Author Topic: Shaking on the downhills  (Read 4049 times)

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Offline sanuk

Shaking on the downhills
« on: October 19, 2010, 10:03:24 pm »
It wasn't such a big deal, but when scooting down some of those Californian hills this summer I noticed the bike would hit a certain speed - maybe about 30- 40 mph - where the front end would start to shimmy, if that's the right word.  A kind of shaking that increased or decreased according to my speed.  I was fully loaded with panniers front and back and it did seem that the weight balance in the front panniers had something to do with it.  I tried to keep it equal but still got that shimmy downhill.   The bike rode perfectly otherwise and I checked all the nuts and bolts regularly.  Didn't do any downhill without the panniers to compare.  Any ideas, solutions?

Offline mu2flyer

Re: Shaking on the downhills
« Reply #1 on: October 19, 2010, 11:15:00 pm »
that's interesting, which bike and set up ????

Offline whittierider

Re: Shaking on the downhills
« Reply #2 on: October 19, 2010, 11:31:20 pm »
The easy quick-fix (but not a permanent fix) is to clamp the top tube between your knees when coasting down those fast downhills.  It not only prevents it, but stops it very effectively.  Some people have lost control and been killed because they did not know that.

A common cause of the shimmy is an asymmetrical fork, something you cannot measure without removing the fork from the frame and putting it in special equipment.  Another one is having a load up high behind the seat, and I get it (even at very low speeds) when I have two full quart bottles in the holder above my very large touring seat bag.  As soon as I swap one bottle with an empty one from the frame (so only one of the bottles back there is full), the problem is gone.  I used to get it on another bike with a certain load on the seat bag, especially if the seat bag was not sinched up tightly by its straps.  One thing people always bring up is a loose headset, but I have worked at a bike shop and test-ridden a ton of bikes, including with loose headsets, and can say I have never found a loose headset to cause the shimmy (although I won't say it can't).

This was all dicussed at length recently at http://www.adventurecycling.org/forums/index.php?topic=6948.0 , with the topic started by someone with a Salsa Fargo that had it.
« Last Edit: October 19, 2010, 11:33:42 pm by whittierider »

Offline Galloper

Re: Shaking on the downhills
« Reply #3 on: October 20, 2010, 08:40:20 am »
As mentioned above by whittierider, this is a problem difficult to identify.   The first thing I'd try would be to take the front panniers off and see what happens.  If you still get the shimmy, try again with more weight in the front panniers. 

Offline John Nelson

Re: Shaking on the downhills
« Reply #4 on: October 20, 2010, 11:18:42 am »
Warning: Experimentation, although necessary, can be dangerous. I would recommend taking the bike to a shop for a full safety inspection first, to miinimize the risk. Certain things like loose headsets that can potentially cause the problem can be fixed.

Another warning: Although stabilizing the top tube with your knees is a correct response to shimmy, braking can sometime make it worse. So resist the urge to brake until you have taken other measures first.

Offline paddleboy17

Re: Shaking on the downhills
« Reply #5 on: October 20, 2010, 01:12:15 pm »
2 properly check your fork will require alignment gages.  Not every bike shop is going to have these, so you will have to call around.
Danno

Offline sanuk

Re: Shaking on the downhills
« Reply #6 on: October 20, 2010, 09:32:04 pm »
The bike's a Koga Miyata and they have sealed integrated headsets.  Anyway there's no noticable play there.  The frame's aluminum and that's supposed to be less flexible anyway.  Less so than steel I've heard.  I don't have any hills around here to try out on without the front panniers and there's no what I would call reliable or totally trust worthy bike mechanics this side of the ocean.  Like I said, it only became an issue on those long winding downhills in California above a certain speed.  I had to keep braking otherwise I'd really lose it!  If I get it back to Europe I'm entitled to a free tune up at the shop where I bought it.  It's due.

