My Sherpa developed a severe shimmy problem under heavy load. Despite applying my best efforts and calling on the collective wisdom of the Forum, the problem persisted. Robin Thorn, his designer and "test pilot" Andy Blance, shipping supervisor Cath Colenso, and the entire Thorn staff became involved in addressing the problem and made every possible effort to help and assist me. They communicated with me nearly every business day by email, and at their expense shipped a Thorn EXP rack and a replacement front fork in a different offset in an attempt to resolve the problem with this one rogue Sherpa. When it became evident the problem could not be addressed, they offered to replace my bike with a Nomad Mk2, since the Sherpa Mk2 was no longer available and the Sherpa Mk3 has a lower load rating insufficient for my expeditionary needs.
My acceptance of their offer did not mean the end of their help and assistance. They volunteered to test the new bike to be certain it would meet my requirements, and Andy loaded the bike with my maximum expected load and set off to do just that, as Robin followed in the company van and Cath caught it all on video, available for viewing here: http://www.facebook.com/Touringbikes
As I write this, the Sherpa is boxed in my livingroom, awaiting collection by Thorn for a complete analysis of the problem. This is simply outstanding customer service, and we can all rest easier knowing that Thorn stand behind their warranty and have gone far beyond the efforts of any other bike-maker I can imagine. My sincere thanks and gratitude to Thorn the company, to Robin Thorn himself, to Andy, to Cath, and to the entire staff for an outstanding job. I can't begin to describe the incredible care and detail with which the new bike was assembled, packed, and shipped; it is simply perfect, and performs as one would expect -- like a Thorn.
I have decided to leave this thread intact, as the efforts I went to in attempting to resolve the shimmy might prove helpful to others investigating any similar problem on their own bikes of any make. In doing so, I want to express my heartfelt "Thanks!" to each and every contributor and correspondent who tried so hard to help resolve my problem and supported my efforts to fix it on my own before I notified Thorn and asked for their help.
A summary of Thorn's efforts begins on Page 9 of this thread, followed by an explanation of Andy Blance's test-load and general-load recommendations in his own words on Pages 9 and 10. I thought them to be of such value, I posted them as a standalone sticky post here for easy reference: http://www.thorncycles.co.uk/forums/index.php?topic=4515.0
When and if the cause of my Sherpa's shimmy can be found, I will append it to the end of this thread.
And now, onto the original thread, as it played out...
All the best,
Sherpa has developed a severe shimmy problem, and I need to call upon the Collective Hive Mind of the Forum for help.
Earlier, I loaded Sherpa in a number of fully-loaded touring configurations and took him on long day rides. No problem. Solid as a rock.
Today, I loaded everything up and planned to take a shakedown tour to the mountains. Big problem. The bike was so unridable I had to walk it down 6% hills, which was no fun in racing cleats. I have given up on the shakedown tour, and am back at home to try and sort things out.
I made a quick little table and attached it as a .jpg so you can see what I carried and where. Compared to a desert crossing, I did not have any water in the MSR dromedary bag atop the rear rack. I would normally carry 6l there, so that is 13.23lbs/6kg saved there. As it was, my entire load *on* the bike was 82.09lb/37.24kg. Add my 172lb/78kg and the bike's weight of 42lb19.05kg (1.4lb was added with the ring-lock) and the total on the tires is 296.09lb/134.30kg as I rode it today. Earlier, he had worked fine with a load of 103lb/46.72kg carrying the extra water.
Here's what I found:
1) The shimmy was not always present. It never appeared below 9mph/14.5kph. It manifested from about 10-15mph/16-24kph, and then subsided until about 18mph/29kph and was present thereafter. It was horrible at 25mph/40kph and I once took the bike up to 45mph/72kph and really wished I hadn't.
2) The shimmy was worse when I removed one hand from the handlebars.
3) The shimmy manifested as a sine wave in the top tube, with an amplitude of about 1.5cm from center, and at a frequency of about 2.7 cycles/second. It was about as quick as you can say "Wogga-wogga-wogga" in one second.
4) Once started, the shimmy was self-perpetuating within a speed range. It didn't quiet down after a couple cycles, but just kept going.
5) I tried riding loose, tense, and hanging on for dear life; none made a difference. Neither did gripping the top tube with my knees or a closed hand.
6) I was riding only on pavement, but road surface did make a difference. Not the immediate surface, but the smoothness. Wavy pavement triggered the shimmy immediately.
Here's what I tried:
1) Adjusted tire pressure. Up to 55psi F/R. then down to 40 F/R. Then differentially, 55/40 and 40/55. No difference.
2) Moved both steerer mounted 1l bottles to rear rack-top. Very slight positive difference, but not enough to matter.
3) Emptied both steerer mounted 1l bottles. Very slight positive difference, but not enough to matter.
4) Emptied all three 1.5l bottles on frame. Very slight positive difference, but not enough to matter.
5) Removed and stashed handlebar bag in bushes. Very slight positive difference, but not enough to matter.
6) Re-oriented rack-top load F/R and then redistributed it side-to-side. Very slight positive difference, but not enough to matter.
7) Checked headset; properly adjusted.
8 ) Checked wheels; properly tensioned and true.
9) Checked tires; properly seated and true except for the usual slight wander of the tread cap.
10) All bags solidly mounted and noise-free.
11) All racks solidly mounted and noise-free. No discernible sway in either rack. The shimmy involved mostly the top tube from what I could see. I took a video, but gave up after a few seconds 'cos riding with one hand was almost impossible. Later, I may try posting the two clips I took.
12) Before emptying my water, all the bottles were full and slosh-free.
13) All heavy objects were placed toward the center of the bike L/R and F/R to keep the polar moment of inertia as low as possible. The load was well-balanced and well-placed.
14) Sherpa's frame appears sound in every way, with no bends, misalignment, or broken welds. I have never taken a fall on him, nor has he toppled over while parked. And yes, the wheel quick-releases are tight.
15) The packing scheme is the same as in earlier test runs, except for lacking the extra 6l of water in the MSR Dromedary atop the rear rack. Earlier, he did fine with a load of 103lb/46.72kg carrying the extra water.
Since everything is a known weight and modular, I think I will add or subtract modules as needed and then race up and down the street in front of the house to see if there is a difference. I've rearranged my schedule for the Big Tour, and right now, I can't take it, so I'm up against the calendar. I love the bike, have a sizable investment of time and money in it, and need to make it workable if at all possible.
This is essentially what happened to my Miyata 1000LT *after* my Great Basin tour in 2010, and led to my purchasing Sherpa. I carried 77lb/35kg of load then, and used the Thorn Low-Loader in front. I broke a weld on the last strut of my alu rear rack and replaced it with the Surly Nice (Rear) rack before retiring the bike due to the shimmy problem, which by that time manifested even without a touring load. Both racks were transferred to Sherpa and appear sound and solid in every way.
My thanks to all in advance. I do so hope I can come to a resolution that will resolve the problem and allow me to use the frame for its intended purpose, as it did so well on earlier test runs. I'm baffled.