Scrub radius is explained in this article - What is Scrub Radius : Sharing: Instructions. The larger the scrub radius is the more feedback and shock load you get. The engineers account for the fact that wheel alignment specifications used by alignment technicians are for a vehicle that is not moving. How much positive scrub radius until screwing things up? 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. Since the scrub radius has to do with steering geometry, such positive offset requirements only apply up front. Some careful measuring shows that my scrub radius is + 2”. A positive scrub radius (i.e. Typically the scrub radius is configured to be as small as possible to minimize the effects under braking, but squirm (from zero scrub radius) is also undesirable. A positive scrub radius tends to be beneficial while braking into a turn. FORUMS. 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. If your car has the more favourable set up of negative scrub radius then installing a wheel spacer can cause bigger issues. Changing the scrub radius, aside from running zero scrub radius, has no impact on the way the tire acts. Whenever people talk scrub radius, this comes up because it sounds like you're increasing "scrub" on the tire. It's how far the centerline of tire moves outside of the steering axis iclination (SAI). Scrub Radius: Purpose 1. The vehicle’s forward motion, combined with friction between the tires and the road, moves the front wheels back (toe-out). Some road feel in the steering wheel is a good thing. Chassis engineers use scrub radius to create the feedback the driver feels through the steering wheel. I drive spiritedly on SoCal freeways that curve a lot. Anyone ever had to solve an issue like this before? We live & learn. 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. 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. FRONT = Springs marked: VA, or (F), or Front. Provide the driver a feel for the road. -5mm, so fitting the ET37 wheels will result in a positive scrub radius of +7mm. 2. The advantage is that there is much greater road feel and feedback so that you can feel when the front tires start to break loose in a corner. Q. Rear-wheel-drive vehicles typically have a positive scrub radius. RE: Effects of improper scrub radius on tire wear patprimmer (Publican) 4 Oct 10 18:41. The positive scrub radius has had some benefits and drawbacks to my driving experience. This is an exmaple of a suspension geometry with positive scrub radius. splitbolt, Mar 22, 2018 #12. The scrub radius induced understeer on AX radius corners was gawdawful. From my understanding, wider wheels won't affect scrub radius, and there's more choices by going 8.5 than 8 for width. An excessively negative scrub radius will require greater steering effort, while excessive positive scrub radius (where the tread center essentially moves further outboard) can not only affect handling and ease of steering, but also over-stress wheel bearings. Even in excess of 80 mph the car is firmly planted and stable. It is only referred to as positive or negative. the kingpin axis intersects the ground inboard of the wheel centre line) will give a toe out tendency on a front axle when subject to a braking force. Since positive scrub radius tends to cause toe-out, you have to think about this in conjunction with the static toe settings (it may be acting to cancel out some toe-in, … 3. On the front suspension, a positive scrub radius gives stability because it causes toe out when braking and toe in when accelerating. Supposedly my scrub radius is way off. Work In conjunction with static toe settings to help bring about a running toe of zero. Most of us have negative scrub radius on our cars because it tends to go hand in hand with MacPherson strut suspension. Effects of positive scrub radius. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. So much for being way off. 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." Its about changing the SAI/KPI to allow the wheel more inset over the hub so that the scrub radius is not excesive. But evidently scrub radius is unlikely to cause excessive wear on the inside 1" of the tire so apparently I should keep looking. Mar 22, 2018 at 3:02 PM #13 #13. splitbolt Well-Known Member. Login Register. A positive radius allows the wheel to roll over on lock making parking situations easier. 1. Suspension | Chassis | Brakes -- Sponsored by 949 Racing Relating to suspension, chassis, and brakes. An increase in positive scrub radius also causes the wheel to gain positive camber when the wheels are being turned, reducing grip in corners on the front wheels and producing understeer. Joined: Mar 7, 2010 Member: #32761 Messages: 5,832 Gender: Male NW Arkansas Vehicle: 2016 TRD OFFROAD DCSB MGM. The diagram shows a positive offset in which the mounting surface is outside of the wheel centreline. 