The new Sherpa Mk3 uses the same Thorn 969 chro-moly tubing as before, but in different diameters and with revised sizes and geometry.
As the brochure indicates (Issue 20, late Spring 2012, pg 16, here: http://www.sjscycles.com/thornpdf/ThornSherpaBroHiRes.pdf
The smallest frames now use standard-size tubing (1" top tube, 1-1/8" down tube).
The small- to medium-size frames use oversize tubing (1-1/8" top, 1-1/4" down tube).
The large frames use extra-oversize tubes (1-1/4" top tube, 1-3/8" down tube).
The frame sizes
have changed, and these tubing diameters are distributed differently
among the frame sizes than they were for the Mk2 Sherpa. With the Mk3 Sherpas, Andy has taken the opportunity to more directly match tubing sizes to frame size and expected rider weight (for example...smaller riders are also lighter and get smaller frames and lighter, smaller-diameter tubing. The smaller-diameter tubing is still proportionally stiff because shorter lengths are used). The cargo capacites differ from the earlier range.
The standard fork is made from Reynolds SuperTourist cro-mo, and are designated ST-1 in the brochure. This is a heavier-duty fork than before and has an uprated cargo capacty according to the load charts in the brochure. It has
bosses to mount Low-Loader pannier racks.
The lightweight fork is made from competition-weight Reynolds 853. This lighter fork lacks
low-rider (Low-Loader) pannier rack bosses. This fork is intended for commuting and light touring. The brochure says this light fork can make the Sherpa a "formidable balloon bike".
Andy notes that both forks will take a 2.15" tire if desired, and both forks have mounting bosses for mudguards.