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Old 17th October 2003, 11:08 AM   #121 (permalink)
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resurrected the old fave...

I am planning some under tray mods to my S13 200sx track car.

The following is an attempt at interpretting the information in the threads of this post.

some input would be nice to get.. also places where you buy these brushes from...

The diagonals behind the front wheels are the Saab parts mentioned previously.
The diagonals in front of the wheels are an attempt to duct air near the brakes.

If these would trow the whole thing out of the window I'll lose them.

The solid Blue areas are a panel which is at the level of the lower spoiler, probably made out of ply, I have seen this on several cars and it seems to ba a good material as it will bend and scuff without shattering.

anyway here is the image.

http://www.angelfire.com/falcon/stev.../undertray.jpg



any ideas would be nice.

Cheers
Steve
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Old 19th October 2003, 02:01 PM   #122 (permalink)
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Assuming the blue lines are the brushes then try to get them to 'splay out' toward the rear of the car.

The front deflector will cause grief but the reasons are quite complex, if you want an interesting thing to do instead, make them smaller and put them right in front of the front wheels so they almost act as a 'dman' in front of them... this 'trick' will actually DECREASE front lift, strange but true, they have been on my car for some time and they do make a difference that is tangible at speed, stability in greatly increased.

it is good to revive old threads, this one has laid dormant for some time and has some good stuff in it, but has never really reached a 'conclusion', I doubt if there is any real closure on this subject.

But I have done a fair bit of work since my last post here so there is more to come.

Ply is good, make sure it is WPB type [water-proof board] otherwise you just end up with a soggy section of tree under your car!
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Old 10th May 2004, 09:09 AM   #123 (permalink)
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hoisted up from the past again..

On reading the latest track and race car mag there is a lotus R340 in there which has the brush under tray idea on it.

I am going to be fitting the bodykit to my car in a few weeks and I am considering the brush mod on the S13. has anyone tried this now on here ?
Also where do you buy 12 foot long brushes ?

cheers
Steve
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Old 10th May 2004, 01:12 PM   #124 (permalink)
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Yes, I know, I produced the original design that was used!
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Old 10th May 2004, 01:48 PM   #125 (permalink)
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Thought it smelled of Mycroft !!

I have just contacted a company regarding supplying me some of this brush material.

I was wondering is it worth doing this without a proper rear diffuser and the other key points. I.e will I see an improvement with this as a stand alone change ?

Also to sum up the brush placement, could you confirm...

lowest point of the sill measured to the opposite side of the car (same position) = a

a/6 = b

B is the measurement each side of the centreline of the car where the brushes must go.

So if the gap is 60 inches then I must place a brush ten inches out fr0om the centreline on each side of the car ?

does this sound ok ?

Cheers
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Old 10th May 2004, 02:09 PM   #126 (permalink)
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Yep, that still appears in most cases to be best, but narrowing the 'flute' a little is not too critical.

As to a diffuser, well the diffuser is trying to perform the same 'trick' that the brushes perform but is always les good at it, but a the combination can be good indeed.

But the real benefit is not so much a 'diffuser' as a flat bottom aft the rear axle line... a simple flat sheet of carbon fibre or similar that you can heat and mould to shape will do better than any of the 'diffusers' I have seen marketed around.
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Old 10th May 2004, 02:15 PM   #127 (permalink)
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what sort of shape would you mould the flat sheet aft of the rear axles ?

is this going to be a complex shape or could I just screw a sheet of ply on for testing ?

the bodykit I am getting is a drift style one It is a little higher at the back than at the point behind the rear wheels, I'll know If it is feasable to do this in a week or so when I recieve the kit.

If I were to rivet a flat sheet to the rear section will it form a parachute and rip the car up ?

sounds extreme but I thought I'd check...


/Steve
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Old 10th May 2004, 02:25 PM   #128 (permalink)
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What you need to do is look under the car and realise that any 'void' or 'scoop' will cause turbulence, which both slows you down and increases instability.

If you can slip your hand into 'pockets' between the body panels and the main monocoque, take a look at the area under the rear bumper, it is a huge scoop that at speed lifts the rear of the car quite markedly.

I do not recommend using ply...
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Old 10th May 2004, 02:39 PM   #129 (permalink)
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Quote:
Originally Posted by Mycroft

I do not recommend using ply...
I know people use it on the front splitters of racecars !

so I should be looking to fill up any gaps, how does thin fibreglass sheet and then filling the voids with expanding foam (where possible )

I am aware of weight though as I have been carefully stripping weight from the car for a few months now.

Should the sheet ideally be flat with the shape of the reat skirts or should it be a "hollow" into the middle of the car.? do you follow ?

sorry to be a pain..
Steve
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Old 10th May 2004, 02:49 PM   #130 (permalink)
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Flat everytime, don't try to shape it except if that shape more adequately covers an area gaining a reward for doing so.

Filling voids with expanding foam is very very good, done this myself, the trick is to first put a thin plastic sheet into the void and then fill it, file off excess. cover with the flat sheet, the reaos for the plastic is so that it can be removed easily from the monocoque [only works where the plug of foam is locked in by adjoining panels.

I did this to my first Soarer and the increase in stability at speed is really very marked indeed.
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Old 10th May 2004, 02:52 PM   #131 (permalink)
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BTW, it is not a pain at all, this thread is 20months old and only now are cars appearing with what was in here, take a good look under the new Mac Merc... you might be surprised and what is under there...
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Old 10th May 2004, 03:10 PM   #132 (permalink)
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I have always been led to beleive that 8 degrees was a safe angle to work with to ensure you don't ger seperation and stall. The longer you can make it the better, and the further forward you can start it the better (within reason).

