Little Red Devil

[Bullet Fast]

Heat wrap the lot and run a flat underbody across the lot. I'm sure you spoken to gav about what I suggested off line. I'm still curious about wether we could do it.

On another note, bike coefficients are terrible and have been since dustbin fairings were made illegal. A few brave designers have tried exhaust gas directions at the major gap between rear wheels and tail sections, namely John Britten, and can be seen by typing same into google. Point is its really not worth anything. Moto GP is nearly open on exhaust rules as I understand and if that has was worth 0.1kph on top speed it'd be pointing at a vacuum generated by the bike. Which it isn't. You'd gain more advantage by pointing your exhaust outlet at competitors cold/ram air intakes so they can't sit close enough to streamline. Now, go back and look at the same Moto GP bikes and production GP bikes... B-) notice anything?

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[kaanage]

I beg to differ - the Britten was VERY fast for the horsepower that it had much. of which was due to the exhaust and hot air from the radiator (which was under the seat) being directed into its wake. It also improved the airflow at the front of the bike by not needing a big ram intake.

MotoGP bikes are built against all sorts of PERFORMANCE constraints, some of them being reduction of polar moment of inertia, keeping the vertical CoG sensible and a foward weight bias. Having a mid exhaust exit aids all of these and if you add that they have enough power that they are still accelerating into the braking marker at most tracks, then you will see that the drag reduction is of a lesser concern than Moto3 bikes which have SFA power and all of them direct their exhaust well back. Dumping the exhaust into the gap between the rear tire and the bike would be silly for all sorts of reasons.

[VWindahouse]

Heat wrap the lot and run a flat underbody across the lot. I'm sure you spoken to gav about what I suggested off line. I'm still curious about wether we could do it.

Yes mate very interested if you can make the front section especially. Iv'e given Gav one of the ABS splitters I have for you to scan. From there just add a further flat section that comes forward another 60mm or so but goes all the way back to the end of the engine bay. At this stage the Lip is about $120 landed and a flat bit of carbon sheet like this is about $500 landed

Carbon fibre front splitter to suit all vehicles - Carbon Mods

So if you can make a complete unit for less I'm in!

[Bullet Fast]

Not usually, if I make it its because it's a prototype. Prototyping is necessarily expensive. Where I win out is during creation of products where rapid Prototyping IS the method of manufacture.

Anyway, it'd need to be fitted to a car in Sydney and measured in place to do the alignment. How do you imagine it being fixed to the car at the front?

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[gavs]

Anth I think RE diffuser you might be thinking along the wrong train of thought.

Diffusers do work on torsion beam cars and they are on cars from the factory, RenaultSport is a great example. When they released the Clio 197 the decided to get the road holding ability the wanted, rather than a ducktail spoiler, they went with a (yes, it's not just for show!) fully functioning engineered under tray and functional diffuser. On the updated Clio (the currently fugly one) they RE tested it and aside from reducing drag (a massive benefit of under-vehicle aero) it was producing 30kgs of negative lift or 'downforce' at 180kph.

But the Clio is multi link you say...

The current shape megane was released globally by renaultSport before any other model because they saw the popularity and benefits of releasing the performance line first. Let the skunkworks get it working great, then adapt it for the masses, a magnificent mantra. This resulted in an aerodynamically driven design which also sports undercar aero and a fully functioning diffuser.

What people recognize as a diffuser is this:
But that is for looks more than anything. A diffuser does 1 thing, it accelerates airflow. It doesn't have to be as aggressive as this f430 one or off a DTM car, look at the pre-2013 v8 supercars with a watts link live axle rear end and dirty great fuel cell, yet their diffusers still worked.

Don't worry about the wheel well or the torsion beam, with that dirty great wing you are running, you will be better off creating a flat undertray that hangs BELOW the current floor, yes, a false floor and then block in the sides with some skirts. When you get to the torsion beam, just make sure that the floor between the rear of the car and the beam and the floor from the front are below the beam, this might require raising the car closer to factory ride heights but you can then tailor your body work to suit your 'ground effects'.
This way you rely on the wing and a front splitter for your 'downforce' and the flat undertray to counter the drag of the car.

Side note, while your bumper holes might get better top speed, you're introducing a whole lot of turbulence from that airflow at the rear of the car, not so bad for drag racing but horrible for circuit work.

[kaanage]

while your bumper holes might get better top speed, you're introducing a whole lot of turbulence from that airflow at the rear of the car, not so bad for drag racing but horrible for circuit work.

