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New 18" Wheels - right rear tire touches body


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Hello,

I recently installed 18" wheels on my Boxster (MY98 base). They are 18x8 et 50 front and 18x10 et 42 rear with Michelin Pilot Sport PS2s. I noticed that in certain circumstances, when I go over bumps at high speeds ( >70 MPH), although sometimes if the bump is big enough it happens even at low speeds, the left rear tire touches the top of the fender. I've had the new wheels installed for about a month and I put about 500 miles on them. The left rear tire has a mark on the outer edge from where it touches. It is not deep, but certainly noticeable. When I first heard the noise I ordered a 5mm spacer kit for the rear wheels. I got the kit and installed the spacers, but the tire still touches. Since it is only the left rear tire, I doubt that it has something to do with the offset or the size, otherwise the right wheel would show the same symptoms. Now I am thinking maybe it is the left rear suspension? Could it be on its way out? I've had the car for about 2 years, and it now has 76K miles on it. I do not know if any suspension work was done on the car - I doubt it.

Is it possible that I might need to replace the suspension? What tests can I run to better determine the state of the suspension and/or the cause of the rubber hitting the fender?

Thank You in advance for all your help.

Edit - I mistyped in the title and put right instead of left. It is the LEFT (driver) rear tire that is touching.

Edited by azzar0
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For those of you who are interested - the answer, aparently, is fender rolling. It is basically a procedure that pushes the inner fender lip inward, thus creating more room for the tire. My neighboor, who knows everything about everything, immediately showed me why it rubs on the left and not on the right - the lip on the right side is not flat but bent upwards, whereas the left fender lip was horizontal. Long story short I used a rod to push the lip in a little bit, took the car out for a drive and it certainly diminished the rubbing, significantly. It still rubs if I go over big bumps (huge ones) - I may need to further roll the fenders (or take the car to a body shop, they have a special tool for rolling). The tool can also be rented from various sites on the internet, but a good ol' baseball bat works just fine if you heat up the paint and do it slowly...

Anyway, mystery solved!

Thanks all for your suggestions.

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If you were already hitting the out fender lip, why would you put on spacers? Your wheels are the wrong offset already. You can probably remedy the problem by removing the spacers and adding more negative camber, though it will increase tire wear.

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I'd be inclined to get it rolled professionaly. There is something just not right about bending the metal on a Boxster with a "rod".

You must really love those wheels to go through that.

For those of you who are interested - the answer, aparently, is fender rolling. It is basically a procedure that pushes the inner fender lip inward, thus creating more room for the tire. My neighboor, who knows everything about everything, immediately showed me why it rubs on the left and not on the right - the lip on the right side is not flat but bent upwards, whereas the left fender lip was horizontal. Long story short I used a rod to push the lip in a little bit, took the car out for a drive and it certainly diminished the rubbing, significantly. It still rubs if I go over big bumps (huge ones) - I may need to further roll the fenders (or take the car to a body shop, they have a special tool for rolling). The tool can also be rented from various sites on the internet, but a good ol' baseball bat works just fine if you heat up the paint and do it slowly...

Anyway, mystery solved!

Thanks all for your suggestions.

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If you were already hitting the out fender lip, why would you put on spacers? Your wheels are the wrong offset already. You can probably remedy the problem by removing the spacers and adding more negative camber, though it will increase tire wear.

Perhaps I don't understand your statement - the correct offset for 10" rear wheels is 47 - 48, as Mike stated. That is POSITIVE offset, not negative. So, since my wheels have a 42 offset, adding 5 mm makes it 47. I am not talking about camber, that I agree that it should be negative, but the actual wheel offset. In any event, I had talked to my local Porsche dealer, TireRack, and a couple of other shops before I ordered spacers, to make sure I end up with the correct value. They all told me the same thing - 5 mm spacers.

