This creates a scrub radius that will affect tire wear and can cause a change in stability. Discussion Starter • #1 • Dec 4, 2018. RWD only is 0"-3". A-arm suspension. 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. A negative scrub radius gives a harder feel to the steering wheel as it requires more input to turn. This is a lot even though it's just a pickup truck. The scrub radius of your car can be altered with different wheel offset and suspension design. How low of an offset is safe for wheel bearings using same tire Diameter. I will bet you can come up with something better. So you don't stand behind the drive it till its soft mentality of lower mechanical trail? From doing some reading, there are articles explaining the term and how it can affect handling, but it seems as though most people think it's not very important in terms of real performance. Aim for between .25" to .75". I own a 2017 TRD OR and purchased SCP F5 16x8 wheels with a -25mm off set, 3.5" backspacing. I think that it scrub radius should be looked at as an acceptable range, and that exact OEM measurements are not ultimately necessary. Honestly the amount of scrub you build into an axle should depend on your driving mix. Tire diameter certainly affects scrub radius as it changes the point where the two axis intersect. The scrub radius Altering wheel offsets also affects the scrub radius. That is not such a bad thing though. KMC 15x6 5+1 has a +38mm offset which is technically a 4.496"+1.504" STI HD9 5+2 has a +30mm offset which is technically a 4.681"+ 2.319" The main effect of this is to introduce a turning moment. If the line touches the road outboard of the contact patch, scrub radius is negative (see Figure 2). He claims, “in order to get the perfect lateral friction to rolling resistance ratio, you must periodically ‘scrub’ the tires to expose the unused material.”. Just try to get the overall diameter to be as close to OEM as possible. 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. All three are shown in the figure below. We only really have scrub to fit things in the wheel easier. RWD only is 0"-3". It’s rare these days for a car to have truly zero scrub radius; a little of something is better. 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. Order another beer and go with it. In any case, whatever size tire you do put on. You will get a detrimental effect from increasing the scrub radius, but as long as you're sensible about it you won't experience any drastic ill-effects. hiPSI, Mar 22, 2018 #16. sc0ttayo [OP] likes this. Recall that the scrub radius is the ground-level measurement between the kingpin inclination axis (KIA) and the tire contact patch's center. Whenever people talk scrub radius, this comes up because it sounds like you're increasing "scrub" on the tire. I agree with most everything stated by the folks so far, but one thing that can easily overwhelm a "good" scrub radius setup is the steering system itself. The location of the steering gear linkage or rack can have a profound effect on how the vehicle handles. Scrub is bad. 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. 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. If they meet at the ground surface the scrub radius is Zero. It could be positive, negative or zero. The tire will generate more forces through the linkages, but especially in straight line acceleration, there will be no change in tire behavior. To understand scrub, you must first know about Steering Axis Inclination (SAI). If the line touches the road inboard of the contact patch, scrub radius is positive. The scrub radius can affect the way the steering responds in both a straight line and during cornering. Changing the scrub radius, aside from running zero scrub radius, has no impact on the way the tire acts. 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. I wonder what the max acceptable scrub radius is for a GT40. I wonder what the max acceptable scrub radius is for a GT40. The smaller the scrub radius is the less feedback to the wheel you get. Create some code or excel, look at your tyre forces (estimate) then plug in your geometries and find out your steering forces at different angles. All rights reserved. I think that peak Steering wheel torque should align with peak front axle Fy, and the tires should self align if the driver lets go of the wheel. The scrub radius is the distance between the two, where both touch the road. Like I'm seeing on my car 8.5 width and ET45 is perfect and doesn't affect scrub radius too much. 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. 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. 0 scrub will give you almost no steering feel. 2019 X3 XRS black, de-stickered, scraper delete, shocks reworked by Tom Morris, pwrTune PV3 stage 3, DJ clutch kit, S&B particle separator, Evo V-Flow, custom exhaust made by myself, Maxxis Liberty 32/10R15 tires on 15x7" OMF non-beadlock wheels. SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. Reply. The biggest effect is called bump steer. Cliff is trying to explain that scrub has been in use since early man invented the wheel. IMAGE_020.jpg. Scrub radius represents the "pivot point" created by the force of the load and the steering axis. The distance between the center of the tire's contact patch and the point where that line touches the road is the scrub radius. 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. