We hear the word ‘differential’ used a lot from drivers and commentators. It’s
something that can be adjusted to help the handling and performance of the car.
But rarely is any detail ever given on what the differential is and why it’s important.
So let’s have a look. But first, a very quick aside to mention Skillshare,
who have kindly sponsored this video. You already know I’m a fan of Skillshare
as an enthusiastic self-learner. Skillshare is an online community that’s host to thousands
of courses across all kinds of topics People often ask me how I make the animations
for my video and this video you’re about to watch is much more 3D than normal, for
reasons that will become clear. I don’t work in 3D a lot but I always try and push
to learn new skills in After Effects, my software of choice.
And if you bang After Effects into the search bar at Skillshare you’ll see hundreds of
courses to help you. To use 3D effectively I had to understand how the camera works in
After Effects so I made sure to familiarise myself with it as I went along.
If you want some straight courses in explaining how to animate confidently, then try Jake
Bartlett’s comprehensive overview for starters. Thanks to Skillshare’s sponsorship you can
get a free two months trial with the link in the description so you can have a feel
for it yourself. Whatever kind of content you’re creating you’ll find ways to get
confident in creating in high quality. OK, let’s find out what a differential does.
OK, so the first thing a differential does is translate the torque of the drive shaft
(which turns this way) through ninety degrees to put torque through the rear axles. Remember
torque is a turning force. I’m going to illustrate this in a very simplified
way throughout. Know that the mechanisms are a bit more sophisticated than the paper cut
outs I’m using throughout this video. So, the driveshaft ends with a pinion gear
at its end, like so. And this locks into a ring gear like this.
So now we see we have turned the turning force around from rotating about this axis to rotating
about this axis. Awesome. Now, we might imagine both rear wheels are
connected by a single axle running between them. The ring gear might attach to this axle
which drives both the left and right wheels equally in lock step.
In this scenario each full rotation of the left wheel is matched by a full rotation of
the right. And we could do that. It seems sensible at
first thought. Except this runs us into problems as soon as we try and turn a corner.
If we imagine the path of a car turning through a bend, it’s clear that the outside tyre
travels a longer path than the inside tyre. It’s tracing part of a larger circle, you
could say. So, really the wheels need to be able to rotate
at different speeds through corners. The outside wheel needs to rotate further than the inside
wheel to travel the longer distance. If both wheels were locked in rotation the
loaded wheel - the outside wheel - would dominate and force the inside wheel to rotate too fast,
which drags the rubber along the track, causing unnecessary wear and introduces friction and
inefficiency to the whole system. This is bad. This is not fast.
So, that brings us to the second job a differential has to do - allow the left and right axle
to rotate at different speeds as and when it’s needed.
The way it does this is by introducing a third type of gear - a spider gear - to interact
with the axles. The spider gear axel is fixed to the ring
gear. So the spider axle rotates as the ring gear rotates. The spider gear can then rotate
around it. The spider gear sits between the two axle
gears and so drives them as the ring gear turns, just as the ring gear did before.
Driveshaft. Pinion Gear. Ring Gear. Spider gear. Axle Gears.
When the car is in a straight line, the spider gear does not rotate on its own axis so it
drives the axle gears equally. Both wheels turn together in step, just as before.
But, when the car starts cornering, say, to the right, the spider gear rotates on its
own axis as it turns on the ring gear. How does this make a difference?
Well, now the spider gear is applying a turning force to the axles in two ways.
Firstly, the rotation of the ring gear dragging the spider gear around turns the axles in
this direction But also, the rotation of the spider gear
itself turns the left axle a bit more in the same direction and it turns the right axle
a little bit less in the same direction. The net result is the left wheel turning faster
and the right wheel turning slower. Brilliant. Problem solved.
Except - and this is where we get a little more technical - this solution has its own
problems with high performance cars like those found in Formula 1. Let’s see why.
What we’ve described so far is an ‘open differential’ system. It’s a clever, simple
mechanism that responds to the resistance of the wheels and reacts mechanically.
The problem is that if the grip under each wheel is significantly different, the open
differential overcorrects and causes massive problems.
An example: let’s say there’s big oil patch down one side of the track where some
unthoughtful driver has tried to get back to the pits on a blown engine.
So the right side of the track is a lot more slippery than the left.
In delivering power through the wheels, the left side has a lot more traction that the
right. But this means the left wheel experiences a lot more resistance than the right.
So what happens is almost all of the power takes the easy route to the right wheel, which
slips, wheelspinning massively while the left wheel gets left with almost no power to keep
the car moving forward. The car bogs down. In this situation it would actually be easier
if the two axles were locked together because then the left tyre could keep some power to
drive the car forwards using the grip is still has.
And it’s not just a situation as crude as one tyre being in oil, water or grass that
can cause a significant difference in traction between the left and right wheels.
When a car pushes through a fast corner we call the outside wheels the loaded wheels.
This is because the car’s centre of mass shifts to the outside and results in a lot
more force pushing on the outside tyres than the inside.
As such, the outside tyres are driving with a lot more grip that the inside tyres due
to their high load so the inside tyres have traction loss and can quickly spin up as if
they were on oil as the power is sent to the ‘less difficult’ wheel.
So F1 cars (and most racing performance cars) have a type of limited-slip differential.
This is a modified type of differential where the differential can adjust itself on a scale
between being fully open (where the two axles rotate completely independently) and fully
locked (where the two axles move in lock step) When one tyre is suffering traction loss,
the limited-slip differential can start to lock the axles such that more power is sent
to the high traction tyre, allowing it to keep pushing and stopping the slippery wheel
from spinning wildly. By starting to lock the slippery axel to the
grippy axel you’re making it harder to turn the slippery axel so the power gets shared
more evenly between the axles. There are quite a few ways to achieve this,
including some entirely mechanical reactive systems but F1 uses an electronic hydraulic
system. This means the onboard computer has been pre-programmed
(or mapped) by engineers to respond to a variety of variables (like how hard the accelerator
is being pushed and how much resistance each wheel is experiencing) to increase and decrease
the lock of the differential accordingly, preventing excessive power going to just one
of the axles. This is done through a hydraulic system that
makes precise changes in fluid pressure to change the friction acting on the system between
each axel and the spider gear. When a driver ‘adjusts the differential’
from their dashboard, they are changing how aggressively the differential responds. A
maximum locking differential will keep the tyres fully locked as if one single axle.
A differential set right down to nothing will act as if fully open - both wheels completely
independent, with no lock kicking in. Through a corner, the differential will open
up as the car turns in, allowing the wheels to rotate freely as their path dictates.
But as the driver applies the accelerator and puts power into the wheels, you’ll want
the differential to start to lock so the power can be delivered properly to the loaded, high
traction wheels and give you good acceleration out of the corner.
The electronic mapping should respond to the car and do all this for the driver.
But, as the driver uses different compound of tyres, or as the tyres wear, or as the
track goes through different conditions - hot, cold, dusty, clean, wet and dry and so on
- the driver will want to adjust the sensitivity of the differential accordingly.
The soft tyre will be more naturally grippy than the hard, for example, so its traction
through a corner will be different and require a looser differential.
The driver will compensate for this throughout the weekend and even throughout a lap as they
feel what’s needed from the car. And that is the basics of the differential.
If you hear that a driver has made adjustments to the diff, they are likely feeling the driveability
of the car through the corner and improving it as necessary.