I don't know much about wheel offsets, and I am the first to admit it, therefore I ask those who know. Please explain why the wheels have the wrong offset, I would like to know. If anything I can go back and question the dealer, should you have a valid explanation.

Thank You.

Edited by azzar0
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For a 986 Boxster - rear wheels that are 18 x 10" wide need a 47 mm offset. If you have added a 5 mm spacer (and longer wheel bolts) then your offset is correct.

Thank You Loren, for validating the offset. Yes, the kit came with GT3 (longer) bolts with red sleeves.

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I'd be inclined to get it rolled professionaly. There is something just not right about bending the metal on a Boxster with a "rod".

You must really love those wheels to go through that.

I am thinking about bringing the car to a shop to have them do it. The basic principle, however, is the same. Something pushes the lip inward; whether that something is a baseball bat or a roller dictates how smooth the surface and the overall lip looks when completed. You still have to apply heat to the paint (so that it doesn't crack), regardless of which approach you take. Aesthetics are not an issue, in my opinion, because you couldn't notice imperfections (should there be any) tucked under the fender.

In any event, I only pushed in very very little, not the entire fender. I will talk to a couple of shops and see exactly how they do it and if the price is right I might have them do it.

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Are you sure these wheels have a positive offset of 42? If so, they should have been rubbing the inside of the wheel wells, not the outside. I have 18 x 10's on the rear of my '02 S with an offset of 47...I am tight on the inside, but certainly not on the outside.

Your statements are confusing to me...

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Are you sure these wheels have a positive offset of 42? If so, they should have been rubbing the inside of the wheel wells, not the outside. I have 18 x 10's on the rear of my '02 S with an offset of 47...I am tight on the inside, but certainly not on the outside.

Your statements are confusing to me...

Yes, I am sure. I am not trying to confuse you :rolleyes:

The difference is that your car is an 02S while mine is a 98 base. I believe later and S models have bigger wheel wells to accomodate larger wheels, that's why you're not seeing an issue with the outside. My wheels are tight on the inside too, but not enough to cause a problem.

:renntech:

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I rolled the front fenders on my '73 911 with a wooden baseball bat. I helps to heat the paint (warm, not hot) to keep it from cracking. Works like a charm with no damage; but do take your time, instead of trying to do it all at once.

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I like the baseball bat technique....

Please post pictures and video if possible :rolleyes:

Just like JMatta said - make sure you heat the paint (with heating gun / hair dryer), then slowly roll the bat and push the lip in. Google "fender rolling" and you'll even see some videos on how to do it (as well as how not to do it :>)

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47mm to 48mm is the ideal offset for a 10" rear on a 986.

Right, which is why I added the spacers. So now the offset is 47 (42 + 5).

If adding a 5mm spacer brings the offset to +47 then the wheel's original offset must be +52 (52-5=47). Offset is the distance between the rim's center-line and the surface which mounts flush on the hub. The spacer "brings" this surface closer to the center-line and pushes the wheels out. So, with spacers, you can only reduce positive offset.

Bringing the wheels out changes suspension parameters such as the roll center or the scrub radius. It also reduces the effective spring rate because the wheel acts on the springs through a "longer arm". It could also put more load on bearings etc. So, generally, it pays to be conservative with offset changes. At 5mm, however, there should not be much of a concern, especially if your final offset is 47 - which corresponds to Porsche's post-2002 986 IXRC option: "Sport Techno wheels" (8Jx18 ET50, 10Jx18 ET47).

Now, if you do have a rim that is ET42 - and I apologize for doubting you again - and by adding a 5mm spacer it rubs less that before on the outside lip, I must have my definitions wrong.

