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Old 10-19-2004, 02:38 AM   #201
Plague
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Quote:
Originally Posted by DaddyMAM
Does anyone know the offset(factory) for an 05 STI ?I know there are some changes in the '05(including the change in bolt pattern)which leads me to believe offset could have been changed as well.I'd check if I had the wheels to look at,but I'm still a couple days off from picking it up from the dealer (it hasn't been delivered yet and there aren't any at the showroom to look at). I'm SO friggin excited!
Factory offset is 53mm. It doesn't jive with the beginning of this thread regarding acceptable offset for 17x8's, but that's what is.
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Old 10-19-2004, 09:46 AM   #202
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Quote:
Originally Posted by Plague
Factory offset is 53mm. It doesn't jive with the beginning of this thread regarding acceptable offset for 17x8's, but that's what is.
.....judging by the date the thread was started I don't think the STi was taken into account.
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Old 10-19-2004, 02:12 PM   #203
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I can see that would be an issue. I was hung up on all USDM WRX and STI wheels having +53mm and therefore thought that it should be 53mm regardless of width. Now that I actually put thought into it, I see that wheel bearing happiness seems to be based on the ratio of load inboard of the mounting flange vs. load outboard. It would make sense that offset should change based on wheel width. Sorry that I sounded flippant.
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Old 10-19-2004, 03:51 PM   #204
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Plague - you were right. If you want to maintain the manufacturer's steering geometry, you keep the same offset regardless. The wheel bearing issue is much less important - a few mm either way will not make much (if any) difference to bearing life, whereas such a change can affect your steering response, castor return, etc. As one who used to design this stuff for a living, I can tell you we would set the geometry we wanted, than package the components including the bearings around the geometry.
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Old 10-19-2004, 03:56 PM   #205
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Quote:
Originally Posted by Plague
I can see that would be an issue. I was hung up on all USDM WRX and STI wheels having +53mm and therefore thought that it should be 53mm regardless of width. Now that I actually put thought into it, I see that wheel bearing happiness seems to be based on the ratio of load inboard of the mounting flange vs. load outboard. It would make sense that offset should change based on wheel width. Sorry that I sounded flippant.
You didn't sound flippant at all.

btw, not to argue, but you were right to think the offset should remain the same in an ideal situation regardless of the width. However, I think the reason he came up with the list is because of the availabilty of various wheels on the market (our specs are relatively un-popular) and clearance issues. The wider you go, the less likely you are able to stick with the stock offset due to rubbing on the inboard surfaces. His post was his thoughts on what is a fair compromise to the stock specifications.

EDIT: Looks like Roger got his post in while I was typing mine.
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Old 10-19-2004, 05:29 PM   #206
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Google "negative scrub radius" to find a lot of articles on this issue - including a ref. to the 05 Legacy line. In the very old days, people went for centerpoint steering for lightness (pre power steering), i.e. virtual "kingpin" line (see my earlier post for how this applies to strut suspension) went through the tire centerline - easy to do with solid front axles and scads of positive camber - which is how vintage cars were. Along with that, btw, came "wheel wobble" which your grandparents would have been familiar with. Once IFS became universal, center point was much harder to achieve, and with SLA (aka double wishbone) front suspensions, positive scrub radius became the norm - this means that the king pin line is inside the tire centerline. In the case of differential braking (i.e. one wheel on high grip surface, one on low) such a geometry tends to pull the steering toward the high grip surface. The point of negative scrub geometry (todays thinking), where the king pin line is outside the tire centerline, is that it gets rid of steering pull under braking (and for FWD/AWD, helps under acceleration.

When you mess with this scrub radius, as you do when you change the wheel offset from the design value, you mess with the steering feel and handling under heavy braking or heavy acceleration conditions. I don't know how to say this politely, but compared to this geometric change, the variation in bearing load is completely and utterly irrelevant.
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Old 10-19-2004, 06:12 PM   #207
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Thanks for the fresh input rogerd. From what I've read centerpoint steering is the ideal. I've seen info on the web about old, old Subarus using this design. Modern Subarus seem to use such a large wheel offset to try and keep as close to this ideal as possible, while using a modern strut assembly.

