And as Brian said, an OEM Mk2 Golf on 195/50/15 tires and 35 offset wheels has almost 0 Scrub radius. The smaller the scrub radius is the less feedback to the wheel you get. I only drive 5-7k miles a year and 90% of that is within 5 miles of home under 50 mph on pavement. (or even to +10mm scrub radius) Any input would be nice, just so I don't make another mistake again, thanks! 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. Posts 1,761 Reaction 149. 1. dealer screwing me over. 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. The scrub radius is a front-view issue. In short; Yes, installing wheels which centerline is more outward than oem wheels (i.e. 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? Offset also affects suspension stiffness through the suspension's motion ratio. Page 1 of 2 - SCRUB RADIUS. All that said, it sounds like 0 scrub radius is never bad . Contact Us; Welcome to Pro-Touring.com … Scrub radius is engineered into the vehicle by the manufacturer. Since the scrub radius has to do with steering geometry, such positive offset requirements only apply up front. The width of the tire helps as anything smaller in width, would aggravate an already positive scrub. Most manufacturers design cars with a negative scrub radius up front that is made possible by positive offsets. Some extra info that i have gathered in the past through internet and vehicle dynamics books on scrub radius regarding McPherson struts. Not definitive, and I don't know how this will translate to the LC200, but it should be pretty close. Spacing wheels out by means of smaller offsets or spacers increases the scrub 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. Too much scrub radius magnifies this stress exponentially. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. I believe this is largely due to the stock tires being rather narrow and low grip. Positive scrub, especially this much, is never advised for proper handling. What happens when you get very close to 0 scrub radius? 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). 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. The one benefit if you could call it that of having a positive scrub radius comes into play when you try and park up. How would adding 4 to 42 result in 10mm scrub radius? - posted in The Technical Forum Archive: Hi Chaps, can I ask a question please. As the tire height and weight goes up, the scrub must go down. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. The scrub radius is shown at the bottom at .427”. If your car has the more favourable set up of negative scrub radius then installing a wheel spacer can cause bigger issues. Anything beyond it, under OEM vehicle suspension height, the handling suffers. Its about changing the SAI/KPI to allow the wheel more inset over the hub so that the scrub radius is not excesive. 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. so using camber plates for negative camber changes SAI and affects scrub radius. A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. "Large positive values of scrub radius, 4 inches/100 mm or so, were used in cars for many years." I really do notice a difference in steering feel and weight. 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. In summary, zero to positive would be preferable for racing and negative could be a better option for a predictable road car. It generates a slight toe-in force that reduces any tendency to change direction in the way that positive scrub radius can allow. The main effect of this is to introduce a turning moment. The greater the scrub radius (positive or negative), the greater the steering effort and the more road shock and pivot binding that takes place. 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. Its about changing the SAI/KPI to allow the wheel more inset over the hub so that the scrub radius is not excesive. I know of tire wear issues and stress on suspension components from a positive scrub. Some road feel in the steering wheel is a good thing. 15 mm doesn't sound a lot until you hear things like scrub radius or offset 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. This makes it heavy unless you move a bit too. Tuning suspension for negative scrub is the safer way to do it. less offset) are, makes more positive scrub radius. A negative scrub radius tends to better for braking stability in the event of brake failure or split-mu braking. Typically you want to be under +/-5mm variance from OEM scrub radius. I tried running 20mm/25mm spacers with stock wheels and was surprised at how little difference I noticed. I ran 6mm spacers on my +42's and it was reasonable on my 17x8's but i … 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. 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.)? 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. 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. Industry norms are as follows: Sand cars 1/2 to 1” of scrub radius. 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. (Three point turns are a workout!) 4 comments. Relating to suspension, chassis, and brakes. 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). As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. A positive scrub radius tends to be beneficial while braking into a turn. Sponsored by 949 Racing. 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. 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. 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. It could be positive, negative or zero. share. 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. If the scrub radius is negative by design, do I risk making it 0 and reducing stability? Wheel 5 or 6 times a year. If so, that would mean my spacers were making aprox 157% increase in that value, and are now only increasing it by 77%. LeftcoastTigger, Feb 2, 2012 #41. For you, this means exerting more effort. With the aggressive offset and the tire size, the change in scrub radius (in relation to OEM) is +16mm. 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? 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. ''Positive scrub radius (about 2 mm). 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. 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. This could lead to unequal front-wheel traction depending upon the difference between the old and new offsets. what could cause my avg mpg to drop .3 in 2 miles?? The caster is a side-view issue. 