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AZBMW Forums › Model Specific Discussion › ///M Cars › Tire selection
Tire selection
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ACK-FIN
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PostPosted: Fri Mar 21, 2014 12:00 am
Post subject: Tire selection

I've found a set of BBS Style 108 in 18x8 and 18x8.5 and am trying to decide on tires. And am also trying to decide if I want to go wide enough to require fender rolling. Since the car rides lower than oem I have no idea what will fit without rolling and what are the limits of width with fenders rolled. I don't think I'll want to go as wide as possible but I might as well know what my options are. I've read a rew different setups people use when going staggered. Why do people use 35 height rear and 40 front? I see this done a lot.

Also, I'm wondering how to price the existing tires and wheels. The tires are at least 95% 17zr40 235 Yokohama S-Drive all the way around and the wheels are, I believe, regular e36 17x7 heavy-ass wheels with some curb rash. Your help will be appreciated.

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M3_Dust
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PostPosted: Fri Mar 21, 2014 12:04 am
Post subject: Re: Tire selection

225 front 245 rear pilot super sports for the win

your signature is stupid as fuck

welcome to the forum

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ACK-FIN
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PostPosted: Fri Mar 21, 2014 12:58 am
Post subject: Re: Tire selection

Thanks for the input and we all know about opinions.

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wkohler
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PostPosted: Wed Apr 02, 2014 6:00 pm
Post subject: Re: Tire selection

Before choosing tires, you should know how tire sizing works.

Take a 235 40ZR18.

The section width (tread) is 235mm wide or thereabouts. Every tire is different.

The Z suggests super extreme greatness performance. Tires with very high speed ratings have this.

R is for radial.

18 is obviously the wheel size. Its hole is designed for the specs of a standard 18" wheel.

I skipped the 40 (aspect ratio) because that seems to be what you're having trouble with. That number refers to the height of the sidewall being a percentage of the section width. In this case, its 40%, or 94mm.

If you have staggered wheels and want staggered tires, here's the deal.

If your front wheels are 8s and the rears are 9s, use the 235-40s in front. In the rear, use a 255/35. 35% of 255 is 102mm. Its less than a 1/4" off but it is essentially the same diameter tire. To have tires of different widths on a car keeping the same diameter, the aspect ratio has to change relative to the section width.

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ACK-FIN
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PostPosted: Wed Apr 02, 2014 10:21 pm
Post subject: Re: Tire selection

wkohler,

Thanks for the info if it was directed toward me. I'll need to go back and read the thread because I have no problems knowing how tire sizing and/or rating works. Aparently I phrased my initial question poorly. I just want to know what will fit w/o rolling or pulling the fenders and then what tires people are enjoying these days.

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wkohler
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PostPosted: Wed Apr 02, 2014 10:25 pm
Post subject: Re: Tire selection

ACK-FIN wrote:
I've read a rew different setups people use when going staggered. Why do people use 35 height rear and 40 front? I see this done a lot.

Mostly that says you don't know how it works, which is why I explained it.

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ACK-FIN
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PostPosted: Wed Apr 02, 2014 11:08 pm
Post subject: Re: Tire selection

OK, I know what the numberrs and letters mean but I don't know the inner diamsions of the wheelwells and I don't remember the offests needed not do I completely understand offset - I've never had to pay attention to it.

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ACK-FIN
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PostPosted: Wed Apr 02, 2014 11:18 pm
Post subject: Re: Tire selection

when one goes from a 17" 40 to a 18" 35 does the outter diameter stay roughly the same (I haven't done the math)? And is their a benfit in using 18" 35 read and 18" 40 front that I don't seem to underrstand? Is it particular to this car or is it 'universal'?

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wkohler
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PostPosted: Thu Apr 03, 2014 2:28 am
Post subject: Re: Tire selection

Why do you say you understand how it works when you clearly don't? I even explained it. I'm happy to keep explaining it for some reason, but don't tell me you don't need it explained because you do.

The rough idea in my experience with tires in the 205-255 range is that for inch increase in wheel diameter, the sidewall shrinks by 5% to keep roughly the same overall diameter.

Example:

You are currently running a 235/45/17 (wrong size for an E36 - should be 225/45/17). If you wanted to keep roughly the same overall diameter with an 18" wheel, you'd get a 235/40/18.

The point is that the aspect ratio is not the sole deciding factor in the size of a tire. It has to work together with the section width as it's a percentage of the section width. It's not a fixed height as you'll note from the math I did in my previous post.

A 225/45/17 is shorter than a 235/45/17. A 255/40/17 is nearly the same overall diameter as a 235/45/17. There are several tire calculators on the Google that you can play with and visually see the relationship about these numbers.

As far as wheel offset goes, it's pretty simple. Wheels have a width (measured from where the bead sits across - basically inside the lip where the tire sits, not from edge to edge). The center of this width is called the center line of the wheel. The term offset refers to how much the hub mounting plate is offset to the center line of the wheel.

A wheel that has a 0 offset will have it's hub mounting plate at exactly the centerline of the wheel. From there, there are positive and negative offsets. As the offset number decreases, the wheel moves further outboard; a negative offset puts the hub mounting plate more toward the center of the car while a positive offset does the opposite.

Here's a couple examples in the BMW world:

E36 M3 DS1 is 17x7.5 ET41

The term "et" refers to the offset. It's derived from the German "Einpresstiefe" which means "insertion depth." The offset on this wheel is +41, meaning that the hub mounting plate is 41mm outboard of the centerline of the wheel.

5, 6, 7 and 8 series cars have lower offsets than 3 series.

The E28 M5 used a 16x7.5 ET20 wheel. This wheel sits 21mm further outboard than the DS1 even though the width of the wheel remains the same.

a 17x8 ET41 wheel will sit further out than a 17x7.5 ET41 wheel since the offset is calculated from the center line of the wheel, which changes every time the width changes. It's pretty basic.

As far as worrying about how much room you have in your wheel wells, that's not the issue. The limiting factors in wheel fitment aren't the physical size of the wheel well rather the distance between the tire and the strut tube in front and the fender lip and in the rear, the tire to trailing arm clearance and of course the fender. None of the other stuff really matters. Put a super wide wheel on in front and try to turn lock-to-lock, sure you'll have some problems, but for most fitments, fenders, front struts and trailing arms are the key.

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7/75 E12 530i | 7/80 E12 M535i | 2/85 E28 Alpina B7 Turbo | 3/85 E28 528i | 2/87 E28 M535i | 8/94 E34 525iT
10/80 E12 M535i | 12/82 E28 533i | 4/83 E28 533i | 3/85 E23 745i | 6/85 E28 M535i | 7/86 E28 535is | 5/94 E34 525iT
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e46tuner
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PostPosted: Thu Apr 03, 2014 2:47 am
Post subject: Re: Tire selection

good info!

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M3_Dust
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PostPosted: Thu Apr 03, 2014 3:03 am
Post subject: Re: Tire selection

wikikohleria

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