Suspension Design Advice
Even those who have already read the manual and started working with their
spreadsheets may still be unsure how to decide what springs and sway bar rates
they should choose. This page is meant to help you with that decision. First,
you must have a good idea what you're trying to accomplish with your suspension
modifications. Most modifications are made to achieve one of several general
goals. Below are some of those goals, and how best to achieve them, and what
problems to watch out for.
Don't understand a lot of these terms? Check the FAQ
for some recommended reading that can help you.
Custom Lowering Suspensions
Tuning Racing Suspensions
Wheel and Tire Fitment Tips
Custom Lowering Suspensions
By far most people who modify their car's suspension do so
for looks. The lowered "look" is very popular, and
most aftermarket springs are designed to allow people to cheaply
and easily do that. If that is what you are interested in doing
be aware that, while there are advantages to doing this, it can
cause a number of different problems; some are minor, some can
be major.
- Advantage: Lowering your car looks cool.
- Advantage: Lowering your car also lowers its center
of gravity (CG) which decreases "weight transfer" to
help improve handling. And it may also lower your car's "roll
center" and change other aspects of your car's suspension
geometry in ways that help handling. But sometimes these changes
hurt.
- Advantage: Lowering your car improves aerodynamics
at high speeds.
- Problem: Lowering changes your car's wheel alignment.
Usually the car's "negative camber" and "toe-in"
are increased. Extra negative camber may be a handling advantage.
Solution: After you have lowered your car, make sure to
get the car's suspension aligned immediately at a good wheel
alignment shop.
- Problem: Lowering the car makes
it more susceptible to "bottoming out." This is when
your suspension compresses enough, usually over bumps (and especially
bumps in turns), to cause your suspension's "bump stops"
to contact. This pounding can damage your car's suspension and
chassis if it is excessive. It can also cause your car to "skip"
over bumps in fast turns, or even increase the likelihood of
your car flipping over! This can be dangerous and isn't the way
to win races either.
Solution: Make sure that you select a spring rate that
is stiffer in proportion to the amount you lower your car. If
you plan to lower your car so much that it only has half the
"wheel travel" it currently has (the distance between
the bump stops with the car at normal ride height), you should
double the spring rates. This will ensure that your car will
not bottom out any more than it currently does.
Example: This example assumes that your car currently
has springs that have a linear (not progressive) rate
of 120 lbs/inch, and has 4 inches of "compression"
("jounce") wheel travel (at normal ride height) before
contacting the bump stops:
- First calculate the amount of weight needed to compress you
current springs four inches:
4 inches x 120 lbs/inch = 480 lbs.
- Then calculate the rate necessary to cause the new springs
to compress 2 inches with the same 480 lbs of weight on it:
480 lbs / 2 inches = 240 lbs/inch.
Your new springs should be 240 lbs/inch minimum. That is a 100%
increase (200% of the original rate). If you lower your car only
1 inch (so it has 3 inches of compression travel), the rate should
be at least 160 lbs/inch (480 lbs / 3 inches = 160 lbs/inch).
That is a 33% increase. If you lower your car by 3 inches (so
it only has 1 inch of compression travel), the springs would
have to be 480 lbs/inch (480 lbs / 1 inch = 480 lbs/inch). That
is a 300% increase.
The problem with most aftermarket springs is that they are usually
only slightly stiffer than the factory springs — usually between
10% to 25% stiffer. This isn't generally enough when cars get
lowered by even one or two inches. So be very careful when buying
springs and don't buy them if the vendors won't tell you what
the spring rate is, and definitely don't buy them if the spring
rate is lower than what you calculate they should be — unless
you plan on driving only on smooth roads!
- Problem: The lower ride height can also cause your
car to scrape "speed bumps," and when crossing over
sidewalks into driveways and parking lots, and to bump into curbs
when parking.
Solution: None, except to avoid those places or not lower
your car so much.
With the above information you should be able to either design
your own lowering springs using the spreadsheets, or analyze
aftermarket lowering springs from different manufacturers using
the "solution" above to determine
which ones are stiff enough to accommodate the amount of lowering.
Tuning Racing Suspensions
Many people compete in car racing that allows them to modify
their car's suspension to improve handling. The Sports
Car Club of America, for instance, allows lowering springs,
different shocks and struts, and different sway bars in its "Solo
II" "Street Prepared" category, and in its "Improved
Touring" series, as well as the various "Production"
racing classes. This usually involves lowering the car, like
above, as well as substantial stiffening of the suspension
— often on the order of 300% or even higher. Some people want
to race-tune their car's suspension for street racing. That isn't
entirely practical, or recommended, but if that's what you want
to do you can follow the same procedures as racers do, but limit
the spring stiffness to something you can live with on the streets
(usually around a 200% increase in stiffness, or a "natural
frequency" of between 1 and 2 cycles/second — see the manual
for more information).
