Author Topic: Sherpa Shimmy resolved with superb warranty response by Thorn Cycles  (Read 55587 times)

ians

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #45 on: June 24, 2012, 05:58:18 PM »
Hi Dan

if you scroll down to the bottom of this page http://cyclotouriste.co.uk/index.htm  you'll find reference to an article describing testing for shimmy.  But possibly you know more than the author by now.

Ian


jags

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #46 on: June 24, 2012, 06:00:15 PM »
This also looks like a great set up,
but can i just ask have you guys concidered  loosing half the load :)
i'm not being a smart ass but what on earth have you got in all those  panniers.
surly for a two or three week tour you could fit everything into 4 panniers.

Danneaux

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #47 on: June 24, 2012, 08:21:38 PM »
Hi All!

Some exciting additions to this thread, and it is frustrating that due to the press of daily life, I have defer my replies till later today. I'll jump on this one, though, 'cos it's core to the discussion...
Quote
can i just ask have you guys concidered  loosing half the load. i'm not being a smart ass but what on earth have you got in all those  panniers. surly for a two or three week tour you could fit everything into 4 panniers.
No, you're not being smart, jags, it is a logical question to ask and I'm glad you did. I've detailed my load elsewhere on the forum with pics, but basically...

1) My two front panniers carry nearly all my "stuff" Why? Because the contents are small and dense and stable and unlikely to change in volume much and I can take either of the two small bags into the tent with me or reach down and filch stuff out of them while astride the bike when stopped:
- Stove
- two alu bowls I use as pots
- 1-liter of stove fuel (white gas or unleaded gasoline as the case may be).
- small toilet kit.
- reserve tools and spares
- spare folding tire, 2 tubes, 2 patch kits
- small paperback novel
- small plastic box with rechargeable batteries
- 1 pr. riding shorts
- 1 pr. casual shorts
- 1 spare jersey
- 2 spare socks
- lightweight fleece jacket
- lightweight long-sleeve wool jersey with nylon wind panels on the front.
- lightweight wool tights
- lycra tights

2) The HB bag carries:
- Some energy bars/snacks in double zip-lock bags to reduce "bear smell" and are eaten away from the HB bag.
- A little sack with my lightweight wind jacket
- A little sack with my sun hat, my light fleece balaclava, and my billed hat
- my wallet and lock keys, spare camera battery.
- my slim little flash-video camera (digital camera goes in my rear jersey pocket)
- "dumb" 3G CDMA cell phone with great battery life and tower reach. Even so, often out of reach for days.
- small sack for prescription meds for thyroid and pollen allergies, toothbrush and paste, floss, and small tube deodorant.
- small notebook and pen, small first aid kit, all-purpose pocket pack of paper tissues.
- small silicone cup, used primarily for snagging water from restroom faucets that are too shallow to refill my water bottles.
- small plastic zip-lock bag with bug spray, sunscreen, lip balm, zinc oxide, another Kleenex packet and two flat dust masks for dust storms.
- Map goes in case on top. LED headlight goes in side pocket with another little bottle of eye drops. Other pocket has two little packs of tissues (serve as napkin, snot rag, and toilet paper in descending order of multi-use).

3) Rack-top drysack carries:
- 3lb/1.36kg winter-weight down sleeping bag good to -15C; use as a quilt in warm weather.
- 4.6oz/.13kg silk liner; use alone as sleeping bag on blistering hot, 80F/27C nights.
- 2.6oz/70g air pillow
- 2.21lb/1kg winter-weight dual-chamber sleeping pad
- 9.2oz/260g sleeping bag stuff sack and lightweight, 35l dry sack

4) Other rack-top sack contains:
- Tent at 3.75lb/1.7kg with fly and footprint and a variety of stakes including deadman flukes for snow and sand and titanium needle stakes for rocky soil

