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 Post subject: Re: Downforce
PostPosted: July 10, 2016, 9:06 am 
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Where are your exhaust pipes? I has to be blown! :roll:


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 Post subject: Re: Downforce
PostPosted: July 10, 2016, 9:31 am 
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carguy123 wrote:
Virtually every diffuser I've ever seen also has several strakes between the 2 end plates to further divide the air and keep it from separating.

Yes I think they would help but making the stakes/vanes rigid if fairly difficult. My side plates are stiff due to the curve and the triangles I added which will become fender extensions. I am not really sure how to add strakes with enough rigidity to be any use without making it extremely complicated. The current version is also very light, without the bar was 7lbs. Probably around 10lbs now as it sits.

vroom wrote:
Where are your exhaust pipes? I has to be blown! :roll:

Well it is sort of.... I directed the exhaust under the car when I remade it to keep the sound levels down/hidden and also that it would act like a blown diffuser. Hard to tell in the photo but the exhaust is directed under the floor to the diffuser.
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 Post subject: Re: Downforce
PostPosted: July 13, 2016, 9:27 am 
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wrightcomputing wrote:
carguy123 wrote:
Virtually every diffuser I've ever seen also has several strakes between the 2 end plates to further divide the air and keep it from separating.

Yes I think they would help but making the stakes/vanes rigid if fairly difficult. My side plates are stiff due to the curve and the triangles I added which will become fender extensions. I am not really sure how to add strakes with enough rigidity to be any use without making it extremely complicated. The current version is also very light, without the bar was 7lbs. Probably around 10lbs now as it sits.


first i am a Gator fan... no big deal...

Second, its time to go carbon fiber (you already have the mold!) or fiberglass. Something very simple like this should be all that you need to make rigid strakes. Have a go pro? Run it as is with some yarn taped on and see how much air disturbance you get. Add strakes accordingly. I am going to assume your main floor is flat right?
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why dont you have a build log link in your sig? :no:

lastly, i assume you have see this??
https://www.youtube.com/watch?v=hgXni_LUgwc

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 Post subject: Re: Downforce
PostPosted: July 13, 2016, 3:30 pm 
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Yes the main floor is flat. Well apart from the fact that it is diamond plate so its flat but knobbly. If i have time I will do some yarn testing and maybe adding food coloring which I saw on F1 a few months back.

Not sure why I don't have the build log in my signature it used to be there. Too busy bragging about wining, anyway that has been fixed.

I have watched a lot of Kyle Drives and Engineering Explained videos. They both offer lots of great info.

The issue with adding the strakes to mine is I made it curved as such while it is easy enough to cut the strake to the same curve it becomes much more difficult to then bend or add the 90 degree for attachment. I did think about how easy it would be to smooth the edges and then remake it out of carbon fiber. I think that may be a future project.

Anyway for now I am pretty happy with it and have to get the entire body finished by Nationals which is about 7 weeks away and I plan to race on 2 or 3 of those weekends. So not a lot of time to get it all done. For now installed and secure is the goal rather then optimizing each part.

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 Post subject: Re: Downforce
PostPosted: July 14, 2016, 9:38 am 
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the reason you want a diffuser angle to be between 10 and 14 degrees for most cars is not to have the air rolling back on it's self .
if the air does this the diffuser becomes nothing more a a device that will add more drag to the car with it then with out it .

ideally you would want the underside of the floor between the axle center lines to be at 1 to 3 degree angle . the angle depends on the size of the underside of the car and it's ride height .
this also applies to the size shape and angle for the diffuser .

we all know the point of a diffuser is to accelerate the air flow under the car .
but most do not know a diffuser is in many ways like a front splitter is not like a wing as it's aid to giving down force but it's a device that speed air flow under the car and makes the hole car more like a wing in it's self .
giving the hole top side of the car down force .

adding more vertical fences is in most cases a smart idea !
to many and it will defeat it's purpose of having vertical fences .
what vertical fences do is directs air flow and also will speed up air flow they alos help to stop air from getting sucked under the car . as air flow is sped up it will want to suck more air in from higher pressure areas like the top side of the car . this is bad real bad !
one way to combat this is to find out what the cars height is at full compression of it's suspension . then make the VF's about 1/4 " off the ground at the car full compression height .

cars with out FB's (flat bottoms ) tend not to benefit much from a diffuser . the reason is the air flow is so backed up or packed up and turbulent by the time it gets to the diffuser that the diffuser it's self can not do it's job to speed up the air flow .

diffusers work best with the uses of a FB and splitter . this is because the air flow starts at the front of a car . so with out the aid of a splitter directing air flow at the front diffusing at the rear becomes almost uneffective .

aerodynamics start to work at or near 60 MPH and above under that speed it's all mechanical grip .
with true DF cars a progressive rate spring with work best . the reason is as speeds increase so does the weight of the car . using a linear rate spring many times will not handle the two types of grip DF and mechanical they tend to be over sprong for mechanical grip and not enough of a spring rate for down force grip .
this leads to lots of hair pulling when your at a track with both very high speed corners that like aero grip and very slow speed corners when mechanical grip is needed .


