ZPOST
BMW Garage BMW Meets Register Search Today's Posts Mark Forums Read


Go Back   ZPOST > BMW Z4 Technical Talk > Track / Autocross / Dragstrip / Driving Techniques
  TireRack

SUPPORT ZPOST BY DOING YOUR TIRERACK SHOPPING FROM THIS BANNER, THANKS!
Post Reply
 
Thread Tools Search this Thread
      11-08-2010, 11:07 AM   #1
3002 tii
Lieutenant General
3002 tii's Avatar
2269
Rep
12,559
Posts

Drives: Z4 M, X5, GX460
Join Date: Apr 2008
Location: CT

iTrader: (99)

Garage List
245 or 255 Square for Auto-X?

Just bought a new set of wheels, 18x8.5 et 42 on all 4 corners. Which tire size would you recommend for auto-x? I was thinking 245 would be the ideal with but that'd require a 35 profile which then puts me lower than stock. I know a 255 will fit w/o rubbing based on others' reporting but I'm worried that it might be overkill for the front?

This is a non-m coupe, on stock oe sports suspension.
__________________
Follow for latest mods
Appreciate 0
      11-08-2010, 07:26 PM   #2
F360C
Major
United_States
366
Rep
1,373
Posts

Drives: E86 S54
Join Date: Apr 2006
Location: Hoboken, NJ

iTrader: (2)

255 all the way!
__________________

07 Z4 M Coupe AW/IR
Appreciate 0
      11-08-2010, 09:17 PM   #3
3002 tii
Lieutenant General
3002 tii's Avatar
2269
Rep
12,559
Posts

Drives: Z4 M, X5, GX460
Join Date: Apr 2008
Location: CT

iTrader: (99)

Garage List
thanks man, i'm thinking the same...
__________________
Follow for latest mods
Appreciate 0
      11-09-2010, 08:15 PM   #4
jbh
Private First Class
jbh's Avatar
18
Rep
156
Posts

Drives: 2007 Z4M
Join Date: Aug 2009
Location: Fort Worth

iTrader: (2)

More rubber is generally better. I run 245 square on my M roadster for autox, 255 would be better if they fit for you.
Appreciate 0
      11-10-2010, 09:02 AM   #5
3002 tii
Lieutenant General
3002 tii's Avatar
2269
Rep
12,559
Posts

Drives: Z4 M, X5, GX460
Join Date: Apr 2008
Location: CT

iTrader: (99)

Garage List
Quote:
Originally Posted by jbh View Post
More rubber is generally better. I run 245 square on my M roadster for autox, 255 would be better if they fit for you.
agreed but also considered the min difference bet 245 vs 255, given the increased weight of larger tires and the slower turn-in...
__________________
Follow for latest mods
Appreciate 0
      11-10-2010, 06:57 PM   #6
F360C
Major
United_States
366
Rep
1,373
Posts

Drives: E86 S54
Join Date: Apr 2006
Location: Hoboken, NJ

iTrader: (2)

Quote:
Originally Posted by Dubbedown View Post
agreed but also considered the min difference bet 245 vs 255, given the increased weight of larger tires and the slower turn-in...
I don't think you will really notice the decreased turn in and that even if their is any. The only way I think your going to get slower turn in is if the larger tires is to big for the rim or it has a larger side wall. A 255 should be okay on an 8.5 rim especially if i am running a 255 on a 8" rim. In addition if you run a 255/35/18 you have less side wall which should help turn in. I am going off an R888 here because it what I know best but the 255/35/18 is .4 of an inch wider then the 245/40/18. In addition because of the smaller side wall it weighs in at 25.8 pounds compared to the 26 pounds of the 245/40/18. Lastly toyo lists the 255/35/18 as being able to fit on the 8.5 rim. So not only is it wider with less side wall flex but it is also lighter. Seems like a no brainer for me
__________________

