Scrub radius Vs Tire width

[Bow Tie 67]

Current setup:

Street 18 x 8 with a 245 tire + 3.195 scrub

Track 17 x 9.5 with 275 40 R888 + 3.695 scrub


I will be widening the front street rims to mount 275 width tires.


My question:


Is there a direct correlation between scrub radius and tire width when it comes to traction.


Or maybe I should put it this way:


Would you rather have the extra rubber with increased scrub vs nominal scrub and a smaller contact area?


I am more interested in performance than daily driving / parking.

[tyoneal]

Current setup:

Street 18 x 8 with a 245 tire + 3.195 scrub

Track 17 x 9.5 with 275 40 R888 + 3.695 scrub


I will be widening the front street rims to mount 275 width tires.


My question:


Is there a direct correlation between scrub radius and tire width when it comes to traction.


Or maybe I should put it this way:


Would you rather have the extra rubber with increased scrub vs nominal scrub and a smaller contact area?


I am more interested in performance than daily driving / parking.
===================================
I Don't think It's a either/or situation. You want as close to, "0", scrub, as you can get.

If the suspension is set up correctly, you can have a wide wheel/tire combination and have very little, to "0" scrub.

Take care,

Ty

[Bow Tie 67]

Well " o " will not give you much feedback when at the traction limit. ( not good when racing )

I guess I should have titled it Scrub vs track width.

[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?

[JRouche]

:woot: 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?

Glad I looked at this thread. I didnt have a clue on what the effects of a zero scrub was. Of all the books I have read they talked about striving for a zero scrub, but didnt talk about the effects. How the car feels.

On my car it just happens to be zero, I didnt plan it, its just the way it turned out with the wheel backspace and size I am using. But Im not liking what Im hearing. I dont want a "light" or "loose" feeling wheel. I like to be able to feel the stress and even the sliding through the wheel. I hate surprises.

So with that, and the fact that Im still in the building, or tuning stages what can I do if anything to bring back some feel if I find it is too light?

I understand more pos caster will give a heavy wheel. But thinking about it, that might be bad. It will be heavy for straight line correcting. But will the heaviness of that amplify the light scrub and make it even more difficult to feel tire slip? ERRR!!! Cars!!! I should stick with my honda civic :)

Oh, and I expect the car to oversteer a lil due to the setup. I might be building for destruction from the sounds of it??????

Solly for the hijack. Maybe the answers will help the cause. Thanks, JR

[pro-tour79]

Scrub radius is mentioned and talked about in Steve Smith's books and Herb Adams of the top of my head, the actual scrub radius is not affected by tire or track width but the wider the tire the more contact patch you have causing friction, this friction is a load force that the driver feels as resistance in the steering wheel AKA road feel, problem is that this force comes from the tire side of the steering system that means the faster the steering box ratio is the easier it is for this force to push back against the driver's input, too much scrub radius can cause the front end to shimy under hard braking, and I'll harp on this one more time , these forces are the reason you don't put a R&P designed for compact/modern cars in our older cars ans as mentioned above by B-alley "stuff can start to break"

[Bow Tie 67]

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
http://www.youtube.com/watch?v=nmo_dkNZIHM&feature=related

http://www.youtube.com/watch?v=W8UiE7yvO_M&feature=related

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

[JRouche]

Scrub radius is mentioned and talked about in Steve Smith's books and Herb Adams of the top of my head, the actual scrub radius is not affected by tire or track width but the wider the tire the more contact patch you have causing friction, this friction is a load force that the driver feels as resistance in the steering wheel AKA road feel, problem is that this force comes from the tire side of the steering system that means the faster the steering box ratio is the easier it is for this force to push back against the driver's input, too much scrub radius can cause the front end to shimy under hard braking, and I'll harp on this one more time , these forces are the reason you don't put a R&P designed for compact/modern cars in our older cars ans as mentioned above by B-alley "stuff can start to break"

Steve? Maybe Carroll. I have read all his books, along with Adams' and Puhns'. And after all the reading I still didnt get a clear idea on how scrub affected the drive. Just how to change the scrub and what causes it. Thanks for the clarification. JR

[JRouche]

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]

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.