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# Over-Drive Tires

 (+11, -4) [vote for, against]

The inside bead is mounted to a fixed hub, and the outside bead is mounted to a hub that is extended/retracted laterally from the center of the axle by a linear actuator. When fully extended the tire would have a spindle-like shape (long-side making contact with the ground) and when fully retracted the tire would have a doughnut-like shape. ***Did not integrate the volume of tire from one position to the other***

(If you trust my physics feel free to skip this part, if not have at it)

Lets assume that the radius of the tire while extended is .2 meters. Lets assume that the radius of the tire while retracted is .4 meters Lets assume the tire has a mass of 10kg

@ 25 m/sec (approx 55 mph) Angular momentum of extended tire (L1) = 100 kg.m2/sec Angular momentum of retracted tire (L2) = 200 kg.m2/sec (Angular velocity was adjusted to reflect change in circumference)

(if you skipped ahead, come on back.)

This has three good affects.

1. Instead of drive ratio being fixed, the variable radius tire could provide a smoother transmission of power, keeping the engine in its most fuel-efficient rpm rang (or most powerful rpm range)

2. When passing (opening throttle to set %) the tires could be extended for a *little* more oompf (turning the 4 X 100 extra kg.m^2/sec into angular velocity)

3. When (panic) braking the tires could be retracted to slow down the tires (if your not a physics major, think of an ice-skater extending her arms while she spins) reducing speed and gaining momentum. (good thing for an impending collision)

There it is. My first Half-baked Idea. Have at it.

 — MikeD, Mar 24 2008

 I cant find a link but I'm positive this has been done on a concept car.

 I can't find a link since I don't know what the tech is actually called.

Hope I'm wrong.
 — Giblet, Mar 24 2008

not sure I understand what the concept is, but if you indeed have variable sized tires then while making a turn, you could increase the size of the outer tires and do away with differentials.
 — FlyingToaster, Mar 24 2008

 If this could work, it would be a great idea. The basic idea (variable tire diameter) is used on top fuel dragsters, but the method used is centrifugal force instead of varying the bead width. The problem is that tires shaped like donuts would corner as well as using real donuts. Ask anyone who has ever driven an ATV with low tire pressure. Even if you can solve this one, you'd have to tackle the effects of having an elastic tread surface to allow for variable tire diameter while containing enough air pressure to lower rolling resistance.

So to pull this off I think you have to stop using air pressure for support and go with some type of elastic solid tire tread supported by a mechanically variable diameter hub.
 — MisterQED, Mar 24 2008

I like it, but you'll still need transmissions.
 — Voice, Mar 24 2008

There was an RC car that did this many years ago. I don't remember what it was called, or if your could modify the wheels 'on the fly' or not.

 //The problem is that tires shaped like donuts would corner as well as using real donuts//

 Good point. Did not consider this. I would say that being able to decrease radius one side would help, but that would increase the tire speed on the inside (no good). Hmmm. Can the inherent handling problem be solved?

 //I like it, but you'll still need transmissions.//

Agreed. Never meant this to supplant a transmission, but to compliment the transmission between the shift points.
 — MikeD, Mar 25 2008

I like it. [+]
 — david_scothern, Mar 25 2008

There was a car toy that would project claws from the tire treads when it got bogged down. Weird to watch.
 — elhigh, Mar 25 2008

+ - a newby completely devoid of troll-like characteristics - what a pleasure! Welcome [MikeD].
 — ConsulFlaminicus, Mar 25 2008

 I think handling and control during the transition from one mode to the other would be impossible. I certainly wouldn't want to drive it any faster than 12 mph, but it might be fun on the sand dunes.

