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Old 07-15-2017, 05:54 PM   #11
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I thought only one wheel would double the revolutions of the driveshaft due to the spider gears spinning?
-Christopher
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Old 07-15-2017, 05:56 PM   #12
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Originally Posted by Tootalltechie View Post
My mistake my tires are 8.25 R 20 and roughly 37" diameter. I was not home when I asked.

Sent from my SM-G530W using Tapatalk

I'm worried that I might have done damage driving home last fall. Driving some mountain down hills at over 65 mph.
with a gas notor as long as you didnt over-rev it or foam the oil / get it hot it should take it.. I dont know much about running on propane to know if it causes hotter cylinder temps etc...

ive built hotrods with 350s in the past that I was taching 3000+ on the highway (they were holeshot cars more than cruisers)..

-Christopher
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Old 07-16-2017, 10:47 AM   #13
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Originally Posted by cadillackid View Post
I thought only one wheel would double the revolutions of the driveshaft due to the spider gears spinning?
-Christopher
Nope, it doesn't work like that, there is no multiplication from the spider gears.
Rob
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Old 07-16-2017, 12:30 PM   #14
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Originally Posted by cadillackid View Post
I thought only one wheel would double the revolutions of the driveshaft due to the spider gears spinning?
-Christopher
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Originally Posted by White Lightning View Post
Nope, it doesn't work like that, there is no multiplication from the spider gears.
Rob
Actually there is some truth to this. The gear ratio comes into play here. When you are going down the road and all wheels are spinning the same speed, you'll get 1 tire revolution for every X driveshaft revolutions. So far, so good, right?

Now, when you go around a corner, the outer tire needs to spin faster than the inner tire. That's the entire reason we have differentials in the first place. For our discussion, it doesn't matter how much difference there is, you still end up with the same amount of tire rotation with the same amount of driveshaft rotation; normally it's split evenly between the tires but around a corner, one gets more, the other gets less.

Following this logic, if we stop one wheel completely (and we have an open differential, not a locking or limited slip one), the differential still does it's job, but now you do indeed get double the tire revolutions for the same number of driveshaft revolutions. Working in reverse (to calculate gear ratio), remember you're getting half the driveshaft revolutions since you're only turning one wheel. Either rotate the tire exactly twice, or double how much the driveshaft turns to get a correct result.
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Old 07-16-2017, 01:01 PM   #15
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Originally Posted by Brad_SwiftFur View Post
Actually there is some truth to this. The gear ratio comes into play here. When you are going down the road and all wheels are spinning the same speed, you'll get 1 tire revolution for every X driveshaft revolutions. So far, so good, right?

Now, when you go around a corner, the outer tire needs to spin faster than the inner tire. That's the entire reason we have differentials in the first place. For our discussion, it doesn't matter how much difference there is, you still end up with the same amount of tire rotation with the same amount of driveshaft rotation; normally it's split evenly between the tires but around a corner, one gets more, the other gets less.

Following this logic, if we stop one wheel completely (and we have an open differential, not a locking or limited slip one), the differential still does it's job, but now you do indeed get double the tire revolutions for the same number of driveshaft revolutions. Working in reverse (to calculate gear ratio), remember you're getting half the driveshaft revolutions since you're only turning one wheel. Either rotate the tire exactly twice, or double how much the driveshaft turns to get a correct result.
oops yep I had that backwards.. double the tire revs is correct..
-Christopher
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Old 07-17-2017, 01:16 AM   #16
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Gentlemen, again, there is no multiplication through the spider gears. If you jack up one wheel, put the drive line in neutral, spin one tire, hold the other in place, and count the drive line revolutions you will get the final drive ratio. Guaranteed lol. I've done it hundreds of times in order to ball park the ratio. All you need from there is a list of the available ratios and you have the numbers you need, along with the tire diameter to figure drive line RPM at any given MPH. Most non lock up torque converters that are factory stall speeds will give you right about 200 RPM above the drive line RPM for a crankshaft RPM. Give it a try.....
Rob
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Old 07-17-2017, 01:51 AM   #17
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Originally Posted by White Lightning View Post
Gentlemen, again, there is no multiplication through the spider gears. If you jack up one wheel, put the drive line in neutral, spin one tire, hold the other in place, and count the drive line revolutions you will get the final drive ratio. Guaranteed lol. I've done it hundreds of times in order to ball park the ratio. All you need from there is a list of the available ratios and you have the numbers you need, along with the tire diameter to figure drive line RPM at any given MPH. Most non lock up torque converters that are factory stall speeds will give you right about 200 RPM above the drive line RPM for a crankshaft RPM. Give it a try.....
Rob
Rob, you are mistaken. If both sides are turning, then yes, this is true. However, if one is stationary then the other gets the difference (hence the term "differential"). Hot-rodders can even tell you if you jack up one wheel, start the car, put it in drive and floor the throttle, you can easily spin that one tire well over it's speed rating.
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Old 07-17-2017, 02:02 AM   #18
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The spider gears do not "multiply" anything. Their purpose is to simply make the differential work. When you're going straight down the road, the spider gears are not turning relative to each other (they are stationary in the housing, going along for the ride). When you take a corner, they turn so as to allow the wheels to rotate at different speed but overall the vehicle is still moving forward at X MPH. Again, no multiplication, just allowing a difference between the sides. The tighter the turn, the greater the difference.

Farm tractors often have independent wheel brake which allow the driver to slow or stop one drive wheel; the intent to allow the outer wheel to help turn the tractor and give a near-zero-turning-radius. Assuming the differential is open (many have diff locks), the same applies. The input shaft still turns at the same speed but the outer wheel spins faster as it is receiving the difference from the non-turning wheel. Still no multiplication.
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Old 07-17-2017, 10:37 AM   #19
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Thank you Brad.
Rob
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Old 07-17-2017, 12:08 PM   #20
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this is exactly what I have experienced.. often if you jack up both wheels on an open dif, the spider gears will turn instead of the ring gear... turning one wheel results in the other turning backwards..

-Christopher
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