Spherical Tyres for Cars

You can save all fuel, because the car will not steer and it will crash into something.

This is a joke, right?

I am not sure that you are right in your assumption of not steering. Even a spherical tire has a transverse rigidity and this what allows a change in direction since in the plane normal to rotating axis the resistance is less. The radius will be more important but steering could be maintained.

No! You fail to take the absence of contact area into your statement.

You can't steer a car that is riding on balls.

I think you make again an error: he mentioned "spherical TYRES".

This means that it is a spherical TIRE filled up, as all tires, with compressed air and which transfers the load via a deformation (elastic) since in fact the contact area depends on the envelope compliance and the internal pressure. This is, for instance, the reason why designs for low consistency ground have large tires in order to have a bigger contact surface and reduce the filling pressure. Auto tires are, because of the high rolling speed and generated - centripetal (!)- forces, quite rigid but a great part of load is taken by internal pressure. This is the reason you are obliged to be sure that pressure is within limits since if it is too low tires deform too much and present bending fatigue with all unpleasant consequences.

Even if the tire would be stiff-which is not the case- it will have a contact with the ground and this contact will be related to a friction and because of it to an adherence to the road and where is adherence there is a capability to steer.

Unfortunately your thought and subsequent justification is -dare I say- wrong.

You know what I advance is only basic physics.

Gee, why not? We steer a lot of posters here who "...ride on balls..."

Now, just hold on there a second. He may be on to something.

Motorcycle tires are more spherical than automobile tires, and we all know motorcycles get better gas mileage.

So imagine the possibilities. Think of a car that made almost no contact with the road: a car that jumps like a cricket. The energy savings would be ginormous.

I'm calling it the Holleymobile.

On a motorbike the plane of the wheels can vary from the vertical, so a spherical or cylindrical profile makers sense. On a 4-wheeler it can't vary (much) from design angle.

But if there were spherical tyres on cars, and they had less contact area (which doesn't necessarily follow) I can't see an obvious reason that would give better fuel economy.

Why should they have less contact area ? Please explain.

I didn't say they would. The OP said that, I was questioning whether it's necessarily true.

I suppose the contact area depends on the diameter, how truly spherical the tyre is, inflation pressure and maybe other factors.

Please elaborate.

I just meant I don't suppose it would be a true sphere (apart from it having an axle shaft of some sort through the middle), but with a flatter section in contact with the road. The bigger the diameter, the greater the contact area, both circumferentially and axially. And higher pressure clearly reduces the way it "spreads" on the road.

What do you think, you brought the concept up!

to reduce area of contact/friction

If you reduce to nil the load then you will have a smaller contact area and of course less friction. Use a balloon to sky hook the car and the problem is solved!

May be we misunderstood your concept and this was the idea of the spherical tire ?

Why would reducing area of contact/friction give better economy? The point of tyres is to grip the road, not slide across. In any case friction force does not vary with the area, only with the downward force (weight).

Which ever is the tire geometry the contact area dependds on load, tire stiffness and filling pressure. The contact is the zone where the load (weight + dynamic effects) are transfered to the ground it will NEVER depend only on the geometry.

I your thoughts you considered a high stiffness tire but this does not change the FORCE transfer and the correlated friction.

Losses at this level are correlated to 2 phenomena:

- the tire own hysteresis

- the tire sliding on the ground.

Why would a larger diameter tire equate to a wider (axial) contact patch, ceteris paribus?

I thought it was fairly clear intuitively, but a bit of math(s) follows.

Just treating it as a sphere (no band of tread round the middle, to look a bit like a ordinary tyre).

If sphere radius = R, and due to the car weight the radius is reduced by a fixed distance h at point of contact, and the radius of the contact patch = r, by the property of intersecting chords r² = (2*R + h)*h. As h << 2*R, r² = 2*R*h so area of patch pi*r² varies as R.

If instead the radius reduction is not fixed, but is a given fraction of R, say k*R, r² = 2*R²*k so area of patch varies as R².

We came to the result that the contact surface transmits the load to the ground and according to what was written before tires are quite flexible and o not offer a too important resistance. This means that the force will be transmitted by the filling pressure and if this is the case which ever the sphere radius is the area is the same for the same load and the same pressure.

Possibly, but I have difficulty imagining the contact area halving if inflation pressure is doubled. Maybe some of the force due to the increased pressure is taken by increased tension in the tyre wall. Also a quick estimate for my car of contact area (4 lots) and recommended pressure comes to rather more than the weight of the car.

