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  #1  
Old 05-06-2005, 08:53 PM
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Scrub Radius

Does anyone know what the factory scrub radius is on the SVX? I know it's negative, but by how much? The reason I'm wondering is because many aftermarket wheels have a much lower offset than the SVX's original equipment. The factory 55mm offset on a 7.5" rim adds up to substantial negative scrub. Would a rim with 38mm offset get you to, (or past,) zero scrub? This seems to be about the minimum offset that people can run with any degree of success.

If nobody knows the answer, does anyone know a quick and easy way to measure it? Crawling under the car with a floorjack and measuring tape, while probably rather accurate, requires me to get off my lazy butt.

Ultimately, I wonder how much, (if any,) fender flexing needs to be done to get into positive scrub territory.
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  #2  
Old 05-06-2005, 10:16 PM
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Hate to be the one but what exactly is scrub radious?
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  #3  
Old 05-06-2005, 10:19 PM
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Quote:
Originally Posted by thundering02
Hate to be the one but what exactly is scrub radious?
Yeah i have the same question. I thought this thread was about cleaning the car
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  #4  
Old 05-07-2005, 12:20 AM
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"Scrub" is a lot like rim offset, but instead of measuring the distance from the hub mounting surface to the tire rim, the measurement is from the center of the tire surface to the intersection of the steering axis and the road surface. On an SVX, the steering axis, or kingpin angle, is an imaginary line from the top of the McPherson strut that passes through the ball joint at the bottom of the hub and continues on to the road surface. With zero scrub geometry, the line would be in the center of the tread where it touches the ground. With positive scrub the line would be inboard of the center of the tread, and outboard with negative scrub. (If the car was parked and you turned the wheels left and right, the intersection of the steering axis and road surface would be the point that the tire pivots about.)

Generally, on performance cars, they give the car positive scrub. This gives the driver good feedback, telling them which tire has the most traction and which way to turn the wheel to correct. On luxury cars [read: the SVX] and lower performance vehicles, they give the car negative scrub. This takes away that feedback, but instead tends to automatically correct for the driver. Most of the current SVX owners probably don't like that, but the little old ladies who bought SVXs when they first came out probably needed the extra help. I believe that Porsches usually have a rather generous positive scrub. SVXs don't.
Attached Images
File Type: gif Scrub Diagram.gif (16.1 KB, 3725 views)
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  #5  
Old 05-07-2005, 12:27 AM
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My guess is that the OEM wheels give pretty close to zero scrub radius. One easy way to eyeball the scrub radius would be to jack the front wheels off the ground, and while someone is slowly turning the steering wheel from lock-to-lock, find the pivot point at the bottom of the tire. The distance from that pivot point to the middle of the tire section width should be your scrub radius.
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  #6  
Old 05-07-2005, 12:02 PM
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Quote:
Originally Posted by mbtoloczko
... One easy way to eyeball the scrub radius would be to jack the front wheels off the ground - while someone is slowly turning the steering wheel...
You read my mind. Again though, I'm lazy.

I've read that the scrub is negative, and that's consistent with the way the car behaves. I was just hoping there was a sleeper on the network who had stashed this little bit of knowledge away.
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Last edited by UberRoo; 05-07-2005 at 12:07 PM.
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  #7  
Old 05-07-2005, 03:34 PM
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My friends Saturns are positve scrub figured that one out by doing a wheel bearing
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  #8  
Old 06-21-2005, 11:38 PM
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The answer (+27mm)

Quote:
Originally Posted by UberRoo
Does anyone know what the factory scrub radius is on the SVX?
I finally took the time to figure it out.

Here's the answer:
-27mm scrub.
(Almost exactly one inch of negative scrub.)

For those of you who are too lazy to do the math: Going from the factory +55mm offset rim to the 'minimum' +38mm offset gets you 17mm closer to zero scrub. That still leaves you 10mm shy of neutral. You'd need a wheel with 28mm of positive offset to reach zero.

Also, the wider your wheels and tires are, the more your scrub offset will affect the vehicle's handling.

Disclaimer:
This is based on my own measurements. I'm quite confident they are fairly accurate, but there are no guarantees. I figure that my margin of error is less than 5mm. Actual error is probably half of that.
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  #9  
Old 06-22-2005, 07:56 PM
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Did you jack the wheel off the ground to get the measurement?
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  #10  
Old 06-23-2005, 02:07 AM
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Quote:
Originally Posted by mbtoloczko
Did you jack the wheel off the ground to get the measurement?
Heh. Sort of.

I placed a floor jack under each ball joint and lifted until the tires were lightly touching the ground. This kept the car from shifting side to side while performing my test. I turned the steering wheel side to side several times and then jacked the car up until I could rotate the wheels. I turned the front wheels 180 degrees to prevent any 'contamination' to the rub marks I'd just made on the tread. After removing the tires it was very apparent from the markings, the spot on the tread which they pivoted about. I measured the width of the tires (sidewall to sidewall, 244.5mm) and divided by two (122.25mm.) From the center of the tread, I measured the distance to the center of the pivot markings (27.0mm.) I did this to both tires and arrived at the same answer plus or minus one millimeter. My tires are basically slicks right now, which made finding the exact center of the pivot markings very easy.