07 Z4 M Coupe AW/IR
Appreciate 0
      11-11-2010, 12:24 AM   #7
Master Apex
Private
United_States
17
Rep
99
Posts

Drives: '17 F80 M3 '07 Z4M '06 E60M5
Join Date: Apr 2010
Location: Austin, TX

iTrader: (0)

18x8.5 et42 with 255 will touch the front strut.
I am running 18x8.5 et38 with 245 and the inside clearance is only about 7mm, so your et42 will reduce the clearance to 3mm, don't think you can use 255 without spacer in the front.
Appreciate 0
      11-11-2010, 10:57 PM   #8
3002 tii
Lieutenant General
3002 tii's Avatar
2269
Rep
12,559
Posts

Drives: Z4 M, X5, GX460
Join Date: Apr 2008
Location: CT

iTrader: (99)

Garage List
Quote:
Originally Posted by Master Apex View Post
18x8.5 et42 with 255 will touch the front strut.
I am running 18x8.5 et38 with 245 and the inside clearance is only about 7mm, so your et42 will reduce the clearance to 3mm, don't think you can use 255 without spacer in the front.
thats weird b/c when i used the wheel offset calculator here:

http://www.e90post.com/forums/forumdisplay.php?f=12

i put in oem settings of: 8.0", et 50, 225/40/18
new wheel: 8.5", et 42, 255/35/18

and it spits out: "2mm MORE clearance on the strut side, and the outside edge of the wheel will EXTEND an extra 14mm"
__________________
Follow for latest mods
Appreciate 0
      12-28-2010, 09:33 AM   #9
cgroppi
Private
cgroppi's Avatar
United_States
2
Rep
64
Posts

Drives: 2007 Z4 M Coupe
Join Date: Mar 2010
Location: Tempe, AZ

iTrader: (0)

They'll fit. I run 285-30 Hoosier A6s square on my M coupe with stock wheels. I need a 5mm spacer in the back for wheel well clearance. The front fits fine.

Shorter tires are typically better for autocross. You get better acceleration because of the gearing advantage, and you lower the car. If you're in stock class, shorter tires are the only legal way to lower the car.
Appreciate 0
      12-28-2010, 10:29 AM   #10
ZStig
instagram 997turbotom
279
Rep
7,376
Posts

Drives: Interlagos Z4MR
Join Date: Jun 2008
Location: Delray Beach, FL

iTrader: (4)

are you serious? 285s in the front? wow
__________________
Appreciate 0
      12-28-2010, 04:25 PM   #11
cgroppi
Private
cgroppi's Avatar
United_States
2
Rep
64
Posts

Drives: 2007 Z4 M Coupe
Join Date: Mar 2010
Location: Tempe, AZ

iTrader: (0)

Quote:
Originally Posted by Roffle Waffle View Post
are you serious? 285s in the front? wow
Yup. Only works with Hoosiers though. You could never get a 285-30 street tire on an 8" wide wheel. You can't even get Kumho V710s in that size on an 8" wheel without some radical methods. Hoosiers pop right on.

Also, remember that this is a kludge forced by SCCA rules. You would not want to run a tire that wide on a wheel that skinny if you were allowed wider wheels.
Appreciate 0
      12-30-2010, 10:06 PM   #12
tarheel91
Private First Class
3
Rep
171
Posts

Drives: ESS Z4 3.0i
Join Date: Nov 2010
Location: Carolina

iTrader: (0)

Wider tire = a more ovaly contact patch that has pressure more evenly distributed throughout. The key to grip is to have the largest, most homogeneous contact patch possible. A wider tire helps with the latter. This means more grip. A wider tire will also generally reach peak grip at a smaller slip angle because of increased lateral stiffness (hence the concept of wider tires breaking away more suddenly) which means BETTER turn in (assuming identical side walls). You will have to fight it more (i.e. heavier steering), but that shouldn't be an issue with power steering.
Appreciate 0
      08-05-2012, 01:15 AM   #13
S2000
Private
United_States
19
Rep
97
Posts

