If the new wheel is wider than the stock wheel, you will loose clearance on both suspension and wheel well sides. What you said about scrub radius reducing with a taller tire is true, however +2.5" of scrub radius -.25" is still bad. So much for being way off. If you have not already guessed, the easiest way to change scrub is by changing your offset with either new wheels, or hub centric wheel spacers. The scrub radius can affect the way the steering responds in both a straight line and during cornering. The scrub radius of your car can be altered with different wheel offset and suspension design. I've never heard the term scrub radius before. Honestly the amount of scrub you build into an axle should depend on your driving mix. If the line touches the road inboard of the contact patch, scrub radius is positive. I think that it scrub radius should be looked at as an acceptable range, and that exact OEM measurements are not ultimately necessary. Discussion Starter • #1 • Dec 4, 2018. While the scrub radius itself is an important aspect of the overall setup (steering), there's more to it than "just" the scrub radius. On a suspension using upper and lower control arms, the pivot points are where the upright connects to the control arms. This will affect how the tire contact patch interacts with the road during cornering, braking, and acceleration, and can cause unique toe characteristics depending on the design. The car is evaluated in automotive design and performance through a series of static and dynamic events which focus on safety, manufacturability and of course out right speed. But necessary for driver feel. Scrub is a bigger problem on production vehicles with relatively low longitudinal suspension rates often coupled with asymmetric and overly compliant steering systems (darn solid axle SUV's). It seems to me that you wouldn't want zero scrub radius for the same reasons you don't want zero toe. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes ‘grabby’ causing tire wear to increase and the steering to become unstable. How would I deal with the extra 1.5" - 2" positive scrub radius? 38.3 KB Views: 364. I have 90 mm, I think that is 3,54" Attachments. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero. If you alter the alignment and the suspension angles by changing the wheel’s width and diameter, it can affect the offset and change the point where the flange centerline and steering axis touch the ground. I think that it scrub radius should be looked at as an acceptable range, and that exact OEM measurements are not ultimately necessary. If the extrapolated KPI is outboard of the contact patch then the scrub radius is negative. Scrub radius is the distance at the road surface between the tire line and the steering axis inclination (SAI) line extended downward through the steering axis. Does this scenario sound like you and your car buddies at the local watering hole after a club meet or romp through the countryside? Since scrub radii tend to have quite small measurements, 10mm is proportionately quite a large change, so will almost certainly have an appreciable influence. Formula SAE and Formula Student are collegiate engineering competitions with over 500 participating schools that challenge teams of students to design and build a formula style car. Suspension and handling properties will not change. Thats what i was thinking about the scrub radius but i was trying to figure a way to have less but still try to keep a 14 to 16 degree kpi which Ive heard is ideal. A-arm suspension. In a straight line the tire tends to be stable and tracks well. Using Shock theapy's assumptions your KMC wheel has a 0.874" scrub radius and the STI 5+2 has a 1.162" scrub radius which is a big difference in scrub radius, this is why you feel the big difference IMO. Essentially it’s defined by the location of the point in space where an imaginary line traced down through the centre of the suspension column crosses the one drawn vertically down through the very centre of the tyre. Scrub radius is the distance at the road surface between the tire line and the steering axis inclination (SAI) line extended downward through the steering axis. The inclination of the steering axis is measured as the angle between the steering axis and the centerline of the wheel, so if your camber is adjustable within the pivot points (i.e. What is acceptable scrub radius? Different wheels or tires from side to side will cause differences in scrub radius as well as a tire that is low on air. My personal opinion...don't go below ET40. Taller wheels than stock will push the scrub radius farther out from the center line which is acceptable to a point. The scrub radius is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. Well, now you know all about scrub, so the next time it comes up at the watering hole you have the confidence in knowing that your drunken compatriot is having more fun with creative banter than tuning and driving his car. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively. I assume a total of 2" wider in the front than it is now, and maybe 1" wider in the rear, so maybe a little more stable, but still well withing what is termed acceptable for scrub radius as far as wear and tear on parts etc? Press question mark to learn the rest of the keyboard shortcuts. Scrub radius will give you bump steer in one-wheel bumps For high-speed stability in the presence of road disturbances, zero or slight negative scrub radius is desirable (negative is outside the wheel center plane) Braking forces introduce steer torques proportional to the scrub radius. The distance between where the SLA and the centerline to the wheel intersect the ground is called the scrub radius. WHEEL ALIGNMENT SIMPLIFIED Wheel alignment is often considered complicated and hard to understand In the days of the rigid chassis construction with solid axles, when tyres were poor and road speeds were low, wheel The main effect of this is to introduce a turning moment. Suspension and handling properties will not change. The line through the steering axis creates a pivot point around which the tire turns. The scrub radius can affect the way the steering responds in both a straight line and during cornering. This man walks into a bar… “NORM!” He proceeds to belly up to the bar and begins to talk about the daily grind and the latest in his relationship with his wife. To understand scrub, you must first know about Steering Axis Inclination (SAI). My advice. Having a small scrub radius is beneficial as it allows the wheel to react less to braking inputs due to the smaller moment arm and increases steering stability under braking conditions. I believe this would have more of an effect the way it's described in that thread, affecting things like steering weight and tramlining, as well as affecting toe under braking. This is not desirable as it makes the steering heavier (more effort to steer) and can cause unwanted bumpsteer while braking. Changing the scrub radius, aside from running zero scrub radius, has no impact on the way the tire acts. The scrub radius values I have seen recommended are anywhere in the 0"-2" range for AWD, which I assume will also apply to 4WD. We live & learn. When dealing with suspension design and dynamics, little things that are easily overlooked like scrub radius can make a greater difference in your car’s handling and feel than all your theory and fervent tuning can accomplish. Small changes in tire width are sometimes acceptable, though they still alter scrub radius, a key figure that determines how stable your car is, or isn't at the limit. SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. Portals do have some benefits, but until the figure out how to move the ball joints out further with them, or I decide to build a 100% crawling RZR, I'm probably just gonna have to leave portals out of the equation. Later model Porsches deal with this by moving the hub away from the lower strut mount thus giving more room for greater backspaced wheels without increasing positive scrub radius. Now, I was able to get through school with a little intuition and a lot of B.S., but these qualities do not always work when I am dealing with inanimate objects that are not humored by my B.S. If the new wheel is wider than the stock wheel, you will loose clearance on … If there is a change in ride height front to rear on a passenger car, it can affect caster and steering. The scrub radius Altering wheel offsets also affects the scrub radius. This means there is less than half an inch between the king pin inclination line and the tire center line where the tire contacts the road. Offset affects the steering geometry's scrub radius, possibly leading to problems with torque steer or self-centering characteristics. Negative scrub also helps reduce torque steer in front wheel drive cars. He was told that after changing to an aftermarket wheel and tire combination, he needs to modify the scrub radius a bit and wants to know what in the world they were talking about. 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