Hello guys, I've just recently started understanding that tuning the scrub radius can make a car more suited for a driver. I need a bit of input on what road-racers find a good compromise.

I kind of roadrace, but the races i do are something between road races and auto-x. I drive an ED6 and it's my daily-driver, so i presume positive Scrub would be way to difficult on my hands. Too negative however makes the steering so dull that I sometimes feel the understeer with my body instead of my hands.

1. has anyone figured an easy way to measure the scrub radius? I'm scratching my head and it doesn't seem easy given the fact that camber changes the center of the contact patch.
2. how much scrub radius (or offset) have you run before it gets unbearable? my targer speeds are 60-100mph and i do need the feeling in the steering wheel especially during hard braking.
3. I can pretty much figure that playing with the scrub radius will alter my camber while in turning, but i really need some input on this as i have no idea of how noticable this change is.
4. I usually run the front at 1/8-inch toe-out and i really feel that this combined with a bit more scrub than stock makes the car kind of darty and road-crest happy. Has anyone seen that or is it just the toe doing that or just the scrub.... or is it both as i think is the case.

thanks for your time.

 

You can read all about it in Carroll Smith's books, but I doubt that'll answer more questions that it raises...

Most of the fast Honda guys I've met are concerned more with fender clearance and track width than scrub radius. Its not that it doesn't matter. Its just that the changes to offset and wheel spacing have more to being practical than finding an ideal. Also, there are other ways to change steering effort and feel... A quicker rack, larger/smaller steering wheel and the amount of caster will all affect the way steering effort and feel.

An "ideal" scrub radius is something I've heard professional racecar builders argue about. There's no one right answer. Just varying degrees of compromise with the other variables and trade-offs.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 1net &raquo;</TD></TR><TR><TD CLASS="quote">2. how much scrub radius (or offset) have you run before it gets unbearable? my targer speeds are 60-100mph and i do need the feeling in the steering wheel especially during hard braking.</TD></TR></TABLE>
If you're running an '88-91 Civic without power steering, you'll get plenty of feedback through the steering no matter what. The steering feel is probably the best attribute of an EF/ED. The make/model/size of tire also has a huge impact on steering feel.

We run our ED7 at autorosses with 13x8" +25mm wheels and at track events with 15x7" +40mm wheels, 225/45 tires in both cases. No lack of feedback through the steering with either setup.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">4. I usually run the front at 1/8-inch toe-out and i really feel that this combined with a bit more scrub than stock makes the car kind of darty and road-crest happy. Has anyone seen that or is it just the toe doing that or just the scrub.... or is it both as i think is the case.</TD></TR></TABLE>
It's more the liberal amount of toe-out than anything. I've found that 0 toe up front greatly reduces the tendency for the car to pull randomly and follow the crown of the road.

Our wide 13" setup does take a bit more effort to turn at parking speeds and will be upset by uneven bumps more easily, but it's not something you'd run on the streets or at high speed events anyway and it isn't a big deal at all on the autocross course.

 

I do have power steering and i know this is part of my little feedback, but won't remove it yet mostly because i plan on putting and lsd in and i don't like fighting the steering... but that's a different story.

I love toe-out. I even love it in the back, but that proved to be way too scary for a daily driver, so front toe out will probably stay at 1/16 to 1/8.
What bothers me is that toe doesn't hurt my cornering capability, so i can leave some toe-out just for the turn-in fun. Offset however seems to affect my dartyness a lot and since i've been running toe-out for more than an year now, changing the offset greatly affects the straight line stability and turn in..... that shouldn't be the case, so I was wandering if what I'm experiencing is actually scrub radius change.
I usually run 195/50/15 on 15x7 wheels and I've tested 40, 36, 28, and 20 offsets. Of those the +20 offset ATS CUP wheels where about the crasiest and the more offset i use the more straight line stability.
I do know that the offset affects track width, weight transfer, and suspension stiffness... that's all quite clear, but i now want to learn about scrub.
I'm pretty much sure it doesn't affect only torque steer and braking stability... actually I'm pretty sure it affects the camber when the wheel is turned and as so it should affect the cornering ability.

Has any of you experiensed such things when changing scrub radius or do the crub change effects get covered up by the ofther things we change when altering offset?

 

Scrub radius is zero when your roll center is located at the ground plane. Unfortunately, this is nearly impossible with the Honda Civic suspension because the control arms are not parallel (infinite roll center) and usually the front control arms are canted upward slightly (depending on how much you've lowered the car).

If you lowered the car to the extreme, you might get a roll center near the ground, but obviously you have many other problems when you do that.

I wouldn't worry about scrub radius too much, and if you are really concerned remember that a stiffer set of springs will reduce the wheel travel and also reduce the amount of scrub during jounce and rebound. So, yeah, get some really stiff springs and you will get minimal scrubbing.