JohnnyU

I did a 40 miler today on Mormon Immigrant Trail by Placerville, CA and on my first serious decent, I had mad front end shimmy. I nearly lost control of the bike, but finally got under control after what seemed for about 30 seconds. I was doing about 40 mph at the time. I clamped my knees on either side of the top tube for the rest of the day and didn't have any more episodes.

Does anyone have any insight for why this may be happening?

Thanks

Homebrew01

Loose headset ?? Apply front brake, rock bike back & forth. Is there any "clunking" movement or is it nice & tight ?

toucci

Every bike will do this to some degree at a certain speed. Whether you notice it or not is up in the air. I have never owned a frame that oscillated wildly at speed but I have ridden one.

Things you can do:
Change your weight distribution ("jockey" the saddle)
Clamp the top tube with your knees (good move)
Change your speed (faster or slower)

edit: here's an article on Sheldon's site: https://sheldonbrown.com/brandt/shimmy.html

JohnnyU

Thanks for the post.

All is well when I test the headset. I may take the bike into my LBS and see if they have any perspective on this.

JohnnyU

Thank you for your post.

I think I just need to "manage" my downhill experiences. The knees on the top tube made a difference. Perhaps too much weight up front exacerbates the situation. I'll check out the link. I'm gratefull for the replys.

JohnnyU

Hammertoe

Here's another idea on why...

https://www.calfeedesign.com/forksymmetry.htm

WheresWaldo

I have seen the Calfee article before and with current technology and the advent of composite forks I simply don't buy his line of reasoning. It simply doesn't make sense for an item that is made in a mold, unless the fixture was built improperly. I think Calfee was looking for an answer that was out of the norm and happened to think this one up and then said to himself, "It could happen!"

MKahrl

Lifting your rear off the saddle will also stop the shimmy. It's a little scary at first but it removes a contact point that the shimmy is pivoting off of.

Dave Moulton

I tried to explain it in simple terms here.

BarracksSi

What about the wheels being out of balance or true?

Brian Ratliff

Actually, it is extremely common in manufacturing for molded components to warp as it cools from the molding temperature to room temperature. Molded parts will always cool non-uniformally, and if you are talking about carbon fiber components, there is the additional variable of rheology of the curing resins, which due to non-uniformity in part thickness, will cause thin parts of the carbon fiber component to cure ahead of schedule and thicker parts to cure behind schedule, so to speak. The molds have to be very carefully designed to minimize these problems. Especially for parts which cannot be post finished with machining or cold setting.

Welding or brazing will also cause parts to move out of alignment. This is likely the reason behind welded steel and aluminum frames taking such a long time to enter the market after 100 years of lugged steel and a shorter time of lugged aluminum.

I actually buy this reasoning for speed wobble. I have one bicycle which has speed wobble starting at around 25-30 mph if I ride no hands. On top of that, it also tends to pull to the right and is very difficult to ride no-handed. I have three other bicycles of various manufacture, design and materials and none of them tend toward speed wobble at any speed, even going 45 mph with no hands.

BarracksSi

This is true with car parts as well. Part of the higher cost of higher-quality parts is to cover the losses of throwing away the parts that didn't come out of the mold very well. Different car manufacturers even have different tolerance standards that their parts must meet before leaving the supplier; some will throw away four times as many parts because they're so picky.

cedricbosch

Winter76

Zoinks, I have problems getting up to 45kmph let alone 40mph. Of course we have no hills here...

cyclezen

anecdotally, a headset which is too tight or is brinnelled (oldstyle with loose bearing) can be a common cause of 'headshake'
'too-tight' even applies in modern 'threadless' and integrated designs (found that out the 'hard' way). I have had to 'readjust' one Cane Creek S2 headset which seems to 'tighten' over periods of months of use, though not recently...
Last year I bought a new Tarmac off the rack and it rode quite 'truckish' and did start a shimmy at about 40 mph. After checking the headset, it did seem too tight. So I readjusted and the handling changed, for the better, instantly. Still was too big a bike though...

I'm surprised 'trail' wasn;t mentioned by some others...
My Limongi, built for me as a 'crit' bike by Pepe, back in '76 has some nasty headshake when it gets much over 40 mph. I still have it, luv it, but know better than to go rippin down extreme downhills. It has a very steep headtube and the original fork (still on it) prolly has too much rake. Makes for a real quick handling bike at 25 to 35, but downright brownstain inspiring at 40+mph downhills.