I'm tempted to remove a wheel and attach a straight-edge to the hub flange so that it nearly touches the ground. The end of the straight-edge should draw an arc on the ground when turning the wheel. From there, the radius of the arc would be added to or subtracted from, (depending on whether the arc is convex or concave in relative perspective, to) the offset of the wheel. The resultant number should be the scrub radius. It's important with this method that the hub be the normal operating distance from the ground or the radius of the arc will change.

I suspect the resultant answer using this technique would be very close to the result of my first test. For my purposes, I'm happy if I'm within a centimeter, but I suspect I'm much closer than that. Nevertheless, I'd like to do another test using a different method just to validate the first test.

I welcome more input - unless it disagrees with me, of course.
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  #11  
Old 06-23-2005, 11:59 AM
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Quote:
Originally Posted by UberRoo
...
I welcome more input - unless it disagrees with me, of course.
Turns out that I agree, so I can give you my input. :-) Last night, for the purpose of finding the roll center of the front suspension, I measured all the pickup points and pivot points on the front suspension. Turns out I was able to get the scrub radius from these measurements. I got a value of -20 mm for the OEM wheels. I'll post the drawing in a day or so. Anyhow, it seems that the scrub radius is indeed negative. I'm kinda curious why Subaru did this.
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  #12  
Old 06-23-2005, 12:40 PM
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Here is an interesting article on scrub radius:

http://www.hrsprings.com/site/techni...rubradius.html
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  #13  
Old 06-23-2005, 07:31 PM
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Well I almost agree.

First we need to seperate the two functions of "Steering axis" and "Scrub radius". The Steering axis is the setting that causes the wheel to point forward. As the steering is turned from side to side the 'SA' causes the front end to rise as the wheel reaches full lock. It is the weight of the car, acting on the 'SA', that forces the wheel to return to the straight ahead position.

The Scrub Radius is as you say, the distance that the center of the tyres contact patch, is located from the point, that the Steering axis meets the ground.

Ideally they should be on the same spot. But as nothing is perfict they are placed at different points to overcome some other problem. Wide tyres are one, along with torque steer, how solid the suspension is mounted, and diagonal brake connectons. All these cause the wheel to move from the straight ahead position.

In the SVX we have all those, the cross connecting of the brakes, require negitive scrub, to keep the car from swerving across the road, when a brake failure causes only one front and one rear brake to work. Without it the car would swerve to that side, so we have negitive scrub to cause that wheel to steer in to the center of the car. This allows the car to hold a straight course, under these conditions.

Front wheel drive, calls for a positive scrub. When cornering the weight is removed from the inside wheel to the outside wheel. If the inside wheel loses traction, the outside wheel will try to steer out, If it has positive scrub the wheel will pull around the axis, to steer in the same direction, as the wheel is turned.

Using a scrub offset, is fine while both tyres have the same grip. If the traction is unever, like braking or accelerating on uneven road surface, the car will go in the direction of the wheel with the most grip. This can cause the car to dive around under brakes, or to torque steer under acceleration.

So there is allways a compromise used. With the SVX it is to use a negitive scrub to nutrailse the effect of the cross braking, and rely on the AWD to cancel the need to use positive, to hold the front wheel drive force.

Harvey.
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  #14  
Old 06-23-2005, 11:43 PM
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So this could be why I occasionally get some torque steer at launch? [38mm offset]
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Old 06-24-2005, 12:42 AM
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I wasn't entirely in agreement either. How scrub radius would influence tire wear, I have no idea. I think he's smoking somethin'. Admittedly there's still quite a few aspects of suspension geometry I don't have a firm grasp on. ...yet.

I think the original design is quite good. For an experienced driver that wants a little more liveliness from their car however, tweaking things can certainly provide that effect. I notice that when the car encounters adverse conditions of pretty much any sort really, it does a mighty fine job of taking care of things all by itself. The problem is that many of those 'adverse conditions' are deliberately induced by the driver. (That's me.) When I purposely throw the car into a corner too hot, or throttle on over a rise, I want the car to behave badly. I find myself counter-correcting to maintain these induced behaviors. Other times, when the 'adverse conditions' are not intended, the car corrects for itself, but always a little more or less than I'd like. In that case, the problem is that I never know how much it's going to correct and how much I need to assist or counter-correct. (It's much like trying to balance something while somebody is randomly tugging on your sleeve.)

If the car just demanded that I take full control of all the necessary corrections, I'd be fine with that, but the current geometry is constantly meddling with every little input I give it. Has anyone every gently modulated the brakes or throttle when somebody in the passenger's seat is trying to drink from an open top container just to mess with them? I think it's really funny, even if it does mean risking them spilling their drink in your car. I feel like the car is doing the same thing to me with the steering, except it's not so funny anymore. ...well actually it is, but not from my perspective.
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