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Old 03-26-2007, 12:14 AM   #1
BIGSKYWRX
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Default thoughts on camber plates and scrub radius/offset

posted this in wheel/tire http://forums.nasioc.com/forums/show....php?t=1227147

thought it might get a little response here- basically w/ camber plates pushing the top of the strut inboard, which should be changing SAI- it then should be changing scrub radius as well???

My thought is w/ running camber plates, run a slightly lower offset- to maintain a closer to stock scrub radius???

Any engineering (or geometry ) types care to take a stab at the change in scrub radius (if any) w/ say a 1" movement of the strut top inboard?
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Old 03-26-2007, 03:40 AM   #2
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Originally Posted by BIGSKYWRX View Post
My thought is w/ running camber plates, run a slightly lower offset- to maintain a closer to stock scrub radius???
bingo.
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Old 03-26-2007, 03:45 AM   #3
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Hi bigskywrx,

That would equate to 7.8mm increase in scrub radius, 2.25deg increase in SAI, and a generous (too sensitive information) increase to roll centre height as a bonus.

Cheers,
Wojtek.
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Old 03-26-2007, 10:46 AM   #4
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I knew it was worth posting in this fourm

Wojtek- thanks for the numbers. An increase in roll center height too-that's worth-

Thanks guys!
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Old 03-26-2007, 09:06 PM   #5
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anyone care to explain what scrub radius and SAI are?

Thanks!
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Old 03-26-2007, 09:08 PM   #6
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Originally Posted by WL Flatout View Post
Hi bigskywrx,

That would equate to 7.8mm increase in scrub radius, 2.25deg increase in SAI, and a generous (too sensitive information) increase to roll centre height as a bonus.

Cheers,
Wojtek.
Wow, that 7.8mm is pretty significant... is it pretty common for camber plates to be able to reposition the strut top inboard by as much as an inch?
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Old 03-26-2007, 09:44 PM   #7
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SGD- click on the link above I've got a couple of pics that illustrate it pretty well.

I'm just going by memory (which sometime ails me ), but 1" (25mm) seems realistic to me.
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Old 03-26-2007, 09:47 PM   #8
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25.4mm to be precise
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Old 03-26-2007, 09:54 PM   #9
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1" is definitely within reason for a camber plate. That would be pretty much the max setting. So basically, this makes 45 the ideal offset when using camber plates at the max setting.

The only drawback to a large SAI that I can think of is a slightly reduced wheel rate of the springs (wheel rate of a McStrut = cos(φ) where φ = the total tilt of the strut when you account for caster and SAI) and perhaps the associated camber change during large-steering-input maneuvers. A higher SAI will also lend itself to heavier steering, but many would welcome that, I believe.

I know all the Rota wheels are 48 offset and many of the aftermarket wheels that are available are also 45. I think when using camber plates you should aim for a very slightly positive scrub radius just to have some good feedback from the tires.

Last edited by nhluhr; 03-26-2007 at 10:02 PM.
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Old 03-27-2007, 12:34 AM   #10
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A little lower wheel rate, but a higher roll center (classified how much higher )

<--- running a 46 et wheel (in the "summer" anyways)
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Old 03-27-2007, 02:18 AM   #11
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yeah I imagine the wheel rate issue is negated by the higher roll center, wider track, and increased camber
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Old 03-27-2007, 05:51 AM   #12
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Quote:
Originally Posted by S.G.D View Post
anyone care to explain what scrub radius and SAI are?
Steering Axis Inclination (SAI) is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle.

Scrub radius is the distance between where the SAI intersects the ground and the center of the tire. 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. Rear-wheel drive cars and trucks generally have a positive scrub radius while FWD cars usually have zero or a negative scrub radius because they have a higher SAI angle. Using wheels with different offset than stock can alter the scrub radius.
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Old 03-27-2007, 06:54 AM   #13
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Quote:
Originally Posted by Mark Avery View Post
Steering Axis Inclination (SAI) is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle.

