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I run a 245/40/15 on my 15x9s, although I've considered a 225. I street drive 205/50/15's, so keeping the tire height similar is a pro for me.
FWIW It seems like tire rack always does some sales at the beginning of the season. You might be able to save $50-100 on a set around March. I think they did $80 off a set of RT660s last year.
At the risk of asking to be spoon-fed, yet absolutely conforming to the "neurotic" caveat in this thread title...
I treat the following statement as gospel, emphasis my own
Originally Posted by emilio700
Radials will deliver optimum grip when mounted on a wheel about the same width as the casing.
TireRack doesn't specifically call out the width of the tire casing. Bending a bit into the world of bicycles, of which we know Emilio is a huge fan, the simplest definition is from Kenda: "Tire casing is the fabric that the rubber compound adheres to". Someone else makes a clarification that I like: "The layer beneath the tire tread is the casing."
Thus, through some mathemagramatical equation, the casing width is the tire tread width. (clarification requested).
Looking at the TR specs and cross-referencing with the tire and wheel combination that SM will be running on their ND MTTC car, the 245/40/15 version of the 660+ is, mathematically, the correct size to start with for my 15x9 wheels, right?
The general consensus has always been 205s on 8s, 225s on 9s and 245s on 10s. The idea being that the wheel is giving as much support as possible to the shoulder of the tire to optimize responsiveness and stability without requiring excessive pressure.
My understanding is that "casing" in this discussion is equivalent to 'section width'...?
A 225/45-15 will generally have a ~9" section width.
I'm running a 245 660 on my 15x9's right now. They work well, are plenty fast with good feedback, and no real complaints. Though as I understand it optimally I'd be running this tire on a 10" wheel, and might go that route later.
TireRack specs this tire as having a section width of 9.8" and a tread width of 8.4"
The 225/45 version of that tire has a section width of 8.9" and a tread width of 7.9"
The 255/40 version of that tire has, amusingly, a section width of 10.2" and a tread width of 8.4"
So to reverse-engineer the thinking of the Miata Gods, it seems that using the TireRack tread width measurement lines up with the "casing width" described above.
You know what's amusing? In school, my quizzes/tests/etc were always dinged for not showing work...
I'm running a 245 660 on my 15x9's right now. They work well, are plenty fast with good feedback, and no real complaints. Though as I understand it optimally I'd be running this tire on a 10" wheel, and might go that route later.
This may be the most straightforward answer, without diving into the intricacies of casing vs section vs tread. You're running the tire I'm considering on the wheel that I have and have no complaints. This should be the KISS answer, I think.
Wow, I actually have a relevant background for this discussion. I designed (heavy truck) tires for a few years so I can provide input on the tire side of things. First, think about tires like a balloon - they want to take on a spherical shape, and the more not-spherical they are (think low-profile tires) the more internal stress there is on the load-carrying components. Second, balloons have very little compression strength. Load path through a tire is from the ground to the tread belts, around the circumference of the tire, and then vertically down to the bead (and rim). So the rim is essentially hanging from the top of the tire.
This is a very long winded way to say that yes, as an approximation, the tread width is about the same as the casing width. And from a structural standpoint having the sidewall be vertical results in the lowest stress on the carcass (casing). How that relates to peak grip I couldn't tell you - that wasn't a primary concern in semi truck tires. Matching the rim width to the casing width seems like the right path, though. If the rim were too small you'd expect the lateral stiffness of the wheel\tire to be super soft and unresponsive. At the other extreme, the tire would become very stiff and have a very steep slip-angle-to-lateral-force relationship, which would probably make the tire pretty peaky and hard to drive.
All of this to say, matching the tire width to the rim width is probably a safe bet.
Wow, I actually have a relevant background for this discussion. I designed (heavy truck) tires for a few years so I can provide input on the tire side of things...
The amount and variety of real-world, relevant experience on this forum is incredible. If I could give you more than one cat, I would have smashed that button to death. This is an excellent breakdown of the structure of the tire and helps my understanding quite a bit. Kudos!
Originally Posted by crxguy52
This is a very long winded way to say that yes, as an approximation, the tread width is about the same as the casing width.
Your wind is much appreciated. Looking at the details on the TireRack site explaining how they measure tread width (which at least in the case of the Falken, is not a number that the manufacturer provides), I think it's safe to say that their tread width number may be smaller than the casing width, but it will never be larger than the casing width, because of the difficulty in figuring out where the tread "ends".
