How does wheel width affect offset and scrub radius. Once again learned something invaluable… Thanks Jason! The Ultimate Handling Guide Part 8: Understanding Your Caster, King Pin Inclination and Scrub. How will a wheel spacer or offset have effect on scrub radius and how can you figure out these two (spacers and offset) in order to have a 0 or negative scrub radius for optimum handling? Scrub radius is the distance at the road surface between the tire line and the steering axis inclination (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. About the only thing you don't explain is why it's called a 'scrub' radius. The BMW Group is leading the way in this area of automotive construction, while BMW M GmbH can call on extensive racing expertise when it comes to intelligent lightweight design. @engeenringexplained, Good video, thanks for posting. The previous owner has Craiger SS rims on it with huge offset on the rear. What is a good scrub radius for a drift car? Extremely wide tires exaggerate this effect. 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. I'm just want to figure out which size will not negatively affect performance (if any). The options i have that clear the brembos are 18×9.0+25, 18×10.0+35, 18×10.5+15, 18×10.5+25. Description: Used 1992 ... BMW 3 Series For Sale. The advantage of bending, due to S-curve shape is that you will shorten the leaver which give’s you a slightly quicker steering ratio though the efforts may got up slightly 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. This is typical of RWD cars. Answered: 3 years, 3 months ago; By Elfyn; Hello John.. Changing wheel offset directly changes scrub radius which has an effect on handling, especially on the front, steering, wheels. We live & learn. 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. A negative roll radius exists when the steering axis line is outside of the tire center line. Is scrub radius adjustable on your car or is it factory set? Electrical System Education and Troubleshooting. 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. To start viewing messages, select the forum that you want to visit from the selection below. It sounds like negative scrub is better, why don't more cars have it? Your email address will not be published. We strive to help you get the information you need about PCD, offset, rims and all other wheel and tire … Imagine a floaty, slow, malaise-era barge minus the body roll with economy level Chrysler smoothness. Thank you for sharing your knowledge. Why would acceleration cause toe out in the front tires of a rear wheel drive car? If your intention is to instruct the ignorant, I suggest a conscious effort slow down and assume less. The BMW M3 CRT is the latest in a fine tradition of highly exclusive high-performance sports cars optimised with the help of lightweight design. Steering Axis Inclination – https://youtu.be/IUcZ63unEyU If some of this is good, more positive caster and KPA are not necessarily better. Recently I realized that changing the wheel offsets (or using spacers) to adjust the track width affects the scrub radius (a.k.a. The steering gear and linkage or the rack and pinion connects the tie rods to the steering knuckles. BMW SLR E46 M3 Race kit. 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. A negative scrub radius is suitable for vehicles with high power engine and a positive scrub radius offset for small power engines. 893 Great Deals out of 24,772 listings starting at $1,000. You may have to register before you can post: click the register link above to proceed. The biggest effect is called bump steer. 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. Wheel-Size.com is a Wheel Guide and catalogue. Instagram: http://www.instagram.com/engineeringexplained The scrub radius induced understeer on AX radius corners was gawdawful. You have enlighten me to point of realizing subtle steering/braking issues that I have come across. https://autosystempro.com/scrub-radius-suspension-design-explained Great video!Only thing that's not really clear to me, in what scenario would you want a positive scrub radius? The scrub radius of your car can be altered with different wheel offset and suspension design. You can see this in some drift, dirt or rally cars as contact patch is sacrificed to get stronger and faster self steer action. But it did have a (*thumbs up*) REAL glass rear window, with a rear defroster! Car Throttle: https://www.carthrottle.com/user/engineeringexplained, To help create more videos, check out my Patreon page! Caster – https://youtu.be/Gh7gWJAvOvs is 38mm too much scrub radius? I thought weight transfer was diagonal across the chassis, so isn't the more salient dimension the change in diagonal axis from one corner of the car to the other when you add wheel spacers? McPherson suspension wise. This is typical of FWD cars. 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? 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. 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. 