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Something often eluded to but not given much explanation is the size of a car’s wheelbase.
Wheelbase length is often a key part of the car’s design package with small variations
leading to big differences in car performance and handling characteristics.
Over the last few years, Mercedes have tended to show up with the longest wheelbase on the
grid and Red Bull the shortest and this remains true even as their overall difference has
tightened. The wheelbase is defined as the length between
the vertical centrelines of the front and rear wheels - essentially marking out the
footprint of the car along its length by measuring the points at which it actually holds the
track. This is a different measurement from the overall
length of the car, though the size of the chassis itself is never completely independent
from the size of the wheelbase. A longer car will mean a longer wheelbase.
And though the wheelbase can technically be whatever length the team demands, it is restricted
by a number of other factors in the car’s design and regulations.
A car is a complete package of interacting features and variables with few single parts
being designed in isolation. The length of the powertrain - the combustion
engine, power unit components and transmission can force the wheelbase to grow or allow it
to be tighter. So choices made in packaging engine components
for weight positioning, cooling or power delivery can echo out into the wheelbase length.
Similarly, the size and shape of the fuel tank force a lengthening of the car, the powertrain
and, as such, the rear axle. In 2010 with the ban on refuelling, the cars’
fuel tanks increased from about 90 litres in capacity to around 210 litres in order
to hold the fuel needed to complete a race distance, though the required size dropped
in 2014 with the super efficient hybrid power units.
In 2019 the maximum allowed fuel increased from 105 to 110 kilograms in order to coerce
teams into driving more flatout and less with fuel management in mind - the relaxed pace,
lift and coast style racing that had been taking the energy out of races.
Teams are not required to build fuel tanks to the full 110 kg capacity though and could
make smaller fuel tanks if they wanted tighter packaging, a lighter car and a smaller wheelbase,
with the side effect being the need to manage fuel more during the race.
As fuel cannot be stored more than 400mm from the longitudinal axis - a line running the
length of the car. This puts a ceiling on how tall you can design and position your
fuel cell, meaning any increases to this size would come from its length.
And while you’re designing and packaging all the components, you’ll often have to
fiddle with your wheelbase just to meet the regulatory requirements on weight distribution.
The F1 car itself must weigh a minimum of 746 kg but this minimum weight must be balanced
in a particular way across its length. The front tyres must always bear a minimum
of 45.5% of the 746kg weight (or 339kg) and the rears must bear a minimum of 53.5% (or
399kg). This means if you move the engine forward
in the car, away from the rear wheels, you might have to move the front wheel forward
to balance the load across the car. All your design decisions have little consequences
like this, which makes figuring out the perfect package a near-impossible task.
So now we’ve understood what wheelbase length is and what internal factors might push that
length one way or another, let’s ask: how does a longer or shorter wheelbase affect
the performance and driving character of the car?
The main benefit of a longer car is that it gives you more space to place aerodynamic
devices and more surface area to control aerodynamic flow.
So between the front and rear axles there’s more room to place turning vanes and bargeboards
up front, more wiggle room to play with your flowing sidepods in order to generate efficient
downforce without airflow detaching or causing problems.
In particular you can really beef up your diffuser size under the car, which is a massive
part of the car’s aerodynamic grip - even more so from 2022.
This is really effective through medium to high speed corners, both giving you the downforce
to plant your car firmly on the desired line at higher speeds and, with its larger footprint,
giving the car a more stable footing as it navigates these parts of the track.
A larger wheelbase is a bit more stable and rides bumps better than a car with a shorter
wheelbase which is more easily unsettled by irregularities in the track surface.
On the flip side though, a shorter wheelbased car is more nimble and can attack slower,
tighter corners better than a longer car can. Think a little bit of trying to get a sofa
through a bend in a corridor. Where a much shorter arm chair can turn much more easily
and navigate several routes around this tight bend, it can be incredibly tricky for the
sofa to find a turning angle that allows it through.
Pivot! Pivot! Pivot! Incidentally, this Friends episode is almost
22 years old now. 22! That makes no sense. 22 years before this episode was 1977. And
I remember 1977; I started going to concerts and I saw the Led Zeppelin…
Anyway. The longer wheelbase gives the car a longer footprint on the ground, meaning
it’s ability to turn nimbly is reduced over its shorter wheelbases counterparts.
Buses, which are arguably pretty long vehicles, actually have pretty short wheelbases for
their length. In fact, the front wheel sit behind the driver, meaning from the driver’s
perspective, they travel past the corner before they start to turn. Isn’t that weird?
Another thing to consider when giving your car a longer wheelbase is the moment of inertia.
This is just the tendency for something to keep rotating once it’s turning.
A longer car will be slightly harder to get turning and slightly harder to stop turning,
meaning its a bit more likely to understeer and a bit more likely to slide and be harder
to straighten up than a shorter car. Hamilton described some of the difficulties
that come with Mercedes going long on their wheelbase.
He prefers a car with a positive front end, meaning more pokey at the front as it steers.
You know, when you turn the wheel, the front of the car just goes in the direction you
want it do and leads the car around the corner. Instead, the Mercedes tends towards understeer,
meaning the front end is sluggish, reluctant to turn in.
Imagine trying to turn an oil tanker around out on the ocean, how the ship seem like it
doesn’t want to turn, preferring its existing direction of travel. It’s like that only
a million times better, obviously. Instead, Hamilton feels the Mercedes is pushed
through the corner from the rear - something the designers have worked on improving over
the years. Managing the longer wheelbase by improving
the aero balance across the car changes its behaviour as it turns corners, changes the
steering angles needed to manipulate the car and the way the car rotates through a turn.
Now, in general it’s pretty much agreed that the cars are too big at the moment and
the good news is that 2022 brings with it a wheelbase restriction of 3600 mm.
For context, that’s about 10cm or 3 inches shorter than Mercedes’ current layout and
2 cm or 1 inch shorter than Red Bull’s. It’s a small start but hopefully something
that can be brought down more in future without compromising packaging and safety structures.