Welcome to the North American Subaru Impreza Owners Club Saturday October 25, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
Click here to visit TireRack
Tire & Wheel Forum sponsored by The Tire Rack

Losing traction? Need new tires?
Click here to visit the NASIOC Upgrade Garage...
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC Technical > Tire & Wheel

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 07-02-2003, 02:49 PM   #1
BriDrive
Scooby Specialist
 
Member#: 21801
Join Date: Jul 2002
Chapter/Region: South East
Location: 90 Degrees North
Vehicle:
2013 Jacked Up T4R

Default WHEEL OFFSET ? Consider This !

Wheel offset:
In the 2002 WRX sedan, the stock wheel is 6.5” wide (165.1mm) and has a positive offset of 53mm.
That is to say, the wheel- mounting surface is 53mm from the centerline of the wheel in a direction to the outside of the wheel.
There has been a mile of threads discussing wheel offset for the WRX and its relationship to different wheel widths. I believe there is some misunderstanding or a lack of conceptualization on the part of applying wheel offset when increasing wheel width. There are two variables to consider when a change of wheel width is contemplated. One variable is clearance of the suspension, the other variable is load displacement over the wheel. Unfortunately, in the extreme, these variables will work against each other.
Let’s look at the load displacement variable first.
Some simple calculations will show that the stock wheel parameters listed above yield the following results: 82.1% of the load is displaced in a direction toward the inside of the wheel (that is, the car side) and 17.9% of the load is displaced to the outside of the wheel. Presumption one is that the engineers have designed this load displacement criteria as a best possible compromise between wheel bearing life, suspension geometry, handling characteristics and suspension clearance during compression and steering.
Taken by itself (that is, load displacement) you would need to simply apply this ratio to any given wheel width. For example, say you want to go to a 7” (177.8mm) wide wheel. To maintain the same load displacement, some simple calculations yield, guess what….a 57mm positive offset. But wait a minute you say, the charts all now finally say that somewhere between 50 and 53mm positive offset is supposed to be optimal for the this width. That’s where I started to investigate why. 57mm is precisely the offset you would need on a 7” wide wheel to maintain the stock load displacement, but heck, all the recommendations were going in the opposite direction…ie from 53mm for 6.5” to maybe 50mm for 7”. So I tested the theory, because I just so happen to own 7” wide OZ Superleggeras. I cut two strips of paper to precise lengths, one 165.1 mm wide and the other 177.8 mm wide. I then precisely marked their centerlines and marked the offsets. Then I laid them against a perpendicular plane ( an imaginary hub ) at their respective offsets. What you will find is that the OZ wheel very closely maintains the inside space of the stock wheel, great for maintaining wheel clearance against the strut under compression and turning, but only 77% of the load is displaced to the inside of the wheel and 23% of the weight to the outside.
I won’t ramble on any farther….Conclusion: There is a balance to maintain in increasing wheel width between load displacement (bearing life we keep hearing about) and inside clearance. If one were only to consider bearing life as important, then you would have to subscribe to the 82.1/17.9 ratio theory. Problem with this, is that as you increased width you would rapidly run out of clearance room. If you were only to subscribe to the clearance is important theory, then you would rapidly wear down the wheel bearing as you increased your width. The theories mutually exclude each other. No-one has ever explained this. So I generally question where folks are getting their information and how it was derived. Ultimately, you could learn Japanese and consult a WRX engineer, but skip all that…If you want the best of both worlds you will have to compromise: I have developed a formula that averages both extremes: the results for my version (I’ll call it Optimal Compromise): 7”-52mm offset / 7.5”-51mm offset / 8”-50mm offset.
How’s it done: calculate ratio for each theory first.
82.1% theory : Step 1: .821 x wheel width (mm) Step2: Wheel width x .5 Step3: Subtract answer in Step2 from answer in Step1.
SpaceMaintenance theory: Step 1:wheel width (mm) – 135.55 mm Step2: wheel width x .5 Step3: Subtract answer in Step 1 from answer in Step2.
Then add result for each theory together and divide by two(2)….Voila!
* Registered users of the site do not see these ads.
BriDrive is offline   Reply With Quote
Old 07-02-2003, 08:09 PM   #2
tora
Scooby Guru
 
Member#: 5851
Join Date: Apr 2001
Chapter/Region: VIC
Location: Vancouver
Vehicle:
09 Forester XT
+ NA Miata

Default

I completely understand...

Good explanation and I like your ratio at the end of optimal compromise.
tora is offline   Reply With Quote
Old 07-02-2003, 08:25 PM   #3
bbbwrx
Scooby Specialist
 
Member#: 32734
Join Date: Feb 2003
Chapter/Region: RMIC
Default

Thanks for putting so much time and effort into this. I don't completly understand but I understand where you are coming from.

