Chad's right about the basic formula for figuring out loft/knot diameter being a 2/1 ratio. This is a great starting point for badger brushes. Typically, depending on the brush shape, (bulb, fan, flat top, hybrid), a variation of 2mm either way is enough to reduce or increase the backbone. Another consideration would be the density of the fibers being used, and type (different shaft thicknesses). Silvertip 2-band badgers typically have thicker shafts than their 3-band bretheren, and thusly have more backbone naturally, but will generally not be quite as dense because you cannot fit as many hair in the knot due to the larger diameter shafts.
Awesome, but how does this translate to synthetics?
Synthetics, especially the newer generation, have a "corrugated" or wavy pattern on each fiber instead of being straight and smooth like a natural hair brush. This leads them to feel much more "scrubby" than a natural hair brush. Since they are basically plastic, they are going to be more resilient, and will be more "springy", and have little to no break in period. As pointed out earlier in this thread, you're going to get a very similar experience on shave 100 as you did at shave one. Synthetics typically are all fairly close in fiber thickness, though types such as the Mother Lode knot (an oversized make-up brush) and Cashmere knot are more thin, leading to less backbone. You would think this would mean they are more dense as well. For the Mother Lode knot, this is absolutely the case. It's supposed to be so dense as to be a lather hog, and has very little backbone, even though it's so dense. Not so much for the cashmere, which is soft yet if the loft is too high, it will end up floppy. I've yet to use a mother lode or timberwolf knot, so I can't give any personal anecdotes on my experiences with them. But I have used plissoft, Blackwolf, tuxedo, cashmere, BOSS/AMAK Gamechanger, and Silk Smoke knots. Here are my notes on their performance characteristics below:
Plissoft: somewhat soft tips, medium to low density, medium backbone. Nothing outstandingly good or bad with this brush. Very springy fibers.
BOSS/AMAK: least soft tips of this bunch, but not unpleasant. More dense than the plissoft, but still nothing more than what would be classified as medium. I use this knot as a baseline for trying new soaps.
Tuxedo: Softer tips than both the Plissoft and BOSS/AMAK knots, medium density, but backbone wise packs quite a punch.
Blackwolf: This is the original "tuxedo" knot that was sold by Peter Wolf of Wolf Whiskers. It's much different than the tuxedo I own, being much more dense, and the tips are MUCH more soft, falling just short of the cashmere. Backbone is moderate...somewhere between the BOSS AMAK and Silk Smoke.
Silk Smoke: I have a bulb in this, 24mm, and I believe Eric set it at 50mm, possibly 48mm. Much more backbone than any of my other synthetics. The tips are about as soft as the cashmere, however. It's an interesting juxtaposition to have tips that soft with that much backbone.
Cashmere: Slightly more soft than the Blackwolf and Silk Smoke, but MUCH less backbone than either. More dense than any of the other synthetics I own with the POSSIBLE exception of the Blackwolf knot. I have both a fan and bulb in this knot type. (the bulb is set much deeper and exhibits quite a bit more backbone than the fan which is set to 52mm loft on a 24mm knot.)
Hope this helps.
