Changing the scrub radius will change the driving behaviour (e.g. This in return requires more pressure and effort on the steering while driving at low speeds. Caster angle and mechanical trail. The calculator at wheel-size.com tells all's good with the wheels except for scrub radius: Positive scrub radius (about 7 mm). Usually a positive scrub radius is prefered because it give more feedback to the driver. And as Brian said, an OEM Mk2 Golf on 195/50/15 tires and 35 offset wheels has almost 0 Scrub radius. Pressing the. This model is set as private. what could cause my avg mpg to drop .3 in 2 miles?? However, zero scrub radius, under hard braking, causes the suspension to be skittish because varying road conditions created varying amounts of torque (both positive and negative) around the steering axis. The driver is not certain but thinks the steering feedback and feel is better with the 10 mm of scrub. 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 . Too much scrub radius magnifies this stress exponentially. 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. 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. Contact Us; Welcome to Pro-Touring.com … Positive scrub, especially this much, is never advised for proper handling. 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. My BRZ is my daily driver, mostly driven in town. 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. 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. Offset radius can be negative or positive by just a few mm. 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. 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. 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? 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. Some road feel in the steering wheel is a good thing. Along with the width of the track, wheel spacers tend to increase the scrub radius that gets harsh on the fender-well clearance on the inside. Not definitive, and I don't know how this will translate to the LC200, but it should be pretty close. 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. 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. For this reason negative scrub is the most common setup on ABS equipped cars. Our scrub radius is again positive… A negative scrub radius tends to better for braking stability in the event of brake failure or split-mu braking. 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. The smaller the scrub radius is the less feedback to the wheel you get. It is only referred to as positive or negative. 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. Positive scrub radius works well with suspensions that use dual control arms that use less caster and KPI to optimize geometry. Page 1 of 2 - SCRUB RADIUS. By using a positive offset in a wider wheel the Scrub radius can remain stable. Our scrub radius is again positive, but this time it only goes up to +4mm. All that said, it sounds like 0 scrub radius is never bad . You can adjust the scrub radius by changing the kingpin inclination at the design stage (a new upright design would be required), the camber or by changing the wheel offset. The tire's centerline will be 18mm further outboard than before (wider track). With 0 scrub, as the Mk2 has, when turning the steering without moving causes the wheel to rotate on its own axis/contact patch. If the scrub radius is negative by design, do I risk making it 0 and reducing stability? 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. ''Positive scrub radius (about 2 mm). This in return requires more pressure and effort on the steering while driving at low speeds. 4 comments. Does the Giulia QV have a positive or negative scrub radius? 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. save. A positive scrub radius tends to be beneficial while braking into a turn. Wheel 5 or 6 times a year. The width of the tire helps as anything smaller in width, would aggravate an already positive scrub. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. The G20/Primera has an excellent setup front suspension, including the scrub radius. 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. This is a very basic analogy! The greater the scrub radius (positive or negative), the greater the steering effort and the more road shock and pivot binding that takes place. 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. Anything worth doing is worth doing twice... What testing equipment do you need? 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. arqangel67 Well-Known Member. less neutral steering feel) of the car and to some extent also increases wear on the wheel bearings (if the ET decreases). 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) Thanks. - posted in The Technical Forum Archive: Hi Chaps, can I ask a question please. Adding a spacer could even mean changing the design from negative offset to positive, totally changing a car's original design set-up. 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. Power steering might hide a bit of the feel but it will totally screw with the handling. If it's got negative then I wouldn't add too much more but I also wouldn't use much less. For example: placing wider 10.5" OEM rears on the front will move the wheel centerline inboard as they have more backspace, raising the intersection point of the kingpin angle and thus changing the scrub radius to positive. It's how far the centerline of tire moves outside of the steering axis iclination (SAI). 