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Kart Setup For Dummies

 
At first glance it looks like setting up a Kart Chassis should be a piece of cake.  Despite lacking the springs and shocks there is almost an overwhelming list of things to change.  It's hard enough for most newbies to learn the driving techniques let alone trying to figure out what axle they need, or whether moving the seat back an inch will help.  Nearly all the information we've seen has been written for someone with 3 years of experience - expecting the new kid (or dad) on the block to learn from a 200 page setup book is like trying to soak up honey with a sponge.  It may be sweet stuff, it just won't soak in.  That's why we wrote "The Chassis Setup Guide for Dummies".

The Basics in a Nutshell

Karts do handle a lot like a car does so the same behaviors of losing grip applies to both.  To the driver they both understeer and oversteer.  

Beyond those fundamentals karts have a 3rd factor that affects turn speeds - binding.   Karts lack a differential so as they go through a turn the rear tires are traveling two different lengths - a bit of a conflict that drops grip.  The result is you go through the turn slower.

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;

  1. Check the Camber.  If wear is excessive set camber to offset wear pattern.
  2. Set the kart on the ground
  3. Fill the tires to 12 PSI
  4. 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.
  5. Put the driver in the kart, roll it about a foot and have them hold the brake steady
  6. 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"
  7.  Measure the distance between the left & right angle at a line across the front of the tires and record it.
  8. 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.
  9. 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.
  10. 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;

  1.  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.
  2. 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.
  3. 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
               
Add Rear Torsion Bar 1 1 -2 -1 1 3 -3
Remove Rear Torsion Bar -1 -1 2 1 -1 -3 3
               
Increase Rear Track Width - 1/4" 1 1 -1 1 0 -1 -1
Decrease Rear Track Width - 1/4" -2 -1 1 -1 0 1 1
               
Raise Rear Ride Height - 1/8" -2 -2 0 2 -1 1 2
Lower Rear Ride Height - 1/8" 2 2 0 -2 1 -1 -2

The Last Word on Chassis Setup

As a novice karter even this guide may be "too much information".  Take heart, the sport of karting is just complex enough that every new achievement is rewarded with great personal satisfaction.  There are several books on the market that go into far more detail, far more then is required.  Of course to justify the price of these books a couple hundred pages may give the buyer a feeling of justification.

We also provide an Advanced Chassis Tuning Guide that covers all the correct tuning parameters in the Subscriber Section of Kartweb.  While that isn't free, the Kartweb Subscriber pages are filled with valuable information and instruction videos not found anywhere else.  In addition, we have over 50 hours of high quality on-line videos filmed at the top races from the last few years.

Our goal is simple;  To give the karter the best information available.