Welcome to the North American Subaru Impreza Owners Club Saturday September 20, 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 03-22-2007, 10:53 AM   #1
BIGSKYWRX
Scooby Guru
 
Member#: 7958
Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:
09 GH B+

Default camber plates- scrub radius- offset

Have never thought of this before (and may be way off base ) but it would appear that for those of us running camber plates it might be advantageous to run a slightly lowere offset wheel (which most of us probably are anyways).

Here's my reasoning- first a diagram of scrub radius



another



If you move the top of the strut inboard (as you would w/ camber plates) to maintain a 0 (or closer to 0) it appears you would need to lower the offset by x amount

What I don't know is how much the top of the strut typically is moving and it's overall effect on scrub radius- any engineers out there
* Registered users of the site do not see these ads.

Last edited by BIGSKYWRX; 03-22-2007 at 10:49 PM.
BIGSKYWRX is offline   Reply With Quote
Old 03-22-2007, 11:41 AM   #2
Mind
Scooby Specialist
 
Member#: 109216
Join Date: Mar 2006
Chapter/Region: MAIC
Location: PA
Vehicle:
08 STI DGM
76-77 Lancia Scorpion(s)

Default

Hmm... I thought the scrub radius 'zero' point was determined more by the steering mechanism, and where the 'steering' axis intersects the ground. I'll have to double check when I get home to see if and/or how the strut angle would change this.

Great info and diagrams. In some of the suspension texts I've been reading they comment that scrub radius ideally should be close to zero, and having a negative scrub radius (tire centerline is inboard of the zero scrub radius point) should be avoided.

Last edited by Mind; 03-22-2007 at 11:56 AM.
Mind is offline   Reply With Quote
Old 03-22-2007, 02:38 PM   #3
BIGSKYWRX
Scooby Guru
 
Member#: 7958
Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:
09 GH B+

Default

Here's one working definition
"Scrub radius is the distance between where the SAI intersects the ground and the center of the tire. This distance must be exactly the same from side to side or the vehicle will pull strongly at all speeds. While included angle problems will affect the scrub radius, it is not the only thing that will affect it. Different wheels or tires from side to side will cause differences in scrub radius as well as a tire that is low on air. Positive scrub radius is when the tire contact patch is outside of the SAI pivot, while negative scrub radius is when the contact patch is inboard of the SAI pivot (front wheel drive vehicles usually have negative scrub radius)"
BIGSKYWRX is offline   Reply With Quote
Old 03-22-2007, 10:38 PM   #4
Mind
Scooby Specialist
 
Member#: 109216
Join Date: Mar 2006
Chapter/Region: MAIC
Location: PA
Vehicle:
08 STI DGM
76-77 Lancia Scorpion(s)

Default

The kingpin axis is defined by the lower ball-joint and the upper strut mounting point. The thing that was confusing me with the diagrams was that one of the pics shows the strut axis to be the same as the kingpin axis, which isn't always the case. But yes, with the strut top moved further inboard, the zero-scrub radius moves outward.

But this leads me to another question... on a stock setup, what offset wheel provides a zero scrub radius?

Maybe this would be better in the suspension forum?
Mind is offline   Reply With Quote
Old 03-22-2007, 10:48 PM   #5
BIGSKYWRX
Scooby Guru
 
Member#: 7958
Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:
09 GH B+

Default

I thought about posting there and should have probably

I'd have to guess that the oe offset (53 or 55 depending on model) is at 0 scrub, but maybe not
BIGSKYWRX is offline   Reply With Quote
Old 03-22-2007, 10:50 PM   #6
silver arrow
Scooby Specialist
 
Member#: 25859
Join Date: Oct 2002
Chapter/Region: South East
Location: Foriderp
Vehicle:
1965 MGB yellow
2008 Black Miata

Default

I'd try Motorsports too. Some pretty knowledgeable people in there too.
silver arrow is offline   Reply With Quote
Old 03-22-2007, 11:10 PM   #7
Mind
Scooby Specialist
 
Member#: 109216
Join Date: Mar 2006
Chapter/Region: MAIC
Location: PA
Vehicle:
08 STI DGM
76-77 Lancia Scorpion(s)

Default

Quote:
Originally Posted by BIGSKYWRX View Post
I thought about posting there and should have probably

I'd have to guess that the oe offset (53 or 55 depending on model) is at 0 scrub, but maybe not
I was just wondering the other day why the '06+ WRX wheels were increased to a 55mm offset... I can't imagine anything being different about the suspension geometries between the '06 and '05, which means they have different scrub radii. I thought maybe the potential for wagons to run with a +0 tire and not rub may have been some motivation... maybe...

