Sherpa Shimmy resolved with superb warranty response by Thorn Cycles

[Danneaux]

Hi All!

Thanks to my testing there's some new ruts in the pavement; unfortunately the results are nearly the same.

1) Sherpa does wonderfully with my front load (L/R panniers, HB bag) alone.
2) He does wonderfully with my front load and all the water (including the two 1l bottles on the steerera nd three 1.5l bottles on the frame).
3) Add either the rack-top load (11lb/5kg) /or/ the two rear panniers 12.5lb/5.7 each,25lb/11.3kg total and he does fine.
4) Add both the rack-top load /and/ the rear panniers, and the wheels fall off the cart, so to speak. Shimmy City. At least the results can be reliably replicated and are repeatable. I repeated all the prior two days' testing and I'm beat. Fit, but beat; interval training is a great builder of stamina.

I'm puzzled, too. I can't account for this happening.

This is with the 700C-sized Tubus Cargo Evo rack. Unlike the Surly, there's no loose-fitting parts and the rack doesn't sway when I poke at it. Moves maybe 2mm with a lot of lateral pressure on the last strut. I'm building an instrumented test rig to see how much pressure is involved, but it seems to be plenty stiff by any reasonable standard.

I've added a few more cells to the matrix of test permutations:
1) Tire pressure makes a difference, but only by stiffening the sidewalls and thereby changing the amplitude and frequency of the shimmy slightly. I used differential pressures F/R to see if there might be any difference due to effective pneumatic trail (thanks, Stuart, I kept your suggestion in mind). With 63mm of trail, the geometry of the bike is spot-on for what it needs. Really, I can't see any reason for shimmy and am utterly baffled by its appearance.
2) I swapped-in the 1.5in slicks from the tandem. No real change. Didn't fit them differentially, though.
3) I swapped places between the front and rear panniers (nearly the same weight in each, I wondered if form-factor might make a difference.
4) I checked the loads in the bags, and sure enough, all the heavy stuff is low and toward the center of the bike fore-aft and left-right.
5) I tightened the compression straps on all bags.
6) On the rear packers, I loosed the built-in compression straps and widened the bags L-R so the weight could drop further down, but at the cost of sitting further from the center, laterally. This left the entire top half of each rear Packer Plus pannier empty, so the weight really is low.
7) I checked the hub cones and the quick-releases. No problem.
8 ) The tires are well-seated on the beads. There is a small wobble in the tread of the Duremes from new, but it has never been a problem.
9) I didn't just check the headset; I re-adjusted it from scratch. It is perfect and still there is a problem.
10) The bike shimmies with hands off the handlebars.
11) The bike shimmies when standing. And, when standing with no hands and the saddle nose grasped between my legs (Not a Good Idea, but worth a try).
11) Removing the Ortlieb underseat bag and moving the rack-top load forward beneath the saddle till it grazed the backs of my thighs didn't make any difference.

Bottom line: The bike just is not happy with a rear load of 36lbs/16kg arranged as it is. I guess the next thing is to dispense with the rack-top load and put the equivalent in my free weights in the rear panniers to see if it is the weight or the placement/distribution back there that is causing the problem.

I've done about as thorough a forensic job as I know, and all the problems seem to stem from the rear load, since no other combination presents any problem at all. By the way, the frame looks perfect, with no sign of cracks, fissures, or fractures anywhere. There's no creaks and such, either.

Man, I just have to be missing something, but what...? Any further suggestions are surely welcome, and I truly thank you all for your generous ideas and thoughts to date.

Logo arrives tomorrow, with its lower load-rail. The rack itself is 3mm lower than the 700C-sized Cargo Evo currently on the bike, and the load rail is 58mm lower yet, so the hooks will sit on the rack 61mm/2.4in lower than they do now. The Logo Evo also has a narrower top deck and the entire rack leans in toward the centerline, effectively triangulating it at the front and rear against lateral movement. The top deck is wedge shaped in plan view, further triangulating it against movement.

Best,

Dan.

[jags]

Dan any chance you could  put up some photos of the bike fully loaded.

[Danneaux]

Dan any chance you could  put up some photos of the bike fully loaded.

