms (in the case of McPherson struts) to alter the roll centre. They're actually
usually sold as 'camber
adjusters' though. I'll get to that later on.
So why would we have lowered the car in the first place? Because of the effect of -
of Gravity - The lower the better, because if the car has a high
centre of gravity then it will roll more and have less cornering power.
The car's centre of gravity works against the cars two roll centres,
and the greater the difference between the two the more the car wants
to roll, so the best thing is to try to have them at the same place.
This can't be done in practice, so it's minimised by making the car as
practicable. So, how low is low enough?
For circuit racing, just enough to keep the car off the circuit. (Well, most of the time anyway!)
For road use, enough to be practical, ie, if you can't drive up the average road gutter with a little room to spare then the car is too low. This might mean that the sides of the car may be about, say, 12cm (5") or so. Any less, and it's too hard to drive around and keep your sanity!
You can of course also move the centre of gravity down a little by moving things like heavy batteries, water overflow tanks, etc, down as far as they'll go - It all helps.
To lower the car, shorter springs are needed. A spring is actually a very clever thing -
Springs - A coil spring is an old fashioned torsion bar all coiled up into a far more practical package. When the coil spring is compressed, the coils move closer together and so have the effect of twisting the solid metal of the coils in relation to each other. So, if you shorten a coil spring it will effectively become a little stiffer as well. Since the coil spring is a long torsion bar all coiled up, if the spring has a lot of coils in it then it will be softer than an otherwise identical spring that has less coils. The same goes for two springs that have the same number of coils but different wire thicknesses - The one with the thinner wire will be softer.
A Japanese enthusiast's Nissan Silvia modification to the rear suspension. Not terribly practical to go shopping with, but certainly a lot easier to work on & adjust.
One important thing when considering
lowering your car is how long the springs will be when they are shortened
- The springs must not be so short as to be loose in their carriers, or one
day they WILL fall out! One way to fix this to is get a double-wound
spring, ie, have the majority of the spring the correct coil spacing, etc,
and the bottom part a lot more coils closer together so they bunch up a lot
faster and fill the gap. When the car is off the ground, the double wound
springs will just fit neatly into the carriers, then as the car settles completely
onto the ground the finer coils will be fully compressed, thus leaving the
upper coils to work as
they normally would.
I do not like the new type of 'progressive rate' springs, as I don't think that they would work as well as normal springs with shock absorbers -
Absorbers - Or to give them their correct name, dampers. They
control the amount of energy going into and out of the springs. Pretty
fancy description, eh? Well, it's not as bad as it sounds. What I mean
by that is the damper damps the suspension going up and down in unison
with the spring by means of a set of hydraulic valves running in special
oil. As a general rule, the stronger the springs the 'stiffer' the dampers
must be. This means that if you have dampers that are meant for normal springs and you
are using lowered, stiff spring then the suspension will not be working
properly. The modern adjustable dampers help quite a lot in this area, but all
the adjusters do is change the rebound (The car body moving 'up' movement)
settings and not the bump. (When the car body is
In the above paragraph on springs, I mention that I didn't like progressive rate springs; the reason is that the damper has to control the energy going into & out of the spring, and if that amount is changing with regard to the spring position, then you can't accurately damp its movement.
Dampers work very, very hard. So hard in fact, that it's possible to make the oil inside them froth up and boil on continuous driving on a bumpy road. The solution came in the form of gas pressure dampers - They use nitrogen gas pressing down on top of the oil to make the oil more stable and so have a much greater resistance to frothing and boiling, thus keeping the control in the damper. The structure of them is basically the same as a normal oil only type damper, but they can be easily picked from the oil only type because the gas pressure inside pushes the damper shaft to full extension. (Though this doesn't help if it's already in the car ...)
Here's the internal view of some various types of dampers -
|This one is a very basic type, and is very common in just about every modern road car. It does the job, but is not very sophisticated.|
|Here is a diagram of how the actual valving in a typical damper works. As the piston moves up or down (ie, the car over a bump/dip) the oil inside the damper body passes through the appropriate orrifice, then is regulated by a 'valve stack', which is a series of sprung restrictors to control the oil flow through the piston, thus providing damping. The degree of stiffness of the 'valve stacks' make the shocky hard or soft, and in fact can be tailored to vary the amount of restriction with piston speed, ie, fast bump, slow bump, fast rebond, or slow rebound.|
|This diagram shows the components of a typical, basic damper. This one is a twin tube type, and it differs slightly from the very basic one described above in that it has a second, larger tube around the main body. This give the damper a larger volume of oil, and it also has the 'bump' valve in the very bottom of the centre tube. This one, from Koni, also has an adjustable stiffness 'rebound' valve (on the end of the piston rod) and is adjusted by means of turning the piston rod. (This is usually done by taking the damper out of the car, collapsing it completely, then turning the 'foot valve' to the setting desired.)|
|This damper is a mono tube gas pressure type, and is
usually inteded for a more high-performance type of car. The bottom
of the damper tube
contains a high-pressure gas, which in conjunction with a floating
piston, keeps the oil under pressure, thus raising the boiling point
of the oil. This is more important for off-road type cars, but it can
help road going, performance, and racing cars. Certainly, the gas pressure
types seem to be more consistant in
Note that this particular diagram shows a gas type that does not have any provision for adjustment of bump/rebound stiffness. The bump & rebound valves are also both situated on the piston.
|This is a twin tube type,
with a larger oil resovoir housed in the outer tube. Again, having
a larger oil supply makes the damper more resistant to fade. As a matter of
interest, having the outer tube also means that the damper is also more resistant to damage from
stones, etc, thrown up by the tyres.
