Chassis set up is like any other physical science - you learn a
little about a whole structure then you break that structure down to
learn it's components. The same thing applies to the 3
behaviors of understeer, oversteer, and binding. But
before we get to those things we need to discuss the tires.
Basic Tire Briefing
Tire Hardness
They can be hard, soft or something in between. Most of the
time, but not always softer means more grip. Rubber compounds
soften up as they get hot. The grip increases to a point as
the rubber gets hotter. Soft tires tend to begin with more
grip but as both a soft and medium begin to get hot, say 170° the
differences in grip aren't all that much different, most of the
time. Hard tires generally will have less grip as they
are formulated for endurance and consistency through heat cycles.
Tire Pressure
Tire pressures seem to have subtle effects that take a while for
a driver to really feel small differences. The simple rule
about tires is more pressure heats up faster, less pressure is
better to avoid overheating and blistering after several laps.
Sidewall stiffness is mostly what determines the pressure range
with some input from the compound. Bridgestone YHB and YHC
compounds have a range of about 9-13 PSI cold. The softer YHB
will favor 9-10 PSI where the YHC will favor 11-12 PSI.
However if you're just out for a few laps to qualify both compounds
will work best at 12-13 PSI but the softer YHB will be just a bit
faster. By the way, Vega and MG both have stiffer sidewalls
then Bridgestone so they run 1-2 PSI less.
Tire pressure is usually set to where a driver wants it then to
come "in". If you want them on right away set them to the max
of say 13 PSI. Don't expect them to be as fast after 6-7 laps.
On the other hand if you're looking for best grip at lap 4-12 set
them at 11 PSI. And saving the best for last maybe lap 10-20
drop them to 9 PSI. That's how Tire Pressure works.
Tires tend to lose air so ALWAYS check them before going onto the
track.
Baseline Chassis Setup
Chassis Setup preparation begins before going to the track.
If you have a chance to have a reputable dealer help you with
getting the more complex elements of an initial chassis setup it's
always well worth the cost - usually under $50. While we won't
dwell on the more advanced tuning of scaling the kart & driver, this
is the ideal starting point. Most retailers will go through an
initial chassis baseline setup as a perk for the sale including
proper placement of the seat. Optimal Seat placement will
depend on the driver as will positioning of ballast weights.
We cover scaling, seat position, and ballast weights in full detail
in the Advanced Chassis Tuning Guide.
A basic alignment is the best place to start for the beginner.
Even if you don't have the latest laser alignment tools (often
overrated) a tape measure, two 30" long pieces of aluminum angle and
a couple of 6" C-Clamps are all you need to set Toe-In.
Inspect the tire wear for Camber. If the tire wears on the
inside, too much camber. Both front tires should show wear in
the same area, mostly at center. A simple setup session
follows;
- Check the Camber. If wear is excessive set camber to
offset wear pattern.
- Set the kart on the ground
- Fill the tires to 12 PSI
- Set the steering wheel to dead ahead and roll the kart about
10 feet. If it travels straight the steering wheel is true.
If not, the direction it rolled to is the side to set toe out to
toe in; Setting toe in to toe out will be done on the other
side. If it rolls straight and you need to adjust toe,
adjust both sides.
- Put the driver in the kart, roll it about a foot and have them
hold the brake steady
- Clamp the aluminum angles to the wheels with the clamp in the
center - be sure the angles are putting equal pressure on the
front & rear of the tire with the angle sticking straight out the
front of the tire about 16"
- Measure the distance between the left & right angle at a
line across the front of the tires and record it.
- Measure the distance between the left & right angle at a point
15" in front of a line across the front of the tires and record
it.
- The two measurements should match. If the 2nd
measurement is greater, the tires are toed out, if it's less they
are toed in. Step 4 tells you which side to adjust.
- Repeat steps 5 - 9 until the front toe is true.
Camber/Castor adjustments will change alignment but not by
enough to worry too much about it. The purpose of this is just
to get you in the ballpark.
Remove front and rear torsion bars if they are equipped (be sure
to take them to the track).
Basic Handling Briefing
Understeer
Understeer takes a bit more information to choose the best
adjustment for correction.
Is it worse on left turns or right turns?
It should be an equal condition on left & right turns.
Otherwise the corner weights are off and the chassis may be twisted.
Chassis twist is normal and can be straightened pretty easily.
Corner weights and chassis twist are discussed at length in the
Advanced Chassis Tuning Guide.
Where is it understeering the most?
- Turn Entry (turn-in) to Apex
- Apex to Exit
- The whole turn
Entry Understeer is usually corrected by either widening the
front track width or adding castor. Too much castor will make
the kart "darty" especially on high speed tracks.
Oversteer
Most karts will be set up a little more oversteer then
understeer. Just like with understeer it takes a little more
information.
Is it worse on left turns or right turns?
It should be an equal condition on left & right turns. If not
check corner weights.
Where is it Oversteering the most?
