drag coefficient and lotus 7 shape

dscotland · Post by **dscotland** » Fri Nov 05, 2004 10:05 am

thinking about the hardtop, but will the weight gain offset the
aerodynamic gain?? (and racing with the roof down is more fun anyway,
otherwise I would have bought a tintop with everyone else...)

Everyone thought that Lotus 7 shaped cars topped out around 100mph, so
imagine EVO magazines suprise when the Caterham RS500 (with mitsi evo
engine) topped out at a frantic 148.4mph. Everyone would need hairdresser
assistance after trying that.!!

(RS500 does 0-100mph in 8.1sec and has 552bhp/ton, so I'll admit it's not
a very representative sample of a Lotus 7...)

"Ian Chapman" <Ian.Chapman@ngc.co.nz>
Sent by: e-admin@mx5club.org.nz
05/11/2004 09:35 a.m.
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Subject
Re: Targa and drag coefficient

Can't quote Cd figures - but any convertible will be faster with roof up
- by virtue of a cleaner air flow along and exiting the car. The shape
of the back of a car is one of the most important things in clean air
flow - obviously frontal area makes a difference. Smooth surfaces and
smooth transitions make a fast car. In ideal conditions a hard top MX5
maybe slightly faster again than a soft top up.

Lotus Sevens even with very grunty engines can do 0-100 in a few
seconds but top out at a bit over 100mph - not a great shape and lots of
exposed bits but have a reasonably small frontal area.

If you want the perfect aerodynamic shape - look to nature - the
classic rain drop. It is free to form whatever shape it wants as it
flows through the air. Look at tapered engine cover of an F1 car.

Many Mini Seven racers liked Riley Elf bodies (remember that cute bum)
because it gave them a few extra mph at the top end. Hmm - so J-Lo
should be a very fast runner!

Ian

Greg · Post by **Greg** » Fri Nov 05, 2004 7:11 pm

hi, all

Not sure on the actual drag coefficients - I think zorruno has that nailed.

I notice that all the pictures of MX5s (Miatas) racing in the Norman Garret
Performance Handbook have hardtops on. I seem to recall a comment in there
about a combination of additional rigidity plus aerodynamics made the weight
penalty worth it.

All depends on how serious one is I guess ...

Regards,
Greg

1990 V Special

Post by **zorruno** » Sat Nov 06, 2004 10:34 am

Greg wrote:

I notice that all the pictures of MX5s (Miatas) racing in the Norman
Garret Performance Handbook have hardtops on. I seem to recall a
comment in there about a combination of additional rigidity plus
aerodynamics made the weight penalty worth it.

I'm guessing it is probably more likely that the race rules forbid
roofless cars. The spec miata races in the US seem to allow roof off
racing, but generally standard classes don't allow it due to the
possibility of things (like arms) flying out of the car in a roll or
stuff getting into it (like gravel...). Not that a hard top would stay
on long in that situation anyway.

cheers
zorruno

EricW · Post by **EricW** » Sat Nov 06, 2004 11:49 pm

An MX 5 with a hard top will, absolutely unarguably, have a lower Cd than
one with a soft top and that car will be better than one with the top down.
It is simply a matter of stream-lining, turbulence and skin friction.

Without testing it, one would imagine that there is a considerable
difference top down, with turbulence occuring behind the wind screen and air
being sucked in to the low pressure area in the cockpit. Indeed the fact
that we have wind blockers fitted to the car, (or get a stiff neck from the
breeze coming in from behind), would tend to bear this out. A soft top will
deflect this flow into a far more laminar configuration, but the
irregularities caused by the hood frames and the texture of the fabric, this
accompanied by drumming of the fabric setting up turbulence, will still
contribute to overall drag. A hard top will reduce those problems and
produce the best laminar flow, albeit in the case of an MX5, still not
spectacular compared to a modern aerodynamic shape, such as an Aston Martin,
Porsche or Lamborghini, but a lot better than your average Toyota Hi Ace.

There ought to be quite a difference in top speed between a car fitted with
a hard top and the same car top down, because the faster you go the greater
the effect of drag. The "law of squares" tell us that if we double speed we
square the drag. Thus, if we have quite a bit of difference in total drag at
60mph between a car with the top down and the same car with a hard top, that
difference will be four times greater at 120mph, as will the total drag on
both cars.

One could experiment with this by driving a set distance, at high speed,
with each of the roof configurations, in the same conditions, and comparing
fuel consumption. It would probably be most instructive.

Regards

Eric

Grant · Post by **Grant** » Sun Nov 07, 2004 8:50 am

I have got the results of very un scientific checks with the roof up and
down. As the roof is very rarely up, and I keep a log, the only difference I
noticed in fuel consumption was when I bought a new set of plug leads (about
10% improvement!). The next time was on a return trip to Christchurch
(700km) with the roof up and immediately noticed an improvement in fuel
consumption. Can't remember the figures now but it was also near the 10%
mark. This at normal legal road speed.
I wonder what effect a windblocker has on the cd.
Now off to Picton on a callout, roof down, bright sunny calm morning and
already 20 degrees out there!

Grant.