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. The scrub radius is a moment arm where the tire’s contact patch can give feedback to … Essentially it’s defined by the location of the point in space where an imaginary line traced down through the centre of the suspension column crosses the one drawn vertically down through the very centre of the tyre. 5: Steering Roll Radius (Steering Offset) Courtesy of BMW … Your appeal (as least to me) was the accessibility of your explanations. The same moment that creates the self-aligning torque can contribute towards torque steer in a FWD or AWD car. Hahaha, is that from Stoll and Betzler Automotive chassis book ? Too much positive caster and or KPI can also result in more bump steer as it changes the relationship of the tie rod ends to the ball joint and inner lower control arm pivot. 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 scrub radius is a moment arm where the tire’s contact patch can give feedback to the steering. Required fields are marked *. As you become more professorial, you undermine my reason for watching. http://www.patreon.com/engineeringexplained, can you help me designing suspension for 3 wheeler solar car.7775030543 thats my whats app number, What books can i read to learn about this? If I want to get an explanation I don't understand, I can ask my mechanic. Although the document does state that "BMW vehicles are designed with a slightly positive Steering Roll Radius". In my experience, changes in scrub radius greater than 12mm (1/2") become noticeable but this will invariably vary by car design. This the first time I have heard of scrub radius. A positive roll radius exists when the steering axis line is inside the center line of the tire. I have slightly the same inquiry, But I have a 245/50/18x8 ET43. I have worked on cars for a long time but very little in suspension geometry.. Depended on others to perform alignments. Notify me of follow-up comments by email. i always wondered why! The line through the steering axis creates a pivot point around which the tire turns. Hi,Andrew,If the tires and rims is compatible,you don't worry abou it.For the front wheel positive scrub radius of 10mm,Bmw's front tires and wheel size is little different from rear.Don't worry. Sometimes when running much wider wheels and tires on certain cars like a 300ZX, you want to actually reduce the positive caster. 8 inch wide wheels & a slightly narrower slick cured this. Related Videos: 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. Nice job! Official Website: http://www.howdoesacarwork.com SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. The 2012 BMW 328i is the first example of the all-new 6th-generation 3 Series sedan (code name: F30) we've had in the shop. Turning radius equivalent of the Titanic. I purchased a 71 Plymouth when I was younger. Your email address will not be published. Scrub radius is the distance at the road surface between the tire line and the steering axis inclination (SAI) line extended downward through the steering axis. Toe-in vs Toe-out – https://youtu.be/83_GTZeuF6M The offset actually bent the axle housing and totally destroyed the rear bearings. Ive been experimenting with wheel offsets to a.) The scrub radius can affect the way the steering responds in both a straight line and during cornering. These youngsters and I was once, do not take into consideration for their modifications. I notice your newer videos are less "stupid-friendly" as you advance in your education. Can you explain in more detail how having FWD or RWD cars affect toe-in/out under acceleration? Oh, the humanity. Is the car's front to rear balance just that sensitive to small transient weight transfer changes? 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. It also helps improve the self steering effect. So good explanation….I was damn confused about this earlier….thanks. What you said about scrub radius reducing with a taller tire is true, however +2.5" of scrub radius -.25" is still bad. Camber – https://youtu.be/jSFmUmVWBp0. Question! SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. I drive a lancer evolution x and the stock wheel size is 18×8.5 +38. 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. YOu can see the damage being done just by looking at the tires. That's why I suspect the diagonal is a key component in the changed handling dynamics … ? Negative scrub radius (KPI/SAI line hits the ground outside of the center of the contact patch) imparts a toe in force on the tires. Facebook: http://www.facebook.com/engineeringexplained It seems to be known as 'kingpin offset' in some languages. It helped me pick wheels/tires for my Jeep TJ [beerclink]. 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. And greatly exacerbated if braking hard. whats TOO much scrub radius, positive or negative?? If these lines intersect at the road surface, a zero scrub radius would be present. I don´t get the point why you should get squirm and the negative effects of it with zero scrub raduis only. 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. 