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Old 01-29-07, 12:56 PM   #1
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Wheel offset vs scrub radius

Scrub radius discussion split from Jimlab's FD Wheel & Tire Sizing Worksheet

FD Wheel and Tire Sizing

FD Wheel and Tire Sizing

FD Wheel and Tire Sizing


Quote:
Originally Posted by jimlab
Scrub radius, on the other hand, changes camber as the suspension travels through its range of motion.
Scrub radius will have no effect on camber.

Scrub radius is the difference in distance between the center of the tire contact patch and a line extending through the center of the kingpin axis to the ground. If these two points are the same then you have zero scrub radius.

If the kingpin axis does not intersect the center of the contact patch, then you have scrub. It is called this because as the steering wheel is turned the tire must literally scrub or roll about an axis that is not in the center of the contact patch, thus "scrubbing" the tire across the pavement. Keep in mind all this is when viewed from the front, and is not to be confused with "trail" which has the same description but is viewed from the side. Trail works like a castor and helps the steering self center.

The problem with using too much scrub radius is that it introduces higher steering effort (not necessarily bad) and a strong tendency for the steering wheel to kick back (bad) or the car to pull to one side or the other (bad) anytime the grip at the front tires is not perfectly balanced. This is an especially disconcerting problem when braking hard over bumps; the steering feels like it's trying to beat you up. If anyone has ever driven a car with lots of scrub in the rain and hit a deep puddle with only one side of the car it probably scared you out of your wits as the car tried to launch itself into the gutter

Last edited by DamonB; 01-31-07 at 01:44 PM.
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Old 01-29-07, 01:04 PM   #2
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Check out this link I just found for the Mazdaspeed MS-01S wheels for the FD3S:

http://www.japanparts.com/db/partsli...dle=2&small=22

They are giving offsets in the +38 to +40 range (wheels out), even in cases where it's not needed for clearance like the "stock" 16x8 size. I wonder if the Mazdaspeed engineers were going for greater wheel feedback or if perhaps these are intended for some sort of adjustable race suspension where you can alter the shock centerlines.

Great thread. Thanks again for your spreadsheet and in-depth explanations.
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Old 01-29-07, 03:32 PM   #3
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Quote:
Originally Posted by DamonB
The problem with using too much scrub radius is that it introduces higher steering effort (not necessarily bad) and a strong tendency for the steering wheel to kick back (bad) or the car to pull to one side or the other (bad) anytime the grip at the front tires is not perfectly balanced. This is an especially disconcerting problem when braking hard over bumps; the steering feels like it's trying to beat you up. If anyone has ever driven a car with lots of scrub in the rain and hit a deep puddle with only one side of the car it probably scared you out of your wits as the car tried to launch itself into the gutter
That's pretty telling - rough roads and driving in the rain are a handful as it is.
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Old 01-29-07, 09:12 PM   #4
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Quote:
Originally Posted by DamonB
Scrub radius will have no effect on camber.
That's not what I learned, but whatever.

Quote:
Scrub radius is the difference in distance between the center of the tire contact patch and a line extending through the center of the kingpin axis to the ground. If these two points are the same then you have zero scrub radius.
Wow, wish I'd posted that...
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Old 01-31-07, 09:19 AM   #5
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Quote:
Originally Posted by jimlab
That's not what I learned, but whatever.

Wow, wish I'd posted that...
If one were to be picky this is incorrect:

Quote:
Originally Posted by jimlab
For those that don't know, the scrub radius is the distance between the extended centerline of the steering axis (where the centerline of the shock would hit the ground)
You must have got your info from someone talking about struts. The steering axis and the centerline of the shock are the same in only one case: Mc-strut type suspension. On A Mc-strut altering the steering axis does change camber because the strut is responsible for both.

For every other type of suspension the centerline of the shock has nothing to do with anything about the steering axis or scrub radius.

Last edited by DamonB; 01-31-07 at 09:27 AM.
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Old 01-31-07, 10:02 AM   #6
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Did you turn into Jeff Hoskinson when I wasn't looking?
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Old 01-31-07, 10:24 AM   #7
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Click the image to open in full size.Click the image to open in full size.


Click the image to open in full size.

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).

