It’s nearly a year since the last in this
discontinuous series. In the last story, Part 6, we covered the ideas of stress
and strain, while in previous parts we’ve covered everything from design to
using taps and dies (see the breakout box below for the full list.) Now it’s
time to look at another fundamental, and something which you almost never see
covered in textbooks.
And the topic? – designing for minimum weight.
Note that here we’re not talking about the
selection of the material (eg carbon fibre versus chrome moly steel), and
neither are we talking about a full scale design analysis that quantifies the
stress on each component. Nope, here we’re talking a much broader philosophy. So
how can designs achieve minimum weight?
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Making
Things, Part 1 - Forces in structures
Making Things, Part 1
Making
Things, Part 2 - Cutting holes to make things lighter
Making Things, Part 2
Making
Things, Part 3 – More on holes
Making Things, Part 3
Making
Things, Part 4 – Taps and dies Making Things, Part 4
Making
Things, Part 5 – Jigs and templates
Making Things, Part 5
Making
Things, Part 6 – Strength of materials
Making Things, Part 6
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1. Make Parts Perform Multiple
Functions
Cars have become fat and heavy. Even a small car
like a Corolla now weighs 1300kg (and I know it’s no longer ‘small’ but still...)
and it appears that engineers don’t even turn a hair at adding brackets,
structures, accessories and redundancies to their designs.
Let’s say you’re building a full car. Obviously an
engineer will have to assess and pass the final design but even within those
constraints, there are still many ways to achieve an outcome. Take for example
the front suspension. What is the absolute minimum that the chassis must provide
in order that it can locate the bits and pieces?
But before that, what are the bits and
pieces that must be located? Considering a double wishbone suspension, the
chassis must provide:
Allied points for consideration in designing the
front end (in a front engine car with front wheel steering!) include the:
-
Engine mounts
-
Steering rack mounts
Sure, maybe it’s all pretty obvious. But now the
key question becomes: how can some of these functions be integrated into the one
assembly? That is, how can a single part perform multiple
functions?
For example, the upper wishbone mounts may be able
to be integrated into a chassis member that also takes the vehicle’s weight
acting through the spring. Now, since this member will have to be strong (it’s
supporting that corner’s weight, remember), can it also be used to support an
engine mount? Or, to take that more broadly, be part of the structure that supports the engine? In turn that would feed the force of the engine’s
weight (inertial as well as static) straight through to the upper spring mount,
and via the spring, through the tyre to the ground. And since the instantaneous
forces acting on the upper damper mount are likely to be much higher than the
forces acting through the spring, how can we use this same member to support the
upper damper mount as well?
From this prelude it’s obvious this member will
have to be strong (and so relatively heavy), but if it can handle the upper
wishbone mounts, the upper spring seat, the upper damper mount and help support
the engine, well, then it’s earning its keep.
You can then do the same mental exercise for the
lower wishbone mounts and the anti-roll bar mounts. In fact, can the anti-roll
bars mounts be integrated into the upper wishbone/engine/spring/damper
structure, so saving some more weight? (Stiff anti-roll bars can develop quite
some force at their mounts!)
In many respects the key point is not as many
people think – how can high loads be well-spread. Instead, it’s how can high
loads be concentrated so that one member can be designed strongly to cope with
all that can be thrown at it – and so other areas can be made far lighter. To
put it another way, the metalwork that supports the engine’s weight will
probably be strong enough to be part of the assembly that also supports the
upper wishbone mounts, the spring mount and the damper. Providing these
functions separately will near double the weight of that section of the chassis.
A final thought: racing cars that use stressed
engines and/or transaxles are following this same
how-can-a-single-part-perform-multiple-functions idea. In that case, the
strength of the engine and/or transaxle can be utilised to become part of the
chassis of the car.