Love to! The trouble is, the option to do so is no longer available to me from my "Global Moderator" account on the Forum, and I have to figure out why. I am pursuing this elsewhere, but in the meantime...I can't post piccies.

Will as soon as I can.

Best,

Dan.

[il padrone]

Nomad is calling.......  calling......... !!

 




Alternatively, take your rack-top load off and put it in:

[Danneaux]

Nomad is calling.......  calling......... !!

But I can't heeeeeaaaaaar it....! 

It'll have to call louuuuderrrrr!

Chuckling,

Dan.

[stutho]

Dan,

My bad.  You should now have the option to attach. 
Sorry.

[JimK]

It'll have to call louuuuderrrrr!

I can imagine these interval work-outs are exhausting, getting a heavy load up and down a hill umpteen times. But I bet you will figure out what's going on and get your Sherpa system working properly.

I think you're smart to try the abstract away the details of the load by using simple chunks of iron and then moving those around in various configurations. Is it the total weight, the height of the center of gravity, or some lack of rigidity in the system. If you have the free weights, maybe you could just take the panniers out of the picture too by strapping the weights, cord through center hole, straight to the rack, with some newspaper to protect the finish. Maybe even newspaper between each pair of disks so the free weights don't jostle around either. Our teenager has a couple 25 pound disks. Doesn't take many of those to simulate a significant load!

This is a tricky puzzle but with your systematic approach, you will get it figured out!

[triaesthete]

Hi Dan
do you think your (unusally?!) high pedalling cadence may be a cyclical factor in all this? You could redo all your tests in a higher gear and really sleep well.
Just a thought
Ian
PS I love the wordplay in the title of this thread.
 

[Danneaux]

Hi All!

Catching up...

jags: Piccies, as requested. They're not as crisp as I'd like 'cos we took them at 21:00 hours on the first day of summer and there wasn't much light for moving subjects. These photos show the whole shootin' match atop the Surly rack. That includes 6l in the MSR dromedary water bag atop the rack in this photo. The big red dry sack contains my -15C 3lb/1.36kg winter sleeping bag, my 2.16lb/.98kg pad, silk liner and air pillow, and weighs 6.15/b/ 2.78kg total. The smaller, black sack is my tent, complete with footprint and dry sack for the inner compartment, and a variety of stakes ranging from Scandium sand hogs to titanium needle stakes, weighs 3.76llb/1.70kg. Total for the sleeping system is 10lb/4.5kg. With six liters of water, the 10l MSR Dromedary sack weighs 12lb/5.46kg and allows about one refill of the 6.5l in bottles on the frame and steerer. The rear panniers are carrying 12.5lb/5.7kg apiece, so 25lb/11.3kg for the pair, and the weight in them is packed low and forward and compressed laterally with the Ortlieb's own compression straps and my added ones that also tie them to the rack.

Since these photos -- and with the switch to the Tubus Cargo Evo -- the tent now goes under the underseat tool bag. The red dry sack goes forward right against the tool bag, putting the dry sack's weight just ahead of the rear axle. The MSR Dromedary has been switched-out for two smaller bladders with the same capacity, one in the bottom of each rear pannier. All panniers are secured with compression straps that also secure the ends of the red dry sack. The dry sack and tent load as a module, held together by webbing and Fastex buckles. They're then cinched down using weblock Arno straps. Neither the panniers nor rack-top load wiggle or bounce in any discernible way.

The Tubus Logo Evo has now made it to Springfield (yes, the one creator Matt Groening has admitted he used as the inspiration for the town in The Simpsons) and should come across the bridge to me here by 16:30 this afternoon. If all goes well, it'll let me drop the bag hooks a total of 63mm lower than they are now, and lower the bag caps to nearly the same level as the rack deck, lowering the rack-top load and letting me move it further ahead. If not...well, the Cargo Evo in the proper 26" size is coming, and it'll drop the lot 13mm lower. Not a lot, but maybe enough. The Cargo Evo's deck is a lot more usably wide than the Logo, so it may be wash. We'll see. The good weather will hold through today, then we've got rain predicted through Monday that will make test runs less fun. I'll be a busy fellow today.

Pete:

Alternatively, take your rack-top load off and put it in [an ExtraWheel trailer]...

