How Scrub Radius Works

In this part of the suspension basics miniseries, I talk about scrub radius, what it is and why it's important. Scrub radius largely affects low speed handling and feel, however it also has a number of structural implications. Probably not a good idea to put those spacer rings on then...

[@KYLEENGINEERS]

Also, it has been brought to my attention that I didn't mention the effects scrub radius has on torque steer. The lateral offset of the contact patch will make torque steer worse, so for FWD/AWD cars you want to keep that number low. Sorry for not mentioning it, but it is a brief summary video and most of my experience is in RWD cars, so I forget to say these things. Apologies and thanks for watching! If you are interested in this more, try here: kzbin.info/www/bejne/nWrQamulf72gjtU

[@mikegrady4]

KYLE.DRIVES, you also don't describe how scrub radius influences dynamic weight movement in a vehicle as wheels are turned. It is a significant tuning point in suspension geometry, especially in rear wheel driven cars on surfaces with lower coefficients of friction.

[@kenlai2200]

hello good day to u , can u plz help i want to upgrade from 17 inch 7 width ET55 215/55/17 to 19 inch OZ rim 8 inch width ET 45 with tyres 225/45/19, the scrub radius shows positive of about 7 mm, can u please help if i can go ahead, thanks

[@dieterreniers1554]

@@kenlai2200 You have to change your KPI angle to fix it. This is the center of rotation for your wheel. Just go with the correct ET. If you want to have wider wheels, you must upgrade with wider J to keep the standard scrub radius (ex 7J to 8,5J). If there's a difference in braking force between the wheels, the car will automatically turn towards the side with the most braking force. With a positive scrub radius, located on the inner side of the wheel, the mechanical momentum will make the wheels turn towards that side. With a negative scrub radius, located on the outside of the wheel, this momentum works from the opposite direction so the wheels turn to the side with less braking force. This two forces balance each other out. I really wonder how you've measured the scrub radius? The only way is to calculate it with the technical plans of your car, because the numbers are so small and 1inch can make the radius from negative to positive... Hope I made it clear for you.

[@NeilChandMusic]

I'm happy both you and EE talked about Scrub radius so close together! Helps me with my understanding :) Not sure why anyone would complain about content stealing (even though this wasn't) but I guess not everyone is here to learn haha

[@Kfugoso]

Been following your videos, makes me understand what goes around in motor sports with race engineers adjusting the suspension setup to the driver's driving style. Read a few books but it puts me in the deep-end i can't fully comprehend yet. but your videos make it easier for me to understand. Did a motor-sports course in Australia but the level of education or having the right teachers for the particular area is limited :(. So i can appreciate having videos like yours around that are taught by the people who understand and had experience in that field.

[@unclegeorge6175]

A brief but excellent presentation. Thank you

[@KYLEENGINEERS]

Well, I was going to wait a few weeks before releasing this, but it seems Engineering Explained has forced my hand on this one...

[@RcHDProductions]

KYLE.DRIVES This isn't about who launches a video first or who is better. This is about you teaching us your knowledge and this video seems forced by the competition. I want to ask you a favor. Don't try to make the same videos of EE. You both are good but I don't see the point of being subscribed to two channels that shows the same. You can make it better than this. I say this with all the respect to you. I only want to receive the best of you. Not a forced response.

[@KYLEENGINEERS]

+Rodrigo Cunillé You misunderstand, this video was already uploaded and ready to launch as part of the miniseries I announced a day before EE launched his video, there's no way I could have time to film, edit and upload a video within a few hours of his launch so I turned mine to published as soon as I saw he released his (basically when I woke up and checked my youtube notifications), unfortunate timing really as I was actually planning on releasing scrub radius first and decided at the last minute to release camber instead. I do note that many people seem to be dissatisfied with the level of detail in this video though, I thought I would only do a brief one as I din't think people would be that interested in the specifics of scrub radius, however the feedback is duly noted and I will be releasing a more detailed look into the effects of scrub radius down the line. You seem like a loyal subscriber so I doubt that you will think I'm lying, but just in case you need proof, check the haircut, beard length, clothing and lighting from the camber video: kzbin.info/www/bejne/q3uwoIKkjcl3rLc Both were filmed on the same day, as well as the other videos to come in the miniseries.

