Related Videos: I drive a lancer evolution x and the stock wheel size is 18×8.5 +38. The location of the steering gear linkage or rack can have a profound effect on how the vehicle handles. the car steers as i brake, but i noticed it is even worse when the surface is not flat. How common is zero scrub radius among race cars – e.g. New Posts; Member List; Mark All As Read; Forum; Technical; Suspension, Brakes and Handling; If this is your first visit, be sure to check out the FAQ by clicking the link above. I have worked on cars for a long time but very little in suspension geometry.. Depended on others to perform alignments. I don´t get the point why you should get squirm and the negative effects of it with zero scrub raduis only. Question! These youngsters and I was once, do not take into consideration for their modifications. We live & learn. I'd like to widen my cars track width by 3 inches on each side so I figured 3 inch wider wheel and 38mm less offset on the wheels (which totals to about 3" unless my math has been done improperly) would put the outer face of the tire 3 inches farther out. Search. https://www.carthrottle.com/post/what-is-scrub-radius-and-why-is-it-important What is a good scrub radius for a drift car? So if I were to push the centerline of the tire further to the outside of the car towards the fender….I'll end up having more positive scrub radius…which seems to have the least desirable characteristics on a performance car. i always wondered why! Can you do a video on push rod actuated suspension please? 893 Great Deals out of 24,772 listings starting at $1,000. To review; the scrub radius is the distance between the centerline of the tires contact patch and the point where the steering axis line (or KPA line extended or Dave Point) hits the ground. 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. Car Throttle: https://www.carthrottle.com/user/engineeringexplained, To help create more videos, check out my Patreon page! It also helps improve the self steering effect. I'm just want to figure out which size will not negatively affect performance (if any). Thanks. The scrub radius is a moment arm where the tire’s contact patch can give feedback to the steering. SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. Wait so is zero a good amount of scrub radius to run for a drift car via 0 offset wheels? And don’t forget to check out my other pages below! Hahaha, is that from Stoll and Betzler Automotive chassis book ? On sale $1,239 Regular price $1,350 Quantity ... offset wheels we can build these in several different widths in order to increase your track width while maintaining scrub radius as close to stock as possible for better, more neutral steering feel and control. Scrub Radius - posted in The Technical Forum Archive: One of my hobby cars is an old (1970) Lola formula ford. BMW SLR E46 M3 Race kit. Your appeal (as least to me) was the accessibility of your explanations. I guess all one can do when fitting non-stock wheels is keep offset close to stock while erring on the more +ve side to avoid pushing scrub radius to zero or into the -ve range in order to best preserve the basic characteristics in the suspension design. Took an endless amount of room to get the bastard to stop. 5: Steering Roll Radius (Steering Offset) Courtesy of BMW … A negative roll radius exists when the steering axis line is outside of the tire center line. Boy I would love to see this explanation as it would pertain to solid front axles that use either kingpin or balljoint knuckles. [Archive] Page 90 General discussions, questions, facts and figures about the 2000 - 2006 BMW X5 This is typical of FWD cars. Steering Axis Inclination – https://youtu.be/IUcZ63unEyU YOu can see the damage being done just by looking at the tires. 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. 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. Can you explain in more detail how having FWD or RWD cars affect toe-in/out under acceleration? If some of this is good, more positive caster and KPA are not necessarily better. The scrub radius is a critical part of the (Torque = radius x Force) function where the torque is applied to the spindle and transferred to the steering linkage. If these lines intersect at the road surface, a zero scrub radius would be present. So if I'm looking at changing wheels on my vehicle and staying he same tire size, but going with a different offset from 55 to 40 I am looking at generating (assuming my vehicles scrub is zero to start) about 19/32's of positive scrub. Nice job! To start viewing messages, select the forum that you want to visit from the selection below. How does wheel width affect offset and scrub radius. Your email address will not be published. This the first time I have heard of scrub radius. Knowledge is king and that is why your called an engineer. Out of topic now but say you got a same wheel size and use a spacer say 25mm with 0 wheel offset, will it have the same effect on wheel bearing(stress) as no spacer but a -25mm offset? Appreciate your time & efforts! 8 inch wide wheels & a slightly narrower slick cured this. @engeenringexplained, Good video, thanks for posting. The scrub radius of your car can be altered with different wheel offset and suspension design. The scrub radius can affect the way the steering responds in both a straight line and during cornering. The scrub radius is the distance at the road surface between the tire center line and the SAI line extended downward through the steering axis. Excessive caster and/or KPA can result in understeer because tipping the tire on edge and lifting it extremely can cause the corner weight on the outside tire to drastically increase, artificially increasing weight transfer. In AWD and FWD cars, you want to try to limit the scrub radius to less than 2” because it increases the effects of torque steer. Keep in mind that a -25mm offset will increase the distance between the wheel flange and suspension components that might rub with inner wheel.. Just trying to take out the 14×6 +38mm offset wheel and put a 15×8 (custom made offset and back spacing) and I don't know how to calculate this.. all I know is that I need a wide track on my 88 vw mk1 with least stress on bearing and optimum handling feel. https://autosystempro.com/scrub-radius-suspension-design-explained Ive been experimenting with wheel offsets to a.) +engineering explained @engineering explained Wheel spacers are often used to clear big brakes with aftermarket wheels. Scrub Radius Scrub Radius Definition Scrub radius is the distance at the road surface between the tire line and the SAI line extended downward through the steering axis. Scrub radius is particularly significant if or when one has a flat, very low tire, or loss of braking on one side. i can feel this when i change to lower offset wheels. Notify me of follow-up comments by email. Although the document does state that "BMW vehicles are designed with a slightly positive Steering Roll Radius". Will this be noticeable in day to day driving and braking? Can you please explain if why are we adjusting the wheels to toe out on FWD cars with McPherson suspension if it tends to toe out during acceleration. That's why I suspect the diagonal is a key component in the changed handling dynamics … ? Scrub radius comes into play in the game of balancing positive caster and KPI. But now the car is easily pivoted and rotated around it's polar moment with the throttle. A negative scrub radius is suitable for vehicles with high power engine and a positive scrub radius offset for small power engines. Many cars do not have zero scrub out-of-the-box. About us. I know a lot of trophy trucks and class 1 buggies use equal length control arms but the suspension travel is so much that it also plays a role on steering linkages and steering angles. The wheel will be sticking out away from car 25mm in both ways but which will have least load on bearing? The main effect of this is to introduce a turning moment. Wheel-Size.com is a Wheel Guide and catalogue. The same moment that creates the self-aligning torque can contribute towards torque steer in a FWD or AWD car. You're incredible. We strive to help you get the information you need about PCD, offset, rims and all other wheel and tire … The line through the steering axis creates a pivot point around which the tire turns. About the only thing you don't explain is why it's called a 'scrub' radius. The roll radius is the distance between these two lines. I used the tire-calculator at Wheel-Size.com to compare the oem set up, (which assumed the oem set up had a zero scrub radius) to the new rims and tires and it calculated the new scrub radius to be positive 21/32″ scrub radius which is approximately 5/8″ or 16.67mm. Helped me pick wheels/tires for my Jeep TJ [ beerclink ] the help of lightweight design suspect diagonal. 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