Goldtrom

Hi Guys,

Sorry to be such a stickler for the facts... but anyhow...

The other day at Armadillo Willys we were discussing what happens if a car goes into a skid... this was of course in reference to auto-xing... I wanted to know what type of wear/damage was happening to the car. I asked where all the energy goes, and specifcally if it was dispated as heat (made sense to me)... I recieved the "oh no, its static versus kinetic friction.." line and "no it doesn heat up". Well this didn't make sense to me, and so I looked it up. Below is a nice link to a reasonable web site, believe it if you want, but your tires will get hot if you go into the skid, your energy has to be dissapated some how, and that is how it does it...

http://www.bsharp.org/physics/stuff/skidmarks.html

Later folks... hope to see you at the next auto-x

-- Robert

pll

hehe, I think I need to start going to the lunches again. I guess all you IT guys didn't take physics

Out of curiosity, where did they say the energy went?

rzrsedg

Hey Robert !

I recall there being 2 questions.

Is there damage to the car?
How did you spin - isn't the friction dependant on temperature?

Damage? Not really. Any damage related to sliding is usually immediately after the slide There is obviously wear on the tires and sometimes - esp in the case of a spin - that wear is uneven (flat spotting).

The second question is more complicated - and I don't think I can answer it.
BUT - the fact that when we slid that the energy was dissipated as heat (and noise) doesn't answer the question of HOW we got into the spin/slide in the first place. The gain in heat to the tires and pavement didn't cause the spin.

Tire grip is dependant on temperature - too cold or too hot and they just don't grip as well as the "just right" temp. This is why Hoosier makes an Autox compound tire - it's grippier at lower temps.

I think that our spins and slides were mostly power slides last time at marina. We were breaking the rear loose by putting the power down too soon. This broke the rear tires loose. My admittedly simple understanding of how this happens is as follows - if you punch the gas the tires brake loose b/c the threshold of motion has been broken (ie - the contact patch of the tire is now moving relative to the ground... this is hard to see... um...

Okay - Let's just take a step back. No, I was wrong. I'm sorry. Take a step forward. Now take a step back... and then a step forward... and then a step back... and then we're cha-chaing.

Okay - it's work time!
A rolling tire moves at the same rate as the surface/pavement.
A skidding tire doesn't move at the same rate as the surface/pavement.
You can "skid" a tire by locking up the brakes - the tire is moving slower than the surface.
You can "skid" a tire by goosing the thottle and spinning the tires - the tire is moving faster than the surface.

Friction: ratio of force on object to normal force
Static Friction: ratio of force on non-moving object to normal force
Kinetic Friction: ratio of force on moving object to normal force

Static friction is higher than kinetic friction b/c when the object is not moving the object "locks" itself to the surface through surface irregularities. Once you get something moving it "glides." Yeah - pretty empirical - but it's the best I can come up with.

Got it?

so what we did is make the rear tires skid by braking the threshold of motion. Once we did this the fronts had more grip than the rears. And when the fronts have more grip than the rears and you're trying to turn... spin time (unless you do something to counteract that...)

Check out the Phyics of Racing pages. Some physicist that likes to race is writing a book (slowly).

-r

DJ Qube

Some of the energy is also dissipated in the form of sound.

Goldtrom

rzrsedg,

I really don't need a physics lesson, but thanks for the info.

My original question at the lunch was and has always been wear and tear on the car.I even specificaly asked "where does all the energy go?". Either I'm saying different things then what I'm thinking (has been known to happen), or your hearing differnt things then what I'm saying. Either way, clearly there was a miscommunication.

Sometimes I wish I tape recorded everything I've ever said, so I can go back and hear how stupid I really sound .

But anyhow, what I took away from our conversation was that there was no heat being released from a skid... That's how I remember it and I'm sticking to it!



-- Robert

chroot

Where the hell was I during this lunch?

I am physics' worst nightmare. Physics runs from me, and makes me pie. It's too bad I missed it.

Rob, when you skid, you slow down. Fact. When you slow down, you're giving up kinetic energy. The decrease in speed is due to friction. Fact. Friction produces heat. Fact.

When you skid, some of the car's kinetic energy is released in the form of heat, and some of it is released by liberating little bits of your tires and spreading them over the asphalt. Remember how I showed you my almost-new-looking 10,000 mile S-02s?

- Warren