A question about static weight and transfered weight

[Paul Kish]

You can put the kart on the scales and add static weight to it and see the weight added on the scales.

You can put a nose on the kart and the air traveling across the nose can add down force.

Those are two different ways of making the spindles and the axle push down harder on the tires.

But we many times talk about weight transfer. You will get weight transfer from acceleratio and you will get weight transfer from braking. When you accelerate and weight transfers to the rear again the axle will push down harder on the rear tires. When you go around a turn centrifical force will transfer weight outward. And again the outside tires can have either the spindle or the axle push down harder on the tire.

All that is leading up to my question. For not know of a better way to ask i'm going to try the question this way.

Are there two kinds of weight factors? One is the kind that occures when you just place static weight on the axle or you apply down force created by the kart traveling thru the air. The second is when weight is transfered. Its the second kind of weight that i'm actually asking about. When weight transfers to the rear of the kart because of acceleration do the actual parts of the kart become heavier??? If the actual parts become then I would think that the tires themselves become heavier. That would mean that weight transfer thru acceleration will actually make the rubber which makes up the tread of the tire become heavier????

Where i'm heading with this is, bet nobody guessed this.... tire growth.

If the tires actually get heavier under acceleraion then the tires will grow more when weight is transfered to them then when they are just rolling without major acceleraton. Also when weight is transfered to the outside during a turn that would tend to make the outside tire grow in diameter and the inside tire reduce in diameter. That would create a sort of automatic stagger in a sprint kart. That would also mean that maby, just maby, air pressure consideations may have more to do with tire growth and the resulting stager much more than we realize.?????

anyhow that's the question.... Do the physical tires actually get heavier and inturn lighter when weight is transfred to them and away from them.

[Ian Morgan]

To get right to the question, the tire and wheel cannot get physically heavier. However, weight transfer will place more weight to some of the tires or take it away from others hence weight transfer. Reading previous post of yours I am sure you have tried to simulate weight transfer and other dynamic effects but what happens when you push down on one corner of your kart, the opposite corner gets lighter, right? It would be great if we could dynamically weigh our karts but that is hard to reproduce accurately and consistantly.

You do bring up a good question on tire growth, though. Yes tires do grow, If you ever watch a twin engine kart get started on a stand, you can see the tire circumference increase. Now a stock 4 cycle will not make them grow as much. IMO, more tire psi will yield less tire growth since the tire structure itself is stronger. I believe there would be alot of variables involved with tire growth during weight transfer like chassis flex, engine torque, the tire structure itself, CG and VCG, etc.

Again, you bring up a very good question. I don't know if this is something to be looked into deeper but it never hurts to try. Im curious to see what other people think and maybe correct a few of my theories.

Ian

[Paul Kish]

Thanks Ian

Then I guess weight transfer must be a purely directional thing and the sole things about weight transfer that you can alter are the direction and how much weight transfers.

The interesting about that is that before you can transfer weight you first have to have weight to transfer. That's where the tough part comes in which is deciding how to move both static weight on the scales and dynamic weight on the track around.

I think I will just forget about transfered weight making the physical tire heavier. It will kind of make things easier to deal with anyway. Now all I have to deal with is tire growth determined by how fast its spinning and weight place upon it via static, down force or weight transfered to it. hummmmm...sounds like transfered weight is still a factor in tire growth. Its just that I no longer need to think in terms of the tire getting heavier just how the weight is applied to it. Easy ain't it?

thanks again

[Steve F]

You are correct in assuming there are two types of weight (force is more correct), static and dynamic. Static weight is just that, the weight placed on each tire when the kart is at rest with the driver in it.

Dynamic load or Force (weight) is the result of the mass of the kart and driver (weight) times the acceleration (g's) placed on that mass. From physics remember that Force= Mass x acceleration.

Once we know what the acceleration forces and the mass of the kart/driver is, we need to find the center of gravity so we can determine the resultant load on the component we are interested in.

Knowing that, and the physical dimensions of where the loads are placed relative to the cg, we can determine the dynamic loads on the components of interest.

Basically, (very basically) we add up the results of the dynamics load and the static load to get the total load on the component.
Remember that the mass of the component does not change, (the wheel and tire's weight dont change,) but the load placed upon them does.

I have calculated the weight transfer on karts and formula cars for my design projects. I will tell you that moving the cg has the the most dramatic overall effect of any change that you can do to the kart.

If you are interested running some of these calcs on your own, check into "How to make your car handle" by HP books, or if you'd rather do the hard work yourself, go get an engineering statics and dynamics book and dive in.

The good news is that working these calculations will help you understand how a kart works. The bad news is that these formulas while reasonably accurate, do not depict the whole story of chassis twist, tire dynamics, caster/dynamic camber change etc etc etc. At some point, calculations become just purely academic exercises to give us engineers confidence that we've designed components that won't fail under "normal" circumstances.

Outside of that, there's no substitution for on track testing.

Steve F

[Paul Kish]

Thank you very much for the reply Steve and Ian.

Paul Kish

[ October 01, 2001: Message edited by: Paul Kish ]

[Rodney Ebersole]

"when weight is transfered to the outside during a turn that would tend to make the outside tire grow in diameter and the inside tire reduce in diameter" The transfer weight on the outside tire may make the circumference greater but I would bet the diameter from the contact patch to the center will be less. which would give you a smaller dynamic circumferance on the outside tire and a larger dynamic circumferance on the less weighted tire on the inside.

[Paul Kish]

quote:

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Originally posted by Rodney Ebersole:
"when weight is transfered to the outside during a turn that would tend to make the outside tire grow in diameter and the inside tire reduce in diameter" The transfer weight on the outside tire may make the circumference greater but I would bet the diameter from the contact patch to the center will be less. which would give you a smaller dynamic circumferance on the outside tire and a larger dynamic circumferance on the less weighted tire on the inside.

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Rodney

the..

]"when weight is transfered to the outside during a turn that would tend to make the outside tire grow in diameter and the inside tire reduce in diameter"

that I wrote doesn't make a whole lot of sence now, to me either. I appreciate your pointing it out so I can straighten it out in my mind.