I would assume Subaru is taking a FWD design approach in the front suspension since those wheels are still providing a good amount of tractive force. So maybe there's some negative scrub radius with the OE offset wheels. But that's just me guessing. Maybe the next time I've got some time I'll take some measurements and try to determine what the scrub radius is. But I know someone on here has had to have already done this...

I know that between my OE 55ET 215/45-17 RE92 and my summer setup, the '04 BBS 53ET 225/45-17 RE070, there is definitely a lot more torque coming through the steering wheel with the summer setup. But there's so much difference between those two tires that I can't assess what is due to what.
Mind is offline   Reply With Quote
Old 03-22-2007, 11:45 PM   #8
DuckStu
Scooby Specialist
 
Member#: 124060
Join Date: Aug 2006
Location: Bloomfield Michigan
Vehicle:
2004 WRX STI
Silver

Default

I figured it a simpler way (not knowing what scrub radius was).

I guessed that tilting the wheels in (more negative camber) would cause the tires to ride a bit on the inside edge (actualy just move the center of drag or center of weight in a bit). So to have the center of drag in the same relative position to the kingpin axis,...you would run less offset (move wheels outwards) to get it back in the right zone.

I just ordered a set of Enkei RFP1's in 18x8 45mm with Michelin Pilot Sport PS2's (235/40/18). The wheel offset is 9mm less than stock, but I run more negatiove camber (-1.5 instead of -.5), so I'm hoping things will be alright.

Got a line on the center caps that finally came out for the 16-18" sizes.
DuckStu is offline   Reply With Quote
Old 03-23-2007, 12:09 AM   #9
snovvbrdr
Scooby Newbie
 
Member#: 121063
Join Date: Jul 2006
Chapter/Region: NESIC
Location: Manchester, NH
Vehicle:
2006 WRX Stg. II
Steel Gray

Default

Those caps are so hard to get in, takes like 10 min. a cap (being careful not to scratch the wheels)
snovvbrdr is offline   Reply With Quote
Old 03-23-2007, 09:36 AM   #10
BIGSKYWRX
Scooby Guru
 
Member#: 7958
Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:
09 GH B+

Default

Quote:
Originally Posted by Mind View Post
I was just wondering the other day why the '06+ WRX wheels were increased to a 55mm offset... I can't imagine anything being different about the suspension geometries between the '06 and '05, which means they have different scrub radii. I thought maybe the potential for wagons to run with a +0 tire and not rub may have been some motivation... maybe...
Well the 02-05 WRX wheels were all 55 et as well, they were 16x6.5 vs 17x7 however. The older GC/GF's ran a 16x7 53 et

Not sure on the offsets of the STi's
BIGSKYWRX is offline   Reply With Quote
Old 03-23-2007, 09:43 AM   #11
Turn in Concepts
NASIOC Vendor
 
Member#: 93646
Join Date: Aug 2005
Chapter/Region: MWSOC
Vehicle:
Many Track Records
Let us help you go fast!

Default

Quote:
Originally Posted by BIGSKYWRX View Post
Well the 02-05 WRX wheels were all 55 et as well, they were 16x6.5 vs 17x7 however. The older GC/GF's ran a 16x7 53 et

Not sure on the offsets of the STi's
The USDM sti's have all had 53mm offsets. The 04's obviously 17x7.5 and 05+ 17x8.

Tony
Turn in Concepts is offline   Reply With Quote
Old 03-26-2007, 09:50 AM   #12
BIGSKYWRX
Scooby Guru
 
Member#: 7958
Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:
09 GH B+

Default

FYI I posted in suspension- Whiteline was kind of enough to do a calculation (based on moving the strut 1" inboard- I think this is close to what many would be moving w/ camber plates).

