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  1. #1
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    Apex ARC8 17" vs. 18", Offset, and Tire Size Questions

    Let me first preface all of my comments and questions by stating I don't know much about wheels or suspension. I'd love to learn though. I've sifted through several threads both here and other forums but I still haven't come to a clear picture about what wheel/tire setup I should use to meet my goals. Those goals are to maximize grip and handling, minimize unsprung weight, and improve ride comfort.

    That said, I drove my father's 2006 ZHP this evening. 30k miles, original suspension. It sits on 17" Style 119 wheels wrapped in 225/45R17 Pirelli P Zero All Season Plus tires and it rides appreciably better than my car on stock 18" style 135Ms wrapped in a mix of Toyo Proxes T1s with a refreshed suspension. I'm guessing that the additional comfort is mostly a result of the tires having a lot more sidewall rather than the suspension itself or slight reduction in unsprung weight, but I could be wrong.

    I had always planned to swap to a square set up with Apex ARC8s at some point to shed some unsprung weight and gain the looks but I'm at the all-too-common crossroads of 17" vs. 18". I really like the look of 18" wheels on our cars. On the other hand, I enjoyed the improved ride of the tall-sidewalled 225/45R17 tires on my father's car. So my first question is, how much would the ride improve with a 17" Apex wheel over an 18" version?

    Next question is offset. The Style 135s are ET47 in the front and ET50 in the rear I believe. The closest offset that Apex makes in a 17x8.5" ARC8 is ET40, but they do make an ET45 in the 18x8.5" wheel. So that's a difference of 7mm in the front and 10mm rear for the 17" and 2 and 5mm in the 18". How important is that difference in offset to handling? I also am vaguely aware of this concept "scrub radius." Is that a big issue to consider with offset in terms of handling?

    Now...tires. If I went with 18" wheels I think I would use 245/35R18 tires but maybe 235/40R18. But that 35 means a short sidewall so less ride comfort. In 17" wheels I'm torn between 245/40R17 and 235/45R17. How much grip will an extra 10mm add? How much does ride quality decrease with decreasing sidewall size? Will any of these wheel/tire combos rub on our stock suspension?

    Finally, weight. Now, either wheel size sheds a lot of weight. My stock wheels weigh ~28 lbs front ~28.5 lbs rear. Conservatively, the weight savings from the wheels is 9.5 lbs in the 18" (28-18.2 = 9.8) and 10.5 lbs for the 17" (28-17.2 = 10.8). Tires I'm looking at the Continental ExtremeContact DW, which in the sizes listed above are (according to TireRack):
    • 235/45R17 - 21 lbs
    • 245/40R17 - 22 lbs
    • 235/40R18 - 22 lbs
    • 245/35R18 - 21 lbs

    Which means the total weight per corner is:
    • 17" - 17.2 lbs + 21-22 lbs = 38.2-39.2 lbs
    • 18" - 18.2 lbs + 21-22 lbs = 39.2-40.2 lbs

    At the end of the day, the weight is a bit of a wash. In both 245mm setups, it's roughly 39 lbs. The only difference is that the weight is slightly more toward the rotational axis, which, if my high school physics taught me anything, means there's lower rotational inertia. How noticeable would that difference in rotational inertia be?

    I know, it's a lot of info, but you see what I'm wrestling with! I'm just trying to find the best performing square Apex ARC8 setup that would have some improved ride comfort. Am I thinking about this right? Any help would be greatly appreciated, especially from guys like Stephen or Derbo with 17" Apex wheels, even if it's a "you're doing this all wrong"!

    EDIT:
    I ended up purchasing 17x8.5" ET40 Apex ARC-8s in Hyper Black through their official group buy for a total of $849 shipped. I wrapped them in 245/40R17 Continental ExtremeContact DW tires from Discount Tire Direct for $482 shipped (promotion). My impression can be seen in my project thread (link), but for ease of reading's sake, here is a quick summary:

    03/27/2016

    I've had about 300 miles on them since they were installed and this set up is great. The car feels more planted in the corners and manages bumps in the road much better than before. With traction control off, I can use the throttle pedal to modulate the car's rotation. I think steering response is ever so slightly decreased which probably down to both a taller sidewall and a softer sidewall. I don't mind it though, just an observation. Slightly reduced road noise. All of that with a marked improvement in ride comfort. I really enjoy this setup, it's precisely what I hoped to get out of it.