Oddly, SLA design was used the most when wheels were 14-15 inches and had little offset. Now most wheels are 16-17 or more and have lots of offset, yet struts are used. Why not use all that room inside the wheel for a true centerpoint setup? Use PAX tires/wheels for even more room.

OMG! I just thought of a reason why dubs might actually be a good wheel size.
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Old 10-19-2004, 09:45 PM   #208
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Centerpoint tends to be unstable as it lacks a "pull" one way ot the other, hence the "wheel wobble" that used to occur. Some amount of scrub radius gets rid of that tendency. Both positive and negative have their believers. Pure FWD with positive scrub has a long history going back to the original Mini - one of the most precise handling vehicles of all time (much better than the BMW built "pretender"). Negative scrub is a comparatively recent trend, driven mostly by braking stability.
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Old 10-20-2004, 11:04 AM   #209
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Quote:
Originally Posted by rogerd
...In the very old days, people went for centerpoint steering for lightness (pre power steering), i.e. virtual "kingpin" line (see my earlier post for how this applies to strut suspension) went through the tire centerline...
I'm guessing that the kingpin line is perpendicular to the ground plane and is considered to bisect the tread width if the wheels are pointed straight ahead (edit: I just realised that it may not bisect the tires on my WRX as the kingpin line might be outboard of the whole tire when viewed from the front). Based on the last few posts, I'm reading that scrub radius is based solely on offset from the centerline regardless of wheel width. This is confusing to me because of this...

Let's say wheel width is 100 units, offset is +10 units. 40 units are now outboard of centerline and 60 are inboard. Increase wheel width to 120 units, offset is still +10 units. Now there are 50 units outboard and 70 inboard. The ratio is now changed - but the scrub radius hasn't? And the car will handle basically the same in regards to turning and stability under braking?

Last edited by Plague; 10-20-2004 at 11:20 AM.
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Old 10-20-2004, 10:51 PM   #210
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Plague: It does not matter how much is inboard or outboard of the wheel mounting face. What matters is where the centerline of the tire is relative to the suspension "joints" (top strut mount, bottom ball joint), as structurally the loads operate through the tire centerline, and are fed to the suspension mounting points. Remember that once the wheels are tightened, the wheel and the hub operate as if they were a single unit.

p.s. the "king pin" in McPherson strut suspension vehicles is virtual - from the front of the vehicle draw an imaginary line through the center of the top strut mount and center of the lower ball joint, and visualise it extended to the ground. It will lean in at the top, out at the bottom, and hit the ground outside the tire centerline. It's the amount of offset between this line and the centerline of the tire (scrub radius) that you need to maintain, if you want to keep the manufacturer's suspension geometry.

p.p.s Trivia - Earle McPherson (for whom McPherson struts are named) was a GM engineer - and for years Ford (esp. in Europe) used his strut design and GM cars did not. Now everyone does - even BMW.
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Old 10-21-2004, 05:45 PM   #211
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Rogerd,

Thanks for being patient with me. I posted without reading the whole thread.

Fortunately I read the thread and got to know a lot more about offset and suspension geometry and how wheel width does not effect geometry if the offset stays the same. I also learned, unexpectedly, what causes torque steer.

Now I think the guidelines in the first post (positive offset decreasing as width increases) could merely be a guide for wheel fitment - what the offset needs to be to keep your wheel/tire combo from hitting parts of your car. But, hold on, that's not true either - since a 17x7.5 +53mm STI wheel/tire will not hit anything on my '02 WRX. Edit: while going to get a soda, I realized that the first post could simply be a guidline to keeping the same backspacing as you increase wheel width. That makes a little more sense, but since a 17x7.5 +53mm fits fine, maybe you don't need to worry about backspacing until you get up to 8" of width. Maybe 8.5." Still, you're going to hit part of your car sooner or later unless you start making Cone Basher type body mods.

