Thoughts on wheel size

[DesertSn8ke]

I have occupied much brain space and time studying Logan's Coupe wheel and tire spreadsheet, and scoured the internet for options. Just to validate my thinking:

If you go from a 11" wheel to a 12" wheel and want to keep the OUTER lip of the rim in the same position relative to the edge of the fender, backspacing needs to INCREASE by 1". So for simplicity sake, a 11" wheel, 0 offset, 6.0" BS/6.0 FS will be positioned relative to the fender lip, the same as an 12" wheel, -25.4mm offset, 7.0" BS/6.0" FS. The difference being clearance between the inner wheel lip and chassis will decrease by the additional 1" width. Offset can be fine tuned from this point of reference for clearance.

Larger diameter wheels don't affect BS/FS/offset requirements to position the wheel relative to the fender lip, but the larger diameter increases the probability of rubbing on chassis/suspension etc.... which may or may not be able to be solved by BS/FS/offset adjustments.

FFR manual suggests 6" BS for 12" rear and 10" front wheels, but does not give guidance above a 17" diameter tire.

On the coupe, it appears many split the difference when moving from 11" to a 12" wheel, adding .5" to both FS and BS in the rear. Often landing in the 6.0" BS/7.0" FS range.

My brakes necessitate an 18" wheel minimum. I have the IRS, and the car will be mostly street driven, and setup at street ride height. I don't want to notch the frame, and will run an 11" rim if that's what's necessary, but it does seem like a 12" with 335/30/18 tire is possible at street height, no frame notch, no rubbing if set up just right.

For the rears I'm looking at a wheel that is 18x12 and is available in both -15mm and -26mm backspace. I'm leaning towards the 18x12, -15mm, 5.9BS/7.1FS
For the fronts it is available in 18x10 -15mm/0mm/+15mm. I'm thinking 18x10, +15mm, 6.1BS/4.9FS
I'm thinking those would get me setup that works front and rear with no spacers and no rubbing. Or at least its in the ballpark per Logan's spreadsheet. Thoughts?

[GoDadGo]

How about these as a possible option, but they are a tad skinnier:

....................... 18X8.5, 18X9.5 and 18X11:

....https://circuitperformance.com/category/circuit-performance-wheels-rims/cp21-wheels/?filters=wheel-diameter%5B18%5D%7Cwheel-bolt-pattern-pcd%5B5x114-3%5D

[DesertSn8ke]

Certainly affordable, but limited sizes. 18x11 is only +20 offset, so seems like one would need around a 35ish mm spacer to get in the range for the back. If I go 18x11 I might just end up running the FFR Halibrands with spacers. The Rocket Racing Attacks are reasonably priced, and can be had in sizes where no spacer should be needed. I'm also considering those. The Attacks are just not quite the look I want.

[GoDadGo]

The Rocket wheels I think would be a fabulous look on a Type-65 Coupe.
Also, like you I am NOT a fan of using spacers and adapters for wheels.

[Dave Tabor]

Rocket Racing Attack wheels.

rears are 18x11, 5.5" backspace.

no spacers, no rubbing, 315mm wide tire.

Dave
Gen III #17

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[DesertSn8ke]

Thanks for the additional pictures of your wheels Dave. I was definitely eyeing your setup and looking through the forum for pics of your car. They fit really nice. Just double checked, on Logan's wheel and tire repository your rears are listed as 18x11, 5.0" backspace. Are they actually 5.5"?

the 18x12, 5.9" backspace I'm looking at would poke out .6" more, and intrude .4" further into the wheel well.
The 11" width is available with -10mm, So 5.61" BS/6.39FS.

[Rsnake]

I have some 18x12 rear wheels being made now and they will have 5.75" back spacing.
I currently have 18x12 with 6.75" bs and 1" spacer for an effective bs of 5.75". I do think 335 tires are
Too big to fit on the coupe without modifications of which I had to do, as in notch the frame. The tires are flush with the fender lip with the current bs so you don't have anymore room unless you want them sticking out past the fenders.
You can go to my YouTube channel at Cobra Dayton Build and search for wheels.