5) The rear panniers contain what I call the Cycle of Life: Food and water in, waste um, "out":
- Two side pockets: small pocket packs of paper tissues, whose shape and size makes them more ideal than rolls of toilet paper.
- Toilet trowel & Steripen UV water purifier and filter plus some pills for if the batteries go dead
- Small roll, bin liners for trash disposal.
- GoPro vidcam and chest harness/helmet mount (18oz/510g).
- Under 1 bag cap-lid: My rain gear (helmet cover, gloves, booties, jacket, pants) @ 2lb/1kg.
- Under other bag cap-lid: Camp shoes (20oz/570g), folding Alite Monarch chair (20oz)
- Food. Varies, depending on whether I am near stores or have to carry a week's supply. At home, I repackage the food to minimum dimensions and go with a lot of dry stuff and food in foil pouches. At the little rural stores I find on my route, most stuff is either canned or microwavable. MW stuff can do, but does not turn out well. So, it is usually cans. Because the bulk and type of food vary so much, it all goes in the rear panniers, which can be expanded or compressed to hold it. It also means there is no food smell in any other bags, so all I have to hoist into a tree at night is the two rear panniers. Bears aren't a problem in the desert, but they surely are in the heavily forested mountains I cross on the way there and back.
- And, of course, the rear bags (Ortlieb BikePacker Plus) weigh something. So do the nylon webbing straps I use to secure the lot. By the way, Arno straps rule.

Besides food, the big weight for me on my extended, self-supported, solo desert crossings is the water. When all bottles are full, I carry 1.7 US gal/14lb (6.5l/6.5kg) on the frame.

I also have to carry extra water for when I'm spending extended time where water supplies have been poisoned by alkali and cannot be made potable (I carry pH strips to test first). To cover that contingency, I have a 10l MSR Dromedary water bag. If I carry 6.5l in it, that allows me to refill all the bike bottles once. This can be on the knife-edge of needed capacity. All the water is for drinking and cooking only. I cook in freezer bags so there are no dishes or pots to clean (the used sacks become trash bags for responsible disposal later). I don't bathe for a week or two at a time (Eww, but true) except for the occasional wipe-down of um, "delicate" areas. I wear really lightweight undershorts that can be rinsed under a water bottle stream and air-dry in a few minutes in the low humidity. The shorts and their synthetic chamois get washed about once a week, and sometimes ride wrongside-out atop the racktop load so the sun can do its work on them. Waterless alcohol-gel hand sanitizer and gel deodorant keep-down bacterial growth in the nether regions. There's many other tricks and tips I've developed over the years that help, often coming from military training practices.

This seems to be the irreducible minimum for what I do, leaving a minimum margin for safety and durability.

I've got to deal with temperature extremes -- hot and cold. June temps are often 9F/-13C at night in snow when crossing the mountains from the Valley to the Central Plateau and then desert regions. Last trip through, the rangers closed the gates at Steens Mountain due to whiteout conditions near the summit where I'd planned to overnight. Once in the desert, nighttime temps of 18F/-8C are common, and my bottle freeze solid if I don't bring them into the tent and place them on the foam sit-pad. During the day, air temps are commonly 124F/51C. My temperature data logger has repeatedly recorded well over 148F/64C on asphalt, which melts and sticks to the tires. This is why I need the chair to keep me up off the pavement with an air-gap when I stop; the foam sit-pad tends to stick to the melting tar, and if I sit on the pavement directly, I get scalded and the tar sticks to me like napalm. There's no shade. I've though about packing a small umbrella and expoxying a 10-24 nut to it for my camera clamp to fix to my rear rack at stops in my chair, but the wind would blow it wrongside-out. I used to just stand around when I stopped for a brief rest, but I may be getting um, "more experienced" (*not* "older"). That's why the chair now holds more appeal.

I am just warm enough if I wear everything I've got when it is coldest (3-5F/-15C--16C) and I'm standing still:
- Jersey, shorts, socks.
- spare jersey, shorts, socks on hands
- all the rain gear
- hats/balaclava/helmet cover
- lightweight fleece jacket
- light wool long-sleeve jersey with wind panels
- tights, wool and lycra
When I'm working (riding), then the layers come off. They go back on when I stop, hence the convenience of storing the two jackets and rain gear under the bag-caps where I can get them by just unsnapping the two buckles and flipping the lid. Really helps to leave the main bags closed if it is snowing, pouring rain, or in a dust-storm.

I almost always toss in a couple pairs of women's pantyhose (tights) as well. They are light, cheap, and provide a couple more layers' warmth for minimum  bulk and weight. I've sometimes cut off the legs and used them as long sleeves to reduce sun exposure (lighter than bigger bottles of sunscreen). They also make it harder for ticks to bite and attach themselves and work great as silt and scum filters for skimming water from cattle troughs before hitting it with the SteriPen and/or pills. I recommend them highly if you can get over the idea of it.

