My next question: Swaying on a metal bridge. - Subaru Outback - Subaru Outback Forums
 
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post #1 of 5 (permalink) Old 08-01-2011, 08:10 PM Thread Starter
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My next question: Swaying on a metal bridge.

What makes one car sway left/right while driving over a metal, open-grate bridge while another car takes it as straight as on a solid road?

I have always wondered this.

Hubby's Impreza wobbles terribly across one of those bridges while my Outback does not. ( I also remember that my '68 Pontiac LeMans did the same swaying action. Very disturbing back when I was a brand new driver)

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post #2 of 5 (permalink) Old 08-01-2011, 08:53 PM
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Try riding a motorcycle across one.
I reckon the grate traps the tires back and forth in the grooves which creates the swerving feeling. Different tires and different suspension geometry makes some cars do the swerve dance differently.
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post #3 of 5 (permalink) Old 08-01-2011, 09:11 PM
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Having ridden a motorcycle across several similar styled bridges, I think it has more to do with tire width than anything, with the tire getting caught in between spikes and wanting to jerk side/side.
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post #4 of 5 (permalink) Old 08-02-2011, 03:30 PM
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Quote:
My next question: Swaying on a metal bridge.
I'm still "stabilizing" from the previous question . . .

This is another interesting one. I don't recall seeing any objective testing/reports about this, but I agree with the Brucey and upflying.

Tires, and the way they contact, and react to, the greatly reduced surface of the open grates (there's only the upper edges, which in total is probably down around ten percent of the tire's normal contact area), are probably the most significant factors. Others might be alignment, weight distribution, the distance between wheels (front to back and side to side) in relation to the pattern of the grate, as well as the size and shape of the grate openings, and whether or not they are all perfectly in line, and level, with each other.

The design of the tire tread, and the tire body, probably are most influential. As the others have mentioned, the longitudinal tire grooves (the ones in line with tire) would tend to follow the narrow upper edge of the grating that runs the length of the bridge. Small variations in the grating's straightness and height could cause one or more tires to move horizontally as it tries to follow the edge. Depending on the car's suspension, height, weight, and even speed, this could set up a rocking or side-to-side motion.

Also, not all bridge grates are built the same so if it is a reaction between the tires and/or the car and the grate, the same car might react differently on bridges with different grates designs (size, shape).

Someone on another forum mentioned that the rocking or side-to-side effect is greater the slower the car is moving. I haven't tried this. Also, if it is the way the tire tread follows the narrow edge of the grate, I wonder if higher tire pressure would reduce the effect.

It's not something that's caused me too much concern, although I can appreciate the probably magnified effect with a two-wheel vehicle. (Reminds me of the earthquake we had here last year -- I was standing on the concrete basement floor, and I could feel my feet moving side to side while I, that is, my head, was still. Weird, and scary, to say the least.)
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post #5 of 5 (permalink) Old 08-02-2011, 10:11 PM Thread Starter
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Thanks all.

I think it makes the most sense in the theory 'how the tire tread reacts to the grate pattern'. I do know that the amount of swaying is distinctively different on the 2 metal bridges we use regularly.

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I'm still "stabilizing" from the previous question . .
cute

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