SCCAForums.com

This post contains a bevy of common misperceptions, let's go through them one-by-one.

Father LeadFoot:

The Miata is narrower than a RX7, but that's where the advantage stops.

It is also shorter in length by a foot, with an ~8" shorter wheelbase.  Big help when turning is the name of the game.

Problem is right now the Miata gives up HP capability as viewed by "Power to Weight Ratio" due to approximately 140lbs too much weight, I'm basing that on what I think the max capabilities of a SC'd miata are currently ~330whp with a 1.2L whipple and about $10k in engine work.

A 330whp Miata @ 2240lbs. has a 6.78 weight:power ratio.  A 400whp RX7 (like the one that won last year and was faster on day 2 this year) @ 2530 has a 6.3:1; even Erik's monster, which takes the delicate hands (and feet) of a surgeon to pilot well on slippery HPT, is only about 5.48:1 (2740lb, 500whp).  These turbo cars also have difficult-to-account-for lag, which a SC'd Miata wouldn't.  A 1.8 with an autocross-centric turbo system like I built for my 240 (but with a smaller turbo, like the disco potato) could do a lot to equalize the power side and keep lag pretty reasonable.

A turbo'd Miata sized to make that power or better would be quite laggy for a AutoX course, but what we're up against is Tubro 20b's, not 13b's. 

There is only a SINGLE 20B that autocrosses in SM2, Erik's car.  Andy's is a 'regular' 13B.

Also how much weight each tire has to support is also higher than a RX7s or Corvettes capability, you couldn't get a Miata lite enough under the SM2 rules in order for that Weight/Contact Patch ratio to match a RX7 on 315s and 335s, but less weight would get it closer. I can deal with close, but not sure I want to continue to build my car while its "no chance"

The RX7 that won this year was over 2700lbs and was on 285's and 315's.  I'm guessing it was a little front-heavy, so each front tire probably had about 700lbs with the rears about 660.  That's 2.45lb. per millimeter width of tire width in front (which you want light to make the car turn), and 2.1lb per millimeter in the rear (which you want as much weight on as possible so power makes it to the ground).  A 2240lb. Miata on 275's all around and 150lb. in the trunk would have 550lb on each front, 595 on each rear.  That's only 2lb. per millimeter of tire up front, and 2.16 in the rear.  In my eyes, the individual and overall tire load looks much better on the Miata than it does on Erik's car.  (It looks even better on a 914-6).

---

--Jason Rhoades

........and I stand humbly corrected. Thank you for that write up though, very informative.

---

Logan Murray
SM2 NA Miata

i believe laggy turbo miatas are a misperception, as well.  in '05, the last year i ran SM2 with the SC '99, i broke my cable mount before my last run on day 1 and jumped into Jeremy Schuster's turbo Miata for my last run.  His car was putting down same power numbers as my 2.0 twinscrew.  His system used Racelogic traction control and was a beast on the track . I actually ran a faster time in his car than in my car and it had no lag.  there was a tad amount of lag, but no more than the amount of "lag" that the supercharger has with air routing through the intercooler.

turbo cars can have lag dialed out if tuned properly.  Jeremy's car is FAST and i bet is much faster now that he's in XP 

---

Dan Pedroza
GS 127
WikiWiki!

sm2dan:

Miatas actually are only 1" narrower than the RX7 when you put on sticky tires.  We measured the STrelnieks RX7 rear end from side to side with 315s to my NC with 245s and it was only 1" savings with the NC Miata.  with 285s, expect that gap to close up.  The Miatas have a shorter wheelbase, which can be an advantage, but 300+hp is more than enough power to beat the RX7s with 285 rubber.

d 

Talk, talk, talk, yak, yak, yak.  Bring it already!

-Andy M.

boxboy:

sm2dan:

Miatas actually are only 1" narrower than the RX7 when you put on sticky tires.  We measured the STrelnieks RX7 rear end from side to side with 315s to my NC with 245s and it was only 1" savings with the NC Miata.  with 285s, expect that gap to close up.  The Miatas have a shorter wheelbase, which can be an advantage, but 300+hp is more than enough power to beat the RX7s with 285 rubber.

d 

Talk, talk, talk, yak, yak, yak.  Bring it already!

