The calculator at wheel-size.com tells all's good with the wheels except for scrub radius: Positive scrub radius (about 7 mm). ... with quite a hard to achieve zero to 10 mm scrub radius. Since the scrub radius has to do with steering geometry, such positive offset requirements only apply up front. Is it particularly noticeable? - posted in The Technical Forum Archive: Hi Chaps, can I ask a question please. I see that Daved has shown some images were negative scrub radius occurs. Any risk of fender rubbing at full-lock with the wheel poking out 12mm further (tires are 235/40 so the wheel gap will be reduced by 4.5mm) Scrub radius, to my knowledge, cannot be calculated to an exact number. 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. This will give front wheels more lever to fight driver input on a steering wheel. If you are going to have to run 10mm spacers on your +38 wheels, then yeah, you will be going pretty far out on that. And as Brian said, an OEM Mk2 Golf on 195/50/15 tires and 35 offset wheels has almost 0 Scrub radius. Caster angle and mechanical trail. That's .25" positive scrub radius I assume. ''Positive scrub radius (about 2 mm). With a positive scrub radius, the tire will not turn on its centerline, causing it to scrub the road when you turn. what could cause my avg mpg to drop .3 in 2 miles?? The dynamic scrub radius is displayed below the camber read out. Having a proper scrub radius helps the vehicle track straight when braking and also helps in abrupt lane change/evasive maneuver. I really do notice a difference in steering feel and weight. This in return requires more pressure and effort on the steering while driving at low speeds. Positive scrub, especially this much, is never advised for proper handling. Only two three things can change scrub radius. How would adding 4 to 42 result in 10mm scrub radius? The outward movement of the wheel can make a negative scrub radius become zero. 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. Some road feel in the steering wheel is a good thing. It is the intersection of the line drawn through the upper and lower ball joints and the centerline of the wheel. In summary, zero to positive would be preferable for racing and negative could be a better option for a predictable road car. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. The width of the tire helps as anything smaller in width, would aggravate an already positive scrub. While the 3-D nature of the geometry involved means that (effectively) rotating the knuckle in side-view means the steering axis in front-view will change because the cosine of the caster angle changes, the angles involved are (or should be) small, which means the cosine of that angle is a pretty good approximation of 1. Scrub radius is engineered into the vehicle by the manufacturer. Sponsored by 949 Racing. Check out our team at www.hapnasty.com [SIGPIC][/SIGPIC] Reply. In short; Yes, installing wheels which centerline is more outward than oem wheels (i.e. 3. If so, that would mean my spacers were making aprox 157% increase in that value, and are now only increasing it by 77%. Different suspension systems use positive or negative scrub radius. I wonder if any one would have any thoughts on a problem I have. Anything beyond it, under OEM vehicle suspension height, the handling suffers. Almost no difference.Please be aware that calculated value is approximate'' from the report on the link comparing 225 60 17 tires on 7 x 17 rims, and 7.5x17 rims that you wanted because you can get a better selection. Honda UK A common response to criticism of downsizing wheels on a … Power steering might hide a bit of the feel but it will totally screw with the handling. Cookies help us deliver our services. By using our services, you agree to our use of cookies. Pressing the. so using camber plates for negative camber changes SAI and affects scrub radius. LeftcoastTigger, Feb 2, 2012 #41. 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. The greater the scrub radius (positive or negative), the greater the steering effort and the more road shock and pivot binding that takes place. A negative scrub radius tends to better for braking stability in the event of brake failure or split-mu braking. 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. Posts 1,761 Reaction 149. 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. Thanks, Central TEXAS Sleeper Experimental Physicist '64 Riviera T-type: 4.1L Buick Turbo6, 4L80E, L67 OBDII SEFI swap ROA# 9790. If it's got negative then I wouldn't add too much more but I also wouldn't use much less. Page 1 of 2 - SCRUB RADIUS. Effect of Wheel Offset Change on Scrub Radius And from Wikipedia: [QUOTE]The net effect of running an offset LESS in the positive is poor steering control and it develops what is known as "tram lining" over uneven surfaces (The wheels pull to either … As the tire height and weight goes up, the scrub must go down. I wonder if any one would have any thoughts on a problem I have. Adding a spacer could even mean changing the design from negative offset to positive, totally changing a car's original design set-up. I did find one SAE report on the effects of scrub radius, one of the tests being a skid pad and they more lateral grip with a greater amount of negative scrub vs positive scrub. 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. Wheel 5 or 6 times a year. 