The guidelines seem to be there just to make for a huge ass thread.

The wheel bearing nirvana thing is just not true. Want to be good to your bearings? Stay at +53mm. The further you get away from +53 (+48, +45, +2!), the worse it gets for your bearings. But wait! I've also read that the bearings don't care much about 5-10mm less offset.

Here's the more important part - since the Dave Point and the tire centerline get closer together when you decrease offset, your handling will change. Whether you like the results or not is a matter of preference.

Good thread.

Last edited by Plague; 10-21-2004 at 06:04 PM.
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Old 10-21-2004, 08:58 PM   #212
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I plan on purchasing 17" x 8.5" +50 offset wheels with 255/40/17 tires for my STi.
I'm going to be using coilovers to gain clearance due to the increased wheel width, +50 offset, and 255mm tires. Is anyone running a comparable setup?
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Old 11-02-2004, 03:52 PM   #213
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I found this hope its helpful somehow::..
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Old 11-22-2004, 05:10 PM   #214
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Yea, there is nine pages to this forum and i do not have enough patience to go through it. My quiestion, i am looking to purchase Axis Spiders, 18x8 and the offset is +40, i understand that that is not in ur range, but the gentlement told me that my car 99 Impreza RS can handle andthing from a 38-42? It seemed to me he was reading this off of some sheet? Is his info valid or should i just get a different wheel.
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Old 12-11-2004, 05:45 PM   #215
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What do you think about the 5Zigen FN01R-C 17x8 +35 wheels?

Will this hurt the bearings?

How will be the track width affected?

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Old 12-13-2004, 02:34 AM   #216
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[quote=tharoka]What do you think about the 5Zigen FN01R-C 17x8 +35 wheels?
QUOTE]

I like them, but the name is too JDM. I mean, 5Z FN01R-C (17x8, +35) sounds like a part number or an algebra equation.
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Old 12-13-2004, 08:11 AM   #217
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Quote:
Originally Posted by aspera
Quote:
Originally Posted by tharoka
What do you think about the 5Zigen FN01R-C 17x8 +35 wheels?
I like them, but the name is too JDM. I mean, 5Z FN01R-C (17x8, +35) sounds like a part number or an algebra equation.
Ok, and what about the bearings and track width?
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Old 12-13-2004, 08:33 AM   #218
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+35 offset it the WRONG offset and the wrong wheel.....have you read/learned NOTHING here??????

+45 would be getting better and +48 better yet
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Old 12-13-2004, 01:19 PM   #219
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ok so quick question... from the diagram posted above it looks like the offset will effect the clearance that the brake caliper has. so my question is this... i recently received an e-mail from a prodrive usa rep basically saying they will be producing the GC-010Es with the 05 STi bp (114.3mm x 5) but the offset is going to be +46mm and not the +53mm... will there be any kind of clearance issues (not just brake clearance)
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Old 12-15-2004, 02:03 AM   #220
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Quote:
Originally Posted by twistedsymphony
I just began looking for new wheels for my car and of course I want the perfect size, offset and all...

so rather than ask around I decided to just educate myself on the topic...

after spending a few hours stairing at various ways to calculate offsets and diagrams of wheel measurments I dont understand why the offset changes with width

the offset is in place and shouldn't be changed as to not put an unusial constant stress on the berings. if the offset is the distance measured from the centerline of the wheel I assume it it such so that the berings (or stresses related to centering the berings) are located at the centerline. if this is so then the offset should never change when you increase the width of the wheel because if you never change the offset the weight (of the car) on the inside of the wheel will always be dispursed the same as the weight (of the car) on the outside

That is odd ! Looking at the picture, if the wheel off-set is measured from the centerline of the wheel then Zero offset is the best for the bearing. Because at zero off-set the centerline of the wheel is at a straight line with the bearing, and the bending force by the wheel to the bearing is at a minimum possible. I think clearance is the main issues here, not related to wheel bearing. Other Subaru product have smaller wheel off-set, but the WRX have higher off-set is just because it need to clear the bigger caliper. That means if it fit, a smaller wheel offset is actually better for the wheel bearing !! Am I missing something here?
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Old 12-15-2004, 05:21 AM   #221
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Quote:
Originally Posted by WRXBob
Am I missing something here?
I think you are. It sounds like you are associating the mounting surface of the wheel with the wheel bearing. If so, the wheel bearing would have to be very very thin. In fact, the brake rotor already takes up that space.

