Oh no!

Talkemada won't accept some basic physics!

You have kind of got it there. The weight of the bridge is the equiv of the force from the tyre (each unit area of the bridge exerts a force due to mass same as each unit area of the tyre exerts as force due to pressure). i.e. double the width and the brdge weighs twice as much. The cables are the equiv of the rim. This is fixed in size/strength. The only thing resisting the force from the weight of the bridge is the cables so now they are twice has heavily loaded, same as the rim is twice as heavily loaded as the tyre exerts twice the force.

Actually the bridge analogy is not bad. Part of what I tried to explain is that more tyre with the same pressure acting on it (per unit area of tyre) will mean more force on the rim. Which is quite a lot like more bridge of the same density means more weight on the cables.

So to carry on that analogy, the rim strength is the strength of the cables holding up the bridge. A small tyre is a narrow bridge; a big tyre is a wide bridge. Increasing the pressure is like adding more material uniformly to the bridge: you can make the narrow bridge much thicker before the cables snap, whereas because the wide bridge has much more area, you can only make it a little thicker (all over) before the cables snap.

Kind of convoluted, but hope that helps.

: P

Talkemada won't accept some basic physics!

Why accept something you don't fully understand?

See, in just a few posts, one person has said 'no that's not what we're talking at all', to two others telling me that I'm on the right track! Can you see how confuddling this is for me? 😀

Seriously, this is not a troll. I simply don't understand it, that's all. I'm sure there's probably other stuff that I have in-depth knowledge of, that others might not 'get'. Make sense?

So, I'm trying to understand how it works. And maybe others would find it interesting, I don't know. But to say 'oh you're stupid you don't understand just accept it' isn't really very helpful. Why not go away and have a think about your abilities to explain stuff?

I'm sort of starting to understand it, but I'm still having difficulty in getting how the bridge analogy applies to a rim. In my head, the bridge analogy is sort of opposite to the tyre/rim scenario. Because I'm thinking that the increased load on each cable is like an increase in pressure. Can you see where I'm coming from? The 'bridge' thing is my way of trying to understand the force/area/pressure thing that youse have explained to me. The next step is trying to work out ho that applies to a structure like a rim and tyre combo. I'm having difficulty in understanding why the tyre beads would be pulling on the rim walls more, if the volume is increased. Surely that force would be the same regardless of the tyre volume, given the same pressure?

I think that's the bit we're all trying to explain and why people keep repeating that pressure and force are very different things.

I weigh 75kg. That means the FORCE I exert on the earth due to gravity is 75kg (assuming g=10ms^-2 and not using Newtons as a unit for the moment)

If I stand on the floor in my Vans, the surface area of the soles is, let's say (25x10cm^2 x2 = ) 500cm^2. So the PRESSURE I exert on the floor is 75kg per 500cm^2, or 0.15kg per square centimetre.

However, if I put on pair of stiletto heels and stand on just one heel, I haven't changed my weight, so the FORCE I exert is exactly the same. But this time all that force is concentrated on an area, let's say, 1 sqaure centimetre in size. The PRESSURE I'm exerting is now 75kg in just one square centimetre.

Does this make sense so far?

: P

It's seems CRC would like me to run a Grand Prix on my rear wheel, having sent me a 700c example instead of the Speed King 2.3 I ordered. Taking the lower tread / fasting running rear tyre concept to the logical conclusion?

However, if I put on pair of stiletto heels

Kinky! 😮

I understand that, I just don't understand how a fatter tyre at the same pressure can exert a greater force on the rim, at the same pressure as as skinny one.

My way of looking at it is like this: The rim always has the same surface area, so the only change in surface area, is in the tyre. The increased volume of air may well be exerting more force because of the increase in surface area of the tyre, but that extra force is exerted outwards in all directions, not just against the rim. So, the force created by the pressure, is still the same regardless of tyre size, no? How does the bigger tyre then become a stronger lever to push/pull against the rim's edge, if the pressure is the same? Surely all that extra force is being used to push against the extra surface of the tyre?

You've got to think of that force becoming tension in the tyre carcass which is then pulling the bead off the rim.

Oh so now 'force' becomes 'tension'?

And you wnder why I'm confuddled?? 😯

You're right that the forces from the air inside the rim do not change – after all, as you say, the rim has the same surface area, the air pressure is the same, so the force it exerts on the rim is the same.

I think you may not be seeing how the forces on the tyre are turned into forces pulling on the rim.

The bigger tyre, as you say, has more surface area so has more force acting on it. However, because the tyre does not expand (or, at least, doesn't expand very much – as someone else said, double your air pressure and your tyre will not double in size), those increased forces don't just act to expand the tyre.

OK, an imagination experiment:

Imagine you have a small cereal box, one of those little variety pack ones. You've taken all the cereal out and left the flaps open. You turn it upside down and sellotape the flaps to a table so it's "open" side down and the flaps are sticking out, taped to the table. Now you drill a hole through the bottom of the table under the box and put a balloon through it. Then you try to inflate the balloon. It will need quite a bit of puff, but sooner or later as you blow up the balloon it will force the box off the table. The force from the air pressure inside the balloon will not be able to expand the box, so it will be released in the only remaining direction, and lift the box away from the table.

Does that make sense?

Where the tyre analogy comes in is that if you instead use a full size cereal box, or maybe a shoe box, instead (and a bigger balloon), you won't need as much air pressure. As before, the air pressure will push against the sides of the box and they'll push back, meaning that the force ends up pushing against the table and lifting the box up.

Granted, this is not an exact analogy, the forces inside the tyre and rim are in slightly different directions, but the general principle is the same…

: P

…and yes, tension is a force. It's a pulling force instead of a pushing force but it's still a force. A force has only magnitude and direction.

: P