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Figuring out scrub radius

SteveW

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Not wanting to overthink this but I am curious about scrub radius on these cars. With the double jointed control arm setup it would be difficult to figure out due to the virtual point that the lower end of the suspension rotates around.

So I thought what about rubbing some chaulk on the contact patch and turn the wheel back and forth while stationary. Is the below an accurate representation of scrub radius?

On the particular setup the car has right now it looks to be about 1" to 1 1/8" positive since the pattern is inboard of the center of the contact patch.

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Norm Peterson

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Maybe.

Negative camber moves any notion of contact patch centroid inward from the center of the tread. Geometrically, the middle of a contact patch area starts losing area at the outside shoulder - your picture is showing this quite clearly. On a force basis it'd be the centroid of the distributed force per unit area over the CP - with negative camber a square inch on the inside shoulder rib will be carrying more force than a square inch near the center of the tread, which will be still greater than the force on a square inch out closer to the outboard shoulder.


Norm
 
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SteveW

SteveW

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Thanks for the reply, Norm. I hadn't thought of what you wrote above but, if negative camber moves the forces inward from the center of the contact patch would it be fair to say that that would negate some of the effect of a positive scrub radius? Meaning counter a "toe out effect" on my car?

What is leading me down this rabbit hole is my car is fun to drive on dry roads, wet roads and is a quick autocross car. A month or two ago we had this heavy frost in town but the roads weren't bad at all so I decided I'd take the car out for a coffee shop run. The sun was up so it had melted all the frost and ice off the road except for what was in the shadows. That is normal and expected around here and I'm very used to that but I'm not normally out in the Mustang. I was just rolling along on flat, smooth roads in town at just 25-30 mph and as the car rolls over there patches of frost it is trying to dart all over the place. It did not want to go straight.

I thought that was very strange and have been wondering what would cause that and thought of scrub radius maybe being a factor, especially if it is excessive. I was thinking it would want to pull to one side or the other depending on if one side of the car was on a slick surface and the other on dry pavement.

1" doesn't seem like it is excessive though compared to what I've seen elsewhere online.
 

BmacIL

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Thanks for the reply, Norm. I hadn't thought of what you wrote above but, if negative camber moves the forces inward from the center of the contact patch would it be fair to say that that would negate some of the effect of a positive scrub radius? Meaning counter a "toe out effect" on my car?

What is leading me down this rabbit hole is my car is fun to drive on dry roads, wet roads and is a quick autocross car. A month or two ago we had this heavy frost in town but the roads weren't bad at all so I decided I'd take the car out for a coffee shop run. The sun was up so it had melted all the frost and ice off the road except for what was in the shadows. That is normal and expected around here and I'm very used to that but I'm not normally out in the Mustang. I was just rolling along on flat, smooth roads in town at just 25-30 mph and as the car rolls over there patches of frost it is trying to dart all over the place. It did not want to go straight.

I thought that was very strange and have been wondering what would cause that and thought of scrub radius maybe being a factor, especially if it is excessive. I was thinking it would want to pull to one side or the other depending on if one side of the car was on a slick surface and the other on dry pavement.

1" doesn't seem like it is excessive though compared to what I've seen elsewhere online.
I'd add onto what Norm said by saying camber and tire type and tire width vs rim width all play into this. Certain tires are much more susceptible to scrub-induced slip angle, and even with the same tire will behave differently at different camber angles.

What you are also feeling is a relative slowing of the one tire vs the other from the frost/slush.
 

Ewheels

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Thanks for the reply, Norm. I hadn't thought of what you wrote above but, if negative camber moves the forces inward from the center of the contact patch would it be fair to say that that would negate some of the effect of a positive scrub radius? Meaning counter a "toe out effect" on my car?

What is leading me down this rabbit hole is my car is fun to drive on dry roads, wet roads and is a quick autocross car. A month or two ago we had this heavy frost in town but the roads weren't bad at all so I decided I'd take the car out for a coffee shop run. The sun was up so it had melted all the frost and ice off the road except for what was in the shadows. That is normal and expected around here and I'm very used to that but I'm not normally out in the Mustang. I was just rolling along on flat, smooth roads in town at just 25-30 mph and as the car rolls over there patches of frost it is trying to dart all over the place. It did not want to go straight.

I thought that was very strange and have been wondering what would cause that and thought of scrub radius maybe being a factor, especially if it is excessive. I was thinking it would want to pull to one side or the other depending on if one side of the car was on a slick surface and the other on dry pavement.

1" doesn't seem like it is excessive though compared to what I've seen elsewhere online.
I know this is an older thread but I really enjoy the topic and wanted to put in my $.02 :)

Steve, I would assume you do definitely have positive scrub radius, to what exact measurement, it's hard to say. Your scenario you provided further proves this.
With negative scrub radius, under loss of traction at one wheel or under braking, the front wheels will want to toe in; therefore keeping the car straight and predictable. This is a good thing for safe, daily driver vehicles.
With positive scrub radius, under loss of traction at one wheel or under braking, the front wheels will want to toe out; therefore making the car dart about in different directions.

Generally for "performance" cars with handling and cornering as a priority, you want zero to slightly positive scrub radius. This will make the car turn in much better.

Out of my own curiosity, is your car on factory wheel sizes and offsets? I've always wondered what the GT PP scrub radius is. (Not exact numbers, but if its positive, zero, or negative.)
 

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Man am I glad to see interest in this thread. Scrub Radius on this chassis has been on my mind since my 2015 GT Premium purchase.
I have reached out recently to "Ford Performance", as well as Dean Martin of KOHR Racing on the exact scrub radius numbers. (I was most interested in the GT350 scrub radius), as I knew that was one of the considerations when they widened the S550 front tread width for the GT350. I have not received any answers.
Using some of my old Stock-Car Racing measurement tricks, I have used laser beams w/tripods from the top and front of the chassis to try to determine the exact scrub radius. All I can say for sure is it definitely appears a half- inch or so Positive.
I appreciate @SteveW in his illustration, and it also confirms my notion of Positive Scrub.

I did read several articles on the roll-out for the new GT350 where Ford did talk a little on the front suspension enhancements, citing they wanted a positive scrub radius:

4. Because of the unique frontend and wider front track width, the tire scrub radius of the GT350 and GT350R have been optimized to be just outside of the front tire’s static centerline. This was done to target the functional centerline of the tire—not the static centerline of the tire—when the car experiences extreme cornering loads.

You can read the whole article here"
https://www.hotrod.com/articles/18-...out-the-2015-shelby-mustang-gt350-and-gt350r/


Front-end geometry has changed dramatically in the last several years, but I can recall:
"Large positive values of scrub radius, 4 inches/100 mm or so, were used in cars for many years." -Wikipedia-

Come-on Ford engineers, tell us what the exact scrub radius is on the S550 GT and GT350 chassis!
 
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Ewheels

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I have a decent amount of "textbook knowledge" on this topic but disappointingly less real world experience. Would you say adding 10mm to the scrub radius via wheel offset would make for a worse driving experience? Or better said, is 10mm too much?

I ask because I am looking into new wheels and cannot decide on 35mm offset or 40mm. (Factory is 45mm)
 
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SteveW

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I gave up on this topic when I decambered the front end at the knuckle and the scrub radius pattern appeared to get worse when it should have gotten better. So, whatever I was looking at didn't correlate.

Then my area got buried with snow and I couldn't take the car out for any kind of over the road testing for a while. Now, it's autocross season and I setup the front suspension for max camber.
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