jimlab

19s.

Note: There would have been a picture of my new tires attached here, but apparently that feature isn't working, or perhaps there are simply too many jackasses in the Lounge chewing up the forum's bandwidth for their own amusement for an attachment to be uploaded before the server times out...

S-03 Pole Positions, P245/35-19s in front, P285/30-19s in the back. They're both 25.5" tall by my tape measure, about 0.2" taller than the 235/45-17s and P265/40-17s (Toyo Proxes T1) that I had before. 26 lbs. for the fronts, 30 lbs. for the rears, on the nose.

Bridgestone recommends an 8.5" wide rim for the P245s, and 10" for the 285s. I'll probably go +45mm offset front and rear, unless someone knows a reason why I shouldn't. Using "Manny-math", I get the following...

Front:
8.5" nominal wheel width + 1" = 9.5" overall
9.5" / 2 = 4.75"
+45mm offset = ~1.77"
4.75" + 1.77" = ~6.5" back spacing and 3" front spacing

Rear:
10" nominal wheel width + 1" = 11" overall
11" / 2 = 5.5"
+45mm offset = ~1.77"
5.5" + 1.77" = ~7.25" back spacing, 3.75" front spacing

Manny said 3.5" (+/- 0.1") available front space in the front of the car and 4.0" (+/- 0.1") in the rear, with 7.5" maximum back spacing front and rear, so those offsets should work well unless my math is wrong. Manny, you want to double check?

Now I have a couple of questions...

#1 - Anyone know what the hub "pilot" diameter is on the RX-7 offhand? I could measure myself, but figured if someone had already gone to the trouble...

#2 - Anyone know the caliper height for the RX-7 AP Racing big brake kit (CP5200 calipers) above the top of the rotor hat? Again, I could measure myself, but I'd have to dig my brake kit of out the boxes...

Thanks in advance!!

maxcooper

#1 - The centerbore is 67.1mm.

-Max

jimlab

Come on Max, you've got the CP5200 calipers... didn't you measure them before you put them on the car?

Thanks for the answer to #1.

SleepR1

Yup

Quote:

Rear:
10" nominal wheel width + 1" = 11" overall
11" / 2 = 5.5"
+45mm offset = ~1.77"
5.5" + 1.77" = ~7.25" back spacing, 3.75" front spacing

Yup

Quote:

Manny said 3.5" (+/- 0.1") available front space in the front of the car and 4.0" (+/- 0.1") in the rear, with 7.5" maximum back spacing front and rear, so those offsets should work well unless my math is wrong. Manny, you want to double check?

7.5 inch rear wheel position backspace should work with 19s (plenty of clearance from the rear trailing arms). Arithmetic looks good Tires might be a bit tall, so I'd roll the front and rear fender lips as a precaution. Of course if your car's a trailer queen...

Quote:

Thanks in advance!!

Good Luck!

jimlab

I have JIC coil-overs, so my ride height is totally adjustable. I should be able to keep the tires out of the fender wells.

I had P235/45-17s (25.3" tall) in front and P265/40-17s (25.4" tall) in back the last time around and never had any trouble with rubbing. In fact, my fender liners were in perfect condition, and I never caught the lip of the fender or cut the tires. 17x8s and 17x9s with +40mm offset, I believe.

The new tires are basically the same diameter (+0.1-0.2"), and although they're a little wider, the biggest difference is in the rear where it doesn't matter as much because the tires aren't turning. The front is only 0.5" wider, as far as the wheel is concerned, so I don't think I should have any problems.

We might have a new "recommended fitment" in the works.

Or garage queen.

Not everyone plans to track their cars, and if they did, they'd probably buy a set of wheels and tires specifically for the track, right? It's not the end of the world Manny, the wheels do come off pretty easily.

jimlab

Manny, do you think I should go with a +51mm offset in the rear instead? It'd max the backspacing at 7.5" (assuming for the moment that I didn't have coil-overs or K2RD trailing arms) and front spacing would be 3.5".

Also, as long as the wheels fit over the hub, it's not a big deal, right? I don't need hubcentric wheels I'd assume.

dclin

iTrader: (4)

FWIW, I think you should go 19X10 +50(ish) in the rear

an X8.5 +45 front and X10 +45 rear would have the front wheels tucked in a little, and the rear pushed out a bit - relative to the fender lip.

Here is 18x8.5 +44 and 18X10 +44. Not a good angle, but you can kinda see what I mean:



Not that noticeable, but knowing how **** I am, and knowing how much more **** you are then me - you might want it to be more even in spacing to the fender lip.

BTW, the pic above had a temp set of of 225/40/18s on the front 18X8.5 (a little narrow for the rim), so it may add to the 'tucked in' illusion. The 245/35s I was waiting on would have filled it out a tiny bit more.

