Only 1g ?

[djones]

Is it true that a car can only pull a maximum of 1g without the help of aerodynamics? Regardless of tyres.

[RH1300S]

No, it's not true

I think you may be thinking of (is it Newton's) the law that relates to friction between SMOOTH bodies.

A nice anecdote I have heard (true or not) - for years scientists were saying that a car could never acclerate at more than 1g - unfortunately no-one told the drag racers that - and pretty soon they exceed 1g without aero downforce.

The mechanical (the compound deforms to grab the road) and chemical (literally sticky) grip of the tyre helps it hold onto the road surface.

I believe that it is quite common for a race tyre (in many series) to be able to exert more than 2g without aero help.

[Tom]

I was on the swings today after a maths exam and a thought occured to me; what kind of force is experienced on a swing?

[MrT]

Centrepetial force some will say centrifugal force but this in the world of physics is not a 'real' force!

[Sawtooth-spike]

Correct me if i am wrong but everything on earth taking 1G's worth of presure as that earth gravity?

[Tom]

1G downwards, yes. Not on the subject of G-force but interesting still. The air pressure above our heads is equivalent to a small car on all our heads.

[modbaraban]

this is deffinitely more than 1G with no wings...

[Saribro]

Let's see if I can make some sense of this:

The basic Newtonian law is Force = mass*acceleration
The force is the frictional force from the tyres, the mass is that of the car, we can express acceleration as a factor 'x' of gravity, do some rewriting:

F/m = x*g

Frictional force is calculated as µ*N, where 'µ' is the frictional constant between the tyre and the roadsurface, and 'N' is the normal force between the car and the road.

(µ*N)/m = x*g (we'll get back to this)

N can be expressed as m*g, so we get:

(µ*m*g)/m = x*g

the masses cancel out, and we are left with:

µ*g = x*g

so the amount of g's a car can pull is all in the tyres. If you have tyres with µ > 1, you can do more than 1g.


Now we'll see what downforce 'D' adds into our earlier equations.
With downforce, N gets another term, so that N = m*g + D
We subsitute it into our earlier equation and get:

(µ*(m*g + D))/m = x*g

To make it easier to interpret this, we'll do some rewriting again:

µ*g + µ*(D/m) = x*g
Since we know the value of N, we can formulate D as a factor 'd' of N so that D = d*N, and D/m = (d*m*g)/m = d*g, resulting in:
µ*g + µ*d*g = x*g
g cancels out, grouping some terms:

µ*(1+d) = x

And here we have it. What downforce essentially does, is increase the frictional factor between the tyres and the roadsurface (lift would result in a negative d, thus decreasing it).
For a tyre, µ is not the same in all directions (think slip angles), and downforce varies with speed, so you can imagine that understanding the grip of a car can get pretty complex.

I hope I haven't made it too messy :).

[ginsu]

Is it true that a car can only pull a maximum of 1g without the help of aerodynamics? Regardless of tyres.

I believe there was an old belief that 'mu' could not be over 1g for a racing tire, but that has long been proven wrong.

Saribro, I agree on all the math, although the downforce and tyre dynamics is simplified. But, how did you type the letter 'mu'?

[ds.raikkonen]

Centrepetial force some will say centrifugal force but this in the world of physics is not a 'real' force!

http://en.wikipedia.org/wiki/Centrifugal_force

1G downwards, yes. Not on the subject of G-force but interesting still. The air pressure above our heads is equivalent to a small car on all our heads.

Actually, the pressure on our head is equal to the weight of an elephant!

[djones]

Thanks for the replies.

It was a Physics professor that told me about it. He did at the time say unless there was aero OR some sort of chemical stickyness so I guess he was actually right.

I didn't say about the chemical stick because I never actually thought about they tyres 'sticking' so thought it was irrelivent.

[RH1300S]

No he wasn't right.........

Tyres produce mechanical grip (he would be right if the tyre was smooth and hard and sitting on a smooth surface).

The road surface is irregular and the surface of the tyre deforms to "grab" the surface. Try and imagine it as if the tyre interlocks with the texture of the road (we are talking very small irregularities here - but they all count). Push the tyre sideways and it will need more than 1g to make it lose grip.

This happens regardless of chemical grip.

