Motions of wheels

[Cristatus]

For example, look at any car. If it is FWD, the front set of wheels will move faster than the rear set, and vice versa with RWD.

With AWD (like on Quattros) you don't get this effect.

Another simpler example: look at a front loading cassette deck where you can see the wheels that pull the tape. The one that is spinning is always spinning faster than the one that is being pulled.

Why is this so? I don't think it's because of friction, because cogs can't lose efficiency because of losing grip, can they?

 

[Peter]

Friction losses mean that the driven surface /never/ reaches 100% of the driving surface's speed. End of.

 

[DrPizza]

For example, look at any car. If it is FWD, the front set of wheels will move faster than the rear set, and vice versa with RWD.

With AWD (like on Quattros) you don't get this effect.

Another simpler example: look at a front loading cassette deck where you can see the wheels that pull the tape. The one that is spinning is always spinning faster than the one that is being pulled.

Why is this so? I don't think it's because of friction, because cogs can't lose efficiency because of losing grip, can they? ??
Let's take a look at rear wheel drive. We'll assume the rear wheels have the same radius as the front wheels.

They spin at the same speed (unless there is kinetic friction involved). Generally, tires are a case of static friction, not kinetic friction. Exceptions are, of course, when you skid to a stop, or when you peel out.

And, back to the cassette: the gear that the tape is winding onto spins at a constant speed. The speed at which the other rotates depends on ratio of the radii to the outer edges of the tape on each reel. When the take-up reel has very little tape on it, the other wheel spins more slowly. When the take-up reel is nearly full, the other reel spins very quickly.

 

[dkozloski]

Most four wheel drive vehicles do not have the same final drive ratios front and rear. This is why you can't use four wheel drive on pavement. All wheel drive cars have three differentials which allow all four wheels to each turn at a different speed to conform with the Ackerman principle.

 

[Peter]

Originally posted by: DrPizzaAnd, back to the cassette: the gear that the tape is winding onto spins at a constant speed. The speed at which the other rotates depends on ratio of the radii to the outer edges of the tape on each reel. When the take-up reel has very little tape on it, the other wheel spins more slowly. When the take-up reel is nearly full, the other reel spins very quickly.

Nope, wrong. The /tape/ velocity is constant, driven by a roller next to the read/write head. The two reels rotate at changing speeds, depending on how much tape is on each.

 

[FrankSchwab]

DrPizza -
No, you've got it wrong. The capstan and rubber pinch roller next to the head always spins at the same speed; it sets the speed of the tape past the head, and that is a constant. The Take-up reel is driven with a slipping clutch so that it maintains a specific tension on the tape, but spins at precisely the speed necessary to pick up the tape that the capstan is driving. The next time you play with a cassette player, stick something into the hub of the takeup reel and stop it; you'll find that the tape continues to move past the head, and the feed reel will continue to move, until the tape gets sucked into the mechanism and stops everything.

dkozloski -
No, four wheel drive vehicles DO have the same final drive ratios front and back. The simplest transfer cases hard-connect the front and back driveshafts which can lead to some humongous torsional stresses on the driveshafts and components as the car turns on hard surfaces (the rear wheels travel a shorter distance than the front). All-wheel drive vehicles solve the problem in various ways - they can turn the transfer case into a differential (in which case the nomenclature "4 wheel drive" is doubly incorrect), they can put a clutch into the transfer case allowing a limited-slip action between the front and rear, or any of several other methods. Having different final drive ratios front and back would imply that, in four-wheel-drive, one set of wheels would always be spinning, in other words, using dynamic friction rather than static friction. Not such a good idea on ice or other slick surfaces.

And Logic1485, I think you've just not done enough observations. If you play a tape starting at the beginning, the take-up reel has no tape on it, and thus a smaller diameter, than the feed reel. It will spin faster as a result. Halfway through the tape, they should be spinning about the same speed. Near the end of the tape, the take-up reel will be spinning slowly, and the feed reel will be spinning fastest.

I have a VCR that uses this principle to know when it is getting near the front/end of the tape in it's hyperdrive rewind/fastforward modes. It measures the difference in RPM between the take-up reel and the feed reel, and can calculate the difference in diameters between the two. About 2 minutes before either end, it slows down and waits for the tape tension to change (or the clear leader to appear) to tell it when to stop.

