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Scrub ?

rdc

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#1
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

Well-Known Member
#2
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

Well-Known Member
#3
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

Well-Known Member
#4
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
 
#5
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

Well-Known Member
#6
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!!!!!
 
#8
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

Well-Known Member
#9
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!)
 
#10
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!
 

Bob_Sheaves

Well-Known Member
#11
Hi Brannon,

Actually, the Japanese use 2 different sets of numbers depending on the meaning....one is homegrown (truely Japanese, used for objects 1-9) and the Chinese (used for everything else and informal talk)....

1= ichi (Chinese) or hitotsu (Japanese)
2= ni (Chinese) or futatu (Japanese)
3=san (Chinese) or mittsu (Japanese)
4=shi (Chinese) or yottsu (Japanese)
5=go (Chinese) or itsutsu (Japanese)
6=roku (Chinese) or muttsu (Japanese)\
7=shichi (Chinese) or nanatsu (Japanese)-sometimes abbreviated as "nana"
8=hachi (Chinese) or yattsu (Japanese)
9=ku (Chinese) or kokonotsu (Japanese)
10=ju (Chinese) or to (Japanese)

For example, when I go into a Shinsengumi (like a 7-11) for 2 packs of smokes, I say "Marlboro nico, kudasai", which means "2 Marlboro, please".

Best regards,

Bob Sheaves

(added as aftewr thought) When I get tired of listening to the arguments at work, there is another very helpful phrase...

Moh kekko desu. (pronounced "Moe keck-ko dess")

....which means "Thank you, I've had enough".......<P ID="edit"><FONT SIZE=-1>Edited by Bob_Sheaves on 05/24/01 11:41 AM (server time).</FONT></P>
 

FABRICATOR

Well-Known Member
#13
HELLO DYLAN,
About the only control arms that don't affect scrub radius are ones that are stationary or, if moving, ones that are equal length, parallel to each other, and share parallel mounting axis.

Trying to get zero scrub radius is not a bad thing. Near zero scrub radius is required for FWD and independently suspended 4WD to avoid torque steer. It is the least abusive of all systems to steering components and spindles. It remains fairly neutral even when slamming into rocks. This geometry works very well in off-road racing. The MacPhearson 4WD Trophy-Truck was a good example. Certain versions of it open up possibilities for sprung and un-sprung weight savings and even longer travel, but that's another story. About the only drawback to this system is less "feel" or driver feedback. But the advantages certainly outweigh this. You will see more use of this in the future.

You are right about scrub. This is something that needs to watched on the street (unless you own a tire store). It's not too critical in the dirt, although it is beneficial to minimize it.
Take Care,
Chuck
 

Dylan

Well-Known Member
#14
chuck:

the location of the upper and lower ball joints in relation to the tire centerline determin the scrub radius. This has nothing to do with the direction the arms go from the ball joints to the frame. scrub radius is dependent only on the hub knuckle asm. and the wheel offset. you can make make longer arms for your IFS truck and drasticly change the geometry but and you will not affect the scrub radius but you are changing the scrub.

zeroing the scrub radius is a bad thing! "neer" zero scrub radius is not a bad thing

a lot of intended driver feedback is caused by trail which is another peramiter of hub knuckle asm. and is also affected by caster.

scrub in the dirt may not be a bad thing for tire wear but it is an indication that you might have a problem with wheel jacking which is a dangerious situation.

? how does scrub radius have anything to do directly with unsprung wheight?

dylan
 

FABRICATOR

Well-Known Member
#15
Hello Dylan,

Your statement that “the location of the upper and lower ball joints in relation to the tire centerline determine the scrub radius” is not entirely correct. Their relationship to the car and the ground also come into play. Assuming your “centerline” starts at the center of the tire-ground contact area and goes up true vertical from there, scrub radius is created by two different planes which are at any amount of angle to a true vertical tire centerline. One of these planes is any angle from true vertical in a forward or backward direction (castor), in relation to the car. The other plane is any angle from true vertical inward or outward from the car (king pin inclination). The intersection or these two different planes form a single line and are the ball joint centerline. Except in the cases I mentioned before such as arms that are at rest or, when moving, form a parallelogram (equal length parallel arms and parallel mounting axis as you might find on some Class 1 or 10 cars) any independent front suspension will involve inward and outward movement of the ball joints independent of each other. Nearly all vehicles with independent suspension have mounting points (axis) of the UCA (upper control arm) and the LCA (lower control arm) that are not parallel with each other in either vertical or horizontal planes. In some state-of-the-art Trophy-Truck applications the UCA and LCA axis are at radical angles to each other (10 to 15 degrees on a horizontal plane). In any case, this all causes changes to the ball joint centerline in at least the forward and backward plane (caster). This greatly affects the “trail” as you referred to (this is usually a term used for motorcycles and is generally one dimensional) and scrub radius.