Glad I could help! Designing tires was cool for like... 6 months. Then it became about as exciting as it sounds. Tires in a designated size - say 245/40/15 - must fit in a dimensional window per industry standards, it's not an actual measurement of the tire (like you've probably surmised). Tire standards are also a hodgepodge of English and metric measurements, it's a mess.
As an aside, I could not for the life of me figure out where these "cat" buttons were. Turns out I'd disabled them somehow in the settings. Anyways, thanks for the cat!
Tire standards are also a hodgepodge of English and metric measurements, it's a mess.
The fact that tire width is measured in millimeters and the wheel dimension is in inches is one of those itches that I must ignore and keep out of my brain.
It's like socket sets. I have to wonder about our sane friends in the metric world - how does one refer to a 3/8" drive socket vs a 1/2" one?
Hey buddy, can you hand me a 14mm deep socket in 7.62 drive?
Oh man, your Escalade would look so rad on 50.8cm wheels, yo!
It doesn't help that "225/45-15" tires from different manufacturers are all different actual widths, shoulder shapes are different, and 'treadwear' numbers are complete fabrications!
I think you're good with either 225/45-15 or 245/40-15 on your 9s... Don't forget the 245 is a little shorter so will shorten the gearing a bit...
How about this one.... At Harley-Davidson, the Milwaukee-Eight and Revolution Max powertrains are in production now... One is designed, manufactured, & assembled completely in imperial while the latter is SI even though they are assembled at the same manufacturing plant and designed by the same core team (that I'm on).. The quantity of unit conversions that I have to make for a typical week is staggering... torque specs, fluid capacities, flow rates, any distance/volume/density measurement, power ratings, heat rejection values, default CAN channel units on ECMs... it... is... painful...
My understanding is that "casing" in this discussion is equivalent to 'section width'...?
This is correct. I'm certainly not qualified to contest crxguy's assertion that Casing width and Tread width are the same, but Emilio did mean section width. In club racing, NASA uses a tire template that must fit over the tire and it goes down the sides of the tire. It's not measuring the tread, it's measuring the section width. Examples here in Dan Howard's thread: https://www.miataturbo.net/race-prep...5-95309/page2/
So in the example of that 245 tire which tirerack lists as a 8.7" tread width but 10" section width, you're looking for that section width number to match to rim width.
This popped up in my feed. I think I owe the miataverse a minor apology. Our recommendation for max performance from a radial has always been to use a wheel at least as a wide as the "actual tread width". Despite repeating it many times over the years, it looks like I flubbed it at least once, stating "casing width" by mistake. Sorry!
The bigger story is that Hoosier is no longer the only DOT tire maker with fantasy tire sizes. Many of the current Super200's are wider than their official size designation. That first number in the tire size is nominally the casing width. So the 245/40/15 RT660 mentioned earlier should be 9.64" casing. But measures 10" casing on an 8.5" wheel. We know from experience that tire is fastest on a 10" wheel which would spread it out to something like 10.7" or 270mm.
So we're sorta revising our legacy recommendation to include casing width as a guide if actual tread width is unknown.
The 245/40/17 RE71RS in our shop:
8.4" mfr spec tread width
9.8" mfr spec sec width measured on 8.5" wheel
9.15" tread on the ground as measured by actual wear area
9.25" molded tread area
FWIW, our next ND wheel will also be available in a 17x9.5".
Oh my gosh! You made a mistake once, years ago on some random forum and it's come back to bite you in the tail. You should be so sorry for giving out free advice and flubbing the punch line once! </sarcasm>
Really though, thank you and Ed for stepping in to my humble thread and clearing this up, with additional new information to boot!
Now I'm sure to run the tire that will make my shitty driving a few tenths faster than it would be on a suboptimal tire/wheel pair!
I placed the order for the RT660+ in 225 today, they’ll be here in about a week, coming all the way from South Bend.
NASA NorCal just announced that their HPDE groups will now get 5x20 minute sessions per day instead of 4x20, as well as having fewer cars. That’s nice, essentially an extra half-day of driving if you run both days.
My plan is to run the first NASA event on March 1&2, then evaluate how the car and I are doing before joining the MTTC events. If the fuel starve issue comes back, or if something expensive shakes loose again, I’d rather it happen at an event where my entry fee has been comp’d (by working tech) than where I’m paying retail price.
My goal this year is to keep my nose clean and put an emphasis on reviewing my data at the trac so I can learn from it. I generally get out of my car, run to the post-session download meeting, and then work race groups until it’s time to go out again. I’m going to try to figure out how to squeeze some review time in there.
Not much in the way of major updates, and that’s probably a good thing.
I bought some RT660+s in 225 and had them mounted up.
