General Offset Question

[TTown]

If I understand offset correctly, the higher the number the more it will pull the wheel in toward the car. So, if that's true, is the reason that stock Ford mustang wheels sit so far inside the wheel-well is that it has too much offset? The corollary being that custom wheels tend to have lower offset to "push" the wheels out flush with the side of the car?

And, what is the manufacturer reason for having the wheels tucked in the wheel-well? Is it for handling and/or to keep wheel sling off the side of the car?

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[ronnieb1958]

If I understand offset correctly, the higher the number the more it will pull the wheel in toward the car. So, if that's true, is the reason that stock Ford mustang wheels sit so far inside the wheel-well is that it has too much offset? The corollary being that custom wheels tend to have lower offset to "push" the wheels out flush with the side of the car?

And, what is the manufacturer reason for having the wheels tucked in the wheel-well? Is it for handling and/or to keep wheel sling off the side of the car?

Good Question. The forum should have a dedicated thread for questions like this and for questions about lowering cars and the mods needed to maximize performance. Basically a novice primer.

 

[abbeynormal]

The stock offset is set to put the centerline of the wheel where Ford thinks it should be for things like scrub radius and wheel bearing loads. When you push the wheels outboard you mess with those things (probably in a negative way) as well as having the tires throwing more trash from the road onto the side of the car.

For the front, you don't have much choice but to start moving the wheel out if you want to go to a wider wheel. This is one of the disadvantages of a strut based suspension.

 

[TTown]

^^ Makes sense. Is it correct to say that stock wheels tend to have a higher positive offset than all of the after market wheels people are throwing on their cars to run wider tires?

 

[abbeynormal]

Yeap, that's what I've seen on these forums. I suspect most people don't care too much of the car starts tramlining a bit. My fox body 5.0 did this and I hated it. I love how stable my 2017 PP is on long trips.

So, since I don't want to change the offset too much, if I get aftermarket wheels, I'm going to shoot for 19x9.5 with 50mm offset and then I'll run 5mm spacers up front. Those might have to be custom wheels to get those specs though.

 

[NightmareMoon]

Idk about any extra tramlining. I run a lot with both stock wheels ad well as 19x10 ET35, which are pishing the limits of flush, and both with the same width tires (285s) in different brands. The lower offset wheels look 1000% better and dont tramline or anything. I drive to events on the stock wheels, usually 30-300 mile trips and daily the low offset wider wheels.

The lower offset is probably worse for hub wear in the long run and my rear fenders are slowly trying to catch up to the rock dings on my front bumper, but its not too bad. I'd do it again in a heartbeat, and Ill probably replace the stock wheels at some point with lower offset wider wheels when the budget allows.

 

[FlowForm_Wheels]

^^ Makes sense. Is it correct to say that stock wheels tend to have a higher positive offset than all of the after market wheels people are throwing on their cars to run wider tires?

Yep, that's correct I would say. Most people that are looking to upgrade to a new wheel set are going to improve the looks by going with a lower offset than what was provided to them from factory. Pushing the wheel out closer to the fenders producing a more flush look (aka going with a lower offset), will help give you a better overall look of the car. Wheels with a higher offset which are sunken inside the fender tend to make the car look unfinished.

 

[Brazos609]

I have never been able to find a diagram showing the scrub radius in factory configuration. Both offset and tire diameter can change this on the wheel side.

 

[TTown]

I'm not sure if I'm reading something into the above that isn't there (in NightmareMoon's comment), but let me ask this: Are there any unintended long-term (negative) consequences of running the wider than spec wheel (other than perhaps more trash being thrown on the car side)? That is, can the wider wheel cause harm over time to the car given it was designed with a narrower wheel in mind?

 

[TTown]

I did find this on a Toyota forum from a random google search. To a non-mechanical guy / casual observer, I could see how wider wheels might throw off the geometry and cause some type of uneven/ bad wear issues over time.

"BenWa, I had a long conversation with a wheel bearing engineer at SKF a couple of years ago about the effect of changing the wheel's offset either by using different wheels or through use of spacers, same thing.

The 2nd gen Taco comes from the factory with a +30mm offset to its wheels. The SKF engineer told me this was the ideal offset that Toyota established to center the load of the vehicle over the wheel bearing module. If you remember, each front wheel bearing module has two side-by-side tapered roller bearings in the bearing module. The engineering design is to split the load so that each of the tapered parts in the module takes about 50% while driving in a straight line.

When you move the wheel outward (change the offset from the +30mm factory design to a lower number, such as +12mm) you proportionally change the loading so that the outermost tapered bearing takes a greater load and the load on the innermost tapered bearing decreases.

Let's do some math for fun. Assume that your truck weighs 4,000 lbs. and that the weight distribution is 50% on the back and 50% on the front. That would would make about 1,000 lbs. of weight on each corner, or 500 lbs. that each tapered bearing part in one of the front bearing's module would be loaded (assuming a straight path). Now, let's change the offset by say 60% from the factory's +30mm to a +12mm wheel or doing the same by using spacers to move each wheel outward by 18mm (0.7 in.).

Now watch what happens to the loading. By changing the offset by 60% the innermost tapered roller bearing's load decreases 60% from 500 lbs. to 200 lbs., and the outer now has to take a load of about 830 lbs., up from the 500 lbs. it previously carried. Keep moving the stance wider and work the math, you will soon see that the outer tapered roller is taking almost all the load and the inner almost none.

