90decibel

Hi folks! I could use some help from the community. I want to build a set of BBS RS for my 993 C2. I am starting from a 16" RS012/013 set. Planning to keep the centers and replace the lips/barrels with a step up to 17". Trying to figure out the most appropriate sizes. Nothing is a direct fit and some level of extra hassle or compromise is involved.

Here are my constraints and goals:

1. The car is at RS height.
2. Daily driver + autocross.
3. Standard (black calipers) brakes. Test fitted the centers. Lots of clearance in the back. Only 2.5mm or so in the front.
4. Open to slight modifications (e.g. rolling the fenders or shaving the backpads of the wheels, but prefer simplicity and reliability over any particular look.
5. The looks are important (that's why the project), but handling should not suffer (i.e. don't want to mess with the steering feel, scrub radius and all, don't want to max out the camber for fittment).

I put together a list of what I consider reasonable options. Please help me decide (or propose other options). Here is the spreadsheet I used for the math (it can be copied and edited).

orangecurry

Can't help directly, but I have 8.5Jx17 ET50 wheels on the front (with 225/45 tires), the car is lowered to roughly RS+10, and there are no clearance issues. Fenders are standard.

Personally, if I were building wheels from scratch, I'd choose 8.5 front (225 width) and 9.5 rear (255 width). It's nearly what I have now, after a journey including most sizes in the OE 17s and 18s, and even 265 are a bit 'overtired' IMO on a lowered and very stiff setup. (only difference to my ideal is that I currently have 9J rears)

Bill Verburg

first I'd look into tires that will fit the proposed wheels

17s are much harder to fit once you go bigger than 225/45 and 255/40

235/40 and 275/40 would be the max on 8. and 10.5 x17, but are taller than I like, you may have a different opinion but unless thee is a rea rneed i'd avoid tires that tall

best compromise might be 235/40 and 265/40, but these are getting really hard to come by

for a 993 n/b w/o ride height restriction
front
the max tire f/s is ~69mm and the max tire b/s is ~180mm this is what a 245/35 x18 on 8,5 ET56, tire height is just a bit < 24.8mm due to tire compression, there will be some rub on the inner wall at close to full lock

if you are only going to use a 7.5 then the max tire is a 225, typical 225/45 x17 is <24.9" OD so not much taller and w/ way less f/s or b/s
a 225/45 on a 7.5 ET45 will have roughly the same tire/f/s as the 245/40 on a 8.5ET56 and so will be the lower limit for that tire on a 7.5, use ET50 it moves the tire/wheel 5mm more inboard and there will be no chassis fitment issues
if using an 8 ET46 is the lower limit for ET, w/ a 225/45 tire use 8 ET50 to move the assembly 4mm furthe rin, this would be an oe fit
Rear

the max tire f/s is ~80mm and the max tire b/s is ~210mm, this is w/ a 285/30 x18 on 10ET65, this tire on a 10 is stretched to ~290mm+/- depending on the specific tire both sides of the tires are very very close to hitting hard things, the drivers side lip and the passenger side inner face are the closest points of contact and the drivers side wheel well lip will need to be massaged at low ride heights

You will probably be able to get away w/ a 10.5 rear but that also depends on the tire used and ride height

10.5 w/ a 275/35 x17 will be stretched out to 284mm, so even this rare bird has a bit more breathing room than a 285/30 x18, you don't want a 275/40 x17 as it's extra height kills acceleration

if using 27535 x17 on 10.5ET70 then match that w/ 245/40 on 8.5ET56 front

orangecurry

Mr Verburg is the man.

Yes this too

90decibel

Thanks for the input! Could you please explain why these specific widths as opposed to 8 and 10, for example?

90decibel

Thanks for the information! I wish I could say I understood all of it :-) What is f/s and b/s?

Could you please dumb it down for me? I am really not that picky about the tire sizes and happy to settle on some reasonable middle ground. I looked at what you recommended as the best compromise (235/40 and 265/40), but they are not available for my go-to tires (I fluctuate between Michelin PS4S, Potenza RE-71R and Falken Azenis RT660). What would be the second best compromise size for 17s?

PS: Also thanks a bunch for the wheel size guide you put together! I used it for my initial research.

orangecurry

Because (as Bill said) your start point should be your favorite/best rated tire (or range of tires) and in a size readily available today, will-be-available next year, and not stupid cost.