BEN
bbbwrx is offline   Reply With Quote
Old 07-03-2003, 02:57 AM   #4
Kostamojen
Scooby Guru
 
Member#: 2272
Join Date: Sep 2000
Chapter/Region: BAIC
Location: haha XD
Vehicle:
2013 Subaru 599 :P
Galaxy Blue Sexy

Default

Your arguement is good.

However, you forgot one thing... Tire clearance.

I myself am running a 16"x7" wheel with a +48 offset with 225 series tires. There is hardly any clearance between the inside of the tire and the rear struts. If there were any less clearance (IE lower offset) it would rub under load. Thats why you cant run say a stock wheel with a 225 series tire on a GC, it just wont fit clearance wise.

Mathmatical theory is great, until you run into the real world...
Kostamojen is offline   Reply With Quote
Old 07-03-2003, 08:25 AM   #5
wrx2.0 555
Scooby Specialist
 
Member#: 21383
Join Date: Jul 2002
Chapter/Region: South East
Vehicle:
2010 Trek
EX-5

Default

Things that make you go, MMMMMMMMMMM!!!
wrx2.0 555 is offline   Reply With Quote
Old 07-03-2003, 08:47 AM   #6
subysouth
Scooby Guru
 
Member#: 5039
Join Date: Mar 2001
Chapter/Region: South East
Location: Ocean Springs, MS
Vehicle:
2007 Outback XT
Grey 5-speed

Default Re: WHEEL OFFSET ? Consider This !

Quote:
Originally posted by BriDrive
Let’s look at the load displacement variable first.
Some simple calculations will show that the stock wheel parameters listed above yield the following results: 82.1% of the load is displaced in a direction toward the inside of the wheel (that is, the car side) and 17.9% of the load is displaced to the outside of the wheel. Presumption one is that the engineers have designed this load displacement criteria as a best possible compromise between wheel bearing life, suspension geometry, handling characteristics and suspension clearance during compression and steering.
Taken by itself (that is, load displacement) you would need to simply apply this ratio to any given wheel width. For example, say you want to go to a 7” (177.8mm) wide wheel. To maintain the same load displacement, some simple calculations yield, guess what….a 57mm positive offset. But wait a minute you say, the charts all now finally say that somewhere between 50 and 53mm positive offset is supposed to be optimal for the this width. That’s where I started to investigate why. 57mm is precisely the offset you would need on a 7” wide wheel to maintain the stock load displacement, but heck, all the recommendations were going in the opposite direction…ie from 53mm for 6.5” to maybe 50mm for 7”.
Nice write up but your theory goes a little henky above. I am not quite sure what you mean by load displacement, but based on your statement you seem to be comparing how weight/area/width is distributed on the inside and outside of the mounting face. To maintain the exact same ratio in front of and behind the mounting face, no matter what the wheel width is, you have to do one simple thing, maintain the exact offset as stock. Because offset is measured from the centerline of the wheel, regardless of the wheel width, the same ratio of width in front of and behind the hub will be maintained as long as the same offset is maintained.

If that is confusing, imagine a 0 offset wheel which places the mounting surface at the exact center of the wheel. Regardless of the wheel width, half of the "load" will be in front of and behind the mounting surface.

There is no hard and fast exact Subaru offset. Or should I say we dont know what it is. Subaru tends to decrease offset on their vehicles when going to wider wheels to allow for wheelwell clearance it appears. If you take the offset of the thinnest wheel mounted on your bodystyle Subaru(assuming similar suspension geometry) that should be close to the ideal offset for that platform.

ss
subysouth is offline   Reply With Quote
Old 07-03-2003, 09:34 AM   #7
Heather
Scooby Specialist
 
Member#: 11627
Join Date: Oct 2001
Location: Chico, CA
Exclamation I like it!

Good write-up. But our WRX wheels are a 55mm offset. I am sure of it, only the Subaru 17" wheels are a 53mm offset. I too think I figured out wheel offset and here's my simpler explanation:

Offset is measured from the center of the wheel. If you go to say a 48mm OZ SL in a 17 x 7 rim the offset moves out towards the fenders, and yes the wheel will line up exactly where the stock wheel does on the inside of the car thus avoiding hitting the struts (since at stock there is precisely a 1/2 gap before the tire would hit the struts). If you stick with a 55mm stock offset with wider rims say 7" or 7.5" then the size of the rim would spread out equally inside and outside the car most likely hit the struts.

All the calculations for this formula are in Fred Puhn's book, "How To Make Your Car Handle"

Anyway, I wanted OZ SL wheels really bad myself but I found in the calculations my husband did from the book that with a 48mm offset on a 17 x 7 wheel the tire would stick out 3/4 of an inch past where it does now towards the outside of the fender. There is only about a 1/2 gap or so with full suspension droop (depending on your choice of tire and your alignment – variables )
So with the SL OZ my tires (in theory only) would rub my wagons fenders. That is not good but it is better then rubbing my struts! That is why, although the 55mm offset is preferred, it is not possible with a wider rim without hitting the inside of the struts.

