Functional flat underbody and rear diffuser design

**s4mmie** · 03-02-2013, 04:00 PM

Has anyone had any experience with the above?

After my last outing at Lakeside, it revealed a big problem with a combo of no weight in the rear and lots-o-lift.
Braking into the corners quickly you could feel the rear end get squirmy, and the rear also squirmed about hooking it along the straight around the dog-leg.

I like the idea of a rear wing for simplicity, but I wouldn't run it permanently and I'm trying to figure out a mounting solution that would let me return the car to stock if need be.

So in the mean time, I've turned my attention to the underbody and diffuser.
I'm currently trying to decide on the best material to use. Two thoughts: aluminium sheeting, or ABS plastic.
I would prefer to use the ABS plastic as it is potentially easier to work with.
Any opinions on if the ABS will have enough rigidity?

I'm also concerned with cooling. I'm thinking of running a naca duct at the front of the gearbox so that air can run down the exhaust tunnel and then out a vent in the rear..

I'm also contemplating removing the tub for the spare wheel and stitch welding a panel in the hole to allow room for a functional rear diffuser.

This is my end game --

**Stuwey** · 03-02-2013, 06:02 PM

I'd be looking closer at your driving technique, tyres/pressures, and suspension.

My brother runs a 1299cc Nb Mini under very tight class restrictions, and he's currently almost 5 seconds a lap quicker than me around Mallala. My first time out and he's done many, but you the the idea. 1299cc/10" wheels, 7,5" brakes/glass, full trim.. ZERO aero enhancements and just 145hp.

More experience, further adjustments first. Do you have a rear sway bar? Ditch it for one meeting to calm the rear, see how you go.

**VWindahouse** · 03-02-2013, 06:40 PM

It's more this track in particular Stuwey as all Polo's with no rear seats and no spare tyre experience what Sam has described above including me and I have all kinds of bracing, sways front and rear set on hard, coilovers with no visible gap between the guards and rubber, and antilift bushes, and wide sticky tyres to boot. What this track seems to exaggerate is lift in the rear that causes those feelings of the rear end becoming light and lose when turning on the main straight through the dog leg which has an uneven surface to begin with. Last time there I pinned it flat through there at 190kph and had a small heart attack as the car skipped over until it found grip again. Even Sean a very well seasoned track whore gets lift on the back straight when he pins it down the hill and he has a fully laden polo.

**shinygti** · 03-02-2013, 06:49 PM

Looks like a good idea and might help with stability. Thinking for easy use and light weight thinking the abs plastic is better idea but where do you get it from? Also have you thought about acrylic ?? You can buy it at bunnings and is cheap so you can experiment a bit with it.

**Eddy** · 03-02-2013, 07:20 PM

Originally Posted by Stuwey

I'd be looking closer at your driving technique, tyres/pressures, and suspension.

More experience, further adjustments first. Do you have a rear sway bar? Ditch it for one meeting to calm the rear, see how you go.

Hoyhoy.

Spot on Stuwey.

**Stuwey** · 03-02-2013, 07:41 PM

Thanks, Eddy. I just think these cars have so much more potential, before you throw more money at it. The 'more more more' theory doesn't work!!!

**s4mmie** · 03-02-2013, 08:16 PM

Always willing to listen to those with experience.

I tend to agree with Ant on this one though.. it seems to be once you get over ~170 that the problems appear. I wouldn't say that you could adjust your driving style to the scenario Ant mentioned above, apart from slowing down and changing pants. I would prefer not to slow down. It's hard to argue with the aerodynamics showing how much lift these suckers have at the rear standard, let alone with ~50kg less in the rear.
In particular, hard braking before a corner almost feels like its going to mono.

I know there's a few schools of thought regards RSB. I feel that it's not necessarily the cause but more not helping the situation. I like it at slower speeds because I can force a bit of oversteer but it may be a little too skittish for the track. I'll give it a try this month, but like I say I think it's more accentuating another problem.
Like I mentioned, the car is very composed below ~170.

Funny you mention a Mini Stuwey, I'm pretty sure we have both (fellow ausminier?) and I know which one I'd rather be carrying speed in

Still can't beat it.

I'm not one to throw money at a problem, but from driving it, it feels like lift more than inadequate suspension or incorrect lines. I'm more than happy to take opinions on board

Also, re tyre pressures, I ran 36 psi instead if 38 and it made a great difference to rear end grip. Not sure I want to go much lower than that though..

**Eddy** · 03-02-2013, 08:22 PM

:

Originally Posted by Stuwey

The 'more more more' theory doesn't work!!!

Hoyhoy.

Spot on again Stuwey...:driver...

**Stuwey** · 03-02-2013, 08:34 PM

I've done a 1:28. My brother has done a 1:23. This Mazda, does a 1:13. Improved Production is VERY limited. It's one of the lowest-tech cars in the country, but it's the fastest by a long shot.

Keep experimenting. Keep developing. Too many people rock up with a shopping list as long as their arm with the biggest, best, most expensive components. Doesn't work, though.