From that, I think 225/45 and 255/40 are your good start point.

I hope you know that you have to make sure the rim width and the tire width work together - this usually means 'similar'.

Next point - as I think Bill also says/agrees with, for the best sporting dynamic from a wheel/tire combo, the sidewall movement should be minimized, and you do that by making the rim wider than the tire.

People use the term 'stretched'. If the sidewall is stretched, it flexes much less than a tire that is the same width or wider than the rim width. Wider tires give more 'comfort'. The resulting sidewall movement/flex acts as another layer of suspension.

(Apologies if I have not explained well!) Bill might just chip-in with some better words

Conclusion - normal rim/tire combo from our start point is 8J/225 and 9J/255 - less sidewall flex means 8.5J/225 and 9.5J/255

Bill?

90decibel

Got it. The algorithm then is as follows: step 1 - pick the tire size, step 2 - pick the rim width that would slightly stretch the selected tire.

Let's try that with some numbers:
Step 1. I am fine with 225/45 for the front and 255/40 for the rear. That's what I am currently running on my stock wheels (7x17 and 9x17).
Step 2. Using this calculator (not sure if it is the definite source) the ranges come back 7" - 8.5" for the front and 8.5" - 10" for the rear. Let's pick the maximum allowed size for that slightly stretched sidewall. It ends up being 8.5" front and 10" rear.

That's where I get confused. When I do the math the result is different from what you suggested (10" rears vs 9.5" that you proposed). Close enough, but I want to make sure I understand what I'm doing here.

orangecurry

That's my opinion and it works for me. But I use this webpage - it's very clear/visual representation, and compares one wheel/tire combo next to another.

https://www.wheel-size.com/calc/

Bill Verburg

f/s - frontspace not shown in the pic but goes outboard from the wheel mounting face instead on inboard, as for b/s the limit is the vertical face plane
b/s - backspace

the wheel has an inherent f/s and b/s as does the tire, the tire's is usually larger but not always

the tire f/s an b/s are the critical #s because it's the tire that rubs, tire bub is the upper limit for either f/s or b/s though it is far more critical for f/s

since you aren't that critical as to maximizing the tires size in 17 I'd stick w/ 225/45 and 255/40 x17 on 8 ET 52 amd 9.5ET60 in 18 225/40 & 265/35 x18 on 8 ET52 and 10 ET65

these specs prove for good fitment at any ride height or suspension spec stock fits were 7ET55 and 9ET55 which would fit as well but provide a bit less grip w/ th tires mentioned above

If you have some other unmentioned specific need the above does have a little wiggle room to go a bit wider on the wheels and a bit more outboard fitment, this is controlled by ET and width choices

90decibel

Excellent calculator! Thank you!

Also, I didn't mean to pick on your recommendation. I was picking on my understanding of the whole thing :-)

orangecurry

apologies from me - I didn't think you were picking - I was just warning you it's only my opinion, and others may disagree. I should have used more words

90decibel

Perfect! Thank you! Now I have my target for the tire/rim combo: 225/45x17 on 8 ET 52 and 255/40x17 on 9.5 ET 60.

The issue now is I can't build rims with these exact specs with the centers I have. Based on the lip size I chose I will either need to deal with 3-4mm of extra stick out or run 5-9mm spacers. That's where my original question comes in. I feel going the spacer route is a safer bet because it allows me to dial in a perfect rub free offset. I updated my calculator with the latest numbers. Could you please take a look? Do you agree with my choice among the options (highlighted in green)?



The only other constraint is the front brakes clearance. Without a spacer I only have about 2mm to the spokes. That is another reason why I thought of going the spacer route. In the back there is plenty of brake clearance and I may opt for shaving the backpad on the centers to deal with the extra poke.

90decibel

I certainly don't want to upset people trying to help me on the Internet out of their kindness in their free time

Bill Verburg

given those choices(if I'm reading it right)

for 225/45 & 255/40 x17 tires on a n/b 993
I'd go w/ 8.5ET55 & 10ET62 or 8ET49 and 9.5ET69

as I said before the tires allow for a little more wiggle room< I don't think that spacers would be needed or wanted