Edited by Costas
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I recently installed 18" wheels on my Boxster (MY98 base). They are 18x8 et 50 front and 18x10 et 42 rear with Michelin Pilot Sport PS2s. I noticed that in certain circumstances, when I go over bumps at high speeds ( >70 MPH), although sometimes if the bump is big enough it happens even at low speeds, the left rear tire touches the top of the fender. I've had the new wheels installed for about a month and I put about 500 miles on them. The left rear tire has a mark on the outer edge from where it touches. It is not deep, but certainly noticeable. When I first heard the noise I ordered a 5mm spacer kit for the rear wheels. I got the kit and installed the spacers, but the tire still touches. Since it is only the left rear tire, I doubt that it has something to do with the offset or the size, otherwise the right wheel would show the same symptoms. Now I am thinking maybe it is the left rear suspension? Could it be on its way out? I've had the car for about 2 years, and it now has 76K miles on it. I do not know if any suspension work was done on the car - I doubt it.

Is it possible that I might need to replace the suspension? What tests can I run to better determine the state of the suspension and/or the cause of the rubber hitting the fender?

You just pushed them out further...

If you were already hitting the out fender lip, why would you put on spacers? Your wheels are the wrong offset already. You can probably remedy the problem by removing the spacers and adding more negative camber, though it will increase tire wear.

Perhaps I don't understand your statement - the correct offset for 10" rear wheels is 47 - 48, as Mike stated. That is POSITIVE offset, not negative. So, since my wheels have a 42 offset, adding 5 mm makes it 47. I am not talking about camber, that I agree that it should be negative, but the actual wheel offset. In any event, I had talked to my local Porsche dealer, TireRack, and a couple of other shops before I ordered spacers, to make sure I end up with the correct value. They all told me the same thing - 5 mm spacers.

I don't know much about wheel offsets, and I am the first to admit it, therefore I ask those who know. Please explain why the wheels have the wrong offset, I would like to know. If anything I can go back and question the dealer, should you have a valid explanation.

For a 986 Boxster - rear wheels that are 18 x 10" wide need a 47 mm offset. If you have added a 5 mm spacer (and longer wheel bolts) then your offset is correct.

Are you sure these wheels have a positive offset of 42? If so, they should have been rubbing the inside of the wheel wells, not the outside. I have 18 x 10's on the rear of my '02 S with an offset of 47...I am tight on the inside, but certainly not on the outside.

Your statements are confusing to me...

47mm to 48mm is the ideal offset for a 10" rear on a 986.

Right, which is why I added the spacers. So now the offset is 47 (42 + 5).

If adding a 5mm spacer brings the offset to +47 then the wheel's original offset must be +52 (52-5=47). Offset is the distance between the rim's center-line and the surface which mounts flush on the hub. The spacer "brings" this surface closer to the center-line and pushes the wheels out. So, with spacers, you can only reduce positive offset.

Bringing the wheels out changes suspension parameters such as the roll center or the scrub radius. It also reduces the effective spring rate because the wheel acts on the springs through a "longer arm". It could also put more load on bearings etc. So, generally, it pays to be conservative with offset changes. At 5mm, however, there should not be much of a concern, especially if your final offset is 47 - which corresponds to Porsche's post-2002 986 IXRC option: "Sport Techno wheels" (8Jx18 ET50, 10Jx18 ET47).

Now, if you do have a rim that is ET42 - and I apologize for doubting you again - and by adding a 5mm spacer it rubs less that before on the outside lip, I must have my definitions wrong.

The wheel in this case is ET42, which is a positive offset of 42mm. Adding a 5mm spacer makes the effective offset 37mm, not 47mm, and obviously aggravated the fender rubbing condition in this case. If you need ET47, you can't get there with ET42 wheels. Simple drawing and definitions here, at tirerack.com.

--Brian

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I had the EXACT same problem with my 2000 S. I added 5mm spacers but that didn't make any difference!

I'll be switching back to the 9 inch wheels, I can't deal with driveability problems. Too bad because those Sport Classic 2's look great.

Cheers

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The wheel in this case is ET42, which is a positive offset of 42mm. Adding a 5mm spacer makes the effective offset 37mm, not 47mm, and obviously aggravated the fender rubbing condition in this case. If you need ET47, you can't get there with ET42 wheels. Simple drawing and definitions here, at tirerack.com.

