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...

[ronvaliquette]

There is a video about SAI(Steering Axle Inclination), that is what many in the USA consider "scrub radius"... I liked your explanation much better, great job!

[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

[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!

[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.

[unclegeorge]

A brief but excellent presentation. Thank you

[benhowe]

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.

[samueltucker]

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

[ananthkaimal]

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

[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...

[churchracing]

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.

[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.

[jabadahut]

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.

[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?

[jitsemiedema]

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.

[batesmclain]

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

[JT-gmfk]

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.

[cantthinkofnameyeah]

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

[tjidde]

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!

[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?