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Color0's Micro RC Blog -- A technical brain dump from the mind of yours truly...
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The Super GT Wall Handling Problem: Solved!

Posted 01-18-2012 at 05:34 PM by color0
As tinkerer first and a racer second, I don't like admitting that the Mosler MT900 is the fastest and only competitive body at the top levels of Mini-Z racing. I admit it's fast: with great corner speed, on-throttle stability and superb wall handling, I'm not surprised that the PN Worlds this past year was contested almost entirely of Moslers. But on the other hand, the Super GT Nissan GT-R and Honda HSV-010 have a little less stability but noticeably more corner speed: whenever I put them back-to-back against the Mosler, the Mosler always wins overall but the GT-R/HSV's fast lap will be up to 2 tenths faster! This discrepancy almost always comes down to wall handling.

Simply put, the GT-R and HSV suck at it. Just take a look at their huge front splitters (image borrowed from Kyoshosan):

The splitter can literally cut through RCP rail foam if you go in hard enough, and so the HSV and GT-R can not ever touch the wall if you want to go fast. The problem seems to be bad enough that Kyosho released bumpers for these bodies, but this doesn't help because the bumpers still have sharp corners, and the bumper truncates early, leaving the wheels still unprotected if you're trying to run wider offsets up front (+3N, etc.) to avoid traction rolling.

The older method that has been explored thoroughly is cutting off the splitter entirely:

Which works -- the body does glide off the walls better -- but considerably weakens the front bumper and reduces the protection of the front wheels (doing this forced me to run +1.5N maximum up front rather than the stock +2N offset). Within just a few races of cutting off the splitter I cracked that GT-R from the ground all the way up through the headlight, shattering the lens! So that's a no go.

Now I think I've discovered (or rediscovered, as the technique isn't new) the ticket to success with the GT-R and HSV, which is the Kyosho clear bumper intended for the Mazda 787 (MZN303-4). It's just the right length and width, and it has rounded corners! Exactly what I've been looking for. The modifications are fairly simple to get it working: I cut off the rear mounting points to shorten the clear bumper, letting me use the front mounting points to bolt it to the MR02/03's extra holes up front.

I notched the clear bumper just inside the outside edge to make some clearance for the front tires when steered. I drilled out the mounting holes in the 03 chassis, then used 2 countersunk 8mm screws and 4 PN blue metal shims (2 per side) to attach. The 2 shims lined up the clear bumper perfectly even with the bottom of the chassis -- you can go higher or lower depending on your preferences.

The PN interchangeable front clip doesn't play well with the gusseting on the bumper so I shaved it off. Next I turned the bumper into a bona fide splitter by cutting out the center section. I smoothed out all the edges (just in case) with 1000 grit sandpaper.

Next: because I didn't want to raise my body, I cut off the front splitter and then shaved the front bumper upwards to compensate for the height of the clear bumper/splitter. It's about 1.5mm worth, you may consider cutting a little more if you find that you can't pop the body on without dangerously bending the clear bumper.

At this point I popped the body on (carefully) to check ground clearance and body clearance. The GT-R's front bumper touches the clear bumper at the front grill area but neither is stressed or bent -- good. This gives a good aero seal without threatening to break anything. I shaved the GT-R's front bumper a little further upwards on the sides to give the clear bumper a little room (~1mm) to bend up in a collision. Ground clearance is perfect, 1.5mm and the splitter lies flat. It also truncates later to protect the wheels better than Kyosho's GT-R/HSV-specific front bumper. Everything's looking good so far!

The bumper/splitter has the perfect shape to glide off walls like an F430. Which is excellent, because the F430 is even better off walls than the Mosler we're trying to catch up to.

It's also wider than the GT-R by about 1.5mm each side, so I can turn to almost my full steering travel (74%) with +3N front wheels and still protect the wheels! With +2N fronts (stock offset) you would be able to use 100% steering travel with absolutely no fear.

And you can see that the clear bumper re-forms a nice short splitter under the front bumper, for downforce. Theoretically the flat underside will also generate downforce through the "venturi effect", but that will have to be tested later.

Initial testing around the dorms has been super promising! The car glances off walls even when the wheels are steered towards it, so wheel protection: check! lfsminiz on the forums has confirmed that the bumper does in fact help on RCP rails, presuming that you don't misjudge the turn TOO badly lol.

So, assuming I don't break anything in the first race, I believe I've successfully tackled the Super GT wall handling problem! I'll cut an HSV the same way too to test: since the HSV has a shorter nose than the GT-R it would be safe to assume that more front splitter area will be exposed = even more downforce! We'll see soon. ^_^
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