Bringing your weight back may have the opposite from desired effect. Getting your weight more onto the bars will help stabilze the front, if done before the shake starts. Bringing weight back while its shaking, could make you go down. Bringing weight back on a downhill most likely means you're gonna stiffarm the bars more, exasperating the 'shake'.
Get as much weight on the pedals as possible, clamp the TT and bend the elbows to keep the arms from fighting the shimmy. make sure your will is uptodate.

Brian Ratliff

I noticed the Calfee site specified that they have "identified a cause" for speed shimmy. I suspect that there is more than one cause which results in similar symtoms.

JohnnyU

I found that Dave Moulton's article describes the conditions for shimmy that I experienced. I'm a tall rider (6'3") on an XL compact carbon fiber frame. I must admit that when this was happening, I had a death grip on the handlebars which seemed to make matters worse.

I'll try the lighter grip and the suggestions to get weight off the saddle this weekend. I also will check the headset to see if perhaps it is too tight.

Oh, and the will looks solid.

superdex

I'm 6'3" and ride a compact frame, and had some shimmy. It was a loose (integrated) headset. I'd start there...

NealH

The nice thing about a Calfee bike, they come with a no compromise policy on shimmy. This means a Calfee will not shimmy, much the same that any competently designed and built bike will not shimmy.

If you should get a Calfee that exhibits shimmy, rest assured the factory will immediately work with you to get it corrected regardless of the cause. They don't tolerate it. No other manufacturer/dealer should either.

nerobro

There are a great many reasons for headshake. Lots of them have been identified. Another reason that hasn't been mentioned is resonance. The front tire of a bike is constantly hunting. left and right. it's never "really" straight. The speed it hunts at varies with forward velocity. The resonant frequency of the forks is defined by the weight of the handlebars, the CG of the bike. Motorcycling tells me that a more flexable frame is more prone to headshake, same for a flexable fork. Your arms are awesome damping devices, typically just having a hand on the handlebars is enough to stop headshake in it's tracks.

Bontrager

I'll bet it was your death grip. Check everything on the bike is straight, tight and true before you go out. Maybe have a friend ride with you to look at your ride while you descend and see if s/he see's anything vibrating or out of whack.

redtires

In light of all of this fork symmetry talk, the only way that it would make a difference is if every last part of the bike was also in perfect (or within .5mm according to the article) alignment. I've designed and built hundreds of custom steel bikes, and the forks that go along with them, and yes, we always worked to have alignments well within the 1mm or better range...but that goes for the ENTIRE frame AND fork. You can not just simply align a fork...the fork and the bike MUST be aligned together, otherwise symmetry is for not. I don't care what anyone at Calfee says, they are not providing a valid service unless they are aligning the entire frame and fork. And that not only covers the the side to side, planar alignment, but you have to make sure the fork blade lengths are identical as well as the seat and chainstay lengths. This can only be accurately achieved AFTER the head tube and seat tube are realized in perfect alignment to the center of the bike. High speed shimmy cannot simply be attributed to one dimension of the bike. One bike may shimmy at a certain speed with one rider on it, yet not with another under as close to identical circumstances. Each case is different and may be attributable to any one of multiple variances.

Honestly, that article that Calfee put out kinda pissed me off, I've always liked their stuff...but that information is just totally misleading. Sending your fork to them without the rest of the frame is useless and is a waste of your money.

Skeleton

I recently balanced my wheels - it worked. My method only works on a wheel that is minutely heavy on one side of the circumference; it doesn't work if the wheel is imbalanced laterally (left or right side).

I placed the bike on a bike stand, put it into highest gear, then cranked the peddles fast by hand. The rear wheel developed a shimmy vibration. I stopped the wheel, then very very gently turned the wheel ever so slowly by hand to try to get it to reveal its weighted side. Then I placed a small counter weight on the opposite (light) side of the wheel. I used a spare cycling computer spoke magnet on the rear wheel on the outer radius of the spoke. After a bit of trial and error testing, I discovered the exact spoke and radial placement for the counter weight. Now the shimmy is gone.

JohnnyU

Wouldn't you feel the out of balance wheel, though in other ways than shimmy?

Brian Ratliff

Just a thought:

Isn't the steering axis with respect to the contact patch of the tire dictated soley through the fork and wheel? The frame alignment will have to do more with whether the steering axis is in line with the rest of the bike, but has nothing to do with where the steering axis falls with respect to the contact patch.

They are simply reporting what they found. I have a little trouble with their alignment of 2mm being the difference, seeing as all the other dimensions on a bicycle are an order of magnitude or more larger than this tolerance. But if they found that 2mm fork misalignment tends a bicycle toward speed shimmy, then why wouldn't they report what they found? Why get pissed off about it?