Differentials...please explain how it works - The Unofficial BMW M5 Messageboard (m5board.com)

young-nyc · 9th April 2002, 17:25

Hey guys..ever since reading automobile maganzines at a young age...i never understood what gear ratio numbers meant..
can someone explain to me how this works?
gear ratios and differentials??
thanks a bunch.

Racin' Randy · 9th April 2002, 17:50

I'll take a shot at this one.

First of all understand that there are two variables when it comes to gear ratios. Firstly, those in the transmission and secondly those in the rear end differential. The rear differential serves 3 purposes. 1) To change the direction of travel of the power from the engine from the drive shaft to a 90 degree angle to the drive axles, 2) To allow a gear multiplication, and 3) To provide either no-lockup, limited-slip, or locked type transfer of power to the left and right axles.
The transmision serves the purpose of multiplying the revolutions of the engine by an internal ratio to aid in the acceleration and cruise aspects of driving. whew!!!!

Ok, back to the rear end differential. Imagine that under the car, your driveshaft is going to turn in a clockwise motion. The pinion gear of the rear end differential is connected to the driveshaft via a universal joint (u-joint) which allows the driveshaft and pinion to be connected, but yet be pliable for suspension travel. That pinion gear is going to then turn a ring gear which is connected to the differential. The of number of teeth of the ring gear divided by the number of teeth on the pinion gear gives you the gear ratio. An example of that would be a 31 tooth ring gear and a 9 tooth pinion gear. 31 / 9 = 3.44444444444 or 3.45.

Now, as the pinion gear turns the ring gear, it is connected to the differential which is basically both a hub for turning and connecting the axles, as well as a series of clutches bathed in oil which apply power equally to the two axles(limited slip) based upon the relative amount of traction available under each wheel. All conditions being equal, then both rear tires would spin at the same speed. A locker rearend is one that makes a solid connection between the axles and allows for no give based upon traction, which is ideal for straight line racing conditions, but basically nothing else. To illustrate this, think of the arc of travel of the two rear wheels of a car going around a 90 degree corner. The inside tire will travel less distance than the outside tire and with a locked rearend, something has to slip somewhere and it usually means having to spin the inside tire around every corner. Impossible in slow speed situations such as parallel parking (well for some). And an unlocked rearend is just the opposite. Basically the rear of the car is driven by only one of the rear tires.

So, in summary, both the transmission and the rear end used different ratio gear sets to multiply (or in the case of overdrive) divide the revolutions of the engine so that you can have both acceleration and top end power.

Hope this wasn't tooooooooooo long winded.

RR....

Angel · 9th April 2002, 17:50

Going to a "lower gear ratio" will increase torque multiplication and provide quicker acceleration. When drag racers talk about going to a "lower gear ratio", they are referring to going from a 2.73 to a 3.23 (for example). This means you are going to a higher numerical ratio and increasing torque multiplication. If you want to retain the same final gearing while going to a larger tire you will need to change the differential ratio.

For example, a 1997 6 speed Camaro or Firebird comes with a 3.42 rear gear. The automatic cars are equipped with a 2.73 or a 3.23. A stock manual equipped vehicle will go through the traps (the timing lights at the end of the track) in 4th gear at roughly 100 mph and 4500 rpm. The driver will have to shift into fourth just before the lights (assuming a 5500 rpm shift point).

With a 4.09 rear gear, the car will now go through the traps at roughly the same mph but at approximately 5500 rpm in 4th gear.

Hope this helps.

tfung · 9th April 2002, 17:56

ahh.. Young, don't you play GT3 on PS2? you should fiddle around with the gear ratios in that game some more...

basically, each gear has a ratio of 1:xx, meaning that each time the engine turns once, the gear will multiple it by that number.. For example, let's make up some numbers here, 1st gear is let's say 3:1, so that means everytime the engine turns 3 times, the driveshaft will turn once... when it gets to the diff, it will be further multipled by the diff ratio... let's say is 4:1... so you have the driveshaft turning 4 times before the wheels would turn once... effectively, it means that the engine must turn 3x4 times before the wheel would make one revolution...

it's sort of like the gears on a mountain bike... the lower the gear ratio, the more power you get to the ground, but you won't go very fast...

did I explain that properly? please correct me if I'm wrong...

young-nyc · 9th April 2002, 18:19

woah

ok i will have to read save this read and re-read it over and over again to get a better understanding of how this works..
Thanks for the detailed information Randy
And more abridged version from Harry =P hehe
and Tfung i will try to play GT3 more often..but it's hard when your PS2 is in the car

AndyMenard · 9th April 2002, 18:26

Although there are absolutely a ton of people on this forum who know and understand stuff like this, and some of them do a very good job of explaining things... from time to time, I like to visit howstuffworks.com to get a simple explanation along with graphics.

Check this out....

http://www.howstuffworks.com/differential.htm

young-nyc · 9th April 2002, 18:29

thanks a bunch Andy...
good to see you're still lurking around on the board =)

Wilkie · 9th April 2002, 19:40

Also consider this simple example.

Look at any high end bicycle, with 10 or more speeds. There are 2 or 3 large sprockets up front, next to the pedals. Consider this the "transmission."

At the rear wheel, there is a cluster of 5 or so smaller sprockets. Consider this the rear "differential," which unlike a car, has selectable ratios-- the smaller the rear sprocket selected, the "higher" gear you are in, and the faster you can go given a fixed rate of pedaling (rpm); the larger the rear sprocket, the faster you can accelerate or go up hills, but your top speed is limited by how fast you can pedal (rpm-limited).

The actual final drive ratio being used at any given time is a function of the front and rear sprockets (tranny and diff) (as well as tire diameter).

BMWsilverstreak · 28th January 2004, 17:01

great explanations! the only thing I would add is that when someone is talking about a "taller" gear/ratio the number is decreasing say from a 4.11 to a 3.42
a 4.11 ratio is a good drag car ratio which means the drive shaft is turning
4.11 times every 1 time the wheel turns. A 3.42 ratio is a "taller" gear ratio
because the drive shaft only turns 3.42 times for every single wheel turn.
When talking about "9 bolt ford rears" these are referring to rears that have
have larger diameters to house the larger ring gears and also have stronger
overall castings to support the amount of torque exerted on the differential.
hope all this helps!

DZeckhausen · 28th January 2004, 17:19

Quote:

Originally Posted by young-nyc

woah

ok i will have to read save this read and re-read it over and over again to get a better understanding of how this works..
Thanks for the detailed information Randy
And more abridged version from Harry =P hehe
and Tfung i will try to play GT3 more often..but it's hard when your PS2 is in the car

To make it even simpler:

With a 2.81 differential, your driveshaft turns 2.81 times for every single revolution of your rear wheels.

With a 3.15 differential, your driveshaft turns 3.15 times for every single revolution of your rear wheels.

(Asuming that the car is going in a straight line!)