Ok guys, I was watching PHR tv last night and they had a segment on Rear ends. I never really understood it until now. But with knowledge, comes more questions. Ok, the driveshaft turns the pinion gear (small one) which turns the ring gear (big one) which turns the axles.

So now the rear end ratio is 4:11. That means that for every 4,11 times the pinion gear turns, the ring gear turns once. Right?

If that is true, wouldn't..say..a 2:73 rear end ratio be faster off the line, because the pinion only has to turn 2.73 times for the ring to turn?

I know this isn't true, because a 4:11 winds higher and will get goin faster than a 2.73, but I don't see how.

Thanks for reading, and any help would be greatley appreciated,
Chris

Yes, no, both assuming a 1:1 top gear ratio. The rear is a multiplier just like different ratios of gears in a transmission. Using a standard M21 4-speed as an example, you can accelerate quicker taking off in 1st gear than you can in 4th gear because 4th gear is 1:1 and 1st gear is like 2.20:1.

Imagine you're going to punch someone in the face and you have one second to do it. You can punch them less than 3 times (2.73 gear) in that one second. Now you're really mad and you can punch them more than 4 times (4.11 gear) in that same one second. Four punches should have more effect than three. The lower ratio gear (higher numerically) allows your engine's RPMs to rise faster in the same amount of time than a higher ratio (lower numerically) rear gear. Hence, your car should accelerate quicker. Make sense?

Check the two pictures for an exagerated example. In the first picture, turning one gear turns the other the same number of revolutions.

http://static.howstuffworks.com/gif/gear-helical1.jpg

In the second picture, turning the bigger gear one revolution will spin the smaller gear 'x' number of times with the same externally applied effort.

http://static.howstuffworks.com/gif/gear-helical2.jpg
graemlins/thumbsup.gif

The gear multiplies the torque at the wheels. The higher the gear ratio number, the more the torque is multiplied.

Think of a hand crank winch.You'll notice that you crank and crank, and the cable reels in very slowly, but it's easy to crank, thanks to the gear reduction. If the gears weren't there, you'd have to crank WAY harder to pull that car out of the ditch. The gears increase the torque output, you crank with little effort, and pull a much bigger load than you could normally.

High number gears do tha same for the car. The engine only makes so much power. So if your car can't get out of its own way, you can make it easier for the engine to move the car fast by multiplying the torque with more gear. A quick example with a stick shift is easy. Start in second. The engine had a harder time moving the car off the line, because the gear ratio is less than in first.

chris

also, its not a 4:11 gear. Its 4.11:1 That's a decimal.

For a 4.11:1 gear, you turn the pinion 4.11 times to every 1 time the wheels turn.

for a 2.73:1 gear, you turn the pinion 2.73 times to every 1 time the wheels turn...

and so on...

Gear ratio determines torque multiplication.

Example : 454 Big Block putting out 500 FT/Lb Torque Peak. Auto trans w/ a converter to get the motor up to peak torque at launch (assume 3000 stall for this case ), Auto trans w. 2.42 first gear ratio.

500 FT/LB x 2.42 x 4.11 = 4,973.1 ft/lb torque after multiplication of transmission and rear.

500 FT/LB x 2.42 x 2.73 = 3,303.3 ft/lb torque after multiplication of transmission and rear.

In either case above, both outputs are going to spin street tires. That is a remendous amount of torque.

Originally posted by Dale McIntosh:
Imagine you're going to punch someone in the face and you have one second to do it... THAT was classic! I'll have to use that analogy the next time I have to explain diffs :D

Another simple mwthod to see it for yourself is find a geared bike. Old 10 speed or a mountain bike with plenty of gears.

Flip it on it's bac so it's up in the air and put the bike in a high gear (numerically higher, think top speed cruising.) Now, grab the crank and spin it as HARD as you can. If you're like me and rarely work out for excercise, you might feel that shoulder in the morning smile.gif

Now, drop it into it's lowest numerical gear (take off gear where the pedal is easiest) and spin the crank as hard as you can. Notice it's remarkably easier?

In the first test, the bike's gears are acting like the 2.73:1 gears. In the second test, they are acting like 4.11:1 gears. The gears give a mechanical advantage to move the car easier and faster through "wasted" engine revolutions. (Wasted meaning, the engine revolution difference between the 4.11 and 2.73)

I hope that makes sense.

I think I got it now. Thanks guys graemlins/thumbsup.gif
Chris

Originally posted by Team140:
</font><blockquote>quote:</font><hr />Originally posted by Dale McIntosh:
Imagine you're going to punch someone in the face and you have one second to do it... THAT was classic! I'll have to use that analogy the next time I have to explain diffs :D </font>[/QUOTE]Sometimes, the injection of a little humor works. :D