    Some people say they experience a slight increase in acceleration. I haven't really noticed it but wouldn't be too surprised if it were true.

    Also I like the slight increased offset, visually. The additional ~10mm fills out the wheel wells nicely.
    I figured I would also add this short Car and Driver article on the performance gains from reduced unsprung weight due to lighter wheels. Discussion of the article is found later in the thread here.

    Last edited by BMWCurves; 10-04-2016 at 08:00 PM.

  2. #2
    Great post! Although I don't have a ZHP (yet), I'm very interested in the answers and comments to your post. Subscribed


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  3. #3
    Join Date
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    Subscribed. Great questions here, wondered the same things as this is on my future list as well.


    Current:
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  4. #4
    Join Date
    Sep 2011
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    William,

    Great post. Let me post my experience and specs before I answer your questions with my own personal opinions.

    I used to run 18x8.5 +40 RPF1 on 255/35/18 Bridgestone RE11s
    I am currently running 17x8.5+40 ARC8 on 245/40/17 Dunlop Star Specs Z1.

    1. I'm not sure if you are looking for a quantifiable number or are you looking for an opinion of "comfort." But if logic serves you well, the diameter of the rim/tire is about 25" total. When you reduce the the rim size by an inch, you have to fill that inch up with addition rubber in order to keep the 25" diameter of the rim/tire setup. This will minimize discrepancies with the the speedometer. That being said, with an addition inch of rubber replacing metal, chances are comfort will be increased because obviously rubber is softer and more pliable than metal. Now, there are different types of compounds of rubber and the sportier ones have stronger sidewalls than the "ECO" ones. My personal downsize to 17" rims did change the diameter of the rim a little bit. Since I went to 245/40/17 instead of the 255/40/17, I went with a 24.72 inch diameter rim/tire setup. This meant my sidewalls only went from 3.51in to 3.86in instead of a half inch (sidewall is measured from edge of rim to edge of tread. A full inch would be both sidewall's added.) I don't think its significant enough to vastly improve the ride but it did give a little tolerance to bumps and potholes on the road.

    2. Offset usually is a larger influence on clearance than it is regarding handling when you are talking about. A wider track will also improve your handling but will compromise some steering input and effort. To keep it simple, I run 17x8.5 + 40 fine without any issues on front and back.

    3. 10mm doesn't sound like alot but its almost half an inch of track on ALL corners. Understand than the 2nd number in a tire size is an Aspect RATIO. It is depending on the 1st number to determine the sidewall height. For example, ZHP stock tires 225/40/18 has roughly same sidewall height as 255/35/18. I am unclear if they will rub as I never had stock suspension and non-factory sizes.

    4. I wouldn't worry much about this. It's not significant enough to worry about it under 100mph.


    If you are planning to get ARC8 for comfort, I would suggest the 17x8.5 with 255/40/17 tires. This is similar size to our 255/35/18s on the factory setup. Also, tires for 17s is significantly cheaper at that size.


    And obligatory 17 ARC8 pic time.



  5. #5
    I cant answer any of your questions but just wanted to say the way you post questions and reply to questions from others is just awesome!!


    Sent from my iPhone using Tapatalk


    2006 Coupe | Black Sapphire Metallic | Natural Brown | Gruppe M Intake | Corsa TSE3 | Michelin PSS on Apex ARC-8 | Koni Yellows |

  6. #6
    Join Date
    Nov 2014
    Location
    Portland, OR
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    Derbo,

    This has a lot of great info, I appreciate it! I'll try and respond point by point:

    1. Yes, I meant comfort from a non-quantifiable standpoint. I doubt anyone here has sat with their car and run over a rough road at a set speed and then swapped out wheel/tire combos and compared logged data from accelerometers, but hey, this place has incredible people so you never know! All your points make sense and they're basically what I assumed. I forgot to mention tire compound as another factor, but you touched on that as well. Are summer performance compounds less pliable/forgiving than all seasons or is there really no general trend i.e. there are both hard and soft compounds in all seasons and summer performance tires? And at the end of the day it sounds like you got a little improvement in comfort, but not much, correct?