A really risky venture is to buy any wheel wider then 7" say a 7.5" or an 8" wheel in theory it will most certainly hit the struts inside and the fenders outside (the wagon at least since that s what I am measuring) but many people here have Rota's in 17 x 7.5" with a 48mm offset and no problems- hence the variables of tire size, tire variations and alignments.

In our calculations we figured that a 17 x 7" wheel of 50mm - 53mm offset is ideal for our cars (Prodrive and Subaru only I'm afraid ) I think you concluded the same thing in the beginning of your thread as well.

Yes in my books (also the MRT WRX Training book) it states that wheel bearings are aversely affected by incorrect offset and it places strain on the steering rack making the car will have a harder to steer feel which will kill your steering rack bushings (Just read the Project WRX in Sports Compact Car in the August issue!) But the wheel bearing and the steering rack bushings lasted 50,000 on Project WRX so you will be fine for a long time if that's any consolation Still I think it's better to go outside towards the fenders then inside towards the struts with new wheels. Really with the little clearance there is you have no choice. Wish I were brave enough to try the OZ SL's I think they look fantastic on our cars
Heather
Heather is offline   Reply With Quote
Old 07-03-2003, 11:26 AM   #8
BriDrive
Scooby Specialist
 
Member#: 21801
Join Date: Jul 2002
Chapter/Region: South East
Location: 90 Degrees North
Vehicle:
2013 Jacked Up T4R

Default

I appreciate the responses you folks have taken the time to make. A footnote to my topic is that I do not intend for my formula to be an end all golden rule…simply one way to balance clearance and bearing life, as the offsets needed to address one or the other go opposite ways.
Kostamojen: I agree and tire clearance is certainly is another factor, which is really and extension of the clearance variable I introduced. You are in a GC also which would require different calcs, but the compromise between bearing life and clearance is still applicable. Mathematical theory is good and can ultimately save time over trial and error if one understands what he or she is trying to accomplish and if the theory being applied is fundamentally sound.
Subysouth: I beg to differ: your point on offset/load displacement only holds true IF AND ONLY IF your offset is always ZERO. 53mm offset on a 6.5” wheel will certainly and definitely have a different load displacement than a 7” wheel with 53mm offset. To validate this, I’ll use a random load number for the right front wheel. Lets say that 960 lbs needs to be displaced over a 6.5” wide wheel (165.1 mm). That equates to 5.815 lbs/mm. On the 6.5” wheel with 53mm offset, there are 135.55 mm to the inside to displace weight and 29.55 mm to the outside to displace weight. That’s 788.22 lbs to the inside and 171.78 lbs to the outside. (This incidentally is the 82.1% ratio I talked about because I used the stock wheel) The important point here is that there are 5.815 lbs/mm displaced. On a 7” wide wheel (177.8mm) the wheel only needs to support 5.399 lbs/mm. If you maintain a 53mm offset on this wheel, there are 141.9 mm to the inside and 35.9 mm to the outside. That’s 766.1 lbs to the inside and 193.9 lbs to the outside. That’s equivalent to a 79.8% ratio. Your last paragraph (…”Subaru tends to decrease offset on their vehicles when going to wider wheels…”) kind of makes my point again. I’m trying to demonstrate: that you can’t go to a wider wheel without adversely affecting the load displacement and make it fit. (The amount of adverse effect is relative, and I place no hard fast rule on anyone’s comfort zone. I am simply demonstrating you can’t have it both ways. My problem is that published wheel offsets for wheel widths are really addressing fit issues, BUT people are misguided in thinking that by veering away from them they are adversely affecting bearing life.
Heather: To address your offset point, I would respectfully direct you to my example for subysouth above.
The size of the rim does not spread out equally independent of wheel width with a fixed offset. The wheel width drives the offset variable. I use OZ 7” 48mm offset like I mentioned prior, with 215/45/17 PolePositions on my 2002 WRX. The farther you go to the outside with a wider wheel (ie. the less offset),the more strain you will put on wheel bearings everything else being equal. Everyone’s cars are somewhat different…I’m not trying to introduce a hard fast rule that anyone can apply to any Subaru….There are too many variables in: suspension,springs,struts,coilovers,ALKs,wheels,ti res,etc….My point to consider with the introduction of my ‘Optimal Compromise’ theory is a simple formula that averages out the opposite effects of achieving load displacement for “bearing wear” and fit. Each WRX owner will ultimately have to compromise to some extent with offset to address their desired application.
One final thought on offset and bearing life…..It’s relative to the extremes in which one departs from a stock load displacement ratio AND how the car is being driven. Lots of track days, lots of autocross, lots of aggressive driving, poorly inflated tires, badly sorted out alignment, camber, toe, incorrectly installed BBK or other brake components…..all of these things work in tandem to wear wheel bearings. Load displacement and how a wheel offset displaces it is only one factor.
BriDrive
BriDrive is offline   Reply With Quote
Old 07-03-2003, 09:33 PM   #9
subysouth
Scooby Guru
 
Member#: 5039
Join Date: Mar 2001
Chapter/Region: South East
Location: Ocean Springs, MS
Vehicle:
2007 Outback XT
Grey 5-speed

Default

Bri I see what you are saying but you are using the discontinuity of the "correct" offset to use it as a proof that it creates a discontinuity. Its like using a word to define itself. Meaning the correct offset says that to correctly distribute width + and - the mounting face is to move the mounting face 53mm outwardof the wheel center no matter what the width is as long as clearance is satisfied.