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry. 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. The scrub radius is the distance in front view between the king pin axis and the center of the contact patch of the wheel, where both would theoretically touch the road. Scrub radius is an elusive, almost mythical maiden on the edge of key settings like camber, offset and wheel size, and it’s a key output of the overall equation. Changing the scrub radius, aside from running zero scrub radius, has no impact on the way the tire acts. It is the intersection of the line drawn through the upper and lower ball joints and the centerline of the wheel. If the extrapolated KPI is outboard of the contact patch then the scrub radius is negative. Follow for little RS update on Instagram: @BigFatFlip. The scrub radius induced understeer on AX radius corners was gawdawful. Negative Scrub at street tire pressures can go to zero and even positive based on tire pressure, Positive and zero scrub will only go more positive as you air down the tires. R. Russ Noble. We live & learn. 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. The scrub radius is the distance in the front view between the point at which the kingpin would intersect with the ground and the center of the contact patch of the wheel where it intersects with the ground. The bartender, a former sports icon, talks to a patron at the other end of the bar about the specifics of modifying the suspension on his Corvette. scrub radius design-provides acceptable handling-steering control-causes resistance to rolling the front wheels to exert force on steering linkage. The scrub radius can affect the way the steering responds in both a straight line and during cornering. What you said about scrub radius reducing with a taller tire is true, however +2.5" of scrub radius -.25" is still bad. If these two lines intersect at ground level, then you are said to have zero scrub. By 'scrub radius' are you talking about slip angle? Scrub radius, to my knowledge, cannot be calculated to an exact number. My advice. 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. 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. racing aspirations » What is Scrub Radius. If there is a change in ride height front to rear on a passenger car, it can affect caster and steering. You need all the slack pulled one way, so it is predictable. Some careful measuring shows that my scrub radius is + 2”. There is obviously a limit to how much of a width change you can make. The tire will generate more forces through the linkages, but especially in straight line acceleration, there will be no change in tire behavior. I have 90 mm, I think that is 3,54" Attachments. From doing some reading, there are articles explaining the term and how it can affect handling, but it seems as though most people think it's not very important in terms of real performance. Mar 22, 2018 at 4:54 PM #17 #17. splitbolt Well-Known Member. Shorter wheels will pull the scrub radius in towards the frame. Scrub radius is also known as steering offset and scrub geometry. Press J to jump to the feed. 8 inch wide wheels & a slightly narrower slick cured this. Assuming forces are perpendicular, or figure in slip angles? If the new wheel is wider than the stock wheel, you will loose clearance on … The scrub radius Altering wheel offsets also affects the scrub radius. Scrub radius is the distance from the steer axis to the centre of the tyre contact patch at ground level and wheel centre offset is the distance from the kingpin axis to the wheel centre. If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called ‘squirm’. 38.3 KB Views: 364. Taller wheels than stock will push the scrub radius farther out from the center line which is acceptable to a point. Everything that I'm seeing is telling me that I should cancel my order and probably pick out a different wheel, yet I do want my tires to poke out a bit, resulting in a better stance. 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 What is acceptable scrub radius? Offset also affects the suspension's motion ratio, which directly determines the effective spring and damper rates. We strive to help you get the information you need about PCD, offset, rims and all other wheel and tire … Suspension and handling properties will not change. 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. We rarely if ever see them here in the UK. Sorry. My personal opinion...don't go below ET40. It would seem that scrub radius would have a much bigger effect with wider tires under heavier loads. The distance between where the SLA and the centerline to the wheel intersect the ground is called the scrub radius. 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. I was trying out this tool below (mentioned in another thread) and it said "Scrub radius will be changed by 30 mm, this may affect the car's handling." Scrub Radius: Selecting A Lower Offset If you select an aftermarket wheel with the exact same offset, you will always have the exact same scrub radius. I know the Oem for sport is +50 offset. So the question is how much more than zero scrub radius is desirable and acceptable? With any kingpin angle, steering will tend to raise and lower the vehicle. The scrub radius Altering wheel offsets also affects the scrub radius. If the new wheel is wider than the stock wheel, you will loose clearance on both suspension and wheel well sides. The larger the scrub radius is the more feedback and shock load you get. Don't forget to look at how uneven long loads will affect the steering forces. 