The tent is a 1-person jobbie; I only stay in it to sleep. If it rains, I ride in the rain and cook in the rain; the tent is just for sleeping. The rest of the time, I am on the bike 16-17 hours/day and usually eat on the bike. The one cooked meal of the day is dinner in camp before I crawl into the tent to sleep.

I've got to deal with winds. When I break camp I try to get in at least 20mi/32km before eating so I can make some distance before the winds start. Afternoon winds are usually steady, blowing the alkali dust at about 39mph/63kph. They're usually quartering or headwinds, but occasionally I'll pick up an oikaze (favorable following wind, as Japanese fishermen call it).

For off-bike recreation, I take landscape and flower photos and soon video and limit myself to reading a chapter a night to make the one paperback novel I take last as long as possible. I also listen to the little 1-AAA battery-powered AM/FM/Weather radio I carry or my little .8oz/22g rechargeable MP3 player (America's "A Horse with No Name" usually seems appropriate).

So, that's what I carry, and where. About the only weight I can lose or vary is the amount of food and water I carry, and there's times I need all of that I can lug.

All the best,

Dan.
« Last Edit: June 26, 2012, 08:11:28 AM by Danneaux »

mickeg

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #48 on: June 24, 2012, 09:19:57 PM »
I will keep this short.  Yes I have considered trying to cut weight.

Several years ago I decided after a canoe trip that my pack was too heavy.  So, I weighed all of my camping gear, item by item.  I then put all the data into a spreadsheet.  From that I could compare weights while deciding what to bring - do I bring this stove at 17 oz (again, sorry not using SI units, am in USA) or this other stove at 21 oz?  Etc.  I have expanded that spreadsheet with biking gear, kayaking gear, backpacking gear, etc.

Right now my gear is about as light as it can get.  I will however start out with 4 days worth of food since I do not expect to see a store for 4 days.  Trip is 2 weeks, but realistically you bring the same gear for a week or for a month or two.  Weather, I expect 30s to 90s (Farenheit) or roughly freezing up to mid 30s C, which complicates clothing and sleeping bag options.  Over the past month and into the foreseeable future, it has rained more days than it has not where I am going, thus the waterproof gear. 

The duffel will be roughly half full most of the time, but it will have room for extra water bottles and food.  I have a piece of cardboard rolled over the bottom half inside the duffel to keep it straight and not sagging, but with the new rack I may put the duffel side to side instead.  When I took the photo, I had four liters worth of empty water bottles in the duffel to help fill it up.

Assuming the two front panniers are 25 liter, the rear are 40 and the duffel if it is only half full would actually only have 15 liters in it, I am pretty close to only using four panniers.

jags

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #49 on: June 24, 2012, 10:03:17 PM »
ok lads point taken ,i'm only a fair weather tourer if that. :-[
but saying that we are all kinda spoiled these days with all this light weight gear on offer.
and all the high tech cloths/ panniers /bikes and what not.
how did the cyclist do years ago when this gear wasn't available.
i often seen lads on old steel bikes with one or two bags touring the world how the hell did these guys do it, makes you wonder.
i'm not saying you won't use all that gear maybe you will maybe you won't. but if wasn't there you would never look for it.
yip we are all home creatures at heart ,guess if that 36inch flat screen tv only weighed a few grams , it would have to come along.

just winding you guys up  ;D ;D ;D
« Last Edit: June 25, 2012, 12:02:30 AM by jags »

Danneaux

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #50 on: June 25, 2012, 04:13:49 AM »
Hi Mickeg,

I am so glad you have joined this thread, as our experiences with Sherpa handling and racks appear similar.

As you have already read to this point, the battle continues to find and address the root cause of my shimmy.

