Slings really can hold a car

The problem with using your climbing gear to pull a car out is that you have no idea what forces it is taking and hence, have no idea if it damaged your gear in a very subtle way. Of course you can see catastrophic failures, but what about just up to that? Ropes with a sheath are what I would be wary of using again, since the internal damage might be concealed pretty well.

It's also amazing how weak a climbing rope is compared to a good tow strap. I tried pulling a stuck car out with an old climbing rope and that snapped like a wet noodle.

Yarp wrote: If it makes you feel better to throw it away then by all means .....

Yeah, I usually figure my life is worth the $5 to $10 in sling material I just used to stress across a possibly oil covered bumper hitch, or sharp edged, chewed up bumper or salt encrusted tow point, or WhoTF knows what else it's undergone in this little episode...

Sure, it "might" be fine and would "probably" never fail under normal stresses but I'd sure feel like a dumbaxe cheapbustard if a climbing buddy or I got hurt or worse because I was too freaking cheap to toss a 20 ft piece of sling.

Toss it in a trunk for later towing use, use it as a dog leash, tie your wife up, recycle it or whatever, but retire it from climbing.

You can only climb on abused gear if it's Made in USA. If it's Made in China, one car pull and it's done. If you can even get your car out with it.

Ray Pinpillage wrote:I'm going to give Corina a pass on the physics, she's a cutie.

Are you quoting her instructors?

Marc R wrote:a fair approx of rolling resistance is something like 2% of the total weight (level ground). that means for a 4000 lb car it would take about 80 pounds of force to start it rolling, which makes sense considering a single person can push a car in neutral.

this is approximately accurate for a vehicle on asphalt, on level ground. i would be surprised if these were the conditions though....

stich, that must have been a pathetic rope. i have a 20 year old maxim 11mm that i have towed a handful of people with. some out of pretty heinous stuckage.

Auto-X Fil wrote:You can only climb on abused gear if it's Made in USA. If it's Made in China, one car pull and it's done. If you can even get your car out with it.

x2 but I use a usa made recovery strap thats actually made for this so what do I know.http://desertrat.com/products/?id=4102

It amazes me how little some people know about physics, yet still climb. Pulling a car with a tow-rope puts nowhere near the weight of the whole car on the rope. Hell, I've even pushed a car along half in & half out with one foot & I'm really strong, but I don't think that one of my chicken legs is pushing 4000 lb worth. The previous guess of 80 lbs is in the ball park at least on level pavement & that is probably high.

On an intersting note, I used an old 10 mm Black Diamond rope to pull boulders along my road to make a guard rail of sorts and snapped the rope on about 1/2 the tries. In that case the boulders of course had no wheels like a car does & therefore lots of friction to overcome, but didn't weigh as much either. It appeared that the rope stretched at the knots and sort of burned itself at the failure point. The trick was arranging the rope so that the boulders didn't tend to dig into the road. I don't really think I'll be using what's left of that rope for any climbing related purposes though.....

i just want to know what a civic is doing at jacks this time of year when its so muddy.

Darren Mabe wrote:i just want to know what a civic is doing at jacks this time of year when its so muddy.

Getting stuck, apparently.

I once pulled a Civic back onto the road that had its front wheels about 2-3 ft below the road level in a ditch. The vehicles were connected with my cordelette which was 7 mm perlon cord tied into a loop with a dbl fish knot. It took almost all the torque my van had. I was sure the cord would come flying back at the windshield at any second, but it held. NO WAY I would use it again though, as the DF knot was about 1/3 of its original diameter. I cut it open and there was evidence of minor sheath melting inside.

The force on the sling would be the normal force of the car (weight ) times the coefficient of friction (very slightly higher than zero) for rolling.

Not impressed, hang your car from a bridge with a sling and I will be impressed.

CorinaCarlson wrote:pull a Honda Civic out of the mud

The force to needed to pull out of mud is far greater than it would be rolling on dry pavement, hello. In this case, the coefficient of friction while rolling on dry pavement (as some have mentioned) has absolutely nothing to do with it. 80 pounds extra my ass. Y'all are on crack.

Edit to add the word "absolutely"

If you're pulling it out of the mud, that means a (relatively) high coefficient of friction. Nowhere near 1, but way higher than .02. I'd say around .2 or .3? Just a guess. Plus if it's sunk into the mud, that adds somewhere between a 5 and 10 degree incline to pull it up out of the mud. So no, it wouldn't be the normal force times the coefficient of friction.

... and climbers wonder why we're all single.

To first order newton's laws applied to a rope pulling a block are T-mu*N=ma where N = m*g (on a flat surface) or what people think of as Rope Force - Frictional Resistance = weight. For a constant velocity pull of a car (where velocity = constant and a = 0) Rope Force = Frictional Resistance.

For a coefficient of friction of .25 to .5 which is reasonable, the sling feels one quarter to one half the weight of the car. For a honda civic this is ~ 600 to 1200 lbs or 2500 to 5000 newtons. Check Dyneema's strength ratings, its not surprising that a sling can be used. For a 22 kN sling you should be able to actually slowly lift a civic off the ground if you attached the sling to a car and hoisted it with a crane.

Now if you gunned the Tacoma you'd be needing to consider the impulse and I'd guess the sling would break.

Tzilla Rapdrilla wrote: Hell, I've even pushed a car along half in & half out with one foot & I'm really strong,

Try that with a car stuck in mud.

Not the same as dry pavement

Phil Lauffen wrote:... and climbers wonder why we're all single.

I'm not single. I got MP :P

Eric Krantz wrote: Try that with a car stuck in mud.

That's not mud; that's officially a quagmire.

Eric Krantz wrote: The force to needed to pull out of mud is far greater than it would be rolling on dry pavement, hello. In this case, the coefficient of friction while rolling on dry pavement (as some have mentioned) has absolutely nothing to do with it. 80 pounds extra my ass. Y'all are on crack.

+1 for sanity, +1 for using y'all

To you chaps talking CoF: That C- you got in high school physics doesn't license you to spray formulae around like they magically apply to complicated problems. This is the real world; not everything is spherical cows anymore. Know the limits of what you know, fools.

Evan Sanders wrote:If you're pulling it out of the mud, that means a (relatively) high coefficient of friction. Nowhere near 1, but way higher than .02. I'd say around .2 or .3? Just a guess. Plus if it's sunk into the mud, that adds somewhere between a 5 and 10 degree incline to pull it up out of the mud. So no, it wouldn't be the normal force times the coefficient of friction.

He's talking about the coefficient of rolling resistance which is a different quantity from the one you're thinking of and is around .01 for cars on pavements. You're correct that it doesn't apply to dragging a car out of the mud if the wheels are dragging and not rolling.

Replace what I have above with that coefficient and you do feel around 1 to 2 percent of the car's mass pushing it on pavement. It would only jump up around the levels for coefficients of kinetic friction you quoted if you were pulling it on the beach (or it was rolling in the mud).

I used pulling a dead weight because, to first order, that's closer to the process of dragging a car out of mud assuming you aren't pulling a ton of mud as well.

In any event, given the strength rating of some slings you could lift the car out which is all the weight.