- Turn Entry (turn-in) to Apex
- Apex to Exit
- The whole turn
Braking
Braking effects of changes to the chassis setup are far more
complex then they would appear. Braking at the last possible
inch to make a turn has less of an effect on the lap time then it
does to make a passing or defense maneuver. Say the end
of the straight speed is 50 MPH or 75 Feet Per Second (FPS) leading
into a 30 MPH turn (45 FPS). You have to scrub 30 FPS off with
the brakes to make the turn. With a rear brake only kart that
may take about 20-25 feet depending on the track. A slight
decrease in braking may be achieved with positive rake in the
chassis height but less then 10%. That may provide 2 feet
shorter braking distance. The period of time you spend braking
under those conditions that added 2 feet of open throttle won't cut
more then .01 seconds off the lap lap time.
Where braking effects are most helpful are to carry braking a
little further into the corner leading to a technique know as
Trail Braking. The fastest drivers are masters of Trail Braking.
Trail Braking affects lap times in two ways; First it allows pulling
the braking point even deeper into a turn, and second it aids in
"rotating" the kart for the apex direction. However the
setup to optimize the chassis for trail braking becomes rather
complex and will be covered in the Advanced Chassis Tuning
Guide.
| Binding Without Data Acquisition it's pretty difficult for a newbie to
tell if the kart is binding. One way you can tell is when
another competitor on an equal package say like a Rotax is clearly
faster coming off a turn.
Binding is reduced by taking weight off the inside rear tire
through a turn. Often you'll hear people say they prefer a
softer chassis - that's because it helps to lift the inside rear
through a turn. Rather then focus on reducing the binding too
much, take note of the effect when you make other changes.
This is more of an advanced setup change.
The Power Cycle
There are more arguments and misunderstanding about the
Power component of chassis setup then anywhere else.
It's all too easy to lump all reasons for a driver being
faster into having more power. That may be true in some
cases, but certainly not all. What makes it even more
difficult to differentiate is the visual effect of two karts
coming out on the straight when the lead kart seems to pull
away much faster down the straight. You may be very
surprised to learn how much of this is really due to chassis
setup and driving. The earlier you recognize that, the
quicker you'll learn to be fast. See the sidebar for a
discussion about driving technique to illustrate this point.
For the novice tuner and driver chasing chassis setup to
control Bind and Power can greatly over complicate the chassis
tuning learning curve. In the Simple Setup Change &
Effect Table below we'll constrain the tuning processes to the
major elements the driver and tuner should focus on. |
Driving Technique and
Power
Laying down the power usually starts just after the apex of
the turn. We've all had lessons in
centrifugal and centripetal forces so you should
be familiar with how acceleration negates centrifugal force -
converts sideways to acceleration force on the tires.
Turn speed is very important and this is where reducing Bind
is crucial. The tires will be at their limits; It
may appear that two karts are nose-to-tail going through a
turn, but if that's the case, the trailing kart is doings
something wrong. If the lead kart is faster the speed
they travel will be at any given point on the track.
Ten feet before the apex they will be traveling slower
through the turn then they were ten feet earlier.
Therefore a trailing kart ten feet behind will be catching up.
The trailing kart appears to be catching the lead kart - if
the catch up too much, chances are they're taking the wrong
line which will blow their exit speed.
Likewise at the apex the lead kart is beginning to
accelerate and should be pulling away from the trailing kart.
The closer to the apex the power is put down, the further
ahead a kart will pull on the drag race down the straight.
Exiting the turn is where binding really begins to rear
it's ugly head. On acceleration the weight shifts from
the front to the back. If the inside rear has say 30 lbs
of weight on it, that may shift to 50 lbs. If the
outside rear has 150 lbs and the inside rear has 50 lbs
binding becomes a real drag. On a 125 Shifter exiting a
60' radius turn, this equates to about 7-9 HP of drag through
the exit of the turn - a distance of about 40 feet. If
that turn leads onto a 400 foot straight, and the average
power constant is 36 HP, the Bind is costing over 1 HP alone
through the entire section. Moreover an added 20% Bind
factor is dropping the apex speed by about 1 MPH.
It takes about 6-7 HP to make up for that 1 MPH loss in the
turn on a 400 foot straight. You can see why Data
Acquisition becomes much more important, and why we'll deal
with that in more detail in the Advanced Chassis Tuning Guide. |
Basic Chassis Adjustments Briefing
There are over 50 chassis adjustment elements on a typical
chassis for setup variation. Some are gross tuning, some are
fine tuning, some may be specific to a brand or model of a chassis.
We will deal with the Top Ten Tuning elements.
Tires
Without a doubt tires are the most important element in tuning.
Grip is all about the tires adhering to the track. Most of the
time a spec tire will be chosen leaving tire pressure as the only
tuning parameter. Tire tuning always seems to happen in 3's...
High, medium, low. Most racing slicks have an operating
range mentioned above in the Basic Tire Briefing. Three things
are important to consider when setting the pressure to a high,
medium or low setting and are given in the order of importance;
- Number of laps in the session. 5 laps or less, run
higher pressure without regard to elements 2 & 3. Building
temperature also builds pressure. For 6-12 laps a medium
pressure is best. For 12 + laps lower pressure is better -
with an increasing importance up to about 20 laps.
