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Learning how to build Lotus Seven replicas...together!
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PostPosted: Tue Sep 16, 2008 10:37 am 
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carguy123 wrote:
I've seen rear mounted radiators before on Rods I don't know how well they work at speed, but they seem to work when puttering around town. And since most of these are running blown big block engines you'd think there'd be enough flow to handle our little engines.

Are you going to put your beastie on the track? A relatively minor rear end tap could put you out of commission as there is no crumple area between the rad and another car like there is in the front.

As far as running coolant thru chassis tubes keep in mind you could get corrosion and eventually failure of the tubes. Not a pretty sight. Also there is the heat transfer.

While that might be good for the coolant I am not sure it would for you or cockpit temps.

How bout side pods for the radiator and shape the pods to smooth out the airflow over the rear wheels? Protection, less plumbing and better weight distribution.


Good points. I did think about corrosion and heat transfer issues. I think with the proper coolant/corrosion inhibitors, that should not be a problem.
Heat transfer can be dealt with with a little insulation. This is the reason I chose the bottom tubes and not the top so my arm won't be resting on a hot tube.

I believe, but I'm not sure, that there is enough pressure differential between the bottom of the car and the back of the car to get good enough airflow at speed withoutscoops. As far as rear end collisions are concerned, yes that would be a problem, however, I don't intend to race this car although I probably will take it on the track from time to time.


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PostPosted: Tue Sep 16, 2008 11:05 am 
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Asphalt Kicker I'm not so sure the bodywork itself is the culprit you are making it out to be, I think it is the blunt nose, cut out sides and exposed wheels that's the real culprit not the hood. OK, that's bodywork but I read your post as being bodywork like the hodd. The hood is horizontal to the air flow.

Now it is true that it could be shaped to direct the flow more advantageously (just look at all the weird shapes they use on F1 cars now) but I believe that his pointy nose would do wonders towards lowering drag.

If you are worried about Lotus 7 looks then your side ducts would ruin the lines as well.

I'll be first to admit that I am not a big fan of the 7's lines, the back end has always looked unfinished to me. But I am a fan of how it performs so I live with the lines. With that said it has been 40-50 years since the original design so isn't it just about time for a little improvement? What's wrong with a pointier nose? Maybe raise the nose and put a front wing ala F1?

In addition to a more sloped, curved windscreen (there go the looks again) how about raising the sides some so that it increases the safety factor, decreases the buffeting and increases the aero effects. I'm not talking FF or anything like that, I'm talking evolution, not revolution.

Rongaudier I think that you would need to make the cooling tubes out of a different material than mild steel to expect any life. It still scares me to think about the structural tubes gradually weakening over the life of the car. Now a finned aluminum tube running alongside the structural tube could work. You could almost make it look like a side pipe


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PostPosted: Tue Sep 16, 2008 11:35 am 
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I'll give 2:1 odds that you'll overheat if you do try using the underbody airflow to feed the radiator. Not to be a 'nay sayer'...But here is why I don't think it will work.

1. I'd guess the pressure under the car isn't as high as you think it is...Especially if you're using a full underbody tray and 'bullet' nose for aerodynamic purposes. In fact , depending on the design, it's likely you'd be trying to pull air from one generally low pressure area to another.

2. The physical amount of air mass flowing under the car isn't very high if your car is low to the ground, and doesn't have a front grill/engine bay dumping high pressure air from the front of the car to the underside.

3. The pressure drop through the radiator would compound this, since it needs a larger pressure differential to function than does air simply escaping the backside of the car. The air is going to take the path of least resistance.

4. A majority of the air mass would not follow the turn require to take it up to the radiator, as making such a sharp turn without a scoop sticking into the airstream would potentially cause a significant turbulent region just inside of the inlet that would prevent additional airflow to the radiator. An 'underbody scoop' would help provide the local pressure necessary to overcome 3 by making up for 1, but would still not help with 2.


Yes the pressure difference is enough to suck some of the air out of the underbody airflow and pass it through the radiator, but I would be highly skeptical that it could possibly pass enough air through to keep the engine from overheating. In the end, you would be miles ahead by taking air from a naturally high pressure/flow area facing the airstream, or creating one with a scoop, above or on the side of the car. However I could be completely wrong...You'll never know for certain unless you try.

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PostPosted: Tue Sep 16, 2008 11:47 am 
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Assphalt Kicker wrote:
the main drag on a locost is the radiator, suspension, fenders, body work and the windscreen!
Yes, I will agree that the physical existence of the car is the the main cause of aerodynamic drag on it...While you listed most of the parts of the car that touch the airstream, thus causing drag, you significantly overlooked the part of the car that is "non-existant" which contributes to a large portion of the drag as well.

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PostPosted: Tue Sep 16, 2008 12:40 pm 
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rongaudier wrote:
I know this subject has come up before, but here it is again. How about the rear mounted radiator idea? It would solve alot of problems for me based on my planned build.

