Cheers!

Just was wondering, if you moved up to bling bling wheels and they wheel offsets were different... say +32 mm or +35 mm instead of + 40 mm on a stock FC.

Would this change the scrub radius enough to affect performance?

88IntegraLS

I once read a post on a Miata forum where one member related an increase in steering effort with lower offset wheels (miatas use something crazy like +50, oh wait the FD uses that same offset).

So what is scrub radius? That is a technical term that I haven't learned yet.

DamonB

Scrub radius is the difference in distance between the center of the tire contact patch and a line extending through the center of the kingpin axis to the ground. If these two points are the same then you have zero scrub radius.

If the kingpin axis does not intersect the center of the contact patch, then you have scrub. It is called this because as the steering wheel is turned the tire must literally scrub or roll about an axis that is not in the center of the contact patch, thus "scrubbing" the tire across the pavement. Keep in mind all this is when viewed from the front, and is not to be confused with "trail" which has the same description but is viewed from the side. Trail works like a castor and helps the steering self center.

Increased scrub radius results in higher steering effort and a much stronger tendency for the steering wheel to kick back when the grip at the front tires is not perfectly balanced. Scrub radius and trail work together to give the car its steering "feel".

So to answer the question, maybe. If half the difference of the new rim width compared with the old is added to the offset, then scrub radius will remain the same. Anything else will change the scrub radius.

jhaywood

Just read this whilst trying to find spec measurements for the FD suspension....I think you are wrong wrt the effect of wheel rim width on offset.
The offset is measured wrt to the centreline of the wheel, so a change in the wheel rim width does not affect the offset!
Half of the difference in the wheel rim width is the change in the change in the backspace of the rim, specifically because the offset hasn't changed!
ie the backspace is half the rim width + the offset.

BTW, do you have the actual value for the scrub radius on the FD, I am assuming that it is zero, as that seems to be the best design, but I haven't found a definitive reference so far.

As an aside:
The effect of scrub radius on the steering feel is a bit of a mind bender, IF both wheels were upright and rotated about the scrub axis AND they continued moving perpendicular to the wheel centre axis, I think there would be no net couple as the couple on each wheel about the scrub axis would be equal and opposite, it isn't until you start to turn and/or add in the effect of castor that the couple on each wheel becomes unbalanced and you start to feel kicking do to the time variable imbalanced couples. At least that is how it seems to me, obviously best to balance the contact patch around the scrub axis (ie zero scrub radius) to get rid of the kicking!

dk_davis

The key point was this sentence, "If the kingpin axis does not intersect the center of the contact patch, then you have scrub.". That statement relative to your question depends upon how close the original offset and the changed to offset are to the ideal offset. If the starting point is optimal then any change will add scrub and depending on amount it may be noticeable. But say the original offset was 5 mm inside optimal and the new offset was 5 mm outside optimal, such as change may not me noticeable as the scrub is still 5 mm.

TeamRX8

iTrader: (2)

often overlooked; it includes tire diameter as well
.