Caterham 7 Cd and frontal area

[edmandsd]

I've read that a Caterham 7 has a Cd of 0.66 - Is this with or without full screen ? Also anyone ever determined the frontal area with/without full screen ?

 

Cheers

David.

 

 

 

Home of BDR700

[Chris K]

I think I remember that .66 was with swept wings n screen and .63 was with cycle wings...

not sure about aero screen etc

 

not 100% sure but a simular conversation occoured on se7ens,net a year or so ago..

 

Chris

 

Now, Beakie, we'll just flip this switch and 60,000 refreshing volts of electricity will surge through your body. Ready?..

[stevefoster]

Would be good to know for pumping in to an AP-22.

Screen must be more than full roll cage OR?

 

 

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Edited by - stevefoster on 5 Mar 2003 14:42:49

[Peter Carmichael]

Dave,

 

You may find yourself getting many different opinions on Cd. You are on the right track by looking at CdA, but this isn't generally a published figure so you need find a researcher who has measured it.

 

Alternatively, wind up your horsepower and measure terminal speed. You get a good indication of CdA from:

 

CdA= power / (term. speed)^3

 

For instance, a Superlight with cycle wings and 135 horsepower (101kW) has a terminal speed of ~120mph (about 140 Caterham speedo mph; 53.63m/s) coinciding more or less with the power peak in sixth gear at 7000rpm. This gives you a CdA of: 0.656

 

I reckon my 250bhp engine would take the same car to 147mph which gives 0.661, so we are definitely in the ballpark.

 

You could take a quick poll of power, top speed, wing/windscreen/wheel format (sounds like fun) and plot the variation.

 

 

 

 

[edmandsd]

Funnily enough I was thinking along these lines and looking at the max speed v power for a variety of production Caterhams.

 

Assuming Cd as 0.66 and working back to determine a constant frontal area for an R500 with 230bhp and 146mph top speed I determined that 160 mph should be possible if power increased to 300bhp, all other things remaining equal. Reducing the Cd to 0.5 increases max speed to 175 mph all other things remaining equal..........if only it was possible !

 

 

 

Home of BDR700

 

Edited by - edmandsd on 5 Mar 2003 16:59:57

[dino ferrana]

OK so we know that it is a very unaerodynamic car and that making it better isn't easy. But what are the worst areas and how could you improve them.

 

1. I reckon aerofoil wishbones with inboard suspension is a start.

2. Addition of aluminium (or carbon) tonneau and boot cover (a la Nurburgring 24hour cars and new R400 racers).

3. Full Aerodynamic undertray incorporating diffuser and nose vented radiator.

4. Long front wings that fit very closely and go close to the ground at the front.

 

Any other suggestions? I know that all mind will be ruled out instantly as being total crap by PC within 20 minutes 😬

[AMMO]

Make a Radical body fit 🤔 😬

 

AMMO

[Peter Carmichael]

I sketched something the other day. Reasonably easy to make a mini Panoz out of a Caterham. Important thing to remember is that diffusers exist principally to reduce drag rather than eliminate lift.

 

Full width splitter at the front. Letterbox air inlet in nose. Diffuse into gap behind the front wheels through full width side pods. All this requires flat undertrays for proper underbody management... etc. etc.

 

dave,

 

Remember it is CdA you want to get down to 0.5, so don't be tempted to make anything bigger than it needs to be. A big smooth shape is as bad as a small rough shape.

[c21vhpd]

Assuming Cd as 0.66 and working back to determine a constant frontal area for an R500 with 230bhp and 146mph top speed I determined that 160 mph should be possible if power increased to 300bhp, all other things remaining equal. Reducing the Cd to 0.5 increases max speed to 175 mph all other things remaining equal..........if only it was possible !

According to Peter's approximation the 21 has a CdA of 0.54 - so it shouldn't be too hard using the smaller 7 chassis to get it down to 0.5... Best of luck

 

Honda Passion Orange, 640kg , and proud of it

[Nifty]

Dino,

 

Look what morgan did with the Aero 8 - now I for one will stick with my brick!!

(Afterall most of my cars have been bricks - 70hp LWB Land Rover!!!!!)

 

Just out of interest, anyone know the CdA for S3 Esprit (normally aspirated) or the original Mini Cooper?

 

[edmandsd]

Peter - I appreciate it's CdA that's important and not just Cd - I was using the formula:

 

Drag HP = (Cd x A x V^3)/(15 x 10^4)

 

With an R500 assuming Cd = 0.66 i'd determined 'A' as 16.8 sq ft. I'd then applied this relative unknow to the same car with 300hp to determine the top speed (V) - All relatively hypothetical I accept, but the numbers don't look silly.

 

You mention that diffusers reduce drag (Cd) - obviously mitigated to a certain extent by their effective frontal area. Does this apply to rear diffusers aswell ?

 

Weren't Freestyle looking into this aspect of the Caterham 7 ?

 

If SBD's Westfield performance is anything to go by I can see why it's worth researching !

