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  1. #1
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    Default How to Measure Scrub Radius & Ideal Radius?

    any suggestions on how to measure scrub radius with basic home alignment tools?

    i'm thinking that if i have the front wheels off the car; support the car on blocks under the LCAs, at ride height. i can drop a straight edge or plumb line directly down from the mounting surface in the center, and draw an arc (by turning the wheels) on the ground surface?



    of course assume the car is on a level surface, been jounced, etc..

    SAI intersection with the ground would be the middle of circle, and scrub radius could be calculated by adjusting the radius of the circle by wheel offset.

    what is the best method to determine the appropriate scrub radius when selecting wheel offset? is zero always best for RWD?

    thanks,
    Scott.


  2. #2
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    No, a 0 scrub radius is not ideal. You do want some to help with feedback from the road.
    Brian


    I have an unlimited budget. That bad part is I have already used it up.

  3. #3
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    Default

    I seem to remember 0.5 to 1 inch. I think I might have that out of the Herb Adams book. Might be totally wrong though.

  4. #4
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    Default

    Just thumbed through Herb's 'Chassis Engineering' tonight. Says minimum scrub radius is better, but no guidance beyond that.

  5. #5
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    I can help you. Give me the measurements that showed on this picture (write in photoshop or something). Green points are ball joints and the right point is the mounting flange. Measure from flange to center of the ball joint.

    Give me the dimensions (height and width) of the tire and backspacing too.

    You'll get a drawing back with scrub and SAI.
    Attached Images Attached Images  

  6. #6
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    My opinion is no less than 1/2" scrub, and no more than 2".
    Matt Jones
    Mechanical Engineer
    Art Morrison Enterprises

  7. #7
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    Quote Originally Posted by Ripper
    ...I can help you. Give me the measurements .....
    thanks. car is at the body shop on the frame right now. i'll take you up on this when i get it back this Summer. i think it's going to be tough though to get accurate measurments, and any error in measuring at the ball joint, will be magnified down at the point of scrub radius. i suppose the best point to attempt to measure from would be the middle of the spherical joint in the BJ?

    Quote Originally Posted by silver69camaro
    My opinion is no less than 1/2" scrub, and no more than 2".
    thanks, Matt. this is for RWD power steering car?

    any opinions on whether my method would work for measuring it?

  8. #8
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    Yes, that is for RWD and PS. That's pretty much all I work with, really.

    Anyway, in regards to your method, I guess it would work if I read it correctly. Personally, I would just use a tape measure and plot the points of your upper and lower ball joints, wheel center, etc. and draw it out on paper. If you measure to a 1/16 or 1/32, it will be plenty accurate. I would think that would be easier, but again, just my opinion.
    Matt Jones
    Mechanical Engineer
    Art Morrison Enterprises

  9. #9
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    Quote Originally Posted by silver69camaro
    My opinion is no less than 1/2" scrub, and no more than 2".
    That sounds very reasonable.
    The theory is, the tire contact patch moves inboard during hard cornering, so some positive offset keeps the contact patch from going to the negative side of the scrub radius, and retains some cornering feel. I don't think that RWD Muscle Cars are very sensitive to offset until you get beyond 2" scrub radius.
    David
    67 Camaro RS that will be faster than anything Mary owns.

  10. #10
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    Default scrub radius picture

    measured my scrub radius.

    my '61 hardtop with the disk conversion is at the body shop, but i measured scrub radius on my '61 convert with the stock brake setup.

    tonight, i used my method i had thought of above, and i think it worked pretty well. i pulled off the front wheels and blocked the LCAs with the measured side right at wheel height. i dropped a plumb line from the center of the wheel at the mounting surface and drew a dot on the surface. i rotated the wheels around and a circle started to emerge. i did this all the way to the stops.

    then, i located where the center of the circle would be by rotating a string around. the circle was not a perfect circle (maybe because of caster, although this car is nearly vert caster).

    anyhow, the radius of the circle to the mounting surface was 3 7/8". the offset of the wheels are 3/4", so the centerline of the wheel would be where the dashed straight line is in the picture.

    so, the scrub radius turns out to be 3 1/8".

    my disk brakes push it out 5/8", so the scrub radius with stock wheel offset is 3 3/4" on my bubbletop (with disk conversion). sheesh. so that explains why the tramlining was so awful when i ran 245/45/17 performance tires like that.

    i suppose i need to get skinnier wheels/tires and push offset in as much as i can to try and get inside of 2". i haven't really looked to see if i have the room yet.




    i also attempted to measure the ball joint locations. i'll post those in the morning, and see how close that method comes. problem with that is trying to measure the center of the spherical joints. it's really not clear where that is. how far under the zerk fitting? these measurements could be 3/8" or 1/2" off, but i gave it my best shot, and i'll post the nums in the morning.

    cheers.

