How to check rear gear ratio ?
11-21-2001, 09:29 PM
#1
Instructor
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How to check rear gear ratio ?
I heard that to check the rear gear ratio of my C3,
all I had to do was jack one rear tire off the ground,
put the car into neutral,
mark the driveshaft with chalk,
spin the raised wheel two complete rotations,
and count the number of times the driveshaft rotates.
Is this correct? And is this accurate?
all I had to do was jack one rear tire off the ground,
put the car into neutral,
mark the driveshaft with chalk,
spin the raised wheel two complete rotations,
and count the number of times the driveshaft rotates.
Is this correct? And is this accurate?
11-21-2001, 10:29 PM
#2
Safety Car
Re: How to check rear gear ratio ? (John Dirks)
That method will work. It might be a bit tough turning one side if you have a good tight posi unit though. Two complete revolutions will only be accurate with one wheel jacked off the ground. You could also just rotate the wheel one time. Then double the number of turns of the driveshaft. IE; If you drivedhaft turns aproximatly 1.5 turns with one wheel rotation, yur probably lookin at a 3.08.
With a tight posi unit, you can also jack both wheels off the ground. When you rotate one wheel, the other follows along in the same direction. make sure its in neutral for this method. With both wheels rotating togeather in the same direction, its 1 wheel rotation and count the driveshaft.
Its when you leave 1 wheel on the ground that the differential spider gears come into play. Thats why with 1 wheel up, its 1 rotation and double the driveshaft count,or two wheel rotations and count driveshaft once. Same result either way.
The problem with vettes when jacked is the droop in the trailing arms puts greater angles on the half shaft u joints. They tend to wanna bind while hanging in droop like this.
If you have a good tach you can also use this calculator to determine you gear ratio. It would require that you enter the other known variables accuratly to get a close guess. >>> http://www.prestage.com/carmath/calc_Gears.asp
With a tight posi unit, you can also jack both wheels off the ground. When you rotate one wheel, the other follows along in the same direction. make sure its in neutral for this method. With both wheels rotating togeather in the same direction, its 1 wheel rotation and count the driveshaft.
Its when you leave 1 wheel on the ground that the differential spider gears come into play. Thats why with 1 wheel up, its 1 rotation and double the driveshaft count,or two wheel rotations and count driveshaft once. Same result either way.
The problem with vettes when jacked is the droop in the trailing arms puts greater angles on the half shaft u joints. They tend to wanna bind while hanging in droop like this.
If you have a good tach you can also use this calculator to determine you gear ratio. It would require that you enter the other known variables accuratly to get a close guess. >>> http://www.prestage.com/carmath/calc_Gears.asp
11-21-2001, 10:30 PM
#3
C6 the C5 of tomorrow
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Re: How to check rear gear ratio ? (John Dirks)
No it's not!
First of all if you have Posi you won't be able to spin the tire!
If you don't have posi you will be way off on the count due to the posi carrier spinning...
The correct way is to get both wheels off the ground and spin the driveshaft. Count the number of times the driveshaft spins for one rev of the wheels.
NOTE!
Watch both of the rear wheels, if you have a very loose, open ( non posi ) diff one wheel may spin the oppisite direction of the other, or not at all.
If BOTH wheels do not spin at exactly the same rate, this will not work and you will have to put the car on the ground and roll it while someone counts drive shaft revs ( don't run them over! :rolleyes: )
First of all if you have Posi you won't be able to spin the tire!
If you don't have posi you will be way off on the count due to the posi carrier spinning...
The correct way is to get both wheels off the ground and spin the driveshaft. Count the number of times the driveshaft spins for one rev of the wheels.
NOTE!
Watch both of the rear wheels, if you have a very loose, open ( non posi ) diff one wheel may spin the oppisite direction of the other, or not at all.
If BOTH wheels do not spin at exactly the same rate, this will not work and you will have to put the car on the ground and roll it while someone counts drive shaft revs ( don't run them over! :rolleyes: )
11-21-2001, 10:56 PM
#4
Safety Car
Re: How to check rear gear ratio ? (427V8)
427 is correct in noting that a posi diff will make it difficult to determine gear ratio with the 1 wheel jacked method. Actually the one wheel method works good on an open diff straight axel car. Just wanna note that he and I are on the same page, just explaining in a different way. Has anyone found a way to jack both rear wheels off the ground without the trailing arm droop? With all my other straight axel cars its easy. I just put jack stands under the axel tubes and let the car down on them. Then I can run the car thru the gears on the stands. Ive not yet found a way to comfortably run my vette drivetrain while jacked on stands. Those sharp u joint angles worried me too much.
The calculator method works good. Just measure your rear tire diameter. Run the car down the road in 1to1 tranny ratio. ( this would be 3rd gear on a typical 3spd automatic. or 4th gear on a typical 4spd tranny. note the mph and rpm at a steady highway speed. bring the data home and put it into the calc i inserted above. if you entries are correct, it should come close to one of the available ratios allowing you to assume what you have.
The calculator method works good. Just measure your rear tire diameter. Run the car down the road in 1to1 tranny ratio. ( this would be 3rd gear on a typical 3spd automatic. or 4th gear on a typical 4spd tranny. note the mph and rpm at a steady highway speed. bring the data home and put it into the calc i inserted above. if you entries are correct, it should come close to one of the available ratios allowing you to assume what you have.
11-22-2001, 05:37 AM
#6
Melting Slicks
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Re: How to check rear gear ratio ? (John Dirks)
why not just look for the code stamped into the case? it is hard to see, but it's there. probably a little more accurate... :yesnod:
