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Learning how to build Lotus Seven replicas...together!
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PostPosted: September 8, 2014, 3:18 pm 
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There have been a few other attempts at the 7`s aero but I dont know anyone that can verify if they are successful or worth the grief.

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PostPosted: September 8, 2014, 3:21 pm 
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cheapracer wrote:
John, your front guards are probably as bad as you can get for lift, in fact if I wanted lift that's how I would design them.

You have the choice of doing a wrap till the front edge is considerably below the axle line or venting them so the air has somewhere to flow through, not ideal but at least you retain the looks. Remember some mesh to stop stones coming thru and the first vent needs to start before the vertical axle line.

One considerable amount of front lift comes from people's heads and the rollbar, you have no rollbar and a windscreen so you are ahead there.

Kick up "diffusers" are pretty well ineffective, they don't hurt but the flat plate surface has no hope of retaining a laminar flow especially short angles over around 6 - 7 degrees and with all the disturbances you have forward of it and from the sides. The effort and cleanliness of your bottom( :shock: ) is certainly worth some speed and fuel efficiency though.


windshields and roll bars cause a lot of drag but really nothing in the way of nose flit to the cars .
when the roll bar and windshield is removed there is hardly any difference in the aerodynamic weight balance on the nose of the car . the change bounced between .001 and .000 for the difference .


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PostPosted: September 8, 2014, 3:39 pm 
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bob wrote:
There have been a few other attempts at the 7`s aero but I dont know anyone that can verify if they are successful or worth the grief.

Attachment:
images.jpg


Attachment:
Caterham_Steve Wilson 01-Aug-08 004edit.jpg



the 1st thing that has to be done is you have to run simulations of the aero package your designing .
just to design the aerodynamics for the yellow sports racer that was not to bad to start with and we have over 350 hours into the design .
any little change in the design changes many other things .
it gets to be like a dog chasing it's tail at times .

the other design we did in my reply took about a 1000 plus hours .


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PostPosted: September 8, 2014, 7:42 pm 
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the aero is pretty bad as a whole,
and due to the style of my particular car it gets worse

Attachment:
p5.jpg


as you can see from the side, i have a front bumper that looks like a football helmet face guard i will eventually place a sheet of aluminum in there to scoop up some air and deflect it upwards.

also notice the considerable venting of the air out from the hood sides through grills.

Attachment:
john camera 020.jpg


here you can see exactly how flat the bottom of the car is.

as far as the front fenders, as they are fitted to the uprights, i don't think any lift of the body is being generated but certanly there is a lot of drag but i live in the desert and stones are the bigest problem, i have however kept the leading edge of the fenders as close to the tires as i can.

overall, i don't think the drag is too bad at all, as at highway speeds my economy is over 40 miles to a gallon


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PostPosted: September 9, 2014, 4:27 am 
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962porsche wrote:
windshields and roll bars cause a lot of drag but really nothing in the way of nose flit to the cars .




Now anyone else in the forum who has more than 1/8 of a clue of simple logic and basic math, below is blantantly obvious example whereas 'X' is air pressure (Cd), 'Y' is lift and 'P' is pivot. The further 'X' is back and the higher it is, the more lift will be created at the nose ('Y').

Attachment:
XYP.jpg


7's have been around for 50+ years being improved and raced, more than one found it's way into a wind tunnel, University studies and more recently CFD and they generally come up with the same results;

Front guards need big changes, preferably run them forward around the tire and from the axle line vertically straight down to the ground clearance limit with a splitter lip forward.

Aerodynamic section covering for the rollbar. There's a few light aircraft guys in the forum already know the huge difference that makes to wing struts.

Inboard front suspension if you're real keen.

Apart from that and doing what John has done cleaning up underneath, there is not much more you can do without upsetting the very basic visual aspects that draws one to a Locost 7.


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PostPosted: September 9, 2014, 7:08 am 
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An interesting thread re splitters from an early thread on here viewtopic.php?t=2302

bob

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PostPosted: September 9, 2014, 8:16 am 
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cheapracer wrote:
962porsche wrote:
windshields and roll bars cause a lot of drag but really nothing in the way of nose flit to the cars .




Now anyone else in the forum who has more than 1/8 of a clue of simple logic and basic math, below is blantantly obvious example whereas 'X' is air pressure (Cd), 'Y' is lift and 'P' is pivot. The further 'X' is back and the higher it is, the more lift will be created at the nose ('Y').

