HANDLING, OR WHAT HAPPENS BEHIND YOU WHILE YOU'RE SAWING ON THE STEERING WHEEL

 by Doug Welch

In an ideal world, your kart is the fastest tricycle in the corners ever to hit the racetrack. While every kart manufacturer approaches this problem from a slightly different angle, the bottom line is that your kart has to go around the corner on three wheels.  A live axle goes straight great but turns for crap.

Many things help get the kart up on three wheels.  The flex of the chassis, the geometry of the front end and flex of the drive train components in the rear, all have significant impacts on how a kart accomplishes it’s mission of getting the inside rear wheel un-weighted.

To understand this problem, let’s first look at some basic parameters in the front.  Increasing the kingpin inclination in the front increases weight jacking in the rear.  It shifts more weight to the outside rear tire, which in turn un-weights the inside rear tire.  Less inclination has just the opposite effect.  Additionally, front-end track width (how far apart the front tires are) can also increase/decreases weight jacking.  For most karts, moving the front tires out increases weight jacking, moving them in decreases weight jacking.  However, too much inclination or too wide a track width can make the kart over steer and feel darty.  So we can’t solve all our problems with front-end adjustments.  In fact, some tuners argue it is best to set everything in the front in a medium position and make all your major changes in the back and in the seat position.

Look at the kart from the back.  Imagine a triangle from the top of the seat to the outside of the tires.  In general, the steeper the angle, the more the weight from the biggest lump in the kart, the driver, is transferred through the tire and down into the pavement.  As that angle lessens (i.e., move the tires out or lower the seat) the more the weight is trying to rip the tire loose from the pavement and make it slide sideways.  That is why narrowing the rear track width increases grip and moving them out decreases grip.

But just moving the tires in and out in the back can screw up other things.  Now imagine you were a very tall basketball player and could look at your kart from above.  Draw a triangle in your mind through the front wheels and to one of the rear wheels.  Moving the wheels in and out in the rear changes that triangle and it is that triangle that is most important in getting the kart to handle correctly.  It is that angle that helps jack the weight as the manufacturer designed the kart.  Many kart manufactures want that triangle to stay within a fairly small-defined envelope and moving the wheels too much will really make a mess of the handling.

Rear axle flex and the associated components can have the biggest impact on kart handling.  Wheels, tires, hubs, axles and bearings are all used to control this flex.

The most obvious component is the axle.  It is fairly easy to see that they come in many different grades.  Some tuners change axles as easily as some change track width.  A quality axle can make these changes easy, simple and quick and most of all, predictable.  In general, a harder axle is better for low grip tracks and softer axles for high grip tracks.  Axle stiffness should be the basis for all other changes.

Hubs of different lengths are available to fine-tune the axle.  A hub can control the stiffness of the axle in the area by the wheels.  A long hub will stiffen the axle where a short hub will allow the axle to flex.

Two overlooked areas for controlling flex are the wheels and the bearing hangers.  First, lets talk about wheels.  Wheels can also have dramatic impacts on handling.  A stiff wheel can transfer a great deal of energy to the tires where a soft wheel will absorb energy.  Additionally, a soft wheel will cause the tire to wear on the inside where a stiff wheel will transfer more energy to the outside of the tire.  The offset of the wheel-mounting surface can have significant impacts as well.  A wheel with a small offset will effectively stiffen the axle, as the hub has to be moved inwards to maintain a certain track width.  Conversely, a wheel with a deep offset requires that the hub be moved outwards to maintain a specified track width.  This movement exposes more of the axle thereby increasing axle flex.

Bearings and their associated hangers also have impacts on the control of rear axle flex.  Under hard cornering, the axle doesn’t just flex between the bearing hanger and the wheel and tire.  Rather, the bearing acts as a fulcrum point and the axle bends inward as well.  The fit of the bearing to the bearing hanger is now critical.  Because of the axle flex, the bearings are no longer parallel to the long axis of the axle.  The bearing must move in the hanger or the axle will bind and rob the kart of both power and momentum.  Additionally, if the bearing moves haltingly in the hanger, it will absorb and release energy uncontrollably, which in turn can cause the kart to hop while cornering.

Another area that is usually completely overlooked is axle, hub and wheel run out.  There is always a certain amount of run out in any manufactured component.  The axle may not be perfectly round or straight.  The hub-mounting surface is not always perfectly perpendicular to the bore for the axle.  The wheel-mounting surface is not always true to the tire-mounting surface.  The accumulation of these small manufacturing tolerances can add up to a significant amount of run out at the tire-mounting surface once everything is bolted together. 

This run out can kill a great handling kart.  In effect, the tire is moving up and down as it rotates.  The driver can sometimes feel this but will often mistakenly think that it is an out of balance condition for it will feel the same as an unbalanced tire.  What happens in the corner is that the kart loads and unloads the tire as it rotates up and down.  This condition will most often upset the kart in the middle and exit of a turn when dynamic loading is heaviest or the driver is trying to apply power.  The rear would feel loose or hopping but its effects would be almost imperceptible to the driver.  The tuner would tend to tighten the kart to the point of binding but never fully solve the problem.

Let’s bring this all back together.  In general, a stiff setup is best on low grip tracks such as early in the year before much rubber is laid down or for cold tracks.  A soft setup is best for high grip tracks such as found in the heart of the season during the summer or at national events such as the ProMoto Tour where many high-powered karts are laying down a lot of rubber.

So how does one go about figuring out which changes to make and when?  It is best to keep the overall rear track width within a fairly small range.  Most sanctioning bodies have an overall width limit so we can only go so wide.  Many turners suggest that you use that restriction as the maximum limit and go no more than 1-1/2” less than that for that is the range in which most karts are designed to work.  Start with the axle you think will work best, medium length hubs and the track width set in the middle of the range.  Then make changes to track width to either tighten or loosen the kart as needed.  As you approach the limits of track width, switch hubs and go back to the center of the track width range and start testing again.

Follow this sequence to see how this works.  You arrive at the track, it’s warm and there is some rubber built up.  Install the medium axle, medium hubs and soft wheels.  After the first session you find the kart is loose.  So you move the wheels inward and still it’s just too loose.  Switch to longer hubs and put the track width back to the middle position.  After testing it’s still loose, so you move the track width in but still not what you want but it’s getting closer. At this point you can either switch to a hard axle and short hubs or change to a stiffer wheel.  Record your data at each change and don’t change more than one thing at a time or you will never figure out what change did what.  Also, record more than just the items on the kart and your lap times.  Record the driver’s comments as well.  A fast but difficult handling kart will wear the driver out in a main event.  You want a fast, easy handling and predictable kart.  Now get out there and test.