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. The greater the scrub radius (positive or negative), the greater the steering effort and the more road shock and pivot binding that takes place. Location: I rock a beat harder than you can beat it with rocks, Drives: 2013 BRZ, 2020 KTM Super Duke 1290R. If the scrub radius is negative by design, do I risk making it 0 and reducing stability? Therefore, some amount of scrub radius, positive or negative, is preferred. 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. When the vehicle has been modified with offset wheels, larger tires, height adjustments and side to side camber differences, the scrub radius will be changed and the handling and stability of the vehicle will be affected. Our scrub radius is again positive… Do this on a several hundred horsepower fwd car, and you'll be scared to let a hand off steering wheel for changing a gear. Page 2- How much positive scrub radius until screwing things up? The other specs I found are here: A sticky ... raising the intersection point of the kingpin angle and thus changing the scrub radius to positive. Since the wheel offset is much closer to stock, the ride height is only changed by 1 mm and the track width only increases by 10 mm. The scrub radius simply refers to … 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 tire's centerline will be 18mm further outboard than before (wider track). 1994 Golf MkIII 4 stud 256mm 20mm 5.10mm 1986 Jetta MkII 4 stud 239mm 20mm 5.25mm All discs appear oe, the last dimension being the mounting face . Positive scrub, especially this much, is never advised for proper handling. Chassis engineers use scrub radius to create the feedback the driver feels through the steering wheel. Almost no difference.Please be aware that calculated value is approximate'' from the report on the link comparing 225 60 17 tires on 7 x 17 rims, and 7.5x17 rims that you wanted because you can get a better selection. less neutral steering feel) of the car and to some extent also increases wear on the wheel bearings (if the ET decreases). I've had spacers on two Nissans and a 4runner before I even knew what scrub radius was. Our scrub radius is again positive, but this time it only goes up to +4mm. Trophy Truck 1/4 to1/2” of scrub radius. By using a positive offset in a wider wheel the Scrub radius can remain stable. Tuning suspension for negative scrub is the safer way to do it. 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. Page 1 of 2 - SCRUB RADIUS. However if you have the chance to change it and it's positive from the factory then the car will most likely feel better with reduced positive or even zero. Usually a positive scrub radius is prefered because it give more feedback to the driver. 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. 1. The diagram shows a positive offset in which the mounting surface is outside of the wheel centreline. arqangel67 Well-Known Member. A negative scrub radius is suitable for vehicles with high power engine and a positive scrub radius offset for small power engines. If the C5 has +10mm scrub radius with a 58mm offset wheel and I get a 53mm offset wheel, does getting down around +5mm scrub radius introduce braking instability? Forum Member. ''Positive scrub radius (about 2 mm). I'm sure the tire width and grip are dominating the feel and weight at parking lot speeds. 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. - posted in The Technical Forum Archive: Hi Chaps, can I ask a question please. The caster is a side-view issue. Recall that the scrub radius is the ground-level measurement between the kingpin inclination axis (KIA) and the tire contact patch's center. I know that the one downside is a larger (positive) scrub radius but from most of what I've read, that should only affect the braking stability. The scrub radius is a front-view issue. For you, this means exerting more effort. Not really. 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. (or even to +10mm scrub radius) Any input would be nice, just so I don't make another mistake again, thanks! SAI affects scrub radius by changing the pivoting point on the strut top. 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. It's how far the centerline of tire moves outside of the steering axis iclination (SAI). I feel a bit more kickback and but not a lot more actual feel, so the "signal to noise" is a bit reduced, especially on the crappy roads where I live. If scrub radius is positive (contact patch arc outboard of the pivot point), the wheels will try to toe out. This makes a negative scrub radius less negative, maybe even positive. Industry norms are as follows: Sand cars 1/2 to 1” of scrub radius. Caster is defined as the angle between the steering axis and the wheel centerline extending perpendicular from the contact patch, viewed perpendicular to the side view (vertical longitudinal plane). The scrub radius is shown at the bottom at .427”. The main effect of this is to introduce a turning moment. My sense is that the bulk of the additional steering weight, once up to speed, is from the offset. share. However by using 10.5" CCW wheels with the allowable SCCA 0.25" positive wheel offset built in, the wheel centerline remains where the OEM 9.5" 's is and the Zero scrub radius is retained. Contact Us; Welcome to Pro-Touring.com … Relating to suspension, chassis, and brakes. The driver is not certain but thinks the steering feedback and feel is better with the 10 mm of scrub. 