How Many G's for Typical Driving?

[sethro5hc]

Hey guys,

As a quick background I'm designing a kayak rack for suv's. We're working to find the perfect material and to do so I need to know what all the rack will be put through. My question is what sort of G forces do trucks/suv's go through on a day to day basis? I don't mean the g's of starting, stopping, or crashing. But the vertical g-forces associated with hitting a pot hole, driving off a curb perhaps, or maybe even just driving right up to the river on an old dirt path with its bumps and things. Would would be the best way for me to find this out? I don't want to design a rack that's just fine and then snaps when somebody hits a pothole. Is there a fairly standard assumption for the largest G's they would see in day to day driving (again talking about potholes and curbs here, not crashes).

Should I just pick a pretty heavy truck and assume it runs off a 6" curb and use that or what? Any help is greatly appreciated here, I just don't want to make too large of an assumption and overdesign this thing, we're trying to cut costs on it as is.

 

[GregLocock]

I suggets you reverse engineer a reputable brand of rack with a known load rating, to double check this.

However- I doubt the body of a car, remote from the suspension, sees more than about a g (that is, have you been pushed down in your seat by a weight equal to your own, or bounced clear of it completely?).

Cheers

Greg Locock


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[Tmoose]

I think I remember an article in Car and Driver back in the era of the ponderous Ford Thunderbirds. Someone was commenting about the 'Bird's handling while achieving over 0.3 g (1/3 g) lateral,or maybe braking acceleration. Supposedly a Ford engineer commented "What? Did you run into something?" I always figured that was commentary about "typical" T-bird owners' driving habits, not the cars' capabilities. After playing with a rental Grand AM with an onboard g-meter I think a similar value (max 0.3) might apply for many US drivers, especially those thoughtful of their passengers.

 

[sethro5hc]

Thanks guys. I'm just trying to think worst case scenario here, without a wreck that is. Think an old SUV with worn out shocks driving off an 8" curb or a driver going over some old railroad tracks at 60 without seeing them. I know I've done some things in my 4runner before in daily driving where it sure felt like a heck of a lot more than just .3 g's. I know with some off roading I've said "ow" after hitting certain ditches/potholes etc.

About the reverse engineering, I tested a similar rack yesterday. Same v-wing design. To give you an idea, imagine a flat 150mm x 150mm square touching the ground. It's about 25 mm thick. Out of it comes wings on either side that go 250mm in the X-direction and about 90mm in the Y-direction, or 8" and about 3.5" for us English unit weirdos. PC+ABS is their material. Wings are about 10mm thick. I stood on both ends of the wings (about 190 lbs) and it bent down until it just barely touched the floor on either side, then immediately sprung back to normal shape when I stepped off. A co-worker who's more like 160 stepped on and it didn't deform all the way to the ground. So that's 160 pounds that it can handle on just ONE of the 2 v-wings that would hold a kayak/canoe. It never cracked, broke, or showed any plastic deformation. So if centered right, that one can hold 320 pounds. Considering even heavy canoes are only in the 80-90 pound range, and you wouldn't expect people to lift one much heavier than that on top of their car, it seems like this company prepared the product for a much larger g-shock than 0.3g's. PC+ABS is an expensive plastic, quite expensive actually. I doubt they would've over-engineered it so much if they didn't think it was necesarry. That's where I got the 3.0g's worst case scenario estimate.

 

[GregLocock]

Not too sure where you got 0.3g from, that is the typical acceleration that a commuter uses.

OK, so it looks like for vertical inputs you want more than 1g and possibly 3g. Short of measuring it I don't think you'll get much closer than that.





Cheers

Greg Locock


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[sethro5hc]

Thanks,
Yea I'm not really concerned about the horizontal G's experienced during acceleration while driving, those G's are forces to be dealt with by the mounting hardware, not so much the rack it self. The downward forces and vertical G's are the ones I'm concerned about.

I know the design will easily hold as heavy of a canoe/kayak as anybody would put up there right now, I just can't have it snapping when they fly over that speed bump they didn't see and get a little air.

 

[NormPeterson]

What about fatigue and the material's resistance to it?

I'm more than a little doubtful that the occurrence of just one near-"design basis" event will guarantee that the sort of drivers/driving involved won't see it happen repeatedly.


Norm