[DesertSn8ke]

Thanks Rsnake. Yes, I found your YouTube in a different thread yesterday, and saw the one new wheel you have so far. They're gonna look fantastic! Thanks for the input on the 335. I think I will stick with the 11" width and tried and true 315 tire. For the front how did you like your 10", 5.5 backspace? I see on your new front you're actually going with a slightly negative offset (-6mm). I think you'll be the first with a negative offset on the front. Any particular reason there?

[Rsnake]

The 5.5 bs on the 18x10 front fit perfect. I just figured I would move the tire out another .25" since I have the room and plus it will make the dish of the wheel a little deeper. Either way you cannot go wrong. I think the 315 on the rear is the correct choice, I went back and forth on the new wheels and what size but since I already had the frame notched out for the 335's I went with it. They do look mean on the car.

[Dave Tabor]

My rears are this (looking at the invoice):

Rocket Racing Wheels TTR19-8116550 - Rocket Racing Attack Titanium Machined

Dave
Gen III #17

[DesertSn8ke]

Ok, fantastic. Per the PN, that is the 5.0" backspace.

[Dave Tabor]

Ok, fantastic. Per the PN, that is the 5.0" backspace.

Correct- that was a typo in my post #5.

Dave

[rhk118]

Correct- that was a typo in my post #5.

Dave

Dave - making sure I'm understand backspacing correctly. If I ran with a 1" rear spacer with your rear rim, then I would need to get the 6" backspaced rim, not 5" without spacer, correct? That would give the tire contact patches the same width? Those rims look great. Do they stick out from under your body in front or back? They look fairly flush on your pictures but everything is at an angle so can't see. Going to have a separate set of track rims but need to run spacers with the Halibrands and don't want to deal with switching out spacers with the rim changes for obvious reasons...

Also is your suspension set up on the lower "street height" mounts or the higher "race height?"

[Dave Tabor]

Dave - making sure I'm understand backspacing correctly. If I ran with a 1" rear spacer with your rear rim, then I would need to get the 6" backspaced rim, not 5" without spacer, correct? That would give the tire contact patches the same width? Those rims look great. Do they stick out from under your body in front or back? They look fairly flush on your pictures but everything is at an angle so can't see. Going to have a separate set of track rims but need to run spacers with the Halibrands and don't want to deal with switching out spacers with the rim changes for obvious reasons...

Also is your suspension set up on the lower "street height" mounts or the higher "race height?"

I think that your understanding of backspacing is correct. I'm THINKING that you are saying you'd buy a 6" backspaced wheel because that is what is available and then use a 1" spacer to get an equivalent backspace of 5". The math is correct but why would you buy a 6" backspace wheel (if you could get one with a 5" backspace?).

If you are om Instagram, I have many pictures of the car @davesdaytona.

Here are a few views of the rear wheel/tire: 2015 IRS, no spacers, Toyo R888R 315/30ZR18. Also a couple of pics of a real Daytona.

Not to confuse things but to give another example of the math- when I first got the car on the road I used these wheels in the rear with a 2" bolt-on spacer:

https://www.americanmuscle.com/solidblack-fr500-18x10-9404.html

The backspacing specs on these are 6.30 Inches- so 4.3 inches effective with the 2" spacer. At that time I was only concerned with how flush the wheel/tire would be to the fenders. Since I was less than a 5" backspace with this setup I was not concerned with a tire rubbing the frame.

My move to the Rocket 18x11s allowed me to sort of split the extra ~1" difference: I brought the backspacing from 4.3" to 5" (wheel moves towards the inside of the car 0.7") and gained 0.3" width to the outside/fender side of the tire. I hope this makes sense- my brain can only speak backspace- offset is too confusing to me. I like the Rockets- readily available and if you don't mount tires on them you can send them back.

My suspension was built using the street height pickups/mounts and my ride height is 4" all around. I've tried it lowered at 3.75" and I rub some speedbumps around here.

