E92 - TMS E92 M3 Roll Cage
By Thorney Motorsport
We seem to have more phone calls and questions on cages and
their fitting than anything else these days, be them for full race car
preparation or just track day use there does appear to be a lot of misunderstanding
about what a cage is designed to do it seems.
In simple terms there are two kinds of cages for cars – a safety cage
and a chassis cage. A safety cage is exactly as it sounds, a cage installed
into the car to protect the occupants in the event of a crash, its purpose is
simply to prevent the occupants getting squashed in the event of a side impact
or worse still a roll over. Pretty much all bolt in cages are safety cages. The
other kind, a chassis cage, is where the whole chassis is essentially tightened
up and strengthened effectively making parts of the bodywork simply there for
show; the whole car is stiffened up around the design of the cage. This is what
we do.
Designing cages is part art, part experience and part
science, the overall aim is to connect up the whole chassis to one controllable
frame where flex and lack of flex and movement can be predicted, controlled and
taken advantage of to improve grip and handling. All road cars have flex
designed into the chassis, sometimes deliberately (flex = more comfortable ride
in some case) and sometimes as a necessity of design (big holes in the frame
for doors, sun roof etc) so the one constant when designing a cage is to
increase the overall chassis stiffness. One crucial aspect is to stiffen the
chassis at the suspension points, by limiting flex here the cars suspension can
be isolated from external movement and a ‘pure’ suspension set up be developed,
whilst road car chassis tend to be quite stiff at the suspension points the
extra stress that full race suspension (especially on slicks) is such that it
is here that we can make the most gains.
Here pictures paint a thousand words so in picture one you
can see the intricate design the cage makes up, the front suspension towers
(where the front suspension bolts to the cars chassis) have the white cage bars
extending from the main cage and the main cage in the cabin forms a multiple
role of both protecting the occupants in the event of a roll over but also ties
together all aspects of the chassis to each other.
Picture 2 shows more detail of the front suspension tower.
Front suspension legs are a key area of both lateral and longitudinal flex,
they literally stick out from the shell with the engine mounted between them so
it is imperative to tie these into the chassis otherwise the whole front end of
the car will feel loose and disjointed from the back, especially under braking.
On road cars this is why a lot of performance cars have strut braces where a
bar of metal is bolted from the top of one suspension tower to another, most
race cars (where the cages have been extended far enough) don’t need them. The
picture shows how we support the suspension on three separate axes, from the
centre, the side and underneath. Picture 3 more clearly shows the two sides where
the cage is welded to the suspension tower, underneath the suspension point is
re-enforced so in effect the front suspension is actually attached both to the
cage and the chassis rail together.
Moving back from the front of the car the next area for consideration
is the A pillar/windscreen and passenger safety cell. Picture 4 shows the A pillar design, the cage
comes across from side to side (known as the dash bar), from the front
suspension support (one three axes) and forms part of the side and roll over
protection, bar A, B and C are both side impact and form part of the roof bar,
strength in this area is important as it protects the occupants legs in the
side of a side or three/quarter impact – common in racing. If you see in this
picture of the E46 M3 crash we suffered even though the impact was over 60mph
sideways into a concrete wall the drivers leg area was completely unaffected.
With the relative high weight and size of the E92 chassis
we’ve designed the side impact protection to be as low as possible to keep the
centre of gravity as low as we can, it also makes the car easier to get in and
out of, for regulations purposes there is a set time limit of ten seconds to
get out, if it takes any more then it in theory is illegal, mind you, if the
car is on fire its a pretty good incentive for even the most immobile driver to
get out pretty damn quick!
A lot of the main design work goes into the main chassis
part of the cage, here the driver needs to be protected by the roof bars, a
roll over hoop which passes under the B pillar section of the car and then all
connect up to the rear suspension towers. At the same time we need to ensure
that there is room for a larger capacity fuel tank (the car has been designed for
sprint and endurance racing where a 110 litre tank is needed), provision for
harness mounts for the driver and passenger (this car will be used for demo
rides for passengers) and of course strengthening and supporting the chassis
and in our case air jack points.
Picture 5 shows how all these parts interlink, A is the roll
over cross brace protection, B ties the rear suspension towers and supports to
the main front hoop of the cage, C ties the suspension towers to the chassis
floor, D ties the chassis floor to the main B pillar hoop and E is the harness
support mounted as close as possible to the seat to avoid harness flex (more
important now with Hans devices compulsory in a lot of race series). The
important feature is that every part of the chassis is literally tied to each
other, this creates both a very stiff platform but also creates structural
connection that we can exploit when setting the car up – the whole car reacts
as one rather than several disconnected flexible points.
Picture 6 is taken from the boot, here you can see in detail
how we build up the rear suspension chassis points, weld in extra strengthening
(some inside the tower where you cant see) then ensure that the whole are is
supported again on three different axes, the suspension is now mounted to the
chassis and the cage which is connected to the chassis itself to the floor, the
side and the cage. Picture 8 shows the same area from a different angle and
also shows part of the fire wall which we need to install – this is to separate
the remotely adjustable suspension (which are high pressure gas filled) from
the driver section, another regulation.
Picture 7 shows the cage from the rear, the symmetry is
perfect.
Once the cage is in we need to paint it, very thin,
lightweight paint is used with a single cover and one of laquer, we want it to
look nice but extra paint means extra weight – not needed, whilst white is a
pain to keep clean its a traditional BMW colour and we like to keep some
traditions and you’re done. Next up is putting it all back together.
Cages, Harnesses, and the LAW. Contrary to popular belief it
is not illegal to have a cage in road car, even one of these silly ‘cosmetic’
cages made of shiny polished aluminium (which are lethal in the event of a
crash as the aluminium conveniently folds itself into knife shapes when it
deforms). However the design needs to ensure that the occupants wont come into
contact with the cage in the event of a crash – not as simple as it sounds if
you keep using the standard static seat belt as they allow the body to move a
lot more than race harness. This means at least 6 inches of clearance from your
head. Using harness in a road car is also not illegal but technically the belt
release button must be a push button type not an aircraft type to pass an MOT,
this is to ensure that ambulance, fire service and police know what to do to
release you, for some reason the DOT thinks all first responders are thick, go
figure. Typically on a road car we tend to advise owners to keep the static belt
for the road and a harness for the track, the reason being that race harness
only work when they are tight and I mean tight, as in hard to breath in fully,
a race harness worn loosely will simply snap your collar bones in the event of
a front impact, these then fold in two resulting in your head hitting the
steering wheel – unpleasant. So, do it right and you’re fine, do it wrong and
at best the police will nick you and at worse you’ll get killed so be careful.
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