Scrub radius Vs Tire width

[Bow Tie 67]

Originally Posted by B_Alley

Scrub radius doesn't have anything to do with the amount of grip a tire will produce. It's the distance (in front view) between your steering axis intersection point and the tire contact patch center point. Since all your forces go through the center of the contact patch (not exactly, but we'll keep it simple for now) the farther that steering axis is from the contact patch, the more torque is going into the steering system and therefore the driver senses more 'feel'.

When designing suspensions and steering systems for our formula cars, I'd always try and keep the scrub radius as small as possible because this meant less torque going into your steering system, meaning you could make things super light! However, too light, and the driver can't feel what the tire is doing at the limit!

I would recommend going with an offset that would give you the largest track width, since that will reduce weight transfer, which increases tire efficiency, which helps rip your face off harder while cornering! Just try not to go too big on the scrub radius because then stuff can start to break! What options are you looking at?

Bobby,

That is what I wanted to hear. I will go with max track width which should put me right at the point where my 275 40 17 tires sit. With these sizes up front there really is not many options without loosing quit a bit of steering. This setup makes parking a two step adventure minimum.

Thanks,
Matt

[B_Alley]

Matt - good luck in the build! Let me know if you have any questions!


JRouche:

The easiest thing you can do to increase positive scrub is to put offset spacers in-between your wheel and hub. We used to make them for our test cars to dial in steering feel. Obviously, with too big of a spacer you won't be able to get enough threads on your lug nuts!

As for Caster, that's another part of the equation! Just like from the front view, steering force is proportional scrub radius, in side view, steering force is also proportional to Mechanical Trail. Thats the side view distance between your contact patch centerline and the steering axis. So, if you increase caster, then you move that point further forward thus increasing that distance and increasing the torque going into the system. Caster is bad for inside rear tire lift, but good for camber gain in roll! How much you ask? That all depends on your tire!

Speaking of tires, want to hear a trip? That 'center of contact patch' is never in the same location. In fact, with I wouldn't be surprised if, under hard corning, that tje tire's contact patch moves laterally 1-2 inches and under hard breaking, it moves 2-3 inches!

Don't believe me? Here's some cool videos, ENJOY!

Lateral Force
https://www.youtube.com/watch?v=nmo_d...eature=related

https://www.youtube.com/watch?v=W8UiE...eature=related

Longitudinal Force
https://www.youtube.com/watch?v=HldiiDmvTxI

[JRouche]

Originally Posted by B_Alley

JRouche:

The easiest thing you can do to increase positive scrub is to put offset spacers in-between your wheel and hub. We used to make them for our test cars to dial in steering feel. Obviously, with too big of a spacer you won't be able to get enough threads on your lug nuts!

Thanks Matt. Im stuck with my wheel placement. Its all in there and tight, no wiggle room. The wheel and tire has to be where its at, no changing that. Im thinking Im stuck with what I have and drive it to the best of my abilities. Thanks for the info. JR

[pro-tour79]

It's Steve Smith, try this www.stevesmithautosports.com

[JRouche]

Originally Posted by pro-tour79

It's Steve Smith, try this www.stevesmithautosports.com

Oh, didnt know he had a book. Solly Carrolls' books are some good reading also. JR

[Bryce]

Question: does static camber affect the scrub radius?

[B_Alley]

It all depends on how you adjust static camber. Most cars do it by moving the inboard upper or lower control arm points. This increases camber and KPI together so theoretically it is the same- BUT now your contact patch is all distorted and its center isn't where it should be. Another thing happens when you increase camber- you change your track width. How much? We would just set our car on some pressure sensitive paper to find the center at different camber settings, but our tires are way different!

Also, when you add camber, you get Camber thrust, which is additional cornering force, even though the car isn't cornering. Sometimes it makes a difference, sometimes it doesn't, again, depending on what tire your using.

What a lot of Race Cars do is have your outboard balljoints mount on a clevis that then attaches to the upright. This allows you to change camber of the tire without changing KPI or messing with toe settings.