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It should be clear for anyone that friction force between a rubber tyre and road's asphalt is much bigger than friction force between the same tyre and the metal wheel around which that tyre is installed.

So, how comes that the tyre does not slide around the wheel during strong braking?

  • Welcome to Motor Vehicle Maintenance & Repair! – kyle_engineer Dec 27 '17 at 1:53
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Empirical data would suggest that you have this backwards. The friction between the tire and rim is greater than between the tire and the road surface. The only examples of vehicles where the tires are literally screwed to the rim to avoid slippage is off-road/rock crawling and drag racing. There may be other situations...

There is a reason why it can be difficult to seat the bead when changing a tire. It's a snug fit! Couple that with the fact that the friction surface area for the beads are long, thin and a continuous 360 degrees and the friction surface for a tire is an oval acting on one small part of the tire at any point in time.

There is no magic keeping the tire attached to the wheel, it's all friction.

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    +1, "the friction surface area for the beads are long, thin and a continuous 360 degrees", there is also the additional ~32 p.s.i. force applied to the bead against the rim, by the air pressure inside the tire, yes? – Jimmy Fix-it Dec 26 '17 at 18:41
  • @JimmyFix-it once the beads are seated can you move the tyre around the rim even if you deflate the tyre? Even partially deflated ie on sand the tyres don’t tend to move... – Solar Mike Dec 26 '17 at 21:57
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    Can't say that I remember trying to spin a deflated tire on the rim with the bead set... I would imagine it could happen if I had recently installed the tire, with the bead freshly lubed (we always lubed the beads with a watery soap solution before installing, I think mostly to help stretch the tire over the rim, maybe it helped hold a tiny bit of air too, to help set the bead?) – Jimmy Fix-it Dec 26 '17 at 22:50
  • @JimmyFix-it perhaps you should try it - personally I have had to re-break the beads to relocate the tyre, when some specify the valve position etc – Solar Mike Dec 27 '17 at 11:10
  • Could you provide some link with the respective friction coefficients? I still find it difficult to believe that friction of rubber against metal is greater than friction against asphalt, even taking into account the bigger contact surface.. – MadHatter Dec 27 '17 at 12:26
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While I completely agree with Tim Nevins' answer, I wanted to throw some simple mathematics in here so we can see where the truth is ...

On this website they talk about the contact patch of a P225/60R16 tire. They state the total road area where the rubber meets the road is calculated to be about 147 square inches with the tire at 32psi. If we were to calculate the area of where the rubber of the bead meets the rim, I think the following could be a fair approximation (as I don't have the exact numbers in front of me).

Given:

  • Rim Diameter = 16"
  • Tire contact area ~1/2" around circumference of the rim
  • Tire contact area ~1/2" at the side of the bead (lip of the rim)
  • Two beads

Area computed

  • Circumference of the rim = 16 * π or 16 * 3.14159 = 50.26544
  • Tire contact around rim = 50.26544 * 1/2" = 25.13272 square inches
  • Tire contact around lip of rim = 50.26544 * 1/2" = 25.13272 square inches
  • Square inches per bead = 25.13272 + 25.13272 = 50.26544 square inches
  • Two beads per tire = 50.26544 * 2 = 100.53088 square inches
  • Four tires per car = 100.53088 * 4 = 402.12352 square inches

Compare the total contact patch area of 147 square inches to the total amount of just over 400 square inches which attach the tire to the rim and I think it's obvious your original conclusion has holes in it. Please note, this doesn't take into account the friction which is produced from the weight of the car as it rests upon the contact patch, nor does it take into account the force applied by the steel band which keeps the bead seated upon the rim. Still, though, if we are looking at just contact of rubber to its mating surface, the area of tire to the rim is quite a bit more than the amount of tire which actually touches the ground.

  • I agree about size of contact surfaces. It is the friction coefficients which leaves me a bit perplexed... – MadHatter Dec 27 '17 at 12:22

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