[@flamingronin]

+KYLE.DRIVES Stop trying to be a hipster Phil

[@shanekoshie7381]

Honestly this was better than engendering explained’s video

[@samueltucker7205]

I know people are hating on your vids but your depth in these topics is awesome and you are very good at explaining things really helpful. +KYLE.ENGINEERS

[@ananthkaimal5941]

could you please tell about different effects of negative and positive scrub radius respectively

[@aedengb9532]

what a wonderful channel honestly

[@churchracing4058]

If you have or could make some videos on roll center, moment center and instant center that would be awesome. Thank you for what you have filmed so far, very helpful.

[@benhowe2087]

There is a lot more to scrub radius. Front wheel drive cars tend to have very small amounts of scrub radius and the pivot point is outside of center. Rear wheel drive cars have the pivot point inside of centerline as in your picture. Manufactures have gone to great lengths to control scrub radius. Audi, Honda use ball joints on top of the wheels and tires. Audi uses two lower ball joints in addition to this creating an imaginary pivot point. Torque steer and darting under heavy braking can be controlled to some extent by reducing scrub radius. Also tire diameter effects scrub radius in addition to wheel offset.

[@galant310]

great job keep it up

[@Robert8455]

Good basic info.

[@nickamarit]

I changed the scrub radius of my car by swapping the rear wheels to the front; the car had stock mild positive scrub radius but now has a good amount of negative scrub radius. The result is that steering weight increased noticeably; there's a lot more feedback; and there seems to also be more bumpsteer. My question is, had I used spacers to make the scrub radius more positive than the mildly positive stock scrub radius, would the steering feel heavier, have more feedback, and have more bumpsteer as well?

[@nwimpney]

I found it a little odd that you didn't mention the feedback effects of scrub radius. Any time there's an uneven force like hitting a pothole with one wheel, the steering will be pulled in the direction of that wheel. Obviously, this is a big thing off-road. Another place where it becomes apparent is when breaking hard, and one wheel loses traction. (The steering will pull towards the wheel with traction)

[@teamoldgoatsracing]

Sorry for digging up a old video. I dont understand a part of this. We have cut up our control arm on our track car so we can relocate the ball joint to fix bumper steer. While we do this we can move the ball joint out or inwards. That should affect scrub radius. The pivot point is the ball joint, not the intersecting line of the strut to the ground. So I dont know how to actually measure the scrub radius we have. Do I follow a line from the top of the strut to the ground thgough the strut. Do I follow a line from the top of the strut through the ball joint. What is the actual best way to determine where the scrub radius is on the ground. I understand the therory, not the actual measuring points.

[@dethkon2284]

So if i pivot the axle around a central point on the body what would happen? bad scrub radius? good? strange changing scrub radius? i want to know!

[@jabadahut50]

is 38mm too much scrub radius? 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.

[@tjidde95]

its great how you educate people about these things and you're doing an amazing job at it kyle! but i feel like you missed the opportunity to go in debt about the toe angle changes scrub radius provides under different acceleration and deceleration forces through play in bushes and other suspension components. In turn, affecting handling to some degree. I believe some car manufacturers design there cars with a negative scrub radius to minimize the steering effects of a punctured tyre at speed. (retaining forwards direction). People often unkowingly change their scrub radius by changing wheel offset like you mentioned but camber changes also have a fairly big impact. I really do hope you revise this video when it suits you!

[@dieterreniers1554]

If you want to have wider wheels, you must upgrade with wider J to keep the standard scrub radius (ex 7J to 8,5J). If there's a difference in braking force between the wheels (caused by road condition for example), the car will automatically turn towards the side with the most braking force. With a positive scrub radius, located on the inner side of the wheel, the mechanical momentum will make the wheels turn towards that side. With a negative scrub radius, located on the outside of the wheel, this momentum works from the opposite direction so the wheels turn to the side with less braking force. This two forces balance each other out and keep you balanced. People who just trow on some spacers for the "right" fitment, are in danger when they need to pull of a emergency stop. it doesn't matter how wide your wheels are, as long as you don't change the center of the wheel

[@batesmclain2499]

what is the site you suggested for suspension design and information? Thanks for the video and inforamtion.

[@jitsemiedema7423]

Hi Kyle. I'm having a question. I'm having an issue with massive understeer at low speed corners. Will a lower scrub radius help? It's a dubble a-arm setup. Already tried multiple camber and caster settings. But this didn't make any difference. Neither did spring rates, and I can't run a front swaybar.

