Wheel Offset Question

[SuperG]

I was wondering when offset is mentioned, such as 38mm-42mm...is that the offset required for a certain sized tire to properly fit the wheel, or the offset for the wheel to properly fit the car?

 

[shoinoff95]

They are speaking about the offset for the wheel to properly fit the car on the hub with enough clearence to clear the fender well and strut towers.

 

[pjtoledo]

Offset also has a major role in how a car handles and steers. With the wrong offset on a steering axle the car will steer itself. You want a real thrill?? Try changing lanes at 70 mph without turning the wheel. All it takes is too little offset and any bump or uneven pavement. Panic stops?? No problem, the car, not you, will choose what lane it stops in. Front brake upgrades have an effect on offset. For instance the center of a rotor on a 92 is quite thin. Stock, you can get away with 36mm offset. Do a 96 upgrade and the rotor hat thickness increases by about 3.5mm. You now have an "effective" offset of about 32mm. The steering will be very tricky on that setup. Tire height is another consideration. While the height of a tire will not change the offset, it will change the intersection of 2 very crucial steering/alignment lines. Those being the actual middle of the tire, and the steering axis. Moral of the story, stay close to stock offset on a steering axle. Yeah, I know this safety stuff is boring.

Perry

 

[SuperG]

It seems 38-42 mm offset is typical...So, offset can/will change for different wheel sizes and tire combinations, right?

So, looking for a 17" wheel and 235 wide tires, I would want to select 235/50-17 to maintain near stock height, right? Otherwise, how do I determine the proper offset?

What is the offset for the stock wheel and tire combo for a '93 ATX?

This would explain some steering changes since I had to rush a set of 225/50-16 onto my car (the tires I ordered did not come in time) while preparing for some high(er) speed driving...

 

[K-Dawg]

Actually I would probably stick with a 38-40mm offset to ensure clearance on the inside with a wider rim. But you'd probably be fine with a 42 on most wheels/tires.

Tire size is more dependent on rim width, so you need to know what size rim you'll be using before you decide on a tire size. I wouldn't use a 235/50/17 on any size for the SHO.

Stock SHO wheels have a 42mm offset.

 

[SuperG]

Thanks K-Dawg, but I am interested in your last statement about not using 235/50-17's on any SHO...what do you mean? There are quite a few folks who have 17 and even 18" wheels. I know several guys with 245/16's on the front...

 

[K-Dawg]

I say that because the 235/50/17 is larger in diameter than the stock tire. Its not that much larger, but I would rather have slightly shorter than slightly larger.

IMO, a 235/45/17 would be more appropriate on a 7.5"-8" rim. You could put a 245/45/17 on some 8" rims. On a 7" rim you could run a 215/50/17, 225/50/17, or 225/45/17.

 

[SuperG]

Hmm, I see your point, but I also see that the 235/50-17 is only about 1/10th of an inch taller. Well, heck, we have a good bit of clearance anyway, unless the car is lowered.

Perhaps the thing that becomes an issue is the clearance inside the wheel well with the wider rim and tire...the tire must be closer to the inner-side of the wheel well...

Anyway, I appreciate the advice. However, I just realized that, although I now know what the offset means, how is it measured?

 

[pjtoledo]

The offset is the diffence between the actual middle of the wheel and the hub mounting surface. To measure on a mounted wheel/tire combo lay the wheel face down on a flat surface. Place a straight edge across the back of the wheel. Take the measurement to the ground, it will be about 9" for a 225 tire. So the middle will be 4 1/2 inches from the back of the tire. Now measure from the back of the tire to the back of the hub mounting surface. Lets say that winds up being about 6 1/8" . The difference between 4 1/2" and 6 1/8" is 1 5/8" Thats the offset. Of course it's expressed in millimeters. So, 225mm (9")overall width divided by 2 equals a 112.5mm (4.5")centerline, then the distance to the hubsurface is about 155.5mm (6 1/8"). That calculates to a 43mm (1 5/8") offset. To check the offset on an empty rim measure from the inside of the bead lips, the lips can be of different thickness and since they are "outside" of the tire don't count. Most rims have the offset stamped on them. Offsets come in negative, like ours where the wheel is displaced into the wheel well, and the opposite is positive where the wheel moves out away from the car. I think thats right, I always seem to forget which way is what. Due to the way our SHOs are constructed the "best fitting" offset is different between front and rear. There is less clearance between the wheel and strut in the rear. Taurus' steering geometry was first designed with tires as small as 195/70-14, a very short tire. The stock SHO tires measured about 26.1",,almost 2" taller so we are actually near the upper limit with just 215/60-16 on slicers. I would, and have, gone to a slightly shorter tire for better performance all around. About tire sizes,,,think basic physics.. Consider the line from the spindle to the pavement as a lever. The longer that line the more leverage will be granted to the pavement, shorter line means the car will have the advantage. Damn, my fingers are getting tired, gotta learn to stop blabbing.

Perry

 

[SuperG]

Wow...thanks for the explanation, Perry! That helps a lot...

So, you would go with K-Dawg's idea of a 235/45-17, rather than the 235/50-17?

 

[RI-SHO]

For those that need a visual aide like me, here it is:

 

[pjtoledo]

SuperG:
Wow...thanks for the explanation, Perry! That helps a lot...

So, you would go with K-Dawg's idea of a 235/45-17, rather than the 235/50-17?

Sometimes,,but not always,,shorter is better. I definately plan on shorter tires, between 25 and 25 1/2". Be wary, the actual size of tires varies a lot. Check the specs for actual section width, tread width, and most important,,load rating!! A general rule is the shorter the side wall, the smaller the load rating, get the numbers before you buy.

Perry

 

[pjtoledo]

RI-SHO:
For those that need a visual aide like me, here it is:

That's a great picture of backspacing. A critical dimension to be sure, but a line indicating the actual center of the wheel is needed to show offset. The difference between a horizontal line thru the center of the wheel and where lines A and B meet is the offset.


Perry

<small>[ December 24, 2003, 09:51 PM: Message edited by: pjtoledo ]</small>

 

[NetNinja]

Interesting stuff.

I want to go to 18's on my SHO but I am looking for a shop that will build me a set that can replicate the slicer pattern.

I have a project car and I want to keep it looking like a big bad wolf in Sheeps clothing.

 

[MeShoHorny]

pjtoledo:

RI-SHO:
For those that need a visual aide like me, here it is:

That's a great picture of backspacing. A critical dimension to be sure, but a line indicating the actual center of the wheel is needed to show offset. The difference between a horizontal line thru the center of the wheel and where lines A and B meet is the offset.


Perry

 

[SuperG]

Thanks everyone...that was really helpful!