Scrub ?

[rdc]

Forgive my ignorance, in other threads, people have refered to "scrub," and "scrub radius." A term I've never heard of. I know it refers to the suspension, but what it exactly is it, and is it particular to a certain type of suspension design. (A-arms, I-beams etc.) Just trying to increase my knowledge.

Thanks in advance

Keep the dream alive !!!

[Bob_Sheaves]

Hi,
"Scrub radius" is defined as the distance between the front view kingpin or ball joint inclination angle and the vertical centerline (including camber) of the ground contact patch of the tire. In an ideal geometry, these 2 lines will intersect exactly at the ground line at the tire static loaded radius. Most modern 4x4 STREET trucks have a (generically called) kingpin inclination angle of around 12.5 degrees or somewhat less.

To picture this, imagine a Ford Twin-I beam front suspension. This design really has kingpins and not ball joints. Standing at the front of the truck, looking towards the rear, at the front suspension at the static loaded height (in other words, the design load point), the driver's (or left hand side) kingpin tilts towards the center of the vehicle. The passenger's side, likewise, is tilted at the top towards the center of the truck. Now, if you extend this centerline of the kingpin to the point where it intersects the ground, you will have the first point to measure from.

The next thing to do is to look at the way the front tire sits relative to the ground. Generally the tire should be as close to "plumb", or vertical, as possible, under all driving conditions. In practical application, this is not necessarily the case however. In order to minimize the "bump steer" (among other things not covered here) as the suspension cycles from jounce (commonly called "bump") to rebound ("commonly called "droop") the front tires will "lean in" at the top slightly, so they roll straight down the road. If you draw an imaginary line through the center of the tire that is parallel to the camber angle, you will get the second point for the measurement. The distance between these 2 gound level points is the scrub radius.

Some other factors that affect this dimention is wheel offset, caster, control arm geometry, etc. In other words, all suspensions have a scrub radius number, and some are bigger or smaller than others.

I hope I haven't bored you too much....

Best regards,

Bob Sheaves

[Dylan]

control arm geometry does not affect the scrub radius.

I disagree with shooting for convergence of king pin centerline with the center of the contact patch because as the force from the tire moves in and out due to varying contact with the road you get a reversal of forces on your steering linkage. if there is any slop (and there always is) or any compliance(and there always is) you will get vibration. keep the scrub radius positive(in most cases), (that is center of the tire outside the king pin centerline at the ground), but small (about 1"). Iv'e seen massive steering components destroyed frome to much scrub radius(4+") .

the second part of the question: scrub not to be confused with scrub radius, is the change in track width throughout the suspension travel. the only system that doesn't scrub is a solid axle. A-arms and beams will both scub more in rebound as they begin to swing under the vehicle, you can compinsate for this a little with a arm geometry but when arms or beams get steep youv'e got a scrub problem( and an even more dangerous problem, wheel jacking, but thats another topic, ie: lifted swing axle bug)

[Bob_Sheaves]

Hi Dylan,

I tend disagree silghtly with your post-re: Control arm geometry. If you take a Honda or Chrysler LH knuckle with the high mounted upper ball joint (above the tread of the tire), this joint location on the control arm is indeed altering the kinpin inclination angle, allowing a far smaller scrub radius when compared to a conventional SLA setup (with the UCA located inside the rim sweep).

Scrub radius is generally applied ONLY to a static condition at the design point, not in a dynamic loaded condition, but I agree that you are correct in stating that too much designed in scrub radius will break parts-completely agree there! (I was trying not to introduce dynamic effects into the explaination at this point.)

I agree with your comments on scrub (sorry, I forgot to answer that part of the question). There is also a way to compensate for the scrub in an SLA design, that is to design the UCA to "pull" the top ball joint inwards- pivoting the GCP (ground contact patch) outwards, effectively spreading the GCP to maintain the track width. This does introduce other, less desireable effects however (beyond the subject here).

Best as always,

Bob Sheaves

[Waldo]

I thought scrub was something you do to your dishes at home...I try to keep up with your dialogue but wow.....technical stuff...I guess that is why I am not a mechanic or fabricator...Late!

BRAAAAAAAAP!

[Donahoe]

For all the guys reading the posts about scrub radius Between Dylan and Bob..... If you want the English version of what scrub raduis is Think of it like caster in a shopping cart wheel.... Its the center line that the front tires will turn on (looking down on the tire from the top) If your scrub radius is off you can break steering parts by putting to much load on them. The less load in turning the better the scrub radius. Dont let these guys freak ya out with big words. We need a engineer to english translater... LOL...

NEVER LIFT!!!!!

[Bob_Sheaves]

Hi Kreg,

Gomen nasai, sensei....(abject bow)

I guess I did get a little deep into it.....LOL!

Best as always,

Bob Sheaves

[Waldo]

Thank you DRE...I feel much better and have gained knowledge in this specific area. By the way (DRE or Bob) what is LOL...Thank you for being caring masters.

Brannon

BRAAAAAAAAP!

[Bob_Sheaves]

LOL=Laugh Out Loud

Best as always,

Bob Sheaves

PS- Gomen nasai, sensei = "Excuse me, master teacher" in Japanese (usually used when you screw up REALLY bad!)

[Waldo]

eich, neich, san, chi, go, roch, hich, hoch, cu, ju...I think that is how you count to ten in Japanese...Is that correct sensei?

Thanks for the response

Brannon

BRAAAAAAAAP!