Now what I am trying to understand is if you have empirical data that says more width in front of the hub or behind is in some way superior. That I would like to hear. You say that Subaru reducing the offset on a wider wheel proves your point of weight distribution, however it also supports Kostas point off strut clearance. Subrau also maintains the identical offset on wheels of varying widths (7 and 7.5" for instance.) which btw supports my point.

Reduction of offset moves more of the width of the wheel in front of the hub, this taken to an extreme can cause nearly immediate bearing failure. You are suggesting a +52mm offset for a 7" wheel while Subaru uses a +53mm. Subaru uses a +53mm offset on some of their 7.5" wheels and you are suggesting a +51mm.

My point is there are great issues to be discussed and debated, but you are placing yourself in the rather unenviable position of arguing with the people who built the cars. And the part that is still eluding me is based upon what principles are you saying your theory would be optimal. Are you saying too much width in front of or behind the mounting face is bad? If so why - specifically?

ss

Last edited by subysouth; 07-03-2003 at 09:38 PM.
subysouth is offline   Reply With Quote
Old 07-03-2003, 09:50 PM   #10
Heather
Scooby Specialist
 
Member#: 11627
Join Date: Oct 2001
Location: Chico, CA
Thumbs up

Good points Subysouth!
Here's a good benefit on the reduction of offset, let's say the OZ SL 48mm offset, it creates a wider track which is good for handling. This wider track of these wheels would give you the same extra 22mm track increase of the sedan in the wagon. Plus there would be no fear of hitting the inside of the struts. It's all about trade-offs. But we need to clearup the Subaru offsets:

WRX wheels 55mm (not 53mm)
BBS 17 x 7" wheels through Subaru 55mm
Subaru 17 x 7 wheels 53mm
STi wheels 17 x 7.5" 53mm offset

But Subaru recommends the 215/45/17 tire with these wheels. The problem of hitting the fenders with the correct offsets from Subaru is when people put on the 225/45/17 tires (which is actually the exact match to the 205/55/16 diameter!)

Heather
Heather is offline   Reply With Quote
Old 07-03-2003, 09:57 PM   #11
subysouth
Scooby Guru
 
Member#: 5039
Join Date: Mar 2001
Chapter/Region: South East
Location: Ocean Springs, MS
Vehicle:
2007 Outback XT
Grey 5-speed

Default

Heather I was not aware the BBS 17x7 had a +55mm offset. Thats good info too. I was thinking of the 16x7 RS wheels at +53mm, but your examples are actually more cogent because youre limiting it to one platform, New Age Imprezas. Thats important too because all Subaru platforms dont run exactly the same offset, but luckily for us most are very close.

ss

edit: and increasing the track can be a good thing as long as you dont get to extreme with offset reduction and get into that bearing damage range. Another way you could increase your track is to get a set of the STi wheels with higher width but the same offset. If you have the clearance and more importantly the $$$$$.
subysouth is offline   Reply With Quote
Old 07-04-2003, 05:42 AM   #12
GravelRash
Scooby Specialist
 
Member#: 16318
Join Date: Mar 2002
Chapter/Region: NWIC
Location: Kirkland, WA
Vehicle:
2002 WRX wagon
Grime over Sedona Red

Lightbulb Excellent thread!

In fact, I wish I'd seen this thread before I spent the last half hour pretty much duplicating its entire content composing a post to the stick Offset thread (just posted a link to this thread instead ).

Bridrive: I hadn't actually thought to analyze it from the load distribution point of view. But I entirely agree with you - at least where wheel bearing issues are concerned. (And thanks to Heather for noting SCCs 50k wheel bearing issue!)

However I do have one technical disagreement with looking at this solely from the load distribution pov (ignoring those pesky clearances for the moment). If you manage offset to maintain exactly the same load ratio then indeed the offset will change slightly as wheel width changes, as you note. The problem with this is that you're also moving the centerline of the contact patch relative to the suspension parameters, affecting behavior like bump steer, scrub radius effects, etc. Of course, the opposite is also true: if you maintain suspension parity by not changing offset as wheel width changes then you change the bearing loading...

But all of that is unfortunately completely overshadowed by the far greater offset changes mandated by clearance issues, as you've analyzed so nicely... To paraphrase, "how wide can you afford to go" given the effects on wheel bearing life?

