Weight in a go kart

Post anything that doesn't belong in any other forum, including gaming and topics unrelated to motorsport. Site specific discussions should go in the site feedback forum.

Post Mon Jul 07, 2008 4:53 pm

I have been having a debate with a friend about how much difference weight makes in karting.

He weighs 60kg and I weigh 84kg.

Around a track I was rougly 0.08 seconds faster (best lap vs best lap) so call it identical lap times.

At the track the owner said they were really close times and I started to explain that down the main straight (it only really had one) my friend consistantly pulled about 3 kart lengths on me. His reply was that the weight doesnt really matter that much and indeed the heavier person will be able to go around corners faster!

Now I dont know much about physics but for a start am I correct is saying that is incorrect and more weight does not = more grip (speed) around a corner?

Also working on the fact the kart weighs about 80kg, as a rough % how much faster will my lighter friends car accelerate?

presumably the lighter you are the better you can brake as well?
djones
 
Joined: 17 Mar 2005

Post Mon Jul 07, 2008 5:06 pm

You're friend is wrong about the ability to go faster in corners.

Aside the tyre load sensitivity, the weight added just requires more force to turn, so in a ideal case where the tyre would not react to weight, everything would already be in your devafor.

But in addition, the tyre's contact path tends to stretch under weight (and loads) and that decreases its ability to actually grip.

So a heavier karter accelerates slower and turns and brakes slower as well....BUT

You have an advantage, in many case the limits of adherence are not reached for the four wheels simultaneously which means you often either over or understeer.

Karters have the privilege to be able to play with the balance of their kart, and you have the advantage of being able to effect more the balance on the kart while you move.

That doesn't mean you friend can't do that, but your position will have a bigger effect on balance.
Ogami musashi
 
Joined: 13 Jun 2007

Post Mon Jul 07, 2008 5:26 pm

Thanks for the reply.

I'm realtively new to karting (have only been about 10 times in my life) and have read about moving your weight around but when driving I can't figure out when and how to do this!?!?


I'm also having problems with a racing line on a praticular set of corners but I'm just about to create a new post for that :D
djones
 
Joined: 17 Mar 2005

Post Mon Jul 07, 2008 8:09 pm

About the weight repartition, the theory is simple, the practice is an art.

If you move your weight (lean your body) forwards, you'll make the car oversteer, if you move it backwards you'll make it understeer.

There're more subtilities with lateral position (because a kart has no differential the iner wheels of the kart, especially the rear one, should lift a bit from the ground, they are designed to do that by themselves but special body placement technique can increase the conerning abilities) but that's a bit hard.


The key thing then is simple: when you sense your kart has either too much understeer or too much oversteer move your body to compensate for it.

Where it becomes an art is to sense when you need it or not and act accordignly.
Ogami musashi
 
Joined: 13 Jun 2007

Post Mon Jul 07, 2008 8:16 pm

So, for instance, sit up straighter (or even lean back a bit) to transfer the weight more to the rear wheels. That should help rear traction in a tricky corner.....
Beer is cheaper than therapy.
DaveKillens
 
Joined: 20 Jan 2005

Post Mon Jul 07, 2008 8:46 pm

http://www.karting.co.uk/KandK/Tech/index.html

I think that you are better driver than your friend djones. 14kg is a lot!
You better begin a diet before your friend improves :lol:

Now, seriously, you may be transferring better the weight to the outer wheels and/or lifting better the inside rear wheel. If your friend learns how to do this, you´ll have no chance.

Ciro is a good reference to ask about karts and I´m sure there must be others in the forum :)
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti
 
Joined: 10 Jul 2007
Location: Argentina

Post Mon Jul 07, 2008 11:16 pm

Belatti wrote:http://www.karting.co.uk/KandK/Tech/index.html

I think that you are better driver than your friend djones. 14kg is a lot!
You better begin a diet before your friend improves :lol:


24kg!! He is really small and skinny, just like an average 14 year old boy sort of thing.

Thanks for all the replys. I will try looking properly into this weight transfer stuff and if anybody else replies with some more tips then all the better :D
djones
 
Joined: 17 Mar 2005

Post Tue Jul 08, 2008 1:06 am

I'm not sure about the weight. Conventional wisdom says the lighter the better, because grip is more or less the same while acceleration should be better for you. The fact that your friend overtook you on straights says to me that he had a better engine.

Now, allow me to give you a long explanation (as usual) on what I know about moving your body in a kart.

A kart seems simpler than a car. Well, I'm not sure, because the whole idea of a kart is that it has no differential and no suspension.