My understanding is that Michelin and Bridgestone employed quite different philosophies with their tyres. Michelin used more chemical grip. Which is why their tyres were constructed differently and why Michelin cars would use visibly different amounts of camber to the Bridgestone cars.

P.S. Road cars will often not manage to achieve 1g.......I SUSPECT (but am not sure) this is not the tyre at fault, but other factors - such as supension compromises that keep them below 1g.

[Ciro Pabón]

If you wish you can download a word document I wrote for general knowledge:

Tire Grip for Road Designers

There, as RH1300S suggest, I made a simple calculation for a dragster, which essentially develops very little or no downforce at all. A dragster goes from 0 to 480 km/h in one quarter of a mile (410 m). Thus:



This is over twice the acceleration of gravity and implies a friction coefficient of 2.2.

I believe your professor is plainly wrong: the weight of the car is not the only thing that limits the adherence, but the interlocking and sticking you get between tires and asphalt. If you could develop a perfect interlocking (think of a "funicular" with gears) you would, in theory, get infinite "friction factor". Same consideration applies if you could develop perfect "stickiness" between the asphalt and the tire.

F1 and dragster tires "work" mainly by adhesion. Therefore, slicks. It is very important for any aspiring driver to read about it.

Perhaps your professor could find interesting the theory of friction developed by Bo Persson, that I've mentioned several times here. I give the links again:

Why Tires Grip The Road: New Theory Reduces Testing

and

Theory of rubber friction and contact mechanics

and

Elastoplastic Contact between Randomly Rough Surfaces

Essentially, what Mr. Persson proved is that if you see the asphalt as a fractal surface (remember fractals?) the area of contact between tyre and asphalt increases proportional to the force you put on the tyre.

There are several "modes" of developing friction, from an "interlocking" mode to a "sticky" mode. The interlocking works at macroscopic scales, the stickiness work at molecular scales.

This means that the tyre "grabs" the small rocks in the asphalt and also "sticks" to the smooth parts of the tarmac. In the following image, which I also repost, the "interlocking" occurs when you see the tyre at the 5 mm level (macrotexture) while the "stickiness" happens at the microtexture level.

[img::]http://www.vti.se/Nordic/2-99mapp/pics/texturer.gif[/img]

Therefore, the references to the "physics of smooth bodies" don't apply: the smoother the surface, the more adhesion you can develop by chemical, electrostatic or even dispersive or diffusive mechanisms (and less macroscopical interlocking or "mechanical adhesion"), which are the five mechanisms developed to explain adhesion.

There is an optimal asphalt-and-rubber texture where the sum of the five modes of adhesion reach a maximum.

I quote from one of the links I give:

..dry-weather tires in Formula One racing ... exude resins and actually even out irregularities in the asphalt, thus considerably improving the area of contact... Racing tires are literally sucked dry.

So, F1 racing tyres work as the feet of a fly: they stick to surfaces, developing coefficients of friction well beyond 1.

Finally, next person who talks about "centrifugal force" should be chastised for not reading enough. You cannot drive properly if you believe in it.

[mep]

As I flew over the text I recognised that you didn't take
weight balance into account.
The F1 car is only rear wheel driven so lets say 40%
of the force on the driven wheels are lost for acceleration.
Some of it can be recovered by the weight shifting caused by
the acceleration.

[ginsu]

To add another aspect to tyre grip:

I recently found out about the 'Hallum Racing Enterprises' tyre model, and this is the first one that i've found that incorporates heating along with mechanical and adhesive grip forces. When you factor in heating you have a lot of energy being dissipated into the tyre as you drive around the track. The energy lost in the tyre is due to 'hysteresis' (which is the time a material takes to return to it's original state once deformed). The higher the hysteresis, basically, the more energy lost in the tire, and the more grip you have. Of course, if there is too much energy going into the tire, then it will overheat, get greasy, and lose grip. So there's obviously a fine line, and when drivers talk about finding the limit, in reality they are really finding the point where the tire is at just the right temperature for maximum grip.

A Bridgestone engineer summed it up the best when he dropped two balls from about chest height. One was made with 'F1 tyre rubber' and the other was normal 'road car rubber'. When he dropped the road car rubber ball it bounced back to about the same height from where he dropped it. When he dropped the F1 tyre rubber ball, it just stuck to the ground like silly putty. Very different response.