And, more subtly, how much tradeoff are you making in compromising the suspension design vs. what you gain from the wider tire / lower profile (stiffer sidewall)? That would be a very interesting experiment: thorough - measured - testing with stock wheel with tire model "A", then slap on a much wider wheel with an equivalently wider tire "A" and repeat the same tests.
GravelRash is offline   Reply With Quote
Old 07-04-2003, 09:27 AM   #13
Heather
Scooby Specialist
 
Member#: 11627
Join Date: Oct 2001
Location: Chico, CA
Default Re: Excellent thread!

Quote:
Originally posted by GravelRash

PART 1) If you maintain suspension parity by not changing offset as wheel width changes then you change the bearing loading...

PART 2) And, more subtly, how much tradeoff are you making in compromising the suspension design vs. what you gain from the wider tire / lower profile (stiffer sidewall)? That would be a very interesting experiment: thorough - measured - testing with stock wheel with tire model "A", then slap on a much wider wheel with an equivalently wider tire "A" and repeat the same tests.
Incorrect in PART 1) When you change the offset to a smaller value then 55mm you move the center of the wheel out further from the center of the bearing brace/axle. This is called a Moment Lever: That is force times length (your moment arm) equals moment. The further you move the wheel out towards the fender, the more load you are putting on the bearings and steering linkage. Therefore mantaining the correct 55mm offset, regardless of wheel width, is ideal. But this not take into account fender rubbing and strut rubbing, but 55mm is ideal for any wheel width for our wheel bearings and steering rack.

PART 2) That's my life's work! If I only had $2,000 for OZ SL's or Subaru UK 17" wheels with the same tires I have now, Michelin Pilot Sport A/S, in 225/45/17 I'd test this like no tommorrow

Heather
Heather is offline   Reply With Quote
Old 07-04-2003, 10:20 AM   #14
subysouth
Scooby Guru
 
Member#: 5039
Join Date: Mar 2001
Chapter/Region: South East
Location: Ocean Springs, MS
Vehicle:
2007 Outback XT
Grey 5-speed

Default Re: Re: Excellent thread!

Quote:
Originally posted by Heather


Incorrect in PART 1) When you change the offset to a smaller value then 55mm you move the center of the wheel out further from the center of the bearing brace/axle. This is called a Moment Lever: That is force times length (your moment arm) equals moment. The further you move the wheel out towards the fender, the more load you are putting on the bearings and steering linkage. Therefore mantaining the correct 55mm offset, regardless of wheel width, is ideal. But this not take into account fender rubbing and strut rubbing, but 55mm is ideal for any wheel width for our wheel bearings and steering rack.

PART 2) That's my life's work! If I only had $2,000 for OZ SL's or Subaru UK 17" wheels with the same tires I have now, Michelin Pilot Sport A/S, in 225/45/17 I'd test this like no tommorrow

Heather
Uh oh Heather has found her stride as a specialist I see. That is exactly correct.

I think maybe a good question to pose here to clarify things is, what is the reason for the offset to begin with? If you know the reason for that it will become clear why it is more important for offset to be maintained no matter how wide the wheel width gets.

However clearance again rears its head and the question does become how much can I safely move off that fixed amount to get what I want in wheel/tire and track width but not wreck the bearings or my suspension response.

Bri your results are correct if you are saying that +51mm on a 7.5 wide tire would be "better" than say a +45mm. But a +53mm would be even better. Maintaining near stock offset(whatever that may be) is the best solution. We cant do that all the time in the real world so the question becomes, how far off ideal can we move?

So long and short your advice is on the right track, in that you are suggesting higher offsets than some wheels out there, but for the wrong reasons. Think about why we have offset wheels and why, once that "ideal" number is established, its "idealness" is non-negotiable.

ss
subysouth is offline   Reply With Quote
Old 07-04-2003, 10:55 AM   #15
Heather
Scooby Specialist
 
Member#: 11627
Join Date: Oct 2001
Location: Chico, CA
Lightbulb We are all on the same page...

Okay now that we all agree the Subaru offsets are more ideal for our cars, where are the 17 x 7.5" wheels with the 53mm offsets?!
Our only choices, that I know of, are STi wheels @ $2,800 a set! Then there's the 17 x 7" 53mm or 55mm offset wheels to chose from either the BBS Subaru wheels, again way too expensive @ $2,600 a set, or the UK 17" wheels (which I like) @ $1,200 a set- better but still not cheap enough! That's it folks- unless anyone knows any other wheels in the correct offset that I am not aware of? Please do tell!

We need Manufacturer's to start making Subaru offset wheels with the Subaru offsets! Toyota and Honda are both somewhere near the 40mm offset range- hence so many 40mm offset wheels.
May OZ be the first to step up to the plate here and make the OZ SL's in 17 x 7 with a 53mm offset? Please

Heather
Heather is offline   Reply With Quote
Old 07-04-2003, 01:15 PM   #16
Chuck H
Scooby Specialist
 
Member#: 20442
Join Date: Jun 2002
Chapter/Region: NESIC
Location: Chelmsford, MA
Vehicle:
KTR tuned 2003 WRX
VF34 equipped, 270 whp

Default

It's an interesting discussion, but has anyone stopped to consider that 55mm may not be the "ideal" offset as far as wheel bearing life, and might just have been used because it fits with a variety of wheel widths on a variety of Impreza models?

