Carpets in the US and carpets in the rest of the world are very different. When Tacony private labeled our Riccar upright for Hoover to sell with a 240 volt motor in Australia, we used the soft, white bristles on the agitator. Wool is predominate fabric of choice for non US homes. In the US, not only are our carpets much thicker and far thicker pads underneath - there are acres of them. If you have 3000 square feet of plush carpet to vacuum, you need very aggressive agitation due to the speed at which you are going to clean.
In the Vacuum Cleaner Museum's 1910 - 1930 rooms, we have wool, oriental type carpets to match the decor of the era. I don't normally use a modern vacuum on those rugs. To be honest, a Hoover 541 does the best job, in the least time, due to its soft brushes. About once a month, I do go over those rugs with a Simplicity Freedom outfitted with the Horsehair brush roller. Even with thousands of visitors each year, coming off a gravel parking lot, the rugs look brand new.
The Engineering Department did a treadmill test with a Simplicity Freedom outfitted with a soft, horsehair bristle brush - on a thin, wool carpet. After 100 hours of vacuuming, there was very little nap removed, about the equal of a tablespoon. That's why Tacony makes different brush strips for their vacuums - so they can be customized to each home. For Berber, Wool, or other carpets that need gentle agitation, the horsehair brushes are ideal. The nylon brushes come in three different 'stiffness' designs. The most aggressive are used on models going into homes where there are shedding pets.