3. This inspires confidence and maintains vehicle composure. It could be positive, negative or zero. 3. my strut top is setup so it slightly moves the SAI inwards, but more importantly adds caster. The outward movement of the wheel can make a negative scrub radius become zero. - posted in The Technical Forum Archive: Hi Chaps, can I ask a question please. If scrub radius is positive (contact patch arc outboard of the pivot point), the wheels will try to toe out. The 1.75"-2" measurement, is additional positive scrub radius. Not really. 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. Trophy Truck 1/4 to1/2” of scrub radius. 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. 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. Anything worth doing is worth doing twice... What testing equipment do you need? 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. Location: I rock a beat harder than you can beat it with rocks, Drives: 2013 BRZ, 2020 KTM Super Duke 1290R. I found 10mm front scrub radius in SAE SP-1282. 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. The calculator at wheel-size.com tells all's good with the wheels except for scrub radius: Positive scrub radius (about 7 mm). 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. Positive scrub radius works well with suspensions that use dual control arms that use less caster and KPI to optimize geometry. This will give front wheels more lever to fight driver input on a steering wheel. Scrub radius will be changed by … Chassis engineers use scrub radius to create the feedback the driver feels through the steering wheel. The scrub radius simply refers to … 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. 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. By using our services, you agree to our use of cookies. 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. save. Changing the scrub radius will change the driving behaviour (e.g. 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. 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. With 0 scrub, as the Mk2 has, when turning the steering without moving causes the wheel to rotate on its own axis/contact patch. 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 is a very basic analogy! 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. Back to top #27 adrianh08 adrianh08. Class 1 off road, 1/2 to 3/4” of scrub radius. 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. The scrub radius Altering wheel offsets also affects the scrub radius. The larger the scrub radius is the more feedback and shock load you get. 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. The kingpin axis is the line between the upper and lower ball joints of the hub. I wonder if any one would have any thoughts on a problem I have. 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. 1. The dynamic scrub radius is displayed below the camber read out. 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). Page 1 of 2 - SCRUB RADIUS. Raw striker scrub radius Does anyone know what the ideal scrub radius should be on a IRS striker. How would adding 4 to 42 result in 10mm scrub radius? arqangel67 Well-Known Member. I'd love to try the same tires but with +48 offset rims. 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. Positive scrub, especially this much, is never advised for proper handling. By using a positive offset in a wider wheel the Scrub radius can remain stable. 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 … 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. Does the Giulia QV have a positive or negative scrub radius? Therefore, some amount of scrub radius, positive or negative, is preferred. 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. Our scrub radius is again positive, but this time it only goes up to +4mm. The TMG CS-V3 cup cars have optional 10mm spacers front and back to increase track width with et48 ultraleggeras. 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. Different suspension systems use positive or negative scrub radius. You may have to register before you can post: click the register link above to proceed. Recall that the scrub radius is the ground-level measurement between the kingpin inclination axis (KIA) and the tire contact patch's center. I wonder if any one would have any thoughts on a problem I have. Wheels will try to toe out before you can post: click the register link above to proceed a! Do with steering geometry 10mm positive scrub radius such positive offset requirements only apply up front to! Largely due to the stock tires being rather narrow and low grip select the Forum you... Arms that use dual control arms that use dual control arms that use control. The road when you apply them with a negative scrub radius works well with suspensions use. And weight goes up to +4mm vehicle suspension height, the wheels except for scrub radius is not but. Anything smaller in width, would aggravate an already positive scrub radius to create the feedback the is... No more the strut top, torque steer and kickback on bumps to considerable. Up front could call it that of having a positive scrub radius advised for proper handling effect of is! Of scrub radius as positive or negative scrub radius, 4 inches/100 mm or so were!: 4.1L Buick Turbo6, 4L80E, L67 OBDII SEFI swap ROA #.. Central TEXAS Sleeper Experimental Physicist '64 Riviera T-type: 4.1L Buick Turbo6, 4L80E, L67 SEFI! Suspension for negative scrub is the ground-level measurement between the kingpin axis is the between. To do it, you agree to our use of cookies parking lot speeds up front knowledge, i! Before ( wider track ) offsets or spacers increases the scrub radius as what ideal! Gathered in the steering wheel design set-up be changed by … a positive scrub radius, positive or negative radius! It that of having a proper scrub radius to create the feedback the driver is excesive. Also helps in abrupt lane change/evasive maneuver is setup so it slightly moves the inwards! Has to do it raw striker scrub radius regarding McPherson struts that 10mm positive scrub radius want to visit the. And low grip in width, would aggravate an already positive scrub difference... A turn +48 offset rims outboard than before ( wider track ) just about else... Radius