Dave
Gen III #17

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[Skuzzy]

Dave, have you measured the scrub radius? That is something which cannot be measured until the car is built and aligned, and yet is pretty important to get right before the car is finished. Very much a chicken and egg thing.

I can only guess Factory Five has dialed in positive scrub radius, in the design, as that would be preferred for road racing and autocross.

[DesertSn8ke]

I had to draw out some sketches on graph paper.... and built my own spreadsheet (always!) to make sure I understood the relationships and how BS/offset and wheel width all relate.

Where it got confusing for me and I had to sketch it out, was when you move between wheel widths, and you want to keep the track width and/or visually keep the wheel in the same place relative to the body. I was debating between running an 11" or 12" wheel in the back. Ultimately gonna stick with 11's. They ones I like have 5.6" BS. They math says they'll be 3mm tucked from what the guys running the FFR Halibrands with a 1" spacer are at. I figure that's close enough to not notice. If I don't like it I can ultimately do a little fine tuning with spacers, but it shouldn't need more than .5" at most. I haven't pulled the trigger and put a deposit down yet though.

[Dave Tabor]

Dave, have you measured the scrub radius?

I have not - this is a front-end thing - but I can tell under hard braking if I space the front wheels out wide.

Dave
Gen III #17

[rhk118]

I think that your understanding of backspacing is correct. I'm THINKING that you are saying you'd buy a 6" backspaced wheel because that is what is available and then use a 1" spacer to get an equivalent backspace of 5". The math is correct but why would you buy a 6" backspace wheel (if you could get one with a 5" backspace?).

Thanks Dave. I have been following you and Greg on IG. Love it and living vicariously through you all haha. I'm making things complicated on myself in that I'm planning 2 sets of rims - the 17" Halibrands from FFR with a more all season tire as "dailies" (it gets cold up here and isn't kind to traction with R888Rs and I want to drive late fall/early spring), and a Track set. Offsets and backspacing between the 2 sets will be all over the place. So my (likely over complicated) thinking on this is I will run 1-1.5 inch spacers in the back permanently and will need these for the Halibrands, but can adjust the backspacing on the race set so they just bolt up to the spacers, then up front no or small "lug centric" spacer to clear my Gordon Levy calipers with the Halibrands and a properly backspaced front race rim so no spacer hopefull needed up front. Those rockets come in a lot of useful backspaces, so could run the 6" backspace on a 1" spacer that also works with the stock Halibrand on the same spacer....so just need to change wheels and not spacers with them.

I've yet to see a picture of a FFR coupe with the FFR Halibrands and no spacers in back, maybe that will work? The original 6 have their tires tucked well under the body, so maybe that looks OK on the coupe with no spacer, just can't for the life of me find a picture anyone who has done that. For now have to get her built and running, but ready to be a roller and need to at least get the Halibrands shoed up. Race rims can come later.

Greatly appreciate the info in here guys, thanks! I think we all go through this.

[Skuzzy]

I have not - this is a front-end thing - but I can tell under hard braking if I space the front wheels out wide.

Dave
Gen III #17

It is a front end thing. For road racing and autocross with a rear wheel drive car, positive scrub radius is preferred. It helps hold a line in a curve when you brake and provides a good centering force when coming out of a curve.

Too much is a bad thing. It can make a car very unstable at high speed and wear tires very quickly and unevenly. Braking, as you noted, will also cause the car to want to skitter around.

The scrub radius is altered by the diameter of the tire, the wheel offset, ride height, and alignment adjustments. Simple enough to measure. With the car at ride height, draw line through the upper and lower ball joints. Note where the line touches the ground. If that point is inside the halfway point of the width of the tire tread, then you have positive scrub radius. If the point lies outside the halfway point of the tread, then you have negative scrub radius.

For street cars a little negative scrub radius is preferred as it makes the car more stable in varying traction conditions. For road racing and autocross a little positive scrub radius is preferred as it helps to keep the car stable in hard turning conditions.

Also note, body roll can change the scrub radius. You never want the scrub radius to transition from negative to positive or vice-versa!