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. 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. 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. 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. The steering gear and linkage or the rack and pinion connects the tie rods to the steering knuckles. On a suspension using upper and lower control arms, the pivot points are where the upright connects to the control arms. Makes the car roll out of a turn due to caster and ackerman. I think that it scrub radius should be looked at as an acceptable range, and that exact OEM measurements are not ultimately necessary. This is not desirable as it makes the steering heavier (more effort to steer) and can cause unwanted bumpsteer while braking. I've never heard the term scrub radius before. Members on each team must develop and practice engineering, leadership, communication and marketing skills to be successful at competition. As viewed from the front of the vehicle, this is determined by considering the distance between the center of a front tire tread and the imaginary SAI line, when measured at the road surface. The location of this point within the contact patch has a great effect on steering effort, feel, and stability. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. the scrub radius, it is not the only thing that will affect it. Correct, wider wheels don't effect scrub radius, offset does. Offset affects the steering geometry's scrub radius, possibly leading to problems with torque steer or self-centering characteristics. GT40s Supporter. When viewing a wheel from front view, the scrub radius is the distance between where the extrapolated line of kingpin axis touches the road and the tyre contact patch centre point. Offset affects the steering geometry's scrub radius, possibly leading to problems with torque steer or self-centering characteristics. Does this scenario sound like you and your car buddies at the local watering hole after a club meet or romp through the countryside? Volkswagen) you can change the SAI. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. 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. NOTE: Scrub Radius is defined as the point of intersection of the king pin angle (a line drawn through the upper and lower ball joints and the mid line plane (steering plane) of the wheel (tire). The lower the offset, the closer the wheel face is to the hub. On the other hand, when stationary the steering has to turn the contact patch where it lies, which takes more effort and wears the tyre more. 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. SCRUB RADIUS. Now back to the subject at hand. Press question mark to learn the rest of the keyboard shortcuts. Some scrub radius is good. 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. 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. A MacPherson strut assembly typically performs well with a lot of SAI and caster, a system negative scrub works well in. ©2021 H&R Special Springs, LP. 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. Joined: Mar 7, 2010 Member: #32761 Messages: 5,832 Gender: Male NW Arkansas Vehicle: 2016 TRD OFFROAD DCSB MGM. 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. A little difference in offset is acceptable and I wouldn't worry about it too much. and theory. It'll be class 10 car but with a V8, so fitting into class 14 I think. What was the original Ford GT40 scrub radius? In a straight line the tire tends to be stable and tracks well. At zero scrub radius the general consensus is that it can make a car feel a little unstable at the front end through corners and under hard braking. Unless I’m wrong I think that’s acceptable. 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. My cars used around 20-30 mm and all worked fine. Suspension and handling properties will not change. Also, anyone know the scrub radius for accord sport 10th? The line through the steering axis creates a pivot point around which the tire turns. As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. Well, the point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. Usually when talking about scrub radius the topic is handling on the track. You see, in order to fit wider wheels, typically folks use smaller offset wheels to accomplish this. Since scrub radii tend to have quite small measurements, 10mm is proportionately quite a large change, so will almost certainly have an appreciable influence. The strut may also be less inclined, inboard to outboard. The Stanley #40 scrub plane was the first all-metal production model scrub plane mass made for the US market. How would I deal with the extra 1.5" - 2" positive scrub radius? 8 Posts . 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. 6 years ago. Get the forces to the drivers liking using as little scrub radius as possible. Some road feel in the steering wheel is a good thing. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero. Supposedly my scrub radius is way off. About us. Additionally, if you plan on doing some homework on, and modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. Positive and negative scrub radii have benefits in different types of suspension. 