 

Home of BDR700

 

 

 

Edited by - edmandsd on 5 Mar 2003 20:24:44

[Peter Carmichael]

Simplistic bernoulli stuff:

 

If air goes fast, it got moving somehow. It got pushed by the air behind it. Therefore the air behind it is at higher pressure. Therefore the fast moving air is at lower pressure.

 

It gets more complex with complex shapes obviously.

 

Diffusers slow down the air. Therefore they produce a high pressure area. You want the high pressure behind you, not in front. A rear diffuser is useful for filling in the hole behind the car. If this is filled with a high pressure area (higher than it was without the diffuser) then you have lower drag.

[Julian Bradley]

But the difference between a full and an aeroscreen MUST be extremely significant - even just in terms as mundane as fuel consumption. The full screen must represent (I'm guessing) some third of the frontal area. Or am I being simple/simplistic?

 

I only mention this as I am going Brooklands this summer, and mine is a 4 speed.

[scooby dooby doo]

aeroscreen seems to be approx worth 20 bhp at motorway speeds. this comes from my own experience, thoughts of others, and things like noticing 4th gear now lasts as long as 3rd used to (6 speed box). I've added power as well since then and the change in feeling from approx 20 more horses is similar.

 

The airflow with an aeroscreen and half doors is much smoother than a screen and full doors (subjectivly from the amount of turbulent buffeting) so I'd expect the frontal area different to be less than expected... IYSWIM...

 

HOOPY 500 kg R706KGU

[EFA]

Peter,

 

"I sketched something the other day. Reasonably easy to make a mini Panoz out of a Caterham."

 

Like this???? here

 

I have costed developing such a body, and reckon it can be doen with a crbon shell for around £10k per car on a volume of 10 cars....... Would include a session with an aerodynamicists wind tunnel using a 1/3 scale model!

 

I'm up for it. Reckon we could hit 200mph with these bodies and some higher gearing!

 

 

On the original topic I belive the 0.66 was a car with a screen with the hod on. Removing the hood had a very detremental effect, although that was still with a screen.

 

Anyone got access to MIRA?

 

Fat Arn

Visit the K2 RUM website

See the Lotus Seven Club 4 Counties Area Website here

 

[yankeedoodoo]

An obvious (to an idiot like me) solution might be to vent the nosecone so that air flows better and doesn't get stopped/pushed by the radiator. I know that if I try this it will fry my engine! Venting the wings/cycle fenders on the rear side like the old Chapparal cars might also promote flow. A few years back some one put a 7 into a wind tunnel and improved it a lot in an hour or two; and it WAS horrendous to start. Has any poster seen that article? I might be able to scrounge it up and fax it if it's of interest.

[Tony C]

In agreement with yankeedoodoo.

Drag can become a horribly complex subject if you allow it

Wind tunnel testing allows quantification and then mathematicians try to rationalise the results.

 

When driving down the road you are basically making a 7 shaped hole in the air.

 

CD is the drag coefficient and is a value which represents the ratio of the pressure differential between the higher pressure at the front and the lower pressure at the back, divided by the air density multiplied by the speed of the car squared (density can be ignored because altitude is not changing significantly).

 

If you maintain the same proportions, making the shape smaller will not affect CD; so frontal area (S) must also be considered.

 

Have to go to work now - maybe more later 😬

[Jason Plato]

here is the article that Tim wrote , what the article doesnt include is a few tweeks that appeared on his car at the next sprint meet we had - 30mm side skirts under the side rails , and closed rear suspension .

 

"Take a look at the extract from the Curborough sprint web site , Tim has been doing some work in the MIRA wind tunnel with his 7 . Makes for interesting reading .

"A Blustery Evening……

Earlier this year MIRA (The Motor Industry Research Association) kindly made their Full Scale Wind Tunnel Test facility available for a Shenstone Club evening, which due to safety and security reasons was limited to 10 members only. Tim Seipel who sprints a Caterham 7, tells us about the evening…

"I was lucky enough to have my car selected for the test session. Following various conversations with Graham Kendall, who not only manages the Wind Tunnels at MIRA, but also competes with daughter Claire (yes that Claire) in the rapid Metro, I prepared a few add on devices in readiness for the limited time available for the structured tests in the Wind Tunnel, whilst some other lucky club members had a tour around the facility. Apart from the obvious Caterham design I had added a full flat floor under tray and rear diffuser. Previous theories on airflow meant that I already raced with the front number plate mounted low down on the front of the nose cone.

Yes I know you are all asking, "Why on earth would you want to do wind tunnel tests on something as unaerodynamic as a Caterham 7?" Well even though it has the aerodynamics of a brick, there are still very useful things that can be accomplished using a test facility such as the Full Scale Wind Tunnel at MIRA. Armed with a multitude of cardboard cut outs and tank tape I was aiming to reduce lift on the front axle to balance the car and hopefully improve the drag at the same time.