Attachment:
XYP.jpg


7's have been around for 50+ years being improved and raced, more than one found it's way into a wind tunnel, University studies and more recently CFD and they generally come up with the same results;

Front guards need big changes, preferably run them forward around the tire and from the axle line vertically straight down to the ground clearance limit with a splitter lip forward.

Aerodynamic section covering for the rollbar. There's a few light aircraft guys in the forum already know the huge difference that makes to wing struts.

Inboard front suspension if you're real keen.

Apart from that and doing what John has done cleaning up underneath, there is not much more you can do without upsetting the very basic visual aspects that draws one to a Locost 7.


if your going to quote then you should quote every thing I stated !
it's great there are also other aerodynamicist on the site I would love to go over my finding of the 7's with them .

what did you get for numbers when you run your aerodynamics simulation of the 7 ?
was it more then the .001 of balance difference front to rear with the roll bar ?


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PostPosted: September 9, 2014, 8:40 am 
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John there is so much more that can be done to the under side of your car to clean up air flow .


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PostPosted: September 9, 2014, 9:07 am 
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bob wrote:
An interesting thread re splitters from an early thread on here viewtopic.php?t=2302

bob


Thanks for that, Bob. I think the Donkervoort setup looks very, very nice. They've smoothed out every piece of the front end.

One reason I'm not putting a lot of thought into aero for my street car is that I just don't have any way to test things. Things can "look great", but be counter-productive and possibly even introduce instabilities into the car. Just looking at the Donkervoort setup makes me wonder what that small adjustable wing (U.S. sense) is going to do to the airflow through the front coilovers and suspension. I'm sure the wings will produce downforce, but is that what's really needed? Would a gentle shaping of the airflow around those parts produce a better result?

The cleaning up of the airflow under (as John H. did) or over (fairing the roll bar) the car are pretty much no-brainers as might be true for lowering and slanting the traditional windshield. Even something as simple as the windshield modifications should be tested IMHO if you're after real results, not looks.

For a true track car, all that work might be worth it, but for a street car, I don't know.

Cheers,

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PostPosted: September 9, 2014, 9:33 am 
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this is the car that is in my simulation program .
because there are some many different designs of the 7 the numbers I get may not have any thing to do with the other 7's out there .
the windshield has a lot of drag from it this is more from the air flow coming around the sides of the windshield then it coming from over the top of the windshield .
because the windshield is centered on the car removing it did not have much effect on the aero balance of the car .

adding a 1.75" round roll bar for a 185 LB 6' person to pass the broomstick test also had little effect on the cars aero balance from 0 MPH up to 125 MPH .
at 125MPH the amount of lift it had on the front wheels is .6 of one LB .
again it's a minimal amount !

as soon as the lower part of the nose is changed the balance of the car does change greatly .

I have tried to transfer the runs I have done in my aerodynamics simulation and they will not go thru .
there is just to much info for this site to handle I'm guessing ?


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PostPosted: September 9, 2014, 10:22 am 
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Now anyone else in the forum who has more than 1/8 of a clue of simple logic and basic math, below is blantantly obvious example whereas 'X' is air pressure (Cd), 'Y' is lift and 'P' is pivot. The further 'X' is back and the higher it is, the more lift will be created at the nose ('Y').


I'm sorry, CheapRacer, this is incorrect. It matters not not how far back 'x' is placed. You are drawing the generation of a torque about the rear wheel. The height of the roll bar matters, it can be placed over the front wheels or the rear wheels and it will produce the same torque.

In addition rules often prohibit fairing of the roll cage tubes and also the attachment of any aero devices whatsoever. It's arbitrary, but I guess it is considered too much of a slippery slope.

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PostPosted: September 9, 2014, 10:50 am 
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I have been trying my best but i just can't get the sim runs to go thru to this site .
so here is a side view of the lotus 7 body I have been running simulations on .
if you look at the nose you will see it's rolls up underside front .
the rolling down of the top side does not compensate for the roll on the underside .
the reason for this seams to be that as the air is running under the chassis it is compressing and packing up right after the roll of the nose .

when I just do a simple change of the chin area and square it off the aero balance changes greatly .
when I added a very small chin spoiler at starts adding aero weight to the nose .
just a 4" chin spoiler across the nose at 40 degrees adds 23.8 LBS to the nose at 125 MPH .
the aero balance weight is what they are in the wheels sitting on the ground .
what I have is static weight of the car then as the car runs thru the MPH up to a set speed of 125 MPH it tells me the aero weight that is getting placed on the 4 wheels .
I have set the cars 1500 LBS static weight at a 50/50 balance .
the run from 0 to 100 is taking 8.4 seconds
the runs from 0 to 125 are taking 9.7 seconds
the weight transfer to the rear wheels is on average 71% to 43 MPH and starts dropping the percentage or rear weight from there .
weight transfer can be altered with suspension spring rate changes and static balance .
this has to do with what type of springs your using linear or progressive rates .
for cars with aero down force progressive rates tend to work best . this is because of the difference in static weight to aero weight or down force on the car . this has to do with mechanical grip at lower speeds to aerodynamic grip at higher speeds .


