alfas
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| posted on 16/6/14 at 06:20 PM |
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aerodynamic drag / air resistance on a full caged Locost
my locost has a rather high full cage, no windscreen only a small wind deflector and front winglets, 205/60r14 semi´s, 3.9 english diff:
it originally came with a mildly up-graded 1300x/flow..now converted to a mildly tuned 1600 x/flow
with both engines the car gets really tired with a speed higher than 70miles...like somebody trys to stop you.
do you think this rollcage is responsible for that, in terms of aerodynamic drag?
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pewe
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| posted on 16/6/14 at 06:28 PM |
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Given most Se7ens have the aerodynamics of a brick I think the roll-cage is pretty far down the scale.
Probably simply lack of power to overcome the overall drag?
No doubt one of the techies will be along soon to give you chapter and verse.....
Cheers, Pewe10
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wylliezx9r
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| posted on 16/6/14 at 06:32 PM |
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Have you got a rear diffuser ? Otherwise the rear tub acts like a parachute.
I spent a lot of money on booze, birds and fast cars. The rest I just squandered.
George Best
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bi22le
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| posted on 16/6/14 at 06:44 PM |
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Good air flow from the engine bay and half tonneau will make big differences.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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Sam_68
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| posted on 16/6/14 at 07:08 PM |
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AFAIK, nobody has put a Locost in a wind tunnel, but a Westfield has a Cd of 0.64 and a Caterham's is 0.70.
The rollcage certainly won't be helping, but neither will it be contributing a disproportionate amount of drag on its own; when your base
drag coefficient is that bad, the effect of an extra bit of scaffolding tube is probably neither here nor there.
As a very rough guide, based upon the Westfield's CdA, aero drag, rolling resistance and transmission loss combined will be absorbing
about 30bhp at 70mph, but the drag curve is rising steeply... by the time you're doing 100mph, that figure will have risen to around 75bhp.
Given that a mildly tuned 1600 Crossflow might only be producing 80-90bhp at the flywheel, you can understand why it feels as though it's
running into an aerodynamic brick wall as you start to go much above 70mph, and is flat out at not much over the ton.
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alfas
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| posted on 16/6/14 at 07:53 PM |
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to reply to some questions:
i have no diffusor installed. underneath the propshaft tunnel is closed..enginebay and rear axle open from bottom.
no side doors, no passenger side cover.
is there a suplier for a diffusor which could be made to fit( with only slight modifications) to a book locost (ron champion)?
and where could i purchase a half tonneau, maybe also half doors (softbits never replied to my requests, aerodynamix half doors are too small)?
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Sam_68
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| posted on 16/6/14 at 08:18 PM |
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Diffusers don't need to be anything sophisticated - a folded sheet of aluminium will do the job (have a look under the back of an S1 Elise if
you want proof!) - so why not just make your own?
Don't expect it to make a dramatic difference, though. You might get a 2 or 3 mph on your top speed, if you're lucky. And
don't expect it to actually act as a diffuser and generate any noticeable downforce - the speeds are too low, the ground clearance too high, and
the airflow too dirty on a 'Seven' for that; it'll just clean up the underbody airflow enough to reduce the drag a bit, that's
all. Simply panelling in under the rear axle will be almost as good.
The best you can hope for with any 'Seven' type is to go from absolutely diabolical to merely dreadful aerodynamics.
What you really want is one of these:
[Edited on 16/6/14 by Sam_68]
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Texan
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| posted on 16/6/14 at 08:39 PM |
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quote:
Originally posted by wylliezx9r
Have you got a rear diffuser ? Otherwise the rear tub acts like a parachute.
Could you elaborate a little?
Do you mean the open area behind the driver catches air and acts like a parachute and therefore a cover over that would help?
I drive therefore I am.
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wylliezx9r
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| posted on 16/6/14 at 08:47 PM |
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quote:
Originally posted by Texan
quote:
Originally posted by wylliezx9r
Have you got a rear diffuser ? Otherwise the rear tub acts like a parachute.
Could you elaborate a little?
Do you mean the open area behind the driver catches air and acts like a parachute and therefore a cover over that would help?
Yes exactly that. If no cover/rear diffuser is fitted, air will enter under the rear and hit the back panel like an air brake. My car felt completely
different after fitting a diffuser especially 70 mph plus, it's like taking the handbrake off in a way. The car now feels much more stable
too.
hth
I spent a lot of money on booze, birds and fast cars. The rest I just squandered.
George Best
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daniel mason
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| posted on 16/6/14 at 08:48 PM |
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The air flowing under the car hits the rear tub if the floor is not flat acting like a parachute but it won't make massive difference. I would
still do it, and under the engine bay,and possibly some vents in the side panels
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Steve ads
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| posted on 16/6/14 at 09:02 PM |
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http://m.youtube.com/watch?v=UPgu7Vylq44 why stop at a rear defuser lol
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balidey
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| posted on 16/6/14 at 09:13 PM |
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Been a long time since I studied it, but the frontal area is responsible for (going from memory now) approx 75% of all aerodynamic gain.
So will a rear diffuser help reduce drag? Yes.
But aero aids on the front will help more.
But all this is rather relative. As said before, most open wheel / open top cars have the aerodynamics of a brick, so all you will end up with is a
slightly aerodynamic brick.
Dutch bears have terrible skin due to their clogged paws
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austin man
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| posted on 16/6/14 at 09:36 PM |
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Also consider what differential you are running as this can and will have a dramatic effecr to top end power
Life is like a bowl of fruit, funny how all the weird looking ones are left alone
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Texan
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| posted on 17/6/14 at 12:10 AM |
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Ahhhh, now I see. i wasn't thinking of an unfloored rear section and I didn't think of the floor as a diffuser.