You can do all sorts of math to determine the "ideal" offset for upgrading wheels, but real world experience tends to say that you can go with 5-10mm "less" offset with no adverse affect on the wheel bearings, assuming that you don't have rubbing issues. Anything more than that and you're really starting to mess up the suspension geometry, though.
Chuck H is offline   Reply With Quote
Old 07-04-2003, 04:30 PM   #17
Heather
Scooby Specialist
 
Member#: 11627
Join Date: Oct 2001
Location: Chico, CA
Default

Yes I knew we would here the other side soon enough. I agree there are hundreds of Rota 17 x 7.5" 48mm offset users on this board that have no apparent problems- who knows down the road? Real world experience over several years of use. They get the benefit of a wider track, a wider wheel and a good strong, reasonably light wheel that is 1/2 the price of anything Subaru sells. If I weren't so caught up on the math I'd try them myself! Rota Tarmacs in grey look just about as good as OZ SL Even Sports Compact Car magazine uses Rota wheels on their project WRX, but like I said, after 50K miles their wheel bearing and their steering rack bushings were shot.
Heather
Heather is offline   Reply With Quote
Old 07-04-2003, 05:06 PM   #18
Dr Ken
Scooby Specialist
 
Member#: 5885
Join Date: Apr 2001
Chapter/Region: NWIC
Location: Portland Oregon
Vehicle:
2000 RS (GME)
SilvAr

Default

At risk of sounding redundant, Cobb's Wheels are 17x7.5 ET+53: clicky
Dr Ken is offline   Reply With Quote
Old 07-04-2003, 05:36 PM   #19
GravelRash
Scooby Specialist
 
Member#: 16318
Join Date: Mar 2002
Chapter/Region: NWIC
Location: Kirkland, WA
Vehicle:
2002 WRX wagon
Grime over Sedona Red

Default Re: Re: Excellent thread!

Quote:
Originally posted by Heather

Incorrect in PART 1) When you change the offset to a smaller value then 55mm you move the center of the wheel out further from the center of the bearing brace/axle. This is called a Moment Lever: That is force times length (your moment arm) equals moment. The further you move the wheel out towards the fender, the more load you are putting on the bearings and steering linkage. Therefore mantaining the correct 55mm offset, regardless of wheel width, is ideal. But this not take into account fender rubbing and strut rubbing, but 55mm is ideal for any wheel width for our wheel bearings and steering rack.

Heather
...which is exactly the way I was thinking before Bridrive popped up And ya know what? You're right, and I shouldn't have been trying to think so hard at that hour . For bearing loads you're exactly correct, it's pure moment arm.

But what Bridrive is showing is that once you move away from 0 offset the percentage loading within the wheel varies with wheel width as you keep offset constant. But those forces are completely contained within the wheel, and that issue should be solely in the wheel manufacturer's lap to deal with, and those internal load changes will have no effect on bearing loads or suspension action.

Thanks Heather for prompting the rethink while I'm awake

Note however that there are two more factors that can affect both bearing loads and suspension reaction in dynamic scenarios, e.g. steering, cornering, and suspension compression/extension.
(Numbered 3 and 4 to distinguish from the aforementioned 1 and 2 )

3) Bearing loads: the design and construction of the tire may be such that under side loads the center of force from the contact patch may not remain in the center of the wheel - in fact it's quite likely that it doesn't - and that factor will vary among tire models in the same size, let alone as tread width changes. In effect this changes the moment arm on the bearings, and the change will be greatest at the same time the bearing is experiencing max cornering forces.

4) Suspension reaction: this effect is closely analogous to the effects you get with large static camber changes, e.g. dial in lots of negative camber to help max cornering and, especially with strut suspensions, you'll almost certainly have very noticeable changes in straight line stability, tram lining, bump steer, etc. because the tires are now riding on their inside edges while running straight down the road. (Never mind tire wear ) At this point the center of force from the contact patch is far away from the center of the wheel (again changing that moment arm on the bearings...).
...
You can experience the same effect in dynamic load scenarios, though more subtly, just from changing the wheel width - but not changing the offset. Think of running with a 4" tread width (appropriate wheel width) vs. a 12" tread width. In straight running, essentially no difference. But as cornering loads are fed through the contact patch - both side load and vertical load, and the suspension moves under the vertical load the geometry of the contact patch will change relative to the road surface, e.g. moving the center of force away from wheel center. And the amount of that movement will quite likely be greater with the 12" tread vs. teh 4" tread. So now you're changing not only the moment arm on the bearings, but also both the moment arm and direction of force being fed into the suspension.
...
For example, suppose that the suspension is set up to increase negative camber as it compresses - compensating for both body roll and tire deflection. If it was designed with the 4" tread in mind it's quite likely that the force center from the 12" tread will be in a much different place, feeding very different loads into the suspension. On a strut suspension this could result in greatly increased bending loads fed into the strut, different directional loads into bushings, and so on. It will also effectively change the suspension geometry relative to the designed scrub radius, bump steer characteristics, etc.