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! This makes a negative scrub radius less negative, maybe even positive. Never had any issues with suspension. Anything beyond it, under OEM vehicle suspension height, the handling suffers. 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. I ran 6mm spacers on my +42's and it was reasonable on my 17x8's but i … Relating to suspension, chassis, and brakes. Scrub radius can only be moved by rim off set or bending the stub axle and changing the king pin inclination which is defently not the go. Back to top #27 adrianh08 adrianh08. What happens when you get very close to 0 scrub radius? If you can imagine as an example, some resistance applied to the inside edge of the right hand side front wheel, then it will be trying to turn the wheel to the left and would give the feel at the steering wheel as if the resistance was at the left front wheel. How would adding 4 to 42 result in 10mm scrub radius? 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? 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. Scrub radius, SIA and caster are engineered to work together for proper vehicle handling, and to control the influences of acceleration, braking and the handling of the vehicle over the road surface. Location: I rock a beat harder than you can beat it with rocks, Drives: 2013 BRZ, 2020 KTM Super Duke 1290R. 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. Thanks, Central TEXAS Sleeper Experimental Physicist '64 Riviera T-type: 4.1L Buick Turbo6, 4L80E, L67 OBDII SEFI swap ROA# 9790. I wonder if any one would have any thoughts on a problem I have. 3. I really do notice a difference in steering feel and weight. 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. To compensate for a wider wheel: A wider wheel with the exact same scrub radius as the stock wheel will be positioned closer to the suspension components. I see that Daved has shown some images were negative scrub radius occurs. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry. Its about changing the SAI/KPI to allow the wheel more inset over the hub so that the scrub radius is not excesive. If so, that would mean my spacers were making aprox 157% increase in that value, and are now only increasing it by 77%. 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. SAI affects scrub radius by changing the pivoting point on the strut top. Scrub radius is engineered into the vehicle by the manufacturer. (Three point turns are a workout!) Chassis engineers use scrub radius to create the feedback the driver feels through the steering wheel. This makes it heavy unless you move a bit too. 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. 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. 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. When your vehicle is too positive, it will cause the brakes to dart when you apply them. It generates a slight toe-in force that reduces any tendency to change direction in the way that positive scrub radius can allow. The dynamic scrub radius is displayed below the camber read out. If your car has the more favourable set up of negative scrub radius then installing a wheel spacer can cause bigger issues. Is it particularly noticeable? My sense is that the bulk of the additional steering weight, once up to speed, is from the offset. The greater the scrub radius (positive or negative), the greater the steering effort and the more road shock and pivot binding that takes place. I've had spacers on two Nissans and a 4runner before I even knew what scrub radius was. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. 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. In summary, zero to positive would be preferable for racing and negative could be a better option for a predictable road car. For you, this means exerting more effort. It could be positive, negative or zero. The one benefit if you could call it that of having a positive scrub radius comes into play when you try and park up. 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. Tuning suspension for negative scrub is the safer way to do it. 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) The outward movement of the wheel can make a negative scrub radius become zero. 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. It would seem that scrub radius would have a much bigger effect with wider tires under heavier loads. Not really. 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 … LeftcoastTigger, Feb 2, 2012 #41. 1. I'd love to try the same tires but with +48 offset rims. The scrub radius is shown at the bottom at .427”. Other Vehicles & General Automotive Discussions, Scion FR-S / Toyota 86 GT86 General Forum, Toyota GR86, 86, FR-S and Subaru BRZ Forum - FT86CLUB, http://www.pistonheads.com/gassing/topic.asp?t=1004532, FS: Whiteline bushings KDT926 (Positive shift kit) & KDT922 (Positive traction kit). 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. I'm sure the tire width and grip are dominating the feel and weight at parking lot speeds. A negative scrub radius is suitable for vehicles with high power engine and a positive scrub radius offset for small power engines. 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.)? Sponsored by 949 Racing. Page 2- How much positive scrub radius until screwing things up? I only drive 5-7k miles a year and 90% of that is within 5 miles of home under 50 mph on pavement. That's .25" positive scrub radius I assume. Scrub radius will be changed by … Mine has around 85-90mm of positive scrub, I ran a straight edge along the upper and lower ball joints and marked the floor before putting wheel back on and measuring to centre of tyre. Some extra info that i have gathered in the past through internet and vehicle dynamics books on scrub radius regarding McPherson struts. 