karting physics 101

8/13/2019 Karting Physics 101

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NATIONAL KART NEWS APRIL 2009 800-942-0033 WWW.NKN.COM22

STORY TITLE

KARTINGPHYsICS 101By Mike Unger

THEREARE3 BASIC

LAWSTHATALLOBJECTS, NOMATTERWHATSIZEORSHAPE,4 CYCLEOR2 CYCLE,

EUROPEANBUILTORAMERICANBUILT,30MMOR32 MM,MUSTWITHOUTANYHESITATION, FOLLOW.

THISISASIMPLEFACT.

Below: The forces

acting on a kart areextreme and not

intuitive. Look at

the distortion on theright rear of this kart

in a left hand turn.

Karting is physical but karting is also about physics.Understanding the basics will help you go faster

I recently received a comment from one of our

eaders asking NKN to explain some of the kartingopics we write about in more technical depth. Ofourse even if you are a beginner you should know

hat while it seems a kart is a simple machine thephysics involved and the how and why a kart doeswhat it does is anything but easy to understand

r explain. That being the case I do believe thatnderstanding the basic laws of physics actingn a kart is beneficial to everyone. So with that

n mind, I will do my very best to explain one of

he most fundamental concepts so everyone cannderstand.

Before we start talking about the physics of

karting, we have to talk about Newtons Three Laws

f Motion. These are 3 basic laws that all objects,o matter what size or shape, 4 cycle or 2 cycle,

uropean built or American built, 30mm or 32 mm,

must without any hesitation, follow. This is a simple

act.

Newtons First Law is often called Newtonsaw of Inertia. It states that an object at rest or in

teady motion will remain at rest or in steady motion

nless acted on it by another external force. Simply

ut, if an object is sitting still in order to make it

move an external force must be applied to it to maket move. If an object is moving at a constant speed,

force must be applied to it in order to make it slow

own.

Newtons Second Law of Motion is often called

he law of momentum. It states that the force acting

n an object is proportional to the mass of thebject multiplied by the objects acceleration. Force

quals Mass times acceleration, F=ma.

Newtons Third Laws of Motion states thatwhen a particle exerts a force on another particle,

the opposing particle exerts an equal and oppositeforce. More simply, for every action there is an equal

and opposite reaction. Put your hand against a brick

wall and push. As long as the wall doesnt move the

force you feel is equal to the force you are applying.

For a kart this means if acceleration is zero theforce being applied to the kart it is equal to the

forces trying to resist it.

Got all of that? Yeah, I know it sounds

complicated but if you can understand these basic

laws and how they apply to your kart, tuning your

kart will make more sense. Lets first talk about justgoing straight. Imagine yourself at the start of a 100


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Figure 1

Figure 2

4 cycle or 2 cycle, laydown or situp all objectsmust follow the basic laws of x2.

Above: You are accele rating downthe straight - the faster you

go the more aerodynamics

pays a part.

mile long straight at a stop with the engine running.

You apply a small amount of throttle and the kart starts

to move. The kart will accelerate until it reaches a

speed at which the force being generated by the engine

working though the chain, sprockets and tires equalsthe external forces acting on the kart as a whole. This

is Newtons 3rd Law.

What are those forces? They are aerodynamic and

mechanical forces. Aerodynamic force, in simple terms,

is the friction caused by the air passing over everypart of the kart. This friction is exponential to the

velocity of the kart. That means that the effect of the

aerodynamic force acting on a kart at 10mph is 100

times greater than acting on the same kart at 1mph.This is why aerodynamics is so important in racing. For

sprint karts, where the speeds rarely get above 60mph,

aerodynamics are not as important as it is for enduro

karts traveling at 100mph.

Mechanical drag is the friction caused by the


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Figure 3

earings, chain, sprockets, and tires. These forces are directlyroportional to the velocity of the object. The faster the

bject goes mechanical friction goes up a linear amount.

At lower speeds around 40mph or so, mechanical drags the dominant resistant force, at high speeds above 70

mph aerodynamic forces are the dominant resistant force.n between those speeds the 2 forces are equally

mportant and depend on your particular kart.