Ahhh...but there may be a happy accident here! Decreasing wheel offset - for clearance issues - as wheel width increases will most likely diminish, or even cancel, these dynamic changes in force center location. But there are still those pesky static / straight running bearing load issues...oh what to do???

Suspension engineer == job security. Heather, your dream job awaits; just make sure the benefits cover therapy
GravelRash is offline   Reply With Quote
Old 07-04-2003, 05:50 PM   #20
Chuck H
Scooby Specialist
 
Member#: 20442
Join Date: Jun 2002
Chapter/Region: NESIC
Location: Chelmsford, MA
Vehicle:
KTR tuned 2003 WRX
VF34 equipped, 270 whp

Default

I haven't read the article in SCC, but I suspect that running much wider, stickier tires is going to wear your wheel bearings and steering bushings much faster than stock, regardless of offset. The better tires allow you to put MUCH higher cornering loads on the suspension, which pretty much has to wear everything faster.

I'm not trying to say that there's anything wrong with sticking with the stock offset. It was obviously designed for a reason. But of all the things to worry about when upgrading, I wouldn't lose any sleep over a couple of mm difference.

Whenever you increase your cars performance, you have to trade off the additional wear and tear against the additional enjoyment you get out of it.
Chuck H is offline   Reply With Quote
Old 07-04-2003, 07:41 PM   #21
Heather
Scooby Specialist
 
Member#: 11627
Join Date: Oct 2001
Location: Chico, CA
Thumbs down

Quote:
Originally posted by Dr Ken
At risk of sounding redundant, Cobb's Wheels are 17x7.5 ET+53: clicky
Beautiful wheels but way too heavy! Yikes!! If I were to increase my wheels to 17", I'd insist on the same or less weight wheels. In suspension theory- Unsprung weight is everything (and I have proof- http://forums.nasioc.com/forums/show...hreadid=384044 ) and 225/45/17 tires almost always weigh 1 pound more then stock size- 215/45/17 is equal in weight.

ChuckH- The SCC article did not even point to the incorrect offset as the reason the wheel bearings and steering rack bushing went out, so I have no proof there. They were using Toyo T1-S tires. I wish I had your attitude about wheels but I really am too caught-up in the math and all the theories to risk it.

Gravelrash- Now you are making my brain hurt I only think clearly in the early mornings when my 3-year-old is still asleep. By now- 4:30- I am shot (7-month's pregnant). I will though re-read what you had to say tomorrow morning I promise. Suspension Engineer huh? Not me! The Moment Arm stuff was a quote from my husband. I had no idea what he was talking about My understanding of suspension is at a entry level, but I do have a passion for it and a strong interest I adore the book How To Make Your Car Handle by Fred Puhn and the MRT WRX in Training manual. Must haves!

All I really want to know is can I buy the OZ Superleggra wheels with 225/45/17 Pilot Sport A/S tires without any fender rubbing or any other issues that could damage my baby WRX wagon? She'd look so good in OZ SL's in grey- the grey that starts with the letter A that I can not pronounce (sounds like a bug).

Heather
Heather is offline   Reply With Quote
Old 07-04-2003, 11:23 PM   #22
BriDrive
Scooby Specialist
 
Member#: 21801
Join Date: Jul 2002
Chapter/Region: South East
Location: 90 Degrees North
Vehicle:
2013 Jacked Up T4R

Default

Everyone....Cool....I'm glad that I'm not the only one at least thinking about this topic.....ChuckH, you are right in many ways...A few mm here, a few there just 'ain't' going to effect most people....I would urge Subysouth and Heather to at least duplicate my paper strips example in the original post. Make strips for any width wheel you desire, mark the centerline and the offsets with dotted lines...Of most interest to me would be for you to create a 6.5" wheel,a 7" and a 7.5" wheel strip and give them all a 53mm (or 55mm,you pick) offset. Then line 'em' up on a perpendicular plane. My math will support the ratio changes. Also it will plainly demonstrate the flaw in your moment arm quote if the offset is the same for each wheel. Wheel Width directly EFFECTS OFFSET...Sorry, didn't mean to shout... Subysouth: Your last questions of the post you made"...empirical data...principles..."resolves me to the fact that I am just no good at trying to demonstrate a point....We can dispose of the "Optimal Compromise" calculations...they weren't intended to be applicable laws...simply formulas that average the need to go out versus the need to stay in...Heather, I've been wanting to say this for a long long time....Just Get the darn OZ's...Use 215/45/17's (pilotsport or poleposition) ...they work tremendous, look good....and my only problem is I can't get Azenis in this width for autocross...If you are not road racing or autocrossing every other week for two years solid, 48's are not going to effect your wheel bearings...trust me, I'm the guy who started this thread...Happy and Safe Independence day Weekend to all of you, my fellow Americans.....BriDrive
BriDrive is offline   Reply With Quote
Old 07-05-2003, 12:16 AM   #23
subysouth
Scooby Guru
 
