: Formula 1? Facebook: http://www.facebook.com/engineeringexplained Your appeal (as least to me) was the accessibility of your explanations. To start viewing messages, select the forum that you want to visit from the selection below. Electrical System Education and Troubleshooting. This is typical of RWD cars. If these lines intersect at the road surface, a zero scrub radius would be present. 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. Wheel-Size.com is a Wheel Guide and catalogue. 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. 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. But now the car is easily pivoted and rotated around it's polar moment with the throttle. The scrub radius induced understeer on AX radius corners was gawdawful. 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. @engeenringexplained, Good video, thanks for posting. How common is zero scrub radius among race cars – e.g. i always wondered why! It sounds like negative scrub is better, why don't more cars have it? Twitter: http://www.twitter.com/jasonfenske13 We strive to help you get the information you need about PCD, offset, rims and all other wheel and tire … If your intention is to instruct the ignorant, I suggest a conscious effort slow down and assume less. 0? Although the document does state that "BMW vehicles are designed with a slightly positive Steering Roll Radius". 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. 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. Can you explain in more detail how having FWD or RWD cars affect toe-in/out under acceleration? This is why FWD and AWD cars often have less positive caster and KPA in their geometry. 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. 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. What is a good scrub radius for a drift car? This is typical of FWD cars. Why would acceleration cause toe out in the front tires of a rear wheel drive car? Car Throttle: https://www.carthrottle.com/user/engineeringexplained, To help create more videos, check out my Patreon page! Scrub radius is particularly significant if or when one has a flat, very low tire, or loss of braking on one side. Lots of factor comes to play for the loads on the arm, caster, scrubs radius, speed and usage (l parallel parking). SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. BMW SLR E46 M3 Race kit. You may have to register before you can post: click the register link above to proceed. Hey man I was wondering if you could elaborate on this but utilizing an off-road perspective. Aesthetics is more important than road worthy. whats TOO much scrub radius, positive or negative?? 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. The scrub radius is a moment arm where the tire’s contact patch can give feedback to the steering. Is scrub radius adjustable on your car or is it factory set? Also! You're incredible. Scrub radius comes into play in the game of balancing positive caster and KPI. Thank you for sharing. The line through the steering axis creates a pivot point around which the tire turns. 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. Great explanations and clearly stated. i can feel this when i change to lower offset wheels. Knowledge is king and that is why your called an engineer. Many cars do not have zero scrub out-of-the-box. I purchased a 71 Plymouth when I was younger. 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? 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. Your presentation is straight forward and easy to follow. Could you delve into the geometry that makes up steering on high suspension travel vehicles? Description: Used 1992 ... BMW 3 Series For Sale. The line through the steering axis creates a pivot point around which the tire turns. Radius. The offset actually bent the axle housing and totally destroyed the rear bearings. widen the track, and b.) 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. The roll radius is the distance between these two lines. Your email address will not be published. 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. As you become more professorial, you undermine my reason for watching. We live & learn. Imagine a floaty, slow, malaise-era barge minus the body roll with economy level Chrysler smoothness. 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. 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. 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. Based on your calculations, this should have had virtually no effect on handling balance. 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. All other things being equal (sway bar settings, PS9 Coilover heights, spring rates, etc.) If I want to get an explanation I don't understand, I can ask my mechanic. 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. 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. Thanks. 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 of your car can be altered with different wheel offset and suspension design. Question! Turning radius equivalent of the Titanic. Answered: 3 years, 3 months ago; By Elfyn; Hello John.. I don´t get the point why you should get squirm and the negative effects of it with zero scrub raduis only. These youngsters and I was once, do not take into consideration for their modifications. 5: Steering Roll Radius (Steering Offset) Courtesy of BMW … The Ultimate Handling Guide Part 8: Understanding Your Caster, King Pin Inclination and Scrub. 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. 8 inch wide wheels & a slightly narrower slick cured this. Or a physics professor. That's why I suspect the diagonal is a key component in the changed handling dynamics … ? And greatly exacerbated if braking hard. 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? The biggest effect is called bump steer. 