dilley
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| posted on 30/12/06 at 04:21 PM |
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top speed and aerodynamics
Ive heard alot of people say that regardless of how much power you run in a 7 it will slow down after 100mph? due to aerodynamics, someone commented
in another thread that a 7 would not keep with an evo after 100mph, has this been proven or is it pie in the sky?
sorry if im talking shite again?
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clutch_kick
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| posted on 30/12/06 at 04:25 PM |
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Theoretically speaking the theory is right. However my old flight instructor used to say that given enough power even a brick will fly.
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novacaine
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| posted on 30/12/06 at 04:28 PM |
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yea the thread about the evo was one i started lol
i have done some calculations about BHP Vs Top speed....
Will post pic of graph in few mins....
And you run and you run to catch up with the sun but its sinking, Racing around to come up behind you again, the sun is the same in a relative way but
your older, shorter of breath and one day closer to death
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David Jenkins
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| posted on 30/12/06 at 04:30 PM |
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Max speed isn't the point of a seven, IMHO. They're all about rapid acceleration and snappy cornering. I probably drive faster in my
tin-top! However, I have more fun in the twisty bits with the seven.
I don't know about aerodynamics, but the airflow around the driver gets a bit fierce after 90mph!
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Jon Ison
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| posted on 30/12/06 at 04:36 PM |
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Mine will pull too around 125 on its own, hit 132 two up with a bit of slipstreaming under the dunlop bridge at Donnington.
The thing is you have braked later, gone round the corner quicker and accelerated away quicker than most "supercars" this is of course on
twisty roads or circuits, in a straight drag race on a straight road sooner or later the "supercar" will pull away but not until you have
reached lost your license speeds.
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novacaine
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| posted on 30/12/06 at 04:43 PM |
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Here is the graph i was talking about,
it assumes that you have built by book dimensions, that you have a windscreen and the car weighs about 600kg
http://www.locostbuilders.co.uk/photos.php?action=showphoto&photo=untitled%20graph.JPG
cheers
Matt
And you run and you run to catch up with the sun but its sinking, Racing around to come up behind you again, the sun is the same in a relative way but
your older, shorter of breath and one day closer to death
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novacaine
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| posted on 30/12/06 at 04:45 PM |
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hold on that link isnt working.....
And you run and you run to catch up with the sun but its sinking, Racing around to come up behind you again, the sun is the same in a relative way but
your older, shorter of breath and one day closer to death
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smart51
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| posted on 30/12/06 at 04:47 PM |
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The power required to overcome aerodynamics is proportional to the cube of speed. If you double your speed, you need 8x the power to overcome
aerodynamic drag. you only need to double the power to overcome rolling resistance.
A seven has roughly 40% more drag than an ordinary car and about half the rolling resistance. At low speeds, the seven goes like stink but it will
slow down more with increasing speed than your ordinary car.
The 100 MPH figure quoted is just nonesense. It is purely down to the power of your enigne. It takes about 78 BHP for a seven to do 100 MPH versus
about 61 for an ordinary car. With 150 BHP, your seven will top out at 123 MPH and your ordinary car 137. At 100 MPH your seven's acceleration
is decreasing more rapidly than your similarly powerful ordinary car. but it will still feel fast. A 95 BHP pinto will feel slow below 100 MPH but a
180 BHP seven will still be pulling strong at 120 MPH
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novacaine
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| posted on 30/12/06 at 04:48 PM |
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try this link instead ....
http://www.custom-concepts.co.uk/untitledgraph.JPG
it might be absolute rubbish but i hope it give somewhat of a guideline
and i have to agree with Dave Jenkins on this one, a 7 is all about the acceleration.
matt
[Edited on 30/12/06 by novacaine]
And you run and you run to catch up with the sun but its sinking, Racing around to come up behind you again, the sun is the same in a relative way but
your older, shorter of breath and one day closer to death
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zxrlocost
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| posted on 30/12/06 at 05:14 PM |
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In my r1 160hp after dynojet
I raced an EVO (03 plate? they are fantastic cars for people who cant drive and need the car to drive for them) from the cannock toll road standing
start I DESTROYED it clutchless upshifts all the way to 122mph on the limiter I think I pulled away so quick that he couldnt keep up and gave up
however i think if he had of tried after about 90mph there wouldnt have been much in it
DILLEY YOU HAVE A TURBO BEC have you been drinking to much over this festive period
you should be moving onto Bugatti bating not asking about evos
PS everyone Im talking about normalish EVOS not MAX POWER 650bhp ones that never get took out
chris
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BenB
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| posted on 30/12/06 at 05:21 PM |
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Acceleration is heavily dependant upon power:weight. Top speed is more dependant upon power and drag coefficient.
