20 mm wheel offset?

[Etron]

hey, my friend got his new First Gen today. and his rims were nice. came with the car. dey are 15 by 7's. So we decided to take it off, because were going to prep it for paint. Then when i read the back of the rim, it said 20mm offset. I was like WTF? he's running 205/50/15 on it. but it looks fine on the car. took me a ride in it and doesnt rub or do anything unusual. kind of just weird that it said 20mm offset on it. mmm?

[jnorion]

What exactly is the question?

[Etron]

I dont really know. i just wanted to know if this was wierd. Is he safe whith this wheel or not.

[jnorion]

If it's not rubbing I don't see why it would matter.

[Etron]

Alright. I mean, i was just worried about something messing up with the wheel bearing with the 20mm offset.

[jnorion]

I'd wait for a second opinion... I'm not an expert on this stuff.

[Etron]

I'd wait for a second opinion... I'm not an expert on this stuff.

thanks.

[goldstar]

A +20 mm offset is probably not a good idea and could lead to problems.

For a calculator to determine the effect of wheel offset on dimensional location of the wheel, check out: www.wihandyman.com/forum/showthread.php?t=12349

Now I'm not familiar with the OEM wheel width on the car in question but I believe that the wheel was +45 mm offset and no wider than 6" (perhaps 5.5"?). The calculator indicates that a 7" wide, +20 mm wheel will project outboard +38 mm (1.5") compared with a 6" wide, 45 mm wheel. That extra projection is actually quite a lot in terms of the increased loading on the wheel bearings, ball joints and control arm bushings that can lead to excess wear and damage.

Additionally, there is the problem of the large change in scrub radius that such a large change in offset causes. For small changes in offset, this is normally not a problem, but it could be in this case. That's another issue, however.

Happy Motoring!

[jnorion]

So in that case a smaller offset would be better, because it would set the wheels in closer and put less stress on the bearings?

Also, what is scrub radius?

EDIT: Now I'm confusing myself, pardon me while I go read about offsets before opening my mouth any further.

[Hraefn]

Low offset does not have to be a bad thing.



235-width tire on 13x8" wheel at 15mm offset (215 tire shown)

[goldstar]

So in that case a smaller offset would be better, because it would set the wheels in closer and put less stress on the bearings?

Also, what is scrub radius?

To begin with, a small decrease in + offset is not harmful and gives the advantage of a wider track that is good for handling. As a rule of thumb, probably any decrease is acceptable as long as the tire/wheel combo will still fit in the wheel well without major modifications. Of course, any change in offset is also determined by the wheel width since the two parameters interact in terms of clearance. However, a reduction in offset to +20 mm from an OEM spec of +45 mm is undoubtedly asking for trouble.

Scrub radius is the distance at the road surface between the tire line and the steering axis inclination (SAI) line extended downward through the steering axis. The line through the steering axis creates a pivot point around which the tire turns. If these lines intersect at the road surface, a zero scrub radius results. When the intersection is below the surface of the road, positive scrub radius results. When the lines intersect above the road, negative scrub radius occurs.

Characteristics of Different Types of Scrub Radius
At zero scrub radius, the car steers easily and will have little or no kickback from bumps. At the same time there will be virtually no road feel or feedback and there will be a feeling of instability while cornering due to the tendency of the tires to squirm. This condition is also known as center-point steering.

A positive scrub radius will make the car harder to steer and there will be more kick-back on bumps. A front tire blowout will yank the wheel extremely hard and given enough positive feedback could pull the wheel out of your hands. The advantage is that there is much greater road feel and feedback so that you can feel when the front tires start to break loose in a corner.

Negative scrub radius reduces steering effort and kickback on bumps compared with positive scrub radius. Additionally, front tire blowouts will act with less force on the steering wheel. There will also be less road feel and feedback, it will be harder to feel the front tires lose grip and torque steer will be decreased compared with the positive state.

In summary, the greater the scrub radius, positive or negative, the greater the steering effort and the more road shock and pivot point binding takes place (although to different degrees). MacPherson strut vehicles most often have negative scrub radius wheras many racing cars will be set up towards the positive side for reasons already discussed. The SAI is not adjustable on most vehicles but changing wheel offset significantly will EFFECTIVELY change the scrub radius. In general, reducing + offset will increase steering effort, possibly making the car harder to control during turning and cornering. Obviously, the grater the change in offset, the more severe the effects will be.

References:

http://www.ingallseng.com/terms.htm

http://www.nichols.nu/tip680.htm

http://www.diamondracingwheels.com/Terms-BS.htm

http://beluap.tr****.com/crash5.html

[Kreator]

heh, if i ever get my pos running, i was planning to run 1" spacers q/ 15x9 -20 offset.... with some body work obviously

road feel all the way :-p track car anyways hehe.

[OCD-SPEED]

Ok so the next obvious question is....If i want to get 16x7's or 17x7s what offset should i look for? An increase in road feel, to me, wouldnt be a bad thing in a front wheel drive. However i dont want to go around breaking joints! WHat do u think gold star?

[goldstar]

I'll start by saying that I was addressing Etron's question on the effects of using a +20 mm offset wheel on a car designed for a +45 mm offset wheel. It was indicated that reducing + offset by 25 mm would move the outside edge of the rim out by 1.5" (with a 7" wide wheel) and the centerline of the wheel out by .98". A number of people have stated that if spacers are used to move wheels out, .5" (12.7 mm) should be the maximum considered. Starting with a +45 mm wheel, .5" spacers would effectively reduce + offset on such a wheel to 32 mm. Now, I have NO idea at what point reduced + offset will start to cause problems. My point was that problems can occur and one should be aware of this fact.

That being said, I'm certainly no expert on what offset is optimal because it depends on so many other factors: principally car model, wheel width and tire size. As stated earlier, any wheel/tire combo that will fit in the wheel well without interference with either the fender or the strut should be acceptable in terms of not causing wheel bearing damage or affecting the scrub radius adversely, and if you're willing to roll your fenders, even less + offset or a wider wheel and tire can be used. Also, keep in mind that for 3rd gens, OEM offsets for DX and LX are +45 mm, ES and P5's are 50 mm and MSP's are 55 mm.

When considering fitting aftermarket wheels the logical place to start is Rishie's wheel/tire guide: www.msprotege.com/forum/showthread.php?t=9129

According to the guide, for P5's using a 215/40-17 or 215/35-18 size, a 7" rim requires +45 mm offset or higher and a 7.5" rim requires +48 mm or higher. For a 205/40-17 size, a 7' rim should be +42 mm or higher (barely making it) and a 7.5" rim should have +48 mm or higher. Rishie goes on to say that sedans can run 2 to 3 mm's less + offset than P5's since they have more fender clearance. These figures should also apply to 16" and even 15" wheels given the SAME wheel widths, tire widths and rolling diameters.

Another good source of information is to ask other Forum members what they are using and what they have found to work without problems. For example, I'm using 16 x 7" wheels, +42 mm offset, and 205/45-16 tires with absolutely no problems or interference of any kind.

Happy Motoring!