Functional flat underbody and rear diffuser design

[Guy_H]

I have done a fair bit of research into this as we are building a Scirocco R track car, we are also blessed with good friends that build some of the fastest time attack cars in the country & work with big $$ Aero engineers.

As much as its a great suggestion, I don't think a full flat bottom is going to give you positive benefits without a balanced wing / splitter / diffuser combo. Obviously you are trying to limit air getting under the car firstly, and that which you allow under, you then manage it. The rear wing is what is going to help with "weight" on the rear end.

Secondly, this is where front spring rates & damping (as well as sway bars) need to be set up to help limit & negate massive weight transfer (a two way differential also helps at this stage by keeping the front straight). A FWD car with a stiffer front end will be faster that one sprung the other way around.

If you are interested, I have a heap of links & you can purchase some very good books about it.

I have 900+ photo's I took of the Essen Motor Show in Germany, a lot studying the under-bodies of the cars (Including the Polo WRC, Scirocco wide body 24hr & the new Poland GTI cup) and the different approaches they use on the aero approaches. If you are around, pop in & I'll show you them .

P.S. to get your true tyre temps, you can't do this with an infra red pyro, on a cool down lap & pull into the pits you lose a huge percentage of the working temperature. Invest in a proper probe that goes 5mm into the surface where the tyre is still at track temperature to see what is really going on.

[VWindahouse]

Wow, hope there's a build thread in the near future for the Rocco Guy?!?

[Guy_H]

I hope so, I love so much the 24 hour GT widebody version, but to make it eligible for time attacks & production racing I can't go that far. We actually have two here donating to the project.

The donor is a brand new stat write-off so can't be registered - perfect for the job!

[Guy_H]

P.S. Sam,

here is some good reading for you:

Improving aerodynamics with aftermarket parts

[VWindahouse]

I really hope it works out, a Rocco time attack car would turn a lot of heads! There is some pretty mad looking aero'd up roccos throughout this link for inspiration Guy!

Improving aerodynamics with aftermarket parts

[Guy_H]

Yeah - I have a lot of close up photo's of the Polish GTI's - actually got to sit in one

[VWindahouse]

Sounds like a very productive "business trip" then

[Stuwey]

I'm not denying the lift and tip toe feeling, but that's part of driving on the limit if adhesion. More track time on different circuits and driver training helps with different approaches to high and low speed cornering, for the kink i back off but get on the throttle very early so the cars as better balanced at speed rather than trailing braking late into the corner. Check some of the my vid and te g-forces, cant recall if ts clear or not. Would also be worth getting a reliable lap timer, creating similar segments amongst us so we can also compare to each others strengths and speeds.

But all keen on downforce stuff, extremely interesting. Though again without a reliable timer it'll be hard to show any tangible gain.

Slow in, fast out. Who'd have thought that would work in a FWD hot hatch.

Drive drive drive drive drive!! More super sprints/'time attack' track days. Less 'happy laps' bs!

[clicht]

Yeah - I have a lot of close up photo's of the Polish GTI's - actually got to sit in one

Like

[kaanage]

If you don't want to soften the rear roll stiffness or increase the front, raise your front ride height or lower the rear. With a torsion beam rear end/mac strut front end, this shifts the bias of the grip slightly towards the rear.

I did this after I spun at turn 1 Phillip Island with lift off oversteer and I only lifted off for a moment at turn in. Since then, the car is much more stable under brakes too (the dive under brakes exacerbates the situation).

Drive drive drive drive drive!! More super sprints/'time attack' track days. Less 'happy laps' bs!

+100 (and kudos for being brave enough to state what I feel)

[s4mmie]

Thanks for weighing in here Guy, that threads a good read.

Hmm that's interesting kaanage as I had actually lowered the front more since last time... I have a much stiffer front ARB and brace to go on the front, so hoping that helps.

