Does more scrub radius mean less grip?

Hello all!
I know this topic about scrub radius has been discussed before, but I still want to bring it up because I'm about to buy tires and I want to avoid a mistake.

I have a 1990 miata, stock subframe, arms and knuckles.
A while ago I bought some very cheap TR wheels from a $2000 challenge guy. 15x8, ET20 wheels and 225 RS3s. At the time I didn't pay attention to the offset and I also didn't put much thought into it, but now I want to make changes.

From day 1 the car was much harder to drive. While it had much more grip, it was an active duty to keep it straight on the highway. Any surface changes and the car would steer according to the ground. Same on truck tracks on old asphalt and when braking hard. For a while I've been thinking this must be the wheel offset, but I wasn't sure if it was the worn out tire (I can't recall how it was when the Nitto's were new)

I reinstalled the stock daisies (45mm offset) with the same all seasons I had when I bought the 15x8s because I needed to drive in the rain and the nittos are beyond the wear bars. And the car drives SO MUCH BETTER. Easier to steer, more predictable, goes straight on the highway, same alignment.

I also notice that when doing tight turns around parking lots, the outer wheel no longer plows the ground. And this got me thinking about the outer wheel plowing in tight autox turns.

The one thing I know about friction, is that static friction is much higher than dynamic, so in my head side scrub = dynamic friction = significantly lower grip and also much higher wear every time the car is not straight.

If the paragraph above is true, then I should be looking for wheels with an offset closer to the stock one. In 15x8 the only thing I could find is ET35, still 10mm out. Before I buy some konigs, I figured I'd ask the smartest people on the interwebs.

Is my setup as suboptimal as I'm making it to be?
How much of a difference would this make considering the wheel is still further out than where it wants to be?
is this worth $700?
Has anyone gone through a change like this and experienced more grip/more stability/something?

(Miata specific)
Since the NB wheels have an offset of 40, would I have to change the control arms and knuckles for an NB in addition to the change in wheels to have an offset that's only 5mm off the stock?

Thanks!




I know that NB wheels have an offset of 40 instead of 44, is there a difference in the mounting points of the control arms?
 

That big jump in tire size is having a big effect. The lower offset of your 15x8s is going to give you more kickback through the wheel, but the tramlining and tendency to follow the pavement is much more likely to be from much wider tire with a stiffer sidewall. Especially if they're running very low on tread. The lighter steering is going to be partially from the offset and partially from the smaller tire and even from a lighter wheel/tire combo.

NA and NB have almost exactly the same pickup points for the suspension - same in the rear, some changes to roll center and caster in the front. Mazda detailed all these changes (2mm here, 3.1mm there) but didn't bother mentioning the change in wheel offset. That says something right there.

You have to drop your offset as you go to wider wheels/tires or you'll end up fouling your suspension. That's why you won't find a +45 15x8.

It's worth noting that Spec Miata racers will go for as low an offset as they can manage IIRC. It's faster even without any negative effects. I'm not sure exactly what you mean about the front wheel "plowing the ground" and how offset affected that.

The best way to test this is with wheel spacers, because that changes the offset and keeps everything else the same.

Sounds like it's more likely an alignment issue or some other suspension issue that either the all seasons are masking or the worn out summer tires are accentuating. I'd at least start there before dumping money into throwing parts at it.

In reply to Keith Tanner :

When I go full lock with the 15x8s, I can hear the outer wheel drag as if it's not following the radius the car wants to follow. Sort of a very slow understeer. If I were to make an extreme example, imagine putting a 10in spacer, the wheel instead of rotating on an axis that crosses it, it would swing around the car. I believe that the lower offset causes some of this, at a much smaller scale. If you've seen a kart wheel when you turn them lock to lock you'll know what I mean. 

I've tried the alignment route many times, and even with perfect numbers it wasn't a great drive. I mean it goes straight, as long as the road is great.

I think you're trying to describe ackermann. I have a hard time believing that an inch of wheel offset would have a discernable difference. More likely you've got a stiffer tire that's not as willing to deform. You've zeroed in on scrub radius as the defining difference when you have massive changes in tire size, tire construction and overall weight.

Yeah I think scrub radius is only a small part of the handling difference with your wheel setup. Increased tramlining comes with increased tire width and scrub has approximately zero effect on traction - if you can force the wheels to point where you want, they'll grip just fine. Karts have a ridiculously huge scrub radius with the whole tire clean outside the scrub point.