SKF told me that the design life of those bearing modules is 100k miles if the factory +30mm offset is maintained. I have a relatively modest offset to my wheels: they are +18mm, or 12mm (0.47 in.) different from factory.

I asked for a gut feeling from SKF on the expected life of my front bearings with the offset I run. As I recall it was something like to expect a reduction to somewhere around 65-70k miles or so, I'll have to dig into my old notes to confirm.

Obviously chuck holes, rocks, high speed cornering, etc. will hammer the wheel bearings too."

 

[draconis123]

That's a good read. I'm interested in the load balance change when you have the smaller offset by wider wheels. I'm assuming adding an extra 2 inches of width would lessen that load change.

 

[FlowForm_Wheels]

I did find this on a Toyota forum from a random google search. To a non-mechanical guy / casual observer, I could see how wider wheels might throw off the geometry and cause some type of uneven/ bad wear issues over time.

"BenWa, I had a long conversation with a wheel bearing engineer at SKF a couple of years ago about the effect of changing the wheel's offset either by using different wheels or through use of spacers, same thing.

The 2nd gen Taco comes from the factory with a +30mm offset to its wheels. The SKF engineer told me this was the ideal offset that Toyota established to center the load of the vehicle over the wheel bearing module. If you remember, each front wheel bearing module has two side-by-side tapered roller bearings in the bearing module. The engineering design is to split the load so that each of the tapered parts in the module takes about 50% while driving in a straight line.

When you move the wheel outward (change the offset from the +30mm factory design to a lower number, such as +12mm) you proportionally change the loading so that the outermost tapered bearing takes a greater load and the load on the innermost tapered bearing decreases.

Let's do some math for fun. Assume that your truck weighs 4,000 lbs. and that the weight distribution is 50% on the back and 50% on the front. That would would make about 1,000 lbs. of weight on each corner, or 500 lbs. that each tapered bearing part in one of the front bearing's module would be loaded (assuming a straight path). Now, let's change the offset by say 60% from the factory's +30mm to a +12mm wheel or doing the same by using spacers to move each wheel outward by 18mm (0.7 in.).

Now watch what happens to the loading. By changing the offset by 60% the innermost tapered roller bearing's load decreases 60% from 500 lbs. to 200 lbs., and the outer now has to take a load of about 830 lbs., up from the 500 lbs. it previously carried. Keep moving the stance wider and work the math, you will soon see that the outer tapered roller is taking almost all the load and the inner almost none.

SKF told me that the design life of those bearing modules is 100k miles if the factory +30mm offset is maintained. I have a relatively modest offset to my wheels: they are +18mm, or 12mm (0.47 in.) different from factory.

I asked for a gut feeling from SKF on the expected life of my front bearings with the offset I run. As I recall it was something like to expect a reduction to somewhere around 65-70k miles or so, I'll have to dig into my old notes to confirm.

Obviously chuck holes, rocks, high speed cornering, etc. will hammer the wheel bearings too."

This is a solid argument for why Forged wheels would be better suited than your OE Cast wheels or the various cast wheels in the aftermarket. There is truth in what you're saying which is why you should take very special consideration to the wheel brands that your purchasing and if they're in fact higher or lesser quality than the OE wheels you have on the car now. Many consider wheel style and the aesthetics, but never construction style and quality processes and how that would potentially affect them.

With each Forged wheel we manufacture, and every FlowForm wheel, we verify load ratings are appropriate before we start selling to them to the public. Each wheel set that is purchased we verify that it's correct for every make/model, and even increasing the load ratings when necessary for certain vehicles of course.
Of course, a high quality wheel comes at a higher price, so you'll have to weight that into your options, but luckily our more wallet friendly line of FlowForm wheels pass the same TUV certification tests that our Forged wheels do so you know the quality is there even at a more affordable price.

 

[TicTocTach]

The most noticeable affect of changing offset on the wheels will be how the scrub radius is changed. The scrub radius is one of those suspension design aspects that Ford was almost certainly trying to achieve with their basic suspension calculations, with any wheel specified for a car being part of that design. There is a pretty basic but decent discussion of scrub radius and wheel offsets on this tire dealer industry website:

http://www.moderntiredealer.com/art...-customer-understand-why-wheel-offset-matters

No connection to these folks, I just found them while googling for an explanation of scrub radius.

I would guess that most OEM's are going to be pretty conservative and keep the wheels / tires well inside the fenders if for no other reason than protecting the wheel openings during strange suspension movements. Wheel / tire interference with the body or chassis parts would be a bad situation to defend if something went wrong and wound up in court.

Pushing the tires more flush with the openings could be done while keeping the scrub radius the same, but that probably means making the wheel wider inside and out. More space on the strut side is usually pretty rare, so the offset changes. It's kind of a trade off here, but it doesn't sound like there are a lot of really negative impacts to reducing the offset by 10mm or so. It would be interesting to see if that change alone made any changes to how a car handled that wasn't just subjective.

 

[Grintch]

The stock offset is set to put the centerline of the wheel where Ford thinks it should be for things like scrub radius and wheel bearing loads. When you push the wheels outboard you mess with those things (probably in a negative way) as well as having the tires throwing more trash from the road onto the side of the car.

For the front, you don't have much choice but to start moving the wheel out if you want to go to a wider wheel. This is one of the disadvantages of a strut based suspension.

Plus you have a lot of room for production/alignment variation before tire clearance becomes an issue.

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