    2. How does a wider track from a smaller offset affect day-to-day road feel vs. during aggressive handling? Would it introduce tramlining? I'd prefer to keep my car relatively stable and comfortable since I don't plan to track the car often or even very soon.

    3. I meant those questions in more of a general decrease of sidewall height, not specific numbers since it is a ratio. As far as the grip questions, I guess I should elaborate: since the contact patch shape will become more square in the rear on a 245mm tire but more rectangular on the front, will that affect stability/straightline grip vs. turn in ability, etc. Shouldn't a more square contact patch shape in the rear provide greater rear straightline traction while a wider contact patch shape in the front provide better turn in grip? Or am I thinking waaaaay beyond what I'll actually notice?

    For my own personal reference, the approximate sidewall heights are ranked in descending order:
    • 235/45R17 - 4.16"
    • 245/40R17 - 3.86"
    • 235/40R18 - 3.70"
    • 245/35R18 - 3.36"

    Stock:
    • 225/40R18 - 3.54"
    • 255/35R18 - 3.51"

    From this I think the 245's on 18s is too short of a sidewall. 235s would be preferable.

    4. I mentioned the rotational inertia question from my own basic understanding of physics and Stephen's mention of it in his review of his 17" Apex wheels here. He seemed very enthusiastic but I can't tell if that's just from losing so much unsprung mass vs the actual shift in the mass' location around the rotational axis. I was just wondering if there would be appreciable difference where the mass is located in a 17" setup vs. an 18" setup but if you think it's only noticeable over 100mph, then it's really not worth considering haha

    I really do like the way your car looks, but still have love for the 18s

    Quote Originally Posted by san View Post
    I cant answer any of your questions but just wanted to say the way you post questions and reply to questions from others is just awesome!!
    I appreciate that! Through my education I've learned that if you have a question, it's probable that at least one other person has the same question as you so you're potentially helping out more than just yourself. How else are we all supposed to learn?

    Some more questions:
    • Has anyone had a negative experience from switching to a square setup? I figured a square setup would give the car a more neutral feel, although I can't really complain about the stock handling as particularly understeer-y, with the added benefit of being able to rotate tires. But maybe it's worth keeping a staggered setup. Someone sway me!
    • Rotating tires: I read somewhere that because of the greater negative camber in the rear that the treadwear is not ideal to be rotated to the front and can introduce NVH through odd harmonics as well as unfavorable handling characteristics. Anyone else heard this? I figure if you rotate often enough you'd avoid this anyways.
    • Has anyone run a 235 section tire? Did you notice a loss in rear grip or overall grip? My father's 540i w/ sport package (also up for sale soon if anyone's interested!) has a stock 235/45R17 setup and it rides even better than the ZHP he just purchased. It definitely is more tail happy than our cars, but I think that comes with a mixture of smaller section tires, heavier weight, and 320 ft-lbs of torque.
    • What sort of issues/handling characteristics does excessively wide front tires introduce? Derbo mentions a 255/40R17 tire since it's roughly a 25" diameter wheel, but it seems pretty darn wide on the front.
    • I guess I'm still nervous messing with the offset too much. A 17x8.5" ET40 wheel will be 0.7mm further away from the suspension strut in the front. The outer rim will poke out 13.4mm more than before (source), while the 18x8.5" ET45 wheel is closer to stock and will be 4.3mm closer to the suspension strut. The outer rim will poke out 8.3mm more than before (source). Am I really thinking about this too much?

    Again, any help or insight is greatly appreciated!