You know the zen approach I favor: be the kart. ;)

Now, imagine you do not have differential and nature has provided you with a rear solid axle. Just like an radio controlled car, btw. :)

You are sitting peacefully in the track, when your driver starts to move and turn your little wheels. You have to "scrub" the inner rear wheel, because you have a solid rear axle, no differential. Both rear wheels, rolling the same distances at the same time, will try to keep you going straight.

So, ideally, to turn around a corner, your driver needs to "lift" your rear inner wheel, converting you, a four-wheel kart, into a tricycle for a few moments.

Martin Lyell shows how it's done
Image

When you drive, the first idea is to "shake the rear bridge" (shake your groovy thing... :)) by moving a little your body towards the outside of the curve.

Notice you do it naturally. Why? Because the steering wheel has an angle with the vertical, when you look at it sideways. When you move the steering wheel your outer arm stretches and pulls your outer shoulder toward the front of the kart. The degree of movement depends on the radius of the curve: the tighter the curve, the less you need to move your body to lift the wheel.

Watch Lyell's shoulders: the guy has moved himself towards the outside of the curve, to take some weight from the lifted wheel.

I confess I even exhale when I turn and I inhale when straightening the steering wheel, like when you do weight lifting, but that's, perhaps, taking the idea to the extreme... In any case, for me, it works. :)

If you cannot lift the inner rear wheel enough by moving your body around, you can change the caster angle, increasing it. The followin drawing, in Word, conveys this idea, even if it's not very well drawn:

When you turn the tyre towards the inside of the kart (front circle) the wheel is "lifted" from the ground: this causes this corner to "go down", lifting the opposite rear wheel
Image

You can also increase the Ackerman setting to achieve a similar effect: that is, you make your inner wheel to turn more than the outer, increasing the lift of the rear inner wheel, making slightly easier for you to "tricycle" around the corner.

I have a few moments (I hope you have them too... :)) to explain something about what happens when you succesfully lift the rear inner wheel:

Think about this for a moment: braking while cornering on a kart causes massive understeer. The kart continues in a straight line. Why?

It's not logical, as Spock would say. You're transferring weight toward the front of the car by braking, aren't you?

Besides, when you accelerate, the kart improves the turning. Again, why?

In a regular car you find the opposite: acceleration causes understeer, because the weight of the car transfers to the rear wheels, don't they?

Notice this: when you lift one rear wheel, all the accelerating or braking force in the rear axle is provided by the outer rear wheel. So, when you have lifted the rear inner wheel, any braking or accelerating force of the rear outer wheel, which is out of alignment with your Center of Gravity (CG) causes a torque that makes you, the kart, to turn around your central axis.

Thus, the understeering caused by transferring weight to the back of the cart while accelerating is dwarfed by the torque of the outer rear wheel.

If you followed me up to this moment maybe you're a little confused. Perhaps this image will help:

When you're on three wheels you oversteer by accelerating (green arrows). The rear inner wheel (lower left square) is up in the air, thus exerts no force.
Image

So, if you brake in the middle of a curve, while "tricycling", you "torque" yourself towards the outside of the curve and you oversteer severely. If you're at the limit, this will "break" the front tyres, making them to slip. Once you've blocked the wheels, the friction go down (the dynamic friction factor is smaller than the static) and you cannot recover until you stop braking and regain your "foothold".
Ciro
Ciro Pabón
 
Joined: 10 May 2005

Post Tue Jul 08, 2008 3:00 am

Ciro, I didn´t get these points
Ciro Pabón wrote: I'm not sure about the weight. Conventional wisdom says the lighter the better, because grip is more or less the same while acceleration should be better for you. The fact that your friend overtook you on straights says to me that he had a better engine.

Aceleration should be better for his friend (60kg) and 24kg (sorry djones) lighter.
I think they have similar engines and the light guy accelerates faster.

Ciro Pabón wrote:When you turn the tyre towards the inside of the kart (front circle) the wheel is "lifted" from the ground: this causes this corner to "go down", lifting the opposite rear wheel
Image

You can also increase the Ackerman setting to achieve a similar effect: that is, you make your inner wheel to turn more than the outer, increasing the lift of the rear inner wheel, making slightly easier for you to "tricycle" around the corner.

I underlined the sentence that confusses me.

In general, new kart chasis are designed to obtain the best results with their default caster. This doesn´t mean that you should´t change it, but in a non experienced case I would start experimenting with toe changes. Have in mind that the more kg the driver weights, the more toe changes when you sit (so be careful when measuring it). Toe in may turn the kart nervous but more sensitive to your commands and depends on your driving style. Remember bad alignment is a way to loose power. As a practical rule they should not be bigger than 2mm in the dry.

Here another page with FAQs from an alignment devices company. Among other things, it mentions rear and front track, another setting to watch to.

Other things my come to my head while discussing about this but I´m tired now and the only TV show I watch is gonna start in a couple of seconds.

