What a drag.

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RICHARD MANSFIELD2
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What a drag.

Post by RICHARD MANSFIELD2 » Wed Jul 08, 2020 11:50 am

Hi,
I run Focus ST 170 wheels on my Puma, as most of you have deduced the spare will not fit in the standard spare wheel space under the car. As a result I carry the spare in the boot, so there is no point lugging round the weight of the steel spare and the tray. Now in the interests of weight and drag etc, a lot of time and design goes into the aerodynamics of a car regarding noise and fuel efficiency. From now on in I admit I know very little about the impact of aerodynamic drag under the Puma. So I imaging there must be some impact on drag by removing the tray and leaving a large void where the wheel and tray would have been. Well now I will open the debate, has any one got any views or experience of drag and its effects under the car. I know about ground effect on F1 cars having watched the program about the early 70's Lotus F1 cars and the discovery of ground effect.

Regards,
Richard.
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Post by mmc757 » Wed Jul 08, 2020 1:35 pm

The level one idea for the underside of the car is to present as little obstruction as possible. Modern cars are typically as smooth as possible down there. Look under any VW and you will see massive plastic trays to keep everything smooth.
Level two is to shape your undercarriage to actively create downforce. Cars with a front diffuser or air-dam are trying to actively create a low pressure area under the car, much like the sloped leading edge of an airplane wing (upside down in the case of a car). Rear diffusers also help with this, although its the wrong end of the car for a Puma to be worried about downforce.
Sometimes, when you have a large obstruction, such as a rear bumper cover, you might think that obstruction may act like a parachute, when in fact if its well-sealed so no air flows through it at all, it can create a shield-like air bubble. Thats why we dont hhave vents in our bumpers to let the air pass through.
So the trick is to think of the bottom of the car in terms of whats actually in the air stream and whats in the bubble.
Its possible to create your own ABS plastic (or other lightweight material) trays under the car like modern cars use, but there's some liability concerns if you lose that plastic sheet on the road.
Most likely, removing your spare isnt having a major effect, or if anything, its a net positive effect.
From a weight and space perspective though, one thing to think about is many modern cars dont even come with spares anymore...Instead, they carry a small 12v air compressor and a bottle of tyre slime.
https://www.amazon.co.uk/Slime-CRK0305- ... 63DN9H97N4
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What a drag.

Post by HampusSoder » Tue Jul 14, 2020 4:45 pm

Well I would imagine drag wise it's better to maybe keep the tray even if wheel is removed. Ideally you would maybe remove the tray as well and make your own flat floor to replace it.

I thought I had the tray left on mine but now when checking I realise I don't have it - and it really is quite a big hole.
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PhilLew
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What a drag.

Post by PhilLew » Tue Jul 14, 2020 6:01 pm

Personally, I’d keep the spare in the tray under the car as an emergency ‘get you home’ option and not bother with the one in the boot.
The steel wheel will work perfectly well, even if it’s a different size, in the same way a space saver wheel does on most modern cars, use the boot for something more useful and change to the other ST170 wheel once you get home.
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RICHARD MANSFIELD2
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What a drag.

Post by RICHARD MANSFIELD2 » Wed Jul 15, 2020 8:50 am

Hi.

Thanks for you replies. I in fact have two ST170 wheels with new tyres. The only space I need in the boot is room for two carrier bags of shopping. The idea of the spare in the boot is to a) save grovelling under the back of the car on a dirty wet road in the dark (worst case scenario) and b) having to change the wheel twice. (saving time and effort).

Regards,
Richard
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Post by gingerdave » Thu Jul 16, 2020 8:30 am

Taking things to extreme,if you look underneath a Lotus Elise/Exige there are no obstruction at all ,just 3 aluminium sheets.No jacking points or even exhaust.Just one flat area.
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What a drag.

Post by DOH » Thu Jul 16, 2020 12:59 pm

RICHARD MANSFIELD2 wrote:
Wed Jul 15, 2020 8:50 am
The idea of the spare in the boot is to a) save grovelling under the back of the car on a dirty wet road in the dark (worst case scenario)
Hi Richard
If the road is wet /dirty , how do you plan to fit the spare wheel form the boot without getting a bit grubby?

The only time I plan to removing the spare wheel would be for a track day, purely for weight saving purposes.

The boot space is very useful for such a small car, it is a substantially larger capacity than a similarly sized MX5.

We have to put up with the letter box rear view through the mirror because of this.

A few on here did cut a series of holes through the rear bumper including PumaJay.

It would be interesting to hear from them to see if it made much difference on track with regard to drag.
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