suggestions for shimmy?

[NME]

Hi All,

Sorry to bring up a well-discussed topic, but after reading through
the history of threads, I'm still not sure what to try. I've had my
Hilsen for about five months now, and I've always ridden with
Sackville TourSacks (the rear panniers) mounted on a Nitto Big Back
Rack. I've got less to carry on my commute now, so I took off the
panniers and decided just to strap my bag to the rack. The problem is
that I get a consistent low-speed shimmy when trying to ride no-
handed, which had never happened with the panniers.


I'm not sure what info is relevant, so I'll give you what I know, just
in case:

-- Me (135 lbs with shoes)
-- 55 cm. Hilsen, 33mm Jack Browns, SKS fenders, Nitto Campee Big Back
Rack.
-- U-Lock
-- Messenger Bag: 10 lbs of macbook, power cord, wallet and phone.
Smallish canvas bag, but extends 2" over each side of the rack.
-- Two bungees to strap down the bag.

What I've tried:

-- strapping the bag every which way, moving it forward and backward,
turning it sideways, multiple bungees to lock it down.
-- tightening the rack attachments.
-- changing the tire pressure
-- a lighter load. A 5lb bag (leather purse/wallet/phone/u-lock)
*doesn't* shimmy.


I'm glad the bike handles so well with the occasional 50 pounds of
books and groceries in the panniers, but it would be a real shame if
it couldn't handle 10 pounds on the rack.

Has anyone actually had luck tightening the headset?
Any other suggestions?

Thanks for your advice!
Nicole

[Ken Freeman]

Nicole, I've had this happen on a Woodrup touring frame with a rack trunk.  I really didn't resolve it, but I did stop using a trunk in favor of a saddle bag that snugged under the saddle and did not attach weight to the rack.  The bike had medium-low trail, so it was decent with a front bag.  I added in the front bag with a rack for a long SAGged tour, and that worked quite well.  It does make me want a bike with a low trail.

The fact that you don't have lateral veering when riding no-handed indicates you don't have significant fork/frame/wheel alignment issues, though few bikes are perfect.

Jan Heine has recommended going to a roller bearing headset like a Stronglight or Miche, and several members of iBob have  stated that it works.  The principle is the same as tightening a headset, but I would not suggest tightening a ball-bearing headset.  The risk is accelerated wear of the bearing surfaces (races and cups), which are not restorable.  Could be an expensive proposition.  I'd only suggest it if I knew that you or your mechanic are very well-skilled.  But knowing the risk you can think about proceeding.

The overall principle is to add drag to the steered part to damp an oscillation (the feeling of shimmy is oscillation).  There are a few ways of adding damping - head bearing drag (tighten or go to a roller), a damping cylinder (not uncommon for motorcycles), or perhaps tires.  This last is a little hypothetical, but suppose your 33's are inflated to full pressure so you're prepared for your 50# load.  When you later ride with only 15# load (purse et cetera plus MacBook), the bike feels lighter because the tires have more pressure than is needed for that weight, and the front tire contact patch is smaller and not as long.  This smaller contact patch has less resistance to force applied at the handlebar, and to the inertia of the front wheel as it rotates around the steering axis.  

Going to the Berto paper on tire pressure (google "Frank Berto tire pressure," www.bikequarterly.com/images/TireDrop.pdf), suppose your total bike weight is 135 +30+15 = 180#, and the front weight is 45% (81#), rear weight is 55% (99#).  For your tires the chart gives target pressures (I use these as minimums) of 48 psi and 59 psi, front/rear respectively.  If you reduce your tire pressures to 50/60 front/rear, you'd get as much tire damping as possible.  I'm not sure if it will be enough to solve the problem you're experiencing, but take it as a suggestion.  If you become worried that the rear is too soft, just up it another 5 to 50/65.  You can use the chart to see what your target pressures are for the heavy load case - you should see bigger numbers.

Another way to look at tire damping (it's a term I've coined) is "self-restoring force," a concept from mechanical engineering and vehicle dynamics.  You can Wiki it, but the math gets beyond me, and I'm an experienced engineer.

Good luck!  There are probably some people here who have tried this as a shimmy correction.  I and others have used it to determine target tire pressure values.

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Ken Freeman
Ann Arbor, MI USA

[Scotty]

My bike developed a frustrating shimmy that turned out to be from spokes loosening up. Retentioned the rear wheel and the shimmy went away. I guess there are probably a hundred different things that can cause a shimmy.

[PATRICK MOORE]

One of my Rivs shimmied mildly with narrow tires; replacing them with
31 mm Kojacks seems to have put a stop to it -- tho' when I replace
the seat wedge with a small front rack bag, it may come back. As
Scotty said, the causes are legion.

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[Jim Mather]

For my commute, I put my briefcase and lock in a grocery pannier on
the back. I don't know if a basket is an option for you, but some
times I put my stuff in a front basket and that carries well too.

jim m
wc ca

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[dougP]

Nicole:

Check the rack struts where they attach the rack. I also have the
Nitto big back rack & have noticed those can loosen up. It's not much
but can allow enough play in the rack for it to move around. My guess
is a bike is more sensitive to weight on top of the rack than on the
sides.

dougP

[Kelly Sleeper]

What I would do .. Right or wrong.. Shrug

Air tires to static inflation for tests.