  7. #7
    Quote Originally Posted by BMWCurves View Post

    I appreciate that! Through my education I've learned that if you have a question, it's probable that at least one other person has the same question as you so you're potentially helping out more than just yourself. How else are we all supposed to learn?
    !
    Couldn't agree more... I am planning to go down the same road as you but maybe a year down the line, so I will have the same questions as you and I am sure I will look back at this thread to make my decision... just like I used your thread on the suspension refresh!




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    2006 Coupe | Black Sapphire Metallic | Natural Brown | Gruppe M Intake | Corsa TSE3 | Michelin PSS on Apex ARC-8 | Koni Yellows |

  8. #8
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    Quote Originally Posted by BMWCurves View Post
    Derbo,

    This has a lot of great info, I appreciate it! I'll try and respond point by point:

    1. Yes, I meant comfort from a non-quantifiable standpoint. I doubt anyone here has sat with their car and run over a rough road at a set speed and then swapped out wheel/tire combos and compared logged data from accelerometers, but hey, this place has incredible people so you never know! All your points make sense and they're basically what I assumed. I forgot to mention tire compound as another factor, but you touched on that as well. Are summer performance compounds less pliable/forgiving than all seasons or is there really no general trend i.e. there are both hard and soft compounds in all seasons and summer performance tires? And at the end of the day it sounds like you got a little improvement in comfort, but not much, correct?

    2. How does a wider track from a smaller offset affect day-to-day road feel vs. during aggressive handling? Would it introduce tramlining? I'd prefer to keep my car relatively stable and comfortable since I don't plan to track the car often or even very soon.

    3. I meant those questions in more of a general decrease of sidewall height, not specific numbers since it is a ratio. As far as the grip questions, I guess I should elaborate: since the contact patch shape will become more square in the rear on a 245mm tire but more rectangular on the front, will that affect stability/straightline grip vs. turn in ability, etc. Shouldn't a more square contact patch shape in the rear provide greater rear straightline traction while a wider contact patch shape in the front provide better turn in grip? Or am I thinking waaaaay beyond what I'll actually notice?

    For my own personal reference, the approximate sidewall heights are ranked in descending order:
    • 235/45R17 - 4.16"
    • 245/40R17 - 3.86"
    • 235/40R18 - 3.70"
    • 245/35R18 - 3.36"

    Stock:
    • 225/40R18 - 3.54"
    • 255/35R18 - 3.51"

    From this I think the 245's on 18s is too short of a sidewall. 235s would be preferable.

    4. I mentioned the rotational inertia question from my own basic understanding of physics and Stephen's mention of it in his review of his 17" Apex wheels here. He seemed very enthusiastic but I can't tell if that's just from losing so much unsprung mass vs the actual shift in the mass' location around the rotational axis. I was just wondering if there would be appreciable difference where the mass is located in a 17" setup vs. an 18" setup but if you think it's only noticeable over 100mph, then it's really not worth considering haha

    I really do like the way your car looks, but still have love for the 18s


    I appreciate that! Through my education I've learned that if you have a question, it's probable that at least one other person has the same question as you so you're potentially helping out more than just yourself. How else are we all supposed to learn?

    Some more questions:
    • Has anyone had a negative experience from switching to a square setup? I figured a square setup would give the car a more neutral feel, although I can't really complain about the stock handling as particularly understeer-y, with the added benefit of being able to rotate tires. But maybe it's worth keeping a staggered setup. Someone sway me!
    • Rotating tires: I read somewhere that because of the greater negative camber in the rear that the treadwear is not ideal to be rotated to the front and can introduce NVH through odd harmonics as well as unfavorable handling characteristics. Anyone else heard this? I figure if you rotate often enough you'd avoid this anyways.
    • Has anyone run a 235 section tire? Did you notice a loss in rear grip or overall grip? My father's 540i w/ sport package (also up for sale soon if anyone's interested!) has a stock 235/45R17 setup and it rides even better than the ZHP he just purchased. It definitely is more tail happy than our cars, but I think that comes with a mixture of smaller section tires, heavier weight, and 320 ft-lbs of torque.
    • What sort of issues/handling characteristics does excessively wide front tires introduce? Derbo mentions a 255/40R17 tire since it's roughly a 25" diameter wheel, but it seems pretty darn wide on the front.
    • I guess I'm still nervous messing with the offset too much. A 17x8.5" ET40 wheel will be 0.7mm further away from the suspension strut in the front. The outer rim will poke out 13.4mm more than before (source), while the 18x8.5" ET45 wheel is closer to stock and will be 4.3mm closer to the suspension strut. The outer rim will poke out 8.3mm more than before (source). Am I really thinking about this too much?