/frank

 

[DrPizza]

You're right, Peter and Frank. I wasn't thinking about while the tape was playing; I was thinking about when the reels were being fast forwarded or fast reversed. I used to work at a radio station that used a lot of reel to reel tapes. I never really watched them while they were playing (yes, I knew the pinch roller fed the tape at a consistent speed; otherwise you couldn't edit tapes by cutting out a part; it would change the playback rate). But, the relative speeds were most obvious during rewind and fast forward - that's what I was thinking about.

 

[dkozloski]

FrankSchwab I've counted the teeth on the ring and pinion gears on both Dodge and Ford trucks that the front and rear ratios were different. Not very much but still different. The rear wheels were overdriven. Both would bind up the transfer case so badly that the only way they could be shifted to neutral was by backing up. Undoubtedly there are four wheel drive vehicles made both ways.

 

[Toastedlightly]

Originally posted by: dkozloski
FrankSchwab I've counted the teeth on the ring and pinion gears on both Dodge and Ford trucks that the front and rear ratios were different. Not very much but still different. The rear wheels were overdriven. Both would bind up the transfer case so badly that the only way they could be shifted to neutral was by backing up. Undoubtedly there are four wheel drive vehicles made both ways.

Those trucks = teh suckage.

 

[dkozloski]

Originally posted by: Toastedlightly

Originally posted by: dkozloski
FrankSchwab I've counted the teeth on the ring and pinion gears on both Dodge and Ford trucks that the front and rear ratios were different. Not very much but still different. The rear wheels were overdriven. Both would bind up the transfer case so badly that the only way they could be shifted to neutral was by backing up. Undoubtedly there are four wheel drive vehicles made both ways.

Those trucks = teh suckage.

Agreed, the only way to go is with FWD Corp. and/or Oshkosh. R.I.P.

 

[jagec]

Originally posted by: logic1485
For example, look at any car. If it is FWD, the front set of wheels will move faster than the rear set, and vice versa with RWD.

err? Only if there is slippage, or one set of wheels is larger than the other.

 

[91TTZ]

Originally posted by: logic1485
For example, look at any car. If it is FWD, the front set of wheels will move faster than the rear set, and vice versa with RWD.

With AWD (like on Quattros) you don't get this effect.

Another simpler example: look at a front loading cassette deck where you can see the wheels that pull the tape. The one that is spinning is always spinning faster than the one that is being pulled.

Why is this so? I don't think it's because of friction, because cogs can't lose efficiency because of losing grip, can they?

Both of your examples are incorrect.

 

[BrownTown]

Originally posted by: 91TTZ
Both of your examples are incorrect.

 

[MrDudeMan]

Originally posted by: Toastedlightly

Originally posted by: dkozloski
FrankSchwab I've counted the teeth on the ring and pinion gears on both Dodge and Ford trucks that the front and rear ratios were different. Not very much but still different. The rear wheels were overdriven. Both would bind up the transfer case so badly that the only way they could be shifted to neutral was by backing up. Undoubtedly there are four wheel drive vehicles made both ways.

Those trucks = teh suckage.

:roll: what an insightful comment. i dont own a ford truck, but they are not bad trucks at all.

 

[bobsmith1492]

Originally posted by: MrDudeMan

Originally posted by: Toastedlightly

Originally posted by: dkozloski
FrankSchwab I've counted the teeth on the ring and pinion gears on both Dodge and Ford trucks that the front and rear ratios were different. Not very much but still different. The rear wheels were overdriven. Both would bind up the transfer case so badly that the only way they could be shifted to neutral was by backing up. Undoubtedly there are four wheel drive vehicles made both ways.

Those trucks = teh suckage.

:roll: what an insightful comment. i dont own a ford truck, but they are not bad trucks at all.

Hm, my Ford "truck" ('89 Bronco II) does not indicate these problems in 4WD even when driven on pavement...

How do you know the overall ratio is different? Maybe the ratio you counted made up for a differing ratio elsewhere?

 

[Witling]

Meanwhile, back at the ranch . . . . Could we have something other than logic 1485's original statement that the wheels turn at different rates to establish that they actually do turn at different rates? That seems to be missing here if one limits the discussion to an ordinary two wheel drive vehicle. And IMO, logic's example of the tape cassette is like the clock striking the 13th stroke: it casts doubt on all that has gone before.

