# Effect of wheels turning at different speed?

1. Dec 24, 2008

### Annon

Hi, I'm new here.

I've something in mind for sometime now, that is what happen if a car has its front and rear wheels turning at different speed? Say, the front wheels can cover 100km/hr while the rear ones can only cover 75km/hr. What happen to the car average speed (I guess its less than 100km/hr)? How is that so?

Also, I wonder if this problem is relevant to kicking vs "crawling" speed in front crawl (freestyle) swimming? I've just started swimming recently maybe I could get some good tips here hehe

2. Dec 24, 2008

### Staff: Mentor

Welcome fo the PF. If the front and rear wheels have enough traction so they are not slipping, then they will have the same rotational rates (assuming equal diameter tires). If there is less traction so that the tires can slip, then you will get vehicle motion that is somewhere between the two rotational rates (i.e., one set of tires will be slipping one way, and the other set will be slipping the other way). The exact resulting speed will depend on the weight distribution front/back, and the amount of traction seen by each pair of tires.

BTW, do you know what the function of the "differential" is in car axles?

On the swimming question, for most speeds of freestyle, the legs and feet only provide a fraction of the propulsion that you are getting from your arms. In long-distance freestyle swimming, the legs are generally just used for maintaining rotational rhythm, and much less for propulsion. In sprint freestyle, your feet and legs provide a fair bit of the propulsion.

I've seen a very good free swim video webste, but do not remember the name offhand. Here's a web page with a nice clear video of freestyle technique, but not much tutorial info that I can see:

.

3. Dec 24, 2008

### Staff: Mentor

BTW, since you are swimming in a fluid, the propulsion is much more about lift and drag, and not about traction. As you get better at swimming, you will pay a lot of attention to the lift generated by different parts of your stroke, and the drag created by various parts of your body at different parts of your stroke.

4. Dec 24, 2008

### Annon

Hmm maybe I should make myself clear first that what I'm imagining is that there are two engines for each front and rear wheels set which make the front and rear wheels turning at different rates.... From here I cant imagine what happen if both rear and front wheels cant slip (the car stretches??) so I imagine that the faster one has to "drag" the slower one (the slower one has to slip) so we get the resulting car speed from somewhere between the two rotational rates like you said. Is my thinking correct?

Just want to make my head clear, how about if we supposedly have a rocket fitted with two different engines each in front and rear side. They burn at different rates so they provide different propulsive force to the rocket. The resulting force working on the rocket equals to front engine force + rear engine force right?

And lastly thanks for your info about swimming But I'm still not sure what analogy is applicable to the freestlye swimming problem.

5. Dec 25, 2008

### Danger

You're right about the rocket. The car, on the other hand, has different constraints imposed upon it. In the situation that you propose, something would break. It happens occasionally with 4 x 4 vehicles when someone uses mismatched front and rear axles. I seem to recall that the first civilian Hummers had a bit of a problem with that.

6. Dec 25, 2008

### Staff: Mentor

Swimming is much like your dual rocket thought experiment, assuming the rocket is still in the atmosphere and experiencing drag. Each rocket engine contributes some thrust (resulting from the expelling of mass out the back of the engine), and different parts of the rocket body contribute drag. To make the best rocket (swimmer), you want to maximize lift(thrust) while minimizing drag.

Last edited: Dec 25, 2008
7. Dec 25, 2008

### nucleus

“I've something in mind for sometime now, that is what happen if a car has its front and rear wheels turning at different speed? Say, the front wheels can cover 100km/hr while the rear ones can only cover 75km/hr. What happen to the car average speed (I guess its less than 100km/hr)? How is that so?

Hmm maybe I should make myself clear first that what I'm imagining is that there are two engines for each front and rear wheels set which make the front and rear wheels turning at different rates.... From here I cant imagine what happen if both rear and front wheels cant slip (the car stretches??) so I imagine that the faster one has to "drag" the slower one (the slower one has to slip) so we get the resulting car speed from somewhere between the two rotational rates like you said. Is my thinking correctv”

If you have an engine powering the front wheels through a mechanical transmission and another engine powering the rear wheels through another mechanical transmission your final speed will likely be less than 75 mph depending on gear ratios, size of engine etc. Regardless your gas mileage will be less.

An engine can either power a load or the load can drive an engine. If you descend a long hill with the transmission in gear you will notice that you need less breaking than if you were to put transmission in neutral. This is a case where the car is driving the engine. In your situation where you have two engines of different sizes, the stronger one would push or pull the other engine down the road depending on which was stronger. The weaker engine would act like a break, slowing you down.

8. Dec 28, 2008

### Ranger Mike

it happens all the time..cars with huge rear slicks and small front tires has wheels that are at different ' speed" but the VEHICLE speed is a result of the drive wheel speed..also, during cornering, the front wheels turn at different rates ( ackerman effect) and the rear wheels will be at different speed, caused by the rear differential..has too otherwise the tires break loose and you loose rear traction. in fact, an open differential will have the right rear wheel rotation forward and the left rear is rotating backwards..that is why these cars left only one tire patch when you did a burn out..
Limited slip differantials live Positraction (GM) Sure Grip (Mopar) have clutches that lock up both rear axels ..when power is applied ..dual tire patchs are left during bunr out..but..when car goes thru a turn, the inside clutch breaks loose and outside axel drives car around the corner

see General Physics forum .i did some research on GoPed gearing..see Thanksgiving Dinner Debate About GoPed Gearing

also see todays post on circle track discussion

Last edited: Dec 28, 2008