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 Post subject: Re: Downforce
PostPosted: July 14, 2016, 7:41 pm 
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Great info thanks for the detailed write up. Currently I need to get the entire body built and get the suspension dialed in a best I can in 7 weeks prior to Nationals. Once everything seems to be working well I am looking forward to spending some time running some tests and optimizing the downforce.

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 Post subject: Re: Downforce
PostPosted: July 15, 2016, 12:40 am 
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962porsche, most experts only offer generalities and "trends" when describing downforce production in automobiles. You give real world measurements to shoot for. I appreciate and admire your sharing specifics.

Jack


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 Post subject: Re: Downforce
PostPosted: July 16, 2016, 1:51 am 
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Quote:
962porsche, I appreciate and admire your sharing specifics.


It's true...
:cheers:

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 Post subject: Re: Downforce
PostPosted: July 17, 2016, 6:42 pm 
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Well despite not being finished I decided to run the car yesterday with the diffuser attached. Unfortunately the diffuser was not strong enough and it buckled when I was driving around 50mph causing the side strakes to push into the wheel and then get ripped off the diffuser. I also think the side strakes were too close to the floor at the rear.

I guess it is good news bad new as it means the diffuser was actually working on a relatively slow track. My max speed was 62 mph but the diffuser broke at around 45-50mph. I would guess is must have been producing at least 50-100lbs of downforce to buckle it the way it did. Which is but means I need to make it much stronger and as I am well under weight anyway I can really make it as heave as I want as it is the perfect place to add ballast.
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Here is a video of the course. You can hear the diffuser hitting the floor on the first big right hander and periodically throught the course after that. It basically fell off piece at a time throughout the first 3 runs.
https://www.youtube.com/watch?v=cW00RomwjAU

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 Post subject: Re: Downforce
PostPosted: July 18, 2016, 8:55 am 
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i would say it was to much air pressure over the top side of the diffuser that bent it .
diffusers them self's do not give down force it's an aid to turn the hole car into a wing type shape giving the hole car down force kind of like a front splitter does .
diffusers only speed up air changing air pressure under the car they are in no way like a wing creating down force all by them self's .

looking at the few photos posted there is nothing at all to stop air from running across the top side of the diffuser . so you may very well have had 50 to 100 LBS of pressure on the diffuser to bent it .
in no way was your diffuser producing that much DF as i'm pretty sure that pressure was the air on the top side pushing the diffuser down .

if there is no way to stop air from getting to the top side of the diffuser then you will need to design a dual element diffuser . often we design them for open wheel cars this way you can control the air flow over both sides of the diffuser .


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 Post subject: Re: Downforce
PostPosted: July 18, 2016, 11:43 am 
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The sides will be boxed in as best I can around the suspension and also there will be fenders around the wheel which should prevent most of the air getting into the sides. Then once I build the rear of the car it will be like a box on the top. Hopefully this will make it much more sturdy. I think I am also going to start again with thicker material and weld steel supports and make a few tweaks using what I have learned so far.

Obviously the diffuser destroying itself was not ideal but easy come easy go and at least I know It needs to be stronger. The mark 2 will be slightly shallower to get closer to the recommended 14 degree angle. Would that be 1. the angle at the rear edge on a curved diffuser or 2. the average angle front to rear?
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 Post subject: Re: Downforce
PostPosted: July 18, 2016, 2:04 pm 
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if the bottom is totally flat (or parallel ) to the ground going to a 14 or more degree angle will be way to much .
you will end up with drag numbers being way to high .
i would do a 12 degree angle at most 12.5 .
what is the width and length between the axle center lines of the car ?
i find .057" aluminum tends to work best .
build it modular this way you will keep it light rigid and repairable if it gets damaged .
the will get it's strength for it's structure not it's mass .
in the photo we have found this tends to be the best way to build them . by doing them like this you have no screw or bolt heads on the underside to obstruct / disturb the air flow .
if and when a vertical fence gets damaged you can simply drill out the rivets and quickly replace the VF that was damaged .
your question number one or two ?
it would be number two as how the angle is set . this is because of the roll were and how it's rolled or even if it's flat . any way works being flat or rolled the transition from the cars bottom to the diffuser is the important area that you should be concerned with.
to sharp of a transition and the air will lift off the diffuser .


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 Post subject: Re: Downforce
PostPosted: July 18, 2016, 3:01 pm 
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I have an racecar aero book which specifically says that a sharp corner works better. I found it very surprising, which is why I remember it, specifically, Figure 12 in this PDF. You might want to review the whole thing because there's a lot of relevant material, http://www.strangeholiday.com/oops/stuff/annurev.fluid.38.050304.092016.pdf

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 Post subject: Re: Downforce
PostPosted: July 18, 2016, 4:47 pm 
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KB58 since you are not wanting the air to separate I can't see how a sharp bend could be good. That would guarantee separation.

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 Post subject: Re: Downforce
PostPosted: July 18, 2016, 4:52 pm 
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if your talking about a tunnel car were the tunnel starts before the rear axle center line then it's a hole other topic .


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