The next phase of my investigation will take place after the Tubus Logo Evo struts arrive by FedEx on Tuesday. Since that is the first rack combo that has (barely) stopped the shimmy, it seems a good baseline on which to build. Of particular note for you and your efforts, the Tubus Cargo Evo proved more rigid and resistant to lateral movement than the Logo Evo, but the lower mounting rails and flat "return" (loop) at the front of the Logo Evo seemed to help more, due to variations in weight distribution (panniers lower, rack-top load further forward). The one thing I really don't like about the Logo Evo? The top is so narrow it doesn't do a good job supporting my rack pack on day rides; a real disappointment there. It is more like a Tubus Fly with lowered side rails than a Cargo or Cargo Evo. On the other hand, with my cap-top (non-roller) Ortlieb BikePacker Plus rear bags, the extended caps and the narrow racktop are all in the same plane and I get much better support for the rack-top drysack and tent sitting crosswise. 

Right now, I have the Surly Nice Rack (Rear), the 700C/28"/26" Tubus Cargo Evo, and the 700C/28"/26" (only available in one universal size) Logo Evo. A 26" Cargo Evo is in-transit (rack top is 13mm lower) and the offset forward struts for the Logo Evo. That's a lot to play with as I continue testing.

I hope your new Logo Evo will work well for you, and bring the results you seek. Certainly, your bike is stunning in appearance and just looks so very nice overall. It could easily pose for a brochure or magazine cover. As for the struts, I found on my Sherpa the Logo Evo's strut mounts were so narrow as prevent spacing them to reach the outside of the seatstay rack bosses, so I fastened them temporarily to the inside, which is too narrow to adequately brace the rack on my bike. My Sherpa is a 560S; your larger size may allow more latitude. I had the option of bending the stays as a pair or ordering the offset stays, and chose to purchase the models with 12mm offset, which will be perfect for mounting in the usual position.

Once I have the new struts installed, my next move will be to concentrate on tires.

Already, I have swapped the two Schwalbe Dureme 26x2.0 rigid (wire-bead) tires for a pair of folding (kevlar-bead) tires in the same size. The difference? The rigid models had a wavy tread cap, while the folders have a more centered tread with far less variation.

After I observe the results of that trial, I plan to rob the 26x1.5 road slicks off my tandem and mount them to Sherpa as a pair. Then (though the width will vary), I plan to put a 1.5 slick in front and a 2.0 Dureme in back, and the opposite -- a 2.0 Dureme in front and a 1.5 slick in the rear. The purpose? To exaggerate differences in trail resulting from variations in head tube angle and axle height.

I am still most puzzled by four things:
1) Why the bike does well with all the weight in the front and middle and *either* the 11lb rack-top load *or* the 25lb rear panniers, but *not both*.
2) Why the bike handles so miserably with no front load and more weight on the back; it is just not tolerating a rear load of any substance.
3) Why the bike initially did so well in road and off-road testing last Fall and Winter with this weight and more, but went bad with less on this last test-tour and thereafter. There are no signs of tube or joint failure, and nothing about the bike has changed. All appears to be solid and properly adjusted. It has never fallen over when parked and I have never fallen while riding it; it has never fallen at all. Everything seems to be properly aligned. Except for stiffening the sidewalls, changes in tire pressure seem to make no difference.
4) The same thing occurred with my '89 Miyata 1000LT following my return from one tour in preparation for another. It was as if it had developed a hinge in the middle of the top tube and all lateral distortion during shimmy took place there. I did take four hard falls on the bike during the last tour on it, and I will check the fork alignment to see if a blade has been tweaked forward or back compared to its mate. That is the only possible explanation I can think of for the Miyata, but none of that applies to Sherpa.

Best wishes for a safe, happy, and enjoyable tour, mickeg. Please let us know afterward how the change in racks does for you and how the bike handles with a load.

jags wrote...
Quote
i often seen lads on old steel bikes with one or two bags touring the world how the hell did these guys do it, makes you wonder.
Yeah, me too, jags. I often think of Ian Hibell and shake my head in wonder. He surely did well, particularly on crossing the Darian Gap and the Gobi Desert.

All the best,

Dan.

Danneaux

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #51 on: June 25, 2012, 06:32:48 AM »
Quote
if you scroll down to the bottom of this page http://cyclotouriste.co.uk/index.htm  you'll find reference to an article describing testing for shimmy.  But possibly you know more than the author by now.

