Wheel Offset - Effects on Steering & Handling

[\dave]

I'm adding a lighter wheelset (18x8 OZ wheels (likely) with 235/50 Pilot Sport 4S - dropping between 9 to 11 pounds per corner over my stock setup). My stock wheels and tires are the same size as the lighter set above, and my stock wheels have a +40 mm offset. The OZ wheels are available in either a +35 mm or +45 mm offset, but not 40.

Both OZ offsets fit my car, but I have not located much information on the steering and handling effects of offset changes, except one article that recommends not straying beyond a 5 mm change without considering effects on alignment/suspension. I don't want to make any changes to the car as I will swap between my two wheel sets. Does anyone know how handling would change with either of the two 5 mm offset changes? Also, would it be feasible to go with +45 and a 5 mm spacer to achieve +40 (the wheels come with hubcentric hub rings)?

Conversely, anyone have an alternate up to 21 lb. 18x8 +40 wheel recommendation which is TUV certified?

Thanks in advance.

Dave

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[Fly2High]

You need to look up track width changes. Adding or removing offset is affecting the track width (the distance from the center of one tire to the other on the same axel).

You will find a lot.

An alternative is to bring the +35 offset to a machine shop and have them cut the 5mm off

 

[boB]

https://en.wikipedia.org/wiki/Scrub_radius

 

[Zathras]

I kinda hate to bring up anything that might complicate your situation, but I was wondering if you considered changing from the 235/50 tire size to something like a 245/45. I would think that would still fit fine on the 18x8 wheel, and bring you a smidgen more grip. (Downside being the speedometer would now be off, but only by a little bit. And probably a slightly harder ride.)

 

[Dana Pants]

An alternative is to bring the +35 offset to a machine shop and have them cut the 5mm off

having personality machines a set of wheels to change offset, I can suggest that taking 5 mm off is a horrible idea. The meat of the wheel was something like 7 mm at the base of the lug nut cone before machining.

 

[Fly2High]

Another thing you can do is:
1. Get the 45mm offset and get 5 mm spacers. Spacers are negative offset. 5 mm spacer will move the wheel outward so this is -5mm of offset. This will result in 40mm of positive offset. If you do not understand what offset is, please search and read. Offset is the distance of the wheel mounting surface from the center line of the wheel. A positive offset moves that mounting surface toward the outside while a negative moves it in towards the wheel well. If positive moves the mounting surface outward, that will make the wheel and tire sit further into the well. This is why you can think of spacer as negative offset.
2. Pick a different set of wheels with your desired offset.

Note: bigger spacers WILL require longer studs. Make sure the number of threads exposed are at least as long as the width of the stud.

Changing offset will:
Affect scrub radius
Affect tire wear
Affect steering effort
Change spring and sway bar stiffness
Alter track width

The reason are explained in part in these links. Some good reading below :

Scrub radius
https://www.moderntiredealer.com/ar...-customer-understand-why-wheel-offset-matters

Changing track width by changing offset

http://forums.pelicanparts.com/pors...ct-difference-wheel-track-width-handling.html

https://trackmustangsonline.com/threads/getting-a-wider-track-width-worth-it.6891/

 

[\dave]

Thanks guys. Looks like the approach is - don't mess with offset at all if you aren't willing to make other adjustments. That answers my concern. It's interesting that offset relative to steering and handling is seldom discussed, from what I've seen, when wheel swaps are talked about in forums - and I've been looking extensively lately. Discussion seems to mostly just center around clearance, and then occasionally realignment due to wider tires...

Zathras - Yep, I considered 245/45, but several factors drive me to stay with stock dimensions: more sidewall (8 mm more) to protect lighter rims; I like the ride characteristics that I currently have; more sidewall is supposed to give more warning before breaking away on a curve (I drive on curvy country roads); I'm going to a better traction tire (to Pilot Sport from stock P Zero Nero all season) and I'm confident that the added traction will more than serve my needs.

Here's something annoying and disappointing that I just ran into. Tire Rack lists the Pilot Sport 4S in 235/50-18 as a 25 lb tire, which in part was driving me toward the PS4S. I just found on the Michelin site that this tire is about 27.5 lbs. I went back and scanned the Tire Rack PS4S table and saw a couple of suspicious data entries. Regarding wheels, the weights of wheels are hard to find, but Tire Rack posts them. I wonder if Tire Racks stated wheel weights are reliable. Any experience out there?

Thanks again,
Dave

 

[Ewheels]

This is the main result of changing wheel offsets.
Factory cars are usually on the negative side because it is safer (safer because your average driver is an idiot). Going with the 35mm offset will be fine, improve your car's turn in, and give you a heavier steering feel; all good things.

 

[\dave]

Eric,

Do you know if the S550 comes with a positive scrub radius? If so, do you know how much? I tried chalking the tires but my results were inconclusive. Also, I can't find anything specific or consistent. I've also found as stated above that negative is generally used for safety now, but also found that (paraphrased) "most cars including rear drive are now being built with positive scrub angles."

Thanks,
Dave

This is the main result of changing wheel offsets.
Factory cars are usually on the negative side because it is safer (safer because your average driver is an idiot). Going with the 35mm offset will be fine, improve your car's turn in, and give you a heavier steering feel; all good things.

 

[Ewheels]

Hi Dave,

I do not know for certain what the factory scrub radius is on the S550. Your typical economy car will FOR SURE have negative (the car will track straight in undesired road conditions) but manufacturers may design "sporty" cars with some positive scrub to give the car better turn in because they know the target audience of that car would prefer it that way.
Without actual numerical values, I can only speak from experience. I run 285/35/19 ET35mm square on the track and it feels great. I've driven this setup on the streets only once and it felt as though the steering was more immediate and direct but that could also be from the wider tire profile.

Hope this helps

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