Now you're tempting me, Pete!  Mightily! I've had my eye on one of those since I saw one in your touring pics. What you might see as a trailer, I see as a Water Wagon, hauling all but one of my bottles and the Dromedary, full to 10l. I'm definitely keeping it in mind, but would prefer to do without a trailer of any kind if possible for the trips I take solo. For tandem camping with 4 panniers, a trailer can't be beat. The big problem for me is the concern about the trailer sinking on the dampened playa rims of dry lakes. That photo of my Miyata I posted elsewhere on the Forum ( http://www.thorncycles.co.uk/forums/index.php?action=dlattach;topic=3990.0;attach=1187 ) showing it standing on its own in Nevada's Black Rock Desert gives an idea. It just sucks the energy out of me to churn through that stuff. If an afternoon thunderstorm kicks up and it rains when I'm out there...good luck getting back out again till it dries enough to "surface". <-- Why I have to carry the extra food and water, 'cos the water on the dry lakes is all alkali-contaminated and can't be consumed. It also means sleeping in a chair, 'cos the tent and bag would be under several inches of water. Yeah, the trailer would be a help and a hindrance. Which would depend on the situation of the moment.

Stuart: You're a gem; thanks so much, great job as always!

JimK: Yes, the free-weights with and without the panniers are a great idea. Implemented and done. Though final testing will have to be with a real load, the free-weights should allow gross changes quickly enough to give me some quick indications.

Ian: Glad you like the title; it came to me as I woggled home from that first test-tour muttering those same words. Excellent thinking wrt my high cadence being a factor. Though I pride myself on being a smooth pedaler (the French term "souplesse" just sounds so much more sophisticated, but I can't pretend..."smooth pedaler" does me fine), it could be a factor. Thinking along those same lines, I revved like mad in low gears to get up to speed without trashing my knees, then dumped it into overdrive and just barely (for me) ticked over at 40rpm. No change. Oddly, I also found that on coast-down with no pedaling input, the bike would sometimes "drop into" the wobble as I decelerated without pedaling.

I don't think it's my cadence, and I now also don't think it is a classic case of shimmy, precipitated by such things as trail and gyroscopic precession of the front wheel and such. When there's a lot of weight on the back (with or without any water bottles, front panniers, or HB bag), shaking the handlebars while the bike is at rest is enough to start a sine wave through the top tube -- much worse with the back brake locked to provide a pivot point beneath the rear tire. More and more, I am thinking it is a case of either weight transfer or related to the polar moment of the loaded bike.

Oddly, this is exactly the reason and identical symptoms/behavior that caused me to retire the Miyata 1000LT and buy Sherpa. Me 'Yata did fine all through Europe and throughout the 2010 Great Basin Tour that included the extended transit of Black Rock. On my return, I overhauled the bike, found the last strut-weld on the alu rear rack had broken, replaced it with the Surly, and then the bike became unridable thereafter due to shimmy, even with just 5lb/2.3kg in a rack-pack. Cost me my planned 2011 tour of the Ruby Mountains and cross-transit of Black Rock. I never did figure what caused that nor how to address it. I don't want that to be the case with Sherpa, but there have to be some related causal factors I'm missing. If only I could see the connection. Does anything jump out that catches your collective eyes?

Thanks so much for the kind and encouraging responses; very much appreciated!

All the best,

Dan.

[jags]

looks great Dan total mystery lets hope the new rack is the answer

[Danneaux]

Hi All,

New Tubus Logo Evo arrived
The Logo finally arrived at 14:42 this afternoon. With various things intervening, I finally got to installing it around 17:30 and it was a 15-minute job to get it on the bike (I'm getting far faster with practice, and discovered a few assembly shortcuts that really helped. Ask if you want them. Having all the needed tools and driver bits assembled at the ready made it faster, too).

