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      10-19-2010, 10:58 AM   #1
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Does Offset Affect Handling?

Upto now, I've only been concerned with offsets to the extent of aesthetics, ie will it sit flush with the fenders or too tucked, and the possiblitly of rubbing if too aggressive. But say you find a fitment that fits, but is more aggressive than stock offset, would that affect handling (for better or worse)?

I know stock offset on the non-m coupe is 18x8 et47 on the front. If I find an 18x8 rim with et35, I'm sure it'll fit but am I hurting performance by going something so far from factory specs? I guess same question regarding rears? The only thing I can think of is increased bearing wear but you'd get the same result from running spacers and that doesn't seem to be an issue for most people...
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      10-19-2010, 11:37 AM   #2
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same question for me too... not sure, but i think for the front, if you mess with offset you also mess with the "scrub radius". from what i understand (which is minimal), for track use, an ideal scrub radius is zero. if the factory offset is already at "zero" for the scrub radius, then it stands to reason that any change in offset is detrimental to handling. can anyone else confirm this???
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      10-19-2010, 11:37 AM   #3
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someone w/ more experience should chime in here, but from my experience w/ rear spacers on my e85 it would add or take away oversteer. I'm sure front spacers would change overall steering geometry while turning.
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      10-19-2010, 01:17 PM   #4
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Found this via Google: http://www.imazda.com/forums/showthread.php?t=509

It makes sense that changes in track width would affect handling, although I don't have the expertise to say why and how. Someone like HACK should chime in here.

Also, the option to get wider wheels should also be considered. Rather than getting a lower offset 8" wide wheel, you could get a 8.5" ET 41 wheel to maintain the same inner clearance while extending the same 12mm outward.

EDIT: Might be worth putting in a call to TC Kline or Fall-Line or anyone else who would have some racing experience with these cars.

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      10-19-2010, 01:43 PM   #5
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Quote:
Originally Posted by dekaliber View Post
Found this via Google: http://www.imazda.com/forums/showthread.php?t=509

It makes sense that changes in track width would affect handling, although I don't have the expertise to say why and how. Someone like HACK should chime in here.

Also, the option to get wider wheels should also be considered. Rather than getting a lower offset 8" wide wheel, you could get a 8.5" ET 41 wheel to maintain the same inner clearance while extending the same 12mm outward.

EDIT: Might be worth putting in a call to TC Kline or Fall-Line or anyone else who would have some racing experience with these cars.
so from this... a slightly positive scrub radius is ideal for track, whereas a more neutral, closer to zero, scrub radius is better for your daily driver. racers - let's hear it! what do you know?

excellent find, dekaliber!
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      10-19-2010, 01:45 PM   #6
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Quote:
Originally Posted by dekaliber View Post
Found this via Google: http://www.imazda.com/forums/showthread.php?t=509

It makes sense that changes in track width would affect handling, although I don't have the expertise to say why and how. Someone like HACK should chime in here.

Also, the option to get wider wheels should also be considered. Rather than getting a lower offset 8" wide wheel, you could get a 8.5" ET 41 wheel to maintain the same inner clearance while extending the same 12mm outward.

EDIT: Might be worth putting in a call to TC Kline or Fall-Line or anyone else who would have some racing experience with these cars.
That's exactly what I'm considering, 8.5" et42 or 9" et35.
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      10-20-2010, 03:59 PM   #7
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Quote:
Originally Posted by shift@red View Post
This is not correct. That wheel offset and size will extend 7mm more but also have 5mm less clearance on the strut side of the wheel.
As far as handling goes, a wider track ALWAYS helps. Pushing the wheels out further, especially with ample contact patch, makes for greater stability. Think about it. If you were to take the same object, but have its wheels set at 3" apart and try to swing it around on a frictional surface vs wheels set 6" apart, which is going to spin more easily? The one with the smaller track.
dekaliber's math is correct, shift@red... the stock front wheel on a non-///m is 8.0" x et47, moving to an 8.5" x et41 will retain inner clearance (which helps reduce extra loads on the bearings) and move the outer edge to +12mm.

this 8.5" x et41 will increase front track by nearly 0.5" over stock.

the contact patch will only increase if a wider tire is used.
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      10-20-2010, 04:57 PM   #8
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Next question, say I go with wider but I choose a lower profile tire to keep the height closer to stock, would that also affect handling?