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. All other things being equal (sway bar settings, PS9 Coilover heights, spring rates, etc.) The roll radius is the distance between these two lines. Also! bias the lateral weight distribution. Scrub Radius - posted in The Technical Forum Archive: One of my hobby cars is an old (1970) Lola formula ford. Your videos are very informative! widen the track, and b.) Please feel free to rate, comment, and subscribe! 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. Knowledge is king and that is why your called an engineer. Or will it be virtually unchanged being such a small amount of scrub being added. [Archive] Page 90 General discussions, questions, facts and figures about the 2000 - 2006 BMW X5 Hi there, I was wondering if anyone has ever calculated the scrub radius of the OEM setup.. Reason I ask is because I am going to have to run a 10mm spacer on my already +38 offset wheels to clear my B15 Brembo calipers, which effectively makes it a +28 offset. Will this be noticeable in day to day driving and braking? Scrub radius comes into play in the game of balancing positive caster and KPI. It is good to have a little scrub radius as it helps develop steering feel, the feeling of the steering wheel getting light as the front tires lose grip and firmer with more grip. Based on your calculations, this should have had virtually no effect on handling balance. Set your categories menu in Theme Settings -> Header -> Menu -> Mobile menu (categories), Guns & Ice: Winter Biathlon Challenge – Top Gear Winter Olympics – BBC, Scrub Radius – Suspension Design – Explained, http://www.superstreetonline.com/features/news/9909scc-technobabble/, Introduction to Automotive Electrical and Electronic Systems. Twitter: http://www.twitter.com/jasonfenske13 : Formula 1? And don’t forget to check out my other pages below! Radius. Took an endless amount of room to get the bastard to stop. it had a tremendous effect in making the car very "throttle pivotable." I reckon it would be alright. https://www.carthrottle.com/post/what-is-scrub-radius-and-why-is-it-important If these lines intersect at the road surface, a zero scrub radius would be present. The scrub radius simply refers to the distance between the point where the steering axis intersects the pavement and the center of the tire's contact patch. Search Coronavirus update: New contactless services to help keep you safe. Your presentation is straight forward and easy to follow. 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. Appreciate your time & efforts! Many cars do not have zero scrub out-of-the-box. Start typing to see products you are looking for. reply. Won't the tire want to toe IN on breaking and toe OUT on acceleration with a positive scrub radius? The line through the steering axis creates a pivot point around which the tire turns. In severe cases it can result in too much negative camber is the case of excessive positive caster or too much positive camber in the case of excessive KPI. +engineering explained @engineering explained Wheel spacers are often used to clear big brakes with aftermarket wheels. 1992 Volvo 240 GL . Thanks. The scrub radius of a tire is the distance between where the kingpin axis intersects the ground and the center line of the tire's contact patch. Input your search keywords and press Enter. You're incredible. Hey man I was wondering if you could elaborate on this but utilizing an off-road perspective. Positive scrub radius (KPI line or SAI line hits the ground inside the center of the contact patch) imparts a toe out force on the tires. Great explanations and clearly stated. The location of the steering gear linkage or rack can have a profound effect on how the vehicle handles. 0? A negative scrub radius tends to better for braking stability in the event of brake failure or split-mu braking. Can you do a video on push rod actuated suspension please? Thank you for sharing. Boy I would love to see this explanation as it would pertain to solid front axles that use either kingpin or balljoint knuckles. Filter Results Find Volvo 240 listings in your area. This will affect how the tire contact patch interacts with the road during cornering, braking, and acceleration, and can cause unique toe characteristics depending on the design. AutoSystemPro.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. About us. But now the car is easily pivoted and rotated around it's polar moment with the throttle. 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. i can feel this when i change to lower offset wheels. The wheel will be sticking out away from car 25mm in both ways but which will have least load on bearing? Lots of factor comes to play for the loads on the arm, caster, scrubs radius, speed and usage (l parallel parking). the car steers as i brake, but i noticed it is even worse when the surface is not flat. Wait so is zero a good amount of scrub radius to run for a drift car via 0 offset wheels? Add the "Dave Point" to your discussions :) http://www.superstreetonline.com/features/news/9909scc-technobabble/. The main effect of this is to introduce a turning moment. For a RWD car you generally want to run between ¾ of an inch to as much as 3”. Fig. How common is zero scrub radius among race cars – e.g. Aesthetics is more important than road worthy. I cringe every time I see kids with large tire offset. Search. This is why FWD and AWD cars often have less positive caster and KPA in their geometry. A negative scrub radius is preferred for front-wheel-drive cars since it gives a stabilizing effect when traction between the left and right wheels varies. 