Fix shimmy with foam?

Hi All,

I've got a carbon frame that is too flexible up front. The top and
down-tubes are too narrow, so there is a shimmy tendency.

I wonder if filling the frame with expanding foam insulation would
alter the elastic properties of the frame enough to limit the shimmy,
or at least move it into a speed range I won't frequent?

Joseph

On 8/19/2011 2:25 PM, Joseph S wrote:
Hi All,

I've got a carbon frame that is too flexible up front. The top and
down-tubes are too narrow, so there is a shimmy tendency.


I would check headset bearings first, then look for cracking in the head
tube area.

I wonder if filling the frame with expanding foam insulation would
alter the elastic properties of the frame enough to limit the shimmy,
or at least move it into a speed range I won't frequent?

Joseph

Probably not.

Warning--the spray-can foam you get at the hardware store doesn't really
expand to completely fill spaces; it tends to leave large voids inside
closed areas. If you take a cardboard tube and spray it full, it will
LOOK like the whole thing got filled, but wait a day and cut the pipe in
two lengthwise.... to see what the interior looks like. IF you used poly
or latex foam, it won't be full.

The commercial polyurethane foam spray systems don't have this problem,
but I still doubt they would alter the issue much. Ideally you could
sell the frame to somebody much lighter-weight than you.

Joseph S wrote:
I've got a carbon frame that is too flexible up front. The top and
down-tubes are too narrow, so there is a shimmy tendency.

I wonder if filling the frame with expanding foam insulation would
alter the elastic properties of the frame enough to limit the shimmy,
or at least move it into a speed range I won't frequent?

In theory, maybe, maybe not. Structural foam or spray
insulation from a hardware store? How do you plan to
pressurize the tubes while obviating voids?

In practice, you'll end with a heavier bike which will very
likely have the same amount of deformation.


--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

On Aug 19, 8:25*pm, Joseph S wrote:
Hi All,

I've got a carbon frame that is too flexible up front. The top and
down-tubes are too narrow, so there is a shimmy tendency.

I wonder if filling the frame with expanding foam insulation would
alter the elastic properties of the frame enough to limit the shimmy,
or at least move it into a speed range I won't frequent?

Joseph

I doubt there would be much damping to low frequency oscillation.
Look at stiffening up your wheels first. A lower front tyre pressure
and a switch from latex to butyl tubes may be sufficient. Interlaced
spoke patterns with stiff stainless spokes will cause lateral
stability problems with high loads unless the interlace is kinked to
remove the spoke bowing. The same problem also occurs with 14swg
galvanised spokes and some hard 15swg galvanised spokes.

On Aug 19, 10:42*pm, thirty-six wrote:
On Aug 19, 8:25*pm, Joseph S wrote:

Hi All,

I've got a carbon frame that is too flexible up front. The top and
down-tubes are too narrow, so there is a shimmy tendency.

I wonder if filling the frame with expanding foam insulation would
alter the elastic properties of the frame enough to limit the shimmy,
or at least move it into a speed range I won't frequent?

Joseph

I doubt there would be much damping to low frequency oscillation.
Look at stiffening up your wheels first. *A lower front tyre pressure
and a switch from latex to butyl tubes may be sufficient. *Interlaced
spoke patterns with stiff stainless spokes will cause lateral
stability problems with high loads unless the interlace is kinked to
remove the spoke bowing. *The same problem also occurs with 14swg
galvanised spokes and some hard 15swg galvanised spokes.

I've tried 2 different forks with different rake and stiffness and no
discernable difference. I've also used 2 wheelsets which both work
fine on my other bike. Checked HS bearings, and checked for cracks.

The simple explanation is the narrow aero-tubes are not rigid enough
for my weight.

My house is 150 years old so I've had plenty of opportunity to
experiment with expanding foams, and I have found a good one that
fills very well. And the front triangle of the frame has openings to
the BB and the head-tube so easy to see that the foam has gone where
it should.

Maybe I should fill it with epoxy instead... I don't really care about
weight. It's the aero properties of the frame I'm interested in.