-Andy M.

I'm waiting for you to bail first.

d 

---

Dan Pedroza
GS 127
WikiWiki!

I am not going to dispute Jason's points for the most part, nor am I quoting his entire post but....

The weight to power ratio argument he made was a bit misleading.  All his number are correct, but if you want to "equalize" them to Andy's ratio the Miata still has to make over 300whp at 1950 lbs or over 350whp to "equal" Eric at the same 1950 weight.  I am not saying weight to power is the right adjustment, nor am I saying that there aren't other factors, it just struck me as "glossing" over the weight to power ratio.

 

---

Chris
51 SM2

What is the ft.lb. of torque to weight ratio. I think that is more important than the hp numbers.

CHRISFP78:

What is the ft.lb. of torque to weight ratio. I think that is more important than the hp numbers.

torque = 5252 x horsepower / rpm   Not sure what theoretical rpm to put in the equation for those theoretical peak hp numbers.

What REALLY matters is the area under the torque curve for the anticpated RPM range.  We are really just trying to compare what we think the results will or more importantly COULD be given fully developed cars.  The only real way to tell is for more cars, prepped to whatever extent possible get out and compete against the "known" cars and see where the current formula is deficient.  I can put forth my theories as can others, but until we get some real results to compare to we are all just guessing.
 

 

 

---

Chris
51 SM2

You really need to look at thrust curves which takes gearing, tire size, shape of the torque curve into account.   I did this awhile ago to do a theoretical comparison (obviously real world accel numbers will not be so good).

-Andy M.

 

boxboy:

You really need to look at thrust curves which takes gearing, tire size, shape of the torque curve into account.   I did this awhile ago to do a theoretical comparison (obviously real world accel numbers will not be so good).

-Andy M.

 

Darn, not only are you faster than me, but smarter too.   At least I am still taller 

---

Chris
51 SM2

jzr:

Also how much weight each tire has to support is also higher than a RX7s or Corvettes capability, you couldn't get a Miata lite enough under the SM2 rules in order for that Weight/Contact Patch ratio to match a RX7 on 315s and 335s, but less weight would get it closer. I can deal with close, but not sure I want to continue to build my car while its "no chance"

The RX7 that won this year was over 2700lbs and was on 285's and 315's.  I'm guessing it was a little front-heavy, so each front tire probably had about 700lbs with the rears about 660.  That's 2.45lb. per millimeter width of tire width in front (which you want light to make the car turn), and 2.1lb per millimeter in the rear (which you want as much weight on as possible so power makes it to the ground).  A 2240lb. Miata on 275's all around and 150lb. in the trunk would have 550lb on each front, 595 on each rear.  That's only 2lb. per millimeter of tire up front, and 2.16 in the rear.  In my eyes, the individual and overall tire load looks much better on the Miata than it does on Erik's car.  (It looks even better on a 914-6).

I'm going to challenge this a little...for the sake of conversation, but please correct me where I am wrong 

Is less Pound Per Millimeter REALLY a situation where "Less is More" You guys are running HUGE wings and splitters to create DownForce which, as far as the tire are concerned, acts as more weight on a tire without the defect of more weight in the car, more pound per mm to increase grip.

Also not taken into account is tire diameter, your 240 and the Vettes and RX7 have ~25.6" tires, where a miata on a 275/35R15 only runs a 22.8" tire. That difference in diameter accounts for A LOT of traction under acceleration and braking. A miata with 300whp would only put out about 240 wtq and it would have issues with putting the power down through that 275 due to such a small diameter.

As others have said we'll need to see someone bring a Miata to the game with a high level of prep, and that is something that hasn't happened yet...but some are close. So Chris, when you're done with that engine work.....we need to see some CRAZY aero work for next year as well, downforce, downforce, downforce.

Now feel free to tear apart any of my logic.

 P.S. Chris are you running an Ubercharger now?

---

Logan Murray
SM2 NA Miata

Father LeadFoot:

snip

As others have said we'll need to see someone bring a Miata to the game with a high level of prep, and that is something that hasn't happened yet...but some are close. So Chris, when you're done with that engine work.....we need to see some CRAZY aero work for next year as well, downforce, downforce, downforce.

