Another question:
Will wider fender be available in the future? The Miata I am picking up currently has 9" wide wheels (225/45 STR setup). There is a good chance that if I put any sticky rubber on there it could get wider (275/35, or 13x10s, or slicks)
Yeah I was planning on the same. And most likely needing to remove that outer lip on the fender to allow wider tires to stick out. I suppose you might be able to run no fenders on track. I think that is what the v8 powered Stalker was doing
ProDarwin wrote: Another question: Will wider fender be available in the future? The Miata I am picking up currently has 9" wide wheels (225/45 STR setup). There is a good chance that if I put any sticky rubber on there it could get wider (275/35, or 13x10s, or slicks)
I would think on track a 1400lb car isn't really going to need much more than an R-comp 205.
But of course Auto-X is a different story.
z31maniac wrote: I would think on track a 1400lb car isn't really going to need much more than an R-comp 205.
This this this. Seriously, 275 is crazy overkill. Remember, the tire is in the airflow, so it will have more cooling flow. I'm planning 225s for the FM2 Exocet, and that's only because it has some serious aero that will get it up to Miata weight in the corners. Even the Atom 500 has trouble getting heat into its 245mm rears, and all the Atom's weight is on it's ass.
For natural aspiration, 195 should be fine. 205 for turbos. 205s fit nicely in the current fenders. 275 is big for a Miata that weighs 900 lbs more.
I'm will not be running fenders on my Exocet. Not required in GA, and unsprung mass is the enemy (the fenders are 1.8 lbs, their mounts are 2.3 lbs). You can cut off the inner sides pretty easily with a cutoff wheel and run some door trim along it to make it look clean.
I disagree. Having run a 1400 lb car on track, they're faster with more rubber. In fact, I've found they're really sensitive to low traction tires.
And a spinning tire is really dirty for airflow. Think about it, the top of the tire is moving forward at 2x ground speed. A set of fenders would certainly clean up the car.
Nationals winning CSP Miatas running the 275s are below 2000lbs. 1500lb (or whatever min weight is on a DP miata w/ this setup) miatas are running 15x10 275s and 13x10 255s.
I bet you if you offered them a 295/30-15 (and it would fit), they'd buy it, and go faster.
Regardless... I'm probably not doing anything that nutty soon, but the car comes with a 9"/225 setup, and I'll probably stick with that for a while. Maybe if I snag another set of wheels, I'll go narrower, but that is unlikely. I'll have to look up the laws regarding no-fenders here. Or maybe just make my own :)
In reply to Warren v:
For auto-x speeds I'd say you are fine with no fenders but... not on a road course. Spinning tires really mess up the air over everything behind them. 4.1lbs per corner is a lot for unsprung weight (can they be fitted to the nose cone instead and clean the air over the suspension while becoming sprung too?).
The backstretch at VIR or front straight at Summit or the back at the Glen are all going to love you more for having fenders than hating on you for adding a bit of weight. Just look at all the trouble open wheelers go to make those front wings... that surely isn't because it's faster not to.
The nice thing about a fenderless (or a bolt-on fender) car is that you have a bit more freedom for diameter. So a 255/40-17 would be an option, for example. And I think there's a 295 in the 25" OD range too. Next problem: finding wheels 
Keith Tanner wrote: The nice thing about a fenderless (or a bolt-on fender) car is that you have a bit more freedom for diameter. So a 255/40-17 would be an option, for example. And I think there's a 295 in the 25" OD range too. Next problem: finding wheels
That, and you don't have to worry about the rear fenders getting distroyed by rock impacts, or the front fenders fatigue-failing and ripping off at speed.
Keith Tanner wrote: The nice thing about a fenderless (or a bolt-on fender) car is that you have a bit more freedom for diameter. So a 255/40-17 would be an option, for example. And I think there's a 295 in the 25" OD range too. Next problem: finding wheels
Get wallet.
Remove credit card.
Call JongBloed.
I think they, and of course a few others, will make custom wheels in nearly any diameter/width you want.
kb58 wrote:Keith Tanner wrote: The nice thing about a fenderless (or a bolt-on fender) car is that you have a bit more freedom for diameter. So a 255/40-17 would be an option, for example. And I think there's a 295 in the 25" OD range too. Next problem: finding wheelsThat, and you don't have to worry about the rear fenders getting distroyed by rock impacts, or the front fenders fatigue-failing and ripping off at speed.
I don't know.
I think about how much rubber/debris/rocks I hear beating up the inside of the fenderwells in the Miata on track...........then think about all that coming into the cockpit. Doesn't seem like the greatest thing.
Well I am about to buy 15x8 or 15x9 wheels to run 225/45/15s I was leaning towards the 9. Some DP guys for AutoX even run 13x10inch avon slicks. That is also what the "Stalker" kit cars were using for track time. Swapping out the 15 or 17 street wheels for 13inc slicks
z31maniac wrote: I don't know. I think about how much rubber/debris/rocks I hear beating up the inside of the fenderwells in the Miata on track...........then think about all that coming into the cockpit. Doesn't seem like the greatest thing.
Just by the angles they cover, the current fender's don't do all that much to block the FUD. I also suspect their aerodynamic contribution is less than believed. For debris, all the UK MX150R racers cut out panels and put it inside the upper tube section. Aero benefit too.
My ideal solution for fenders is some sort of aero-focused sprung fender (attached to the body) in front of the tire, and a small rear fender down low to catch the debris. Some states have specific angles for fenders, so the current fender design will continue to be available next year.
I dig the paneled in look and the winglet fenders on the red one
I think the benefit of a kit car and simple fenders is you can put whatever you want on there. If you're putting wider tires on a car you built yourself I think you can find and figure out how to mount wider fenders. It doesn't seem like much of a hardship.
