Reducing scrub radius vs. track width

[baz67]

I just ordered my front wheels and I gave up some track width in order to reduce scrub radius. Both were small reductions. One of my goals was to increase the overall stability of the front end. I am curious what everyone elses opinion would be on what I did. Any and all are welcome.



Brian

[zbugger]

What exactly did you do? Did you just get narrower wheels? What's the balance gonna be between front and rear tires?

[Salt Racer]

[Devil's voice]...Build longer A-arms and revised knuckles. You shall have both small scrub radius and widest track possible....[/Devil's voice]

[baz67]

Damn Allen with the hard questions. 17x8.5 with 5.375 BS. There is a .250 overall track reduction(.125 per side), but also it reduces scrub. I am putting a 255/40 17 tire on it. That has a 25" diameter compared to a 25.7 on a 245/40 17. The shorter tire increases scrub, but I should be able to tuck it in more with the deeper BS and not lose turning radius or have clearance issues. My rears are 315/35 17 so that is one of the reasons why I wanted to widen the fronts. I looked into fitting 275s, however there was too many trade offs to fit them.

Fikse could not get me the 5.250 BS I was looking for. I only had the choice of 5.125 or 5.375. The 5.125 widened track, but also increased scrub radius. I chose the 5.375 for the reduction of scrub radius and the option of putting a .125 spacer in to achieve the original BS I wanted.

I guess I am just curious on what everyones opinion in loosing track to gain a recuction of scrub radius.

Brian

Brian

[Salt Racer]

On a typical Camaro (3600-lb running weight, 53/47 distribution, 59" front track) setup for very slight understeering, 0.25" less track width results in 3 lbs increase in lateral load transfer @ 1.0G cornering load.

I'd take reduced scrub radius anyday.

[wally8]

Salt Racer,

Could you expound a bit on your penchant for decreasing scrub radius?

I know we talked about it a bit in another thread and I said we use it for a jacking effect on circle track cars but: We're talking rims with only 1 to 2 inches of backspacing, lots of steering angle, quite a bit of caster and it's still only a very minimal effect overall. It really falls under the optimization heading not a mandatory setting in circle track.

I've read others who say that as long as you're not parallel parking, who cares about scrub radius. It seems many would say that you need at least some scrub radius to get the proper "feel". It would seem then that there might be other things that are more important, like increasing the track (that would have been my choice).

I don't want to highjack the thread, I'm just curious.....


Wally

[Salt Racer]

Well, I guess I should have been more specific. Sorry about that.

For track-only cars that never see bumpy surface at super high speed (like circle track cars), then my answer would have been the opposite. I may go with even shallower BS so track would increase at least an inch overall.

For other applications, I'm concerned more about steering stability. I strongly believe one of the reasons why late model vehicles drive so nice is b/c of small scrub radius.

1/8" reduction in scrub radius is very small and has minimal effects, but so is 3-lb change in lateral load transfer @ 1.0G load. Old cars typically have 3~4" scrub radius, and generally accepted max. number for cars driven on street is 2.0". I'd like to see smaller number on cars with wide front tires, like 0.5~1.25".

My car originally came with wheels with -0.3" offset, and crappy brakes. Somebody packed the drum on one side full of grease, and the car pulled to one side quite badly under braking. I cleaned it up, bled the wheel cylinder, and adjusted the shoes - it got better but the problem never went away for some reason under panic stops. It was nerve wrecking to drive the car on highway.

Then I swapped in my current wheels with +0.5" offset. The tires are shorter than original by 1.3" or so, so I'd say the reduction in scrub radius was 0.7" or so with KPI I have. The car was much more manageable under hard braking. The difference was greater than I had expected, to be honest. I would think shorter sidewall, grippy tires would compound the problem like this if the scrub radius was kept the same. Now that was with over-boosted power steering. I can't imagine what it would have felt like doing panic stops with original wheels and manual steering. I have also experienced similar things with my Jeep.

So that's my reasoning. For what Brian's trying to do with his car (driven regularly on street, frequent track visits), I'd go for smaller scrub radius.

[Steve1968LS2]

I just got done polishing a story on scrub radius.. learned a few things I will employ the next time I buy wheels..

Should be in the June issue..

[Steve1968LS2]

I strongly believe one of the reasons why late model vehicles drive so nice is b/c of small scrub radius.