I'm thinking instead of 225/40 and 255/35 stock, I'd do square setup of 245/35 on a 8.5" rim, which would be about 0.3" shorter than stock (seemed better to go 0.3" shorter than 0.3" taller with 235/40 for potential rubbing purposes).
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      10-20-2010, 09:05 PM   #9
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Quote:
Originally Posted by Dubbedown View Post
Next question, say I go with wider but I choose a lower profile tire to keep the height closer to stock, would that also affect handling?

I'm thinking instead of 225/40 and 255/35 stock, I'd do square setup of 245/35 on a 8.5" rim, which would be about 0.3" shorter than stock (seemed better to go 0.3" shorter than 0.3" taller with 235/40 for potential rubbing purposes).
you may already know much of this, but i'm just trying to be thorough...

first the obvious - the values sent to the speedo will be higher than standard (i.e. the wheel will be spinning faster, so actual speed will be lower than recorded, and the actual trip will be shorter than recorded). also, we know for a fact that the 235 and 245 tread widths will work in the rear.

ok, now for tires -

going to a shorter side wall will increase sidewall stiffness, therefore giving the driver an enhanced feel or perceived increase in feedback. it will also require the suspension to pick up the extra give needed to accomplish a compression in the suspension overall. remember, suspension engineers not only figure shock and spring compression into their equations, but also sidewall compression (i've heard 15% - 20% of the suspension's initial compression is accomplished with a tire's sidewall!). usually i would consider going to a shorter sidewall a bad idea, but as long as they aren't runflats, it's probably just fine for this setup. a shorter sidewall will also reduce rotational mass, assuming that the non-stock wheel choice doesn't eat up the difference. this aspect is good from a suspension AND from a braking perspective. overheated brakes suck big-time! also, a smaller overall diameter is easier to accelerate from the axle's perspective, obviously increasing 0 - whatever speeds. interesting note: most racers want a very stiff suspension on their cars so there is no diving, squatting, or rolling during on track manuvers; however, the sidewalls on these race-ready rides are usually equal to, or taller than, their production brethern. whether this is a class restriction, or for performance gain, a racer will need to answer that...

front clearance -

235 vs. 245 - follow my math on this - if others on this forum are having front inner liner clearance issues with 235's over 225's on an 8.5" x et35 wheel, then a 245 tread width will as well. pulling the offset in to an 8.5" x et40 will give the same wheel clearance inside, but still move the outside of the wheel out giving more front-end track (THANKS DEKALIBER!), without having the 10mm extra tread width per side of the wheel's centerline produce inner liner clearance issues at lock. inner wheel clearance should not decrease, so choose the offsets carefully! i measured inner clearance on the stock 8.0" x et47 wheel with 225/40R18's to be 10mm. under latteral load, the wheel on the outside of the turn will straighten to zero camber, while the wheel on the inside of the turn will GAIN MORE NEGATIVE CAMBER. with a wider tire and less clearance, an inner tire sidewall-to-strut collision may occur.

great question!... had you not asked this, i would never had sat down and put it all together.
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      10-20-2010, 09:19 PM   #10
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Quote:
Originally Posted by v3.0si View Post
you may already know much of this, but i'm just trying to be thorough...

first the obvious - the values sent to the speedo will be higher than standard (i.e. the wheel will be spinning faster, so actual speed will be lower than recorded, and the actual trip will be shorter than recorded). also, we know for a fact that the 235 and 245 tread widths will work in the rear.

ok, now for tires -

going to a shorter side wall will increase sidewall stiffness, therefore giving the driver an enhanced feel or perceived increase in feedback. it will also require the suspension to pick up the extra give needed to accomplish a compression in the suspension overall. remember, suspension engineers not only figure shock and spring compression into their equations, but also sidewall compression (i've heard 15% - 20% of the suspension's initial compression is accomplished with a tire's sidewall!). usually i would consider going to a shorter sidewall a bad idea, but as long as they aren't runflats, it's probably just fine for this setup. a shorter sidewall will also reduce rotational mass, assuming that the non-stock wheel choice doesn't eat up the difference. this aspect is good from a suspension AND from a braking perspective. overheated brakes suck big-time! also, a smaller overall diameter is easier to accelerate from the axle's perspective, obviously increasing 0 - whatever speeds. interesting note: most racers want a very stiff suspension on their cars so there is no diving, squatting, or rolling during on track manuvers; however, the sidewalls on these race-ready rides are usually equal to, or taller than, their production brethern. whether this is a class restriction, or for performance gain, a racer will need to answer that...