Scrub radius is the distance between where the SAI intersects the ground and the center of the tire. 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. Rear-wheel drive cars and trucks generally have a positive scrub radius while FWD cars usually have zero or a negative scrub radius because they have a higher SAI angle. Using wheels with different offset than stock can alter the scrub radius.
for pics, check the link

http://www.familycar.com/Alignment.htm
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Old 03-29-2007, 10:19 AM   #14
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thanks!!
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Old 02-16-2009, 10:29 PM   #15
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Ok, I've been doing a little research on scrub radius and trying to learn some stuff about it but its pretty complicated. Seems like some of you really know what you are talking about lol. I have an '04 WRX that I just bought new wheels for. The tire size is 235/40-18 and the stock tires were 205/55-16. I measured them, and the new tires have a 5/8" larger diameter. From the research I have done it seems I have made the scrub radius more positive. Of course the car handles kinda funny, and tracks a lot more. My question is what can I do to correct this and make it handle like it did with the stock wheels? or is there anything I can do?
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Old 02-16-2009, 11:15 PM   #16
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^The offset is really the only thing that would affect the scrub radius. Did you switch to stickier tires? On my summer tires my car tramlines like crazy, but on winter tires there's absolulely zero tramlining.

Last edited by Mind; 02-16-2009 at 11:32 PM.
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Old 02-16-2009, 11:30 PM   #17
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camber plates pushed in would get the scrub close to stock

nothing to do w/ your situation, but here's a good technical read on this very subject

http://zzyzxmotorsports.com/library/...h-20070918.pdf
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Old 02-17-2009, 01:56 AM   #18
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However, for a lot of us, thinking has switched on the plate adjustment issue. While moving the strut tops inwards does raise the roll center, which is a positive thing, it also increases SAI, which is a big negative. SAI causes dynamic camber loss.

So, in other words, instead of trying to restore your scrub radius with offset changes, don't screw it up in the first place and you'll be better off anyway.
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Old 02-17-2009, 02:34 AM   #19
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Quote:
Originally Posted by williaty View Post
However, for a lot of us, thinking has switched on the plate adjustment issue. While moving the strut tops inwards does raise the roll center, which is a positive thing, it also increases SAI, which is a big negative. SAI causes dynamic camber loss.

So, in other words, instead of trying to restore your scrub radius with offset changes, don't screw it up in the first place and you'll be better off anyway.
which is why increasing positive caster is also such a good thing
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Old 02-17-2009, 02:39 AM   #20
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Yup, exactly.

That's why I like the plates I'm using. They start off at maximum caster gain. Then I can move them inwards to gain camber only the amount I absolutely have to to get the camber I want. Best of both worlds.
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Old 02-17-2009, 02:41 AM   #21
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Why i like the Ground Control plates, independent camber and caster.
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Old 02-17-2009, 09:42 AM   #22
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Quote:
Originally Posted by williaty View Post
However, for a lot of us, thinking has switched on the plate adjustment issue. While moving the strut tops inwards does raise the roll center, which is a positive thing, it also increases SAI, which is a big negative. SAI causes dynamic camber loss.

So, in other words, instead of trying to restore your scrub radius with offset changes, don't screw it up in the first place and you'll be better off anyway.
you have a way of quantifying that?- camber plates are going away any time soon
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Old 02-17-2009, 10:05 AM   #23
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Stretch did a huge writeup on IWSTI about this a few months ago. Actually went through the calcs.

I am going to play with this myself next month. Swapping sides on my RCE plates and rotate the plates as well. Coupled with my new control arms and 6 gun hubs I should hit close to 8 degrees of caster and be able to run a ton of camber as well.

Last edited by Daishi00; 02-17-2009 at 08:10 PM. Reason: my spelling is the sucketh
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Old 02-17-2009, 10:35 AM   #24
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Quote:
Originally Posted by williaty View Post
However, for a lot of us, thinking has switched on the plate adjustment issue. While moving the strut tops inwards does raise the roll center, which is a positive thing, it also increases SAI, which is a big negative. SAI causes dynamic camber loss.

So, in other words, instead of trying to restore your scrub radius with offset changes, don't screw it up in the first place and you'll be better off anyway.

+1 for that. I do not consider myself an expert by a long ways, though I have been studying the formulae for suspension geometry for quite some time. I have been down this road on other builds and I know that all of the components involved come to a point where you can no longer compromise any single factor without screwing up the whole lot. The one bit of information I have not yet been able to find is the Steering Axis Inclination angle (in stock form) on the '07 wagon. If anyone has a line on this could you let me know ?

Thanks BigSky for even touching on this subject as it is so often disregarded.
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Old 02-17-2009, 10:39 AM   #25
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Yup, exactly.

That's why I like the plates I'm using. They start off at maximum caster gain. Then I can move them inwards to gain camber only the amount I absolutely have to to get the camber I want. Best of both worlds.
Which plates are you using ? What do you think about the Vorshlag plates ? I know I had them on my BMW and loved them. I just saw the ones for the Subaru and they look to be the same. I think they increase castor by .8 but I could be mistaken.
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