Fuji1986

My touring bike is a 1989 Miyata 618GT. I rode it cross-country 20 years ago, but it shimmied constantly beneath 40 lbs of gear (packing too much in the handlebar bag probably didn't help). One day, going down a 20-mile descent in the Cascades, the shimmy got so bad the bike nearly dumped me on the pavement at 40 mph. (Ever see those videos of old suspension bridges swaying in a windstorm to the point they collapse? It felt like that.)

Anyway, I'd like to do a refresh job on this frame, but I wonder about that incident so many years ago. Those frame tubes were literally whipping back and forth like the shaft of a golf club when hitting a ball. Even though the frame looks structurally sound, is it possible the steel could be compromised? In other words, how much flex can steel frames absorb before a stress fracture or crack begins? I'm not sure how to look for that sort of thing beneath the paint. Considering that the frame rides fine and I have no more plans for loaded touring, maybe I'm overthinking it.

bulgie 

Steel can flex and spring back until it reaches its yield strength, at which point there's "plastic deformation", i.e. it "takes a set" and doesn't spring back to its original shape. Shimmy NEVER does that, ever.

Millions of cycles of stress below the yield strength can cause a fatigue crack, but I'm highly doubtful that shimmy could cause that either. Fatigue is hard to predict, highly variable with small changes, like a nick in the tubing, or localized overheating while brazing. The way to find the fatigue endurance of a structure is to exceed it, in other words destructive testing. I don't know of any way to determine what the "remaining" fatigue life of your frame is, but I strongly encourage you to "just ride it". Maybe, if you're worried about it, look at the frame now and then to see if fatigue cracks are starting. Steel frames are usually very mellow in their failure mode — after a crack starts, it grows slowly, and gives you lots of warning. Usually.

Shimmy is a complex situation, difficult to predict and/or cure. Sometimes the same change that causes it on one bike cures it on another bike. But I bet if you rode the frame unloaded or lightly loaded it will no longer shimmy.

I once made his'n'hers custom touring frames for a couple that wanted to tour around the world, but they were afraid that a heavily-built frame would make it dead-feeling when unloaded, so they wanted a compromise, a lightly-built loaded touring bike that would feel lively with no load. We discussed it at length and I thought they were OK with taking the risk that it might be on the flexy side for 'round the world. But then they came back in a panic because they felt the bikes were nealry unrideable once loaded up. They wanted new frames built, and of course they didn't want to pay for them — it was my fault! I saw the way they loaded them, too much weight, and all on the back and in a high-mounted handlebar bag. I convinced them to try a 3-pronged approach:

1. CARRY LESS. Long-distance tourists (especially first-timers) often end up mailing unnecessary stuff back home once they realize they didn't need that stuff. So get serious about paring down your list of must-haves before you leave.
2. Put most of the weight in front low-riders, with mostly large low-density stuff like clothes and sleeping bags in back. Minimize weight in the handlebar bag.
3. Ride a lot with the full load, to get used to it. A bike that seems unrideable at first, starts to feel normal after a while. You can adapt to almost anything. Long-haul tourists have often remarked that they don't like how the bike handles unloaded, after they get completely used to how it handles loaded.

They accepted the challenge, did all 3 things on the list, and then they were very happy with their bikes and had a great tour. Phew!

RCMoeur 

Looking at the title, I kept waiting for the "Asking for a friend" in the initial posting.

I've never had much of an issue with speed wobble in a frame or bike myself, but when people talk about their experiences and their voices get all quiet and the color runs from their faces - I have no reason to doubt them if they say Mr. Happy Bike turned evil on a fast downhill.

Trying to summon my modest LRFD design knowledge, it takes many many small flex cycles or a few big ones to start compromising a steel structure with reasonable safety factors.

79pmooney

Miyata - quality bike, quality tubing, quality workmanship. Skinny tubing. Yes, do the wrong combination of things and add a driving oscillator and shimmies can happen. But major shimmies of skinny tubing aren't huge stress producers. All I would do is periodically, like when you have it on a stand anyway, look it over reasonably carefully, looking for cracks. Perhaps with a small flashlight. Give the head tube and lugs and nearby top and down tubes extra attention. But don't sweat it otherwise (unless you are planning to regularly do these shimmies). And steel is wonderful. It nearly always shows visible cracks well before it breaks. You often feel it not being "right" or hear a new sound.