Remove load and ride.. Test for free handed shimmy. If it shimmies does hand on top tube stop it?

Does coating make a difference.. Coast a bit faster slower etc. Try the other direction ..

Remember Wind will make a difference.

Once base line is defined make changes.

Now ..if needed adjust headset like new install. Good time to clean and lube as well.
Tune up wheels etc.

Try different tires and wheels if available. And different air pressures.

Remember low trail bikes have shimmy and other problems of their own.. So you can ignore those jabs and move forward.

I can make every bike I've ever ridden shimmy free handed at some given speed under some given conditions.

If you work through the basics you'll fix it and be ready for fall season to boot.

Kelly

[NME]

Thank you all for your really insightful comments! I tried a few more
things, including some your suggestions -- made sure the wheels were
aligned, checked tire pressure, etc.

I noticed that the problem was much worse when I was on smooth
asphalt, but would almost disappear when I was riding on gravel,
cobblestones, or bricked sidewalk, so it occurred to me that the
problem was this "harmonic" vibration -- not that the bike was out of
alignment, but that it was too perfectly aligned. So on a whim, I
*loosened* the bolts to the rack -- not enough that the rack wobbles
or is in danger of coming unscrewed, but just enough that there can be
some miniscule "give" between the rack and the frame. My idea was
that if I could disrupt the perfect transmission of this vibration,
then it wouldn't radiate to the front wheel, but would cancel itself
out.

It was the exact opposite of DougP's suggestion and the opposite of my
original inclination, but it worked! It went from violent shimmy to
completely smooth. No shaking at any speed.

Is this perhaps another version of the "self-restoring force" Ken was
talking about? (The maths are also beyond me.) In any case, if the
problem happens again in another context, I'll be sure to look into
"tire damping" (nice term) as a first resort!

--

Thanks again, everyone! I'll finally be posting pictures of my AHH
soon, now that I've got it properly scratched up and broken in. Five
months in Berlin, and besides the daily commute, a few wonderful day
rides already, and and now some inn-to-inn rides planned for fall.

Nicole


P.S. in answer to Ken's question -- neither me nor my mechanic are
well-skilled.

[Ken Freeman]

Congrats!

I don't think its related to the self-restoring force.  That is supposed to be generated in the tire/road contact patch.

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[Ken Freeman]

Sorry, "self aligning torque."

But is the rack secure?

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[Earl Grey]

Congrats. Nice outside-of-the-box thinking. Reminds me of the violent
front brake shudder I cured by toeing the brakes OUT as far as I
could.

Gernot

[dougP]

Nicole:

Congrats on solving the problem. This is a great example of the
elusiveness of sources of shimmy and their cures.

dougP

[Marc Schwartz]

Dear Nicole &tc

Perhaps cork or leather fender washers (as sold by Velo Orange, Rene Herse (Boulder), and others) at the rack mounting bolts would provide sufficient damping to deal with the shimmy, yet allow enough tightening torque on the bolts to prevent them from working loose over time due to road vibrations.

Fall comes to New Mexico; Smell the green chile a'roasting!!!

Marc
______________________________________

On Sep 7, 5:33 pm, NME <nicolemea...@gmail.com> wrote:
> Thank you all for your really insightful comments! I tried a few more
> things, including some your suggestions -- made sure the wheels were
> aligned, checked tire pressure, etc.
>
> I noticed that the problem was much worse when I was on smooth
> asphalt, but would almost disappear when I was riding on gravel,
> cobblestones, or bricked sidewalk, so it occurred to me that the
> problem was this "harmonic" vibration -- not that the bike was out of
> alignment, but that it was too perfectly aligned. So on a whim, I
> *loosened* the bolts to the rack -- not enough that the rack wobbles
> or is in danger of coming unscrewed, but just enough that there can be
> some miniscule "give" between the rack and the frame. My idea was
> that if I could disrupt the perfect transmission of this vibration,
> then it wouldn't radiate to the front wheel, but would cancel itself

> out.dear

>
> It was the exact opposite of DougP's suggestion and the opposite of my
> original inclination, but it worked! It went from violent shimmy to
> completely smooth. No shaking at any speed.
>
> Is this perhaps another version of the "self-restoring force" Ken was
> talking about? (The maths are also beyond me.) In any case, if the
> problem happens again in another context, I'll be sure to look into
> "tire damping" (nice term) as a first resort!
>
> --
>
> Thanks again, everyone! I'll finally be posting pictures of my AHH
> soon, now that I've got it properly scratched up and broken in. Five
> months in Berlin, and besides the daily commute, a few wonderful day
> rides already, and and now some inn-to-inn rides planned for fall.
>
> Nicole
>
> P.S. in answer to Ken's question -- neither me nor my mechanic are
> well-skilled.

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[Ken Freeman]

I like that idea, absorb some of the resonant energy present in the loaded rack, which is presumably vibrating.

[NME]

Ken: The rack is secure! It's not quite loose enough that I can
unscrew the bolts by hand or that I can rattle it by shaking it. In
any case, I'll be sure to keep an eye on it every couple of weeks and
tweak as needed.

Marc: Good idea with the cork fender washers. I'll invest in some
the next time I make it back to the CONUS in the spring. It'll be
about time for a VO schwag run by then in any case.