speed wobble, geometry, and dampening

I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his Spectrum
website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding no hands on
one of my steel bikes. It is not severe, though is repeatable. This is a manuver I often do as a
descent begins to pull on a vest or something, and the shimmy only occurs on this one bike of mine.

The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is the
trail calculation?) Will the result be within "normal" for a road bike? And is it possible that this
change in trail with the added dampening of carbon will change the parameters of this speed wobble?

Thanks for anyone's insight.

Clifford Lee

 

Invisionsf writes:

> I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his
> Spectrum website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding
> no hands on one of my steel bikes. It is not severe, though is repeatable. This is a manuver I
> often do as a descent begins to pull on a vest or something, and the shimmy only occurs on this
> one bike of mine.
>
> The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
> My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is
> the trail calculation?) Will the result be within "normal" for a road bike? And is it possible
> that this change in trail with the added dampening of carbon will change the parameters of this
> speed wobble?

I don't know if a change in fork will solve your problems, but I solved the shimmy problem on my
bike by lowering the stem. I'm sure Brandt could explain the "whys" of this precisely, but as a
practical matter, dropping the stem 0.75" or so elimated the problem. It might work for you.

 

Clifford Lee wrote:
> I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his
> Spectrum website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding
> no hands on one of my steel bikes. It is not severe, though is repeatable. This is a manuver I
> often do as a descent begins to pull on a vest or something, and the shimmy only occurs on this
> one bike of mine.

Many cases of shimmy result from insufficient spoke tension in the rear=20 wheel. Because you
notice it in the front, most people don't think to=20 look at the rear end for the cause, but that
is very common.

Sheldon "'Dampening' Can Be Helped By Installing Fenders" Brown
+----------------------------------------------------+
| I=92m not convinced that this was the right time | to attack Iraq, but I would like to express
| my | support for, and gratitude to U.S. and allied | fighting forces, and to wish them all
| success. |
+----------------------------------------------------+ Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

 

Invisionsf wrote:
> I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his
> Spectrum website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding
> no hands on one of my steel bikes. It is not severe, though is repeatable. This is a manuver I
> often do as a descent begins to pull on a vest or something, and the shimmy only occurs on this
> one bike of mine.
>
> The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
> My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is
> the trail calculation?) Will the result be within "normal" for a road bike? And is it possible
> that this change in trail with the added dampening of carbon will change the parameters of this
> speed wobble?
>
> Thanks for anyone's insight.
>
> Clifford Lee

Trail calculation: rad = wheel radius under normal load off = fork offset ("rake" in bikespeak)
theta = head tube angle from horizontal

trail = (rad * cos(theta) - off)/sin(theta)

I don't know your wheel radius, but if it's a 700-23, 333 mm is fairly typical. Your original
trail is 54.75 mm, after the change it's 56.84 mm. (I know the four-place precision is meaningless
here) This assumes the geometry of both forks keeps the true head tube angle at 73 degrees. Trail
increases by slightly over 2 mm. I'll leave it to others to speculate how much this will change
the wobble.

Dave Lehnen

 

My bike has the shimmies at 30 + mph, esp when going downhill. stem has been changed w/no help. Fork
has been changed w/no help. Rear wheel has been changed and I cannot tell a difference. FWIW I can
almost repeat the same type of "feel" of the shimmie by moving one side of the handlebar when going
slow. Kinda difficult to expain, but I "vibrate" my right hand on the hoods and the front end
shimmies very similar to what it does/feels at high speed. B

 

#D>From: [email protected] (Invisionsf)

>My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is
>the trail calculation?)

Here's a trail calculator:

http://www.kreuzotter.de/english/elenk.htm#D --Tom Paterson

 

Sheldon Brown writes:

> Many cases of shimmy result from insufficient spoke tension in the rear wheel. Because you
> notice it in the front, most people don't think to look at the rear end for the cause, but that
> is very common.

As long as no spoke becomes slack under wheel loading, spoke tension has no affect on wheel
elasticity, the modulus of elasticity of steel (spokes) is independent of preload. On the other
hand, tire inflation has a large effect on lateral elasticity, varying linearly with inflation
pressure. I don't believe we are talking about a wheels with rattling spokes.

Jobst Brandt [email protected] Palo Alto CA

 

I observed:

>>Many cases of shimmy result from insufficient spoke tension in the rear wheel. Because you
>>notice it in the front, most people don't think to look at the rear end for the cause, but that
>>is very common.

Jobst demurred:

> As long as no spoke becomes slack under wheel loading, spoke tension has no affect on wheel
> elasticity, the modulus of elasticity of steel (spokes) is independent of preload. On the other
> hand, tire inflation has a large effect on lateral elasticity, varying linearly with inflation
> pressure. I don't believe we are talking about a wheels with rattling spokes.

Perhaps we are. Then again, a highly dished wheel with sub-optimal tension may well have left spokes
going slack due to dynamic stresses.

I've seen this problem in practice on customers' bikes, and have corrected it by tensioning and
truing the offending wheel.