Joseph

On 8/19/2011 4:12 PM, Joseph S wrote:
.....
My house is 150 years old so I've had plenty of opportunity to
experiment with expanding foams, and I have found a good one that
fills very well. And the front triangle of the frame has openings to
the BB and the head-tube so easy to see that the foam has gone where
it should.


Not what I've seen.

Try the experiment I said: get a cardboard tube about 12 inches long,
fill it completely with either latex or poly foam, let it cure for a
couple days, then cut it open lengthwise. Only the very ends will really
be filled.

The foam cures by outgassing, and the foam in the center stays liquid
too long (because there is no open air for it to out-gas to) and it all
collapses. Only the ends will have foam filling them.

The only way you can really fill a given volume with that foam is if you
spray it in thin ~half-inch layers, and let each layer cure for a day
before spraying the next thin layer on,,,, which isn't really possible
in a tube.

On Aug 19, 3:20*pm, AMuzi wrote:
Joseph S wrote:
I've got a carbon frame that is too flexible up front. The top and
down-tubes are too narrow, so there is a shimmy tendency.

I wonder if filling the frame with expanding foam insulation would
alter the elastic properties of the frame enough to limit the shimmy,
or at least move it into a speed range I won't frequent?

In theory, maybe, maybe not. Structural foam or spray
insulation from a hardware store? How do you plan to
pressurize the tubes while obviating voids?

In practice, you'll end with a heavier bike which will very
likely have the same amount of deformation.

No doubt. I've used several kinds of the expanding foams and I predict
that they wouldn't do jack. It's still a fun idea, I'm all for trying
fun and dumb, but in this case, reversing it is a royal PITA.

The OP needs to indeed examine the frame under strong light, headset
check, and have a shop put a FAG on it to check for alignment. Could
be a build flaw, if so--warrantee it. If not, a commodity oversize
tube alu frame would be a quick fix. Or support your local steel
framebuilder. Or mine, Homegrown, where you can get it built for your
particular mass and riding style.

On Aug 19, 10:12*pm, Joseph S wrote:
On Aug 19, 10:42*pm, thirty-six wrote:



On Aug 19, 8:25*pm, Joseph S wrote:

Hi All,

I've got a carbon frame that is too flexible up front. The top and
down-tubes are too narrow, so there is a shimmy tendency.

I wonder if filling the frame with expanding foam insulation would
alter the elastic properties of the frame enough to limit the shimmy,
or at least move it into a speed range I won't frequent?

Joseph

I doubt there would be much damping to low frequency oscillation.
Look at stiffening up your wheels first. *A lower front tyre pressure
and a switch from latex to butyl tubes may be sufficient. *Interlaced
spoke patterns with stiff stainless spokes will cause lateral
stability problems with high loads unless the interlace is kinked to
remove the spoke bowing. *The same problem also occurs with 14swg
galvanised spokes and some hard 15swg galvanised spokes.

I've tried 2 different forks with different rake and stiffness and no
discernable difference. I've also used 2 wheelsets which both work
fine on my other bike. Checked HS bearings, and checked for cracks.

The simple explanation is the narrow aero-tubes are not rigid enough
for my weight.

Ah aero, you should have said earlier. So yes you really need to
stiffen it up. Be prepared for a wild guess - ceramic paint. It
should practically do nothing, but if you are prepared to fill it with
a relatively soft foam I think you should be looking at other ideas.
You might just want to beef up the most limp sections with fibreglass
and resin.

My house is 150 years old so I've had plenty of opportunity to
experiment with expanding foams, and I have found a good one that
fills very well. And the front triangle of the frame has openings to
the BB and the head-tube so easy to see that the foam has gone where
it should.

Maybe I should fill it with epoxy instead... I don't really care about
weight. It's the aero properties of the frame I'm interested in.

Joseph

What make and model aero frame is it?

On 8/19/2011 4:12 PM, Joseph S wrote:
Maybe I should fill it with epoxy instead... I don't really care about
weight. It's the aero properties of the frame I'm interested in.

Epoxy mixed with carbon fiber "hairs". The latter are nasty to handle,
however.

You could wrap the outside of the junctions of the head, top and down
tubes with another layer of CFRP - ugly but would stiffen up the front end.

--
Tºm Shermªn - 42.435731°N, 83.985007°W
I am a vehicular cyclist.