Scrub radius, the neglected alignment setting

[brods]

One alignment term rarely brought up during DW discussions is scrub radius.



Scroll down to scrub radius on this link for a description http://www.familycar.com/alignment.htm



To use the shopping cart example, it would be like mounting the wheel off to one side of its bracket instead of in the center. Now the king pin axis (the vertical axle the wheel swivels about) is no longer lined up over the wheel centerline. When you push the cart the drag on the wheel will want to steer the wheel to the side its offset on. The further the wheel is offset the larger the scrub radius and the more steering force it will generate. Hopefully that made sense.

"The smaller the scrub radius, the better the directional stability. Scrub radius is dramatically increased when aftermarket wheels and tires, that have additional offset, are installed. They may cause the centerline of the tires to move further away from the spindle. This will increase the scrub radius, which in turn reduces the directional stability. <font color="red"> A large amount of scrub radius can cause severe shimmy after hitting a bump. That is why four-wheel drive vehicles with large tires use a steering damper to compensate for an increase scrub radius. " </font>

Putting on a taller tire HAS to change the scrub radius one way or the other, unless it is corrected with wheel offset (if possible). Most people with DW have installed taller tires, yet I have never seen anybody compare their before and after scrub radius.



One problem when trying to figure scrub radius is finding the SAI (steering axis inclination) spec for the axle. Without knowing that you cannot figure the scrub radius change when swapping to taller tires. Anybody know what the 05 Dodge solid axle SAI and scrub radius distance is?



Here is an old link that is the best description of DW I’ve ever come across: http://www.4x4wire.com/lists/jeepoffroad/200202/msg00084.html

It is easier to understand if you take a bicycle rim, get the wheel spinning and hold each end of the axle. Now angle the axle like in the post and you'll see the rim wants to turn just like he says.



I would think that the larger the scrub radius, the greater the force added to the system during an event, making things worse.



Any thoughts?

 

[Matt400]

Good post, and another reason to stay as close as possible to the OEM back space on the wheel.

 

[rbattelle]

I agree, an excellent post! If I understand things correctly, scrub radius would remain factory if the wheel is not changed during the swap to larger tires.



-Ryan

 

[BigEasy]

rbattelle... you're correct. the scrub radius is the distance from the kingpin/balljoint axis to the vertical centerline of the tire.



bigger tires and wider wheels simply create more leverage on all the frontend parts, which are too small from the factory , thus magnifying DC's poor design :-{}

 

[brods]

I agree, an excellent post! If I understand things correctly, scrub radius would remain factory if the wheel is not changed during the swap to larger tires.



-Ryan

It would change. The SAI is a line through both ball joints. The ball joints are angled so an imaginary line drawn through their centers will point somewhere near the center of the tire contact patch. How far off center is the scrub radius. Now if you keep everything the same and install taller tires you will raise the axle. Now the imaginary line will have to be longer to reach the tire contact patch and since its angled that means it moves over as it gets longer. So now the point where the line goes through the tire contact patch has moved over toward the outside of tire.



This link has a better picture. http://bodyshopbusiness.com/bb/bb030524.htm

Imagine moving the whole suspension up by adding taller tires. You can see how the line will move to the outside of the tire centerline.



What does that mean? Well it depends on what the scrub radius was originally. If it was positive, then as the tire gets taller the scrub radius will decrease and then go negative. If it was negative to start then it will get more negative.

 

[Hohn]

THAT link seems to indicate that the definition of SAI is not relevant to a solid axle front ends



Scrub radius (based on what I know it to be) would NOT change with the fitment of larger tires. It will only change with different wheels specs.



jlh

 

[Matt400]

Some clarification can be found here



Also here is the scrub measurement:

#ad




As you can see, using rims with less than the OEM 6" back spacing will increase Scrub Radius as it moves the tire center line out away from the steering knuckle pivot point.



Also remember wheel width will play a roll too because if you move to a 10" wide wheel and maintain the 6" back space you still moved the tire centerline out away from the pivot point.



Scrub Radius measurements are generally used to diagnose odd front end faults, its not something you can put a wrench on and change, it does change with wheels that have different specs though.

 

[rbattelle]

It would change. The SAI is a line through both ball joints. The ball joints are angled so an imaginary line drawn through their centers will point somewhere near the center of the tire contact patch. How far off center is the scrub radius. Now if you keep everything the same and install taller tires you will raise the axle. Now the imaginary line will have to be longer to reach the tire contact patch and since its angled that means it moves over as it gets longer. So now the point where the line goes through the tire contact patch has moved over toward the outside of tire.

It's a little confusing to think about. Certainly a taller tire (assuming no change in backspacing) will move the intersection of the vertical line passing through the center of the wheel and the SAI up. If scrub radius is defined as the horizontal distance between the SAI and the wheel center at the road surface, then larger tires would change it (it would get smaller).



I think that's what brods means...



-Ryan

 

[brods]

Ryan, yes that is it. Thanks to ECappleman’s recent alignment spec post we know our trucks SAI is 13 degrees, so for every 1” increase in tire diameter, the intersection where the line through the ball joints hits the ground moves outward almost 1/8". So going from a 31" tall tire to a 35" tire moves the intersection outward about 5/8".



Is that a lot? I don’t know, I'm no expert, just reading up on things and this caught my eye some years back. The alignment texts mention that toe is tied to scrub radius too. Since positive SR (scrub radius) makes the tire want to toe out while driving straight, they recommend the toe be set toe in. Likewise negative SR makes the tire want to toe in while driving straight ahead, so they recommend a toe out setting. So if we make mods that changes our SR we also should think of tweaking the toe setting too.



I am hoping someone who knows suspension design or alignment specs can give some input. GM says a large scrub radius is bad, but doesn’t say what “large” is. We also need to find out what the stock RAM 4x4 scrub radius is so we can figure out how changes in tire height, wheel width and wheel back spacing will effect it.

 

[rbattelle]

It all comes down to one thing: change any one item on the stock suspension, and everything else changes with it. Including all the alignment specs. If you put on a larger tire and align the truck to factory specs, it will not necessarily be correctly aligned.



In an ideal world, I suppose any change at all to any suspension or wheel part would be corrected out of the system by re-aligning every suspension component back into a position that puts it at the same relationship to every other component as stock. In reality, that would be quite costly and time consuming.



-Ryan

 

[Hohn]

Couldn't you get zero scrub with enough backspacing, at least in theory?



jlh

 

[Matt400]

Couldn't you get zero scrub with enough backspacing, at least in theory?

Sure can and many of the front wheel drive cars today do with the lower ball joint tucked inside the wheel.



In the link I posted above you will find this statement-

It should be noted that as scrub radius approaches zero, road feel decreases significantly, which can be detrimental to providing the driver with tactile input for steering.

 

[JHardwick]

So going from a 31" tall tire to a 35" tire moves the intersection outward about 5/8".

Just over 7/16" actually.

 

[Matt400]

While it is possible to calculate Scrub changes form different tire sizes, those changes are very small and not worth noting IMO because wheel widths and back spacing are the main factors that will make more significant changes to this measurement.