  11. #11
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    Default

    for any Fletch fans....

    a movie quote:

    Autopsy Doctor: "Have you ever seen a spleen (scrub radius) this big?"
    Fletch: "No-hoh... not since breakfast"



  12. #12
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    Quote Originally Posted by TheMonkey
    thanks. car is at the body shop on the frame right now. i'll take you up on this when i get it back this Summer. i think it's going to be tough though to get accurate measurments, and any error in measuring at the ball joint, will be magnified down at the point of scrub radius. i suppose the best point to attempt to measure from would be the middle of the spherical joint in the BJ?
    As silver69camaro writes, it doesn't have to be THAT exact. Small misses don't affect that much since distances aren't that big. I would say that you'll get within +-1/8" and that's more than enough since tire flex also changes scrub...


    1/2-2" seems quite OK. My new front end will get app. 1 7/8" scrub with 10" wheels and Corvette C4 uprights. However, don't overestimate scrubs affecting on driveability. There are other parameters to consider too.
    This car;

    has about one mile of scrub radius and it's still one of the fastest cars in its series (not many rules, except for a certain weight/power ratio)

  13. #13
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    Default

    Scott, I have American 15X7 wheels with a 4-3/8" backspace.
    From what I can see on my car the biggest backspace in a 15" wheel would be 5".

  14. #14
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    again, i did my best to measure the ball joint locations, but it's tough because i don't know exactly where the center of the spherical joint is. laterally, i think i got it real close, because the center of the bolt could be measured as the distance from the mounting surface of the wheel. vertically was tougher for 2 reasons; 1) not sure where the center of the sphere is vertically, and 2) the tape did not have a straight shot to any reference points (imagine those situations where you need a 3rd hand and you keep hearing the tape measure making that noise when it keeps collapsing).

    upper BJ was 6.25" in from flange, and 19.75" off the driving surface. lower BJ was 5.25" in from flange, and 9.5" off the driving surface.

    so, if SAI moves 1" for every 10.25" drop, then as it drops 9.5" from the lower BJ, the SAI intersects ground 0.93" in from lower BJ, or 4.32" from flange. adjust for 3/4" offset, that comes up with 3.57" stock scrub radius.

    this compares with the 3 1/8" calculated using the plumb line above, or 0.45" different.

    effort wise, i think the plumb line might take a couple extra minutes, but looking at the picture, the margin for error might be 1/8"? as for the method of measuring BJ locations, unless i can figure a better way to measure BJ height, i would guess margin for error would be as high as 1/2". but, it sounds like from comments above, that either of those margins for error is splitting hairs.

    from my experience of tramlining (feels like a kid suddenly grabs steering wheel), i'm going to do everything i can do reduce scrub radius. i still need to get caster into positive territory with a custom UCA, so maybe that will help with the cure.

    thanks for listening.

  15. #15
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    Default

    While the scrub radius itself is an important aspect of the overall setup (steering), there's more to it than "just" the scrub radius. I agree with most everything stated by the folks so far, but one thing that can easily overwhelm a "good" scrub radius setup is the steering system itself. And, when you go up in tire width, things get amplifed pretty fast, I have seen this personally on my own 69 coupe when going from 235 front tires, to 275 front tires.

    The tramlining you report is probably not helped by the "little bit bigger" than you'd like to see scrub radius, around 3.5" or so isn't great, but it's not death either. I'd suspect that the steering gear itself has a big contribution if you are experiencing significant tramlining. Keep in mind that the cars were originally designed for skinny, non-grippy tires. Couple that with the fact that the gear is probably old, etc, it all adds up. I'm not trying to sway you away from reducing the scrub a bit, just keep in mind (as I am sure you already are) that there's more to the story.

    Case in point, and friend built a very, very serious street car from a clean sheet design (it was/is basically a GT1 car with a steel body on it, super cool). He was running very large front tires, and on the street, he was getting some pretty significant tramlining. On the track it wasn't a problem, road courses are pretty smooth, but on the street, he hated it. After years of driving the car, he changed the valving in the rack (I think, it may have been the P/S pump), and it transformed the car and almost eliminated the issue entirely. If I recall correctly, he had a pretty tidy scrub radius on the car, somewhere right around 2" or so.

    I haven't studied the steering setup on a 61 bubbletop (by the way, no fair, that is THE next car I want, the ultimate cruiser), but if it's anything like I've seen on many of the older cars, it's very probable that this is a serious contribution. Might want to look at bump steer too if you haven't already. Nice investigation so far, my hat's off to you.

    Mark

  16. #16
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    I would not go with smaller tires.
    Make sure your suspension is in good condition then play with different toe or caster settings. Tire type also makes a difference.

    What are your settings now?
    David
    67 Camaro RS that will be faster than anything Mary owns.

  17. #17
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    Quote Originally Posted by David Pozzi
    I would not go with smaller tires.
    Make sure your suspension is in good condition then play with different toe or caster settings. Tire type also makes a difference.