In most applications the UCA and LCA are not equal length. This causes changes in the inward and outward angle of the ball joint centerline. It may seem like this would not affect scrub radius but it does. This can best pictured by the fact that, among other things, the tire contact area changes. An example is when the upper ball joint moves outward in relation to the lower ball joint, the center of the contact area moves to the outside of the tire. Technically the scrub radius could be considered the same as always, but to what point or plane? The tire may not be perpendicular to the ground at this point. Keep in mind that even an imaginary vertical wheel centerline cannot always be true vertical. This may be where the most confusion is. The tire contact "patch" is actually changing all the time. We can see camber changes but not caster. We don't see it because the tire spins and always touches on the bottom. In reality, it moves all over because the car is moving all over the place. If the tire were not spinning you probably could easily watch the contact area move in both planes.

Longer UCAs and LCAs if used for a given amount of wheel travel will reduce scrub radius because they reduce inward and outward ball joint movement.

Front wheel drive (FWD) geometry is close to zero scrub radius because it has to be. Braking torque works fine with a high or low amount of “trail”. However putting power through a wheel in a high “trail” situation would be dangerous and it would be very difficult to steer under acceleration. To control torque from stopping and torque from propelling the vehicle, a near zero scrub situation is needed. Try turning you or your neighbor’s FWD car tire on your driveway and then looking at the pattern. You will most likely see a symmetrical rotation which involves all the lines of whole tread width forming clean circles or arcs around a center spot. Non-FWD cars that do not use near zero scrub will leave a patch of smeared rubber like a thumb with ink on it. (Keep in mind that more and more cars and especially SUVs, like Jeep Cherokees, Toyota trucks and SUVs, etc. use FWD geometry. This is what Bob Sheaves was referring to. From a chassis standpoint, they are literally 4WD vehicles without the 4WD components.) This is also why all FWD cars and independent 4WD cars and light trucks use completely different kinds of wheels. They are very near to a total offset. To put it simply, these wheels do not swing as they turn.

Intended driver feedback is determined by caster angle or what you might call “trail”.

Wheel jacking is generally a reference to a rear axle control problem and usually involves leaf spring “wrapping”, or some other component which is allowed to “climb” or twist. This is precisely why control arms used on FWD and 4WD cars and light trucks, such as an S-10 blazer or P/U, are built much more rigidly than the same part for a RWD vehicle of a similar make and model. This is also why MacPherson Struts work so well. They’re an inexpensive way to eliminate slop and twisting from rubber bushings and gain a tremendous amount of rigidity and precise control over wheel movements. These struts gain these attributes through leverage and not brute strength. It is this lack of brute strength and lack of long travel which keeps them off the race course. It must be understood that these principals and geometry cannot be mixed up with each other. Lack of “trail” on a traditional, standard suspension on a RWD car or on a motorcycle would be quite unstable.



I did not say that scrub radius had a direct relationship with unsprung weight. I stated that the type of suspension which uses a near zero scrub opens the door to it. This again goes back to FWD geometry. We can get more into that later.

I will state again that FWD or near zero scrub geometry is superior in all aspects including strength, weight, harshness to steering components, stability, and longevity. And also that you will see more of it on the street and on the race course.


Your argument is correct for a suspension at rest but not for a moving vehicle. What I have mentioned is a very brief simplification of what’s really going on. That is why even to this day there is a different geometry used by almost every car manufacturer. It’s amazing how radical the differences are between some makes and models. Some Audi’s use two upper ball joints and control arms to get “better” geometry. Many Mercedes Benz and believe it or not, the Chevrolet Monte Carlo share an odd “road grader like” approach to it all. Some cars raise up or down 2 inches or more on one side while sitting still just by turning the steering wheel. Maybe this could be measured to figure out the scrub radius. (LOL)

I apologize for my mis-spelling, looks like we all could improve in that area.

Take care,
Chuck
 

Dylan

Well-Known Member
#16
Chuck
Thanks for the input but I respectfully disagree. Check out Milliken and Milliken p.710 for a sketch of the geometric construction of scrub radius vs. king pin location.

Unfortunately the original question, what is the difference between “scrub” and “scrub radius”? hasn’t gotten any clearer throughout this thread
To separate the two:
Scrub= A characteristic of A-arm geometry.
Scrub radius= A characteristic of knuckle geometry.

Dylan
 
#17
Even though this thread is entirely confusing...I like the gentleman's duel going on!

BRAAAAAAAAP!
 

Cameron

Well-Known Member
#20
I always have to scrub my knuckles when I get through working on the car!!!
I hate to have dirty hands!!!
And don't forget under the finger nails!!!

<font color=red>Cameron</font color=red>
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<font color=yellow>BRAT Racing # 936</font color=yellow>
 
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