Drift Physwics.......?

[hayai32]

Ok, so ur in a drift..... Why is it that when your rear wheels are spinning faster they have less traction (ie; increase throttle = decrease traction)? What is the scientific explanation?

I guess that having ur wheels spinning faster decreases a quantity of the co-efficient of friction and hence reduces traction but what quantity? Maybe contact area of tyre decreases...

This is for a physics assignment by the way.

Thanks in advance for any help.

Col

[Bax]

Well if you know how a tyre works.

And if you know why they lose traction..

They lose traction because the car is sideways for one. The car has enough power to start them spinning could be another.

A tyre will have trouble gaining traction while its spinning, even moreso if its going sideways.

[KamikazeR33]

the directional force of the wheel is going the opposite way to the force thats being placed on the car by inertia of the car losing traction (going sideways)

therefore the car will keep traction lost until the directional force meets up with the inertia of the car (going in a straight line)

[ferni]

tyres only have so much grip ... once you apply a certain amount of friction then they loose their grip and slide...

if they grip x amount, once you apply x amount in a rotating manner then they will loose traction

if you apply x amount of force to them laterally they will loose traction

the reason cars can loos traction easier when going around a corner is because the tyres are getting force put on them in a rotating AND lateral way....

its pretty simple, im not physicist

[hayai32]

I didn't write the Q very clearly b4, sorry...

I understand how and why they break traction. I need to know why a wheel spinning at 120kph(wheel rotation speed, not car speed) has less lateral traction than a wheel spinning at 90kph. eg; When you initiate a drift... if you apply 100% throttle the tail will hang more than if you apply 75% throttle.

I cant get away with saying "The faster it spins the less traction" (this is what i have already). I need to know why a faster spinning wheel has less lateral traction....

[Laurence]

While I can't think of a simple answer to your question I think you also need to consider the "system" that is a drifting car. Remember there are two wheels spinning at different speeds relative to the road surface. So it may be that partly the effect is due to a "caterpillar track" between the two wheels creating a moment (heh )

http://www.ac.wwu.edu/~vawter/PhysicsNet/T...ces/Normal.html

http://www.ac.wwu.edu/~vawter/PhysicsNet/T...ionalForce.html

[drifto]

The angle of the tail hanging out is increased because you are accelerating the car forwards and turning it around the corner. The spinning back wheels have less friction then the front wheel ( i will try and explain that in a sec ) The back therefore is thrown out from the centre of rotation of the car , ie the back tries to continue straight ahead while the front turns and the car goes sideways.

To materials in contact with each other have a coeficient of friction.

When a force is applied to those to materials that is trying to move there surfaces along each other, ie accelrate, turn, brake. No slip will happen until you reach the coefficient of friction for those materials.

Once you reach it, slip will happen which immediately lowers the coefficent of friction and therfore causes more slip and so on. Providing more force to the situation by applying more throttle only increases the slip and lowers the coefficient futher.

The only way to regain the original static friction is to take away the force that is causing it, but you have to take more away fisrtly because you lowered that threshold immediately as you broke traction originally. After it is reestablished more force can be applied up to the point of breaking traction again.

In practise this is why accelrating from standstill in the wet without spinning is quicker than just sitting there spinning on the rev limiter.

Put a piece of wet paper on a surface like glass. Push it until it moves and you will be able to continue pushing it with less force than you needed to get it going in the first place.

Proffesor DRIFTO

[V8skylineMAN]

tail hangs out more due to centrifugal force of car going round corner.......faster spinning tyre has less traction due to increased heating of the rubber.....hotter the rubber, the slipperier(sp) it is..... this is the way i see it , anyways

[Majanal]

Was going to have a crack at answering this one... but sat and thought for 10 mins and decided that it would take way to much typing and then I still wasn't sure if I had the right answer... hence I appoligise for not writing anything.

How ever I would imagine it to be a combination of many forces, and what each thing is designed to do... for eg. the rear wheels (forget HICAS just for a minute) are designed to push the car forward. Speed into coner would also be a factor.

I would say come out to the Drift day at Oran Park this weekend and do some research! Except that you live in Adelaide!

Dan.

[hayai32]

spoke to my phys teacher. he thinks that the reciprocal to the downward (well on an angle) force exerted by the tyre to the ground causes an elevation and hence reduces contact area of tyre so lateral friction is reduced. (as surface area is a dirctly proportional quantity in the coefficient of friction)

[Duncan]

hmm, sounds like you need a new physics teacher

interesting that no-one has put up a 100% beleivable explaination yet, since the amount of throttle makes a heap of difference there must be a good reason :confused: Drifto may have got it but I didn't understand

Is it simply that a tyre spinning faster has less total grip, and therefore less grip to provide for lateral movement?