--Brian

Hi Brian,

I used this wheel offset calculator, in addition to talking to various other dealers and shops:

http://www.1010tires.com/WheelOffsetCalculator.asp

The offset calculator came back with the same results - EXTEND the wheels out 5 mm

I am going to a Porsche clinic this Saturday and I'll ask my mechanic again about the details...

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I am assuming that you are correct, that the wheels you have in hand are 18x10 ET42. I am also assuming that in order to fit your Boxster, you need ET47.

The bottom line, no matter what on-line calculator you use, or who you talk to, you can't make your ET42 wheels into ET47 wheels. You'd actually have to machine 5mm off of the mounting surface of your ET42 wheel to get ET47 (and that is definitely not recommended!). Following the simple logic first proposed by jmatta in an early post: If your tires were rubbing on the outside to begin with, and you added a wheel spacer to "push them out," then you just made the situation worse.

I think where people can get confused is in the symantics of "adding" a spacer. For instance, it's incorrect to think that if you "add" a 5mm spacer, you should "add" 5mm to ET42 and "get" ET47. If you're still not convinced, what is your offset if you "add" a 42mm spacer to your rear axle and use your ET42 wheel? Is it ET42 + 42 = ET84? (No, it's ET0, or zero offset.) Also, I think that calculator in the link uses some potentially confusing terms, like "extend," which I think may have burned you.

--Brian

Edited by Q-Ship986
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I am assuming that you are correct, that the wheels you have in hand are 18x10 ET42. I am also assuming that in order to fit your Boxster, you need ET47.

The bottom line, no matter what on-line calculator you use, or who you talk to, you can't make your ET42 wheels into ET47 wheels. You'd actually have to machine 5mm off of the mounting surface of your ET42 wheel to get ET47 (and that is definitely not recommended!). Following the simple logic first proposed by jmatta in an early post: If your tires were rubbing on the outside to begin with, and you added a wheel spacer to "push them out," then you just made the situation worse.

I think where people can get confused is in the symantics of "adding" a spacer. For instance, it's incorrect to think that if you "add" a 5mm spacer, you should "add" 5mm to ET42 and "get" ET47. If you're still not convinced, what is your offset if you "add" a 42mm spacer to your rear axle and use your ET42 wheel? Is it ET42 + 42 = ET84? (No, it's ET0, or zero offset.) Also, I think that calculator in the link uses some potentially confusing terms, like "extend," which I think may have burned you.

--Brian

Interesting. So bare with me... Again, I am not arguing, nor am I questioning your logic, I am just trying to understand the complexity of the offset issue. If my understanding is correct, the definition of a wheel offset is the distance between the center of the wheel and the outer edge of the wheel, where the center of the wheel represents ET0. If the offset is negative than you would need to 'shave' the wheel in order to fit (because you are too close to the strut housing). If the offset is positive, then you would add a spacer to fit - is this correct? If the wheel says ET42, doesn't that imply a positive offset? If it was a negative offset shouldn't it be marked as ET -42? Following the notation logic, since you are 42 mm out and you need to be 47 mm out, doesn't it follow that you would need to add 5mm in order to accomplish that offset?

I think the confusion is between negative and positive offsets. It appears that most people I've talked to assume a positive offset whereas you and jmatta assume a negative one.

Also, if it was truly an offset issue, wouldn't I see this problem on BOTH sides? It is clearly happening only on the left side, not at all on the right side. Upon close inspection, I was able to confirm that the right fender was already rolled (the inner lip is tucked in) while the left fender has not been rolled (the inner lip is sticking out). My suspicion is that the left fender was replaced at some point in time, hence the difference; which would, in turn, explain the issue.

With that said, I am more confused than ever :huh:

Thank you for your input and patience

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