Differential Question Related to Fuel Economy

[dfeldt91]

So, my understanding is if one wheel is easier to rotate than the other then it will rotate that wheel. My question is does having one tire not rotating cause the other to spin at a faster rate than if both were rotating for a given engine RPM?


The reason I am curious is because if this is true then we should be able to remove the CV axle on a front wheel drive car on one side then have the flange the axle mounts to stuck via weld or something to cause the other wheel to spin faster for a given engine RPM.

Perhaps I am not understanding the differential correctly...or maybe this is possible.


Can someone who knows differentials chime in and inform me?

[Gasoline Fumes]

I think it'll work, but the differential's life will probably be shortened. And there will be more friction.

[darcane]

Originally Posted by dfeldt91

So, my understanding is if one wheel is easier to rotate than the other then it will rotate that wheel. My question is does having one tire not rotating cause the other to spin at a faster rate than if both were rotating for a given engine RPM?


The reason I am curious is because if this is true then we should be able to remove the CV axle on a front wheel drive car on one side then have the flange the axle mounts to stuck via weld or something to cause the other wheel to spin faster for a given engine RPM.

Perhaps I am not understanding the differential correctly...or maybe this is possible.


Can someone who knows differentials chime in and inform me?

Yes, that will cause the free axle to spin faster. Specifically at twice the speed that the differential carrier is spinning at.

This will make accelerating from a stop more difficult and you likely won't be able to use the top gear or two in your tranny. Expect the life of the tranny to be greatly reduced due to the higher than normal loads on various bearings and gears.

[spacemanspif]

I must need to brush up on my differential understanding if you guys are correct.

My understanding is:

When an open diff is off the ground, 1 wheel will turn with the engine no more or less than if the wheels were on the ground. Most factory 4x4 trucks are open front diffs and lead to a lot of guys getting stuck when 1 front wheel is off the ground becuase that becomes the one that the engine turns due to the design. Being 1 wheel drive should not have any ill effects on the diff becuase the 2 axles are not tied together like they are with a limited slip setup. I thought most front drive cars are open diff and worked the same as an open diff rear axle setup in old, rear drive cars. All diffs have a wheel that is the primary turning wheel, on my Monte Carlo that was the passenger one. I want to say that you are better off using the wheel the diff wants to drive first but really can't think of a reason why other than the spider gears doing more work...maybe. I can't see any way the wheels would ever turn faster than they do in normal operation, the driveline is still governed by the gears in the trans and diff and the wheel speed is still dependent on the engine RPM and gears, there is no way to cause the wheel to spin faster.

Again, this is MY understanding, please correct any misconceptions I have as I am always willing to learn. Most of my statements are based off my understanding of Chevrolet rear drive platform...I'm not all the interested in front drive platforms others for daily duty beaters.

[dfeldt91]

Thanks for the input! I have two saying it might work and one saying it might not.

Mike, I am not sure where I heard that if one wheel is held still it causes the other to spin faster. Perhaps I misinterpreted what was said or heard at that point a long time ago.

I was thinking(If it is possible, although it may just not be) of getting a small diesel car removing the cv axle on one side and welding to the flange to keep it from rotating to cause the other wheel to spin faster therefore cutting the engine RPMs for a given speed. Maybe my understanding of diffs is just wrong.

Darn, I thought I might have been on to something

[Fat Charlie]

That's my understanding as well- but pressing the brake enough to keep the hanging wheel from spinning freely ought to get things caught up again. And then do try to be more careful for the rest of the day.

[Gasoline Fumes]

I just happen to have a Suzuki Swift GTi diff handy. Yes, the speed will double. But the spider gears were never designed to be used that much. They aren't spinning while driving in a straight line. They don't even have bearings.

Primary turning wheel? Never heard of such a thing and I can't see how that would be possible or desirable.

[redneck]

.

No.

The speed does not double.


20 tooth side gear to a 10 tooth spider gear to a 20 tooth side gear = 0 mechanical advantage.



>

[elhigh]

The speed does double. One wheel fixed and one wheel free, the free wheel will turn at double the differential's speed.

The ring gear is attached to the spider carrier. Normally, with both wheels on the ground, everything turns at the same rate: the ring gear/carrier, and the axle gears are all moving along. In a turn your speed doesn't change because, as one wheel must necessarily slow down on the inner, smaller circumference of the turn, the outer wheel must speed up for the outer, longer circumference. The increased torque on the inner wheel gets multiplied into greater speed on the outer wheel. Net torque applied to the drive wheels remains the same (minus the gearing losses) and your speed through the turn doesn't change.

With one wheel in the air, that wheel turns at double speed, so if you rev the engine and see it indicating 30mph according to the speedo, the wheel is turning at 60mph worth of revs. As the spider gears are attempting to push the fixed axle - and failing - those gears are also turning. So in addition to the revs applied to the ring gear turning the free axle, the spider gears rotating against the fixed axle are also pushing the free axle even further. Double speed.

Very simple demonstration:

Drop a rolling pin on the floor.
Drop a 6' long board on the rolling pin. One end right there on the pin. Mark the spot on the floor where the handle of the pin is, call that 0 feet. So the end of the board and the pin are both at 0 feet.
Push the board forward, rolling on the pin, until its other end drops off the board.
Where is the pin? Six feet from where it started.
Where is the far end of the board? TWELVE feet from where it started.
2:1 ratio. The board is one axle gear, the floor is the other, and the rolling pin is the spiders. Now wrap it all up into a circle and the floor is your fixed axle and the board is the free axle, and instead of pushing the board you push the pin. As it goes around, it moves the board forward at twice the pace.

In order to see something besides 2:1, the axle gears would have to have different tooth counts. You will never ever ever see that. For one thing it would make the spider gears VERY unhappy, and for another it would make the car behave differently turning one way vs. the other way, which would make YOU very unhappy.

Hmm. There is a way to see a different ratio. If the car is running a "spool," which is to say the axles are fixed directly to the ring gear and cannot rotate at different speeds relative to each other. You won't see that outside drag racing. If one wheel is turning and the other not in a situation like that however, that means an axle has snapped and you will hear lots of bad language.

NOW, regarding your original question: that will destroy a differential in fairly short order.

Those spider gears aren't really intended to work that hard for that long. You spend the vast majority of your time driving in a more or less straight line. The spider gears absorb little corrections as you're keeping it between the lines, probably not making a hundred full turns in total over a mile of typical interstate. And the net load on them is distributed evenly, you go to work and go home, making the same turns but in the opposite direction.

If you weld one output in place so the spiders are continually walking around it, overdriving the other output at a 2:1 ratio, then they will be put into a duty paradigm never imagined by the original engineers, and it will all be on one side. It'll get pretty darned hot inside that diff.

I have seen diffs fixed to act as spools, both as cheapo spools for drag racers on a budget, and in the bizarre-yet-awesome Amazonas motorcycle that used a complete (!?) VW Beetle drivetrain. The spiders were welded directly to the axle gear on one side, and a chain sprocket attached to the axle stub. Funky but functional. Funnier yet, that left reverse gear available and on a bike like the Amazonas, that could be pretty useful.

[markweatherill]

Sounds like a new way to experience 'torque steer'.