[Laurence]

I think really some testing a datalogging is required.

It would be interesting to see the difference it all really makes.

Let's not forget the handbrake factor where the wheel is spinning at 0rpm and the car goes more sideways... Or does it

[Babalouie]

Hmm..interesting question...hard to describe in technical terms

One thing's for sure though, and that is that even when the wheel is locked, it is still providing some friction....so by increasing wheelspin (relative to roadspeed), then surely there is less friction?

[hayai32]

nah what my phys teach told me was correct. It has to be a decrease in contact area of tyre as perpendicular forces do not directly affect each other i.e. decrease in lateral traction is not directly related to wheel spin speed. The only change as a result of increased spin speed that could affect the lateral traction is a change in contact area of tyre. If ne1 knows this is wrong please post, cheers

[hayai32]

got diagrams but not sure if they are posting

[hayai32]

p.s. what dorifto said is spot on and supports this.

[Majanal]

Like I said, there are many contributing factors, suspension geometry and set up play a huge part, I know that you can have a tyre wear evenly. This might be due in part to differing contact areas through out a drift perhaps?

If I understand correctly the second diagram of the tyre and contact points would lead to mean that eventually the tyre would leave the ground? This is not possible I'm sure.

[GTRgeoff]

From a Mechanical Engineers pov, and a concept well understood in racing, a tyre has a maximum grip in any direction, referred to as a friction circle, or more correctly as a friction ellipse. The ellipse is because lateral traction is not equal to longitudinal traction in all tyres.

Drifting is essentially braking in reverse, and Jim Clarke was one of the first racers to develop a high degree of understanding in racing. When you apply a cornering force with no acceleration or deceleration you can achieve the total grip available in that direction, but when you apply torque through acceleration or deceleration you now have to resolve the forces in the two directions using a simple Euclidian geometry calculation in vector mathematics.

When the forces exceed the traction available through (lets say static as the rolling tyre has certain slip characteristics that overly complicate the discussion) friction then you get wheelspin and a higher sideways slip angle as front tyre drag becomes a more relevant force. Now the other interesting phenomenon is the difference between static and dynamic friction. Once traction is lost and wheel speed increases relative to the ground, the coefficient of friction decreases. Tyre engineers and chemists put a lot of effort into ensuring there is a fairly linear decrease in the dynamic coefficient of friction as the relative speeds increase to generate predictability. the last thing you want is fro the tyre to grip suddenly or to completely lose traction without some indication.

Now you have another phenomenon that is related to the contact patch, the pressure in the tyre, and the angle of contact at the contact patch which in itself is essentially flat. As the tyre accelerates, the contact patch grows longer because the tyre "grows" (watch a drag rail do a burnout) due to the centrifugal forces of the tread band. The angle of contact approaches zero but never quite does as if it were to, there would be zero contact patch and zero friction. Think of an integration summation formula. The contact patch also grows narrower as the centre of the tyre bulges further. It doesn't actually "lift" itself due to the angle of contact. Temperature plays a part as previously mentioned as well in the differing cf.

As with any problem such as this, locate your points of contact, draw free body diagrams in the plane of contact at each wheel/contact point and look at the magnitude and direction of forces closely and the solution will reveal itself very rapidly, but the key is the reduced contact patch and the variation of dynamic friction.

How do I know this? My undergrad thesis was on the drag losses in a moving wheel and the tyre companies where not very forthcoming with specification detail so I had to develop it myself from first principles and verify through testing. This also meant that I had to understand and read through the many other phenomenon that occur within tyre design and development. I convinced the Head of the school of engineering (a physics professor) and the head of the school of mathematics so there must be something right. I continue to study, with a master of engineering, and now toward employment in race engineering after being exposed to the possibilities by one of the forum members.

Get a new physics teacher if he did those drawings or found them satisfactory. Remember there are four contact points in the car diagram not one and the tyre theory is fundamentally incorrect. He did get the decrease in contact patch right, but for the wrong reason. Also the coefficient of friction is not a static figure with tyres after loss of traction.

[hayai32]

Get a new physics teacher if he did those drawings or found them satisfactory.

I drew them. lol. I only have to go into a little detail and my phys teacher didn't have a prob with them. It's a 1000 word essay so i can't really get everything in. u know how yr 12 is. If ur teach says it's OK don't change it. I'm just ignoring a whole stack of stuff and trying to put what i think has the most importance.

I'd post the essay up but i don't want it to get torn appart. I'd rather just think it's good.

cheers 4 that last post though skylinegeoff, that was gold.

[Duncan]

Geoff, excellent post

do you have a higher quality scan of that article, its a bit hard to read?