1. Pros: The car feels much more stable at all speeds, but especially at high speed cornering. arqangel67 Well-Known Member. Positive scrub is generally preferred in racing. I wonder if any one would have any thoughts on a problem I have. Cars with strut suspension are more likely to have a negative scrub radius, and cars with double A-arm suspension are more likely to have a positive scrub radius. The scrub radius is the “lever arm” of longitudinal forces applied at the contact patch (when braking). I haven't checked clearance yet, but the extra 0.25inch on both sides should clear the suspension components I'm assuming. Sponsored by 949 Racing. The outward movement of the wheel can make a negative scrub radius become zero. I tried searching if anyone has done 18x8.5, 50mm offset, can't seem to find anyone that has done it on this forum. Unless I’m wrong I think that’s acceptable. Most rear wheel drive vehicles have positive scrub radius. It helps driven front wheels find the straight-ahead, which is good for corner exits or if you have a sudden tyre deflation. 4. This is aided by other mods I’ve done, but my shocks and struts are stock. A negative scrub radius tends to better for braking stability in the event of brake failure or split-mu braking. I think 2" positive scrub radius is fine. I also found that the OEM scrub radius is approx. Check out our team at www.hapnasty.com [SIGPIC][/SIGPIC] Reply. The 1.75"-2" measurement, is additional positive scrub radius. FAQ’s Terminology Micro Cellular Jounce Bumper Scrub Radius Spring Rate 101 TECH NOTES H&R Technical Information Sheet (PDF) FAQ’s 1) Which are the front and rear springs? Scrub radius, to my knowledge, cannot be calculated to an exact number. I've researched a bit about the affects of positive scrub on FWD and the effects don't look too encouraging, however as it's only going to be used on track I'm sitting on the fence on going for it. Assist Is providing stability under adverse road conditions. 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. H&R springs fit the same way that stock springs do. Offset also affects the suspension's motion ratio, which directly determines the effective spring and damper rates. Page 1 of 2 - SCRUB RADIUS. ClioSport Moderator Alien Green 200 Sep 14, 2016 #13 Well I ran 15mm front spacers on my cup at one point, and it was awful to drive. x Scrub Radius x Riding Height x Set Back x Thrust Angle x Steering Center x Toe Out on Turns Camber When camber specifications are determined during the design stage, a number of factors are taken into account. If you had positive scrub radius, (towards the inside of the wheel center) and you had different braking in the two wheels, it would jerk the wheel out of a straight line, making the car unstable. A negative scrub radius is suitable for vehicles with high power engine and a positive scrub radius offset for small power engines. The scrub radius is shown at the bottom at .427”. On some cars 5mm of extra positive scrub can be nasty, maybe others cope or even benefit from extra 20mm, I just wanted to know what people think as being a good setup for the clio 1*2 range . I don't have data to back this up, just my gut feel, but if the diagonal line from the king pin inclination angle falls within the footprint of the tire, unless you have some really wide tires, the scrub radius is acceptable. At the same time, a blowout or a failure of one front brake could yank the wheel hard enough to pull it out of your hands. 8 inch wide wheels & a slightly narrower slick cured this. All that said, it sounds like 0 scrub radius is never bad . Extreme positive scrub radius can make the steering very heavy, too, and it was only really viable on old cars with very thin tyres. A negative scrub radius gives a harder feel to the steering wheel as it requires more input to turn. F40 Model Year: 2019 + Previous Generations; F20 / F21 Model Year: 2012 - 2018; E82 / E83 / E87 / E88 Model Year: 2004 - 2011; 2. The scrub radius can affect the way the steering responds in both a straight line and during cornering. It is positive when the angle is inclined inward toward the center of the vehicle and negative when the angle is inclined outward. The smaller the scrub radius is the less feedback to the wheel you get. NorthloopCup . Guess I could go back to the T/As to see what happened then but that sounds like an expensive experiment (!) Offset affects the steering geometry's scrub radius, possibly leading to problems with torque steer or self-centering characteristics. The distance between where the SLA and the centerline to the wheel intersect the ground is called the scrub radius. Project vehicle load to larger Inner wheel bearing. If scrub radius is positive (contact patch arc outboard of the pivot point), the wheels will try to toe out. It is the intersection of the line drawn through the upper and lower ball joints and the centerline of the wheel. 2) Which end of the spring is up? - posted in The Technical Forum Archive: Hi Chaps, can I ask a question please. F44 Model Year: 2020 + Previous Generations; F22 / F23 Model Year: 2014 - … This ... A smaller offset will create a scrub radius which may affect the handling of the car, but there are a number of reasons why you may need to choose a smaller offset. The manufacturer’s specification will be slightly positive (toe-in) to compensate. REAR = Springs marked: HA, or (R), or Rear. Posts 1,761 Reaction 149. In a brake in bend scenario, this gives an understeer tendency which is desirable from a stability point of view. The tire will generate more forces through the linkages, but especially in straight line acceleration, there will be no change in tire behavior.
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