    Again, any help or insight is greatly appreciated!



    1. Certain summer tires may be stiffer than others. For example, I use Extreme Summer Performance tires. Most of these tires are treadwear 200 and their sidewalls are significantly stiffer than say the Bridgestone S04 (regular summer tires). I also want to note, that I noticed the Bridgestones I've used tend to have softer tires than the Dunlop counterpart. I don't have any quantifiable numbers to prove so but that is what my butt says. Haha.

    2. Tramlining definitely is more of a concern when you lower the car and increase negative camber. Larger contact patches will also affect this regardless of the width of the track. When you have LARGE offset changes you will see the effects of tramlining. IMO, 10mm is not significant enough to be bothered by the minor difference.

    3. By going with a square setup you naturally introduce a more neutral steering feel. From a factory standpoint, understeer setups are the safest to deploy due to the biggest unknown factor: Driver capability.

    For your sidewall checks, I honestly prefer to keep it at 255/35/18. I currently run 245/40/17 but I will be going back to 255/40/17 after I waste this set at the track.

    4. Yea, honestly the change usually is minor if the weights are identical.



    Other questions:
    1. Square setup = more neutral feel, Tire rotation is possible to a larger degree especially if you have non-directional tires.
    2. My rear camber is -2.0 and front is -3.0. I rotate as often as possible so I may not be a good help here. Improper wear can contribute to bad NVH or tramline or unwanted tracking. It's up to you if the ability to rotate is worth having the POSSIBILITY of having these issues. It's not always going to happen and its dependent on how the tires wear.
    3. Remember that 235 is usually measured from outer edge of a rim to the edge of the inner edge of the rim. A 235mm from bridgestone MAY not be the same as dunlop 235mm. I know for a fact the Dunlop Z1 SS 245mm is more like a 255mm if properly measured. They are cheating! haha.
    4. Yes you are. Pushing out the rim is actually a good idea in terms of handling and looks. German manufacturers put a 12mm gap in the fender to allow for proper snow chain clearance. You can run a 10-15mm spacer to push out the stock rims to give a better look and wider track.

    Anyways, I hope that helps.

  9. #9
    Join Date
    Jan 2011
    Location
    Fort Worth
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    555
    I made the switch to 17x8.5 et40 Arc-8s with 245/40/15 Conti DWS-06. I was previously running stock with Goodyear Eagle F1 Asymmetric all-season.

    Ride comfort improved slightly. The rear-end is more willing to step out, but I think that has more to do with the switch in rubber than the switch to a square setup.

    Overall, I'm happy with the switch and the look.







    Stock springs, koni yellows. I have a subwoofer in the trunk, and a set of 135s in the backseat, so the rear is lower than normal.

  10. #10
    Join Date
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    Quote Originally Posted by Go Horns! View Post
    I made the switch to 17x8.5 et40 Arc-8s with 245/40/15 Conti DWS-06. I was previously running stock with Goodyear Eagle F1 Asymmetric all-season.

    Ride comfort improved slightly. The rear-end is more willing to step out, but I think that has more to do with the switch in rubber than the switch to a square setup.

    Overall, I'm happy with the switch and the look.







    Stock springs, koni yellows. I have a subwoofer in the trunk, and a set of 135s in the backseat, so the rear is lower than normal.
    Looks great. This is what I'm thinking about as well...minus the sub

    Is that hyper black?

    Sent from an HTC One

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