Huge wheels and debead risk: Real, outdated, or bunk?

So after watching Brake_L8's video on the C8, I notice it has 21s in the back. I remember when that was a specialty wheel size mainly intended for use in rap videos, and now here they are on a mass-produced high-end sports car that means business.

Some "conventional knowledge" has built up among gearheads that the risk of a tire debead (under hard cornering and especially with some slip angle) begins to escalate sharply when you go beyond 18" wheels, due to the ratio of bead size to tire diameter IIRC (and also the contact patch's leverage on the bead?). This was most prominent around the time when the R35 first launched - with knurled bead surfaces on the rim to prevent this. Is this still an issue, something that was once an issue and isn't anymore, or a freshly baked old wives' tale with nothing behind it? Just curious.

If we were talking about escalades and 28's yeah, probably a problem. On a ground up design sports car with modern tires spec'd for it? I highly doubt it.

I was also thinking that making the tire beads stiffer and less elastic could mitigate the issue, maybe a question for the next GRM "ask a tire expert" article?

I seem to recall the R35 knurling as being to prevent/reduce the tire from slipping on the rim during extreme straight-line acceleration/deceleration, not debeading during cornering.

I also don't recall having heard this particular 'conventional wisdom' before, but at first blush it doesn't make much sense to me. You actually now have more total contact area between the bead and rim, at the same (or at least very similar) pressure, resulting in greater total friction force at the bead. And the contact patch leverage on the bead would seem to be primarily the result of the sidewall height, and having little to do with overall diameter. Feel free to prove me wrong, but it sounds to me like a bunch of people making up and/or parroting pseudo science to justify their fear of change.

Sidewalls and beads on low profile performance tires are ridiculously stiff.  Not a problem in my experience.  I don't think I would do a clutch dump burnout on the way out of Discount Tire, but after a few heat cycles, the beads are almost glued on.

I have a car with 30" wheels.  They are also clincher wheels, which was surely meant to keep the tire on the wheel.  I would be very surprised to see a modern tire on a modern wheel break a bead solely due to tread/road surface adhesion.

Knurled beads are to prevent slipping, they wouldn't do much of anything against debeads. I could see issues with rubber band tires on comically oversized aftermarket wheels, but these have just as much sidewall height as other comparable size tires, do I don't see any donation changes in leverage. Actually the current z06 tire is 335/25/20 versus the new 345/25/21, so the new tire actually has taller sidewalls. For reference, the new GT3 has also gone to 21" rear wheels, though a narrower size on a narrower wheel.

The question that I have is how common is the ability to mount tires on a 21 inch wheel? 

Anyone that can mount a 22" tire can do a 21". All the machines I have ever used just keep allowing bigger sizes, they don't have exclusion zones in between sizes. 
 

In a former work life I built machines that made tire beads and apex's (the little triangle of rubber that attaches to the bead and helps to establish side wall stiffness). The beads are made from high tensile steel wound many times (depends on the tire, in the range of 20-40) around to form a hoop. It is rigid as can be. Before they are wound a layer of river is extruded so the bead is adhered together.  It is really cool stuff. You can find videos of copy machines online. One exception to the tire bead thing is Goodyear, they make theirs different and I know nothing about it. 

In my 40+ years of working on cars and tires, the only one I have seen de-bead had broken wires in the bead.  

I've had tires come in that were burned right off from running flat, and the beads were still in place.  

Debeading of tires doesn't happen without a wheel getting broken or badly bent, or an off road guy doing off road things with very low pressure.

As far as mounting, the bigger issue is width. My friend's machine does up to 24" wheels IIRC. I think he got that size mainly for the extra width capacity but even then mounting 315s on 12" wheels is really pushing its limits. 345s on a 13" wheel wouldn't remotely work.

It's definitely possible to debead a tire on-road, but it requires something to be wrong, most likely low pressure, combined with limit handling. So pretty unlikely all things considered. In this case, it was some idiots autocrossing a fwd car with like 20psi in the rear tires. One of them had a big spin and knocked off one of the rear tires.

Streetwiseguy said:

In my 40+ years of working on cars and tires, the only one I have seen de-bead had broken wires in the bead.  

I've had tires come in that were burned right off from running flat, and the beads were still in place.  

Debeading of tires doesn't happen without a wheel getting broken or badly bent, or an off road guy doing off road things with very low pressure.

Have seen plenty of tires come off the rim from being driven flat, but as you say have also seen a few times where there are two little pieces of sidewall still firmly seated on the wheel, and that's all.

Not bringing rallycross into it, where I have seen people debead rally tires at 35psi, because that is far off the pathological end of the curve.

I've also never heard this "conventional wisdom" and I've been modifying cars since 1998.

Streetwiseguy said:

In my 40+ years of working on cars and tires, the only one I have seen de-bead had broken wires in the bead.  

I've had tires come in that were burned right off from running flat, and the beads were still in place.  

Debeading of tires doesn't happen without a wheel getting broken or badly bent, or an off road guy doing off road things with very low pressure.

It's rare but every once in a while a special kind of jack ass manages it..............................

I used to pump the tires on my mothers' Pinto to 45-50psi because I was forever popping the beads on them. 

Because the car had very soft suspension with a bit of technique you could get the rear wheels to lock first; I was forever doing late nite drives sliding the car into corners on rural roads.............some intersections had pavement seams that were just thick enough that it'd pop the bead with normal tire pressure. 

To your point about off road; out at the lake I'd slide off the gravel roads onto the pave road.........I stopped routinely doing that after I had to buy a couple if rims.