Ideally, somebody familiar with the actual WRX bearing would draw it on the diagram you just posted. That way we could see where it is and how big it is.
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Old 12-15-2004, 06:08 AM   #222
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Quote:
Originally Posted by Uncle Scotty
+35 offset it the WRONG offset and the wrong wheel.....have you read/learned NOTHING here??????

+45 would be getting better and +48 better yet
Dear Scotty,

I try to learn, hence the question. In the begining of the sticky tread "wheels that fit the STi", it was estated that the FN01R-C 17x8 +48 wheels fit the STi vs the 5zigen usa info who stated the oposite. The result was at least one member who bought wrong wheels following the sticky, and he was forced to use spacers.

Sorry if it does sound as dumb question, but since 5zigen USA stated in their aplication list the FN01R-C wheel +35 for the STi, I thinks that the question makes sense at least...
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Old 12-15-2004, 10:17 AM   #223
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Quote:
Originally Posted by tharoka
Dear Scotty,

I try to learn, hence the question. In the begining of the sticky tread "wheels that fit the STi", it was estated that the FN01R-C 17x8 +48 wheels fit the STi vs the 5zigen usa info who stated the oposite. The result was at least one member who bought wrong wheels following the sticky, and he was forced to use spacers.

Sorry if it does sound as dumb question, but since 5zigen USA stated in their aplication list the FN01R-C wheel +35 for the STi, I thinks that the question makes sense at least...
I think he was a little frustrated because the info you seek is already in this thread. ....although I still think he can be a little grouchy at times.

btw, if the other member bought wheels from 5zigen with the same offset you describe (+35), wheel spacers will only compound the problem. .....they will force the wheels "out" (make the offset lower). Not that I would recommend it, but you would have to remove material from the mounting surface of the wheel to raise the offset (move the wheel "in").
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Old 12-15-2004, 10:27 AM   #224
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Quote:
Originally Posted by WRXBob
That is odd ! Looking at the picture, if the wheel off-set is measured from the centerline of the wheel then Zero offset is the best for the bearing. Because at zero off-set the centerline of the wheel is at a straight line with the bearing, and the bending force by the wheel to the bearing is at a minimum possible. I think clearance is the main issues here, not related to wheel bearing. Other Subaru product have smaller wheel off-set, but the WRX have higher off-set is just because it need to clear the bigger caliper. That means if it fit, a smaller wheel offset is actually better for the wheel bearing !! Am I missing something here?
I won't claim to be an expert (because I'm definitely not), but I think the point is that the offset is factored into the design of various components and how they operate on the vehicle. Once you alter that, you change the affect on those parts and the driving characteristics of the car.
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Old 12-15-2004, 11:06 AM   #225
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Quote:
Originally Posted by Kean
btw, if the other member bought wheels from 5zigen with the same offset you describe (+35), wheel spacers will only compound the problem. .....they will force the wheels "out" (make the offset lower). Not that I would recommend it, but you would have to remove material from the mounting surface of the wheel to raise the offset (move the wheel "in").
Hi Kean, thanks for your answer.

Maybe its my speeling (or comprehension, my English knowledge is limited, sorry), but what I tryed to say is that what he bought were 17x8 +48 wheels.

And by the moment, I have not be able to find along this tread if 5zigen Usa has commit a mistake displaying the +35 wheels as an aplication for the STi or they are correct and no special damage will be caused to the bearings (of course, I asume there will not be clearance issues with the Brembos), which is the info I was looking for.

Last edited by tharoka; 12-15-2004 at 11:13 AM.
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