I remember both those toys. The one with the claws was 'the Animal.' The other one I don't remember the name of, but it had slices in the rubber tire that allowed the sliding external hub to expand the tire's outer radius.
 — RayfordSteele, Mar 25 2008

Thank you much [Consul] I try, I try. I'm pretty pleased with myself too, actually. Fist one and I got a whole croisant!
 — MikeD, Mar 25 2008

Interesting idea, wonder how much , after working out the kinks, this would help..
 — XValentine, Mar 25 2008

 i think you overlooked some of the side effects of this such at increased tire wear and adding weight to the wheel. the added weight i would think would neglect any benefit that would be gained by putting the engine at it's optimal rpm.

but as for cornering grip you can always get bigger tires and stickier tires.
 — F_R_O_G, Mar 26 2008

 //the added weight i would think would neglect any benefit that would be gained by putting the engine at it's optimal rpm// Tire weight is really only going to come into effect during changes in speed. Frictional Drag is going to be the same, no matter how much tire is in contact with the road (The more tire surface area touching the road, the less force per square whatever) The main Idea is

 1. Variable Radius Tires equals, (more or less), Variable drive ration (In every gear) (The big trucks have 19? gears for fuel effeciency)

 2. The tire could act as an angular momentum resevoir.

Unless I am mistaken, the extra weight (if there be any at all) really wouldn't come into play much, unless I have an unrealistic concept of the fuel requirements to exert an extra 400 L, as compared to the fuel economy of having all the gear ratios in the transmission, plus half of all those ratios, plus everything between. I think the only draw back I'm seeing here is A. handling and B. the possibility that this is already baked.
 — MikeD, Mar 26 2008

 I am afraid that the concept of angular momentum and the conservation of such has been missed. Cute little ice skater not withstanding. While the extending of the arms of the rotating ice skater does slow the rotational speed, it does not in fact slow down the arms and hands of the ice skater.

 Let us consider.. If an object on a 20 inch string is rotating about a central axis at lets say 1 rev per sec, and if the string is then shortened to 4 inches the object's rotational rate will increase to 5 revs per second. However, when taking the path into account, 2 x pi x radius, times the number of orbits per second, we find that the distance/sec traveled by the object remains a constant, therefore the change in rotational rate due to the increase in radius of the tire will have no impact on the speed. ie. point three, panic braking, throw it out.

MikeD, I thought I taught you better than that.
 — Ozone, Apr 07 2008

What are you talking about? Are you claiming that variable sized tires won't change speed? If so, you are wrong, as I said, the system is used on top fuel dragsters.
 — MisterQED, Apr 07 2008

 OK, let's keep this simple

Object A is moving at 10 pi ft per sec As it passes within 1 foot of object B (tire close in) object A is captered in a cirular orbit around object B. Momentum is conserved so the object circles object B at the rate of 5 revs per sec. (circ = 2 pi r, = 2 pi , 10 pi / 2 pi = 5). Next, object A is moving at 10 pi ft per sec. As it passes within 5 feet of object B (tire stretched) object A is captured in a circular orbit around object B. Momentum is conserved so object A circles object B at a rate of 1 rev per sec. (circ.. = 2 pi r = 10 pi, 10 pi / 10 pi = 1) The angular velocity is much different, but the tangential velocity, where the rubber meets the road so to speak, is the same.
 — Ozone, Apr 10 2008

 Yes, but angular velocity is what gearing is all about. Shift gears in your car and see if the angular velocity of the tires changes? Well chances are it didn't.

 The purpose of gearing is to keep rpm sensitive motors within their correct range.

 Now imagine if you could shift in tiny increments applying enough gas the keep the revs constant, and guess what, you speed up.

Nice "blind with science" attempt though, but angular momentum is only conserved in a static system.
 — MisterQED, Apr 10 2008

 Often the simplest design is the best. The benefits from the design are nice. I'd hate to see what happens if any of these moving parts broke. Not to mention the additional weight of these moving parts, which will have to be quite strong to support a car. The belts inside of the tire will start to slip quite quickly as well, unless I'm misunderstanding.

Conversely, a regular tire doesn't have to rely on moving parts. A Constant Velocity Transmission will already keep engine rpm's at optimum power, why risk a tire flying off? Ask a helicopter pilot what a "Jesus Bolt" is and why they cost 7 grand. He'll tell you it holds the rotors on, and keeps him from meeting Jesus.
 — RXAaron, Apr 11 2008

 I like this idea very much but would extend it further so that the steering would use this method. With a small radius on the inside and large on the outside the car would lean into corners.

You might get seasick trying to park tho'.
 — Gordon Comstock, Apr 11 2008

 +

 Only negative is the steering affect... and why have four gear boxes when you can have one...

There will be a volume change when the actuator moves so a pump will be required to maintain the correct tyre pressure (or did someone mention that already)...