Plus it's better than fitting ridiculously wide fenders on the car from the get-go when the majority of people will just be running ~205's.
Nothing wrong with having options. Westfield offers both a standard (<=205) and a wide rear fender.
When the Caterham CSR was developed, EVO magazine had a number of good techy articles on the car. One of the things they covered was the aero testing of fenders. The more the fenders wrapped around the front of the wheel, the better. Also, the little lip on the backside was developed. You can see the before/after.
CSR
Classic
Of course, these cars are always going to be horrendous from an aero standpoint. But if it's not that difficult to improve them, then improve them! And I tell you that you'll want eye protection no matter what.
Keith Tanner wrote: And I tell you that you'll want eye protection no matter what.
No kidding. Unless I'm getting pictures taken of me in the car, I wear a full faced helmet when I drive. In just about every way, I treat these things like motorcycles. "I had to slide through the turn officer, or I would have high-sided!"
As far as aero, the guys I'm developing the car with work with Fluent and other CFD packages for their day jobs. We're definitely going to have at least some pretty pictures of airflow, and at best some excellent aero properties.
Warren, what type of power levels do you think will be necessary to achieve your sub 4 sec 0-60 goal?
z31maniac wrote: Warren, what type of power levels do you think will be necessary to achieve your sub 4 sec 0-60 goal?
Depends on gearing, actually. If you don't need to shift, you only need to maintain 0.7g. If you need a 500ms shift, you need to be closer to 0.8g. Lets assume 1700lbs with a driver and get really theoretical here:
With one 500ms shift, you need >213 whp.
With no shifts, you only need >186 whp.
Allowing a little headroom, a 5 speed Miata should be able to hit 4.0 seconds with ~240 whp, accounting for any sort of turbo lag or torque dip that would drop the acceleration below 0.8g.
A 6 speed Miata with a stock rear end hits 60 in 3rd! Two 500ms shifts means you effectively have 3 seconds to hit 60. You'll need >248hp to make up for the shifts.
In an lsXocet, you could do the launch and 0-60 run in 2nd. If you can put the power down effectively, you'll only need ~250whp to do 0.60 in under 3, assuming a nice torque curve.
In reality, you'll need a bit of headroom (15-25%) to account for traction issues, torque dips, and whatnot. Keep in mind that power required to accelerate is a function of speed. You need half as much power to accelerate at 30mph, for example. The torque needed at any speed would be (Power@60)*(Speed/60) * 5250/RPM. For example, a T56 and 3.42 rear end will put you at 5300 RPM @ 60mph in 2nd. For that 3 second run, the torque required throughout the run would be 245 ft-lbs. The LS3 will do 300 ft-lbs at 1200 RPM. Better practice your clutch finesse!
The 0-60 in under 4 is not an issue on a drag strip. I want to be able to do effortless, repeatable <4sec accel runs on a road course-type surface, which will require a bit of suspension work, setup, and good ergonomics. The balanced F/R weight distribution limits the car to a theoretical 2.6 time, but that's on a very grippy surface with drag radials.
Where is your spring rate cheat sheet Warren ? :)
Are you including drag in those #s? Have you done a coastdown test to determine the CdA of the car?
I quickly plugged in some #s into a accel spreadsheet I had from powertrain class in college.... and using Saturn drag #s (likely a lot lower Cd (0.345), but bigger A (1.83sq m)), it shows a 1700lb w/ driver exocet with FMII turbo kit (this dyno: http://www.flyinmiata.com/tech/dyno_runs/1995_new_vs_old_FM_II.pdf) going 0-60 in 3.7 seconds, and doing the 1/4 mile in 11.4 @ 135.2 mph
Obviously just a ballpark, but interesting.
FWIW, the Saturn #s from school were about 0.8-1 second too slow, and about 5mph high on the trap speed. Not sure where the trap speed discrepancy comes from, but the time discrepancy is from the launch, as it basically calculates using a rolling start at 0.1kph, so power values are effectively 0 at that point (~11 rpm). Using a constant accel (spinning tires), or constant ~2000rpm power figure for the first .5-1 seconds (slipping clutch) would probably be a lot closer to reality.
Wow, been a while since I've plated with these formulas. This is kinda fun
I was missing multiplying by velocity to get from Road Load Force -> Road Load Power. I estimated a launch of ~.8g until around 3500rpm. Also used 0.65 as the Cd and 1.6sq m as the area.
Ends up with a 60ft time of around 2.05 seconds. 3.4 seconds to 60 mph + a shift. 1/4 in 11.6 @ 119mph. I think a .500 shift is a little long. A .15 shift would be around 3.55 to 60 and a 12.2s 1/4.
Though I barely understand what you guys are yakking about, this is what makes the Exocet so much more appealing than the bulk of the home built 7's out there. This thing is actually engineered (and re-engineered, by Warren and his band of elves) to be something other than a pile of parts you put together.
What you're witnessing right now is engineering masturbation Soo many estimates and rectally-obtained precise measurements. Also because of the way that 0-60 is typically tested. There can be a 0.3 second variation in the numbers from the same test depending on which magazine is printing the numbers, as they calculate them differently. Seriously.
As they say, in theory there's no difference between theory and practice - but in practice, there is.
It's the second round of chassis work that's caught my attention with this car. I never felt the original was much more than "look, it's like a Atom and it's easy to build...and now my next project". The improvements Warren et al have managed to pull off kinda illustrates this. I think the reborn US-built car is going to be a pretty interesting one. We just have to see if the real world experience backs it up. Can't wait to find out.
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