(engines revving)
- You guys want some LSD?
Heck yeah, I do!
I'm talking limited slip differentials!
(electronic music)
When you're turning, your outside wheels gotta go further
than your inside wheels
and because each path's a different length.
Each wheel has to have the ability
to rotate at a different speed.
And when I say differentials,
I'm not talking about how we as people
are differential and unique and
that's what makes us beautiful.
I'm talking about the little gear train
inside what we call a differential.
The technology of differentials
dates back thousands of years to some time
between 100 B.C. and 70 B.C.
and no one knows actually who invented them.
So I'm just gonna go ahead and take credit.
Say that I did it.
Yeah, I don't know, just one day it just came to me.
In 1876, James Starley of England
invented a chain-drive differential for use on bicycles.
In 1887, David Shearer used a differential
in Austrian steam car.
And in 1932,
Porsche used the first limited slip differential,
but enough about how all these other folks
invented modern differentials,
what the heck are they? - [Crowd] Yeah.
- Turning gets a little tricky if your drive wheels,
for this example your rear wheels, are on only one axle.
They can't rotate at different speeds
'cause they're attached
so when you turn, your car explodes!
Not really, it's just super inefficient,
it's not gonna handle well,
and it's gonna stutter and skeeter everywhere,
just like my favourite Muppet.
So here's an example we can all relate to.
You know how we were all in a marching band
or we've seen marching bands
do cool formations at halftime shows
or we used to bully a band nerd in high school
but then magically switched bodies with them
so we had to learn band culture
while they learned being a jock
wasn't all it was cracked up to be.
Well, all these band geeks are shoulder to shoulder
and they're doing a turn,
and the band person closest to the axis of the turn
is just barely lifting their feet in tiny steps,
while the band people on the outside are taking big strides.
That's like the wheels on a car.
On a sharp turn,
the inside wheels need to be able to take baby steps
and the outside wheels need to take big strides
so that they can hit that tuba solo.
(tuba clanging)
The non-drive wheels don't have a problem with any of this
'cause they don't need to be connected on one axle
and they're just free to move on their own,
but the drive wheels need to be connected,
otherwise your car's one-wheel drive.
But the wheels on the axles also have to be allowed
to move somewhat independently.
Ay caramba.
Relax, that's what the differential's for.
You know what else is like a differential,
subscribing to our channel, do that now.
Inside the differential, we've got this pinion gear.
Pinion gear is attached to the drive shaft
and turns this ring gear.
The ring gear causes anyone that watches it
to receive a weird phone call and then die seven days later.
This ring gear is connected to a spider gear
that rotate with the ring gear
but can also turn on it's own axis.
This spider gear is meshed with the two side gears
and those two side gears, well, they become the two axles.
When the vehicle's going straight,
the spider gear and the side gears and axles
move as one solid unit,
but when the vehicle makes a turn,
the spider gear rotates on it's axis
and the side gears can move at different RPMs.
A lot of times, there'll be a second spider gear
to handle additional load
and in heavy duty vehicles,
you might have four spider gears.
Oh-oh, my spider gear is tingling.
Oh no, I just had my belt one notch too tight,
that's a false spider alarm.
Now while we're looking at the open differential,
I wanna show you this.
That pinion gear is much smaller
and that ring gear is much larger,
so they turn at very different speeds.
This ratio's called the axle ratio
or sometimes it's called the final drive ratio.
Final drive ratio directly affects
your car's acceleration and top speed.
A higher final drive ratio like four to one
will give good acceleration but lower top speeds
and a lower final drive ratio like three to one
will have a higher top speed
but probably slower acceleration.
The gear ratio,
just like we discussed in the transmission episode,
is the number of teeth on the driven gear,
divided by the number of teeth on the drive gear.
Now the differential I just described
is called an open differential.
An open differential's great 'cause it's inexpensive,
it's light weight and requires very little maintenance,
just like me, but unlike me it's also got a drawback.
If you're driving in a situation
where one of your rear wheels loses it's traction,
(tires skidding)
like if you're on ice, gravel or a slick patch of baby oil,
engine power is always gonna follow
the path of least resistance,
so if the one drive wheel's traction's
different than the other,
most of the engine power is gonna go to the wheel
with the least traction.