The first series of tests consisted of a splitter below the number plate, an additional spoiler below and blending. I won't bore you all with the full technical results but in summary these changes actually reduced aerodynamic drag and lift at the front. The action of reducing front lift actually increased rear lift by pitching the car around its centre of gravity.

The next areas of attention were the front cycle wings. The theory was that air flowed both above and below the wing surface acting exactly like the wing of a plane. This caused a depression on the top surface relative to the air below causing lift. Blanking off the gap between the wheel and the wing did indeed reduce lift and drag, but without increasing rear lift. These results were further enhanced with the addition of a low sharp edge towards the back of the wing in an attempt to trip the air up and not follow the contour causing depression.

At this stage CD (drag) had been reduced by 3% and front lift decreased by a whopping 14kg.

How could I not resist looking at the front suspension? Now obviously I couldn't remove it all so I made covers for all the exposed round section tubes to change them to pear drop section. Although improvements were made, they were nowhere near as big as the effort in making the new sections!

Another few kilos of rear lift were lost by adding wheel deflectors to the lower edge of the rear wings in front of the rear tyres, but at the expense of a little drag. Then came the addition of side skirts to the car. I set these to give ground clearance at all times under racing conditions. These had an overall reduction in lift of 9 kg but a slight drag increase.

As I stated earlier I had already fitted a very pretty carbon fibre rear diffuser. However the shape of this was wrong based on textbook theory. The MIRA experts decided it was not long enough and the diffuser angle was nowhere near the optimum range. Copious additions of very stiff cardboard went some way to correcting this situation. But no matter what we tried, the best solution was to consign it to a very large skip! (The diffuser, not the car!)

From a suggestion by one of the group present, we also added additional blending profiles to the windscreen posts. Although not strictly legal in my racing class, it did improve drag very slightly.

Lastly for amusement I removed all of the weather equipment (roof and doors). This increased the drag force through the proverbial roof requiring a further 10bhp at 100 miles/hour, but actually created some more rear down force.

I would like to thank Graham Kendall, Ivan Starkey & Paul Atkin for making this very interesting visit happen and for the opportunity to complete these tests, which sadly I have not had chance to convert into road/race-worthy modifications."

 

Dave

 

 

[Old Septimus]

Didn't Lotus themselves do a design study which gave the 7 a smoother front with pontoon wings like a Testarossa (the proper one not the new thing)? CC banned it because it would take the car into the Elan market. He believed in market segmentation, the 7 was for anoraks and nutters and the Elan was heading for the hairdresser tendency.

[J.R.]

An easy and empirical way of testing any proposed aerodynamic improvements is to measure how far the car will coast in neutral from 60mph or 100mph.

 

This should be done before and after the mod on the same stretch of road and with same wind etc.

 

You wont be able to boast what the reduction in CD or A or CDA is but you will know that for instance changing full screen to aero improves aerodynamics by X %.

 

This is a method used by "do-ers" rather than "thinkers" *smile*

[Laurence Wilson]

Several Japanese circuit Sevens show practical experimentation. There are numerous images available on the web; I've linked to a few below.

 

Image One

Image Two

Image Three (same car, last in line)

 

I believe I’d be correct in suggesting that the rear wing may be situated to aid extraction of the under body air from the diffuser. Note too, the skirt as mentioned above.

 

The last image shows the cycle wings also feature an attempt to relieve the high-pressure air trapped underneath.

 

Q. Would wings reaching down the front of the wheel not offer a better alternative? These surely would reduce turbulence caused by the tire and eliminate the ingress of air over the front of the wheel? Something similar to a Gurney (sp) following the circumference of the leading half of the tire, moulded about the wing, may also create vortices acting to extract what high pressure air did arise? Could also apply to the rear arch?

 

I’d hazard a guess that the emphasis has been on reducing drag, as opposed to the following car that looks to have focused on achieving down-force.

 

Image Four

Image Five

 

I’d be interested if anyone could give some insight to the ideas behind many of these alterations. We haven’t encountered aerodynamics at college ☹️.

 

A healthy dose of to highlight my ignorance .

 

Peardrop

[Tony C]

If you're interested, a very good, easily understandable book on the subject is - don't laugh 😬

 

"Aerodynamics for Naval Aviators"

ISBN 0-89100-370-3

You can safely ignore the bits on Induced Drag 😬

 

Available for about 12 quid from most good pilot stores.

 

Waiting for my BRG SV kit. 😬 DELIVERY BROUGHT FORWARD, only three weeks to go now - WHOOPEE

[Julian Bradley]

I like JR's solution. Good idea, irritatingly simple. More representative from 100, I'd guess.

[Tony C]

Agree completely

 

All this faffing around with formulae is all very well, but is of very limited practical use.

Formulae can be used as "pictogram's" though, to get an idea of what factors are involved, but that's about it at our level.

 

Oh well, back to the drawing board then 😬

[NoLimits]

60 - 50 does not allow you to calc Cd etc. It does work though in terms of Data loggers etc. As it calcs rolling resistance combining all the forces attempting to slow you down.

 

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