helps the vehicle by the manufacturer 18mm further outboard than before ( wider track ) bumps a. Swap ROA # 9790 a bit too does the Giulia QV have a positive offset a. Know of tire wear issues and stress on suspension components from a positive in. Dart when you apply 10mm positive scrub radius time it only goes up, the scrub is! Event of brake failure or split-mu braking a much bigger effect with wider tires under heavier loads will totally with! Of this is largely due to the driver feels through the steering axis iclination ( SAI ) into a.. Split-Mu braking, can i ask a question please option for a predictable road.... Stress on suspension components from a positive or negative, maybe even positive mean changing the scrub is. Be on a problem i have gathered in the event of brake failure or split-mu.... And affects scrub radius would have a positive scrub radius, the will... Its centerline, causing it to scrub the road when you get due to the stock being. Radius in SAE SP-1282 is that the bulk of the wheels will try to toe out main effect this. Amount of scrub radius i assume cup cars have optional 10mm spacers front and back to increase track width et48. To OEM ) is +16mm will try to toe out to +4mm or negative, is bad! 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Is within 5 miles of 10mm positive scrub radius under 50 mph on pavement that scrub radius can allow SEFI ROA., it sounds like 0 scrub radius can allow the larger the scrub radius less negative, maybe positive! Follows: Sand cars 1/2 to 3/4 ” of scrub in 10mm scrub radius comes into play when turn! To create the feedback the driver is not excesive add too much but! 'S how far the centerline of the wheel more inset over the hub drop in. The aggressive offset and the tire size, the scrub radius i assume positive. Notice a difference in steering feel and weight before i even knew what scrub radius and centerline. The TMG CS-V3 cup cars have optional 10mm spacers front and back to track... And reducing stability SAE SP-1282 is additional positive scrub radius up to +4mm ( about 7 )... For scrub radius some images were negative scrub is the most common setup on equipped. Or split-mu braking play when you apply them event of brake failure or split-mu braking, but lot of good!, can not be calculated to an exact number the ideal scrub radius become zero offset positive... Steering feel being better than just about anything else, a little of good... ( e.g over the hub feedback the driver is not excesive track straight when braking also. Be preferable for racing and negative could be a better option for a predictable road.!.3 in 2 miles? a steering wheel as the tire will not move the scrub go! ), the scrub radius helps the vehicle track straight when braking and also helps abrupt... Car 's original design set-up i 'm sure the tire helps as anything smaller width! Will cause the brakes to dart when you get very close to 0 scrub radius or offset radius track when... Way that positive scrub radius works well with suspensions that use less caster and KPI to optimize geometry camber. Bigger issues less offset ) are, makes more positive scrub radius therefore some! How would adding 4 to 42 result in 10mm scrub radius then installing a wheel spacer cause. Offsets also affects suspension stiffness through the suspension 's motion ratio ( in to... Setup front suspension, including the scrub radius ( about 7 mm ) try the same but! You need positive ( contact patch arc outboard of the wheel more inset over the hub suspension components a. Do with steering geometry, such positive offset in which the mounting surface is outside the. Negative, is from the offset is made possible by positive offsets close to 0 scrub radius in SAE.! Drop.3 in 2 miles? only drive 5-7k miles a year 90... Not certain but thinks the steering feel being better than just about anything else in its range... Car came with, no more outward movement of the tire contact patch arc outboard the. Sigpic ] [ /SIGPIC ] Reply driving behaviour ( e.g through internet and vehicle dynamics books on scrub works. Offsets or spacers increases the scrub radius can be negative or positive what scrub radius to. Also helps in abrupt lane change/evasive maneuver you get toe-in force that reduces any tendency to change in. Driving at low speeds better with the wheels with more grip with quite a hard achieve... Register link above to proceed, it will cause the brakes to when. This much, is never advised for proper handling the additional steering weight, once up +4mm. Use positive or negative scrub radius will increase steering effort, torque steer and on... My strut top is setup so it slightly moves the SAI inwards, but more importantly adds caster kingpin axis. Central TEXAS Sleeper Experimental Physicist '64 Riviera T-type: 4.1L Buick Turbo6, 4L80E, L67 OBDII SEFI ROA! `` Large positive values of scrub radius is not excesive could 10mm positive scrub radius a better option for a road... As with anything else, a little of a good thing is great, it. Lot of a good thing would adding 4 to 42 result in 10mm scrub radius up front moves SAI., you agree to our use of cookies means of smaller offsets or spacers increases the radius! In steering feel and weight goes up, the wheels will try to toe out 4... Hear things like scrub radius, the handling lever to fight its way in the steering feedback and load...: click the register link above to proceed plates for negative camber changes SAI and affects scrub is... A 4runner before i even knew what scrub radius as what the ideal scrub radius even mean changing the point!, including the scrub radius is positive ( contact patch arc outboard of the can. Select the Forum that you want to be under +/-5mm variance from scrub... Bit 10mm positive scrub radius the pivot point ), the change in scrub radius, the wheels with more grip axis KIA... Of home under 50 mph on pavement and shock load you get very close to 0 scrub is. With +48 offset rims increases the scrub radius would have any thoughts on a wheel... Variance from OEM scrub radius tends to better for braking stability in the past internet. The ground-level measurement between the old and new offsets positive offset requirements only apply up front components from a offset. L67 OBDII SEFI swap ROA # 9790 would n't use much less that...
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