With some expected problems...
There were a few disappointments with the Logo Evo, largely expected. The top deck is in a different place, and so is the light mount. Thank goodness I left a little slack when I ran the taillight lead. The top deck sure is narrow. This caused a problem I had worried about: The two front stays are too close together to fasten to the outside of Sherpa's seatstay bosses using the straight stays, so I temporarily put them to the inside. I've already placed an order for 12mm cranked (offset) stays from Tubus to mount the front stays properly to the outside of the seatstay bosses. I couldn't tell how far off it would be till I saw it installed and could take accurate measurements. Right now, I think everyone with a Sherpa and a Logo Evo would need the 12mm offset stays to do the job right, or would need to bend them that far. Order fulfillment should be Tuesday for those, so it was inside or nothing for now. The Logo Evo is not as stiff laterally as the Cargo Evo by a long shot, as it lacks the inverted "U" rear strut (why did Tubus leave it off? Or why not simply add a lower rail to the Cargo Evo?). It is possible to wiggle it laterally over a range totaling 5.7mm with 5.09kg of lateral pressure on the last strut. It does set the bags lower, which lowers everything. Bound to help.

I got the mounting hooks adjusted; the Ortlieb BikePacker panniers needing to be moved rearward just .5in/1.3cm for heel clearance, not a bad tradeoff for sitting 2.5in/63mm lower overall than on the 700C-sized Cargo Evo. Having a flat "return" at the front of the rack along with the lower pannier placement allowed me to shove the tent fully beneath the underseat bag and still clear the backs of my thighs nicely by 2-3 inches. The drysack containing the sleeping bag, pad, liner and air pillow shoves right up against the back of the underseat bag, very close to centered over the rear axle. The lot is secured fore-aft and side-to side and doesn't wiggle.

My theory that dropping the panniers would effectively extend the narrow rack deck proved out. This was the most stable rack-top configuration with cap-top Ortlieb Packers of any tried to date.

No water bag, but an additional 5lb/2.27kg in free weights in each rear pannier. The whole works is shown in the accompanying photo-collage, with detail shots of the rear load since that seems to be the source of the problem. It looks pretty tidy.

And...it helped!
I took the lot out after dinner tonight and did many runs over the next two hours. For the first time, I had no frank shimmy. It came very close, but didn't actually shimmy at any speed up to 22mph/35.4kph, nor on coast-down. When I deliberately induced shimmy by alternately tugging the 'bars, it was of lower frequency and amplitude than before and self-recovered.

But why? And not enough
That's great news in itself and the first encouraging sign since the debacle began, but I still don't know why the bike is so very sensitive to rear loads. If anything, it should be the front that is problematic, and that end is solid as a house. I still cannot understand how the bike did so well initially last fall and through the winter's day-ride testing with a full-up weight of 134lb/61kg on the same surfaces and even really bad gravel, then developed the tendency to shimmy now with less weight. Same as the Miyata.

This all still leaves me unable to depart on schedule, and I've already missed my initial routing window. ODOT (Oregon Dep't of Transportation) opened Old McKenzie Highway 242 to all traffic today, and it was flooded with sightseers; no place for me on a loaded bike with narrow two-lane, blind hairpin corners and no shoulders and very limited sightlines. I know I don't relax much when I drive it, looking as I do for cyclists potentially around every corner -- and finding them in the worst places.

The big problem is I've only got four days' worth of prepacked lightweight food aboard with one days' emergency rations (if rationed), and I need more. I don't have water to refill my bottles. I can't duplicate this loading scheme with the Cargo Evo 'cos the bags sit too proud of the rack for the rack-top load to clear the underseat tool bag and it leaves the racktop load unstable.

Why's Sherpa so rear-load sensitive?
I'm more than a bit at sea on resolving this and am about out of ideas. By all I know about frame geometry, this shouldn't be a problem, and the tubing specs are dead-on for handling such a load, particularly compared to the '89 Miyata with 700C wheels and conventional tube diameters. Sherpa is textbook-spec for the job.I've never fallen on the bike nor has it fallen on it's own. Nothing appears amiss or broken, cracked, or misaligned. It rides wonderfully unladen or with a 25lb/11kg, but so do my other rando/touring bikes.

Any and all ideas are welcome. Nothing's off-limits in terms of suggestions. I suppose it is possible that in my frame size, Sherpa is just not the appropriate tool for the job. Still several online magazine tests and reviews indicate the tubing in the extra-oversize diameters (like my 560S Sherpa) have the same tubing spec as a Raven Tour.

