Challenger does not make a 17" machine. You may or not be aware that different manufacturers use different numbering designations for their machines.
Some are referring to the width of the pad driver; others to what is supposed to be the cleaning path.
My Orbitec CX-20 (their biggest, with the onboard tank & sprayer system) has a 17" pad driver. The cleaning path is about 20" (probably slightly less) given the side-to-side movement of anything orbital.
The Orbitec CX-17 has a 15" pad driver.
My standard Challenger has a cleaning path of about 15", while the pad driver measures 13 1/2". The pad driver on the Challenger MAX is 20". I don't think they use a different number based on the cleaning pad, which is probably in the 21 - 22" range.
Mark's information on the Challenger is exactly right.
The Max has a 22" path... it is massive! The Challenger is designed for residential and office cleaning work, and the Max is better in large commercial settings.
So if we have a challenger and bought 21 inch pads, does that mean we really aren't getting the full clean out of the pads? What is the ideal size of the pad for the regular challenger to clean with?
I have a bunch of pads, no idea of their sizes, but i also have newer 21 inch glads, and 21 inch microfibers, along with yellow challenger pads(no idea on the size), and 100% cotton(smaller of them all).
The extra material out beyond what is under the pad driver really doesn't clean any carpet, but may do a nice job helping to remove soil from dirty baseboards.
I sometimes buy 16" and 17" pads from Argo (www.argoco.com) and these work fine. (Not enough material flapping around to clean baseboards, though.)