2. Look at What Others Have Done
Above I made a snide comment about current car
designers. However, it’s absolutely vital when designing anything for minimum
weight that you look hard at what other designers and engineers have done. It’s
not for nothing that at the pits of any race meeting you’ll see, where possible,
race car engineers walking around looking at other cars. And don’t limit your
inspection to current machines.
Lightweight engineering of vehicles started over
100 years ago with fixed wing aircraft. In addition to using a self-made
internal combustion engine (one exceptionally light for its developed power),
the Wright brothers also knew all about lightweight designs that were
sufficiently strong. The fact that they didn’t have Kevlar thread and carbon
fibre skins is completely irrelevant – the physical forces acting on the various
surfaces are just the same today.
In addition to fixed wing aircraft, the airships
of the 1920s and 1930s show exceptional use of aluminium space frame structures,
while stand-out race cars (including the Maserati ‘bird-cage’) and the first
monocoque race cars are all food for thought. The 1930s and 1940s transition
from steel chassis cars (often with wooden bodies) to full steel monocoque cars
(and the same occurred in railway rolling stock - Budd was the leader in the US)
also makes for very interesting study. That’s especially the case as the new
bodies had to have clear and exceptional advantages over the old for them to be
readily accepted.
Designs that have changed little over time are
particularly interesting. We’re so used to assuming that tomorrow’s designs will
be better than today’s that many people forget that some designs are nearly
fully optimised, and consequently have changed little. Except in materials, a
current diamond-framed bicycle is little different to its brother of the late
1800s. In fact, to go further, many of today’s avant-garde bicycle use designs
that are clearly fundamentally weaker than traditional bikes.
When looking at the work of other designers,
consider two points. One, the idea covered above, is: have they made a
single part perform multiple functions? If they have, it’s likely that
the finished result is lighter.
The other is: have they got a better ‘take’
on the fundamental forces involved than later designers? This one sounds
a bit silly: I mean, how can blokes working without computers have any idea? But
you might be surprised: they might well have - either through
build-it-and-then-work-it-until-it-breaks engineering, or lacking the luxury of
super new materials, by analysing stuff until the last gram of surplus weight
cries for mercy. These ideas are perhaps best illustrated by small racing
sailing boats: the sailing rigs take enormous forces, are light, and use
materials that are often quite pedestrian.
It sounds a bit ho-hum, but a browse in your local
major reference library on the following topics (and don’t forget, universities
and colleges will normally have no problems with an ‘outsider’ simply looking at
the books) is likely to open your eyes to the possibilities:
First aircraft (eg search under ‘Wright
Flyer’)...
...Sailing boats and other wind-driven craft
(especially in sailing rigs and hull materials)...
...Airships (search under ‘Hindenburg’ – this pic is
of the USS Shenandoah)...
...Monocoque racing cars...
...Tubular frame racing cars...and...
...History of bicycles (this one is from 1897!).
3. Use Material That’s Only Strong Enough to do
the Job
subtitled: don’t be lazy...
Let’s think about a D-shackle that holds a sway
bar in its pivoting bushes. I don’t think I’ve ever seen one that isn’t pressed
from a single piece of flat steel bar. But if you’re after minimum weight,
that’s crazy!
Why? Well, the forces acting in the shackle are
not the same throughout the shackle. The flat ends, those that have the bolts
through them, are subjected to bending as the sway bar tries to move away from
the mount. So these parts need to be strong enough to resist these forces. But,
because it’s in tension, the curved part of the shackle can be made from much
thinner material. If appropriate sway bar links are used at the ends of the
‘bar, the curved part of the bush shackle will never be subjected to bending.
Now a sway bar bush shackle that’s fabricated from
three parts, or that has had the curved part reduced in thickness, sounds a bit
anal: how far is this bloke going to go? But that‘s why I’ve put the subtitle:
don’t be lazy. If you want a competitive advantage, or to go where others have
not, every single fine detail needs to be examined.