SAI increases by 2.25" which equates to a 7.8mm increase in scrub radius, so a mid 40ish offset wheel would maintain a pretty close to oe scrub radius
BIGSKYWRX is offline   Reply With Quote
Old 03-26-2007, 10:56 AM   #13
fliz
Scooby Guru
 
Member#: 30342
Join Date: Nov 2002
Location: #blp
Default

Quote:
Originally Posted by BIGSKYWRX View Post
Well the 02-05 WRX wheels were all 55 et as well, they were 16x6.5 vs 17x7 however. The older GC/GF's ran a 16x7 53 et

Not sure on the offsets of the STi's
I've seen both +53 and +55 on older WRX's. My '02 has +53.
fliz is offline   Reply With Quote
Old 03-26-2007, 11:00 AM   #14
BIGSKYWRX
Scooby Guru
 
Member#: 7958
Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:
09 GH B+

Default

hmmm- my 02 was 55 et, as well as my 04- hard telling what they were thinking.
BIGSKYWRX is offline   Reply With Quote
Old 03-26-2007, 12:21 PM   #15
Uncle Scotty
Scooby Guru
 
Member#: 16200
Join Date: Mar 2002
Vehicle:
OK Houston
we have an Uncle

Default

Quote:
Originally Posted by BIGSKYWRX View Post
FYI I posted in suspension- Whiteline was kind of enough to do a calculation (based on moving the strut 1" inboard- I think this is close to what many would be moving w/ camber plates).

SAI increases by 2.25" which equates to a 7.8mm increase in scrub radius, so a mid 40ish offset wheel would maintain a pretty close to oe scrub radius
Mike...I've done a lot of research off this board since i've had a WRX and I believe that one of the reasons that there are so many people running wheels down to +40-+42 is that THEY discovered this long ago and many of them use camber/caster tops.

There seem to be a great many things that are done in other parts of the world that 'we' don't know about or are dismissed, off hand, by the plodding masses here


Quote:
I've seen both +53 and +55 on older WRX's. My '02 has +53.
My '02 had +53 wheels, stock
Uncle Scotty is offline   Reply With Quote
Old 03-26-2007, 01:35 PM   #16
Mind
Scooby Specialist
 
Member#: 109216
Join Date: Mar 2006
Chapter/Region: MAIC
Location: PA
Vehicle:
08 STI DGM
76-77 Lancia Scorpion(s)

Default

Quote:
Originally Posted by Uncle Scotty View Post
There seem to be a great many things that are done in other parts of the world that 'we' don't know about or are dismissed, off hand, by the plodding masses here
I'll bite... like what?
Mind is offline   Reply With Quote
Old 03-26-2007, 02:23 PM   #17
waktasz
Scooby Guru
 
Member#: 86356
Join Date: May 2005
Chapter/Region: Tri-State
Default

^^

D,

BOVs are actually good for your car and make power.
waktasz is offline   Reply With Quote
Old 03-27-2007, 01:19 AM   #18
Uncle Scotty
Scooby Guru
 
Member#: 16200
Join Date: Mar 2002
Vehicle:
OK Houston
we have an Uncle

Default

Quote:
Originally Posted by Mind View Post
I'll bite... like what?
...do your own reasearch...I charge $100/hr
Uncle Scotty is offline   Reply With Quote
Old 03-27-2007, 01:53 AM   #19
chimchimm5
Scooby Guru
 
Member#: 88501
Join Date: Jun 2005
Chapter/Region: BAIC
Location: NorCal
Vehicle:
2011 WRX Hatch
Dark Grey

Default

Could it be that the zero scrub stock is +55, and the +53 is a "within tolerance, more available wheel" because erring towards outwards/positive scrub radius is better?
chimchimm5 is offline   Reply With Quote
Old 03-27-2007, 11:03 AM   #20
Mind
Scooby Specialist
 
Member#: 109216
Join Date: Mar 2006
Chapter/Region: MAIC
Location: PA
Vehicle:
08 STI DGM
76-77 Lancia Scorpion(s)

