Dan is probably the best to give advice on running max weights off road. I rarely ride off road but am much more careful to avoid bad potholes with the bike heavily loaded than not.
I know the answer is a bit fuzzy wooly but life is just sometimes like that.
Good thoughts, Andy, and I agree completely -- there's no hard answers to the question of how far you can push a bike's limits on rough roads without causing damage. For example, coming from a roadie background, I've never owned a mountain bike. I've always ridden and toured off-road on road bikes. I'm extremely easy on my equipment and have gotten away with it due to a number of factors:
1) I unload the bike as much as possible, so easing the strain on the frame. I do this primarily by standing or crouching on the bike when descending, allowing it to pivot at the bottom bracket and only lightly restraining the front end with a couple fingers formed into "o" shapes around the brake hoods. Unorthodox, but it works for me, and throughout much of the 1980s, I rode a 23.5lb touring bike with 56lb/25kg loads on really rough logging roads and fire trails with never a single failure of any sort except to the last strut of two triangulated aluminum rear racks (only half-welded to begin with, and each having a large stress-riser), and returned with my wheels true within a half-millimeter -- using 23mm road slicks. As much as possible, I try to let the bike "float" beneath me as I ride, seated or standing, and this helps greatly to ease stresses on the frame and components.
2) I take great care to use cinch straps on my bags,
making sure they secure the bag and contents to the rack. I have found this greatly reduces second-order vibrations by preventing the bags and their contents from lofting in and with the bags, only to crash down on the rack (and, by extension, the frame to which the rack is mounted). It makes a tremendous difference over time, and many of the stresses that can damage a frame are cumulative. When I see failures resulting from repeated stress, I shrug and say, "Just one brick too many". Anything one can do to reduce second-order vibration and pounding pays huge dividends in preventing fatigue failures.
3) I watch where I ride very carefully, and one of the great joys of off-road riding for me is "reading" the road in real-time as I pass over it. I class gravel as either benign or malignant, and aim for the benign as I pick my way.
4) I don't weigh a lot, being spot-on for my height and age group at 172lb/78kg. This is a sensitive subject, but rider weight is also cargo as far as the bike is concerned, and eats into ultimate luggage capacity. If one of average weight for one's size -- or less -- you'll have more leeway for carrying cargo. If you're well over the average for your size, then there is less headroom in the equation for carrying cargo. I do think human weight
can be a bit easier on the bike than cargo because it is sprung weight -- the rider can ease the loads momentarily. Of course, it can go the other way as well and then it is a perfect storm for failure. For example, when I was leading tour groups in the late 1970s/early 1980s, we had a range of riders. I was pretty light then, 5ft11in/180cm and weighing 145lb/66kg. A large fellow in the group was 6ft2in/188cm and 380lb/172kg. Unfortunately, he was not an accomplished rider, and always seemed to hit what others missed. It all came together in a sad way one day when he rode up a 3cm driveway lip at moderate speed with a handlebar bag. I looked in horror as the fork folded rearward right below the lower headset cup and the top and downtubes buckled. Fortunately he wasn't hurt, but the tire ended up right against the downtube. Of course, this was at a time when touring bikes has standard-diameter tubes and -- at least in America -- "touring tires" meant something like 700x25C. I often thought if Charles' bike had had oversized tubing and 26x2.0 tires (like many Thorns!), he'd never have had a problem.
An extreme example, perhaps, but it does illustrate how all the variables can come together to make a problem for one person and not another.
Another example: For my 2010 Great Basin tour -- a mix of pavement, goat trails, lots of extremely bad gravel, wet and dry playa, and cross-country through areas where rocks were the size of shoeboxes -- I used a 1989 Miyata 1000LT with conventional tubing and 700x32C road slicks. The bike weighed 109lb/50kg all-up with full water tanks and the only failure I had was a broken weld on the rearmost alu rear rack strut, and that only on the last day. I really think the key was riding style and tying the bags to the racks very tightly with cinch-straps. I also made sure no fasteners worked loose -- that's a direct route to fatigue failures of frame and components alike. It really is possible to get away with heavy loads on light bikes and skinny tires on terrible roads for very extended periods -- look at the late Ian Hibell (who traveled light but sometimes carried enormous water stores) and thousands like him over the years. However, it sure doesn't allow as much leeway as when you have some "padding" or "headroom" in the equation -- oversize steel frame tubing and wide, low-pressure tires. As a generalization, overbuilt frames will last longer,
provided they are overbuilt as a system. If you overbuild one part on an otherwise light frame, the tubing will sometimes fail just before it, due to the stress riser caused by an abrupt change in section width or gauge. Stress -- particularly cyclical stress -- is cumulative.
Steel doesn't fatigue, right?
Well, yes, it can given the right circumstances -- for an example, try bending a paper clip too many times. A friend owned a used-bike shop for 25 years, and instead of sending all his damaged frames (mostly buckled top and downtubes and forks all damaged by curb-strikes) to the metal-recyclers, he gave the nice ones to me with the proviso that I never sell or ride them. Agreed. I cut up any number of them, and used many to learn and hone my lugged- an fillet-brazing skills. When I was a regular member of a particular listserv, members would occasionally send me their out-of-warranty failed frames for analysis or possible repair (tube replacement). There was one prominent brand of some repute that had a problem with the lower end of the seat tube cracking clear through above the bottom bracket. There was a direct correlation between the tubing gauge used (too light) and rider weight/style (big guys who were mashers in their pedaling styles). There were a lot of cracks in the seatstays just ahead of the right-rear dropout and sometimes in the dropout itself as well. Seatstay and dropout replacements are relatively easy on brazed frames. Seat tubes...not so much. I wrote those off, but the rest of the frame tubing found its way into my various projects over the years, where it happily does just fine. I have a 1972 Windsor Professional (a Colnago copy made in Mexico of Columbus tubing) I bought used that has had its head tube professionally replaced at some time in the past -- as did most Windsors of that model and era. The problem? The head lugs were overheated during brazing, leading to early failure.
So, to bring this around to Thorn-related discussion and the Standard vs "X" model Nomads in particular:
Frank, I think if one were of average size (as you are) and ride relatively slowly (as you do) and only carried really heavy loads occasionally, you'd do fine with an "X" model Nomad and likely wouldn't risk a failure. The real thing to worry about would be the handling when fully loaded. That's where I think the difference in tubing would make a real and immediate difference in feel and how pleasant the bike was to ride with a heavy load. It would handle these big loads much less well than the standard Nomad Mk2.
Hope this helps.
Best,
Dan. ("No warranty, express or implied")