So here we are applying a small amount

f throttle accelerating up until the kart will

ot accelerate anymore. The point when the

peed is not increasing anymore is calledteady state. At this point is where Newtons

aws become clear. The force generated by the

engine is equal to the aerodynamic and mechanical forces opposing the kart. In

order to accelerate to a higher speed, an external force greater than the opposing

forces must be applied to the kart. Since we have only applied a small amount ofthrottle, more force can be applied by pushing the accelerator pedal more. The

engine will make more power, the chain and sprockets will apply more torque to

the axle the axle and tire will translate that torque into a force that will act on the

road. Since the force being applied to the road is greater than the aerodynamic

and mechanical forces acting against it the karts speed will increase. As thekart accelerates the mechanical and aerodynamic forces will increase with

the speed of the kart. At some point the force being applied by the engine will

equal the mechanical and aerodynamic forces. This is when the kart will stop

accelerating and reach its maximum velocity.

Now with that all understood lets

think about how to increase this karts topspeed. Here we are on our super long

straight trying to go a little faster. Gearing

is critical here. Since most all engines

reach some maximum horsepower

peak before they actually stoprevving it is important to look

at the dyno curve of your

engine. If you are

at max speed at

15,500 rpm butyour engine

makes best

Notice the lateral tire forces actingon the right rear tire in this picture.


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IFYOUARERACINGSPRINTKARTSI DONTRECOMMENDYOUMESSWITHBODYWORK

CHANGES. THECOSTISTOOHIGHANDTHEBENEFIT

ISTOOLOW. SAVEYOURMONEYFORTIRES.

Notice the inside rear tire barely touches the

ground. This is necessary for a kart to turn.

power at 15,000 rpm you are giving up speed

because you are not using all of the power available

to you. In this case dropping a couple teeth will

increase your speed. The opposite is also true. Ifyou are not reaching the max horsepower peak of

your engine before the kart stops accelerating you

need to increase the number of teeth on the rear

sprocket.

Other things to consider while on this long

straight is improving aerodynamics, a matter ofbodywork shapes. If you are racing sprint karts I

dont recommend you mess with bodywork changes.

The cost is too high and the benefit is too low. Save

your money for tires. On the other hand if you are

racing enduro karts with open bodywork rules paycareful attention to your karts bodywork. Keep

edges from flapping in the wind and try to close up

all the gaps as tight as possible. Since nobody thatI know of has the money to test a kart in a wind

tunnel pay careful attention to what bodywork thewinning karts have, especially if that winning kart

doesnt have the most powerful engine.

The other way to increase your top speed is

to reduce mechanical drag. Things that contribute

to mechanical drag are bearings, chain, and tires.Bearings should be taken care of and be in good

running order at all times. If they sound like a coffee

grinder, they need to be cleaned or replaced. Your

chain and sprockets also need special care and

attention. Chains wear out over time and a wornchain will tend to have tight and loose sections in it.

A good chain lube is also important here. Lube your

chain every time the kart goes out and pay careful

attention to how freely the rear wheels spin when

the kart comes off the track. You will need a goodsticky chain lube that will not sling off over the

course of a race. At the end of a race when

the chain is hot it should spin smoothly with

the sprockets. If it doesnt your chain lubeisnt doing its job. Sprocketswear out over time as well

and can cause

excessive drag.

Keep an eye

on them andreplace when

n e c e s s a r y .

Ti res a lso

c o n t r i b u t e

mechanical running resistance as well, the softerthe rubber the higher the drag in general. The same

is true for tire pressure. Higher pressures tend to

have less drag. But before you go increasing your

pressures and changing to hard tires remember weare just talking about going down a 100 mile long

straight with no turning involved. Read on to get the

whole story.

Since we dont race our karts on a 100 mile

long straight piece of asphalt the previous several

paragraphs only tell half of the story. Now you needto go around a turn. For this exercise, imagine you

are in the middle of a medium radius right hand

turn at around 35mph. You are applying the throttle

but only enough to maintain speed, the steering

wheel in turned and the kart is not understeering oroversteering. The setup is perfect.

OK, what does Newton say about this situation

and what can we learn from it? Well just like the

long straight when you are in the turn and things are

at steady state all the forces acting on the kart have

to balance out. That is, they are equal and opposite.So what forces are we talking about? The weight

of the kart works though the Center of Gravity andacts straight down to the track surface. Its equal

and opposite force is the right and left tire force

pushing up on the tires. The lateral forces acting

on the kart are the reactant forces acting though

the Center of Gravity and the Tire Grip force actingat the point at which the tire touches the road. The

reactant force acting through the CG is trying to

push the kart the opposite direction of the turn.

This force is the mass of the kart times the lateral

G of the kart at any given time. The Tire Grip Forceis the equal and opposite force keeping the kart on


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karting physics 101

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