Welcome to the North American Subaru Impreza Owners Club Saturday December 27, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
Click here to visit TireRack
Tire & Wheel Forum sponsored by The Tire Rack

Losing traction? Need new tires?
Click here to visit the NASIOC Upgrade Garage...
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC Technical > Tire & Wheel

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 10-16-2006, 04:14 AM   #1
caterpill
Scooby Specialist
 
Member#: 27136
Join Date: Oct 2002
Cool Is there a way to calculate/maintain the inner strut-tire clearance with wider rims

Is there a way to calculate and/or possibly maintain the inner strut-to-tire clearance while increasing rim/tire width? While reading the stickied "Offset Information Thread", I read this;

"Please people, do not just quote random specs- offset is directly related to rim width- you can certainly use a +55 6.5" wide rim on most late model Subarus, but you cannot use a +55 8" wide rim! Offset Nirvana is within a few millimeters of:

Width .... Offset

6.5" ....... +53 to +55
7.0" ....... +50 to +53
7.5" ....... +47 to +50
8.0" ....... +43 to +47

Anything within sighting distance of these ranges is happiness for your wheel bearings."

I assume that this relationship between rim width and offset is to maintain suspension/scrub radius geometry, and essentially the inner clearance of the strut and wheel/tire assembly.

I currently have a 17x8 +48 with 215-width tires, and would possibly like to explore the realm of wider shoes to both widen the track minutely and increase contact patch. However, I currently have coilovers and the tire is very close to the coilover springs. I wanted to possibly move up to a 17x8.5" or even 17x9" with some 225/235 tires, but without the tire rubbing the coilover from the resultant increase in width.

For instance, if I were to upgrade from a 7.5" wide rim to a 8" wide rim, the offset would change from +50 to +47. However, if I understand correctly, rim width means the rim will increase equally on both sides? So a 8" +50 rim will be 6.35mm wider on both sides of the mounting flange compared to a 7.5" +50 rim? Ok. But that doesn't take into account the 3mm difference in offset relationship. So if I am thinking correctly, if someone was to upgrade from a 17x7.5" +50 to a 17x8" +47, the tire would be 3.35mm closer to the strut?

I was just confused with the offset calculations. How would I know what offset would allow only a change in clearance of the fender to tire and not strut to tire?

It's a little tricky to explain but I tried my best. hehe.

Any help would be greatly appreciated.

Thanks.
* Registered users of the site do not see these ads.
caterpill is offline   Reply With Quote
Old 10-16-2006, 02:27 PM   #2
Patrick Olsen
NASIOC Supporter
 
Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
Vehicle:
1997 Legacy 2.5GT
QuickSilver Metallic

Default

What you're looking for is backspacing, which is the distance from the mounting face of the wheel to the inner lip of the wheel.

Backspacing = (Advertised width + 1")/2 + offset/25.4mm/in

So, your example wheels would be...

17x8" +50mm: BS = (8" + 1")/2 + 50mm/25.4mm/in = 6.47" (164.30mm)
17x8" +47mm: BS = (8" + 1")/2 + 50mm/25.4mm/in = 6.35" (161.30mm)
17x7.5" +50mm: BS = (7.5" + 1")/2 + 50mm/25.4mm/in = 6.22" (157.95mm)

So the answer to your question...
Quote:
Originally Posted by Caterpill
So if I am thinking correctly, if someone was to upgrade from a 17x7.5" +50 to a 17x8" +47, the tire would be 3.35mm closer to the strut?
... is YES.

Of course, that's not the final answer on how close things will be to the strut - unless you're pimpin' the do-rift-o style stretched tires (say, 195s on your 8" wheels...) the wheel is not going to be the closest thing to the strut - the sidewall of the tire is. So what you're concerned with there is the section width of the tire, which you should be able to get on the manufacturer's website or Tire Rack or Discount Tire Direct, etc.

Buuuuut... It's not that simple. A given tire will fit on a range of widths, and the section width of the tire is going to change as you put the tire on different width wheels, and the manufacturer's specs are only going to list the section width when mounted on one wheel width. So the 215/45s you have on 17x8" wheels are going to be wider than the same tires mounted on 17x7" wheels. The thumbrule (not exact, mind you) is that for every 0.5" increase in wheel width, the section width will increase by 0.2" (or 0.1" on each side of the tire).