Yep, you don't want zero SR because that would leave you with very little feedback. I must admit it is something I didn't consider when doing cars before. Modern cars run less than 2" of SR I believe.

[Salt Racer]

I just got done polishing a story on scrub radius...

So you probably saw pics of rusty undercarriage of Riviera and my fine artwork on a piece of cardboard then.

Modern ABS-equipped cars have much smaller scrub radius, actually. C5, for example, has +0.275" up front. FWD and AWD cars, as well as IRS-equipped RWD cars, have small amount negative scrub radius for similar but different reasons. It would have been nice to include those info, but I guess that would be somewhat beyond the scope of the article.

For visual effect, I recommend watching movie Lethal Weapon 4. There's a scene where bad dudes in a 4WD SUV pushing some poor guy's car on rail road track. You can see the front wheels of SUV swivelling back & forth. That's happening due to scrub radius.

[David Pozzi]

Salt Racer,
I believe my Camaro had a little over 2" scrub radius, it wasn't too bad considering but the power steering probably covered up some of the kick back. I'd like to have less SR when I put it back together. I think it's great for straight line stability. When you corner hard, the tire contact patch moves inward the most just as you lose traction. The driver feels the steering getting light and knows he's at the limit. Some drivers need to feel this to know what the front tires are doing. I learned to drive with overboosted PS so it isn't such a big deal to me.

[wally8]

Salt Racer,

Thanks for the extra info. I'm inclined to agree with the 1 to 2 inch numbers. I would think something around 1.5 would be quite tolerable on the street to the average enthusiast. 4 is way out there and those kind of numbers can jack as much as 30lbs onto the left rear with enough steering angle.

I agree with David that driver feel is a really important consideration, so in the end it should be whatever you feel the most comfortable with (within reason).

I would stilll trade out scrub radius for track width any day though (again, within reason). Of course this is SOP for circle track.

David,

Good point about CP deformation. That could be a good case to have at least a little scrub radius.



Wally

[David Pozzi]

I should correct myself, the tire contact patch moves inward when cornering hard, the rear part of the contact patch moves forward when the tire begins to lose traction. So there is less tire trail, it is felt as lighter steering.

another thing is if you have lots of scrub radius, the tire moves more front to rear and it takes a lot more room for clearance.

[USAZR1]

I am putting a 255/40 17 tire on it. That has a 25" diameter compared to a 25.7" on a 245/40 17.
Brian

Huh? That sounds like a typo,Brian. A 255/40 is taller than a 245/40.
Strange but my 275/40/17's are listed at 25.7".

[baz67]

Clint it is not a typo. You are correct. I just worded it funny. It says what you think is correct.

Brian

[Steve1968LS2]

So you probably saw pics of rusty undercarriage of Riviera and my fine artwork on a piece of cardboard then.

No, we photoshopped all that out.. you now have a chromed out undercarriage ;)

Actully it was cool, shows the car is actually DRIVEN. Good pics by the way and I feel the story will teach a lot of people something new. It is definetly an ignored aspect of suspension and handling

[Salt Racer]

...a 25.7 on a 245/40 17....

It's a typo - should have read 245/45 17.


David & Wally,
I agree about the steering feedback. Personally I think C5's 0.275" is bit too small. I think the production car can get by with it b/c of Magnasteer.

My car currently has 2.375" scrub radius. It drives good, but it can be further improved. Once I relocate and get back on my feet financially, I'm planning to build new knuckles and long A-arms. 275/40R18s on Kinesis 18x10s up front. I'm shooting for 0.75" scrub radius.

No, we photoshopped all that out.. you now have a chromed out undercarriage ;)

Ouch! People would think I'm a closet show car freak!

The car wasn't that rusty until last August. Bonneville flooded last year and I had to drive my car in 2-inch deep salt water. Oh well, at least I had a good time out there.

[USAZR1]

How do you figure scrub radius?

[Steve1968LS2]

How do you figure scrub radius?

Wait till the June PHR and find out ( )

Craig Morrison and Katz Tsubai did a great story with diagrams and lots of pictures and in this case a picture (or diagram) IS worth a thousand words..

A quote that sums it up:

Scrub radius is the measured distance at the road surface between the Steering Axis and the centerline of the tire

[dennis68]

Imaginary line running through both the ball joints and where the center of the tread on the front tires intersects at the ground...the difference is scrub radius. Did you guys just say that?

I miss all the good stuff.