Since I’m neurotic, I took the rear shocks out and tested for clearance. There was some, but it was fairly easy to pull out the old fender roller, so I did. There’s more now, but my cheap fender roller gives inconsistent results, so it’s not as perfect as I’d like.
The fronts are pretty much rolled flat. They’ll be fine.
My dog Victoria does not approve of my shenanigans.
First event of the season is next weekend. I’ve spent part of the day cleaning out and organizing the trailer. I found a loose nut on the winch, winch was resulting in intermittent dysfunction. A dab of Loctite and a twist of a wrench and hopefully I won’t be pulling in a dead car again any time soon.
Last weekend, I used some contact cement and 3M spray adhesive to glue up the Air2Gel foam layers and add a Nomex cloth covering. So far only the seat bottom is done, I’ll knock out the back tomorrow. It’s not even mediocre upholstery work, you can see where some of the spray adhesive soaked through. But it’s good enough for a race car.
First event of the season is in the books and I'm generally pleased with the results. Actually, I'm thrilled with them. With the exception of my Solo 2 data logger refusing to connect so that I can download data... Italian software, UGH.
The RT660+ were very nice. I now see just how dead my ancient RS4s were. I started them at 26psi each session and ended at about 30psi after the cool-down lap and slow drive back to paddock.
The fuel pump cut out on me twice. Once coming out of T4 and once while sitting at idle in the paddock. So it's not related to fuel slosh or air getting into the system! Coasting out of T4, I hit the button to switch over to the second power source and the pump came right back on. Both failures this weekend were with the ECU-controlled power circuit. I checked the ECU-to-relay wires and all that this winter, so I don't think it's a loose wiring connection, but I'll definitely get in there and check again.
One of the Miata gurus I trust gave me a couple suggestions: he mentioned that the DW200 can draw 16A or more, which would be right above the limit of the 15A circuit breaker I have in there. I sized this based on the current draw chart provided by DW, but perhaps they're optimistic. I wonder if sustained use at or slightly above the current limit of the circuit breaker has damaged the breaker enough to cause intermittent failures? The fact that I had zero failures while using the new "bypass" circuit breaker is relevant.
He also reminded me that the bulkhead connector at the fuel tank is a weak point. I know I checked mine out for damage as part of the initial K-swap, but I haven't looked at it recently. He suggested replacing it with a bulkhead connector from Ballenger. I think he means this one: https://www.bmotorsports.com/shop/pr...oducts_id/5164
Has anyone used one of those or an alternate?
Looking at ProWireUSA's link to information on the ampacity of wires in free air, I feel I can upgrade the circuit breakers on the 16 AWG Tefzel fuel pump wires (M22759/16-16) to 25A and still be far below the 150C limit of the wire. That still leaves the bulkhead connector as the weak point, but I have the circuit breakers on hand.
Whelp, I’m writing today off as just one of those days. I’m at Thunderhill, my car is intact, I had one good session, and I’m fine. But I’m done for the weekend.
The first session out, I went out late and felt like I had a flat ln the right front, it was just pulling in that direction and even though I’d topped it off, it had been low when I rolled the car out of the trailer. Came in, checked pressure, it was not flat, but maybe just a touch lower than the others. Must have been in my head.
Second session out, had a good time, spun coming out of 8 because … I’m sn idiot and got distracted by a new shift point. But the steering wheel was definitely aimed right going down the front straight, maybe a touch worse than it had been in the first session.
Fount an alignment nut loose on the front lower control arm, right side, forward. Tightened it up, had a Spec Miata crew reset the toe on that wheel. Missed a session with the screwing around.
Session four I was just driving shitty. Car felt better, but not quite right. Maybe it’s in my head. Nothing was loose no matter how I pulled on the front wheel.
Session five felt great, car behaved, I kept my head together, but still, despite having set the toe and the steering wheel being straight driving in the paddock, down the front straight the wheel was definitely turned right and something was off.
Pulled the wheel and found the problem.
The orange 863 bushings on the front upper control arm did what they’re known to do. I haven’t “gotten around” to replacing them with the upgraded black ones because it’s an involved process and I didn’t think I was the kind of high-power, hard-braking driver that would chew these up. And I swear it looked fine recently…
The FUCA has shifted forward, and is probably able to move under load. Suboptimal.
Known problem, my own fault for not jumping on the upgrade when it came out, and really my fault for not having seen this before coming out this weekend.
I’ll be replacing bushings next, and I look forward to using those fancy new super-strong alignment bolts from my favorite purveyor of Miata goods.
Anyone recognize the car across the way here? It’s always fun to watch @codrus drive his not-Miata at the pointy end of a very competitive class!
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