offset versus scrub radius Options

[byndbad914]

...will taking out that scrub radius and going to zero have any significant effect on steering effort?

Speaking from understanding and not actual experience so apply the appropriate amount of salt here, back in the 70s car mfrs put zero scrub into some large, land yacht car designs and the feedback was there was no feedback - the car was completely neutral at the steering wheel and therefore people would be wandering around the lane trying to keep the car moving straight. So, it is my understanding you do not want zero scrub, but a little offset inboard on a car like ours and the images JP attached above. Some modern cars have the offset outboard of the wheel centerline but I have heard that is not best for old tech vehicles and may have had something to do with working well with ABS but don't quote me on that. And again, no experience with that type of scrub.

Scrub is not a bad thing when, like all things, it is used in moderation. One of the main things scrub will do is heat the tires up, so on circle track cars, we used to futz with the scrub both left and right (we would run two different scrub amounts since we only turned left) at each and every track to get the tire temps to be optimal. Too much scrub will just cook tires and build your forearms into an impressive, Popeye-like shape (IMG:style_emoticons/default/smile.gif) Too little was also not optimal. I am trying to recall if we ever were able to get to zero scrub, been a long time now, but I don't think so, I think it was always some "positive" (inboard) amount.

[stownsen914]

I believe all car suspensions have some scrub radius designed in, so the steering has some feedback (consistent with byndbad's post above) - you need at least some. With a strut suspension, you get increased scrub radius as you put wider wheels (or wheel spacers) on - more than you need or ideally want. You'll get increased steering effort with increased scrub radius. With that said, it does seem to work out OK. People have been putting wide wheels on the front of 911s and 914s forever, and it seems to work out OK. Steering effort will be increased though.

I have 11" wide slicks on the front of my 914, and I've always found the steering surprisingly light. Then again, my arms hurt after a day or two at the track (IMG:style_emoticons/default/smile.gif)

On a related note, scrub and scrub radius are different things. Scrub is side to side movement of the wheel as is goes and down through its range of suspension travel. Scrub radius is the distance shown in the diagram in post #4.

Scott

[tomeric914]

Whatever you use, make sure your front wheels are as close to the strut as possible without rubbing on the strut or fender.

Wait a minute Andy, haven't you said to push the wheels out to fill the flares in other posts? Maybe that was for the trailer queen crowd? (IMG:style_emoticons/default/smile.gif) I agree with you here that the wheels should be as close to the strut as possible to limit SR.

[J P Stein]

Let's keep it simple.
A certain amount of Scrub is built into suspensions......split hairs if you want but it is measured as scrub radius.....the radius describes the arc of the center of the wheel rotating around the pivot point as located by (on a 914) strut.

SR provides steering feed back to the steering wheel and gives resistance to the act of turning the steering wheel....a good thing (in smallish bites) to prevent the "numb" feeling of a 58 Buick.

For track/DE guys, excessive SR is no big deal. When LARGE (AX style) steering input comes to the fore, things change for the worse.
My experience comes from here:


A 10 inch wide wheel with 5 inches of backspace.
A previously non-existent push in the tight stuff.,,,,that I was unable to eliminate thru normal methids....or abnormal, for that matter.
Sure, I got heaps of oversteer on the wide sweepers....but....

After fighting it for a season, I switched to 8 inch wide front wheels and problem solved.....the car responded to normal suspension tuning ...not perfectly, but something I could work with.. Do your own math as to the increase of SR over stock(a 5.5 inch wide wheel) & make up your own mind about the cause/effect.