If the brake on one front wheel is not working, with positive scrub radius, stepping on the brake will cause the steering wheel to try to rip out of your hand. Negative scrub radius will minimize that effect.

Scrub radius is designed at the factory and is not adjustable. If you have a vehicle that is pulling even though the alignment is correct, look for something that will affect scrub radius.

Last edited by wanklin; 01-31-07 at 10:31 AM.
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Old 01-31-07, 10:56 AM   #8
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Quote:
Originally Posted by DAMON
Scrub radius will have no effect on camber.
Camber affects scrub radius because the introduction of camber shifts the centerline of the contact patch. To say that they are in no way related seems intuitively wrong. Perhaps you are just pointing out that scrub radius in itself is not a determinant of camber.

Anything that affects camber also affects scrub radius.

Last edited by wanklin; 01-31-07 at 11:21 AM.
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Old 01-31-07, 11:16 AM   #9
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Quote:
Originally Posted by JIM
Scrub radius, on the other hand, changes camber as the suspension travels through its range of motion.
I think you are correct in your correlation between camber and scrub radius, but off on the causal connection. That is to say that camber affects scrub radius, not the other way around, though that may be what you meant. As you pointed out, various amounts of camber are introduced as the suspension goes through it's full range of motion since the suspension does not move on a perfectly vertical plane.

With that being said a larger diameter tire would decrease scrub, but ofcourse cause sensory issues.

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Old 01-31-07, 11:37 AM   #10
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Quote:
Originally Posted by wanklin
Anything that affects camber also affects scrub radius.
Nope.

Only if it's a Mc-strut which your pics show. The FD is not a Mc-strut.

Intuitively right:

With a Mc-strut a static camber adjustment changes the included angle of the steering axis (the strut centerline) vs. the spindle. This will cause a change in scrub radius as mentioned earlier but this is only true of Mc-struts.

The FD is a double wishbone suspension. With this suspension the steering axis angle vs the spindle does not change because they are permanently defined by the locations to where they attach to the upright. No matter how we adjust static camber the included angle between the steering axis and the spindle will not change since the entire upright which contains them is what moves. Since they don't change scrub radius doesn't change.

Last edited by DamonB; 01-31-07 at 11:45 AM.
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Old 01-31-07, 11:47 AM   #11
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No, Damon's right. What I learned about scrub radius as a young lad applies to cars with struts, and it's my opinion that scrub radius is far more important on cars with suspensions of that type, because the turning wheels are also typically (note that I said typically Damon, not always) the drive wheels. If I made a mistake, it was in the use of the word "shock" in place of "strut".

That said, I'd like Mr. Hoskinson Jr. to stand up and explain to everyone how the "kingpin axis" is relevant to modern suspensions. If we're striving for perfect accuracy, then "ball joint axis" would be the proper term because there are no king pins on modern cars and king pin is also used to refer to a 5th wheel or semi trailer coupling.

And people wonder why I rarely bother to help anyone any more...
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Old 01-31-07, 12:20 PM   #12
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Quote:
Originally Posted by jimlab
That said, I'd like Mr. Hoskinson Jr. to stand up and explain to everyone how the "kingpin axis" is relevant to modern suspensions. If we're striving for perfect accuracy, then "ball joint axis"
If you wanted to strive for perfect accuracy the modern term is neither. It's "SAI": Steering Axis Inclination I didn't figure anyone would know "SAI" but thought they might "kingpin axis".

Many modern cars have a steering axis that is described by more than 2 joints so the steering axis doesn't intersect any balljoints at all, it's defined elsewhere in space (Audi calls their design a "virtual steering axis" but other manufacturers are common). Thus "SAI" came into vogue.

The pic attached is from a toy model but illustrates a virtual steering axis that doesn't intersect any of the actual joints.
Attached Thumbnails
Wheel offset vs scrub radius-4link6.jpg  

Last edited by DamonB; 01-31-07 at 12:31 PM.
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Old 01-31-07, 12:25 PM   #13
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Quote:
Originally Posted by DamonB
Nope.

Only if it's a Mc-strut which your pics show. The FD is not a Mc-strut.

Intuitively right:

With a Mc-strut a static camber adjustment changes the included angle of the steering axis (the strut centerline) vs. the spindle. This will cause a change in scrub radius as mentioned earlier but this is only true of Mc-struts.

