Likewise, I'm not looking to prove anyone wrong, rather I'm more interested in discussing the tech behind it. Like anything, there's many factors at play. Is something enough of a concern to pay attention to or address, that is always open to ones interpretation and even I'll take liberty on things in my own modifications. Yet some of these things are firmly understood with real implications. The degree of which may or may not matter to the individual depending on driving style and tolerance. OEMs do sweat these details.
To answer your questions:
1) Do we really know what "proper" offset is? I know we started with +60 and 31.5" wheels, but what criteria was Toyota trying to adhere to?
Yes, we can assume that stock is proper to the suspension design. The other data point I'm using is the Rock Warrior wheels at 50mm offset, designed for a 32.7" tire (@gaijin keeps reminding me that I scramble my numbers on, thanks!). The 50mm offset there is not by chance. From there, we can linearly interpret for larger tire diameters. This picture should help and is the literal depiction of why it is what it is.
View attachment 1904871
If one increases the tire diameter, you can see that the zero scrub radius intersection point won't be at the contact patch of the tire. And one should push the tire further away from the chassis to meet that point again. That intersection point is important as that's the design point at which forces on the tire will minimize steering feedback, ie pulling on the steering wheel. It's also important to turning traction and handling. In a tight turn, tires will scrub (hence "scrub radius"), i.e. squealing in a parking lot). There's more, but I won't go farther than this, and there's plenty of documentation out there.
2) How sensitive is the LC to being slightly out of optimal? Another site I read talked about cars in the old days commonly had 4" of positive scrub radius.
Not as sensitive as passenger cars, and especially not like sports cars. 4" doesn't mean anything as that was integral to the design of that particular vehicle. Trucks/SUVS naturally are setup to greatly reduce feedback. Otherwise off-road, the steering would be kicking back hugely on every bump. But it also depends on ones driving style. The harder one drives, the greater the forces at the tire. Bigger forces, greatly reduced, may still result in kickback or pulling at the steering wheel. So when one says they have no symptoms, it may also be they drive like a granny or cannot perceive it. In suspensions, everything is a convolution too. Simplifying the discussion too much dismisses that alignment, caster, tires, conditions, can all contribute or help the situation.
3) In this case I am NOT running spacers, so my tire diameter is .4" larger (.2" radius) and my +20 vs your +35 is .6". Now, admittedly, that does increase the positive scrub radius, but very little. So, is it enough to really matter? I honestly don't know, but my intuition says it shouldn't. If I look at the symptoms of too much positive scrub radius (pulling under braking), I don't seem to have them.
My setup does indeed have too much scrub radius by ~10mm. Yours is off by ~22mm.
Marcuson inadvertently noticed .75" (19mm). I notice 10mm, but I drive hard and tracked sports cars in a previous life with lots of wheel offset combinations so I know what it feels like. I've considered going down to .75"spacer to optimize my setup, but am staying with 1" spacer adapters as they are stronger and safer for towing heavy, and I'm okay with the minimal symptoms.
4) If I look at the symptoms of too much positive scrub radius (pulling under braking), I don't seem to have them.
Note that it will pull only when forces are not equal between tires. Braking, and hitting a rut or bump on one side. It's when forces are unequal, that the pulling is more pronounced than they would otherwise be with proper offset.
Hope that helps. Getting to "proper" in not important as long as it works for you.
*Note that all these specific numbers only apply to pre-'16 models. The suspension upright (and geometry) was slightly changed on later model years. Yet the general trends are the same.