Bye bye!
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti
 
Joined: 10 Jul 2007
Location: Argentina

Post Tue Jul 08, 2008 7:22 am

More heavier you are. Thew harder it is to steer. I have driven a kart with twin engines and it's very very difficult to steer it. Though the weight distribution would be different when it's a heavier person, but surely lighter weighter is a benefit. I remember during national karting championship drivers try to get as light as possible.

Thread starter you seem to be very quick. ;)
Racing cars are neither beautiful nor ugly, they are beautiful only when you win races.
HKS
 
Joined: 5 Mar 2007

Post Tue Jul 08, 2008 6:20 pm

Is it possible that someone is to light to lift the heavy cart?
Maybe a very light person is not able to lift the rear wheel just by moving a bit with his shoulders.
And so he loses very much time during acceleration out of a corner compared to a heavier person.
Where the engine of the heavier person is not able to reach the max top speed during straights.

I think the track layout plays also a big role.
If the circuit is very twisty with only short straights one could get an advantage.
Or the disadvantage from being heavier is ruled out.
mep
 
Joined: 11 Oct 2003
Location: Germany

Post Tue Jul 08, 2008 7:34 pm

I believe THE LIGHTER THE BETTER, always!
If inner rear doesn´t lift because a skinny driver, then modify the chassis setup.
Colin Chapman was no fool :wink:
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Belatti
 
Joined: 10 Jul 2007
Location: Argentina

Post Tue Jul 08, 2008 9:15 pm

Belatti wrote:Ciro, I didn´t get these points
...

Aceleration should be better for his friend (60kg) and 24kg (sorry djones) lighter.
I think they have similar engines and the light guy accelerates faster.


Sorry, I don't know why I ended thinking djones was lighter. My mistake.

Ciro Pabón wrote:When you turn the tyre towards the inside of the kart (front circle) the wheel is "lifted" from the ground: this causes this corner to "go down", lifting the opposite rear wheel


Well, I don't know how to explain it better. Imagine the chassis is supported on blocks: when you turn the steering wheel inwards, the larger the caster angle the more the tyre "goes up", relative to the chassis.

I'm exaggerating here, but imagine for a moment a caster angle of 90 degrees (the kingpin would be horizontal): the steering movement would be only vertical. If you steer with that angle, the only thing you would do would be to lift up or push down the kart.

When the chassis is not supported on blocks, but the kart is on the ground, if the front left wheel "moves up" (relative to chassis) because of caster angle, then the chassis will go down in that corner.

By logic, if you "push down" the left front corner, the rear right wheel will "go up". Imagine a table with four legs: if you make one leg shorter, the diagonally opposite leg will lift.

I don't know if this is more confusing, but you have TWO adjustments to make: the king pin inclination (KPI) and the caster. The KPI is this:

Image

Caster and KPI work together to have a "jacking effect".

To answer mep's question, if you're a light person and you cannot reach "equilibrium" in the corner, increase caster or KPI. If you have a larger caster angle, when you move the steering wheel the car will squat more in the front, raising the rear.

I disagree respectfully with the idea of "leaving caster alone". When you drive in the rain, the tyres will not produce enough grip to lift the rear inner wheel: you HAVE to graduate caster or KPI. I've seen lots of people making the same mistake (and I've won a few races in the rain because of that). Actually, I don't worry so much about toe or camber as much as caster. A track with faster curves requires larger caster to lift the rear; a short track, with harder curves, requires less caster.
Ciro
Ciro Pabón
 
Joined: 10 May 2005

Post Tue Jul 08, 2008 11:09 pm

Thanks for going into such depth Ciro, even if most of it is going over my head :D

In its most basic form am I correct in thinking:

Braking - lean back.

Turning left - lean right

Turning right - lean left

?????

ie. always lean the oppsosite way to what the karts weight is going???
djones
 
Joined: 17 Mar 2005

Post Wed Jul 09, 2008 10:05 am

mep wrote:Is it possible that someone is to light to lift the heavy cart?
Maybe a very light person is not able to lift the rear wheel just by moving a bit with his shoulders.
And so he loses very much time during acceleration out of a corner compared to a heavier person.
Where the engine of the heavier person is not able to reach the max top speed during straights.

I think the track layout plays also a big role.
If the circuit is very twisty with only short straights one could get an advantage.
Or the disadvantage from being heavier is ruled out.


Added with load transfer (that accounts the weight already in place)i don't think this is a problem indeed.

You just have to be a little bit more aggressive and if the grip doesn't allow it has said modify the set up.
Ogami musashi
 
Joined: 13 Jun 2007

Next

Return to Off topic chat

Who is online

Users browsing this forum: Google [Bot] and 1 guest