The FD is a double wishbone suspension. With this suspension the steering axis angle vs the spindle does not change because they are permanently defined by the locations to where they attach to the upright. No matter how we adjust static camber the included angle between the steering axis and the spindle will not change since the entire upright which contains them is what moves. Since they don't change scrub radius doesn't change.
Would you happen to have a pic handy to illustrate this? While I understand your logic, I need visual clarification and I am oceans away from my car at the moment. Right now I'm trying to visualize the configuration and mounting points at the upright and the computer at this library is pretty worthless for downloading a manual.

I hear ya Jim. King pin is an outdated term wich is still commonly used in the motor world for whatever reason. So Damon's either been reading some outdated books or hanging around old track guys too much ;o)
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Old 01-31-07, 12:40 PM   #14
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Pic attached.

On a double wishbone camber is adjusted by changing the length of the upper or lower arm or by changing their relative mounting position to eachother in order to cause the upright to tilt one way or the other and adjust camber. From looking at the pic lengthening the lower wishbone results in negative camber (essentially how an FD would do it). However the scrub radius doesn't change because lengthening the lower wishbone also causes the entire upright to tilt as well, so as we adjust camber the SAI changes the same amount as does the spindle since they are rigidly connected to the upright. Thus no change in scrub radius.
Attached Thumbnails
Wheel offset vs scrub radius-waf29.gif  
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Old 01-31-07, 01:02 PM   #15
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Quote:
Originally Posted by DamonB
Pic attached.

On a double wishbone camber is adjusted by changing the length of the upper or lower arm or by changing their relative mounting position to eachother in order to cause the upright to tilt one way or the other and adjust camber. From looking at the pic lengthening the lower wishbone results in negative camber (essentially how an FD would do it). However the scrub radius doesn't change because lengthening the lower wishbone also causes the entire upright to tilt as well, so as we adjust camber the SAI changes the same amount as does the spindle since they are rigidly connected to the upright. Thus no change in scrub radius.
Thanks for the pic. It seems too simplistic given that you are changing the pressure vectors by manipulating the arm lengths; causing a new and ambiguous virtual steering angle. It seems that increasing arm length would have a number of affects on suspension geometry and range of motion, however small. I guess the real trick is in engineering a vertical member designed to occomadate all of this. Perhaps saying that there is zero change is a bit strong, maybe "very little" change would be a more correct order of magnitude?

Last edited by wanklin; 01-31-07 at 01:12 PM.
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Old 01-31-07, 01:23 PM   #16
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Quote:
Originally Posted by wanklin
Perhaps saying that there is zero change is a bit strong, maybe "very little" change would be a more correct order of magnitude?
No matter what design of suspension as soon as you mount a pneumatic tire on it the contact patch is going to move around when the tire is loaded. Given the choice you pick the lesser of evils.
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Old 01-31-07, 01:26 PM   #17
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The same would hold true if it were a solid suspension with zero range of motion. I'm not talking about tires....
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Old 01-31-07, 01:32 PM   #18
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Quote:
Originally Posted by wanklin
The same would hold true if it were a solid suspension with zero range of motion.
No such thing on a vehicle.

We speak of karts as having rigid suspensions but even they in fact have 3 dimensional suspension geometry, it just happens to only exist in the steering.

Last edited by DamonB; 01-31-07 at 01:48 PM.
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Old 01-31-07, 03:30 PM   #19
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Ofcourse it exists, It's just not particularly useful and is therefore devoid of practical relevance.

I'll do a little research and see how convinced I am on your analysis of the double A-arm geometry, but for now, thanks for the enlightenment ;o)
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Old 01-31-07, 04:23 PM   #20
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Quote:
Originally Posted by Damon
On a double wishbone camber is adjusted by changing the length of the upper or lower arm or by changing their relative mounting position to eachother in order to cause the upright to tilt one way or the other and adjust camber. From looking at the pic lengthening the lower wishbone results in negative camber (essentially how an FD would do it). However the scrub radius doesn't change because lengthening the lower wishbone also causes the entire upright to tilt as well, so as we adjust camber the SAI changes the same amount as does the spindle since they are rigidly connected to the upright. Thus no change in scrub radius.
Click the image to open in full size.