Sevens have a poor drag coefficient (to put it mildly) and although they have a good power:weight ratio they usually don't have humungous amount
of power.
If you get something that is effectively a covered Seven (like a Phoenix / Fury) it will weigh more (therefore accelerate slower) but the top speed
will be better for a given power engine due to the reduced drag...
The difficult bit is that when you start using huge amounts of power, you usually need loads of downforce to allow you to use it. Creating downforce
creates drag, so higher power can equal increase drag (hence a Lambo Countach has the same drag coefficient as a VW minivan).
Clear as mud huh!
[Edited on 30/12/06 by BenB]
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froggy
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| posted on 30/12/06 at 05:49 PM |
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from experience i added a dyno proven 90hp to a car i had and it gained only 10mph top speed
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Syd Bridge
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| posted on 30/12/06 at 06:11 PM |
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'Theoretically speaking'
Given the same power, there would be few(if any) 'tin tops' that would keep up with a Se7en type car in any circumstance.
Even though the drag of a 7 windscreen is high, the frontal area overall would be much less.
Cheers,
Syd.
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C10CoryM
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| posted on 30/12/06 at 06:20 PM |
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Have to remember that most sevens are well under 200hp. The only reason 200hp is fun is because its in a light car. A 200hp camaro isn't
nearly as fun.
This is fine for acceleration at low speeds when its power/weight, but when it become power/drag 200hp is not really exciting. At that point the
high drag of the seven** sucks up all the power to push through the air. My first vehicle was a 70 C10 with a mild V8. In top gear at 120mph the
truck stopped at 4500rpm. Engine just flat out did not have the power to push through the air any faster, or to raise the engine rpm. Add enough
power and even that brick will go faster. Proven by a 72 C10 which was the worlds fastest drag truck. 1st truck to do over 200mph in the 1/4 mile.
http://www.dragracecanada.com/canadian/cool/photos/017.jpg
Anyhow, best to stick to lower speed events in a low horsepower seven .
Cheers.
** I have not actually seen any evidence of the sevens poor drag co-efficient. Just going by hearsay. Does anyone have some further info to
edumacate me?
"Our watchword evermore shall be: The Maple Leaf Forever!"
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dilley
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| posted on 30/12/06 at 06:24 PM |
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I can keep with a gsxr 1000 all the way to 151mph and I hit the rev limiter, running 220 bhp at the wheels. so how does that equate to what people
say?? it doesnt mak sense to me
Just like to add all testing done on track, and I do not condone speeding unless ive been down the pub(joke)
[Edited on 30/12/06 by dilley]
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zxrlocost
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| posted on 30/12/06 at 07:28 PM |
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you have got 220bhp at the back wheels
can keep up with a gsxr1000 up to 150
and your asking about drag?
I think the point is some havent got even 100bhp 7's
and after 100mph they may not have the affect yours does
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DIY Si
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| posted on 30/12/06 at 07:35 PM |
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It's all about the power required to force a brick/7 through the air at speed. 7's aren't geared for really high speeds (160-180 I
mean here). Just adding power will make you faster, but if the gears allow 20 mph then that's it. This is fiddily to explain with just words.
Most 7's will top out at 120 ish due to either gears or power. Assuming the gears aren't a problem, then more power = more top end, but
sooner or later, you'll run out of gears. An example is my mini. 150 odd bhp, but only does 110 due to hitting the red line in 4th. Given a
5th/longer 4th it would do much higher. Dilley can go so fast due to having a silly/brilliant engine!
I'll apologies now if all that sounds stupid, but as I said, I'm finding it hard to type what I mean.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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craig1410
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| posted on 30/12/06 at 08:25 PM |
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Have a look at Wikipedia here for some examples of drag coefficient (Cd) and more importantly
CdA which multiplies Cd by drag area. The Caterham Seven is listed as a Cd of 0.7 which is indeed pretty bad. Unfortunately they don't give a
measure of the frontal drag area for the Caterham. The Subaru Impreza has a Cd of 0.33 and a CdA of 7.72 ft^2 so while the Cd is half that of the
Caterham, the frontal drag area is probably also about half, especially if you remove the Caterham windscreen and use an aeroscreen instead as many
do.
I know Seven's were never known for high speeds but this is probably more to do with the fact that they were so quick on a modest amount of
power (say 75BHP) that people were disappointed that they couldn't go on to higher top speeds. (eg. 0-60 in 6.5 seconds but only 110MPH just
didn't seem "normal!"