[@RobHTech]

I'm confused. If I go from +55 offset to +38 offset, then I move the tire/rim outward 17mm, which leads to a more positive scrub radius? And, would that change in scrub radius at +17mm outward be sufficient to cause suspension operation problems and stability of the vehicle and steering?

[@JT-gm4fk]

Whats considered too much positive or too much negaticlve scrub radius? Is there a number on both sides that you do not want to reach? I'm looking at around a 6mm positive with one setup and a -4mm with another.

[@cantthinkofnameyeah7249]

Sounds like scrub radius is the point the tire turns at. So what is scrub radius for back tires?

[@Kalimerakis]

For FWD torque steer: If one would offset the contact patch to the inside that would give a light uncontrolled steering feel under acceleration?

[@KYLEENGINEERS]

+Kalimerakis Perhaps this video is for you! kzbin.info/www/bejne/nWrQamulf72gjtU

[@Kalimerakis]

thank you, great video!

[@gorrilaunit99]

scrub radius also affects toe-in/toe-out effects under acceleration/braking need to redo the video

[@nirmalparmar4224]

If RWD car has front track-53 and rear track-53 then How it reduce oversteer?how?

[@satrioharyoprakoso6342]

anyone knows what is the effect of scrub radius to fuel economy of a car? my logic thinks that more scrubbing that happened will increase the amount of fuel consumption of a car because of the energy loss in the scrubbing phenomenon. If anyone has some idea about this can you guys help me?

[@Drum_x_Life]

No man The scrub radius create stress on you steering rod The fuel economy car is from rotolance resistance of tyre Drag resistance Dimension of front rear of car Scrub radius stresses your steering rod as bump steer

[@A_backyard_cricketer]

please tell me the name of that site i m not getting in video.

[@BasuSatwik]

Motoiq

[@2Phast4Rocket]

EE seems to explain the scrub radius and the effects of positive and negative scrub radii better. So if you are trying to compete, explain it better.

[@0Chrispy0]

a shame this is so short. would've liked to see it touch upon the impact on steering, ackerman, and being magnified by caster

[@jakevanderrol]

yeah me too. I am keep to learn more about how it affects the car feel as stated in the video

[@KYLEENGINEERS]

+d3c0y hmmm ok, I assumed that people wouldn't be that interested in scrub radius and just wanted a quick summary, hence why I made this video so brief, but if it looks like there's interest I can definitely see about making a more detailed one.

[@jakevanderrol]

KYLE.DRIVES I have been building a 240Z (McPherson strut front) with flares and have gone from a 14x6" 0 offset wheel to a 16x9.5" -19 offset. So if you would like to use that as your example that would be even better! I like trying to design and setup my own car so I absolutely love your videos and the level of technical detail in them so i say always go all out! It would be great if you could show us a way of measuring the numbers involved on a real car too. I was watching the latest installment of Project Binky and they were calculating their Ackerman with some lasers and tape, but didnt go into how they did it. Double thumbs up for all the videos on aero, I watched them multiple times and learned a massive amount from them.

[@nfwaeft]

Hi, does this apply to bicycles also?

[@sultanofswingdrift3021]

john smith caster aplies to bikes,scrub is on for 2 wheels on the same axis..

[@Drum_x_Life]

Man on bycicke there is caster no king pin

[@AR-hj8xi]

Hey kyle. Is it just a coincidense that Engineering Explained just did this topic. Scrub radius.

[@KYLEENGINEERS]

+Adnan Rahman Well, I assume so, unless he saw my last video which mentioned I was doing scrub radius as part of my suspension miniseries and reactively made his video... I basically got up, saw he had his video and then switched mine from scheduled to public so no-one could be like "you stole this material from Engineering Explained" as there is no way I could've had time to film, edit, upload and publish this video within a few hours of him releasing his. Also, if you note I am looking the same and wearing the exact same clothes with the same lighting as the video I uploaded yesterday, which was shot on the same day. So, short answer, yes, coincidence facebook.com/Kyleengineers/photos/a.458300720882246.107135.448718915173760/992267397485573/?type=3&theater

[@flamingronin]

+KYLE.DRIVES I call bullshit, you definitely stole his material. Plagiarism detector scored your video 100% plagiarised. #philisadirtythief

[@AR-hj8xi]

+Daniel Luong calm ur life.

[@KYLEENGINEERS]

+Adnan Rahman Don't worry, he's my resident friendly troll, has hilarious comments on almost all of my videos!

[@flamingronin]

+KYLE.DRIVES I don't know what you are talking about. I've never commented on your videos before.