Still, I would do 19X8.5 +45(ish) and 19X10 +50(ish) if it was moi.

*oh, and for reference, the rears are 285/30/18s in that pic

SleepR1

Looks good to me?!

jimlab

No, I can see what he's saying. With a +45mm offset front and back, front spacing on the rear wheels is 3.75" which would stick out noticeably further than the front wheels.

Going with a +51mm offset in the back would pull the back wheels in a little and put more of the tire under the car.

SleepR1

yup

jimlab

Max, I got a couple different measurements from my hubs, brakes and wheels. I don't have the numbers here with me at the moment, but I believe that the hub area which the brake rotor fits to is in the 72mm range. The diameter of the hub left sticking above the brake rotor was in the 67mm range as you listed above. On the backs of my wheels, however, the hub opening was 2.85 inches, which corresponds to about 72.4mm.

Also, the hub extends a minimum of 5/8" above the top of the brake rotor's hat (I am using the diagram from the Fikse site to record measurements) so a minimum clearance of 3/4" is probably safe here.

The same goes for the AP Racing caliper. The maximum distance from the center of the hub to the outside of the caliper body is 8". It's a little less, but I figured that was safe. That's equivalent to a 16" diameter circle, which is why these calipers require 17" wheels, obviously.

The height of the caliper "above" the rotor is equal to the height of the hub, at least in front. 5/8", so an absolute minimum of 3/4" for proper clearance of the wheel spokes, I'd assume.

I'm going to list the clearance as 3/4" minimum front and rear, even though the front calipers will be larger than the rear. Anyone think this is a bad idea? I have a set of rear calipers and rotors that I can measure. I wanted to end up with one set of measurements for clearance, though, if possible. That would make the spoke design identical for front and rear.

jimlab

Measured the rear brakes tonight.

The minimum clearance above the wheel nut is about 3/4". I believe that the axle may protrude slightly, so probably a little more is required. My old Konig Villains have about 1 3/16" of clearance in the center hole.

They also have 1 3/16" of clearance on the spokes, below the mounting "pad" of the wheel. In other words, they'll clear a set of AP Racing calipers easily, so 1" - 1 3/16" is what I'm looking at for caliper clearance on the new wheels, probably.

The caliper (rear) is also about 3/4" taller than the hub mounting pad (that the wheel would bolt to) when the rotor is installed, so plenty of clearance front and rear with 1" - 1 3/16" clearance on the spokes.

The clearance diameter (to clear the caliper on the outside) is a minimum of about 7" on the outside. If the RS/RZ calipers are identical and the rotor is just larger in diameter, then there are no problems whatsoever.

The hub diameter in the back is about 67mm, or 2.64". I measured the center opening on one of the rear wheels, and it's the same as the front... 2 13/16", or about 2.8125" / 71.4mm. I think I'll stick with that to ensure that there's no problem with clearance. The wheels are lug-centric, not hub-centric.

Obviously, bolt pattern is 5 on 114.3mm or 5 x 4.5". Ford pattern.

And, while I was at it, I measured and weighed my P275/40-17s (Nitto 555R drag radials) on 17x9s, and P235/45-17s (Toyo Proxes T1s) on 17x8s for reference.

The rear tire and wheel combination weighed ~46 lbs. The fronts weighed ~46.4 lbs. My front P245/35-19 S-03s weighed 26 lbs., so I could have a 20.5 lb. wheel and still end up at the same weight. The P285/30-19s were 30 lbs. each, so I'm only allowed a 16 lb. wheel in the back. Yeah, right.

As for height, I knew that Toyo didn't follow P-metric sizing very well. The P235/45-17s were only 24 11/16" (24.6875") tall, when math says they should be 25.33" tall when uncompressed by vehicle weight. The 19s are another 0.8" taller, overall.

In the rear, the P275/40-17s were 25 7/8" (25.875") tall, when math says they should be about 25.66". Guess Nitto doesn't measure so well, either.

The P235/45-17s had a section width of about 9.5". The P245/35-19 replacements have a section width of about 10.25". The P275/40-17s had a section width of about 10 3/8", and the P285/30-19 replacements have a section width of about 11.5", so roughly an inch wider, front and rear.

Figured these measurements might help someone, even if they're not having custom wheels made. I still plan to go with +45mm offset in front (6.5" backspacing on a 19x8.5" wheel) and +51mm offset in the rear (7.5" backspacing on a 19x10" wheel).

maxcooper

It's best to have the wheels be both hub and lug centric if you are having them custom made anyway. I am almost 100% sure the centerbore is 67.1mm (67.07mm, I think, more precisely). I have hub rings that say 67.1 on them next to the diameter symbol. And I remember hearing John Purner list that off as the centerbore (off the top of his head ) when I ordered my wheels. I believe the Eclipse has the same centerbore if that info is more available; it might be useful for further verification.

