Dave,
Others have answered most of your questions going forward, so I will just do a quick recap before delving into the differences between the racks, as I own both Thorn and Tubus (and Surly, and others) and can do a quick comparison for you.
• The Nomad is the only Thorn in the current lineup with 6mm rack mounts. All others are 5mm.
• Thorn, Tubus' Evo series and Surly's Nice racks all accept either 5mm or 6mm rack bolts. The Tubus Classic series may accept 6mm, but the hole is a little smaller, and the powdercoat might scratch a bit when the bolt is turned into the hole. However, I see no problem in using 6mm bolts on it. So: All the racks accept either 5mm or 6mm bolts.
• Tubus has recently (last 18 months or so) overhauled their lineup, updating many of their models to the new "Evo" (Evolutionary) design. These have a "3-D" investment-cast lower mounting point that is stiffer and offers better bracing at the loss of a built-in secondary mount for the rear fender stays. The hardware and design for the upper stay attachment has been streamlined and offers two ways for attachment. Some racks have been also been modified at the forward part of the upper deck. The Cargo, for example, loses its two "bullhorn ends" in favor of a more conventional "closed return" of the tubing on the top deck. Functionally, the old and new Tubus racks perform the same, but I generally prefer the newer designs incorporating these evolutionary changes. They are a bit more developed than the originals and are are sleeker to my eye. I also prefer a closed return for the top deck tubes, as it provides another lashing point and causes less rubbing to cargo strapped atop the rack.
• Tubus racks are very adjustable with a minimum of fuss and spare stays in a variety of lengths and bends can quickly solve even difficult fitting challenges without fabrication.
• mickeg makes a good point: Tubus' racks are available in a variety of designs. Some may be better suited for your needs than others. Some are targeted toward the minimalist and place and emphasis on weight savings over cargo capacity. Others are more general-use in design, while the remainder are geared toward loaded touring, making them less versatile for other needs. For example, I like my Logo Evo very much, but the design leaves a top deck too narrow to support a rack-top pack. I modified mine by using P-clamps to secure a sheet of DuPont Zytel to make a wider top deck to adequately support my rack pack. Tubus racks are good, but should be chosen to match your needs for ultimate success.
• Thorn's own Expedition racks have been superb in my own use. I was initially put off by what seemed a greater weight and less versatile design, but have not found them lacking. They do have features that are sometimes overlooked:
REAR:
• Heat-treated.
• Top deck is extended so a taillight is not obscured by a rack-top load or panniers.
• The rack includes a choice of either another steel tube cross-brace or a versatile taillight mounting plate for the ends of the top rails. The taillight mount is pre-drilled so a variety of lights can be easily attached, or new holes can be drilled. Like Tubus, replacement parts are available in case of repair or modification.
• Extremely rigid due to direct triangulation and very thick mounting points.
• Nice, wide top deck supports a rack-top load nicely.
• Rear rack has threaded bosses to allow direct fitting of the mudguard (fender) stays, making for rattle-free fitment.
• A much thicker, more durable coating than is found on Tubus racks.
• Mounting brackets require some fabrication to fit: They must be bent in a bench vise, then cut to fit. They are made of thick stainless steel with multiple holes, so no drilling is needed. Once fitted, the rack cannot be transferred to another bike of a different size or make unless new mounting brackets are bent, cut, and fitted, so less convenient than Tubus in this regard. The upside if you leave it on one bike is it becomes a virtual extension of the frame.
• In my own deflection tests, Thorn's Expedition rear rack was most resistant to lateral deformation and remained the most resistant to lateral sway. Several Tubus designs came in a close second (depending on design), and the Surly Nice Rack (Rear) was last, mostly due to the narrow squashed-tube mounting points. Some Tubus designs were lightest, the Thorn a bit heavier, and the Surly was heaviest of all but least rigid in my tests. The same observations held true for the front racks I've compared and tested: Lightest was the Tubus (Tara and Duo models, in that order), Thorn was a bit heavier, and Surly heaviest of all but least resistant to side loads in my own testing, again mostly due to their mount design, which is very robust in resisting vertical loads over lateral loads.
• All these racks have coated tubing. In my testing, Tubus' finish was most frangible and subject to damage, but they offer the possibility of clear, stick-on tapes to prevent damage caused by hook vibration. The tapes must be applied when the racks are new. My luck with them was uneven. Both Surly's finish was much more resistant to vibration-caused damage, while Thorn's finish was thickest and most robust. Ortlieb's glass-filled nylon pannier hooks are more likely than some other brands to cause damage (Ortlieb and Tubus are allied companies and Tubus racks are designed with Ortlieb bags in mind, but are versatile enough to fit all brands). Some Forum members have had good luck attaching various kinds of tubing to protect their racks of all brands, padding them out from 10mm OD to 16mm and dispensing with the sometimes loss-prone sizing adapters found on most Ortlieb pannier hooks.
FRONT:
• Like Tubus' Duo lowrider, the Thorn Low-Loader MkV is intended for attachment only to forks with bosses on both sides of the fork blades. Thorn's mounts are a fully threaded cylinder brazed onto the fork blade without piercing it, so match this requirement.
• Thorn's Low-Loader can be modified to fit by bending it.
• It will accept 5mm or 6mm bolts.
• Is heat-treated.
• Like Thorn's rear rack, it is an extremely rigid design with a more robust powder coating than Tubus.
• As I recall, Thorn's heat-treated chromoly steel rack tubes have a 1mm wall thickness. The inner brace on Thorn's front racks is made of anodized aluminum, as are the spacing blocks.
In actual use, racks appropriately chosen from the Tubus lineup, the Surly, and the Thorn racks all perform well for most needs and the vast majority of users are satisfied with their purchases. If lightweight is the highest priority, then choose carefully from the Tubus lineup knowing some models favor light weight over cargo capacity and vice versa. Surly make just one model front and rear rack, as does Thorn. All three have proven "strong enough" and breakage is rare these days. Tubus does offer a 5-year warranty against breakage in use, but have denied claims arising from blowover and fallover. I've been very pleased with my Thorn Expedition and Low-Loader Mk V racks for heavy expedition use and have no complaints whatsoever about their performance.
Best,
Dan.
