Rear-End Ratio Confusion

[bordin34]

I am am confused about what ratio my rear is in my Charger. It is an 8 1/4. When I turn the wheel one turn, the driveshaft turns 1 1/4 turns. According to my math that gives me a 0.80 rear-end ratio. It is an open rear, I have it so that the other wheel will not turn.

[daves68]

It's driveshaft to wheel ratio. The number of times the drive shaft turns to make the wheel rotate 1 complete revolution.  Example:  If the drive shaft turns 2  3/4 times to make the wheel turn 1 time, that is about a 2.75:1 ratio.  Try it again, you're on the right track.
Dave

[FLG]

Count again, you probably missed counted by one turn.

[bordin34]

I have counted at least five times already. To one turn of the driveshaft there is about a little more than 3/4 turns of the wheel. I will count again right now....

[bordin34]

I just checked again and got the same revolutions. I made sure it was a full turn of the driveshaft. Wheel 1 turn driveshaft 1 1/4 turns.

[tan top]

hmmmm    ..don't  turn the wheel but turn the drive shaft ........... so put a chalk  mark  or  piece of tape on the wheel & drive shaft  ..then turn the drive shaft   & count how many times the drive shaft goes round .. to one full revolution on the wheel

[Nacho-RT74]

I guess open differential Right ?

[bordin34]

It is an open differential. I have both wheels off and the drum off of one side. There is one drum on it, so I set the parking brake so that wheel doesn't spin.

[mikepmcs]

It's because with an open differential you have to make 2 complete turns of the wheel and count the driveshaft revolutions. Or multiply what you have by 2.

v/r
Mike

[bordin34]

So that would give me about a 2.50 ratio. Maybe it has 2.76.

[Nacho-RT74]

thats why I was asking... yes, need to multiply by 2

[bordin34]

I turned the wheel twice and the driveshaft turned 2.5 times.

[John_Kunkel]

In the 8 1/4" the closest ratio to 2.5 is 2.45.

[bordin34]

That seems really really low. I don't think that was even an option to have it that low. Maybe that is why my 0-35 takes so long and it comes alive above 50.

[FLG]

Got the same thing here bordin...slow as hell of the line but much quicker once those RPM's are up there

[375instroke]

This is confusing to me.  I don't see how you multiply the 1.25 turns by 2 to get 2.5?  When one wheel is not moving, and the other is, the spider gears walk around the stationary side gear, and rotate the moving side gear.  That means the moving side gear, and hence the moving wheel, is moving twice as fast as the ring gear, so you would need to divide the driveshaft to wheel ratio by 2 to get the differential ratio, not multiply it.  This is a weird one.

[bakerhillpins]

You have it correct, you are just not thinking about the fact that under normal operation of the vehicle both wheels are turning. Therefore the ratio he measured would only turn both wheels half as much. So if 1.25 turns of the DS make 1 turn of one wheel, then under 2 wheel operation 1.25 turns of the DS will turn both wheels .5 turns.

Hence you multiply by 2 to get both wheels to turn once.

[375instroke]

Damn.  You're right. 

[bordin34]

I guess it has a 2.47, 2.41 does not exist.

[Mike DC]

Yeah, it's in the mid-2's somewhere.

If the driveshaft is giving the rearend 10mph worth of rotation but you've stopped one wheel, then the other spinning wheel is gonna get 20mph worth of rotation.  (That's why the RR  wheel always spins SO damn fast on an open-diff burnout.)

[bordin34]

I believe I have 2.47, so now I know why it accelerates so slow. I also have a new problem to fix on the highway I was turning about 2500rpms I figured I had 3.23's so that would be about right, with 2.47's it should be way less, now I don't think my trans is shifting out of 2nd gear.

[FLG]

What speed? At 80 mph im doing 3k, with 28" tires...so id assume my ratio is about 3.23. What speed are you going?

[bordin34]

At about 65mph. 3.23 at 65 turn 2500, 2.47 at about 65mph in 2nd gear turn about 2900, in 3rd it should be 2000.

[John_Kunkel]

I guess it has a 2.47, 2.41 does not exist.

That was a typo on my part, the actual ratio is 2.45; I corrected my original reply.