Ian
Thanks, Ian, for the reference; much appreciated. As it happens, I have now read the article and it appears Jan (Heine) is no closer than the rest of us in finding the definitive case or cure of shimmy. I do agree with his recommendation to use needle/tapered roller-bearing headsets, but for different reasons (longevity); a happy byproduct of their design and long life is greater friction. I do have one bike that developed shimmy only because of a brinneled (pitted) headset race; I happened to replace it with a Galli Supercriterium model with ti races and the problem -- nonexistent before -- was cured and never returned. The cause was definitely the pitted headset in that case. Not so Sherpa, whose headset is smooth and properly adjusted.

As part of his research, Jan (CompassCycles) posted two YouTube videos showing a bike where shimmy could be induced with the rider sitting upright and slapping his thighs. See: http://www.youtube.com/watch?v=vSNjpQPdrX4&feature=plcp ...and... http://www.youtube.com/watch?v=8pzc2aYVFEg&feature=plcp

At its worst, this is about the frequency and amplitude of my shimmy, but mine occurs when I am riding on the brake hoods or anywhere on the handlebars as well as when I sit up or even stand. It just happens when the rear load combines the rack-top mass and the loaded rear panniers together. With one or the other and a front load...no problem.

Any other ideas occur? I'm going to play with changes pneumatic trail due to differences in tire size as soon as the rack struts arrive.

All the best,

Dan.

julk

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #52 on: June 25, 2012, 08:39:51 AM »
Dan,
I am having difficulty thinking of anything else which would help you in the quest for a solution.

One idea which popped into my head is that the shimmy is symmetrical, do you ride with exactly the same weights on the left and right sides of the bike?
Have you tried riding with more weight on one side? This might unbalance the shimmy and confuse it!

I sometimes take just one rear pannier for a shopping trip and end up riding carrying maybe 15 to 20 pounds on one side. My bike seems to cope with this and handles normally.

Just a thought.
Julian.

mickeg

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #53 on: June 25, 2012, 03:12:13 PM »
Dan,

Thanks for the kind comments on the look of the bike.  I bought the frame and fork used from someone that had the wrong size.  The components were all my selection.  It looks unusually clean right now because I gave it a very good cleaning as part of a detailed inspection I did of it after a bad crash I had in late May.  Drive train looks unusually clean now too, I switched two chainrings less than 100 miles ago so they are quite clean, it is now setup for half step gearing (46/42/24 on front, 11-32 eight speed rear).

I use a different rack around town than I use for touring.  And touring I do not use a rack top bag, so the width of rack top is less critical to me for touring.  I might actually start using the Surly rack around town since it is the perfect width for one of my rack top bags.

I doubt that changing tire size will impact rake and trail enough to be noticeable in handling.

I will not be concerned if I have to mount the brackets to the seatstays on the inside instead of outside.  If the seatstays had zero flex, you are right that attaching to the outer should be more sturdy.  But, any flex in the seatstays may negate that making the specific attachment point less critical.

I think that sometimes shimmy which is a resonance problem may come and go with weight distribution.

I do not think spoke tension was mentioned, is your spoke tension right?  I build up my own wheels and just use feel for spoke tension, I do not even know if any local shops here have a spoke tension gauge.

On my LHT, the shimmy was worse on pavement than on gravel.  I think that the gravel allowed the tire to move slightly side to side as it rolled on loose sand grains, acting like a shock absorber on the resonance.  That leads me to ask, have you tried to evaluate shimmy at higher and lower tire pressures?  This is just a thought, I have never heard of tire pressure being a factor in shimmy, maybe it is not?

Also I suspect that the farther aft the center of gravity of the rear weight (panniers and rack top bag) is set on the rear rack could also impact resonance and shimmy.  If you have excess heel clearance, maybe try moving rear panniers forward to get the center of gravity closer to the center of the bike?

Good luck.



mickeg

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #54 on: June 25, 2012, 03:20:11 PM »
Dan,

Thought of this after I posted my note a minute ago.  Since you have another set of wheels (on the tandem), if they have the same hub spacing, you can swap wheels to see if that changes things.  If you try this be careful you do not accidently put your derailleur cage into the spokes in the event that the rear hub dimensions are different.  If your shifters can be run in friction mode, you might not need to make any other adjustment changes for a short term test.

If swapping wheels fixes the problem, then it becomes an issue of trying to figure out why?