Now feel free to tear apart any of my logic.

 P.S. Chris are you running an Ubercharger now?

We need more than one somebody.  Maybe we should get a count of who is coming to national events in SM2 Miata's next year. 

I am coming and bringing a co-driver, I assume Paul is coming back.  

Paul you bringing a co-driver?

Dan are you going to let Andy talk that way to you and not come defend your honor?   Bringing a co-driver?

Logan are you in? Co-driver? 

Shaikh you have a turbo car don't you?  Co-driver?

That's potentially 10 chances to show what the platform can do.   

I have a '99 ubercharged car in the garage right now, that is serving as the donor for my '92.  So I will be running the ubercharger soon.  As soon as its in and running well I can start working on the splitter I will need to balance out the wing I just ordered.  Its pretty much the same wing Andy runs, although in deference to my lower power, I may not add the little Gurney flap to the second element. 

---

Chris
51 SM2

Father LeadFoot:

I'm going to challenge this a little...for the sake of conversation, but please correct me where I am wrong 

Is less Pound Per Millimeter REALLY a situation where "Less is More" You guys are running HUGE wings and splitters to create DownForce which, as far as the tire are concerned, acts as more weight on a tire without the defect of more weight in the car, more pound per mm to increase grip.

You have a point here, you just don't happen to realize how you are missing yet another competitive advantage.

 The amount of wing area is the same for all competitors, regardless of vehicle weight.  The amount of downforce available is a function of pretty much just speed, area, and how well engineered your stuff is.  Something missed by most is the fact that a constant value of downforce is MUCH more valuable to a lightweight car than it is to a heavier car.  Using the previous examples:

Let's say a well-done 8 square foot 2-element wing makes 100 pounds of downforce at 50mph (a reasonable figure).  On Erik's car:

Rear wheel loading goes from 660 to 710, an increase of 7.5%

That same wing on a Miata increase rear wheel loads from 595 to 650 - an increase of 9.25%.

Thus, the same allowance is worth almost 25% more to a Miata than to the heavier RX7!!!  And that's using a relatively heavy Miata as an example - an NA built one would see even greater relative gains.

Also not taken into account is tire diameter, your 240 and the Vettes and RX7 have ~25.6" tires, where a miata on a 275/35R15 only runs a 22.8" tire. That difference in diameter accounts for A LOT of traction under acceleration and braking. A miata with 300whp would only put out about 240 wtq and it would have issues with putting the power down through that 275 due to such a small diameter.

There's a lot to tire dynamics, but generally a taller sidewall is thought to help power application (see: dragsters).  The 275/35-15 actually has a TALLER sidewall than the 315/30-18 Erik runs.  And it's a lot taller than the 285/30-18.

Apart from that, the tire itself is just another gear between the crankshaft and the pavement; the forces it sees can be altered to be anything you want based on gearing.  Take another look at Andy's thrust graph and absorb it - that is basically a weight-adjusted graph of how much theoretical force the tire should be able to put upon the earth.  The contact patch doesn't know if it's a foot or mile from the hub face; it only knows how much weight is on it and how hard it's being asked to move the ground underneath.

---

--Jason Rhoades

Father LeadFoot:

 

Also not taken into account is tire diameter, your 240 and the Vettes and RX7 have ~25.6" tires, where a miata on a 275/35R15 only runs a 22.8" tire. That difference in diameter accounts for A LOT of traction under acceleration and braking. A miata with 300whp would only put out about 240 wtq and it would have issues with putting the power down through that 275 due to such a small diameter.

 

 

Diameter has nothing to do with contact patch size. CP is completely dependent on tire pressure and vertical load. Wider tire = shorter, wider CP than a narrower tire.

  [insert plagiarization here] A shorter contact patch at the same slip angle begins to slip at roughly the same distance from the leading edge as with a longer contact patch. But the shorter contact patch has more of its length stuck to the road than the longer, narrower contact patch; and therefore a larger portion of its overall area is gripping. A larger portion of contact patch area gripping means more lateral grip. So for the same load and same slip angle, a wider contact patch generates more grip than a narrower contact patch. [end plagiarization] Point is that it has nothing to do with tire diameter.