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Last edited by 962porsche on September 9, 2014, 11:39 am, edited 1 time in total.

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PostPosted: September 9, 2014, 10:53 am 
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I have tried to transfer the runs I have done in my aerodynamics simulation and they will not go thru .
there is just to much info for this site to handle I'm guessing ?


We've had nice CFD pictures posted before, do you need to reduce the file size of the pictures or do you think it's the extension of the file is is complaining about?

If you look in the upper left corner of this forum's pages you can see what a nice nose looks like. That is Chet's car and it has a 7'ish nose I guess. The Locosts and many of our cars use a nose that is taller because modern engines are several inches taller than the traditional Ketn crossflows etc. I think the old nose is a lot more attractive, so it has a lot going for it.


I guess I'm willing to discuss what I am doing with my version of Car9 for aero / downforce. I'm not done yet so there is no idea of how well this will work out. On top of that I am planning a 7 style car that will hopefully wear a pretty outfit like a Lalo or DBR1.

My goal is to reduce lift and if possible produce a rearward weight balance at higher speed, because cars are more prone to oversteer at high speed.

So I have raised the bottom of my front bulkhead a couple of inches. This lines up the bottom with the front lower control arms better, so a double win. I have also raised the bottom of the roll hoop bulkhead about 4". The lower chassis rails are about normal, but they were rotated slightly so that the front tilts up to meet the front bulkhead

This gives a profile to the flat floor and it will have a curve upwards towards the rear starting a little after the dash bulkhead. There are no vertical skirts on the sides of the car because it is too low for that in track setup. However with the 7 style body it will have dual exhaust and it's required underneath support arranged in a way to provide a horizontal "shelf" sort of to provide a high resistance area for flow trying to enter or leave the underside of the center section of the car. The shelf would be perhaps 4" wide. When running the pretty bodywork, like DBR1 or Lalo, this would be evan wider and more pronounced.

The diff bottom hangs down into the area I would like to be aero and so there will be something like "area ruling" to help flow here. That means the vertical cross sections where the diff are will have corresponding bulges to make up for the area absorbed by the diff. The rear body behind the wheels tapers a bit so the diffuser rises as much as possible to make up for that.

All this effort and I haven't been able to figure out an inboard suspension yet for these cars... :(

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PostPosted: September 9, 2014, 1:39 pm 
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Cheap,

i did the inboard front suspension as well.

962P,

what do you think i can do apart from changing the pan for a shorter one and filling in the hole?

boy this thread took off like a rocket---------or a locost, just because someone asked about a rear defuser.

exelent!!!!!

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PostPosted: September 9, 2014, 3:01 pm 
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horizenjob wrote:

It matters not not how far back 'x' is placed.


If I put the math up based on a car with suspension, torque forces around the rear axle and a CoG to work around blah blah ..... I'm not interested, neither are others viewing who want the simple version and that's why the simple drawing. But no problem, we'll just work with the height, doesn't change the facts, only the values.


962porsche wrote:
adding a 1.75" round roll bar for a 185 LB 6' person to pass the broomstick test also had little effect on the cars aero balance from 0 MPH up to 125 MPH .
at 125MPH the amount of lift it had on the front wheels is .6 of one LB .
again it's a minimal amount !


This has been physically tested and proven in wind tunnels, what your software says is irrelevant to real world results - get a refund maybe?

It's not even a complicated math equation if you know the tubes resistance. But who cares, it's simple for our purposes, there is undeniable resistance from the roll cage (you added the crossbraces didn't you) and huge amounts of it at 125 mph, and as it's up much higher than the rest of the car it can do nothing other than rotate the car backwards on it's Y axis. That is an undeniable fact.

And that means the front lifts and the rear squats, but at speed it's mostly the front due to air that's happy to travel under the car and aid in the lift.

Anyone of you in your Locost or a car with a sunroof can merely stick a long object straight up at any speed and see the result, i.e., rotation backwards around your shoulder's fulcrum point.


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