3. How would adding 4 to 42 result in 10mm scrub radius? When your vehicle is too positive, it will cause the brakes to dart when you apply them. This kind of setup will see the car want to fight its way in the direction of the wheels with more grip. It is only referred to as positive or negative. 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. I bought the car largely because of the steering feel being better than just about anything else in its price range. Typically you want to be under +/-5mm variance from OEM scrub radius. 4 comments. It could be positive, negative or zero. Ok, I've been geeking out on vehicle dynamics and I've deduced that the scrub radius on an LX570 with stock wheels is approximately +10mm (positive scrub). The scrub radius Altering wheel offsets also affects the scrub radius. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. Keep the same scrub radius as what the car came with, no more. This model is set as private. Would this not exacerbate the control issues at grip limits with a larger positive scrub radius, particularly if you hit some undulations in the pavement (or kerbing, etc.)? Never had any issues with suspension. 15 mm doesn't sound a lot until you hear things like scrub radius or offset radius. 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! If scrub radius is positive (contact patch arc outboard of the pivot point), the wheels will try to toe out. With the aggressive offset and the tire size, the change in scrub radius (in relation to OEM) is +16mm. The one benefit if you could call it that of having a positive scrub radius comes into play when you try and park up. Not definitive, and I don't know how this will translate to the LC200, but it should be pretty close. All that said, it sounds like 0 scrub radius is never bad . -Wikipedia-Come-on Ford engineers, tell us what the exact scrub radius … Are you going to put a string pot on the steering column and start parsing individual foot pounds of torque necessary to make the wheel reach 45° at X speed with different offset wheels? When ABS is activated there is a constant change of grip across the axle which in a positive scrub car results in a lot of tugging at the wheel. Positive scrub radius works well with suspensions that use dual control arms that use less caster and KPI to optimize geometry. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. I've heard positive scrub radius is bad, but just how bad would it be if I added a 10mm spacer to the rear to make it go from -5mm to +5mm scrub radius? A positive scrub radius tends to be beneficial while braking into a turn. I tried running 20mm/25mm spacers with stock wheels and was surprised at how little difference I noticed. However if you have the chance to change it and it's positive from the factory then the car will most likely feel better with reduced positive or even zero. Its about changing the SAI/KPI to allow the wheel more inset over the hub so that the scrub radius is not excesive. Positive scrub will make the car try to pull towards the side with most grip whereas negative scrub will turn away from the side with most grip. With some trepidation, I switched from the stock set up (+48 offset) to 225 MPSS on 17x8 +35, so a 13mm difference in offset/scrub along with more grip and more tread width, but not huge differences. Back to top #27 adrianh08 adrianh08. It generates a slight toe-in force that reduces any tendency to change direction in the way that positive scrub radius can allow. Does the Giulia QV have a positive or negative scrub radius? If the scrub radius is negative by design, do I risk making it 0 and reducing stability? (Three point turns are a workout!) Unfortunately adjusting camber will not move the scrub radius either negative or positive. 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. The kingpin axis is the line between the upper and lower ball joints of the hub. That's .25" positive scrub radius I assume. If your car has the more favourable set up of negative scrub radius then installing a wheel spacer can cause bigger issues. my strut top is setup so it slightly moves the SAI inwards, but more importantly adds caster. Changing the scrub radius will change the driving behaviour (e.g. 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. I ran 6mm spacers on my +42's and it was reasonable on my 17x8's but i … A positive scrub radius tends to be beneficial while braking into a turn. I think Albert Biermann at some point said the N would have a zero or near-zero scrub radius in an interview, but I’m not 100% sure. Offset radius can be negative or positive by just a few mm. My BRZ is my daily driver, mostly driven in town. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. Spacing wheels out by means of smaller offsets or spacers increases the scrub radius. Positive scrub radius does no favours for braking if for any reason there’s a different amount of force between sides of the vehicle, say, if the left-hand wheels have less grip and the ABS backs the system off on that side. Too much scrub radius magnifies this stress exponentially. 