Can you please elaborate on why these wouldn't help on a circuit?

[gavs]

I just had a response written and goddamn tapatalk deleted it when my phone died!

Because I can't be bothered typing it all again, I'll try to summarise it instead.

The area directly behind the car is a dirty great vacuum, Anthony has changed the characteristics of this vacuum by the addition of the roof-top wing, which will also affect how the vortices come off the rear of the car. By adding high-velocity turbulent air (not controlled at all) into this vacuum will completely upset what is a fairly consistent aerodynamic profile on the car. This can greatly affect the handling and balance characteristics of the car and in all honesty, my personal thoughts are that it will probably negate any negative lift (downforce) generated by the wing entirely. That said, without a proper CFD or wind tunnel evaluation, I couldn't be 100% on that.

As I said, fine on a drag car where you are looking for outright acceleration and speed without any need for handling balance but on a car that is meant to brake and turn corners, I think that there could be an adverse side affect of an extra 8kph in straight line speed...

[kaanage]

OK, that makes sense in context of the wing losing efficiency - the issue is actually the increase in pressure in the wake from the bumper bar exits. This reduces the pressure differential between the upper and lower wing surfaces => less downforce.
I think either extensive track testing or time in a wind tunnel would be needed to quantify the interaction since the wing is nowhere near the bumper and has very little (if any) overhang.

In a drag car, you'd delete the wing and have the holes (and possibly a reverse spoiler on roof to direct air down into the wake) for minimum drag .

[VWindahouse]

Thanks Gav, appreciate the input! Couple of things though and certainly not saying you're wrong here. Yes something like the Clio diffuser was my first intention. Minus the huge budget for part development and wind tunnel testing of course

I look at cost v's benefit with all this as well as being within the category rules and the rules say wheel well stays and a custom diffsuer can't go past the rear bumper. Plus I can't have a flat under body panel between the front and rear axles which isn't such a big deal as the Polo has two plastic panels running that length of the body either side of the exhaust tunnel and the exhaust itself is up quite high and not in the flow of air.

I also look at all the top hatchback time attack cars from all around the world to get ideas as they do spend massive coin on aero as there are basic principles when dealing with the very unique combo of FWD and hatchback chassis. Most of the fastest are sadly Civic's not Clios, Polo's or even Golf R's for that matter.

Now the biggest issue with any hatchback is rear end lift and drag. Usually caused by the same inherent design issue which causes a parachute effect at the rear from the air travelling under the car which there is a lot as these hatchbacks also have a very high COG. My Polo still has a high COG and it can't go any lower. In reflection banging on the wing was just masking the real issue as the lift is not caused by the top of the car.

So for your last comment can't say I agree. You are assuming the Polo's overall aero dynamics are well designed from factory. Well it's far from. Here are some wind tunnel testing of the MK5 Golf 3 door GTI (same chassis and shape as my car just a larger version)



So at 200kph the rear experienced 37kgs of lift and only 2kgs at the front.

And here are the other variants from VW that suffer the same massive lift in the rear end

Golf R32



36kgs of lift at the rear and 3 at the front

Mk6 Golf GTI



A little better at 33kgs at the rear and 5 at the front.

So there is one thing all these very similarly shaped hatchbacks have in common and that a very short and squat rear that catches the underside air flow like a parachute as I said before. This also causes a high pressure build up right where there should be any at all. Just for comparisons sake the Audi RS3 has only 21kgs of rear end lift



and the the best hatchback I can find is the Focus RS with only 4kgs lift at the rear



So VW obviously didn't spend a heap of time in a wind tunnel when designing the shape of hatchbacks in question.

My point is for what ever increase in rear end turbulence can't possibly out weight the drag and lift effect caused by entire rear bumper especially the lower valance plus the vertical bits of metal I have disposed of. They all act as a wind brake because on the Polo there are big gaps that air travels either side of the wheel well only to get caught up at the end. Cars that have a flatter underside that fills the gaps between the beam and the bumper won't have this issue.

Here is a sub 1 minute Civic at Tsukuba which has a well designed fiffuser that looks to be at the optimum 7-8* but because it also has provision for excess air to evacuate out of the bumper.



So until I win lotto and can afford a fluid dynamisit like Andrew Brilliant and a wind tunnel this crude yet very common hatchback mod will do the trick

[VWindahouse]

Actually maybe something as simple as this be worth a go



Thoughts?