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Dusterbd13
Dusterbd13 MegaDork
1/24/18 9:15 p.m.

Thinking WAY too much about this....

 

Will a flat bottom hel or hurt autocross times? What about a splitter? Ricer wing?

Also, in doung a lot of reading, it appears that a big front splitter with no rear aero causes dramatic oversteer. Would that work to offset the understeer of a spool? 

Patrick
Patrick MegaDork
1/24/18 9:19 p.m.

You need a way bigger wing than the amc

SVreX
SVreX MegaDork
1/24/18 9:29 p.m.

In reply to Dusterbd13 :

I went through the same mental gymnastics as you for years with the Mumpkin. 

I started building a flat bottom, but it creates as many difficulties as it does opportunities. Exhaust routing, heat dissipation, keeping the rear diff cool, service access, suspension travel, etc, etc 

There is no doubt there can be a payoff, but at autocross speeds I made the decision there was more value in other areas. No flat bottom allowed me to set the car lower, control cooling better, keep large exhaust, etc. 

Similar answer for a wing. The downforce created at autocross speeds is minimal, unless the wing is absurdly huge (which then also adds weight). 

I ran  a huge splitter on both the Mumpkin and the RX8 without a wing. The cars did fine (again, at autocross speeds). 

It would be a completely different answer if the average speed was 80+ mph for extended periods. 

SVreX
SVreX MegaDork
1/24/18 9:32 p.m.

If you are looking for Concours points, they are all fair game, and can all earn you points. If you are looking for performance advantages, it's a stretch. 

DeadSkunk
DeadSkunk UberDork
1/24/18 9:56 p.m.

I chose to mimic what a lot of autocrossers do to CSP Miatas, a splitter and a big rear spoiler. Since we didn't run with and without it we'll never know how effective it was. I took four runs in the car and I think it helped. The rear certainly felt well planted on corner exit, turn in was good and the car would start to slide at both ends, all pretty neutral. I also mimicced spring rates and sway bars from CSP cars. Front bar 26 mm, no rear bar, front springs about 1.5X the rear rates on Bilstein shocks. 

Dusterbd13
Dusterbd13 MegaDork
1/25/18 5:59 a.m.

In reply to DeadSkunk :

I remember seeing you at the challenge, but can you elaborate on the rest of your setup? 

 

And just to make sure im understanding: a splitter wont be enough to offset a spool, right?

 

And pat: think b17.

¯\_(ツ)_/¯
¯\_(ツ)_/¯ UltraDork
1/25/18 6:28 a.m.

Most of my autocross speed aero knowledge comes from Formula SAE- this is about the "aero elements to weight/car size" ratio you are aiming for in order to be effective:  

Barring that, the CSP guys do seem to have a consistent layout for Miatas that you could mimic, but they also have limits on the size and type of elements which you do not.  If you want to make actual wings, you can use blue foam as core and wrap it in fiberglass for relatively cost effective composite pieces, maybe even make a hot wire cutter with the wing profile you want.  I don't think anybody has done true multiple element wings for the challenge before, you could be the first!

On another note, I maintain that the wing I made out of the door skins for the parking lot ghettocet would have dropped time on the autocross course if we had been able to figure out a way to install it cheeky

Dusterbd13
Dusterbd13 MegaDork
1/25/18 6:54 a.m.

Ive been debating just what i want to make the wing out of. Blue foam/glass or plywood and aluminium like a ww2 aircraft wing.

GTXVette
GTXVette Dork
1/25/18 7:02 a.m.

Your efforts will be better put to use in Chassis set up, And a Bunch of engine!!!!!!!!!!!

One of those Huge Tractor Wings may help some in a Faster turn but as said under 60 not so Much.

Dusterbd13
Dusterbd13 MegaDork
1/25/18 7:08 a.m.

In reply to GTXVette :

But monster aero will look cool if nothing else.  Also, lack of better sway bars may be offset by aero. 

But youre right. I need to tedt this thing before chasing problems that may or may not exist. 

DeadSkunk
DeadSkunk UberDork
1/25/18 7:52 a.m.

In reply to Dusterbd13 :

Since I've never driven a car with a spool I can't tell you if the splitter would offset it. My car does have the Torsen diff though.

My set up was a set of Ebay coilovers ( the blue springs- $60) on the stock Bilstein sport shocks. The kit came with 450F/350R springs, threaded collars and upper and lower spring perches. I used everything but the rear springs. I had a pair of slightly softer rears (325#) that I substituted. I was trying to get as close as I could to a 1.5/1 ratio front to rear. The rear sway bar was there, but I disconnected one end link. The front sway bar was a modified Ford Ranger unit of 26 mm diameter. It's too wide , so I cut the ends off and welded some flat bar in place. It was installed with the Ranger bushings ground down to fit and NA Miata end links. I also used extended ball joints to increase front camber. The car ran 3 degrees in front and 2.5 in the rear. Tires were SpecMiata Hoosier take offs set at 30 PSI. 