Drives: E86 Z4M, E60 M5, E92 M3
Join Date: Dec 2008
Location: Austin TX

iTrader: (1)

Quote:
Originally Posted by DSC_OFF View Post
thats weird b/c when i used the wheel offset calculator here:

http://www.e90post.com/forums/forumdisplay.php?f=12

i put in oem settings of: 8.0", et 50, 225/40/18
new wheel: 8.5", et 42, 255/35/18

and it spits out: "2mm MORE clearance on the strut side, and the outside edge of the wheel will EXTEND an extra 14mm"
The formula calculates only the effect of wheel width and not counting the tire tread width. It should have calculated both and give the "greater" of two.

For example you put
OEM setting 8", et 42, 225/40/18
New super wide tires (on the same wheel) 8" , et 42, 295/30/18
It says "same clearance" which is WRONG.

We know the tire width will extend more to both inside and outside , but formula ignores the tire width and only reported the tire diameter effect.
Appreciate 0
      08-05-2012, 05:20 AM   #14
pmmjarrett
Retired Paratrooper
pmmjarrett's Avatar
United_States
5
Rep
146
Posts

Drives: 2006 BMW Z4 3.0i Sport
Join Date: Jul 2012
Location: North Carolina

iTrader: (0)

Quote:
Originally Posted by DSC_OFF View Post
thats weird b/c when i used the wheel offset calculator here:

http://www.e90post.com/forums/forumdisplay.php?f=12

i put in oem settings of: 8.0", et 50, 225/40/18
new wheel: 8.5", et 42, 255/35/18

and it spits out: "2mm MORE clearance on the strut side, and the outside edge of the wheel will EXTEND an extra 14mm"
That calculator is flawed. It only calculates the wheel clearances and ignores tire sidewall clearances completely.

The only dimensions it's calculating for tires is the diameter and it's using that to calculate speedometer error.
Appreciate 0
      08-05-2012, 09:30 AM   #15
F360C
Major
United_States
366
Rep
1,373
Posts

Drives: E86 S54
Join Date: Apr 2006
Location: Hoboken, NJ

iTrader: (2)

Quote:
Originally Posted by Roffle Waffle View Post
are you serious? 285s in the front? wow
I am running a square 275/35/18 set up currently for my track set up. It took a couple minor mods to make them fit but nothing too serious




__________________

07 Z4 M Coupe AW/IR
Appreciate 0
      08-05-2012, 09:06 PM   #16
Kgolf31
Brigadier General
Kgolf31's Avatar
459
Rep
4,531
Posts

Drives: 2007 Z4MC, 2012 128i
Join Date: Sep 2008
Location: Ohio

iTrader: (4)

R-Comps and Street Tires are 2 different enemies.

R-Comps you can get more tire on a wheel. Like said earlier, it's not abnormal to see +30mm of tire on a wheel with R-Comps versus STs.

I run 235s on a stock wheel up front. If you run 245s on a 18x8 wheel, you'll actually lose speed. It's too much tire for that wheel.

Given a 1/2 inch is 12mm, I'd only suggest going 245s on the 18x8.5 wheel. With 255s, you'll be putting too much tire on that wheel, and the tire itself will roll too much across the wheel and you'll loose alot of turn-in.

Again, from personal experience. I still don't know how you guys are getting that much tire in front (and rear). I'm close the the strut in the front with 235s + 5mm spacers.

In the rear I rub with 265s. With a 5mm spacer it's better, but I'm convinced I rub in hard corners.

These are with RS3s
Appreciate 0
Post Reply

Bookmarks

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -5. The time now is 03:30 AM.




zpost
Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
1Addicts.com, BIMMERPOST.com, E90Post.com, F30Post.com, M3Post.com, ZPost.com, 5Post.com, 6Post.com, 7Post.com, XBimmers.com logo and trademark are properties of BIMMERPOST