During the British Grand Prix, Kevin Magnussen was yeeted into the barriers in what seemed
like a fairly low energy accident. Nonetheless, his wheel appeared to break loose from the
chassis - something we see a lot less of these days thanks to mandatory Wheel Tethers.
This particular incident was different to the Daniil Kyvat accident which happened a
few laps later in which his wheel hub stayed attached to the car but the rubber tyre was
ripped loose. Wheel tethers cannot stop this from happening.
Wheel tethers are only designed to prevent a
Loose wheel Watch out for Loose Wheel!
I don’t care about Lucille She lied
So the wheel is attached to the wheel hub, which is part of the suspension assembly which
is attached to the chassis. Until the late 90s, that was basically it.
So if the suspension broke in an accident or due to failure, the part with the wheel
attached could just go flying away from the car at extremely high speeds where it could
hit another driver, a spectator or one of the many circuit workers positioned close
to the track. Two marshalls were killed by flying tyres
in 2000 and 2001 each as single wheel tethers failed to keep wheels attached.
Sometimes it’s kind of difficult to understand just how dangerous flying tyres can be so
we need to quickly discuss momentum. Whether something hitting you does damage
or not is very dependent on momentum. Momentum is an object’s mass (*) multiplied
by its velocity. Both are important. Momentum is how an object transfers its kinetic energy
to you when it collides with you. A bus hitting you at just 10 kilometres an
hour is going to do you more injury than a pillow flying at you at a hundred kilometres
an hour. Even though the pillow is going ten times
faster - motorway speed, even - it weighs ten thousand times less.
And as momentum is mass times velocity, the ten times slower bus is hitting you with a
thousand times more momentum and carrying a hundred times more kinetic energy.
A formula 1 tyre is about ten kilograms in mass - a big less at the front and a bit more
at the back, but ten is a nice round number. If it flies off a car and hits something at,
say, 100 kilometres an hour – which is not an absurd estimate as these cars can get over
300 kilometres an hour – that tyre is carrying momentum of about 280 kilogram metres per
second (with kinetic energy of about 4000 Joules)
Don’t worry about all the units and stuff for now - just remember “280” and “3920”
as a sense of scale. I’m aware these are just numbers and it’s
very hard to convey in animation exactly what this momentum FEELS like. How do I convey
a sense of weight and impact?
This is just 4kg weight. It’s less than half the weight of an F1 tyre and while most
adult humans can lift it fairly easily, it’s as heavy as four bags of sugar and you wouldn’t
want it dropping on your bare foot. If we drop it from about 1 metres of the ground
we can do a simple calculation to work out that the dumbbell will hit the ground with
18 kilogram metres per second of momentum and be carrying about 40 Joules of kinetic
energy. If you were lying on the ground and the weight
fell on you if could easily break a bone, crack a rib, give you some serious bruising.
And the weight falling a short distance has over 15 times less momentum and 96 times less
kinetic energy that a formula 1 tyre hitting something at 100 kilometres an hour.
So I guess what I’m saying is - we really don’t want these tyres flying all over the
place. Which brings us back to wheel tethers.
Wheel tethers were made mandatory in 1998 and are essentially just a way of tying the
wheel to the chassis so that even if the suspension breaks, the wheel stays attached to the car
instead of flinging off god knows where. One end of the tether is connected to the
wheel hub, the part of the car you screw the wheel onto. The tether feeds through a suspension
arm and is then attached to the survival cell at the car end.
The tether itself is kind of like a high tech rope made of Zylon fibres.
Zylon is a very light weight, man made fibre polymer which has a number of extreme characteristics
such as its resistance to high temperatures and use as ballistic protection including
in bullet proof vests and around a driver’s cockpit to prevent penetration.
It also is extremely resistant to being stretched and, most importantly for wheel tethering,
has an extraordinarily high tensile strength. That is - how much force can the Zylon rope
hold before it breaks? The wheel tethers are just 110 square mm in
area and yet can support over 70 kN of tensile force, meaning a single wheel tether can suspend
nine formula 1 cars without breaking. And each wheel must have THREE tethers attached
to it. Now this isn’t to increase the overall tethering
strength but to allow for redundancies. Back when we only had the one tether - pre
2011 - we’d still quite frequently see wheels go flying as the zylon rope could still break
if cut or twisted by a particular circumstances. So in 2011 a second mandatory tether was added
and in 2018 a third tether was required. Each tether MUST be proven to function completely
on its own, the only reason for extra tethers is to allow for one - or even two - to fail
and still keep the tyre attached to the car. As such, no more than two tethers can pass
through the same suspension part and must be fastened at both ends completely independently
of each other, 100 millimetres apart. This way, even if one attachment point fails or
is damaged, the other tethers will not be affected.
You’ll only really see the Zylon if there’s a tether failure as the tether is entirely
inside other parts of the car and the zylon is kept inside an outer skin to protect it
from damaging UV light. There are two important things the tethers
can’t do with respect to keeping the wheel and car connected.
The first is to keep the tyre attached to the wheel rim. If there’s a tyre failure
or an accident that causes the tyre to detach from the rim and go flying, there’s no way
to hang on to that tyre. Fortunately, the rubber tyre by itself is much lighter and
flexible on its own and less likely to cause damage.
The second thing the tether cannot do is prevent a wheel from detaching if improperly screwed
onto the wheel hub. Mechanics with wheel guns will firmly connect
the wheel to the hub and the hub is the tethered part of the car. If the wheel nut is loose,
the wheel can detach from the hub and cannot be stopped.
It’s for this reason that the FIA are extremely strict about making sure cars do not leave
the pitlane with an insecure wheel nut. So all this makes Magnussen’s separated
wheel particularly rare and my guess is that the tethers were all broken by twisting and
shearing as the right front suspension dug into the tyre barrier and the rest of the
car twisted around that pivot point. Rather than a high energy collision, it was
more of a ripping, tearing action at slower speeds and as such, the tyre didn’t really
go anywhere. Who knows how many lives or injuries wheel
tethers have saved over the years. We’ve seen some truly high energy impacts over and
over again and in almost every case, all four wheels have remained tethered to the survival
cell and kept from doing damage away from the car.
It’s simple, unseen innovations like this that keep F1 incredibly safe despite its high
energies.