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Wheel Offset?


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9 replies to this topic

#1 blitz

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Posted 12 February 2005 - 10:26 PM

Example:

Stock wheel - 16" x 6.5 53mm offset
Wider wheel - 16" x 7.5 ? offset

Assuming that clearance is not a problem, does changing to a wider wheel require a corresponding change in offset?

If so, why?

#2 Setright

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Posted 13 February 2005 - 03:33 AM

Nice question, no room for heresay. We must answer "why"!

Well, fitting a wider wheel alone shouldn't give rise to "rubbing" problems between tyre and fender/suspension. If you fit a wider tyre too, then offset will become more important to avoid this aspect.

Ultimately, the offset should not change. This is because the suspension and not least the wheel bearing expect their loading to act through a certain point. Obviously, a different offset will alter this.

It is important to remember that the extra width of the wheel is evenly distributed about the center of the wheel. Therefore, it is also even about the offset plane, because that is measured from the center of the wheel.

So, assuming you are running a 205/50R16 tyre, and intend to do so on the new wheel too, then the offset should remain at 53mm.

If you are planning to go for a butch look, and fit 225/40R16 rubber, then you need to check how much clearance you have between present tyre and the suspension strut. The new tyre will stretch 10mm further inward. And outward, so watch the fender edge too. A 48mm offset would probably be good enough to compensate.


Personally, I would advise against fitting wider tyres. The ideal wheel width represents 85-90% of the tyre width. A 205 tyre will work best on a 7.0-7.5 inch wide wheel.

I run 195/60R15 Potenza 720'ies on a 15x6.5 wheel for summer. And 195/60R15 Blizzak LM18's on the stock alloy 15x6.0 in winter. Ideally, my winter wheels would also be 6.5inches wide, but the added "slack" provides less twitchy handling. This can be helpful on packed snow ;-)

Offset of my summer wheels is 50 vs the stock 55. This is not by choice, but this was the biggest offset available - unless you wanna pay for Prodrive wheels.

#3 blitz

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Posted 13 February 2005 - 03:49 PM

Setright, thanks for the detailed answer. You gave me what I needed to know which was basically that the offset needs to numerically stay the same in order to retain the steering balance.

The '00 OBS shoe-horns 205/60/15 onto the stock 15" x 6" alloys. :confused: I think I'll pick up a set of 16" x 6.5" WRX pull-offs for summer. That'll save bucks and retain the stock geometry.

#4 //-SCOTT-\\

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Posted 17 February 2005 - 02:21 PM

I know this is an important thread and one that may be referred to by others.
I, personally, dont agree that the wheel offset number will stay the same. It needs to change as the width changes for various reasons.
My buddy explained it best in the write up below, so I'm just going to paste it here for review, etc.




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!


#5 blitz

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Posted 17 February 2005 - 04:54 PM

Paul, thanks for that information. My main concern in posting this question was in regards to the effect that different offset would have on handling and steering response, but that variable's suspiciously absent in your info. Not important?

My intent wasn't to change the offset so much that it would have appreciable impact on wheel bearing life and the clearance thing I can work around by eyeball, so it comes down to handling. I'm anal about "improvements" I might make, ...actually being improvements. :D

#6 Setright

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Posted 18 February 2005 - 02:12 AM

I disagree with Scott.

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Posted 18 February 2005 - 06:42 AM

When I re-read everything, I think I agree with both of you.

This is why...

Your point is just correct. If you could go to a bigger wheel with the same offset and have no rubbing issues then great.

However, more often than not, an increase in wheel width may present inherhent rubbing issues. Then, you could take my buddies equations and find a compromisable offset to fit your needs.

OR....I'm still misunderstanding something here and causing lots of confusion.:banghead:

#8 Setright

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Posted 18 February 2005 - 10:38 AM

To clarify my point: The offset is in relation to the CENTER of the rim. Nevermind the absolute width, it's the offset that denotes the length of the lever that "carries" wheel forces into the hub.

#9 Porter

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Posted 18 February 2005 - 10:57 AM

The major factor that plays into wheel offset choices is not tire load per se, nor is it specifically clearance, though clearance is obviously necessary and must be the first consideration.

The primary focus in choosing an offset should be STEERING AXIS. Moving the contact patch in or out on the assembly affects the axis of steering greatly. The factory places the steering axis roughly in the center of the contact patch, perhaps a bit inside. Making adjustments to camber to the positive side (+55,+57) to make the tire load even will pull the steering center to the outside of the tire, causing very strange steering response. Making the offset lower, i.e. +48 when paired with a 7.5" width, keeps the steering center closer to the factory axis and preserves the steering response as designed.

I know of several autocrossers with STis that are running VERY low offsets (+10, +14) on very wide wheels (10", 11") and are not experiencing real-world tire load problems or steering axis issues.

Don't forget the impact that camber has on tire loading, and the change that occurs during cornering.

#10 blitz

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Posted 18 February 2005 - 11:06 AM

To clarify my point: The offset is in relation to the CENTER of the rim. Nevermind the absolute width, it's the offset that denotes the length of the lever that "carries" wheel forces into the hub.


I like the idea of optionally being able to express the offset as a percentage of wheel width. It gives a good indication of distributed forces (leverage) and the resulting influence on the wheel's directional path.

I can understand that the standard spec is given as a distance for reasons of calculating fitment and clearance, etc. This thread's really helped me to arrive at that intuitive grasp of exactly what I was hoping to find.

It's common in rural areas now, but years ago one of the unique aspects of living in a hip-hop city was witnessing the bizzarre vehicle wheel fitments for no other purpose than to make visual statments. I'm always fearful of driving alongside some of these vehicles because the handling is so obviously treacherous. That ain't my thing tho. My idea of modification is to keep the essential look of the vehicle while making incrimental functional improvements AND simultaneously being careful to minimize any downside. I'm a geek.




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