This particular one shown here is adjustable for rebound stiffness, by means of a small adjuster screw in the very top of the shaft, so you don't have to take the damper out to adjust it.
problem of the varying volume inside the damper body has been solved
by Penske dampers, with the very clever system of having the damper
rod go all the way through the damper body. By doing that, there is
no change in the internal volume no matter where the damper rod is
in its stroke, so there is no need for moving internal pistons, etc,
to maintain an even pressure. They can run a much lower internal pressure
to keep the oil from boiling, so the seal friction is far less than
a conventional damper.
And yes, they're expensive ...
Anti-Roll Bars - Often incorrectly called sway bars - are also used to help keep the car flatter when cornering. They do this by means of a special torsion bar that runs across the car from side to side and is connected to the suspension near the wheels. It's made so that when one wheel goes up, the anti-roll bar tries to make the opposite wheel do the same. So, it has the effect of altering the effective spring rate of the real springs. Picture the car halfway through a corner - The inside wheels way below the body of the car and the outside wheel buried up in the bodywork - The roll bar will be trying hard to make the wheels level and so adding pressure to the outside wheels and taking it away from the inside. "This sounds pretty good!", you might think. Why not put a pair of massive roll bars on the car to keep it dead flat when cornering? Not so fast .... With (too) large roll bars, if one wheel is upset when cornering then it will pass that upset to the opposite wheel and so upset that entire end of the car. Not good! I believe that roll bars are to be used to balance the car once the major items are sorted out, and not an excuse or 'band-aid' to make it feel flat when cornering.
A Package -
This is what's needed to make the car go well around corners. It must be
sought and pursued with great vigour at all times! The only acceptable result
is a perfectly handling car so you can fully exploit the cornering power
of the car, and thus
Ok, in the real world this doesn't happen. A car is several hundred cubic feet of compromises, and we can only do what we can do with what we have to work with. If you are lucky enough to own a popular car that is well supported by aftermarket suspension people, then you will no doubt have a good basis to start with once you want to get some improved parts for the suspension. If you don't, or like me have a racing car that is basically unique in the world then you will have to start from scratch and hand make a few parts and do a lot of head scratching to work out what to do.
So where to start?
There is a very simple way to find out what the car is doing and to find out if what you are doing is making progress. (Errr, forwards, that is!) You'll need a stopwatch, a camera, a tyre pressure gauge, and a tyre pyrometer is highly desirable. (In english, that's a tyre temperature sensor, )
The stopwatch is for lap times, plain and simple.
The camera is to take pictures of the wheels when the car is cornering so you can work out what the suspension is doing.
The tyre pressure gauge should be pretty obvious, but make sure you get a good one and keep it as different gauges will give different readings. If you use the same gauge all the time then the readings will be consistent.
The tyre pyrometer is to check the tyre tread temperature readings in three places across the tyre. The outer, middle, and inner areas are what you want to measure. In the perfect set up, the three temp readings would be the same. This doesn't often happen. You are looking for just two things when you get the temp readings -
1. To compare the outer readings to the middle to see if you have the correct tyre pressure. If the tyre has too much then the middle of the tread will be working harder than the outsides and so run hotter. Vice versa for a tyre with too little pressure in it.
2. To compare the readings across the tyre so that you can see if the tyre has a good camber angle to the road surface. The inner area of the tyre must always be warmer than the outside, but not by a large amount, say, more than 5°C. (Out of a total of about 60°-90°) When measuring tyre temperatures, you must run the car until the tyres are fully warmed (say 3 - 4 laps) and then come into the pits without slowing down too much so the tyres stay warm. The probe must be pushed firmly into the surface of the tyre so you get a true reading of the rubber temp and not one affected by a cooling breeze across the tyre. Do them quickly and write them down and keep them.
Ok, if you don't have a tyre pyrometer, borrow one. If you can't borrow one, then you'll have to rely on some clever detective work when you look at the tread depth remaining & scuffing on the tyres as they wear down. Trust me, the money you spend on a type pyrometer and what you will learn from it will pay for it in saved tyres! When it comes time to buy one, I'd suggest getting a spike or needle type rather than the newer infra-red type, as they'll give a more correct reading even if the car has been sitting for a minute or so.
(That's all I've done for now - More to come later, including more pictures.)
For more motorsport links, try the motorsport section on my links page.
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