Danneaux

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #55 on: June 25, 2012, 04:03:11 PM »
Quote
if they have the same hub spacing, you can swap wheels to see if that changes things
An excellent, idea, mickeg!

The front wheel will swap right over, with identical spacing and Sun CR18 rim. The rear will have to be a tire swap, however, because the rear hub has much wider spacing and the Arai drag brake. The tire will go right over, though.

At this point, I really am just looking for change of some sort, either good or bad. Once I see what else affects the resonance of the frame (besides rear load mass and placement), I will have more direction. To that end, Julian's idea of trying a single pannier is also helpful  (thanks, Julian!).

It is pouring rain at the moment, which complicates setup and testing, but hasn't stopped me. The weather 'round here is not reliably good until after the Fourth of July.

Pete, I especially like Jerome's hat-shaped helmet. I also notice he's moved his cargo weight toward the center of the bike as far as he can, so shimmy likely wasn't a problem for him (he just had to deal with little things, like having enough to eat and drink; how'd he ever?!?).

All the best,

Dan.

Thanks, fellows; your efforts mean a lot to me.

Danneaux

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #56 on: June 25, 2012, 11:50:27 PM »
 :o "Arghhhhhhhhhhhhhhhhhhhhhhhhh!" >:(  <-- Pure frustration.

Goodness, this is hard!

New Cargo Evo arrived. Same as the first Cargo Evo, despite a different description and stock inventory number in the REI catalog. Methinks REI is now stocking the large model as a "universal fitment" (as it says on the tube of parts attached to the rack) despite the two catalog entries. Makes sense and reduces inventory for them. Not so great for me, who required two go-'rounds to figure that out. Nobody at REI seems to know anything about it.

The offset Logo struts are at this moment at FedEx in neighboring Springfield, just 5.9mi/9.5km away. No, can't collect the envelope; hafta wait for delivery sometime before 16:30 tomorrow. Meantime, I can still try the too-large Cargo Evo, the Logo Evo, and the old Surly with various tire combinations and see what comes of it.

Onward!

Best,

Dan.

jags

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #57 on: June 26, 2012, 12:01:14 AM »
hope you get it sorted soon Dan you must be cracking up  at this stage ::)

Danneaux

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #58 on: June 26, 2012, 12:14:34 AM »
Quote
you must be cracking up  at this stage
Well past, jags, well past. It is driving me crazy, and it's not a very long trip to get there.

If you see a man, running through the streets naked while playing an harmonica, chances are it will be me.

The neighbors will tell reporters, "Oh, he was always such a nice, polite, quiet boy growing up. Then...he got involved with those bicycle-things, and look what happened...".

Dan.
« Last Edit: June 26, 2012, 12:16:22 AM by Danneaux »

Danneaux

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Re: Wow! Whoa!...woe. Severe Sherpa shimmy under load.
« Reply #59 on: June 27, 2012, 05:35:37 AM »
Hi All!

Executive summary: Thankfully, I can report success - the Sherpa Shimmy is banished! At least for now, pending further testing.
My faith in the value of good research methodology has been confirmed by this experience. Because at present I am uncertain exactly what made the difference, I will continue testing tomorrow to make sure the success is repeatable at higher speeds (Green Hill, here I come) and then I will deconstruct what I have to see if the shimmy reappears.

Yes, with this much weight, yanking the handlebars back and forth rapidly can induce a brief oscillation visible as a low-frequency, low-amplitude sine wave in the top tube, but it is self-damping almost immediately after the inducement has ceased. The same effect can be achieved at rest so long as the rear brake is locked, forcing the rear tire to serve as a pivot point for "winding up" the top tube, which serves as a self-correcting linear spring. Neither of these deliberately-induced states manifested as shimmy when riding.

For those hardy souls who wish to learn more, here's a summary of today's efforts in excruciating detail:

In our last installment, I got Sherpa to the point where he just barely didn't shimmy with the minimal load I can take, but no more. He wasn't very pleasant to ride, and I wouldn't have had the water or food needed for extended desert crossings.

Here's what I did today, in order; something or some combination has worked:

1) The 12mm offset front stays arrived this morning for the Tubus Logo Evo. They were the perfect size to reach the outside of the rack's mounting holes and the outside of Sherpa's seatstay bosses, remaining parallel. As expected, they made the rack much more rigid at the front than it was before. Still, the rear of the rack is as flexible laterally as the Surly Nice Rack (Rear). The Cargo Evo is much more rigid laterally overall, thanks to its inverted U-shaped rearstay configuration. The rear panniers are mounted to the lower rail of the Logo Evo, as designed.