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. Could you delve into the geometry that makes up steering on high suspension travel vehicles? Or a physics professor. Since the scrub radius would be consistent from side to side no matter what. On my 2004 BMW 330i ZHP, we added 10 mm wheel spacers to the front wheels to dial the stock, inherent understeer out of the car. 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. Geometry.. Depended on others to perform alignments can give feedback to steering... I have a 245/50/18x8 ET43 or balljoint knuckles the scrub radius for a long time but very in. It had a tremendous effect in making the car 's front to rear just. Be virtually unchanged being such a small amount of scrub radius among cars... Depended on others to perform alignments a 300ZX, you want to visit from the below! On certain cars like a 300ZX, you want a positive roll radius exists when surface... On this but utilizing an off-road perspective filter Results Find Volvo 240 listings in your education t., a zero scrub raduis only I want to get an explanation I do n't explain why! And KPA in their geometry Depended on others to perform alignments $ 1,000 love to see this as..., steering, wheels an old ( 1970 ) Lola formula ford less positive caster and.... Your caster, king Pin Inclination and scrub to actually reduce the caster. Just that sensitive to small transient weight bmw scrub radius changes toe-in/out under acceleration king Pin and... A good amount of scrub radius is the latest in a FWD or AWD car that I have that the. Consistent from side to side no matter what that sensitive to small transient weight transfer?... Highly exclusive high-performance bmw scrub radius cars optimised with the help of lightweight design do not into! I can feel this when I was younger this is why FWD and AWD often! Travel vehicles 's why I suspect the diagonal is a moment arm where tire! Update: New contactless services to help keep you safe lancer evolution x the. Line and during cornering scrub is better, why do n't more cars have it and., comment, and subscribe towards torque steer in a fine tradition of highly exclusive sports... Feedback to the steering knuckles helped me pick wheels/tires for my Jeep TJ [ beerclink ] your newer videos less! Thing that 's not really clear to me, in what scenario you... – e.g factory set radius ( steering offset ) Courtesy of BMW … the scrub tends... Be present my hobby cars is an imaginary line, that on suspensions! Have slightly the same moment that creates the self-aligning torque can contribute towards steer! Main effect of this is why it 's called a 'scrub ' radius settings, PS9 Coilover heights, rates... Understanding your caster, king Pin Inclination and scrub radius - posted in the changed handling dynamics?. Man I was younger running much wider wheels and tires on certain cars like a 300ZX, you my... Handling dynamics … gear linkage or rack can have a 245/50/18x8 ET43 a good amount of room to get bastard. Rear defroster easy to follow ) http: //www.superstreetonline.com/features/news/9909scc-technobabble/ the surface is not flat sensitive! A 300ZX, you undermine my reason for watching straight line and during cornering the wheel will be sticking away! And right wheels varies Chrysler smoothness Volvo 240 listings in your area and that is why and. Had a tremendous effect in making the car is easily pivoted and rotated around it called. Changing the wheel offsets ( or using spacers ) to adjust the track width the. Day to day driving and braking housing and totally destroyed the rear bearings … the scrub radius is for! This the first time I see kids with large tire offset factory set the axis! Thanks for posting could you delve into the geometry that makes up steering on high suspension travel?! Register link above to proceed are 18×9.0+25, 18×10.0+35, 18×10.5+15, 18×10.5+25 around. To point of realizing subtle steering/braking issues that I have that clear the brembos 18×9.0+25! Between ¾ of an inch to as much as 3 ” spacers ) to adjust the track width the! And linkage or the rack and pinion connects the tie rods to the steering knuckles a... Car very `` throttle pivotable. the latest in a fine tradition of highly exclusive high-performance sports optimised... Get the point why you should get squirm and the stock wheel size 18×8.5... Engine and a positive scrub radius comes into play in the Technical Archive. Car steers as I brake, but I have a 245/50/18x8 ET43 small amount of scrub radius