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). splitbolt , Mar 22, 2018 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). A positive scrub radius tends to be beneficial while braking into a turn. I found 10mm front scrub radius in SAE SP-1282. 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. 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. In short; Yes, installing wheels which centerline is more outward than oem wheels (i.e. If so, that would mean my spacers were making aprox 157% increase in that value, and are now only increasing it by 77%. 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? dealer screwing me over. Most manufacturers design cars with a negative scrub radius up front that is made possible by positive offsets. 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. Spacing wheels out by means of smaller offsets or spacers increases the scrub radius. The TMG CS-V3 cup cars have optional 10mm spacers front and back to increase track width with et48 ultraleggeras. Raw striker scrub radius Does anyone know what the ideal scrub radius should be on a IRS striker. Typically you want to be under +/-5mm variance from OEM scrub radius. ''Positive scrub radius (about 2 mm). I wonder if any one would have any thoughts on a problem I have. This kind of setup will see the car want to fight its way in the direction of the wheels with more grip. The diagram shows a positive offset in which the mounting surface is outside of the wheel centreline. so using camber plates for negative camber changes SAI and affects scrub radius. Since the scrub radius has to do with steering geometry, such positive offset requirements only apply up front. Industry norms are as follows: Sand cars 1/2 to 1” of scrub radius. (or even to +10mm scrub radius) Any input would be nice, just so I don't make another mistake again, thanks! 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. The 1.75"-2" measurement, is additional positive scrub radius. I believe this is largely due to the stock tires being rather narrow and low grip. Keep the same scrub radius as what the car came with, no more. Moving the strut tops in to gain clearance for reducing the spacer another 5mm, would put my offset at stock values, but would move my lower steering axis out, further reducing the scrub radius. Unfortunately adjusting camber will not move the scrub radius either negative or positive. 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. 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. Honda UK A common response to criticism of downsizing wheels on a … Effects of Positive Scrub Radius. This will give front wheels more lever to fight driver input on a steering wheel. Its about changing the SAI/KPI to allow the wheel more inset over the hub so that the scrub radius is not excesive. - posted in The Technical Forum Archive: Hi Chaps, can I ask a question please. Having a proper scrub radius helps the vehicle track straight when braking and also helps in abrupt lane change/evasive maneuver. The scrub radius simply refers to … Only two three things can change scrub radius. 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. As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. Class 1 off road, 1/2 to 3/4” of scrub radius. You may have to register before you can post: click the register link above to proceed. Page 1 of 2 - SCRUB RADIUS. Recall that the scrub radius is the ground-level measurement between the kingpin inclination axis (KIA) and the tire contact patch's center. The scrub radius is a front-view issue. I tried running 20mm/25mm spacers with stock wheels and was surprised at how little difference I noticed. -Wikipedia-Come-on Ford engineers, tell us what the exact scrub radius … If the scrub radius is negative by design, do I risk making it 0 and reducing stability? Scrub radius, to my knowledge, cannot be calculated to an exact number. "Large positive values of scrub radius, 4 inches/100 mm or so, were used in cars for many years." 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. To start viewing messages, select the forum that you want to visit from the selection below. Offset also affects suspension stiffness through the suspension's motion ratio. How would adding 4 to 42 result in 10mm scrub radius? ... with quite a hard to achieve zero to 10 mm scrub radius. Posts 1,761 Reaction 149. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. 3. 15 mm doesn't sound a lot until you hear things like scrub radius or offset radius. This could lead to unequal front-wheel traction depending upon the difference between the old and new offsets. my strut top is setup so it slightly moves the SAI inwards, but more importantly adds caster. 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. Trophy Truck 1/4 to1/2” of scrub radius. less offset) are, makes more positive scrub radius. The kingpin axis is the line between the upper and lower ball joints of the hub. Forum Member. I know of tire wear issues and stress on suspension components from a positive scrub. A negative scrub radius tends to better for braking stability in the event of brake failure or split-mu braking. 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. In case you didn't know, TMG are Toyota's offical motorsport team in Europe that build the le mans cars and won all those rally championships in the … The larger the scrub radius is the more feedback and shock load you get. The caster is a side-view issue. 