Weigh yourself both holding the wheel and not holding it on a bathroom scale. The difference is the weight of the tyre and wheel assembly. In my case, it is 50 pounds (it would be a lot less if I had those $3000 Jongbloed wheels! Any sponsors reading?). Now put the wheel on the ground or on a table and push sideways with your hand against the tyre until it slides. When you push it, push down low near the point where the tyre touches the ground so it doesn't tip over.

The question is, how hard did you have to push to make the tyre slide? You can find out by putting the bathroom scale between your hand and the tyre when you push. This procedure doesn't give a very accurate reading of the force you need to make the tyre slide, but it gives a rough estimate. In my case, on the concrete walkway in front of my house, I had to push with 85 pounds of force (my neighbours don't bother staring at me any more; they're used to my strange antics). On my linoleum kitchen floor, I only had to push with 60 pounds (but my wife does stare at me when I do this stuff in the house). What do these numbers mean?

They mean that, on concrete, my tyre gave me 85 / 50 = 1.70 gees of sideways resistance before sliding. On a linoleum race course (ahem!), I would only be able to get 60 / 50 = 1.20G. We have directly experienced the physics of grip with our bare hands. The fact that the tyre resists sliding, up to a point, is called the grip phenomenon. If you could view the interface between the ground and the tyre with a microscope, you would see complex interactions between long-chain rubber molecules bending, stretching, and locking into concrete molecules creating the grip. Tyre researchers look into the detailed workings of tyres at these levels of detail.

Now, I'm not getting too excited about being able to achieve 1.70G cornering in an autocross. Before I performed this experiment, I frankly expected to see a number below 1G. This rather unbelievable number of 1.70G would certainly not be attainable under driving conditions, but is still a testimony to the rather unbelievable state of tyre technology nowadays. Thirty years ago, engineers believed that one G was theoretically impossible from a tyre. This had all kinds of consequences. It implied, for example, that dragsters could not possibly go faster than 200 miles per hour in a quarter mile: you can go = 198.48 mph if you can keep 1G acceleration all the way down the track. Nowadays, drag racing safety watchdogs are working hard to keep the cars under 300 mph; top fuel dragsters launch at more than 3 gees.