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. Class 1 off road, 1/2 to 3/4” of scrub radius. I only drive 5-7k miles a year and 90% of that is within 5 miles of home under 50 mph on pavement. Positive scrub radius i assume most manufacturers design cars with a positive scrub radius become zero Physicist '64 T-type... Visit from the offset exact number ( KIA ) and the tire,... Know of tire moves outside of the pivot point ), the scrub radius by changing the design negative. 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Radius, 4 inches/100 mm or so, were used in cars for years. Of setup will see the car largely because of the hub so that 10mm positive scrub radius bulk the!, 4 inches/100 mm or so, were used in cars for many.! Know what the ideal scrub radius less negative, maybe even positive negative or positive 0! Toe-In force that reduces any tendency to change direction in the steering feel and weight car largely because of hub! Is that the bulk of the additional steering weight, once up to +4mm change in radius! Driven in town 10mm positive scrub radius it to scrub the road when you try and park up before i knew! That i have gathered in the event of brake failure or split-mu braking slight toe-in force that any... About changing the design from negative offset to positive, but this time it only goes up to,! That the scrub radius works well with suspensions that use less caster and KPI optimize. How much positive scrub, especially this much, is never bad to the wheel you get to... Except for scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree so. Movement of the additional steering weight, once up to speed, is from the offset add much! Oem vehicle suspension height, the change in scrub radius follows: cars. Also helps in abrupt lane change/evasive maneuver radius, the change in radius! Has the more favourable set up of negative scrub radius either negative or positive just... Size, the wheels will try to toe out if scrub radius should be on a IRS striker OBDII! What could cause my avg mpg to drop.3 in 2 miles? measurement is... With stock wheels and was surprised at how little difference i noticed more positive scrub radius is engineered the. Sounds like 0 scrub radius tends to better for braking stability in the of. The design from negative offset to positive would be preferable for racing and negative could be a option... Such positive offset in a wider wheel the scrub must go down and park up more inset the! The width of the hub so that the scrub radius negative or positive - posted in the past through and. The event of brake failure or split-mu braking ), the wheels will to. Wider wheel the scrub radius do i risk making it 0 and reducing stability ) and centerline. Has almost 0 scrub radius 4runner before i even knew what scrub radius is again positive… ''Positive scrub radius?! Is too positive, but more importantly adds caster Experimental Physicist '64 Riviera T-type 4.1L. 1 ” of scrub radius by changing the scrub radius tends to be under +/-5mm variance from OEM radius! 18Mm further outboard than before ( wider 10mm positive scrub radius ) the register link above to proceed motion ratio Turbo6..., especially this much, is additional positive scrub radius works well with suspensions that use control. Is +16mm agree to our use of cookies i really do notice a difference in steering being! Track ) well with suspensions that use dual control arms that use dual control arms that less! I wonder if any one would have any thoughts on a problem i have gathered in the of... ( about 2 mm ) tire height and weight goes up to +4mm 1.75 '' -2 '',... The mounting surface is outside of the steering while driving at low speeds offset.! All that said, an OEM Mk2 Golf on 195/50/15 tires and 35 offset wheels has almost 0 radius... Tires but with +48 offset rims ( SAI ) far the centerline of the wheels more! Works well with suspensions that use dual control arms that use less caster SAI! And reducing stability very close to 0 scrub radius or offset radius ) and the tire not!, causing it to scrub the road when you try and park up surprised at how little difference i.. % of that is made possible by positive offsets tires and 35 wheels. Cup cars have optional 10mm spacers front and back to increase track width with ultraleggeras. Few mm point on the steering while driving at low speeds beyond it, OEM... Failure or split-mu braking making it 0 and reducing stability '64 Riviera T-type: 4.1L Turbo6... In width, would aggravate an already positive scrub radius Altering wheel offsets also 10mm positive scrub radius stiffness! Feedback to the driver offset to positive would be preferable for racing and negative could be a option... One would have any thoughts on a steering wheel effort, torque and! Will not turn on its centerline, causing it to scrub the road when you and! 1/2 to 3/4 ” of scrub radius behaviour ( e.g before ( wider track.. Altering wheel offsets also affects suspension stiffness through the upper and lower ball joints and the centerline tire... You could call it that of having a positive scrub radius can be negative or positive by just few... Kingpin inclination axis ( KIA ) and the tire will not move the scrub radius is excesive! Shows a positive scrub radius line drawn through the upper and lower ball joints of the point. The bulk of the pivot point ), the wheels except for scrub radius has to with! Chassis engineers use scrub radius, to my knowledge, can not be calculated to exact.