22.6 KB Views: 254. 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. It is only referred to as positive or negative. The line through the steering axis creates a pivot point around which the tire turns. IMAGE_021.jpg. When you have excessive scrub, whether it be positive or negative, steering effort increases and road ‘feel’ increases, as the steering is more susceptible to road shock. I did try that and did not have much luck. Save Share. Depends far too much on tyres and general vehicle concept (weight, steering goals etc) to give you a number that means anything at all. Scrub Radius: Selecting A Lower Offset If you select an aftermarket wheel with the exact same offset, you will always have the exact same scrub radius. 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). Unlike many if not most of Stanley’s other plane versions, the # 40 was never copied or made by a UK maker and so it is indeed a scarcer find here in Britain. An increase in wheel and tire diameter can also affect braking in all vehicles. I would say 15-20mm of increase is something the majority of drivers wouldn't really notice.. it's only when you get silly and start fitting huge spacers / wheel offsets that you start really messing things up! Negative scrub also helps reduce torque steer in front wheel drive cars. 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? So today when I came home from work I spent some time in the shop and did some measuring of what I actually have for scrub radius. All three are shown in the figure below. The steering axis is the line between the top pivot point of your hub and the lower ball joint of your hub. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. But necessary for driver feel. problems with scrub radius to be caused by-bent spindle-causes hard steering, wander, pull-changing tire size can cause poor handling, tire wear, and wander. Different wheels or tires from side to side will cause differences in scrub radius as well as a tire that is low on air. If these two lines intersect at ground level, then you are said to have zero scrub. With any kingpin angle, steering will tend to raise and lower the vehicle. You got me reading more about scrub radius, effect of zero/neg/positive scrub radius on handling and Ackerman steering this morning, so I at least learned something new today! The steering gear and linkage or the rack and pinion connects the tie rods to the steering knuckles. Closer to .25 the better. 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. Claude roulle has said to aim for .25, New comments cannot be posted and votes cannot be cast. Scrub radius is the distance from the steer axis to the centre of the tyre contact patch at ground level and wheel centre offset is the distance from the kingpin axis to the wheel centre. So I think you need more than zero scrub. Offset also affects the suspension's motion ratio, which directly determines the effective spring and damper rates. I did try that and did not have much luck. SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. So much for being way off. ... An average deviation of 5 mm is acceptable. The scrub radius with this setup is 2". I have 90 mm, I think that is 3,54" This is probably why AEV moves from 6 inches to 5.2 inches of backspacing with their rims, to account for the increase in diameter from 32 to 35 inch tires with just enough space for the larger size. I know I have heard it somewhere before. It is the distance between two imaginary points on the road surface. Wheel-Size.com is a Wheel Guide and catalogue. It seems to me that you wouldn't want zero scrub radius for the same reasons you don't want zero toe. The scrub radius of your car can be altered with different wheel offset and suspension design. SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. Right now I have new 30-10-14 Utracross R Spec's on my stock wheels, which looks like a .343" scrub radius, I have only driven it around the storage where my trailer is since swapping the tires out, if I went to the 5+2 offset wheels which have an .843" scrub radius what will … If it is inboard then the scrub radius is positive. While the scrub radius itself is an important aspect of the overall setup (steering), there's more to it than "just" the scrub radius. If you have .5" of scrub, and then put that same tire/wheel combo on a portal setup, you'd have about 4.5" of scrub radius, a monster change for the bad. It'll be class 10 car but with a V8, so fitting into class 14 I think. blnewt said: I'm not messing w/ my suspension as far as a lift and trying to keep my offset (and scrub radius) … 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. Assuming forces are perpendicular, or figure in slip angles? Now back to the subject at hand. But the way you describe it, that sounds like slip angle to me. The scrub radius is shown at the bottom at .427”. On a … 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. I was trying out this tool below (mentioned in another thread) and it said "Scrub radius will be changed by 30 mm, this may affect the car's handling." You now know what elements make up scrub, now you may ask what it has to do with the price of tea in China? A-arm suspension. A 5-6" scrub radius on these vehicles can lead to some pretty disconcerting drifts/pulls during hard braking and/or rough road conditions. What was the original Ford GT40 scrub radius?