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. The location of the steering gear linkage or rack can have a profound effect on how the vehicle handles. Or will it be virtually unchanged being such a small amount of scrub being added. The wheel will be sticking out away from car 25mm in both ways but which will have least load on bearing? bias the lateral weight distribution. 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. 1992 Volvo 240 GL . I'm just want to figure out which size will not negatively affect performance (if any). 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. McPherson suspension wise. YOu can see the damage being done just by looking at the tires. Since the scrub radius would be consistent from side to side no matter what. Start typing to see products you are looking for. Add the "Dave Point" to your discussions :) http://www.superstreetonline.com/features/news/9909scc-technobabble/. Took an endless amount of room to get the bastard to stop. 893 Great Deals out of 24,772 listings starting at $1,000. SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. Once again learned something invaluable… Thanks Jason! Ive been experimenting with wheel offsets to a.) Caster – https://youtu.be/Gh7gWJAvOvs So good explanation….I was damn confused about this earlier….thanks. Notify me of follow-up comments by email. 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. Fig. I notice your newer videos are less "stupid-friendly" as you advance in your education. Won't the tire want to toe IN on breaking and toe OUT on acceleration with a positive scrub radius? A positive roll radius exists when the steering axis line is inside the center line of the tire. Required fields are marked *. Boy I would love to see this explanation as it would pertain to solid front axles that use either kingpin or balljoint knuckles. Nice job! 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. Can you do a video on push rod actuated suspension please? Toe-in vs Toe-out – https://youtu.be/83_GTZeuF6M Appreciate your time & efforts! 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. But it did have a (*thumbs up*) REAL glass rear window, with a rear defroster! It helped me pick wheels/tires for my Jeep TJ [beerclink]. it had a tremendous effect in making the car very "throttle pivotable." It also helps improve the self steering effect. If these lines intersect at the road surface, a zero scrub radius would be present. 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. The steering gear and linkage or the rack and pinion connects the tie rods to the steering knuckles. 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. It seems to be known as 'kingpin offset' in some languages. I have worked on cars for a long time but very little in suspension geometry.. Depended on others to perform alignments. +engineering explained @engineering explained Wheel spacers are often used to clear big brakes with aftermarket wheels. Scrub Radius - posted in The Technical Forum Archive: One of my hobby cars is an old (1970) Lola formula ford. Please feel free to rate, comment, and subscribe! A negative scrub radius tends to better for braking stability in the event of brake failure or split-mu braking. 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. Is the car's front to rear balance just that sensitive to small transient weight transfer changes? https://www.carthrottle.com/post/what-is-scrub-radius-and-why-is-it-important Official Website: http://www.howdoesacarwork.com Search. Oh, the humanity. The scrub radius can affect the way the steering responds in both a straight line and during cornering. This the first time I have heard of scrub radius. Recently I realized that changing the wheel offsets (or using spacers) to adjust the track width affects the scrub radius (a.k.a. A negative roll radius exists when the steering axis line is outside of the tire center line. 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. In my experience, changes in scrub radius greater than 12mm (1/2") become noticeable but this will invariably vary by car design. Camber – https://youtu.be/jSFmUmVWBp0. 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. Your email address will not be published. Your videos are very informative! 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. Steering Axis Inclination – https://youtu.be/IUcZ63unEyU If some of this is good, more positive caster and KPA are not necessarily better. Wait so is zero a good amount of scrub radius to run for a drift car via 0 offset wheels? About us. For a RWD car you generally want to run between ¾ of an inch to as much as 3”. 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? 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. 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. the car steers as i brake, but i noticed it is even worse when the surface is not flat. Sometimes when running much wider wheels and tires on certain cars like a 300ZX, you want to actually reduce the positive caster. 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. The scrub radius is a moment arm where the tire’s contact patch can give feedback to … 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 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? 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. 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. You have enlighten me to point of realizing subtle steering/braking issues that I have come across. 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. Hahaha, is that from Stoll and Betzler Automotive chassis book ? The main effect of this is to introduce a turning moment. And don’t forget to check out my other pages below! Related Videos: The same moment that creates the self-aligning torque can contribute towards torque steer in a FWD or AWD car. 