Offline DwarvenChef

Re: Shaking on the downhills
« Reply #7 on: October 21, 2010, 03:25:03 am »
I'm interested in this topic as I'll be going along the Cascades route from Truckee to Auberry, are these some of those hills you mentioned?? I worry about my daughters bike and her ability to control it. We are going to be practicing on local hills but I don't know where they are at the moment...
Slow is smooth, smooth is fast.

Offline tonythomson

Re: Shaking on the downhills
« Reply #8 on: October 21, 2010, 06:11:51 am »
To both above - just slow down a little and enjoy the rest/scenery especially if the rider is inexperienced with touring and a loaded bike.  On tour you will have plenty of opportunities to experiment with moving your load around (there are endless threads on here about loading a bike) Despite 1000's miles of touring experience it still takes me a day or two to get things how I want them so don't worry if you are not satisfied straight away.

Good luck and enjoy
Just starting to record my trips  www.tonystravels.com

Offline sanuk

Re: Shaking on the downhills
« Reply #9 on: October 23, 2010, 07:34:45 am »
No, I didn't get up to Truckee.  That - Sierra/Cascades - was supposed to be my route back north but it looked way too steep coming up from SF.  And beyond Truckee looks like you need to be Lance Armstrong.   I had trouble just dragging my butt to SF with all those roller-coaster hills along the Californian coast!  Maybe OK if you're under 30.

Offline Westinghouse

Re: Shaking on the downhills
« Reply #10 on: October 27, 2010, 07:18:53 pm »
I have read about this problem before. Of all 37,000 miles of cycle touring I have done through nineteen countries, none of my bikes ever did this in the least, and I assure you I have gone speeding down many long steep hills and mountains, including the Rockies and the Alps. There must be something wrong with your front forks as stated in one of the posts above this one. When it gets bad you might stop pedaling, and raise your right knee, and press it as hard as you can against the top tube with your foot remaining on the pedal. That was what I read anyway.

Offline sam21fire

Re: Shaking on the downhills
« Reply #11 on: November 01, 2010, 11:07:54 pm »
I had this same problem but noticed that the amount of shimmying was different on different trips.  I found that two things seemed to contribute to the problem... slightly loose front rack bolts (the mid-way or top bolts vs. the lower bolts) and unbalanced weight loading between the panniers.  When I tightened the bolts properly and paid closer attention to the balance the shimmy disappeared.  I figured this was a much better and easier solution that finding my fork was misaligned.

Good luck!
Sam

Offline Pat Lamb

Re: Shaking on the downhills
« Reply #12 on: November 02, 2010, 10:09:36 pm »
You might start with the rec.bicycles.tech FAQ on shimmy (http://draco.nac.uci.edu/rbfaq/FAQ/8h.5.html) and see if that helps. 

Also, http://www.adventurecycling.org/resources/201004_TouringBikeBuyersGuide_Schubert.pdf suggests that light, flexible frames may contribute to shimmy.  That's something Bruce Gordon also implied, although he's changed his web site and I can't find the citation.

I've found a loose headset, out-of-true wheel, and unbalanced front panniers can initiate or aggravate shimmy.  Worst I ever had, though, was a headwind on a long downhill with traffic.  The traffic had me tense, a gust started it, and I had to slow way down (40 to 17 mph) to relax enough to make it go away.

Offline sanuk

Re: Shaking on the downhills
« Reply #13 on: November 07, 2010, 04:40:41 am »
Thanks for the input.   As my headset isn't loose, front forks as good as new and I doubt the front wheel is out, I put it down to the weight distribution in the front panniers.  Next time I find a hill I'll check it out.

Offline paddleboy17

Re: Shaking on the downhills
« Reply #14 on: November 08, 2010, 01:09:21 pm »
I have also gotten shimmy when the front wheel is not properly seated in the fork.  This was definitely speed sensitive.  I stopped riding, opened the quick release, made sure the wheel was in its proper position, closed the quick release, and found the bikes handling to be much better.

So maybe the fork is bent ever so slightly or the lugs were not optimally positioned before braising?
Danno