My Mooney has seen several hard crashes in its 50K and 45 years. I trust it completely. I did retire my Miyata 610 after I shortened the wheelbase most of a foot but I am sure it could have been straightened and ridden. (And in the 1930s, had it existed, it would have been. Depression years and that straightened Miyata would have been fine ride.)

RCMoeur 

OK, I'll ask. There wasn't a torch or cutoff wheel involved?

Andy_K 

I've had it happen twice. Once was on an aluminum Giant TCR I rented in Utah. That time wasn't terrible, but enough to know I didn't enjoy the experience. Then I had a decade or so of blissfully wobble-free riding, before my Pinarello Montello did it to me on a long, straight downhill coming off of Bald Peak. That one was really, really bad -- like, I was convinced that the bike was possessed and trying to kill me. I somehow managed to bring it to a stop without hitting the pavement, but by the time I did I was on the wrong side of the road and terrified. I think I walked it down the rest of the hill. It took about two years after that before I could descend a straight hill at anything more than about 20 miles per hour, and for most of that time I felt like every bike I rode was starting to wobble on straight downhills. I had no problem descending curvy roads, but straight ones just freaked me out. I eventually figured out that I was locking my elbows and holding the bars too tight. I don't think that's what caused the initial incident, but the initial incident definitely altered my riding ability. Once I managed to get myself to relax things clicked back into place and I started rebuilding confidence. I still don't trust that bike, even though I'm quite certain there is nothing mechanically wrong with it.

Bikedued

Injury to yourself, or the bike? Sorry had to ask. :-)

bikingshearer 

Andy_K , I had not thought about the straight road factor until your post. I've had a small handful of shimmy incidents over the last mumble mumble years. Always at 40 mph +/- and, come to think of it, always on straight or gently curving roads where you don't have to brake unless and until your personal Chicken-O-Meter siren starts blaring. No crashes, although it sure felt like I was about to once or twice. The last one was about four years ago on my beloved Cinelli. I used the "clamp the knees hard on the top tube" trick. It did not completely stop the shimmy by itself, but it greatly reduced it and made the bike controllable while I gently slowed down to the point where the shimmy demon decided to leave me alone.

My guess - and it's only a guess - is that on twisty descents, between the leaning and braking and accelerating and weight shifting, the harmonics or oscillations or whatever are never constant enough long enough to get into a Tacoma Narrows Bridge-type feedback loop. Of course, my knowledge on the subject is purely anecdotal and is based on no scientific knowledge or training whatsoever, so once again, my opinion is worth exactly what you are paying for it.

Steve B.

Shimmy was from the load on a bike not designed for fully loaded touring, if that’s how you were using it. Photos I can find seem to show a bike more suited to credit card touring, so I suspect has a build not designed for the loads you had on it,

Unless you rode this bike every year as a loaded touring bike, for the past 30 years, I would not expect and damage or unusual wear and would keep riding,

79pmooney

Nah. Just pure cold setting. Of the fork, top and down tubes and two posts of a chainlink fence.

Fuji1986

To the frame tubes. Sorry that wasn't clear!

79pmooney

I routinely brake enough to keep my speed to the high 30s/low 40s on my Ti Cycles going down that hill. (You are talking Bald Peak Road, no? Or Laurel?) When that bike was new, I was 17 years younger, a lot stronger and a lot more confident. I did that hill (Bald Peak) on that bike very close to as fast as I have ever ridden. No speedo but certainly faster than the 49 mph I pegged on my Mooney on Laurel the previous year. (Bike was brand new and I had a huge tailwind!) But if I am not confident - becoming much more so as old injuries affect my life more and more and crash recovery takes far longer - the bike wobbles if I push it.

Fuji1986

The 618 was the second-tier model that year and definitely less stout than the flagship 1000 (thinner seat stays for one thing). It's more capable than a credit card tourer but maybe not for a 4,000-mile camping trip. (I remember it wiggling every time I took my hand off the bars to grab my water bottle.)

