You really want to dampen the vibrations you have. i.e. reduce the Q factor
of the resonator. You can do this by introducing a dissapative element
(e.g. sand) or by coupling to a resonator that is not a harmonic of what you
have. I suspect the foam might do well at reducing the Q factor. Getting the
expanding foam in to the middle of a 78 inch bar might be a bit tricky,.

I've seen contractors use expanding foam inside the hollow rungs of aluminum
ladders. (This is definitely a case where large amounts of added mass is not
an optimum solution.)

Michael Daly wrote:

On 22-Mar-2004, "Gordon Niessen" wrote:

The expanding foam would stick to the inside of the tube and become
rigid. This should stop the vibration. And would be much lighter then
sand

Take the foam's Young's modulus and calculate how much steel it is
equivalent to. Then determine how much it increases the stiffness
of the combined tube + foam. It won't do much.

You don't want to make it stiffer. You want to reduce the resonant
frequency, not increase it. That means adding mass. Check any
book on structural dynamics.

Mike

Dan wrote:

You really want to dampen the vibrations you have. i.e. reduce the Q
factor of the resonator. You can do this by introducing a dissapative
element (e.g. sand) or by coupling to a resonator that is not a
harmonic of what you have. I suspect the foam might do well at
reducing the Q factor. Getting the expanding foam in to the middle of
a 78 inch bar might be a bit tricky,.

I've seen contractors use expanding foam inside the hollow rungs of
aluminum ladders. (This is definitely a case where large amounts of
added mass is not an optimum solution.)

Michael Daly wrote:


Use the expanding foam. Fill one end with foam and then put the end
cap back on. The foam will expand to the other end. You may want to
have something to catch the excess so it doesn't get on you car.

You could easily test the sound deadening properties on a small length
of pipe before filling your car rack. But I have seen it used to stop
noise on metal stairs and around drain pipes. Should work fine on the
inside of pipes.


--
Gordon Niessen
If you aren't on the bleeding edge, you are history.

Gordon Niessen wrote:

Dan wrote:

You really want to dampen the vibrations you have. i.e. reduce the Q
factor of the resonator. You can do this by introducing a dissapative
element (e.g. sand) or by coupling to a resonator that is not a
harmonic of what you have. I suspect the foam might do well at
reducing the Q factor. Getting the expanding foam in to the middle of
a 78 inch bar might be a bit tricky,.

I've seen contractors use expanding foam inside the hollow rungs of
aluminum ladders. (This is definitely a case where large amounts of
added mass is not an optimum solution.)

Michael Daly wrote:


Use the expanding foam. Fill one end with foam and then put the end
cap back on. The foam will expand to the other end. You may want to
have something to catch the excess so it doesn't get on you car.

You could easily test the sound deadening properties on a small length
of pipe before filling your car rack. But I have seen it used to stop
noise on metal stairs and around drain pipes. Should work fine on the
inside of pipes.

It might work but, then again, it might not. (Boy, if that doesn't cover
all the bases, I don't know what will! g)

I'd imagine that sand is a far better vibration dampener than lightweight foam.

Larry

"Lawrence Glasser" wrote in message
...
Gordon Niessen wrote:
You could easily test the sound deadening properties on a small length
of pipe before filling your car rack. But I have seen it used to stop
noise on metal stairs and around drain pipes. Should work fine on the
inside of pipes.

It might work but, then again, it might not. (Boy, if that doesn't cover
all the bases, I don't know what will! g)

I'd imagine that sand is a far better vibration dampener than lightweight
foam.

Probably, but I'd be tempted to try foam if I hadn't already had success
with sand. The bars would be a lot lighter.

Lloyd

On 23 Mar 2004 11:55:44 -0500, "Gordon Niessen" wrote:

Use the expanding foam. Fill one end with foam and then put the end
cap back on. The foam will expand to the other end. You may want to
have something to catch the excess so it doesn't get on you car.

Or you may want to have the racks off the car, perhaps standing vertical to
aid in the distribution of the foam. Perhaps it is possible to have a long
"straw" to get the foam to the centre and further along the bars. Would it
be worthwhile to have a string hanging down inside the bar in case you want
to remove the foam?

You could easily test the sound deadening properties on a small length
of pipe before filling your car rack. But I have seen it used to stop
noise on metal stairs and around drain pipes. Should work fine on the
inside of pipes.

And it will keep wasps from building nests inside - my use of foam around
the house. :-) Anyway, I guess the foam is light enough not to be a factor
if it doesn't work, but it also may restrict trying another method.

Now, do you suppose this will work with the rectangular Thule bars or is it
specific to round Yakima bars?

My understanding is that we are dealing with racks that have a fixed point
on the vehicle. If there isn't a fixed point, then rack noise can sometimes
be reduced/eliminated by simply moving the rack back or forward a couple of
inches on the vehicle.
--
cheers, Stephen