Windshield torx bolts torque specs? I have a bad Light bar vibration.

SigMtnMan · 10-11-2009, 09:01 AM

Anyone know the torque specs for the 6 bolts on either side of the body/windshield?

I am getting a violent harmonic vibration at 55-60ish from the light bar portion of my gobi rack.

Sgalway · 10-11-2009, 03:10 PM

I don't think the bolts being tighter will help. You are seeing one of two things: Vortex induced vibration (VIV), or your tires are out of balance just a little bit and you are seeing a harmonic vibration in the rack.

Balancing = You probably can't feel it, but when you hit the sweet spot the rack sure does.

VIV = It's a common problem and easy to fix. Take some small diameter rope or string, and wrap it around the tubes similar to the wraps that are on your radio antennea. You want good spacing between the wraps. This will change the shape of the vortex shedding pattern and keep it from ocsillating.

**1222** · 10-11-2009, 04:01 PM

Hu? Is that true (VIV)? I’ve never heard of something like that, but I’m always wanting to learn more.

Sgalway · 10-11-2009, 05:45 PM

Quote:

Originally Posted by 1222

Hu? Is that true (VIV)? I’ve never heard of something like that, but I’m always wanting to learn more.

Sounds crazy, but believe it...

Vortex-induced vibration - Wikipedia, the free encyclopedia

Look at your radio antenna, there is a winding of thicker material going the length of the wire. The strakes help to reduce the vibration, thus reducing the humming that can come from un-straked antennae.

What is happening is that a low pressure von Karmen vortex street is being formed behind the tubular which is ocsillating from side to side. The ocsillating low pressure is inducing drag on the tubular thus causing the vibration.

This video (although not exactly round) illustrates how the vortex vibration sets up. This computer simulation was part of a job I did a few weeks ago. It is part of a submerged leg of an offshore oil platform.

[YT]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/VN64We4b2yg&hl=en&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/VN64We4b2yg&hl=en&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>[/YT]

**1222** · 10-11-2009, 05:54 PM

Do you work at area 51?

Thanks.

alaskanthunder · 10-11-2009, 06:04 PM

Yeah, I learned about that when riggin' up the straps to roof top carriers. Put a little twist in the tie down straps and you won't get any wind vibration at high speeds. The wind funnels through the twists reducing the vibration.

**1222** · 10-11-2009, 07:39 PM

Damm, thanks for that tip. If I ever run into a situation like that I’ll remember this and give it a try.

SigMtnMan · 10-12-2009, 10:45 AM

Thanks for the input. I reckon I will start with getting the tires balanced and go from there.