-Max

jimlab

Only problem is that these people don't deal with metric. I have to convert everything to inches, as far as I know. +45mm means nothing to them, 6.5" backspacing does.

My old wheels are not hub-centric, BTW. I don't think it makes that big a difference unless you use flat lug nuts which don't center the wheel on the lugs.

maxcooper

Hub-centric makes them easier to install and may take some load off the lug studs. My point was that if you are having them custom made anyway, you might as well get them perfect.

67.1mm = 2.6417322834645669291338582677165"

2.64" or 2.65" should work fine for the centerbore.

-Max

jimlab

Yeah, but when you're spending this much on wheels, you kind of want them to fit... I'd hate to go too tight on the measurements and have something come out wrong.

jimlab

Uh-oh... better recheck the math.

If I want +45mm (+1.75") offset in the front and +51mm offset in the rear (+2.00"), then my backspacing on 8.5 and 10" wheels is not 6.5" and 7.5", is it?

Front
19 x 8.5"
8.5" / 2 = 4.25"
4.25" + 1.75" offset = 6.0" backspacing

Rear
19 x 10.0"
10.0" / 2 = 5.0"
5.0" + 2.0" offset = 7.0" backspacing

Why am I using a nominal wheel width of 1" larger, except to compensate for the section width of the tire to ensure that it will clear? The actual wheel, however, is not as wide as the section width of the tire. Therefore I should not be using the nominal width to calculate backspacing, correct?

For example, with a nominal width of 11", the rear wheel's centerpoint would be 5.5". Adding 2.0" offset, rear backspacing would be 7.5"... but, the actual wheel has a nominal width of 10", doesn't it? So 7.5" backspacing would put the front spacing at only 2.5".

Or does the lip of the rim extend beyond the nominal rated width of the wheel, and that's where the extra "inch" comes in?

I'm banking an awful lot of money on getting this right. There is no option to send the wheels back if they don't fit. Can someone please shed some light on what's going on here?

Then again, Fikse seemed to be using the same math on their web site, back when they had the fitment table and offsets listed. I just want to make double sure before I go sending off measurements.

jimlab

Unless someone finds an error with my math, here's what I've got...



BTW, I realize that the width shown by "B" isn't precisely accurate, and should actually be bead-to-bead instead of lip-to-lip, which is the arbitrary wheel width used in the math above to calculate backspacing. However, I really don't feel like messing with it any more, and they know what I'm talking about anyway, so...

maxcooper

The actual wheel size is about an inch larger than the nominal size. My 18x10" +50-ish (guess) CCWs have exactly 7.5" of backspacing (measured). I think those wheels are actually 11.125" or so wide. With the 285/30-18 tires, the wheel is actually wider than the tire section width. If you can simply specify the backspacing rather than the offset, 7.5" is perfect for the rears.

-Max

jimlab

Thanks Max.

foko

i just thought i'd add that empiric evidence is occasionally useful:

i have the "cooper" fitment (18X10 +55 all around, 285/30 V700) on my track car. they DO occasionally scrub under high G conditions. tire edge touches wheel well lip. many people run this setup without scrub. point is.....no matter how perfect your measurements....you might have trouble when you are at the limit.

good luck
fabian

maxcooper

I've caught the fender lip a few times with my 285/30-18, 18x10" setup also. It is possible to set it up so that you can do whatever you want without grabbing a lip (ride height, spring rates, bump stops), but it does take a little trial and error to get there. All cars, tracks, and drivers differ somewhat, so it's hard to give specs that would work for everyone.

-Max

jimlab

Does anyone know of any 19x8.5 (+45mm) and 19x10 (+51mm) wheels available?

I'm still waiting to hear back from Boyds design department on the sample CAD design and approximate weight of the finished wheels, but I have a feeling they're not going to be able to meet my 20-22 lb. limit.

I can always go with a Kinesis/Fikse/HRE combination. It's not cost that's a factor, it's appearance, and secondarily, weight.

ptrhahn

iTrader: (9)

Jim,
I notice Forgeline has a new line of wheels called VR1. Their website lists this as a Viper wheel (19x10, and 19x13), but since i see them listed on the Pettit site, i'm guessing they've expanded the line... note that they are available polished, are nice looking IMO, and the 19x10 weighs 20lbs. Thats not bad.

http://www.forgeline.com/wheels/VR1.html

P

jimlab

Interesting. Thanks Peter!