Sheldon "Empiricist" Brown +----------------------------------------+
| All theory, dear friend, is grey, | but the golden tree of actual life | springs ever green.
| --Goethe |
+----------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone
617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

 

Invisionsf wrote:
>the added dampening of carbon will change the parameters of this speed wobble?

I think you can dismiss this one.

 

On 21 Mar 2003 22:35:24 GMT, [email protected] (Invisionsf) wrote:

>I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his Spectrum
>website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding no hands on
>one of my steel bikes. It is not severe, though is repeatable. This is a manuver I often do as a
>descent begins to pull on a vest or something, and the shimmy only occurs on this one bike of mine.
>
>The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
>My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is
>the trail calculation?) Will the result be within "normal" for a road bike? And is it possible
>that this change in trail with the added dampening of carbon will change the parameters of this
>speed wobble?
>
>Thanks for anyone's insight.
>
>Clifford Lee

check the dish of your front wheel - Ъ×

 

> I don't know if a change in fork will solve your problems, but I solved the shimmy problem on my
> bike by lowering the stem. I'm sure Brandt could explain the "whys" of this precisely, but as a
> practical matter, dropping the stem 0.75" or so elimated the problem. It might work for you.

Given that the poster's shimmy occurs only when he's riding no-handed, dropping the stem might not
have a very significant effect. :>)

--Mike-- Chain Reaction Bicycles http://www.ChainReactionBicycles.com

 

Mike Jacoubowsky writes:

>> I don't know if a change in fork will solve your problems, but I solved the shimmy problem on my
>> bike by lowering the stem. I'm sure Brandt could explain the "whys" of this precisely, but as a
>> practical matter, dropping the stem 0.75" or so elimated the problem. It might work for you.

> Given that the poster's shimmy occurs only when he's riding no-handed, dropping the stem might not
> have a very significant effect.

With all the tests we've seen on this subject, wheel balancing, wheel unbalancing, filling the tire
entirely with water and other things, I don't think there is a clear solution. Greater frame and
fork stiffness will increase the resonant frequency to a level where it cannot initiate. I have seen
more effect without changing anything other than inflation pressure.

My bicycle has always shimmied, if I let it, and it hasn't impaired my riding at any speed. I coast
down hills no-handed at maximum speed and, when I tuck in, I have both hands on the stem.
Considering that shopping carts occasionally shimmy, should make apparent that it is an instability
common to wheels operating with trail, something older cars with king-pin steering knuckles suffered
from often.

Jobst Brandt [email protected] Palo Alto CA

 

On Sun, 23 Mar 2003 23:13:57 -0500, Mike Jacoubowsky wrote:

> Given that the poster's shimmy occurs only when he's riding no-handed, dropping the stem might not
> have a very significant effect. :>)

However, a shimmy is a resonance response to the bike as a spring-mass system driven (sideways) by
the rider's balancing motions and the geometry's response. The shimmy frequency is very acutely
dependent on a whole range of factors, from tire pressure to riding style, rider weight, position,
etc., etc.

Changing _anything_ might very well stop a shimmy. Or start it. And if you have the nerve to unload
the saddle during a shimmy (say a downhill, fast speed shimmy rather than a no-hands slower one), it
will stop instantly.

A bad downhill shimmy may feel like you are losing control, but if you get out of the saddle, or
even ease up that death-grip on the bars, it will go away and not cause you to crash.

As Jobst suggests, it is not that big a deal. My old road bike did it all the time, the new one,
not at all.

If it concern you, then change anything you wanted to replace anyway. It'll probably fix it, and if
not, then at least you got your new part.

You might be able to parlay this into a whole new bike if you play it right with your
significant other.

--

David L. Johnson

__o | "What am I on? I'm on my bike, six hours a day, busting my ass. _`\(,_ | What are you on?"
--Lance Armstrong (_)/ (_) |

 

Clifford:

I believe that increasing trail (reducing rake) can magnify shimmy. The increased trail increases
the lever arm between the steering axis & the tire contact point. Any force input through the
contact patch will be transmitted as a greater torque around the steering axis.

MOO, Matt

Invisionsf wrote:

>I have read Jobst Brandt's article in RBT FAQs and I have read Tom Kellog's article in his Spectrum
>website regarding speed wobble (aka shimmy). I experience shimmy at 30 mph while riding no hands on
>one of my steel bikes. It is not severe, though is repeatable. This is a manuver I often do as a
>descent begins to pull on a vest or something, and the shimmy only occurs on this one bike of mine.
>
>The frame is a 56 cm dedaccai zero steel frame with 73 degree parallel angles and 45 cm fork rake.
>My question is will a carbon fork with 43 cm of rake add a significant amount of trail? (what is
>the trail calculation?) Will the result be within "normal" for a road bike? And is it possible
>that this change in trail with the added dampening of carbon will change the parameters of this
>speed wobble?
>
>Thanks for anyone's insight.
>
>Clifford Lee