Here are some figures from the Caterham website:
105bhp, 6.5sec, 110mph
125bhp, 5.9sec, 112mph
140bhp, 5.1sec, 122mph (550Kg)
140bhp, 4.9sec, 125mph (500Kg)
160bhp, 4.9sec, 128mph
200bhp, 3.7sec, 140mph
260bhp, 3.1sec, 155mph
Note, these are all car engines so have comparable characteristics.
Edit: Just found this:
http://www.mayfco.com/caterham.htm
Shows Caterham Super 7 Cd to be 0.62 , area to be 15.82ft^2 and therefore CdA is 9.81ft^2.
Not that far removed from the Subaru mentioned above.
[Edited on 30/12/2006 by craig1410]
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Jon Ison
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| posted on 30/12/06 at 09:11 PM |
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This tin top gets up too 200mph
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craig1410
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| posted on 30/12/06 at 09:26 PM |
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Sounds really nice while it's doing it too...
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C10CoryM
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| posted on 30/12/06 at 09:30 PM |
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Im no aerodynamicist but I have done some messing around. Far as I've seen most of the drag comes from the air leaving the body, and the big
empty hole behind the car trying to pull the body into it. The CdA seems less important to me than reducing that drag from the rear. Which is why
the wedge shaped cars of the 80s have lost favour. I would guess the worst of the sevens drag comes from the open cockpit (and windscreen if
equipped) and the big,flat back panel. Add to it the wide open grille that is rarely vented correctly and yes, for a small car I can see the drag
being quite high. On the average seven I would guess the air under hood tries to get out under body, and worse, through the trans tunnel. I'd
would be curious to see if there is a high pressure area in front of the rear panel because of that. That high pressure area, and the known low
pressure area behind the car may be nasty for drag.
As I said, this is all my own ideas. No calculations to back it up, but it makes sense to me. I do intend to have "proper" ducting on my
seven and play with the aero a little bit. My 100hp engine is going to need all the help it can get .
Cheers.
"Our watchword evermore shall be: The Maple Leaf Forever!"
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alister667
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| posted on 30/12/06 at 10:18 PM |
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Without doubt the easiest way to achieve these speeds is to fit a bike engine and clocks and not re-calibrate the clocks!
A mate with a blackbird engined Westfield did this (he just hadn't got round to re-caling the clocks), took a friend of his out who came back
ranting and raving about doing 170mph down small back roads.
120 in a BEC feels nuts anyhow!
http://members.lycos.co.uk/alister667/
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craig1410
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| posted on 30/12/06 at 10:33 PM |
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quote:
Originally posted by C10CoryM
The CdA seems less important to me than reducing that drag from the rear.
At the risk of being pedantic, the CdA is all important since it defines what speed you get for a given amount of horsepower (or forward force to be
more precise, which comes from the engine torque at the given RPM).
However, you can improve things by either reducing frontal area (ie. the A bit of CdA) by removing windscreen, mirrors, lights etc.) and/or by
reducing drag coefficient (Cd) by improving airflow around the frontal area you have. The latter being the bit you are talking about. The Cd of 0.62
for the Caterham is in road legal condition with windscreen, lights and mirrors fitted.
If you want top speed then you should fit an aeroscreen, fit a cover over both cockpit and luggage bay, minimise the radiator intake, use long front
cycle wings which go almost to the floor, remove the passenger seat and a whole host of other things to reduce the disruption to airflow.
I agree that the air leaving the vehicle is very important for Cd which is why the teardrop shape is very good but there is only so much you can do
before your Seven becomes something else entirely...
Cheers,
Craig.
[Edited on 30/12/2006 by craig1410]
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Wadders
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| posted on 30/12/06 at 11:16 PM |
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Sorry but i just can't accept that, who was riding the bike, big daddy with giant haystacks as pillion?
Originally posted by dilley
I can keep with a gsxr 1000 all the way to 151mph and I hit the rev limiter, running 220 bhp at the wheels. so how does that equate to what people
say?? it doesnt mak sense to me
Just like to add all testing done on track, and I do not condone speeding unless ive been down the pub(joke)
[Edited on 30/12/06 by dilley]
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dilley
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| posted on 30/12/06 at 11:26 PM |
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speak to jack at holeshot racing, or pop down on a nice dry sunny day and ill take you out, that quote was with 2 in the car, looks like I need to
take sombody out on this site to varify a few things you about Sean??
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