Yes, I could make this bracket from
a piece of bar, or tube, or an I-beam. But where does it have to be
strong and where can it be relatively weak? The answer is not going to be found
in an off-the-shelf pre-formed steel shape.
And it’s in this area more than many others where
composite plastic materials have a major advantage: their thicknesses can vary
at will. Intersections can be thicker; the far ends of cantilevers thinner. But
even with traditional materials like steel and aluminium, much better matching
of strength to application can be made by careful fabrication, or if the budget
extends that far, by dedicated castings. It will be slower to produce and be a
more complex job, but it will be likely as strong while being a lot lighter.
A final simple example: a car chassis made from
steel tubes shouldn’t use the same wall thickness and tube diameter throughout.
4. Make Things Small
Smaller things are lighter than larger things. And
they’re smaller not just because there is less material in them, but because the
stresses are less and so the strength of the material can be kept lower. In cars
(and all machines that move), lighter also equals less inertia – easier to get
moving, easier to stop, and easier to change direction. In turn that means
to achieve the same performance, less engine power, smaller brakes
and smaller tyres and wheels can be used. All in turn lighter!
Going small applies whether you’re talking engine
selection, wheel diameter, in-cabin space, fuel tank size – literally anything
and everything. Alec Issigonis, the designer of the original and iconic Mini,
used 10-inch wheels. Then and now, such small diameter wheels are pretty well
unheard of. But by making the wheels and tyres small, he was able to capitalise
on interior space (and of course the transverse front-wheel drive engine and
incredibly compact suspension also had much to do with this!). As a result, the
whole car size could be reduced.
Making things smaller is the easiest way of saving
weight: even without changing anything else at all, the gains can be immense. So
rather than saying: what do I need to perform this function?, say instead:
what’s the smallest part I can use to perform this function?
5. Weigh Everything
Absolutely vital for lightweight design is a set
of accurate scales. The maximum mass capability of the scales will depend on
what you’re building, but you really do need to be able to read down to tens of
grams.
So: do I use an off-the-shelf through-bolt here or
do I have a threaded shaft custom made - one that is thinner in the middle? (And
‘custom made’ might be as simple as grinding away part of its thickness.)
This bracket could be made this way
or this way – and just by plonking the raw materials on the scales
you’ll get an immediate indication of the likely weight outcome.
Major items – like wheels, tyres, springs,
dampers, seats, half-shafts, (full!) radiators and oil coolers - must always be
weighed. For these big items, use scales designed for measuring human body
weight – cheap and easy. No wrecker or retailer of new goods is going to stop
you taking sample weights – and on a lightweight car, a saving of 20kg might
improve your power/weight ratio by the equivalent of 5hp at the engine!
If you’re building a full car, keep a running
tally of the overall weight. This will comprise two columns – one estimated and
one actual. As you progress in the build, more and more ‘estimated’ will become
‘actual’. If you want to get really motivated, work it out in terms of how much
horsepower you’re effectively losing for each additional 10 kilograms.
Conclusion
Clearly if you’re modifying a car, or even
building a car, it’s vital that safety is maintained. I am not talking about
necking-down through-bolts on suspension bushes, for example. (But keep in mind
that many ‘camber kits’ do just this!) However, without compromising safety, or
requiring that you use exotic, hard to handle materials, it’s possible to take a
lot of weight out of a creation.
But the first step is that you become absolutely
focussed...
Doing
It
One
of the few cars we’ve featured over the years where the modification process had
minimum weight as a major aim was Ian Richards’ Daihatsu Mira – see
Mighty Midget
Ian
weighed literally everything coming out of the car and going into it. In
addition to replacing steel body panels with carbon fibre, and some glass with
acrylic, he also looked at the mass of such things as the steering wheel,
steering column and engine cam cover (new, lighter replacements made) and even
lost 1.8kg by removing surplus steel brackets.
The
result was nearly 300 hp/tonne from just a 1 litre turbo
engine...
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