- Track surface smoothness. The more pressure a tire has
the higher (and longer) it bounces over bumps. It's really
important on tracks where the bump factor in a turn preceding the
longest straight is high. Bumpy tracks, lower pressure.
- Ambient & Track temperature rule of thumb: Below 60°
high pressure. From 60-90° medium pressure. Above 90°
low pressure.
|
Sometimes the rolling diameter of two tires
(even from same production batches) are different. While
that's not really a problem for the front tires (even with
front brakes) the unintended tire stagger plays a huge role on
rear tires. For example if the
left tire measures 34.2" circumference at 11 PSI, and the
right tire measures 34.6" circumference at 11 PSI what happens
down a 600' straight?
The left tire turns 210.6 revolutions.
The right tire turns 208.1 revolutions.
Now we know that can't happen on a live
axle - the tires are skidding a total of 2.5 revolutions down
the straight - and you know that's robbing speed! |
Scrubbing New Tires
Almost all new tire compounds make judicious use of
"volatile lubricants" in the compound that tend to evaporate
with heat. The first heat cycle on tires can
dramatically change the grip throughout the tire life, and
often for the worse over the long term.
Scrubbing tires is recommended for all but the Vega
Yellows. Vega Yellows have a special "Qualifying rubber"
on the outer surface that's about as sticky as you'll find on
any tires. That sticky compound also amplifies chassis
bind significantly.
To scrub the tires take them out for about 3 laps at an
easy pace - check the temps by hand - if their luke warm
(under 120° F.) take another two laps. No sliding the
kart! After they've reached between 125-135° wet the
tires down with wet rags to cool. This way they'll
actually have more grip in qualifying and at the end of a the
final race. |
To cure the unintended stagger measure the tire
circumference at equal specified pressures. On new tires only,
if they deviate by more then 2/10's of an inch add about 10 Lbs of
pressure for each minus 1/10" variance to only the smaller tire...
NEVER MORE THAN 60 PSI!
and let them sit in the sun for 20 minutes or so. This
will stretch the smaller tire.
Tires also need
maintenance between track sessions. Wiping off any gravel or
buildup of rubber when the kart comes in with hot tires can go a
long ways to sustaining grip and balance.
Last on Tires are balance. Balance factors change as a
factor of the square of the speed - and aren't too critical at
speeds below 50 MPH. Typically a mounted tire will have about
1/2 oz variation at the rim, and that's not excessive at most sprint
tracks. If a tire has more then 1 oz it will affect
performance, mostly on higher speed turns. Imbalance can be an
issue on a light weight Cadet class when rubber build up occurs.
Chassis
Tuning Elements
The remaining Chassis Tuning Elements are listed in the table
below. You may want to "Cut & Paste this table into an MS Word
or MS Excel file to print and take to the track.
Chassis Tuning Elements are placed in rows; The value in
each of the columns corresponds to how much a given element will
change a given handling effect on most chassis. Not all
chassis are the exactly the same but most are pretty close. If
you're into the math, you'll also find that adding up all the
numbers in a given row always equals zero. It's pretty simple
- what ever you take from one characteristic it returns an equal
amount to another. The key to chassis set up is in finding the
optimal balance that works at any particular track condition.
|
Simple
Setup Change & Effect Table |
|
Chassis Element |
Understeer Effect |
Oversteer Effect |
Brake Effect |
Bind Effect |
Power Effect |
|
Entry |
Exit |
Entry |
Exit |
|
Add Front Torsion Bar |
-4 |
-3 |
3 |
4 |
-1 |
2 |
-1 |
|
Remove Front Torsion Bar |
4 |
3 |
-3 |
-4 |
1 |
-2 |
1 |
|
|
|
|
|
|
|
|
|
|
Increase Castor |
-4 |
-2 |
4 |
4 |
-1 |
-3 |
2 |
|
Decrease Castor |
4 |
2 |
-4 |
-4 |
1 |
3 |
-2 |
|
|
|
|
|
|
|
|
|
|
Increase Camber |
-2 |
-1 |
2 |
1 |
-1 |
-1 |
2 |
|
Decrease Camber |
2 |
1 |
-2 |
-1 |
1 |
1 |
-2 |
|
|
|
|
|
|
|
|
|
|
Increase Front Track Width - 1/4" |
-3 |
-1 |
3 |
1 |
-1 |
-2 |
3 |
|
Decrease Front Track Width - 1/4" |
3 |
1 |
-3 |
-1 |
1 |
2 |
-3 |
|
|
|
|
|
|
|
|
|
|
Toe on center |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
Slight Toe In |
1 |
2 |
-1 |
-2 |
1 |
1 |
-2 |
|
Slight Toe Out |
-1 |
-2 |
1 |
2 |
-1 |
-1 |
2 |
|
|
|
|
|
|
|
|
|
|
Raise Front Ride Height - 1/8" |
-1 |
-2 |
1 |
2 |
-1 |
-1 |
2 |
|
Lower Front Ride Height - 1/8" |
1 |
2 |
-1 |
-2 |
1 |
1 |
-2 |
|
|
|
|
|
|
|
| 