Clearly the tyre wall has hoop tensile stress everywhere except the bit above the road contact area. Your analysis (I think) would require the stress to be zero there, but maybe it isn't.

False dichotomy. Neither changing at a fixed ratio of the spherical radius, nor the same change in height is correct for the addition of the same load to inflated spheres (thin nonrigid skin) of different radius inflated to the same pressure.

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When you place an inflated sphere on a flat level surface, when it first touches the surface the contact is essentially a point, assuming the sphere is a sphere and the surface is flat. This leads to the weight of the sphere deforming the wall near the point of contact. The pressure inside opposes the deformation. The area of deformation grows until the forces in opposite directions cancel.

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The hoop stress is essentially orthagonal to the load/normal at the point of contact, so that isn't a good place to search.

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If the pressure were to remain constant, then the contacts patch would be the same for the same load and same pressure for spheres of different radii. However, since the volume difference makes up a larger portion compared to total volume for smaller spheres, the contact patch will be slightly smaller for smaller spheres. The amount smaller will depend on a couple different factors and which will not allow it to be well approximated as proportional to change in a single variable.

OK point taken.

One more thought I had - for a conventional tyre the wall tension is constant, say T, everywhere except above the road contact area. I'd expect tension is zero along a vertical line above the centre of the contact area (ignoring caster and camber), but increasing to T as you go out to the limits of the contact area. That means the average tension is > 0 over that part of the wall, so total upward force is that + the force from the road. In that case the pressure needs to be a little higher than calculated from vehicle weight/contact area.

At the contact area center you still have stress in the tire since pressure acts on the sphere wall and pulls it in both directions axial and transverse.

The tire internal pressure will be modified depending on the volume change due to the deformation under load.

I was basing my remarks on a conventional tyre, not a spherical one.

Your 2nd para - TINAC made that point in #27.

I mentioned the volume change and it is a function of the tire stiffness since if pressure grows also the tire volume will increase. I do agree with what TINAC wrote but I only wanted to make a point that not only the radius makes the difference.

Sorry if I did not understand your reference. I hope you do not mind.

Not at all, thanks for your comments. You've probably added more to this discussion than I have, but I think it's been kicked to death by now .

It is quite normal when the discussion leaves the "opinion" level then most of the participants are no more interested.

I suppose if the inflation pressure is low and/or elastic modulus x wall thickness is high such that the volume of by the sphere is only greater than the minimum (unstretched) full volume by an amount less than the volume reduction that occurs as the.wall deforms under load, then there would be areas in the contact patch with reduced tension, perhaps zero or even under slight compression, then concentric.rings as the wall buckles. This could happen as the flat surface deforms the wall since the flat section has less area than the surface, so long as the.surrounding skin did not exhibit excessive strain, and the skin in the contact patch had stretch less than that from the relaxed state.

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I know, I know, beating dead whores is passe'. But really, no one is looking. Who's it going to hurt anyway?

Maybe though with less grip a fatality will soon follow, Mildred.

This entire thread is a waste of time.

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What is the difference between the tires on a mountain bike, and a hi-speed racing bike? Mb has wider tires to get a better grip on soft or un even surfaces. Racing bikes have narrow hard tires to minimize rolling resistance.

Tires designed for high mileage vehicles are also narrower, and at higher inflations to accomplish the same thing.

But, on the crop of high powered vehicles, the more tire rubber they can put on the road, the better. Somehow the larger contact area helps to keep the tires from spinning when the gas pedal is depressed.

Pressure definitely has a huge effect. Rubber is not a great spring, I.e. not much of the energy required to bend the sidewall as well as the tread area is recovered when as that portion of the tire returns to the undistorted shape.

This leads.to wheels that have less tire material from high mileage or designed foow rolling resistance, tend to give improvements in fuel economy (conversely, this is a reason mileage drops typically with new tires). This is also the major factor in increased rolling resistance at lower temperatures, the reason some semi trucks are switching to super singles instead duals on each axle and the main reason higher pressure leads to lower rolling resistance.

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It isn't so much the size of the contact patch, but the energy used in changing the shape of the tire that dominates.rolling resistance.

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Road bikes utilize thinner tires for a number of reasons, the two big reasons are weight and aerodynamics. Rolling resistance wouldn't be all that different for thin tires compared to wider similar construction tires at similar pressures pressures. The additional weight would dose.net please most cyclists and the width of excessive would result in a noticable increase in effort required to attain the same high speed.