Default

Quote:
Originally Posted by waktasz View Post
^^

D,

BOVs are actually good for your car and make power.
...don't forget S-techs FTW!!pne!
Mind is offline   Reply With Quote
Old 03-27-2007, 11:28 AM   #21
Uncle Scotty
Scooby Guru
 
Member#: 16200
Join Date: Mar 2002
Vehicle:
OK Houston
we have an Uncle

Default

Quote:
Originally Posted by chimchimm5 View Post
Could it be that the zero scrub stock is +55, and the +53 is a "within tolerance, more available wheel" because erring towards outwards/positive scrub radius is better?
http://img.photobucket.com/albums/v1...scotty/que.jpg
Uncle Scotty is offline   Reply With Quote
Old 03-27-2007, 11:43 PM   #22
subber
Scooby Newbie
 
Member#: 63243
Join Date: Jun 2004
Chapter/Region: SCIC
Location: SoCal
Vehicle:
2004 WRX
WRB

Default

Quote:
Originally Posted by BIGSKYWRX View Post
FYI I posted in suspension- Whiteline was kind of enough to do a calculation (based on moving the strut 1" inboard- I think this is close to what many would be moving w/ camber plates).

SAI increases by 2.25" which equates to a 7.8mm increase in scrub radius, so a mid 40ish offset wheel would maintain a pretty close to oe scrub radius
I tend to imagine that as you move the top of the strut (using camber plate) towards the engine, the entire wheel (and tire) moves with it which means the scrub radius does not change - in fact, if it does change it seems scrub radius will tend to increase or become more positive. You would have to reduce the wheel camber using the camber bolts in order to meaningfully affect the scrub radius. I suppose Whiteline did that in their calculation.?


Last edited by subber; 03-28-2007 at 12:09 AM.
subber is offline   Reply With Quote
Old 03-27-2007, 11:58 PM   #23
BIGSKYWRX
Scooby Guru
 
Member#: 7958
Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:
09 GH B+

Default

Your tire/wheel will have gained camber (obviously ) but the center line of your tire/wheel wouldn't have moved. The SAI has increased though and the line through the strut has now "inched" outward
BIGSKYWRX is offline   Reply With Quote
Old 03-28-2007, 12:10 AM   #24
subber
Scooby Newbie
 
Member#: 63243
Join Date: Jun 2004
Chapter/Region: SCIC
Location: SoCal
Vehicle:
2004 WRX
WRB

Default

I edited my post with a pic. I'm sure I'm missing something or my logic is flawed somewhere... I dont think the centerline of the wheel will change unless you change wheel offsets... I suppose the center of the contact patch would have moved inward somewhat, so then do you equate the wheel centerline with the center of the contact patch?

This is very interesting. I hope I'm wrong though since if you are correct, this will allow me to justify getting lower offset wheels!
subber is offline   Reply With Quote
Old 03-28-2007, 12:19 AM   #25
BIGSKYWRX
Scooby Guru
 
Member#: 7958
Join Date: Jul 2001
Location: Plains of Eastern Montana
Vehicle:
09 GH B+

Default

As the top of the strut is tipped inward- you SAI is being increased, but the center line of the wheel hasn't changed- you have changed the static camber, but not the centerline- they only way to do this is w/ a change of track- lengthening/shortening control arm or changing offset.

I'm probably not explaining it very well.

http://www.familycar.com/classroom/Images/Align_SAI.gif
BIGSKYWRX 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
thoughts on camber plates and scrub radius/offset BIGSKYWRX Brakes, Steering & Suspension 91 08-12-2009 11:28 PM
Scrub Radius Gocasen Brakes, Steering & Suspension 1 01-13-2008 09:55 PM
Impreza scrub radius? Grant Brakes, Steering & Suspension 0 04-17-2007 11:58 AM
Need a little clarification on Scrub Radius / 2-way camber adjustment caterpill Brakes, Steering & Suspension 1 02-10-2006 04:40 AM
Lowering and effect on scrub radius chimchimm5 Brakes, Steering & Suspension 1 02-07-2006 04:41 PM


All times are GMT -4. The time now is 10:16 AM.


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.