I drive therefore I am.
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onenastyviper
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| posted on 17/6/14 at 07:05 AM |
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quote:
Originally posted by balidey
Been a long time since I studied it, but the frontal area is responsible for (going from memory now) approx 75% of all aerodynamic gain.
So will a rear diffuser help reduce drag? Yes.
But aero aids on the front will help more.
But all this is rather relative. As said before, most open wheel / open top cars have the aerodynamics of a brick, so all you will end up with is a
slightly aerodynamic brick.
Yes but don't forget that how the flow is returned to the open stream is also important as well.
"If I knew what I was doing then it wouldn't be called research would it?...duh!"
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Sam_68
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| posted on 17/6/14 at 06:22 PM |
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quote:
Originally posted by balidey...the frontal area is responsible for approx 75% of all aerodynamic gain.
Do you mean for a typical car?
Surely, in terms of general principles it depends on the bluffness of the shape: very bluff shapes, the majority of the drag is pressure drag (ie.
frontal area), whereas for very slender shapes, the majority of the drag tends to be viscous (skin friction)?
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alfas
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| posted on 17/6/14 at 08:21 PM |
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quote:
Originally posted by austin man
Also consider what differential you are running as this can and will have a dramatic effecr to top end power
see my first posting --> all info there
205/60r13 tyres, 3.89 diff, 4 speed box
so if i interprete all the above posts correctly:
non of the aerodynamic things mentioned for rear or underbody, will have a severe effect on top-speed, max. gain would be around 5mph or less.
interestingwise i have driven similar powered westfields and sylvas WITHOUT any aerodynamic modifications..and all those cars never had this
problem...with or without windscreen....but standard rollover bar fitted.
that was the reason i was thinking about the rollcage on my locost acting like a "brake"
[Edited on 17/6/14 by alfas]
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eddie99
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| posted on 17/6/14 at 08:39 PM |
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Having done a lot of aero research (CFD and wind tunnels) and investigation/proving out on tracks, the cage adds drag but not significantly. Best
improvements you can do on a seven are at the front.
http://www.elitemotorsporteng.co.uk/
Twitter: @Elitemotoreng
Facebook: http://www.facebook.com/pages/Elite-Motorsport-Engineering/153409081394323
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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mcerd1
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| posted on 18/6/14 at 09:03 AM |
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quote:
Originally posted by eddie99
Having done a lot of aero research (CFD and wind tunnels) and investigation/proving out on tracks, the cage adds drag but not significantly. Best
improvements you can do on a seven are at the front.
Eddie, how much difference do your oval wishbones actually make ? (or is that a trade secret  )
-
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smart51
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| posted on 18/6/14 at 09:55 AM |
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a 15mm oval tube has mabe as low as half the drag of a 25mm round tube. 60% because it is thinner plus a bit more becuase it is cross section is long
and thin rather than round.
I've done a quick calculation and an MSA compliant roll hoop, with stays and a diagonal, made of 50mm tube uses 31 BHP to overcome drag at 100
MPH. (28 BHP for 45mm tube). If you were to wrap the tube in something aero profiled, you could reduce that drag from 31 BHP to 3 BHP.
Similarly, a set of round tubed wishbones is going to cost you about 3.5 BHP at 100 MPH. Aero profiles will save you around 3 BHP. Oval tube may
save 0.5 BHP per wishbone.
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alfas
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| posted on 18/6/14 at 10:31 AM |
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quote:
Originally posted by smart51
....
I've done a quick calculation and an MSA compliant roll hoop, with stays and a diagonal, made of 50mm tube uses 31 BHP to overcome drag at 100
MPH. (28 BHP for 45mm tube). If you were to wrap the tube in something aero profiled, you could reduce that drag from 31 BHP to 3 BHP.
......
does this mean: my estimation with the roll hoop being responsible for my problem is correct?
(by the way: my engine has "estimated" 90BHP only)
[Edited on 18/6/14 by alfas]
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smart51
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| posted on 18/6/14 at 10:42 AM |
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My calcs are for the roll hoop on its own. Obviously it will be attached to the rest of the car on the road and that will have some influence.
A general rule when sorting aerodynamics is to fix the front first, then the middle, then the back. Once you have disrupted the air flow, it hardly
matters what you do. As far as a car with no windscreen is concerned, the roll hoop is in effect part of the front of the car. At least the bit
that's not near your face.
You could try sellotaping a V shaped piece of card to the back of your roll hoop to see if it makes any difference. Sure, it won't be in any
way optimised, but it could make some improvement. Then you'll know if the hoop is a significant factor.
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alfas
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| posted on 18/6/14 at 11:16 AM |
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do you see a possibilty to make a small sketch where this v-shaped cardboard should be fixed?
[Edited on 18/6/14 by alfas]
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bi22le
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| posted on 18/6/14 at 01:31 PM |
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I assume on the trailing edge creating a tear drop. The tail that you have just added will tidy the air up after the tube
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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smart51
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| posted on 18/6/14 at 09:46 PM |
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quote:
Originally posted by alfas
do you see a possibilty to make a small sketch where this v-shaped cardboard should be fixed?
[Edited on 18/6/14 by alfas]
See the pic on this page. The tail points backwards.
http://nptel.ac.in/courses/Webcourse-contents/IIT-KANPUR/FLUID-MECHANICS/lecture-31/31-2_control.htm
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