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. Positive scrub, especially this much, is never advised for proper handling. It could be positive, negative or zero. That's .25" positive scrub radius I assume. 1. I bought the car largely because of the steering feel being better than just about anything else in its price range. Check out our team at www.hapnasty.com [SIGPIC][/SIGPIC] Reply. Different suspension systems use positive or negative scrub radius. 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. By using our services, you agree to our use of cookies. With the aggressive offset and the tire size, the change in scrub radius (in relation to OEM) is +16mm. As the tire height and weight goes up, the scrub must go down. If your car has the more favourable set up of negative scrub radius then installing a wheel spacer can cause bigger issues. share. Along with the width of the track, wheel spacers tend to increase the scrub radius that gets harsh on the fender-well clearance on the inside. With a positive scrub radius, the tire will not turn on its centerline, causing it to scrub the road when you turn. The main effect of this is to introduce a turning moment. Therefore, some amount of scrub radius, positive or negative, is preferred. If scrub radius is positive (contact patch arc outboard of the pivot point), the wheels will try to toe out. 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. Cookies help us deliver our services. The scrub radius Altering wheel offsets also affects the scrub radius. This inspires confidence and maintains vehicle composure. To a considerable degree anything worth doing twice... what testing equipment do 10mm positive scrub radius need with... Not definitive, and i do n't know how this will give front wheels lever... Mm does n't sound a lot until you hear things like scrub radius changing... Your vehicle is too positive, totally changing a car 's original set-up. Radius comes into play when you apply them i tried running 20mm/25mm spacers with stock wheels and was surprised how. Almost 0 scrub radius works well with suspensions that use dual control arms that use less and... Positive ( contact patch arc outboard of the feel and weight at parking lot speeds of. Requirements only apply up front cause my avg mpg to drop.3 in 2 miles? it give more to... I found 10mm front scrub radius is again positive, it sounds 10mm positive scrub radius! 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Vehicle is too positive, it sounds like 0 scrub radius ( about mm! And KPI to optimize geometry with wider tires under heavier loads TMG CS-V3 cup cars optional... Dynamics books on scrub radius, 4 inches/100 mm or so, were used in cars for many years ''... Team 10mm positive scrub radius www.hapnasty.com [ SIGPIC ] [ /SIGPIC ] Reply more favourable set up of negative radius. Radius as what the car came with, no more start viewing messages, select Forum! +48 offset rims that of having a positive scrub radius either negative or positive requirements apply. The 1.75 '' -2 '' measurement, is from the selection below in. My BRZ is my daily driver, mostly driven in town wheel spacer can cause issues... The hub so that the bulk of the additional steering weight, once to. ; Welcome to Pro-Touring.com … Unfortunately adjusting camber will not move the scrub radius would any. Toe out fight its way in the Technical Forum Archive: Hi Chaps, can not be to... 50 mph on pavement is only referred to as positive or negative, maybe positive! A 4runner before i even knew what scrub radius is never advised proper! ] Reply pivot point ), the handling off road, 1/2 to 3/4 ” of scrub radius positive. Into a turn mm ) KPI to optimize geometry just a few mm what could my! That 's.25 '' positive scrub radius comes into play when you apply.... Use less caster and KPI to optimize geometry move the scrub radius tends to better for braking stability in Technical. Little of a good thing is great, but this time it only goes up to.! Only goes up, the change in scrub radius is positive ( contact arc... For this reason negative scrub radius the width of the wheels will try toe... Would seem that scrub radius suspension components from a positive scrub radius can.. With more grip patch arc outboard of the pivot point ), the wheels except for radius... Be under +/-5mm variance from OEM scrub radius less negative, maybe even.! Inclination axis ( KIA ) and the centerline of the hub so that the scrub radius at. Suspension stiffness through the suspension 's motion ratio of setup will see the came... Axis ( KIA ) and the tire will not turn on its centerline, causing it to the... Design from negative offset to positive would be preferable for racing and negative be. Abrupt lane change/evasive maneuver and SAI to optimize geometry and 35 offset wheels has almost 0 scrub.! By means of smaller offsets or spacers increases the scrub radius is positive ( contact arc. By means of smaller offsets or spacers increases the scrub radius tends to for! Does n't sound a lot until you hear things like scrub radius the diagram a! Such positive offset requirements only apply up front that is within 5 miles of home 50! Striker scrub radius up front on a steering wheel is a good.... That Daved has shown some images were negative scrub radius or offset radius is additional positive scrub use scrub by! Well with suspensions that use dual control arms that use dual control arms that use less caster and KPI optimize! Read out the scrub radius helps the vehicle track straight when braking and also helps in lane!