I will say this thread has brought some interesting opinions.
Let me preface the following sentences with: I don't claim to be an amazing race car driver so I appreciate the feedback from those with more experience and I like to learn. And, I'm not having a go at anyone, we're all VW enthusiasts trying to help one another out. These are merely my observations.

With that in mind..

I find it curious that Eddy agrees with Stuwey about the 'more more more' approach not working, when the little blue girl has an insane amount of custom suspension and trickery...
I also find it curious that Stuwey posted a picture of a reportedly very quick Mazda, which has a whopping front air dam and rear ducktail... all aero mods, and at the end of the day that's what we're talking about here, whether it be in the form of a wing or diffuser.


Like I said, not having a go, just a little confused.




I fail to see how driving ability could change the physical grip limitations of the car at a certain speed following the right line through a corner. There is just logically a point where no matter who was driving it, you'll max out your current grip for a corner at a certain speed. Obviously slowing down will fix it, but that's not the point is it?

For example, would you rip all the aero off an F1 and when it goes spiraling off the track tell the driver he needed to practice more?

[VWindahouse]

Wow, this thread has gone a long away from the OP's question about what material would be best to make up a home made diffuser.

[Guy_H]

Best info above is practice, practice, practice.

The problem is isolating the issues you are experiencing. My suggestion (seeming you are in Brisbane) is to do one of the Paul Stokell driver training days at QR or at Lakeside.

This will give you:

A- some driver tuition one on one on braking points, lines, balancing the car for corners & when to get on the gas.
B- You can let one of the instructors drive your vehicle & give you valuable feedback on where they think your car is lacking or has limitations.

Paul himself has won several national championships in FWD cars.

Probably your best investment of cash at the moment.

Track & Training Days

[VWindahouse]

Best info above is practice, practice, practice.

The problem is isolating the issues you are experiencing. My suggestion (seeming you are in Brisbane) is to do one of the Paul Stokell driver training days at QR or at Lakeside.

This will give you:

A- some driver tuition one on one on braking points, lines, balancing the car for corners & when to get on the gas.
B- You can let one of the instructors drive your vehicle & give you valuable feedback on where they think your car is lacking or has limitations.

Paul himself has won several national championships in FWD cars.

Probably your best investment of cash at the moment.

Track & Training Days

I totally agree with that. I did a morning of driver training theory at Lakeside last year and it was very good as they went over each corner in detail etc. However the instructor did say you don't let off the accelerator through the kink near the end of the straight which by then I'm in 5th gear and last outing hit about 190. Here is a map of Lakeside for those who have never been there



It's taken a few outings to Lakeside to muster up the courage to do pin it flat at those speeds through there as there is a big concrete wall not 5 meters or so off the right hand side of the track to great you if you get it wrong and many have. The G forces pull towards that direction which is not a nice feeling. To add to that there are imperfections in the track surface right at the turning point which actually lift you off at the moment those G forces start to act. In my limited opinion and experience when this is combined with the natural lift the polo encounters (as seen in the thread I posted about aero which shows the rear end lifts at 36kgs at 200kph added to the fact we've taken out another 50 plus kgs from the rear) it all combines for a brief holy crap moment. With speaking to a couple of people who attend happy laps that do regular timed events there and clock very impressive times they both say until they got their aero sorted ie. front splitter, adjustable rear wing and wind vortex generator they suffered the same thing.

[kaanage]

Wow, this thread has gone a long away from the OP's question about what material would be best to make up a home made diffuser.

Sure, but a diffuser is going to do SFA in solving the instability issue - you have listed the extent of aerodynamic changes needed in your post above which is far more dramatic than a rear diffuser alone.

In your situation, the solution is to either man up and hold it flat (easier said than done, I know since I don't do it at turn 1 PI where I probably should) or to lift a BEFORE the corner and then get back on the gas as you turn in.

Stability vs agility is an area where you are always compromising on a track setup and the optimal for a fast flowing track is not the same as for a tight twisty one.