    What are your settings now?
    David
    in the convertible, it felt fine with boring radials on stock rims. when i put a fat sway bar up front, it actually felt great and shot the outside hub caps off on my first clover leaf.

    then, i installed:

    new control arm bushings
    new ball joints
    new outer tie rods
    disk brakes (pushed scrub out exactly 5/8")
    245/45/17 all 4 corners; tires are BFG KDW (directional tires, old tread pattern)

    not replaced: inner tie rods, center link, idler arm, pitman arm, steering box.

    set up camber at 0, caster orig spec was -1.5* and that did not work, so i went for positive and could only get close to vertical, but not in positive territory. i set toe both at 1/16" in & 0" straight. felt the same, but hard to read toe while tramlining is a prob.

    tramlining was unbearable. bumpsteering does not seem to be an issue. i am not able to fit stock wheels over disks so i can't compare if it's just the tires or not.

    i started the bbl top project, and just put new drum brakes & stock wheels tires back on convertible.

    my plan on the bubbletop (when i get it back from body shop) is to put the disks on there, put on SC&C adjustable UCAs with positive caster settings. i am planning on a steering box rebuild with tighter ratio (which i didn't figure could be the problem until Mark mentioned it above). centerlink, idler arm, and pitman arm will be replaced along with the rest of the front end kit.

    if positive caster solves this, then great, if not, i'll go with a skinnier tire and max offset (ugh).

    what would you suggest for alignment settings? i was thinking about 4* positive caster, 0* camber, slight toe-in.

    i would really love it if i could use my existing 245s. they are paid for and look great. is caster my biggest demon? i hope.

  18. #18
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    Quote Originally Posted by TheMonkey
    upper BJ was 6.25" in from flange, and 19.75" off the driving surface. lower BJ was 5.25" in from flange, and 9.5" off the driving surface.

    I still need the measurements from of the tire. In the attached drawing, I assumed it was 9,5" wide and 25" high and backspacing was 5".
    As you can see, SAI is 5,6degrees and scrub is just above 4" (with the tire in the model), however if you give me the correct measurements, you'll see how much scrub you have.

    EDIT: forgot to attach image...
    Attached Images Attached Images  

  19. #19
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    Nov 2006
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    Quote Originally Posted by Ripper
    I still need the measurements from of the tire. In the attached drawing, I assumed it was 9,5" wide and 25" high and backspacing was 5".
    As you can see, SAI is 5,6degrees and scrub is just above 4" (with the tire in the model), however if you give me the correct measurements, you'll see how much scrub you have.

    EDIT: forgot to attach image...
    Ripper: offset of stock wheels & brakes is 3/4".

    so, the upper BJ is in 5.5" from center of tire, and 19.75" above driving surface. the lower BJ is 4.5" from center of tire, and 9.5" above driving surface.

    see pic (not to scale). if my geometry is accurate, you should come up with 3.57" scrub radius. thanks for the help.


  20. #20
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    Nov 2006
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    Quote Originally Posted by Mean 69
    While the scrub radius itself is an important aspect of the overall setup (steering), there's more to it than "just" the scrub radius. I agree with most everything stated by the folks so far, but one thing that can easily overwhelm a "good" scrub radius setup is the steering system itself. And, when you go up in tire width, things get amplifed pretty fast, I have seen this personally on my own 69 coupe when going from 235 front tires, to 275 front tires.

    The tramlining you report is probably not helped by the "little bit bigger" than you'd like to see scrub radius, around 3.5" or so isn't great, but it's not death either. I'd suspect that the steering gear itself has a big contribution if you are experiencing significant tramlining. Keep in mind that the cars were originally designed for skinny, non-grippy tires. Couple that with the fact that the gear is probably old, etc, it all adds up. I'm not trying to sway you away from reducing the scrub a bit, just keep in mind (as I am sure you already are) that there's more to the story.

    Case in point, and friend built a very, very serious street car from a clean sheet design (it was/is basically a GT1 car with a steel body on it, super cool). He was running very large front tires, and on the street, he was getting some pretty significant tramlining. On the track it wasn't a problem, road courses are pretty smooth, but on the street, he hated it. After years of driving the car, he changed the valving in the rack (I think, it may have been the P/S pump), and it transformed the car and almost eliminated the issue entirely. If I recall correctly, he had a pretty tidy scrub radius on the car, somewhere right around 2" or so.

    I haven't studied the steering setup on a 61 bubbletop (by the way, no fair, that is THE next car I want, the ultimate cruiser), but if it's anything like I've seen on many of the older cars, it's very probable that this is a serious contribution. Might want to look at bump steer too if you haven't already. Nice investigation so far, my hat's off to you.

    Mark
    Mark- thanks for the encouragement and compliment on choice of project.

    when i settle on ride height in my current project car, i'll address bump steer, but my probs i had already experienced were happening without any suspension travel.

    you make an interesting point about other components.... i'll be replacing my steering box with a quick ratio. i'll give these tires a try first, and settle on thinner front tires if all else fails. i guess i would just have an extra set rears to burn through then.

    i wonder.... would rear steer cars be more susceptable to tramlining? afik, rear steer just increases deflection through turns, and it seems that most tramlining complaints come from the bmw & audi guys that increase tire size. just kind of thinking out loud. (obviously bmw & audi are not rear steer)

    thx.

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