It slips! - [Crowd] Aw.
And one way to fix this is with a limited slip differential.
Limited slip differentials
are a great way to limit loss of traction.
They're used to limit the amount of independence
between the two axles
so that the amount of power sent to the wheel
with the least traction is limited
and the wheel with the most traction
receives continuous power
in order to get the vehicle moving.
When it comes to a limited slip differential,
there's several kinds.
There's geared, clutch or plate,
and viscous coupling, and electronic
and then within those four major methods,
there's other layers like wheel speed monitoring
or torque vectoring and for right now,
let's just talk on a clutch-based system.
Clutch type LSDs are probably the most popular.
In these, the spider gears sit between
a pair of pressure rings.
These pressure rings are fitted to the outer casing
and the side gears are fitted into the rings.
A pair of clutch packs makes up the friction discs
and steel plates
are then fitted on both sides of the pressure rings.
The clutch packs fit together a lot like sinking rings
from our old manual transmission video
and the clutch pack should be pressed together at this point
and the frictional force
is gonna make it move as a solid unit,
turning as one axle as the vehicle moves forward,
but if one wheel loses traction on a patch of icy baby oil
that engine power going to it
will meet with the resistance from the clutch pack
as it tries to keep moving as one unit.
It's gonna divert the energy to the drive wheel
for more traction
and that's basically a clutch back LSD for ya.
Toyota's got a unique auto LSD system
that uses an open differential
but it applies the brakes
automatically to the wheel with the least traction,
creating resistance to make the wheel with traction
the wheel with less resistance.
Pretty sneaky fellas.
Another way of solving the problems from open differentials
is with a locking differential.
This is particularly useful for off road vehicles
where you could expect one wheel to lose traction
because it's in the air.
(engine revving) (cameraman laughing)
Locking differentials have a mechanism
that will literally lock the axles together
when you need it, in this case,
both wheels receive equal power
so that the wheel with the most traction
will always have power
and now if you really wanna go nuts,
you can throw on your welding helmet and,
well, I mean, that's enough honestly,
if you're wearing a welding helmet
and you go anywhere where there's no welding going on,
people are gonna think you're nuts,
but no, put on the welding helmet
and weld those spider gears together.
Anybody know why we do that?
Does anybody?
Nobody?
What is this, just an Up To Speed thing,
this doesn't work on my show? - [Man] You suck!
- Well, it's for drifting.
A welded diff keeps both of the back wheels
spinning at the same speed
and you don't have to buy a new rear axle.
Welding the diff minimizes power loss from the drive shaft
and makes sure both wheels are always getting power.
When you're drifting,
the last thing you need is for one tire
to be able to follow a different path from the other.
You want both of 'em burning rubber.
A welded diff is a backyard fix to make oversteering easier
and make it easier to go into a drift.
Torque vectoring differentials,
which I briefly mentioned when talking about LSDs,
groovy man, are the most advanced and complicated,
kind of like me.
They use electronic clutches and a separate controller
to forcibly slow the wheel on the inside of the turn
sending torque to the outside wheels powering the vehicle.
Now all of this is for a basic rear wheel drive vehicle
or front wheel, whatever, it's no judgment,
but what about four wheel drive and all wheel drive?
Just so the video's not 28 minutes, basics.
Full time four wheel drive and all wheel drive vehicles
have differentials on the front axle
and one on the rear axle.
They also have a third differential
in the middle of the drivetrain.
This allows wheels to move at different speeds
and prevents drivetrain binding.
All wheel drive may have an open
or limited slip differential in the middle,
but the four wheel drive has a locking differential
for when four wheel drives engaged,
the center differential locks
and all four wheels get sent equal power!
But we'll get to that later.
Differentials!
Thanks for watching this episode of Science Garage, guys.
We can't do this without you.
Subscribe to Donut, hit this beautiful globe.
We got new shows coming out almost every day of the week.
Check out this Up To Speed on Koenigsegg.
You know what'd be cool, welding the diff on a cop car.
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Don't tell my wife she's driving around with a welded diff,
she's doesn't know.