But those tire rules are either written specifically against the 15x9 205 SM/R7, or the rule maker needs a new set of calipers. Too close to be anything else. Still seems like a Rube Goldbergian approach, it's not like a tire manufacturer is going to retool their tires multiple times during a season.

I imagine the rules were written very much with the 205 Hoosier on a 9" wide wheel in mind.


I am not surprised by some of these changes at all.

Fun fact: 205/50-15 VR1s on a 15x8.5 would not be narrow enough.

I don't really have a dog in this fight, i just measured them for Marcus today for shits 'n' giggles. Nobody races on VR-1s so it's just amusing, not relevant.

How far off is it?

I came up with 241.5mm using my probably not-perfectly-accurate measuring methods. I held a level against the tire about 1" down the sidewall and ran a tape measure up from there to the same "height" with a helper holding another level on the other side. Came up with 9.5" on the button.

Wheel width is crucial. I'll move my 205 R7s to 8" or even 7.5" if i have to, in order to stay under 2401 with my current power level.

If you have ~160+whp to fit the ratio, running over 2401lbs allows 257mm of tire.
You get a .1 P:W comp weight penalty reduction at 2451lbs.

Splitter .5 penalty basically only costs 5-7whp, so I think it will always be worth using.

Sport brakes on NA is free. "Brake modification exception: The exception to this rule is that brake calipers/brackets and rotor size may be updated or backdated to any generation model of that vehicle, provided that the donor vehicle is eligible for competition in ST5."

I’m not surprised by wheel/tire rule. Greg specifically asked me what size wheel I was running with my 205 R7s at tech during ESC. As soon as I told him 15x9 he said that will be changing.

Also Toyo calipered my setup at tech during ESC and started laughing when they read the side wall and asked what size wheel. I wonder if they had any input?

Now I get to find out how much of my car gets to change with only two months until the 2018 season... great.

Wooooooooooooooooooooooow.

Should have told Greg that wheel width isn't regulated and told Toyo to stop touching your car if they're not an actual tech official.

Ridiculous.

Toyo rep just happen to be sitting in on the ST4 pre-race driver's meeting at WSC when Greg was speaking about the future of the rules..a certain un-named purple colored tire brand was topic of discussion..

Looking back, I probably should have just kept my mouth shut, rookie mistake. We were told in the drivers meeting that Toyo has the right to caliper our tires for "data analysis". I really think Toyo has some skin in this rule.

So far I don't have to change the brakes (I have sport brakes) and based on how I read the rules the NB subframe I picked up is useless.

I have 4 sets of 15x9 6ULs 2 sets mounted with 205 R7s, 1 set mounted with 225 RC1s, and 1 set 225 H20s. Based on how I read it, my R7s are useless (until I verify by building the Jig) and I have to add weight to my car to use my other tires. I am not going to replace 4 sets of wheels anytime soon (hopefully).

Current Setup:
2340/148 = 15.81 - 0.4 (weight) - 0.7 (A-Arms) -0.5 (splitter, why not) = 14.84

"Less than 2450" is only -.4
"Less than 2250" is -.5
If you go up to 2401lbs you can use your 205 R7s(and 225s) and add more power(if possible).

Good catch updated my numbers.
My immediate thought is to strip as much weight out of the car and go with the ESR ballast system up to 2401. Then switch to E85 and see where I can put boat anchors in the data plot based on gearing. Decisions, decisions, I need to sleep on this.

My plans:
14:1 class base
A arms -.7
Splitter -.5
15.2:1

Option A: Run What I Brung. AKA: Use the car as-is
Less than 2450 -.4
2251lbs @ 15.6 = 144.29 average whp. I made 143 peak last month. I can add a $200 BP5A cam and be good 2 go.
I'll mount my 205 R7s on whatever it takes to measure <226mm. 7, 7.5, 8". 9" measures 230mm, FYI.

Option B: The Rocketship. AKA: Build a ~165 whp motor.
Less than 2550 -.3
2451 @ 15.5 = 158.12 average whp.
225 R7 on 8 or 9" to meet the 257mm rule.

2018 Average Power rules negates that idea.
Redline over 7k gets Peak Power + the next 7 highest readings at 250 rpm increments up to 2k below the peak, divided by 8.



7.2.1 For vehicles with a redline greater than 7000 rpm: The seven (7) highest data points of the above sixteen (16) will be used in the calculation below: Avg HP = Max HP +(sum of the seven highest data points) 8
7.2.2 For vehicles with a redline between 6000-7000 rpm: The five (5) highest data points of the above sixteen (16) will be used in the calculation below: Avg HP = Max HP +(sum of the five highest data points) 6
7.2.3 For vehicles with a redline less than 6000 rpm: The three (3) highest data points of the above sixteen (16) will be used in the calculation below: Avg HP = Max HP +(sum of the three highest data points) 4

I cant even be mad, they actually thought-through the average pw/wt for 2018. They caught what everyone was doing in ST1-4 and fixed it. I need to really read the rules a couple times and build a game plan for 2018.

Interesting the MX-5 Global Cup Car starts with a 0.0 mod factor.

If that's true, it's too bad. Hoosiers are too wide for their size and PT points were adjusted to throw a bone to Maxxis and Toyo, but part of the reason people don't run Toyos in non-spec classes is that Toyo contingency is nearly nonexistent.

I completely agree. If there is 5+ starters I can 100% guarantee everyone is running Hooisers. Their contingency is a lot better. Less than 5, I run my Maxxis.

C/P from my FB post.

These numbers are suspect as the kid's in bed and I can't make a jig. I clamped a bar offset 30mm to another bar and marked the midpoints of the tires, then doubled them to get these measurements. Don't take them as anything near gospel

205 RR on 15x8 @ 27psi - 210mm @ 30mm
205 SM7 on 15x9 @ 22psi - 236mm @ 30mm

So a 2400lb car can run a SM7 on a 15x9, I believe these are the same carcass as R7s. Since the limit is 257mm and my tires are ~10psi low I think they are pretty safe. I'll air em up and make a better jig tomorrow and see if anything changes.

Do you mind re-measuring the 205 RR at 23mm?

As long as we know these aren't official, I don't mind sharing.

205 R7 on 9" = 230mm @23mm.
205 SM7 on 9" = 236mm @ 30mm (FatKao)(My R7 was within 1mm at 30mm)
245 RC1 on 10”: 249mm @23mm. 254@30mm
245 Rival S on 9”: 242@23mm. 249@30mm
225 Sur4g on 9”: 228@23mm. 231@30mm.

225 RC1 on 9”: 225@23mm. 230@30mm.

205 R7 on 8 = 226mm @23mm. Meets “equal to” rule.
205 RR on 8 = 210mm@30mm (FatKao)
205 RE71R on 8”: 208@23mm
205 VR1 on 8”: 218@23mm