 

If you are making the argument that the 275 shortens your gearing too much then I'd agree - but thats nothing that can't be changed under SM rules anyway so the point is moot.

---

Nick Jackson
04 S2000 - BHP Brakes
71 240z w/some stuff

SerNick:

Diameter has nothing to do with contact patch size.....

No, but it has a lot to do with contact patch shape.  Larger diameter tires have a longer contact patch, which aids in acceleration & braking.  There's good reason why drag racers use tires with huge diameters.

---

Steve Garnjobst
#75 XP
'99 Mazda Mutant

MNbiker:

SerNick:

Diameter has nothing to do with contact patch size.....

No, but it has a lot to do with contact patch shape.  Larger diameter tires have a longer contact patch, which aids in acceleration & braking.  There's good reason why drag racers use tires with huge diameters.

Yeah, sidewall height to maximize flex ;)  Otherwise they'd be running 40" wheels.

This is all fun mental gymnastics. What OD tire do you run in XP?

 

The bottom line here is still that folks don't like to ballast. All this other stuff is mouse nuts. If the Miata was 2300# stock, nobody would be complaining. If you take the name off the car, and think that you can have a 350+whp 2240# car on 275s, it sounds pretty darned fast. Would you allow Andy's RX7 to lose almost 300# if he agreed to run 275s? If you think he'd still be competitive, then you have to think a Miata is competitive right now.

---

Randy Noll

rnoll98:

The bottom line here is still that folks don't like to ballast. All this other stuff is mouse nuts. If the Miata was 2300# stock, nobody would be complaining. If you take the name off the car, and think that you can have a 350+whp 2240# car on 275s, it sounds pretty darned fast. Would you allow Andy's RX7 to lose almost 300# if he agreed to run 275s? If you think he'd still be competitive, then you have to think a Miata is competitive right now.

I'd take that trade off...only it has to be a 22.8" 275mm tire as well. The car would be more difficult to drive with 400rwhp, 2" less tire width and 2" less diamter and less pound per mm force working on the tires. He would have probably finished a couple spots back from where he did this year, no knocking his talent, but I don't think the car would be as competitive as it is now.

I actually think the hot ticket on the miata might be the 285/30R18s A6, only problem is that it'd have the ride height of a Ford Ranger and may come into contact with the unibody at full lock. 

Chris, I won't be going to Nationals next year, looking at 2009 though when my car will be somewhat competitive. I won't show up to run in the 15th to 20th positions.

---

Logan Murray
SM2 NA Miata

Father LeadFoot:

I actually think the hot ticket on the miata might be the 285/30R18s A6, only problem is that it'd have the ride height of a Ford Ranger and may come into contact with the unibody at full lock. 

Who cares about full lock?  You only need to turn them enough to get through the tightest section of course, which should be nowhere near full lock.  The fast SP and SM C5's only get about 3/4 turn of the wheel (which is only about halfway to regular full lock) before the wheels commit seppuku on the control arms.  My 240 was similar (tire into toeboard) and I suspect the RX7s are too.

---

--Jason Rhoades

jzr:

Father LeadFoot:

I actually think the hot ticket on the miata might be the 285/30R18s A6, only problem is that it'd have the ride height of a Ford Ranger and may come into contact with the unibody at full lock. 

Who cares about full lock?  You only need to turn them enough to get through the tightest section of course, which should be nowhere near full lock.  The fast SP and SM C5's only get about 3/4 turn of the wheel (which is only about halfway to regular full lock) before the wheels commit seppuku on the control arms.  My 240 was similar (tire into toeboard) and I suspect the RX7s are too.

I could care less about full lock as well.....I was merely mentioning the two downsides to a tire that size on a miata, and they would probably contact the unibody more around 3/4 lock as you state.

---

Logan Murray
SM2 NA Miata

I actually get about 2.5 of my total 2.75 turns before the wheel contacts the UCA.   It took some iteration on the UCA to get that clearance, though. 

-Andy M.