Fork rake - 2mm?

In the past 2 years, I've had 2 instances of shimmying, but with
2different forks/rakes on the same bike. I've played with different
rake, fork style (bladed, non, etc) and both times, it was at about
40+mph (one during a team time trial on the flats, the other on a team
training all-out sprint) and both times, it was a front shimmy. I too
was concerned about the rake. I checked the wheels (diff wheels both
times), spokes, tires, pressure, skewer, headset and so on.

Now, I hit 40+ not on a daily basis, but often enough that if it would
be chronic problem I'd know it. I have begun to think that it is a
resonance thing. Hitting something on the road, pebble or something, or
even just a mini road rut can start the sympatheitic vibration, and with
those kind of speeds and wheel RPMs, it magnifies and starts this
resonant shimmy. I just stop pedaling, loosen up my grip on the bars,
feather the brake, and it diminishes and I go on, and even get back up
to those speeds again, without a recurrance.

Anyway, my experience, FWIW...

Dave

richard said:

I have an Independent Fabrications Crown Jewel, 55 cm, on which I
replaced the steel fork with a carbon fiber. I had reason to believe
the rake was 43 mm, so I bought a Reynolds 43 mm fork. All is well,
EXCEPT...

The past two years, decending a hill in Dubuque, IA, I have experienced
violent shimmying. The sensation is as if a REAR tire has blown and the
wheel has broken loose. The first time (last year), I nearly was lucky
to regain control in time to keep from going down along with a crowd
that no longer liked me. This year, I was ready for it...

This hill has a very smooth surface, but there is a gentle sweeping
curve. Each time, the shimmying occurred at somewhere over 35 mph.

This bike has been ridden at speeds in excess of 40 mph on other
occasions without this shimmy...

Since last year, the wheels and tires have changed. There seems to be
no play in the headset nor the hubs.

I did not experience the shimmy the year I went down this hill with the
steel fork. HOWEVER, on that occasion, it was raining and unseasonably
cold, so the hill was taken quite slowly.

Could this shimmy be the result of a fork with 43 mm rake instead of 45
mm rake? (Better yet, would the extra 2 mm solve the problem?)

Thing is, after buying and installing another fork, I'd have either a
LONG drive to Dubuque or a long wait till next year to confirm...

richard said:

I have an Independent Fabrications Crown Jewel, 55 cm, on which I
replaced the steel fork with a carbon fiber. I had reason to believe
the rake was 43 mm, so I bought a Reynolds 43 mm fork. All is well,
EXCEPT...

The past two years, decending a hill in Dubuque, IA, I have experienced
violent shimmying. The sensation is as if a REAR tire has blown and the
wheel has broken loose. The first time (last year), I nearly was lucky
to regain control in time to keep from going down along with a crowd
that no longer liked me. This year, I was ready for it...

This hill has a very smooth surface, but there is a gentle sweeping
curve. Each time, the shimmying occurred at somewhere over 35 mph.

This bike has been ridden at speeds in excess of 40 mph on other
occasions without this shimmy...

Since last year, the wheels and tires have changed. There seems to be
no play in the headset nor the hubs.

I did not experience the shimmy the year I went down this hill with the
steel fork. HOWEVER, on that occasion, it was raining and unseasonably
cold, so the hill was taken quite slowly.

Could this shimmy be the result of a fork with 43 mm rake instead of 45
mm rake? (Better yet, would the extra 2 mm solve the problem?)

Thing is, after buying and installing another fork, I'd have either a
LONG drive to Dubuque or a long wait till next year to confirm...

I would suspect the stiffness of the fork rather than the geometry, 2mm
difference being not very much at all. There are a lot of bikes out
there with slightly mismatched head tube angles and rake, and not many
of them shimmy (although they may try to make a sudden dive for the
ditch when standing up, like my touring bike does).

Do you know the head angle of your bike? If so, compare it to the fork
rake used on other bikes of the same angle - it's published on most of
the manufacturers' websites. There is some room for manoeuvre; it's not
quite as tight as "a 73 degree head must have a 43mm rake" (or whatever).

richard said:

I have an Independent Fabrications Crown Jewel, 55 cm, on which I
replaced the steel fork with a carbon fiber. I had reason to believe the
rake was 43 mm, so I bought a Reynolds 43 mm fork. All is well, EXCEPT...

Do you still have the steel fork? I don't think the 2mm is the problem, but
the mass/resonance probably is.