After putting in a fair bit of research on double wishbone geometry I am going to have to flat-out disagree. The steering inclination angle on a double wishbone setup (as well as the caster) is determined by draving a line through the center of upper and lower joints. The vertical plate that you speak of does in fact keep these mounting points stationary; however that doesn't mean anything about the angle by which these two points correlate. By lengthening the lower arm you would move the lower joint outward and and the steering inclination angle would follow.

With that being said, you could effectively rezero scrub radius by increasing your lower arm length until the line intersectine the two ball joints intersects the tire center line. This would also have the effect of increasing the length differential between the lower and upper arms which would increace the amount of negative camber that is introduced during the suspension compression stroke. I believe this would be benificial for cars running higher rate suspension setups as it would introduce more traction around corners.

At the end of the day, it is the angular relationships and pressure vectors that mainly determine suspension geometry, not the spacing of joints.

Last edited by wanklin; 01-31-07 at 04:33 PM.
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Old 01-31-07, 04:53 PM   #21
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Quote:
Originally Posted by wanklin
After putting in a fair bit of research on double wishbone geometry I am going to have to flat-out disagree.
You did a "fair bit of research" in less than an hour?!! Heh.

Quote:
Originally Posted by wanklin
you could effectively rezero scrub radius by increasing your lower arm length until the line intersectine the two ball joints intersects the tire center line.
Nope. You couldn't effectively do anything in regards to scrub by adjusting any of the arm lengths.

As you increase the lower arm length the contact patch is moving outward as well since the upright must tilt. Since the upright is tilting the SAI is tilting an equal amount. Since the upright is tilting the spindle is also tilting an equal amout. End result: no change at all in scrub.

Here's a good quote from here. Note that "SLA" stands for "Short/Long Arm" and is a particular type of double wishbone suspension, which also happens to be exactly what the FD has up front:

"MacPherson strut equipped vehicles usually have a negative scrub radius.Even though scrub radius in itself is not directly adjustable, it will be changed if the upper steering axis point or spindle angle is changed when adjusting camber. This is the case on a MacPherson strut which has the camber adjustment at the steering knuckle. Because camber is usually kept within 1/4 side to side, the resulting scrub radius difference is negligible. Negative scrub radius decreases torque steer and improves stability in the event of brake failure. SLA suspensions usually have a positive scrub radius. With this suspension, the scrub radius is not adjustable."

The SLA suspension does not have adjustable scrub radius built in as a Mc-strut does. You can change the scrub radius by using different tire heights or wheel offsets on either type, but there is no way to inherently adjust the scrub of the suspension itself on an SLA suspension because the relative angle between the SAI and the wheel spindle are forever frozen in place by the upright. This is not true of a Mc-strut where adjusting camber also changes the included angle between the SAI and the spindle.
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Old 01-31-07, 05:13 PM   #22
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ahhh yes! I see exactly what you are saying now! and am in complete agreement. I just needed to challenge this idea before I could accept it. Thanks for setting me straight. lol

Man when you're writing two research papers a week you learn how to research quickly, doesn't mean you know everything. Hell, if I knew everything I would have little reason to keep coming back to this board. Good thread good thread.

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Old 01-31-07, 05:18 PM   #23
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Quote:
Originally Posted by DamonB
The SLA suspension does not have adjustable scrub radius built in as a Mc-strut does. You can change the scrub radius by using different tire heights or wheel offsets on either type, but there is no way to inherently adjust the scrub of the suspension itself on an SLA suspension because the relative angle between the SAI and the wheel spindle are forever frozen in place by the upright.
Well you could make custom uprights in conjuction with arm length changes, but how many people will actually go this far....
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Old 01-31-07, 06:32 PM   #24
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Quote:
Originally Posted by wanklin
Well you could make custom uprights in conjuction with arm length changes, but how many people will actually go this far....
Not easily done. These things are taken into account when the suspension is designed because you must alter the design of the upright to change them. As soon as you poor metal to cast that piece the decision has been made and there is no going back.
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Old 01-31-07, 06:35 PM   #25
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Such is life
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Old 01-31-07, 06:35 PM
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