2) I swapped the rigid (wire-bead) 26x2.0 Schwalbe Duremes with the folding (kevlar bead) version in the same size. As with the rigid tires, I followed Andy Blance's recommendations and set maximum tire pressure as per his chart on page 6 of the Nomad brochure (Issue 17c, Summer 2012,
http://www.sjscycles.com/thornpdf/ThornRavenNomadBroHiRes.pdf ): F: 45psi/3.1bar R: 51.5psi/3.5bar. Actual section width and profile at this pressure is 47mm (100% section width/profile).
RESULT: Reduced nervousness and the feeling I was farther from shimmy.

3) I then added weight to the rear panniers, ballasting the rear panniers so they weighed as follows:

Left-Rear: 17.27lb/7.84kg
Right-Rear: 17.17/7.79kg

(Paired rear panniers: 34.61lb/15.70kg...some rounding error, but I weighed the pair directly).

All other loads on the bike remained constant in mass and placement as before.
RESULT: Same as 3) above.

Feeling I was about topped-out on trials with this setup (tired and not wishing to see my teeth spread on the ground like a spilled box of Chiclets), I then decided to change tires. Off came the 26x2.0 folding Duremes and on went the (Trek) Matrix Road Warrior skinwall slicks, labeled 26x1.5, but with an actual section width/profile of 37:37mm in front and 38:38mm rear when inflated to Andy Blance's recommended maximum of: F: 63psi/4.3bar R: 70psi/4.8bar

Result?
I then went out on the unladen bike to check the feel, and it was transformed! From touring bike to high-speed randonneur in one go, even with the heavy Rigida Andras! The difference in acceleration was absolutely amazing. Of course, I reset my computer to the new tire size (and recalibrated it again when I piled on the load a little later so I could account for extra tire squash due to the greater load) so my speed measurements were accurate. For those interested in coast-down data, the 2.0 Duremes were about on par with the slicks in coast-down. I'm intrigued enough that I want to replicate those tests later, but it looks as if nothing much is given away by the fatter tires on coast-down. The difference in acceleration and lively feel made the slicks the  hands-down winner in that category. I could easily see Sherpa as the bike for my next 24hours/400km ride through the mountains. The fenders had ungainly clearances, but who cares when the bike flies on acceleration and winds-up far more quickly even though coast-down is very similar if not almost identical? Leaving the fenders high means no adjustments when swapping to 2.0s again.

Some caveats wrt the swap from 2.0 to 1.5 tires:

1) It is important to remember the change in feel is not due simply to the slick tires being narrower at ~37mm vs ~47mm width.

2) The slicks are also lower in profile by the same 10mm amount in radius (remember the typical 1.0 relationship between tire section width and profile), twice that amount (20mm) in diameter (which is why I had to recalibrate the computer).

3) The lower profile effectively changes the geometry of the bike by changing the pneumatic trail, as discussed in my earlier thread here:
http://www.thorncycles.co.uk/forums/index.php?topic=4245.0

Just how much did the lower-profile tires affect my Sherpa's frame geometry? On my 560S with the same frame dimensions and fork offset as before, trail was reduced from 62.93mm to 58.68mm, effectively moving the bike from a high-trail state, smack-dab into the neutral-trail/neutral-handling zone. The bike was noticeably less "twitchy"/maneuverable at low speeds, and a bit less "on-rails" stable at higher speeds, just as expected. It was more inclined to lean into corners and less likely to prefer the "sit up straight and steer the bus" approach. Wheel flop at low speeds was considerably reduced. A very sweet ride indeed, and very like my 1983/84 Centurion ProTour 15 go-fast randonneur bike, which uses 700x32C tires and a geometry different from my stock Sherpa with 26x2.0 tires.

4) The slicks are also treadless; these in particular have a bare skim or rubber atop the casing for tread and are completely smooth and unbelted. No wonder the tandem flies on them.

5) The slicks are also considerably lighter. They are mounted now and I overlooked weighing them before mounting, so that task lies ahead.