I took 4 runs in the car and it felt really well planted to me. Alan McCrispin was smiling like the Cheshire Cat after his first run and we didn't adjust anything for subsequent runs except the tire pressure. As the tires heated we had to reduce back to 30 PSI twice.

Dusterbd13
Dusterbd13 MegaDork
1/25/18 7:55 a.m.

Do you have any pictures of the sway bar? And what year ranger?

¯\_(ツ)_/¯
¯\_(ツ)_/¯ UltraDork
1/25/18 8:04 a.m.

You mention a spool- I think you may have to ignore some of the conventional Miata autocross wisdom and run a decently stiff rear bar.  If you can't unload the inside rear tire the turn in is going to be lousy, and steady state cornering might be bad too if your tires don't tolerate much slip angle.

DeadSkunk
DeadSkunk UberDork
1/25/18 8:04 a.m.

Dusterbd13
Dusterbd13 MegaDork
1/25/18 8:08 a.m.

In reply to DeadSkunk :

Do you mind if i copy you? Also, what year/configuration of ranger?

Nonack: spool is a possibility.  Not a guarenteed thing yet. But good call on really stiff rear.

DeadSkunk
DeadSkunk UberDork
1/25/18 8:08 a.m.

I don't know what year the Ranger was, I just crawled around under trucks at the junkyard to find one that was close enough to work. If doing it over, I'd make the flat bars longer , by at least an inch. You can avoid the extended ball joints ($160) if you're willing to cut and weld the front lower control arms and make them 5-7 mm longer (or more, I suppose). Substituting the rear springs also added $110 of FMV to my budget.  

I shared this so you, or anyone else can copy it if you choose to. The car was tied for 2nd fastest time in the autocross, so I think it worked well. Nothing says there isn't a better way to do it though.

Dusterbd13
Dusterbd13 MegaDork
1/25/18 8:15 a.m.

In reply to DeadSkunk :

Thanks!

Im going to do this. I really appreciate the help. 

DeadSkunk
DeadSkunk UberDork
1/25/18 8:19 a.m.

A word of caution. While the picture of the two bars makes them look about the same width, they're not. The Ranger bar (on the right) is wider at the ends and at the primary bends. With 7x15, 38 mm offset wheels the wheels do touch the bar at full lock. This wasn't a problem on the autocross course , but it was a little disconcerting when I was turning to load the trailer the day we departed for Gainesville.frown

Dusterbd13
Dusterbd13 MegaDork
1/25/18 8:22 a.m.

The wheel size is stock nb miata 15, right?

DeadSkunk
DeadSkunk UberDork
1/25/18 8:33 a.m.

In reply to Dusterbd13 :

No. Stock is 6x15, 38 offset. Mine were 7x15, 38 offset, so they would be 1/2" closer to the bar. That, combined with the wider bar created the interference.

sleepyhead
sleepyhead HalfDork
1/25/18 8:59 a.m.

you can dig through the whole thread, or just focus on stafford's comments, here

while participating in that thread, I realized that: the CSP-like spoiler, if carefully constructed, could be re-mounted to into a "drag setup" similar to the drag mustang on page two.

Keith Tanner
Keith Tanner MegaDork
1/25/18 9:23 a.m.

For those who don't think aero helps at sub 80 or sub 60 mph speed - try it. I put a wing  and splitter on my car and it dropped my time around our local track by something like 1-2 seconds. Average speed: 45-48 mph. And that was a setup built for road course work.

For dealing with a spool, that's going to mostly be at turn-in. I think the big rear sway idea would probably work, coupled with a big DIY wing and a splitter that isn't so big that you keep cutting cones in half. CSP guys are limited in what they're allowed to do, but they all put on as much aero as they're allowed.

GameboyRMH
GameboyRMH MegaDork
1/25/18 11:19 a.m.

Yeah it's hard to go wrong with a small splitter and a wing or big spoiler. It's easy to go wrong with a spool though! If you're running one then you may even want to increase rear anti-roll stiffness to keep it from fighting you on every turn too much.

sleepyhead
sleepyhead HalfDork
1/25/18 12:06 p.m.

the other thing I thought of, while running errands...

the splitter/flat floor and a diffuser out back will help the drag times too... since they should combine to lower the drag of the car

Duke
Duke MegaDork
1/25/18 12:08 p.m.

What's the go-to for a Challenge-friendly Miata splitter these days? 

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