- Advantage: The stiffer suspension helps improve response and limits
body lean in turns, and "dive" and "squat" under braking
and acceleration.
Complication: Going too stiff can ruin handling on bumpy tracks.
- Advantage: Lowering your car improves aerodynamics
at high speeds.
- Advantage: Lowering your car also lowers its center
of gravity (CG) which decreases "weight transfer" to
help improve handling. And it may also lower your car's "roll
center" and change other aspects of your car's suspension
geometry in ways that help handling. But sometimes these changes
hurt.
- Problem: Every car and every track seem to require
a different setup, and every change affects other aspects of
the car.
Solution: These spreadsheets allow you to instantly calculate
the effect that, say, stiffer front springs will have on the
car — how that change will affect the car's "favored speed"
(a function of the front springs relative to the rear springs,
and the car's "unsprung weight"), and how much softer
the front sway bar should be to retain the current handling balance.
Or, if you want to change the handling balance, simply change
one number in your spreadsheet, and it will calculate what components
need to change in order to accomplish that.
- Problem: The track time (and parts) required to "sort
out" suspension modifications is expensive.
Solution: Use these spreadsheets to calculate your best
estimate of what your car's setup should be before you
get to the track. You will very likely be far closer to your
car's ideal setup than you would be from just guessing. Make
the mods before you go to the track. Then load the spreadsheet
into your laptop computer and you can make quick re-calculations
at trackside to suit conditions, helping you save even more valuable
time in car setup!
In addition to the above concerns, the questions of handling
balance and body roll are important to racers. The manual has
more info on determining how much body roll is acceptable, and
how to determine how much body roll you have now, as well as
suggested values to use for various types of cars when deciding
the handling balance (oversteer and understeer). But here is
a quick summary:
Body Roll
Every car is different, but in general body roll is excessive if:
- You can't adjust the suspension far enough to achieve adequate
negative camber. This is determined through tire testing. If
the outside shoulder of the tires wears too quickly, or gets
too hot, yet you have already adjusted the suspension to obtain
the maximum negative camber possible, you need to decrease body
roll through stiffer suspension components.
- The response of the car when maneuvering is too slow. This
is more subjective, but in general somewhere between 1 and 4
degrees of body roll is normal in racing. Just don't go so stiff
you lose handling in bumpy sections.
- The car doesn't make good use of the tires facing the inside
of turns. They tend to run at more extreme angles to the ground
than the outside tires do. This is because manufacturers favor
the outside tires since they are capable of performing more work
than the inside tires. But the ultimate cornering speeds are
achieved when both the inside and outside tires are working at
their best levels. An absolute minimum of body roll is required
for this, unless you have a solid axle suspension in which case
this isn't an issue. In any case, this is less important than
the above concerns.
Handling Balance
The spreadsheets describe your car's handling balance with
a ratio called "Total Wheel Rate Ratio Front/Rear"
stated as a numerical ratio to "1." If you like the
handling balance of your current suspension, don't change this
number in the ratio displayed by the spreadsheet. If you think
it under- or oversteers too much, you should decide on a new
number. If it understeers too much, make the "front"
number smaller (which signifies softer front sway bars and springs,
or stiffer rear sway bars and springs). If it oversteers too
much, make the number higher (which signifies softer rear sway
bars and springs, or stiffer front sway bars and springs). The
best thing to do is to experiment with your current suspension.
Try removing the sway bar at the end you want to have stick better,
then drive it that way. If you like it, calculate the resulting
ratio using the spreadsheet and enter that value here. If not,
make a bigger change and test that setup.
If you really don't know where to start the following table
can be used. It is meant ONLY as a GUIDE and gives typical ratios
that I have found work well on competition cars I have personally
tested and driven. These are here ONLY as examples and you should
NOT assume that they will work in the same way for your car.
This is, unfortunately, the least scientific part of suspension
tuning. Too many things can affect how a car handles to make
it easily reduced to a number, but at least you'll have a good
starting point. Front engine/rear wheel drive cars often shift
from extreme oversteer under power to severe understeer in turns.
For that reason I don't feel it would be appropriate to list
any value — you'll have to decide on a ratio that best suits
your driving style, track conditions etc.
|
Car Type: |
% Weight
Front, Rear: |
Total Wheel Rate Ratio
Front/Rear: |
Front wheel drive
Mid-engine
Rear engine |
61%, 39%
44%, 56%
39%, 61% |
0.4 : 1 to 0.8 : 1
1.2 : 1 to 1.4 : 1
1.4 : 1 to 1.6 : 1 |
|