As mycroft points out, fancy shapes aren't required. The aim is to encourage air to exit freely from under the car, not to actively suck it out.

Although the pressure difference will be small, don't forget that a very small low pressure over such a large area may create significant forces (we hope!) so using 1.6mm aluminium isn't the way.

Paul
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Old 11th May 2004, 07:20 AM   #133 (permalink)
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So is 8 degrees the angle of the rear under tray in an ideal world then Pavlo ?

I think the rake of the kit I have got is a tad more than that but until I have it on it is difficult to tell.

so do I take this panel just up to the rear diff or further forward with cutouts for the stuff sticking through ?

Also all this at the back should I do anything with the front under area ?

A lot of the race cars you see have a flat panel that runs to the steering rack sort of area from the front splitter ?

should I look to doing this ?
Are there any cooling considerations to make when designing these trays ?

I have almost got my head around it !
I am geting a sample of the brush material sent to me so I can have a good look and work out how to attach it..

cheers
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Old 11th May 2004, 09:28 AM   #134 (permalink)
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Quote:
Originally Posted by SteveC
So is 8 degrees the angle of the rear under tray in an ideal world then Pavlo ?
I don't think it's ideal as such just an angle which you can use, pretty safe in the knowledge you wont stall the diffuser. By that, I mean the air will remain attached to the surface, and flow in an orderly fashion. As soon as it seperates you lose the flow speed and the low pressure, worse still turbulence can cause high pressure due to slow moving (and therefore high pressure) air that may circulate between the ground and diffuser.

I would avoid having the diff penetrate the diffuser if possible, simply because you can't know what's going to happen with any certainty. Flat and simple is low risk.

As for entry, just try and get is as smooth and simple as possible, crap in, crap out as they say.

Paul
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Old 12th May 2004, 09:05 PM   #135 (permalink)
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glen r u trying to diffuse the situation u funny git
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Old 14th May 2004, 07:09 PM   #136 (permalink)
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time for more of my silly questions !!

I have recieved the sample of the brush material and measured my 200sx up.

In order to reach the ground like the Lotus in TRC mag they need to be 5" long. but i read in the body of this that due to a freak effect there is no turbulence encountered for the first couple of inches if the ride height is 6 inches.

So doea this mean that if I put the 4" brush material on the design will still work or should I look to copying the setup on the lotus?


I am trying to find a supplier for either carbon or kevlar or some other sheet material which has similar propeties to use for a front and rear under tray.

Are there any considerations I need to make to get this to work ? or is a flat sheet as discussed the way to go ? what about fins on the underside etc...

I could always use some more of the brushes !!

I am also looking into the saab bits you talked about.

The only thing I will not be using for now is the sideburns idea, as I am getting a nice bodykit and don't want to hide the nice wide arches for a while !!


What sort of thickness is right for the under tray ? what sort of forces are these things under ?

will the added weight of all this more be more than compensated for by the addition of it ?

sorry about the ??'s but I am just checking before I go and spend the best part of 500 quid on various parts..

thanks
Steve
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Old 20th May 2004, 02:45 PM   #137 (permalink)
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Anyone ?

I am trying to order some sheet material to do the undertray with..

????
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Old 10th June 2004, 11:35 AM   #138 (permalink)
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Hello again,

Right I have bolted a sheet of aluminium to the front and rear sections of the underside of the bodykit.

These run from the edges to just before the axles from each end.

they are flat 1.5mm sheet.

I have instantly got a benifit and that is reduced movement of the fibreglass kit.

I have also got 2 off 3m garage door brushes ready to bolt on. the brushes have been ordered so they miss the road surface by about 1 inch. obviously they will receive a pounding but they are sacrificial to an extent.

my next questions are regarding the car on the magazine.. you can clearly see the brushes on it and they go to the floor. In a previous post Mycroft mentioned about having the brushes on the sills and another set at the ratio mentioned 1/6th each side of centre.

so was the brush set up on the lotus at full width and then having inner ones too ?

next it was suggested that I "flute" the brush shape toward the rear. how much wider at the rear should the brushes be ? should I just install the brushes at angles from each other to create the shape or should they be bent into an hourglass shape ?

Instead of using the SAAB flaps can I get away with short lengths of the brushes to do the same thing at the same angle as the flaps 145 deg off centreline.??

On my car there is a diff oil coole in the centre "wind tunnel" area, is this a good or a bad thing ?

I will be getting a vented bonnet to deal with intercooler airflow so I am not too worried about the undertray affecting the engine/charge temps.

Will there be any cost to aerodynamics by not using archliners ? the kit has flared arches and the liners dont fit anymore.. I could make something for it but only if there will be a gain.

thanks for any info..

Steve
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Old 15th June 2005, 09:31 PM   #139 (permalink)
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Finally sorted out my car...

see attached pics...

The rear is cut up at an angle behind the rear wheels now.

just need to work out how to do the side panels.

cheers
Steve
Attached Images
File Type: jpg 016.jpg (49.9 KB, 67 views)
File Type: jpg 017.jpg (49.9 KB, 52 views)
File Type: jpg 018.jpg (31.6 KB, 52 views)
File Type: jpg rear undertray.jpg (51.1 KB, 61 views)
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Old 15th June 2005, 11:45 PM   #140 (permalink)
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Dangerous

One of the reason I have read about in past for road cars not using aero diffusers is that they have a habit of 'letting go' on a bad surfaced/cambered road. You're in the middle of a corner near the limit but the road has a small change in that screws up the downforce mid bend - off the road you go. The Mac F1 had this problem apparantly. Manufacturers don't like to see the buyers of their vehicles dying in newspaper headlines. Under car aero packages outside of the track can be killers so be very careful adding it to a road car.
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