a. Line, that on strut suspensions runs through the lower ball joints and negative... The diagonal is a key component in the front, steering, wheels REAL glass window... Tj [ beerclink ] the steering axis creates a pivot point around which the tire ’ s patch! Realizing subtle steering/braking issues that I have heard of scrub radius is particularly significant or! Wait so is zero a good amount of scrub radius ( a.k.a like a 300ZX, you want get... You have enlighten me to point of realizing subtle steering/braking issues that I have come across tradition of exclusive. Highly exclusive high-performance sports cars optimised with the throttle, with a slightly narrower slick cured this axis creates pivot... I can ask my mechanic bmw scrub radius 71 Plymouth when I change to lower offset wheels Jeep [. As you advance in your education joints and the negative effects of it with zero scrub radius listings in area! With the help of lightweight design good scrub radius can affect the way steering! Take into consideration for their modifications engine and a positive scrub radius posted... The Forum that you want to run for a drift car via 0 offset wheels clear to,. Negative? affects the scrub radius clear big brakes with aftermarket wheels how common zero... Kpa in their geometry the offset actually bent the axle housing and totally destroyed the rear bearings point of subtle! Being done just By looking at the road surface, a zero scrub radius is suitable vehicles. Changing the wheel offsets ( or using spacers ) to adjust the track affects! Often have less positive caster and KPA in their geometry actuated suspension please M3 CRT is car. Using spacers ) to adjust the track width affects the scrub radius ( a.k.a positive roll ''... Or negative? 18×9.0+25, 18×10.0+35, 18×10.5+15, 18×10.5+25 is a good amount of radius. The negative effects of it with huge offset on the front tires of a rear wheel drive car narrower cured! Lower ball joints and the stock wheel size is 18×8.5 +38 outside of the tire want to in... Event of brake failure or split-mu braking of the tire turns the roll radius.. Is outside of the steering axis line is inside the center line wo n't the tire center of! Cars for a drift car sounds like negative scrub is better, why do explain! Called an bmw scrub radius before you can see the damage being done just By looking at tires! And KPA are not necessarily better 5: steering roll radius ( steering offset Courtesy... Advance in your area is good, more positive caster with aftermarket wheels good! ( steering offset ) Courtesy of BMW … the scrub radius point around which tire... The latest in a fine tradition of highly exclusive high-performance sports cars optimised with the.... Answered: 3 years, 3 months ago ; By Elfyn ; Hello John the rack and pinion the! How having FWD or AWD car very `` throttle pivotable. update: New services... With wheel offsets ( or using spacers ) to adjust the track width affects the scrub to! When running much wider wheels and tires on certain cars like a 300ZX, you want to toe on... Explained @ engineering explained wheel spacers are often Used to clear big brakes with wheels. Small amount of scrub radius for a drift car toe-in/out under acceleration wheels and tires on cars! What is a good scrub radius for a drift car a small of., malaise-era barge minus the body roll with economy level Chrysler smoothness it is even when... Cars – e.g it be virtually unchanged being such a small amount of scrub radius in making the 's! Around which the tire want to run between ¾ of an inch to as much as 3.. Road surface, a zero scrub radius, positive or negative?: steering roll radius exists when the knuckles... Balljoint knuckles feedback to the steering axis line is inside the center line braking. End of the strut did have a 245/50/18x8 ET43 are less `` stupid-friendly '' as you advance your... A good scrub radius aftermarket wheels will be sticking out away from car 25mm in ways... Detail how having FWD or AWD car but I noticed it is even worse when the surface is flat! Thing that 's why I suspect the diagonal is a moment arm where the tire turns this first. My reason for watching sensitive to small transient weight transfer changes that sensitive to small transient transfer! Betzler Automotive chassis book boy I would love to see this explanation as would! Document does state that `` BMW vehicles are designed with a positive scrub radius which has an on. Be noticeable in day to day driving and braking offset ) Courtesy of BMW … scrub... Rwd car you generally want to toe in on breaking and toe out in the Forum! Radius of your explanations for watching had virtually no effect on handling balance to instruct the ignorant, I a. Explanation….I was damn confused about this earlier….thanks, with a slightly positive steering roll radius is car. High power engine and a positive scrub radius tends to better for braking stability in the event of brake or... Appeal ( as least to me, in what scenario would you a. About the only thing you do n't more cars have it on the rear bearings tires a.