Yet another offset question

[DonDavis]

I've read, I've browsed, and I've searched. I've gathered from many posts that:

a.) There seems to be a pretty large variation of what offset works for our cars for a given wheel width and tire width.

b.) There seems to be a good number of people who report positive results (or at least no negative ones) with varying offsets.

c.) There seems to be no hard and fast rule as to "what works."

From this I conclude:

a.) I'm confused, because I like rules and having things clear-cut when it comes to stuff like this.

b.) In trying to come up with an internal guideline for myself in choosing a set of aftermarket wheels and tires, I was wondering if this sounds correct...

As a general guide, should the offset should be reduced by 1/2 the increase in tire or wheel width? In other words, if stock rear offset on a '91 is +60mm and stock tire width is 225 and I wish to upgrade to 255 should my ideal offset be somewhere between +40mm - +45mm on 9" wheels?

And at what point (wheel and tire width with a given offset) do certain combinations begin to rub in front at stock ride height? Any kind of general rule there?

At what point and in what way will too much offset affect handling?

Still scratching my head...
Don

 

[2slow2speed]

The shape of the spokes is a *very* important factor as well.

For example there have been cases where a wheel that fits a 97+ OEM brakes will fail to clear the earlier models brakes because the rotors are smaller which in turn means that the caliper location is closer to the center of the wheel causing interferance with the spokes of the wheels.

Ken

 

[nsxtasy]

As a general guide, should the offset should be reduced by 1/2 the increase in tire or wheel width?

I'm sure you mean wheel width, not tire width. What this will do - and I'm sure you're aware of this, in coming up with this formula - is, it will keep the inner face of the wheel in the same location (plane) while moving the outer face outwards. This will prevent interference by not moving the inner face inwards. However, if the outer edge of the wheel/tire moves outwards, that can also result in a tire rubbing problem, particularly if your tire outer diameter is larger than stock. That (in addition to the spoke interference which Ken mentioned, which obviously is going to be different for different wheels) is why these "rules" are not hard and fast.

 

[DonDavis]

OK, I can live with the "greyness" of the topic, I guess. (My major beef at the moment being that I don't have $3k-$4k to plunk down on some really cool sneakers right now.)

Ken, hmmm.

Let me ask this, then. Given that by putting wider aftermarket wheels on the car we are always, to some extent, changing the suspension geometry, how can you know what's too far off from stock in terms of handling and performance? Are we fairly safe in assuming that any extra wheel width (say a change from 8" to 9") should be "taken from" the positive offset, so that the innermost position of the wheel - and therefore tire - remains unchanged in relation to the suspension (i.e., the extra inch would be towards the outside of the car). By this logic, for a stock '91 rear wheel, that extra inch in wheel width would then reduce the positive offset from 60mm to somewhere around 35mm? Or would a better goal be to split the difference and keep the wheel centered in relation to where a stock wheel would be? Or should I just shut my pie hole and keep my stock wheels which aesthetically are not eye candy but perform pretty damn well?

Those are important factors to me and what "works" for one driver may not be what another driver wants. I'm having a hard time figuring out any kind of guidelines that will help me choose wheels that look good, don't break the bank, won't have negative effects on the handling of my dream car and oh, yeah, that fit, don't rub, and clear the calipers.

Don

 

[nsxtasy]

Those are important factors to me and what "works" for one driver may not be what another driver wants. I'm having a hard time figuring out any kind of guidelines that will help me choose wheels that look good, don't break the bank, won't have negative effects on the handling of my dream car and oh, yeah, that fit, don't rub, and clear the calipers.

The problem is that most of what you are asking is very subjective. One person will claim that wider wheels and tires improve handling, another will claim that there is no difference, and another will claim that handling is degraded. Same thing for qualities like "look good", "don't break the bank", etc. So there is no absolutely correct answer to your question.

Incidentally, with regard to your question about widening the wheels while keeping them centered (meaning, maintaining the same offset), that will move the inner face of the rim inwards, which may (or may not) create a rubbing problem as a result.

A few personal opinions, which you or others may or may not agree with:

- Performance differences are far greater between different types (models/brands) of tires than between different sizes of the same tire

- Replace wheels primarily for looks, rather than for performance gains

 

[DonDavis]

Ken, thanks for your time and insight on an otherwise over-discussed topic!

Don