fha772
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| posted on 1/6/15 at 02:34 PM |
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Rear diffuser design...
Hi all,
What's the best design for a rear diffuser?
Would long, with a shallow angle be better than short with a steeper angle.
Also, are there any advantages to fitting fins to the diffuser?
Cheers Frank.
http://www.ppcmag.co.uk/forum/viewtopic.php?f=47&t=6743&start=105
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loggyboy
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| posted on 1/6/15 at 02:47 PM |
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I suspect there is no simple 'one size fits all' approach. I will depend on car dimensions, ride height, desired effect, frontal and upper
rear aero design.
Just copy one from a known functional design (S2 elise, race cars, ferraris etc), at least that should give you a broad starting point.
You could also design one with adjustable sections to test, but the chances of you actually noticing it without complex data reading and wind tunnel
tests is unlikely.
Mistral Motorsport
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coozer
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| posted on 1/6/15 at 02:51 PM |
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Full width of the floor and the kick up should be around 14°.
I just made mine extend flat from the floor and kicked it up so it missed the fuel pump and put the ribs so they where flat with the floor as well, if
that makes sense..
Some description and pictures here:
http://www.locostbuilders.co.uk/viewthread.php?tid=192273
1972 V8 Jago
1980 Z750
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coozer
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| posted on 1/6/15 at 02:52 PM |
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The fins stop the low pressure air bleeding out the sides. I have to say it made a big difference to mine, smoothing out the back end and making it
feel more planted.
1972 V8 Jago
1980 Z750
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Texan
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| posted on 1/6/15 at 03:34 PM |
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I've read that 8-12 degrees is the optimum and that above 12 degrees you begin to get separation of the air from the diffuser which means the
longer, shallower version should be the best.
I drive therefore I am.
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fha772
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| posted on 1/6/15 at 04:29 PM |
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quote:
Originally posted by Texan
I've read that 8-12 degrees is the optimum and that above 12 degrees you begin to get separation of the air from the diffuser which means the
longer, shallower version should be the best.
That's the sort of thing I thought.
I know that each diffuser, for each model of car would have differences, but I thought there would be a set of general rules that apply to all
diffusers...
http://www.ppcmag.co.uk/forum/viewtopic.php?f=47&t=6743&start=105
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coozer
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| posted on 1/6/15 at 04:37 PM |
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I got the 14° as the optimum out of a feature in the Track Driver magazine.....
However it all depeebds what's hanging down on your car, the bend in mine was to clear the swirl pot on the tank and the injection pump.
1972 V8 Jago
1980 Z750
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bi22le
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| posted on 1/6/15 at 06:55 PM |
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There is an optimum design but i would say something is better than nothing.
My striker had holes and vents all over the back to stop the parachute effect from the back end. I still noticed a big difference when i fitted mine.
At high speed it just seems to pull nicer and high speed cornering was better.
Still nothing compared to a good geo set up though. Still the best 250 quid i will ever spend.
I want to flat bottom the front now. I think that will make a big difference.
[Edited on 1/6/15 by bi22le]
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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CNHSS1
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| posted on 1/6/15 at 07:18 PM |
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A full flat floor into diffuser is ideal and sub 13 degree is the norm. I got Simon McBeath to write a consultation paper on my previous hillclimber,
really useful and a knowledgable and helpful guy.
As has been said most diffusers even if they dont work as a diffuser will reduce inherent lift from the parachute effect. At speed this will be
significant so to reduce it will give a nett gain and in effect add downforce by removing unwanted lift.
In general longer shallow angled diffusers will work and be 'less peaky' in their gains than short rapidly rising diffusers. The rate at
which they rise can mean that either higher speeds are required (7 types aero isnt conduscive to 150mph speeds to feed a diffuser) or a cunning way of
feeding and accelerating the air into and out of the diffuser. Google pics of the new Empire Evo single seaters and the Gould GR59 and youll see
theyve created 2 diffusers either side of the tub where the side pods wouldve been. Big long throats feeding into realitvely shallow diffusers to
accelerate the flow.
Its worth noting too that the diffuser design needs to be relevant to the speeds at which you wish it to work. An LMP1 car diffuser will produce
bugger all effect at 50mph, but will be producing 1000kg of downforce at 150mph. A hillclimber willbe working at 45-50mph but be maxed at 120mph as
thats as fast as the car needs to go.
In the case of undercar aero its generally a case of making the flow less disturbed to see a gain, rather than looking for perfection. A 7 types above
car aero is crap in reality so any tidy housekeeping will improve high speed stability.
The straighteners in the diffusers perform two tasks, to straighten flow and add strength for little weight. The side fences on a diffuser are
essential though, without them there can be no true venturi effect merely a reduction in parachute effect
McBeaths book is good as its theory and applied theory, whereby he talks of the compromises needed rathe than most aero books (and ive got most of
them) that are purer aero theory but dont then relate ut to differing car types as such. A perfect F1 design floor and diffuser is of no use on a car
that is 100mph short of its working speed range.
If you can find online any of his articles for Racecar engineering in the MIRA tunnel, they are great reading and dispel many pure theory ideas as
they just dont work in the real world. I spent a day in there with him and a couple of cars (one a friends) and was hooked
Good luck, fascinatjng subject :-)
"Racing is life, everything else, before or after, is just waiting"---Steve McQueen
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