front clearance -

235 vs. 245 - follow my math on this - if others on this forum are having front inner liner clearance issues with 235's over 225's on an 8.5" x et35 wheel, then a 245 tread width will as well. pulling the offset in to an 8.5" x et40 will give the same wheel clearance inside, but still move the outside of the wheel out giving more front-end track (THANKS DEKALIBER!), without having the 10mm extra tread width per side of the wheel's centerline produce inner liner clearance issues at lock. inner wheel clearance should not decrease, so choose the offsets carefully! i measured inner clearance on the stock 8.0" x et47 wheel with 225/40R18's to be 10mm. under latteral load, the wheel on the outside of the turn will straighten to zero camber, while the wheel on the inside of the turn will GAIN MORE NEGATIVE CAMBER. with a wider tire and less clearance, an inner tire sidewall-to-strut collision may occur.

great question!... had you not asked this, i would never had sat down and put it all together.
wow one of the most informative and helpful posts. i knew of the speedo issues and the increased harshness due to shorter sidewalls but i thought clearance would not be an issue.

ugh, now i feel like i'm just not even trying but the wheels i had in mind were 18x8.5 et42 but i guess i have no option besides the 225 if i want square? i figured 225 up front is fine but the loss in rear traction outweighs the benefits of countering understeer with a square setup.
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      10-20-2010, 09:26 PM   #11
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not so fast! we just need to dial in the offsets.

if you would - help me out with the square setup... what are the benefits of this over staggered?
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      10-20-2010, 09:29 PM   #12
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Quote:
Originally Posted by v3.0si View Post
not so fast! we just need to dial in the offsets.

if you would - help me out with the square setup... what are the benefits of this over staggered?
aside from getting more life out of the tires by rotating, i always assumed square setup would provide more neutral handing coming from understeer.
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      10-20-2010, 09:30 PM   #13
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I've been doing a fair bit of research on offsets lately in order to dial in the look that I'd like to go for with my wheel purchase for next year. This is a good read.

http://www.turnermotorsport.com/html...clopedia.shtml

"Most BMWs will take a 15-20mm spacer when used with the stock wheel/tire setup. This is because the German TUV (similar to our own DOT) still requires adequate clearance for snow chains. These are still widely used in Europe, especially in The Netherlands, Norway, and Sweden even though they are obsolete or irrelevant in other markets. The TUV also requires that every car in Germany be equipped with either dedicated snow tires or snow chains for winter driving."
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      10-20-2010, 10:06 PM   #14
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Quote:
Originally Posted by Dubbedown View Post
aside from getting more life out of the tires by rotating, i always assumed square setup would provide more neutral handing coming from understeer.
personal experience (three 3'ers)... BMW + rotating = lots of road noise + SHORTER tread life. most modern BMW's (including ours) have high negative rear camber and a high amount of toe in. this means that with a stock alignment, the car is literally dragging the rear tires down the pavement. this dragging creates small grooves and ripples that when moved to the front of the car create resonance, or in other words, noise. the act of rotating the tires forward will also force a different set of grooves and ripples to occur over the top of the ones put there when they were on the rear. this causes increased cupping and feathering (yes, even more than the already absurd amount already found on modern BMW's). completely stock, 25k mi is tops anyway.

i'm pretty sure i've even seen a bulletin from BMW on not rotating somewhere. i'll see if i can find it tomorrow.

also, a little more math... bridgestone claims their 225 tire section widths are at 9.1" and their 245's are at 9.8"...