6) The slicks' shorter sidewalls and narrower width and lower profile and lack of tread and higher absolute pressure *together* mean there is less sidewall "wiggle" than on the 2.0s. In other words, the 1.5 slicks' pressurized tire casing is noticeably and measurably less flexible laterally than the 2.0 Duremes.

I reloaded the bike with the same load and placement as before, and there was no sign or shimmy and everything felt much more stable and less "nervous" at speed, and there was less wheel flop at 2.5-8.5mph/3.9-13.7kph. Again, all as expected by my trail calculations. Nearly drunk with joy at the first truly positive change since testing began, I replicated all the prior loaded testing with the 1.5 tires and the bike passed with flying colors and no shimmy. Besotted with happiness, I then decided to deliberately add the 10l MSR Dromedary loaded with 6l in the *worst* possible place -- *atop* the rack-top load (above the sleep system dry sack and tent, shown in the photo below). The MSR Dromedary with 6l weighed 13.8lb/6.3kg, right up-top. Even with the water bladder placed high, the bike handled well at 25mph/40kph on smooth and wavy and rutted asphalt, and at much slower speeds it also did well on deep grass, across a stretch of muddy dirt, on gravel, and across a former plowed field.

The only anomalous observation came after repeated high-speed braking at the end of each speed run, when it became evident the bolt-on skewer for the SON28 dynohub loosened. It is possible it was the core culprit in the original shimmy, but this seems unlikely as the hub's skewer bolt felt adequately tight when I removed the front wheel to change tires. In the even it is a contributor, I have replaced the bolt-on skewer with a Shimano q/r unit I have measured and calculated to maintain adequate skewer clearance to allow the hub to "breathe" properly and prevent suction of water past its shielded cartridge bearings' seals. Schmidt Maschinenbau specify...
Quote
Schmidts Original hub dynamo SON 28 fits a fork designed to accept an axle of 9 mm diameter and a width of 100 mm between dropouts. The electrical connections should be on the right hand side (to prevent unscrewing of the hub). The hub is secured using the included skewer set. It fits the same way as a quick-release, but fastens with a 5 mm allen key (recommended torque moment 6 - 8 Nm). Apply a little grease on thread and screw-head but not on the shank(to prevent clogging up the pressure compensation system leading into the hollow axle). If the skewer tension is too low, the axle may move inside the fork end causing a rattling noise. Alternatively a lever-type quick-release may be substituted, or a proprietary security fastener such as Pitlock.
Because the bolt-on skewer was found loose after the *final* (successful) testing, I cannot discount the possibility it was loose originally, accounting for the small positive change I attributed to the switch to the folding Duremes (I had to re-tighten the skewer after the tire change). With the q/r in place, I now have a baseline for running the original tests once again for comparison. Given Sherpa's initial ability to handle very large loads with aplomb and the subsequent appearance of shimmy, skewer tension cannot at
this time be dismissed as a possible contributor. Certainly, for failsafe tensioning of the wheel, the q/r skewer's over-center action is ultimately more secure than the bolt-on variety, particularly given the M5x.8 thread pitch used and the low recommended tightening torque of only 6-8Nm (4.42-5.9 ft-lbs). By the way, the SON's bolt-on skewer was tightened to spec with a torque wrench each time, and before the appearance of the original shimmy.

Tomorrow's going to be a very long day of testing up and down some very steep grades with maximum full load as if for an extended desert crossing. I'll be cautious with my run-ups and coast downs and check the load and skewer tensions after each run. I should know something more definitive afterwards. If all goes well, I will swap the Surly rear rack for the Logo Evo and see what effect it has. Yes, it is worth the effort for standardized, repeatable results. It wasn't so much fun during periods of pouring rain today, but tomorrow is supposed to be better, and my spirits are much higher as a result of today's efforts and positive results.

Thrilled as I am with today's outcome, I still want to get to the bottom of identifying the culprit or contributors and address them so I can cure the problem at the root and not simply apply a plaster to the problem.

I'd also like to get those folding or rigid 2.0 Duremes back on for the places I'll be going, and to also determine if the rigid versions are truly warped/belt-shifted/whatever or if something else was in play.

Getting there!

Best,

Dan.
« Last Edit: June 27, 2012, 09:51:40 PM by Danneaux »