A new line of inquiry: tires
I'm now wondering if the wobble is due to the tires. The rigid (wire bead) Duremes have always had a wandering tread cap and the reflective bands are off-center, though the bead is equidistant from the rim edge. In the early days of Kevlar-belted tires, the belts would sometimes shift, and there were even problems with the tread de-bonding from the belts. My family had a light truck once that went through whole sets of tires 'cos the belts would separate, causing a steering shimmy until they were replaced. Might it be possible these Duremes with the wandering tread caps are no problem with lighter loads, but distort just enough under extra weight to cause a shimmy? Maybe at the rear, which carries more weight? I have a pair of brand-new folding Duremes, and will swap them on tomorrow and have another go.

Best,

Dan.

[julk]

Dan,
a glimmer of light at the end of the tunnel.

Tyres, maybe worth swapping front and rear tyres over, I know mine wear differently.

Why not try an e-mail to the designer Andy Blance at Thorn? He must be able to offer some guidance.
Julian.

[jags]

well lets hope the problem is solved, the bike looks fantastic loaded up ready for war 
very best of luck with it Dan sure hope your tour goes smooth i'm sure it will.

[jags]

oh just seen this Dan might be of help on the tyres  .
http://www.bikeforums.net/showthread.php/794790-Schwalbe-Marathon-XR-tires-various-sizes-all-new

[mickeg]

I just ran across this thread.  I have had two bad shimmy problems during my 50 plus years of biking.

First shimmy experience.  As a kid with a newspaper delivery route, I had those really huge steel wire baskets over the rear wheel on a single speed coaster brake bike.  Some kids where chasing me and I was pedaling as hard as I could to try to get away from them, the baskets that were full of newspapers started swaying back and forth (resonance) and it literally launched me off of the bike.

Second shimmy experience.  Long Haul Trucker, Surly racks front and rear.  Very bad shimmy but I could ride it.  Was much worse on pavement than on gravel, fortunately most of the trip was on gravel.  After a few days I figured out that my front pannier weight center of gravity was a too far to the rear, moving the center of gravity forward to the location of the hub fixed the shimmy.  Still felt a bit wobbly and flexible, but the resonance was gone.

Next tour, no shimmy problems.  This was with the Surly racks on a Thorn Sherpa (610S).  On this trip I found that if I stood on the pedals to try to accelerate instead of staying in the saddle, the bike felt really flexy, so I did not do this anymore.  I suspected that the flexible feeling was the Surly rear rack, but can't say for sure.  I decided the Surly front rack is too heavy, replaced it after this trip.  This trip was on gravel with 26X2.0 Dureme on front and Extreme on rear.

During the past month I have been getting ready to go on a tour with my Thorn Sherpa, Surly rear rack and Tubus Ergo front rack.  Handled great with about 11 pounds (sorry, I am in USA, I don't think in kg terms) in each front pannier and maybe 16 pounds in each rear for my test ride.  Sherpa felt great with new Marathon 26X1.5 tires which I bought just for this trip.  Change in tires is since this trip will be almost all on pavement so I did not want the 2.0 width tires.  Decided to test the LHT with this weight distribution, it also felt great with these four panniers but decided to use the Sherpa anyway.  When I added a duffel with about 16 pounds to the load on top of the Surly rear rack (in addition to the panniers), it suddenly felt much more flexy.  It did not have the resonance problem that I had before with the LHT, but it still felt a bit too flexible.  For this reason, started thinking about a different rear rack. Photo below was taken yesterday with the Surly rear rack and duffel on top.

I ordered a Tubus Logo EVO two days ago.  If it does not arrive on time for my trip, the Surly rack will work for me, as I did not get a resonating shimmy.  But, I hope the new rack gets here in time.  I hope I have better luck than you for mounting brackets.

I concur with your opinions on the lack of triangulation stays in the Surly rear rack.  That concerned me when I first opened the box it came in several years ago, it reminded me too much of the system I had used as a kid to carry newspapers.  But I used the Surly rack anyway.  I went with the Logo instead of the Cargo specifically because I wanted to have the lower pannier position and because when I looked at the drawing of the Logo from the rear, it just looked so sturdy.
http://www.tubus.com/documents/1323435489_LogoEvo_BM.pdf