9.8" - 9.1" = 0.7" ~ = 18mm

18mm / 2 = 9mm = difference in section width from centerline over 225's

(8.5" x et35) outer limit = 18mm over stock (which might rub with 235's)

18mm - 9mm = 9mm

9mm gain of outer rim movement over stock on an 8.5" wheel = et44

8.5" x et44 outer position = +9mm (closer to fender)
inner position = +3mm (closer to strut)

+9mm outer + 9mm of tire = 18mm overall exterior movement = no more fender gap

+3mm inner + 9mm of tire = 12mm of scary close tire-to-suspension dimension

this puts your inner tire sidewall VERY close to, if not actually on, the strut in high load situations. i would not recommend a 245 treadwidth for the track in front. it may be passible for use on the street though.
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      10-20-2010, 11:07 PM   #15
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by the way... i've measured my bridgestones to have an actual 'overhang' from the edge of the rim by 8mm. this is really close to matching the 9mm in my post above.
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      10-21-2010, 08:06 AM   #16
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The incredibly neutral setup of the E46 chassis is complimented by a square setup. Many E46 M3 track junkies swear by it. For our cars, however, I have first hand experience with the square and slightly staggered setup and MUCH preferred the staggered setup. The ease at which our shortened wheelbase car rotates makes having a bit more rubber in the rear advantageous. I run a 255/35/18 and 275/35/18 setup, so we're not talking about a major difference in sizes, and I love it. I ran 265's on all corners for two track days and didn't like it one bit. That said, when I ran a square 255 setup on my E46 330Ci, I couldn't have been happier.

OP. The answer to your question, as many have noted, is yes. Offset does affect handling. That said, you're never going to feel it on the street and would probably have a hard time feeling on the track. All of the various rubber bushings and stock performance parts, designed with comfort for the typical driver in mind, are going to be the limiting factors that impact handling way before offset does. But everyone loves a properly offset wheel... looks much better.
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      10-21-2010, 08:42 AM   #17
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this is good news for dubbedown! if JCz04Bimmer was able to run 265's on the front of his ///m, certainly a 245 square setup will fit on a non-///m. clearly my math earlier is missing a variable...

JCz04Bimmer - what specs were the wheels on the square setup? maybe we can reverse engineer this?

great to hear from a racer and be able to apply the experience!
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      10-21-2010, 08:45 AM   #18
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Quote:
Originally Posted by v3.0si View Post
this is good news for dubbedown! if JCz04Bimmer was able to run 265's on the front of his ///m, certainly a 245 square setup will fit on a non-///m. clearly my math earlier is missing a variable...

JCz04Bimmer - what specs were the wheels on the square setup? maybe we can reverse engineer this?

great to hear from a racer and be able to apply the experience!
btw noticed you're new on zpost, welcome!
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      10-21-2010, 10:14 AM   #19
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thanks!

also found this, courtesy of JCz4Bimmer's comments...

http://www.zpost.com/forums/showthre...ghlight=square

there's a lot of info in there about front tire and suspension setup for the ///m's. one of the posts describes needing spacers and 3 degrees of negative camber to get a 255 treadwidth to run smoothly for the front. i think it's safe to say that if you want to retain the stock sport suspension, a 235 treadwidth is safe.
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      10-21-2010, 10:24 AM   #20
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Quote:
Originally Posted by v3.0si View Post
thanks!

also found this, courtesy of JCz4Bimmer's comments...

http://www.zpost.com/forums/showthre...ghlight=square

there's a lot of info in there about front tire and suspension setup for the ///m's. one of the posts describes needing spacers and 3 degrees of negative camber to get a 255 treadwidth to run smoothly for the front. i think it's safe to say that if you want to retain the stock sport suspension, a 235 treadwidth is safe.
but if i run 235, what's the lowest offset i can go on a 8.5" in the front, 45mm? maybe i will go staggered after all....
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      10-21-2010, 11:27 AM   #21
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Not sure about my offsets, but I'm running 245/35x18 on 9.5" all around on a NON-M. Best handling set-up I've found yet but the semi-pro driver in the family hasn't gotten his paws on it since I changed the fronts.

With the obvious power difference of the M, maybe you want staggered, I don't.
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      10-21-2010, 12:05 PM   #22
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Not sure about my offsets, but I'm running 245/35x18 on 9.5" all around on a NON-M. Best handling set-up I've found yet but the semi-pro driver in the family hasn't gotten his paws on it since I changed the fronts.

With the obvious power difference of the M, maybe you want staggered, I don't.
and that was stock suspension, no change in cambeR?
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