Art "If it ain't broke..." Harris

Well, the 3 replies so far mention resonance. I guess the very smooth
road would encourage the resonance (vs a rougher road shaking things up
to the point where the vibrations cancel each other out). Again, the
shimmy occurred on a road that was almost linoleum smooth...

richard said:

I have an Independent Fabrications Crown Jewel, 55 cm, on which I
replaced the steel fork with a carbon fiber. I had reason to believe
the rake was 43 mm, so I bought a Reynolds 43 mm fork. All is well,
EXCEPT...

The past two years, decending a hill in Dubuque, IA, I have experienced
violent shimmying. The sensation is as if a REAR tire has blown and the
wheel has broken loose. The first time (last year), I nearly was lucky
to regain control in time to keep from going down along with a crowd
that no longer liked me. This year, I was ready for it...

This hill has a very smooth surface, but there is a gentle sweeping
curve. Each time, the shimmying occurred at somewhere over 35 mph.

This bike has been ridden at speeds in excess of 40 mph on other
occasions without this shimmy...

Since last year, the wheels and tires have changed. There seems to be
no play in the headset nor the hubs.

I did not experience the shimmy the year I went down this hill with the
steel fork. HOWEVER, on that occasion, it was raining and unseasonably
cold, so the hill was taken quite slowly.

Could this shimmy be the result of a fork with 43 mm rake instead of 45
mm rake? (Better yet, would the extra 2 mm solve the problem?)

Thing is, after buying and installing another fork, I'd have either a
LONG drive to Dubuque or a long wait till next year to confirm...

You can typically get rid of the resonance by resting a knee against
the top tube. Not an ideal solution, but it will work. Make sure you
gently rest on or both knees against the top tube when you're
descending > 35 mph.

I don't think it's the rake. It may be poor alignment in the frame, or
somehing else wacky.

richard said:

Well, the 3 replies so far mention resonance. I guess the very smooth
road would encourage the resonance (vs a rougher road shaking things up
to the point where the vibrations cancel each other out). Again, the
shimmy occurred on a road that was almost linoleum smooth...

richard said:

I have an Independent Fabrications Crown Jewel, 55 cm, on which I
replaced the steel fork with a carbon fiber. I had reason to believe
the rake was 43 mm, so I bought a Reynolds 43 mm fork. All is well,
EXCEPT...

The past two years, decending a hill in Dubuque, IA, I have experienced
violent shimmying. The sensation is as if a REAR tire has blown and
the wheel has broken loose. The first time (last year), I nearly was
lucky to regain control in time to keep from going down along with a
crowd that no longer liked me. This year, I was ready for it...

This hill has a very smooth surface, but there is a gentle sweeping
curve. Each time, the shimmying occurred at somewhere over 35 mph.

This bike has been ridden at speeds in excess of 40 mph on other
occasions without this shimmy...

Since last year, the wheels and tires have changed. There seems to be
no play in the headset nor the hubs.

I did not experience the shimmy the year I went down this hill with the
steel fork. HOWEVER, on that occasion, it was raining and unseasonably
cold, so the hill was taken quite slowly.

Could this shimmy be the result of a fork with 43 mm rake instead of 45
mm rake? (Better yet, would the extra 2 mm solve the problem?)

Thing is, after buying and installing another fork, I'd have either a
LONG drive to Dubuque or a long wait till next year to confirm...

richard said:

Well, the 3 replies so far mention resonance. I guess the very smooth
road would encourage the resonance (vs a rougher road shaking things up
to the point where the vibrations cancel each other out).

No, the input frequencies causing the resonance will be near the resonance
response (classically, exactly at that same frequency, but the bike-rider
system is a bit more complicated than a standard second-order
differential equation). It's the small side-to-side motions we make to
stay upright that are magnified by the resonance of the system, along with
things like the wind and our own response to the developing shimmy (death
grip on the bar does more to increase the shimmy than to keep you on the
road).

Changing _anything_ can bring the system into a state where it will shimmy
at a particular speed. Changing anything else may well fix it. Quickest
fix is to get your weight off the saddle. Shimmy will stop.

--

David L. Johnson

__o | Do not worry about your difficulties in mathematics, I can
_`\(,_ | assure you that mine are all greater. -- A. Einstein
(_)/ (_) |

One of the cycling FAQs suggests that this is really not a well understood phenomenon and is quite unpredictable and no sure means of remedy. Otherwise, it would be possible to, for a given bike, make certain measurements and do something to make this impossible for that setup.

I've had my share of shimmy experiences and learned some of the techniques to mitigate them, such as trying to place weight at the pedals, resting knees against top tube (which may not be possible due to bike geometry), and finding a new weight distribution between front/rear.