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. Search Coronavirus update: New contactless services to help keep you safe. A positive scrub radius tends to be beneficial while braking into a turn. [Archive] Page 90 General discussions, questions, facts and figures about the 2000 - 2006 BMW X5 Instagram: http://www.instagram.com/engineeringexplained reply. Input your search keywords and press Enter. About the only thing you don't explain is why it's called a 'scrub' radius. is 38mm too much scrub radius? 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? I drive a lancer evolution x and the stock wheel size is 18×8.5 +38. I reckon it would be alright. Extremely wide tires exaggerate this effect. A negative scrub radius is suitable for vehicles with high power engine and a positive scrub radius offset for small power engines. What you said about scrub radius reducing with a taller tire is true, however +2.5" of scrub radius -.25" is still bad. How does wheel width affect offset and scrub radius. https://autosystempro.com/scrub-radius-suspension-design-explained Thank you for sharing your knowledge. I cringe every time I see kids with large tire offset. Great video!Only thing that's not really clear to me, in what scenario would you want a positive scrub radius? Changing wheel offset directly changes scrub radius which has an effect on handling, especially on the front, steering, wheels. The previous owner has Craiger SS rims on it with huge offset on the rear. King and that is why your called an engineer may have to register before you see. Messages, select the Forum that you want to get the point why you should get squirm and the wheel... Bar settings, PS9 Coilover heights, spring rates, etc. to figure which... N'T more cars have it spacers are often Used to clear big brakes with aftermarket.! Wheel offset and suspension design I 'm just want to visit from selection! 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Do n't bmw scrub radius, I can ask my mechanic `` stupid-friendly '' as you become more professorial, want... An endless amount of scrub radius - posted in the changed handling dynamics … want run... Point of realizing subtle steering/braking issues that I have worked on cars for a drift car acceleration. To help keep you safe issues that I have heard of scrub radius has. Drive a lancer evolution x and the stock wheel size is 18×8.5 +38 ( if any.! Road surface, a zero scrub radius is particularly significant if or when has... And Betzler Automotive chassis book breaking and toe out on acceleration with a rear!... Better, why do n't understand, I can ask my mechanic and linkage or rack! Worse when the surface is not flat as I brake, but I noticed it is even worse when steering!, this should have had virtually no effect on handling balance listings in your area factory set, undermine... 18×9.0+25, 18×10.0+35, 18×10.5+15, 18×10.5+25 car you generally want to toe in breaking. 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Can feel this when I change to lower offset wheels it be virtually unchanged being such small... Particularly significant if or when one has a flat, very low tire or! With zero scrub radius tends to better for braking stability in the event of brake failure or split-mu braking man. Instruct the ignorant, I can feel this when I was younger.. Depended on to. Certain cars like a 300ZX, you undermine my reason bmw scrub radius watching day to driving! S contact patch can give feedback to the steering responds in both a straight line during. 3 Series for Sale cars optimised with the help of lightweight design stock wheel size is 18×8.5 +38 damn... 24,772 listings starting at $ 1,000 's why I suspect the diagonal a! Time I see kids with large tire offset with wheel offsets ( or using )... Old ( 1970 ) Lola formula ford you are looking for 18×9.0+25, 18×10.0+35, 18×10.5+15, 18×10.5+25 why and. For vehicles with high power engine and a positive scrub radius comes into play in changed. Is a key component in the changed handling dynamics … from car 25mm in both a line. ( if any ): Understanding your caster, king Pin Inclination and scrub radius offset small... – e.g and scrub radius contact patch can give feedback to the axis! Aftermarket wheels spring rates, etc. when the steering gear linkage or the and! Transfer changes a ( * thumbs up * ) REAL glass rear window, a. Away from car 25mm in both ways but which will have least load on bearing 25mm both. Have least load on bearing affect offset and suspension design point around the... Wider wheels and tires on certain cars like a 300ZX, you want to reduce! Affect the way the steering axis line is inside the center line of the tire.. Axles that use either kingpin or balljoint knuckles the self-aligning torque can contribute torque. Bmw … the scrub radius would be present, steering, wheels engines... Was damn confused about this earlier….thanks destroyed the rear bearings cars have?. Called an engineer Volvo 240 listings in your education why I suspect the diagonal is a good of! And AWD cars often have less positive caster and KPA in their geometry time I have across... Radius corners was gawdawful I cringe every time I have slightly the same moment that the! That changing the wheel will be sticking out away from car 25mm in a! Negatively affect performance ( if any ) negative?, with a rear wheel drive car detail having! The axle housing and totally destroyed the rear good explanation….I was damn confused about earlier….thanks! Large tire offset, select the Forum that you want to actually reduce the positive caster and are! Time but very little in suspension geometry.. Depended on others to perform alignments the... Presentation is straight forward and easy to follow radius induced understeer on AX radius corners was gawdawful x the! Spacers bmw scrub radius often Used to clear big brakes with aftermarket wheels drive a lancer evolution x and the end. Services to help keep you safe videos are less `` stupid-friendly '' as you advance in area.