So, to go back to your question, if you mount the same tire on the 17x8" +47mm as you've got on the 17x7.5" +50mm, the wheel lip will move 3.35mm closer to the strut, but section width of the tire will also change (it'll get fatter by 0.2"). So the sidewall of the tire will be about 5.8mm closer to the strut (the 3.35mm + 0.1"). Make sense?

Oh, and 9" wheels would be a waste if you're only going to put 225s or 235s on them. See earlier comment about do-rift-o stretched tires. I have 255/40s on 17x9"s on my Mustang and the sidewall is pretty much vertical. If I lay the wheel down, the lip of the wheel touches the ground, not the tire. A 225 would look stupid on a 9" wheel, and a 235 wouldn't be much better.

Pat Olsen
'97 Legacy 2.5GT sedan

Last edited by Patrick Olsen; 10-16-2006 at 10:03 PM. Reason: To clarify some things...
Patrick Olsen is offline   Reply With Quote
Old 10-16-2006, 06:10 PM   #3
Zalthras
Scooby Newbie
 
Member#: 102022
Join Date: Dec 2005
Chapter/Region: NWIC
Location: Portland (Tigard), OR
Vehicle:
2006 wrb WRX
1987 panda FX16

Default

I think your questions can be easily answered if you play with this:
http://www.rims-n-tires.com/rt_specs.jsp
Zalthras is offline   Reply With Quote
Old 10-16-2006, 09:43 PM   #4
Patrick Olsen
NASIOC Supporter
 
Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
Vehicle:
1997 Legacy 2.5GT
QuickSilver Metallic

Default

Ooooooh, that's a good one, Zalthras. People always throw out the Miata wheel/tire diameter calculator, but I've never seen the site you posted before! I'll have to remember that one. The only downside for this particular discussion is it uses a "generic" tire with a set section width, so it doesn't show how that changes.

Pat

Last edited by Patrick Olsen; 10-16-2006 at 10:04 PM.
Patrick Olsen is offline   Reply With Quote
Old 10-16-2006, 10:36 PM   #5
kenny08
Scooby Specialist
 
Member#: 79907
Join Date: Jan 2005
Chapter/Region: SCIC
Location: socal
Vehicle:
02 blue wagon
18g goodnesss

Default

im running 235/40/18 on a +51 18x8 rim, and im just BARELY clearing the strut, barely.
kenny08 is offline   Reply With Quote
Old 10-16-2006, 11:53 PM   #6
ride5000
Scooby Guru
 
Member#: 32792
Join Date: Feb 2003
Chapter/Region: NESIC
Location: lincoln, ri
Vehicle:
2003 GGA MBP
12.9 / 105+

Default

Quote:
Originally Posted by Zalthras View Post
I think your questions can be easily answered if you play with this:
http://www.rims-n-tires.com/rt_specs.jsp
nice link
ride5000 is offline   Reply With Quote
Old 10-17-2006, 03:36 AM   #7
caterpill
Scooby Specialist
 
Member#: 27136
Join Date: Oct 2002
Default

Quote:
Originally Posted by Patrick Olsen View Post

Oh, and 9" wheels would be a waste if you're only going to put 225s or 235s on them. See earlier comment about do-rift-o stretched tires. I have 255/40s on 17x9"s on my Mustang and the sidewall is pretty much vertical. If I lay the wheel down, the lip of the wheel touches the ground, not the tire. A 225 would look stupid on a 9" wheel, and a 235 wouldn't be much better.

Pat Olsen
'97 Legacy 2.5GT sedan
Oh, I do know that 225/235's on a 9" wheel would be a waste. Sorry, those numbers were just thrown out for example. I wasn't thinking of stretching tires.

Also, I needed to figure out if my calculations were on the right track. I've never fully understood how it all comes together.

Thank You Again for the help.
Dan
caterpill is offline   Reply With Quote
Reply

Thread Tools
Display Modes

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
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
is there a way to take off the side moldings off the 08 sti? swizz977 Interior & Exterior Modification 1 08-13-2009 10:32 PM
Is there any way to tell what the motor is without the sticker? darksc00by Subaru Conversions 27 11-11-2007 12:03 PM
Is there a way to fix/maintain the vf39? snowcrzy Factory 2.5L Turbo Powertrain (EJ Series Factory 2.5L Turbo) 9 10-16-2006 11:01 PM
Is there a way to vote in the presidential election without leaving my house? eurojax Off-Topic 8 10-01-2004 01:16 PM
is there a way to d/l the UTEC software to practice? wrxtuner02 Engine Management & Tuning 2 12-05-2002 02:41 PM


All times are GMT -4. The time now is 07:04 PM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.