Member#: 5039
Join Date: Mar 2001
Chapter/Region: South East
Location: Ocean Springs, MS
Vehicle:
2007 Outback XT
Grey 5-speed

Default

Quote:
Originally posted by BriDrive
Of most interest to me would be for you to create a 6.5" wheel,a 7" and a 7.5" wheel strip and give them all a 53mm (or 55mm,you pick) offset. Then line 'em' up on a perpendicular plane. My math will support the ratio changes. Also it will plainly demonstrate the flaw in your moment arm quote if the offset is the same for each wheel. Wheel Width directly EFFECTS OFFSET...
Hmmmm put as simply and kindly as possible - youre wrong Bri. Wheel width does not effect offset. Offset is designed in by the wheel manufacturer. If you are saying wheel width SHOULD affect offset please explain.

To demonstrate the lack of importance of that parameter lets use an age analogy. Ann is 6 years older than Tim. At twelve tears of age, Ann is 100% older than Tim. At 18 Ann is only 50% older than Tim. Because Ann is now only 50% older than Tim versus when she was 100% older, does not affect the fact that she is still only precisely 6 years older?

You have demonstrated the discontinuity created by increasing wheel width and then comparing the ratios. You believe this is an important parameter, I do not. It does not affect or have anything to do with the reason for offset to begin with. An ideal offset at 6" is still going to be the ideal offset at 12."

Why do wheels have (x) offset to begin with? Think about it, youll get it.

ss

edit: And there is no flaw in Heather's moment arm assessment, the wheels offset is inherently linked to its behavior as a moment arm. Thats a big hint.

Last edited by subysouth; 07-05-2003 at 12:21 AM.
subysouth is offline   Reply With Quote
Old 07-05-2003, 08:23 AM   #24
aspera
Scooby Specialist
 
Member#: 7879
Join Date: Jun 2001
Location: east of Zzyzx
Default

I have some offset questions.

Why is offset refered to as ET? What do the "E" and the "T" stand for?

Why is getting offset down to the millimeter so important? When backspacing was common you only had to get within about a half an inch or so.

What kind of driving damages wheel bearings more:
A. driving long distances on the interstate?
B. autocrossing for a few minutes?
C. driving on rough roads regularly?

Why is the Subaru offset so large compared to other carmakers? I think it has more to do with counteracting the front wheels toe-in under power than anything else. The front control arms flex forward and toe-in under power, while the high offset wheels steer outward (toe out). Some of the play in the bushings gets cancelled and the car feels tight.
aspera is offline   Reply With Quote
Old 07-05-2003, 09:28 AM   #25
Heather
Scooby Specialist
 
Member#: 11627
Join Date: Oct 2001
Location: Chico, CA
Question wait a minute...

Quote:
Originally posted by subysouth
Another way you could increase your track is to get a set of the STi wheels with higher width but the same offset.
Wait a minute. The way I understand track it is the distance from the center line of the front wheel to the center line to the other front wheel. If the offset is maintained at 55mm, regardless of the width of the wheel and tire, the track will also remain constant and therefore unchanged.

The wheelbearing issue boils down to this: Some cars like old Fords and GMs you can change the offset of the wheels up to +/- 1 inch with no adverse effects because the wheel bearings and live axle bearings are over-engineered and can take the additional moment stress, according to the book How To Make Your Car Handle. Whereas Subaru wheel bearings are not over-engineered and can not take much additional load, according to the MRT WRX in Training book, "Subaru wheel bearing are very weak."

Note: Steering linkage is another matter all together which is also adversely effected by incorrect offsets and tracks.

How far away from the stock offset we can go before the wheel bearing and steering linkage are adversely affected is unknown. There are no studies done, that I know of, on this issue (other then folklore on this club ). So to venture away from the Subaru specific offset is to take a risk that may, in the near future, cost you thousands of dollars in repair bills. Without more solid data on this topic, I am not willing to go there. I will wait for 53mm or 55mm offset light aftermarket wheels come to market.

Heather
Heather is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
wheel offset difference, which way to go in this situation? irish44j Autocross 5 11-22-2009 11:55 AM
Considering this EVO, good idea on this particular car? MF-DIF Off-Topic 19 03-04-2008 03:39 PM
Anyone know the offset of this wheel? subysouth Tire & Wheel 9 08-28-2005 10:48 PM
what determine wheel offset? can I take vw wheels and rotors? etam Brakes, Steering & Suspension 9 05-27-2002 02:32 PM
WRX wheel offset w/body kit, does this look funny to u? JayGold Newbies & FAQs 1 06-13-2001 08:46 PM


All times are GMT -4. The time now is 04:15 PM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.