In the years since that trip I did several shorter tours with maybe 25-lb loads, and though not rock solid it performed well. It's actually a fairly "young" frame, since I bought it in 2000 as new old stock, and it's touring years were pretty much confined to a decade. After that it served as a one-speed grocery bike, barely ridden. I'd say it has only 10–12K miles on it. So a refurbish (some new parts and a paint polish) feels right so long as the frame is sound.

Mad Honk 

I had a frame flex on me going downhill and it threw me into a cornfield. It never hit the road with me on it again. I should have just cut it up and dispose of it, But I had a young fellow that asked for it so I gave it to him. I'm like Calvin, I don't trust a bike that is trying to kill me. Smiles, MH

RCMoeur 

I'll bet you didn't even use a string and ruler to check your work. Seems sloppy.

mpetry912 

Ferrous materials deform under strain according to Hooke's law of plastic and elastic deformation - see chart below.

Shimmy or frame wobble is usually caused by a pendulous load that gets in resonance with wheel rotation and can grow to very high (scary) amplitude.

However I sincerely doubt that you've strained the material beyond the elastic limit ("C" in the chart below) Had you done so, I doubt you'd be here talking about it.

What I'd suggest is that the frame is fiine - but if you're going to do heavy touring, look at better racks, especially where the racks attach to the frame.

putting 40lb of weight on a rack that attaches to the brake centerbolt is quite likely to result in oscillation at some speeds, in resonance with wheel rotation.

maybe a newly built rear wheel and / or new front wheel would help as well. Or front racks in the "low rider" mode.

but to answer your original question, I wouild bet the frame is OK.

/markp

unterhausen

Shimmy is much more likely from a load on the back. My '80s road bike would shimmy reliably at about 40mph if I put my weight too far back. In the absence of a crash, shimmy is unlikely to cause any damage to the frame since virtually all of the motion is in the headset.

mpetry912 

Yes. I lost a good friend to uncontrolled shimmy on his motorcycle. He had a large "trunk" mounted pretty high up and had commented on it prior. he was on a trip with a load, and probably rolled into a turn and lost control, went into oncoming traffic.

Most of the time frame shimmy is a resonance of some kind, in sync with wheel rotation. Make sure your rear rack is solidly mounted, preferably clamped to the stays, not just the brake bolt. And consider front low - riders, they help

/markp

Fuji1986

Thanks for the response. I've checked all the common stress points where steel frames fail (the rare times they do) and I'm not seeing anything that concerns me. On that tour I had a Blackburn Expedition rear rack and low riders up front, plus a big h-bar bag. No matter where I put weight, the bike just complained. Anyway, that's a long time ago and I suspect my bike camping days are behind me. My Miyata would probably thank me for that!

RH Clark

Ya'll riding too fast and too heavy. I don't bounce as well as I used to. I ride slower now and lighter on bikes not designed for it. My Surly LHT has never had the shimmy, but then again, I go slower.

cyccommute 

Speed wobble. AKA death wobble


This article does a pretty good job of detailing why it happens. TL;DR version: It’s due to the gauge of the tubing being too light and too flexible for the job. More modern bikes than your 618 have stiffer frames all the way around.

The good news is that the speed wobble isn’t likely to go outside the elastic region of the graph in mpetry912’s post. It’s scary and may make you want to change your shorts afterward but it won’t damage the bike. If it happens again, just put your knee against the top tube and that dampens the vibration enough to stop the wobble.

52telecaster

I've toured on purpose built touring bikes and sport/racing bikes. I find it I keep the gear down to where I can use front lowriders, a saddle bag and handlebar bag I have almost no handling problems. With heavy loads in back I almost always don't like it. Ymmv but I bet that miyata could still tour.

RH Clark

Me too. If it is still true and shows no signs of damage the steel didn't exceed its capacity to spring back into shape. I have an 83 Miyata 610 as well as the Trucker but only use my Miyata for light trips. I could tour on mine if I leave the kitchen sink at home and take it easy like an old man, which I am rapidly becoming.

Andy_K 

Yeah, that's kind of what I thought too.

BTinNYC 

There's a really nice 63 cm Centurion Prestige frame, stripped and hanging from the rafters of my garage. Those long super thin tubes provided a terrifying unforgettable shimmy on descents. Never again.