 

[gerwen]

FrankSchwab has it right:

In a straight line, all four wheels will turn at the same rate, provided they are all the same diameter. If the front moved at a different rate from the back, it would create instability because of the loss of traction as two wheels are forced to slip. It would also be very stressful on the drivetrain, and cause increased tire wear.

In a turn, the wheels will turn at different rates, which is why we have differentials. Differentials allow two driven wheels to turn at different rates. AWD vehicles have 3 diffs. One for between the wheels of each the front and rear end, and one in the centre to allow differences between the front and rear.

If two driven wheels (or two ends of the vehicle in the case of 4wd) are forced to turn at different rates without a differential, one wheel must break traction, so it can either spin faster or slower than the ground it is on. On dirt (4wd territory) this isn't a big deal, as it is low traction and the forces required aren't that great. On pavement, the force required can be much greater, especially with big 4wd truck tires. This can put immense stress on the drivetrain.

Which is why most 4wd vehicles have a way to disengage and engage 4wd mode (as well as locking and unlocking the diffs). This way you can avoid breaking your drivetrain when on a high traction surface.

I think the op merely had his eyes lying to him when he observed the wheels spinning at different rates. I suspect even the different wheel turning rates in a turn are undetectable by the human eye.

 

[jagec]

Originally posted by: bobsmith1492
Hm, my Ford "truck" ('89 Bronco II) does not indicate these problems in 4WD even when driven on pavement...

How do you know the overall ratio is different? Maybe the ratio you counted made up for a differing ratio elsewhere?

I'm guessing that the differential would be different in the front and rear.

You still shouldn't use 4wd on the pavement

 

[Grumpy642]

 

[Grumpy642]

New 4x4's are made to be driven on or off road. My Xtera has 4 wheel low and 4 wheel high. High is for highway use when you might lose traction. Although I don't think it is true 4 wheel drive. True 4 wheel drive is rare for all the reasons you have said. Put it in the mud and let them spin. Not all wheels will.

 

[Biftheunderstudy]

The locking differential is because if your really stuck and one tire loses traction the other tire just stops. However if your differential is locked and you try to turn your vehicle will basically go in a straight line since the wheels have to be able to rotate independantly to turn correctly.
The idea is that you get twice as much traction with the diff locked and hence many 4WD vehicles have the option to turn it on and off, old vehicles you had to get out and flip a switch on the hub.

Clever auto manufacturers came up with something called positive traction or possy-trac(or something, not sure how you would spell that), basically if one tire loses traction the other tire will still be driven, sort of like an automatic differential lock.

The only times when your tires are spinning at different rates is when you're turning.

 

[bobsmith1492]

Originally posted by: Biftheunderstudy
The locking differential is because if your really stuck and one tire loses traction the other tire just stops. However if your differential is locked and you try to turn your vehicle will basically go in a straight line since the wheels have to be able to rotate independantly to turn correctly.
The idea is that you get twice as much traction with the diff locked and hence many 4WD vehicles have the option to turn it on and off, old vehicles you had to get out and flip a switch on the hub.

Clever auto manufacturers came up with something called positive traction or possy-trac(or something, not sure how you would spell that), basically if one tire loses traction the other tire will still be driven, sort of like an automatic differential lock.

The only times when your tires are spinning at different rates is when you're turning.

"Limited-slip differential?"

 

[gerwen]

Originally posted by: bobsmith1492
"Limited-slip differential?"

Yep. For an interesting read on differentials and how they work, with good diagrams and animations:
http://auto.howstuffworks.com/differential.htm

 

[dave518]

If 4 wheel drive is so bad and would make the tires spin at different speeds on high traction surface, how come 4 wheel drive trucks do so well drag racing? Diesel pick up trucks for example
If their tires were slipping the whole 1/4 mile they wouldn't be able to keep a straight line or drive well at all

 

[jagec]

Originally posted by: dave518
If 4 wheel drive is so bad and would make the tires spin at different speeds on high traction surface, how come 4 wheel drive trucks do so well drag racing? Diesel pick up trucks for example
If their tires were slipping the whole 1/4 mile they wouldn't be able to keep a straight line or drive well at all

That's if all four wheels are LOCKED together, and you're doing something where they should be spinning at different speeds, such as turning. Since most 4wd systems have differentials, and drag races are straight-line, neither of these situations apply.