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While there is a very small amount of sliding (at small scale) as a tire distorts rolling under load, it isn,'t the the same as sliding resulting from heavy.application if brakes at speed. Narrower tires can offer reduced resistance but it isn't typically a large reduction in rolling resistance.

Just stopped by.

Has anyone considered the implications of having 4 spheres under a car?

Let's see. A 30 inch tire is about 8 inches wide. Add some tunring space for the tires and you still are nowhere near the space required for 4 SPHERES under the fenders.

That's 22 extra inches of space you don't have. That's 11 inches under the car 11inches outside the body.

This is a non-starter, still.

Have fun.

IF OP is getting his idea from the spherical vehicle tyres shown in the movie I ROBOT

I would first advise him to leave science fiction as just that -FICTION.

I agree with Lyn that there is no practical method to steer because the

System is not considered practical.

After skimming all of the above, I failed to see anyone note that the Sunracer vehicles have very thin hard tires for the very purpose of reducing rolling friction and the same concept applies for high pressure tires on touring and pro-racing bicycles.

With respect to the "no turning ability" statements, apparently those folks have never watched a GPMoto unlimited class motorcycle road race. The basically spherical and elliptical tires on those bikes allow them to be driven through curves at over 150 mph with a rider and bike lean up to a measured 60 degrees from the vertical. (And yes it does appear to violate all laws of physics until you recognize that even though the bike and rider are almost horizontal, the total weight of the bike and rider is still applied vertically downward on the bike tires to provide traction to resist their lateral G load from the turning.)

Next, if you take a look at some automotive tire history, you will find that the first pneumatic high pressure automobile tires (after they replaced the original hard rubber tires) were very close to spherical because at that time they were based upon bicycle tire designs; and the fact that, because they were high pressure and there was no "belted tire design" the near spherical shape due to the high inflation pressure was unavoidable.

The real problems with a spherical or a thin line contact tire on an automobile is principally due to the high vehicle weight and as a result of that are two fold:

First, the tires will have to be inflated to a very high pressure to support a 3000 lb vehicle and maintain their shape and, be of a very hard unobtainium rubber (or polymer) compound to survive the lateral turning loads of the vehicle without the material being simply abraded off. A compound like that will also have a very low coefficient of friction (similar to the high carbon black content tires of some 50 years ago that wore pretty well; but, whose grip on wet asphalt was negligible when turning at any substantial speed).

Second, the very small patch size would have a very short tread life and be subject to loss of traction due any small area of water, oil, sand etc.

Bottom line, correctly designed spherical tires on an automobile will reduce the rolling friction and give increased fuel mileage, but you would have to turn very gently, replace them frequently and when turning be diligent in watching for any sand, oil etc on the road (much as motorcycle riders already must do). As an example of something currently very close to that design, take a look at one of the small high pressure limited speed and distance spare tires supplied with most current automobiles.

While technically it is true that a sphere is a degenerate form of a torus, it is not correct to call a toroid (non degenerate) a sphere, or even 'nearly spherical'.

The geometry of a tire and the rim upon which it is mounted can reasonably be described as toroid, not a spheroid.. Note, the shape is homeomorphic with a donut, not a cupcake.

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Even though you failed to notice the discussion about bike tires, there is a fair amount there. While higher pressure does tend to reduce rolling resistance (up to a point), a skinnier tire does not reduce rolling resistance in and of itself.

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I am further very skeptical of the example you provide at the end.. You seem to imply that cars would get better fuel economy if all four wheel were swaped with reduced-size spare tires.

Uh, you first. Have any basis for that? a if that wasn't what you meant, what did you intend to communicate with that 'example'.?

It occurs to me as well that there would be a different wear pattern imposed on the road surfaces, one which they were not designed to endure.

First, I have already been advised about my incorrect application of the term spherical, no excuse, I simply became so focused on the small contact area issue that I just went into a term repetition mode when referring to the shape. Bottom line, I clearly need to be more diligent in checking my posts before submitting them.

I admit I overlooked the earlier bicycle post. By the time I first viewed this thread there were so many posts that simply weaved back and forth around the principle issue that I skimmed through a bit.

My comment regarding the small spares was only meant to refer to their more circular cross section than a standard tire. My statement was not intended to suggest it had any actual benefit as to rolling friction.

I agree that the increased pressure is the principle factor in reducing tire flex and rolling friction.