The off road truck racers use rear mounted radiators with large fans. But I think they do it much more to keep dirt out of the radiator than worrying about streamlining. So yes, it is possible to get enough air through the core if you use a fan(s).

How about taking advantage of the low pressure area that builds behind the car? Furthermore, by reducing pressure under the rear of the car (as illustrated) rear end lift should be reduced at high speed.

I would measure the air pressure under the car first to see if it really is a problem. With all the chance of venting along the sides of the car I don't think high pressure would be a problem.

With all the turbulence at the rear of the car -- is there that much lift? I haven't seen any indication from anyone that they are having lift problems with a Locost. I don't think the cars are fast enough to generate dangerous amounts of lift.


Also the use of a pointy nose should reduce form drag somewhat.

Formulas 1 cars are very pointy and I keep picturing F1 cars that use dual rear mounted radiators in pods that have a Cd of .7 to 1 So right now a Locost has about the same Cd as a car that has rear mounted radiators.

But more advantage comes from streamlining the rear of the car. If you drop an empty conical paper cup it will fall pointed end first because the flat end has more aerodynamic drag.


As far as coolant lines are concerned, how about using round tube in lieu of square tube in the bottom of the trans tunnel? Coolant could be routed through these tubes. The radiator could be covered in the back with a louvered panel.

Or just laying in some copper pipe (better heat dissipation) in the transmission tunnel. I have a short section about this in the radiator article on my website.

A rear mounted radiator will put even more weight on the rear end. But I guess you could mount the gas tank up front! :shock:

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PostPosted: Tue Sep 16, 2008 12:51 pm 
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It's hard to say exactly how high the pressure is just by looking. However, consider that lift at speed is caused by the differential pressure between the top and bottom of the car. Air flowing over the top of the car has a further distance to go than the air flowing under the car thus creating the pressure differential (see Bernoulli's law). I would say a flat, smooth bottom would allow more, than less air to flow due to reduced turbulence and resistance. The one factor you mentioned that I hadn't considered is the resistance caused by the radiator and louvered cover itself. That may, in fact be sufficient to cause the air to "bypass" as you suggested.


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PostPosted: Tue Sep 16, 2008 1:09 pm 
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How bout this?Image


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PostPosted: Tue Sep 16, 2008 1:18 pm 
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Be sure to put a cog in there to run a fan over the driver!

Or, you know, use an electric fan :lol:

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PostPosted: Tue Sep 16, 2008 1:20 pm 
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rongaudier wrote:
However, consider that lift at speed is caused by the differential pressure between the top and bottom of the car.
Consider that many cars can produce downforce at speed via the same principle.

Your updated design would definitely not have any issues with cooling.

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PostPosted: Tue Sep 16, 2008 1:22 pm 
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Driven5 wrote:
While you listed most of the parts of the car that touch the airstream, thus causing drag, you significantly overlooked the part of the car that is "non-existant" which contributes to a large portion of the drag as well.


Ok, you lost me there.


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PostPosted: Tue Sep 16, 2008 1:31 pm 
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I know you know this. Perhaps if I worded it less philosophically, and more clearly...What does a locost lack that "aerodynamic" cars have?

Hint:
It significantly smooths out the airflow behind the windshield, and it isn't a guy with a blad head.

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PostPosted: Tue Sep 16, 2008 1:48 pm 
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Driven5 wrote:
I know you know this. Perhaps if I worded it less philosophically, and more clearly...What does a locost lack that "aerodynamic" cars have?

Hint:
It significantly smooths out the airflow behind the windshield, and it isn't a guy with a blad head.


Ok so I get it now. You mean a rear spoiler right? ':wink:'

Yeah, I'd like to have an option for one of those. Not a blad head but one of those missing thingies.


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PostPosted: Tue Sep 16, 2008 2:13 pm 
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rongaudier wrote:
How bout this?Image

Too late, someone's already ahead of us on this one. Just got to remove that fog light .............. or maybe it's the tractor beam projector.


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PostPosted: Tue Sep 16, 2008 2:15 pm 
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I was thinking along the lines of a rear radiator with cycle fenders on the rear wheels. THen try to get high pressure air from between the body and the rear wheels to flow into the radiator.

Not much room between the tank and radiator perhaps.

Trying ti get air from under the car to the rear might work. It isn't just trying to get air from one low pressure to another, but trying to get the low pressure behind the trunk to extend under the car. If there is a passage way, it will do this.

Someone with car must be able to whip up some kind of a water manometer and run some small plastic tubes around their car with duct tape. Or maybe they could lend me their car for the winter...

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PostPosted: Tue Sep 16, 2008 2:33 pm 
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horizenjob wrote:
It isn't just trying to get air from one low pressure to another, but trying to get the low pressure behind the trunk to extend under the car. If there is a passage way, it will do this.
Yes some air will flow through your passage way...but realistically how much?

Also remember to consider where the hottest air will be found over a blacktop surface on a hot sunny day vs where you're drawing your air from.

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