Houston we have a problem! (the bent frame thread)

[linexsa]

great explanation, i was just doing some thinking, and that scenario came into my head. makes perfect sense to me.
yall remember the commercials where they used to drop the truck to prove how tough it was? kinda reminds me of that. i just tried to find those commercials, they are hard to search for.

 

[gotSVT]

Ergo, you have a big, nasty impact between the rear axle and the frame rail, which kinks the frame (and ANY frame would kink), and makes you a Raptor with the dump truck look.

My point is most frames don't bend THIS easy. How do I know? I've done a lot of frame work. None bent. NONE. Your physics lesson is interesting but you were not there. Will something bend if you hit it hard enough? Of course it will. It simply wasn't that kind of a hit. Actually mine started to bend BEFORE I hit any kickers. Explain that? Still want to blame my driving? I was the only one who drove through with zero flats, zero break downs and did the whole run.

 

[BlueSVT]

The 56 foot drop only replicates the same VELOCITY of the impact... that doesn't take into account the intertia of the 6000lb truck and the aftermath of THAT hitting the bump stop... different all together.

The suspension hits the bump stop ALL the time, whether you believe it or not, even under normal use. While PARKED it's only 4" off the bump stop! That means that you'll hit the bump stop going through a parking lot and hitting a speed bump when cruising at 10MPH most likely.

Do this:

Take some chalk, and completely cover the bottom of your bump stop before you go off-road next time. I can guarantee that you'll see transfer between the bump stop and axle even with LIGHT off-roading.

Some have said that if you're hitting the bump stop, you're off roading too hard or exceeding the design limitations of the truck... that's bogus. If that's the case, it's even more reason to believe the design is poor.

 

[GRT4DRT]

The way off this island:
1. Slow down for big bumps.
2. Make sure suspension travel is > than bumps and go fast.
3. Hit big bumps at a sharp angle or make sure they are gradual rises.

The question was WHEN.... sorry wrong answer....

 

[linexsa]

The 56 foot drop only replicates the same VELOCITY of the impact... that doesn't take into account the intertia of the 6000lb truck and the aftermath of THAT hitting the bump stop... different all together.

this was where i was going with my thoughts.

 

[Bad company]

My point is most frames don't bend THIS easy. How do I know? I've done a lot of frame work. None bent. NONE. Your physics lesson is interesting but you were not there. Will something bend if you hit it hard enough? Of course it will. It simply wasn't that kind of a hit. Actually mine started to bend BEFORE I hit any kickers. Explain that? Still want to blame my driving? I was the only one who drove through with zero flats, zero break downs and did the whole run.

I don't know how you drove your truck, or when it bent. If your frame bent before the kicker, than obviously something else happened. I'm not blaming any driving, in fact I admitted that i would probably have been caught unawares as well.

All I know is, one driver specifically said his bent over a kicker, and described the scenario. I surmised that that was much more severe than a jump, and showed that in the scenario as described a frame crash was inevitable, and damage certain.

If you frame is straight before the bump stop, and bends down after there are only three scenarios that can cause that:

1. Buckling due to rear end impact (easy to eliminate)
2. Excessive downward forces aft of the bumpstop (possible but unlikely)
3. Large force upward at the bumpstop where we have stated conditions and the likely culprit.

 

[gotSVT]

I can guarantee that you'll see transfer between the bump stop and axle even with LIGHT off-roading.

Heck try a bridge expansion joint or rail road tracks.
<------ or the road I live on.

 

[Bad company]

The 56 foot drop only replicates the same VELOCITY of the impact... that doesn't take into account the intertia of the 6000lb truck and the aftermath of THAT hitting the bump stop... different all together.

The suspension hits the bump stop ALL the time, whether you believe it or not, even under normal use. While PARKED it's only 4" off the bump stop! That means that you'll hit the bump stop going through a parking lot and hitting a speed bump when cruising at 10MPH most likely.

Do this:

Take some chalk, and completely cover the bottom of your bump stop before you go off-road next time. I can guarantee that you'll see transfer between the bump stop and axle even with LIGHT off-roading.

Some have said that if you're hitting the bump stop, you're off roading too hard or exceeding the design limitations of the truck... that's bogus. If that's the case, it's even more reason to believe the design is poor.

I agree with your post entirely regarding the bump stops. In fact they are supposed to be used, and are in important part of the suspension. they are designed to be the last line of defense in preventing a metal to metal impact. Using them isn't a bad thing. However they are simply overwhelmed in the kicker at 60MPH example.

The design of the truck is not poor, nor is the comparison bogus. I said that for the first 12" of travel the two are equal. Enough to compress the tire, springs, shocks, and bumpstop, bend the axle (elastically) and then need some more space from your frame.

See the post three above your last post BlueSTI where I explain the force necessary from your suspension to avoid crashing.

The inertia comparison is also valid. In one case (the drop) the suspension is compressed between the ground and a truck falling at 60 ft/s and trying to cushion the impact. In the other, the suspension is compressed between the truck at rest and the ground rising at 60 ft/s. The trucks mass is equal, the required acceleration is equal, so the force must be equal. F = MA This is only valid for the height of the bump, and has limitations as the bump gets bigger.

But read the other way of looking at it, I think it's clearer.

 

[Bad company]

As Gilligan would say:

"Good stuff Professor. Now when are you gonna build a frickin radio and get us of this GD island!!!"

Gilligan was an idiot.

Or maybe a genious for wanting to stay on the island with Mary Anne and no competition.



Only two real solutions:

1. More suspension travel.
2. Drive over big, sharp bumps slower.

 

[Bad company]

Ok, try this:

For the 12" solid kicker at 60MPH example:
The suspension has .0166 seconds to move the rear of the truck up 3" (see earlier posts for math). To do so it needs to exert 170,000 pounds of force to the truck frame. This is impossible, and therefore you have crashing between axle and frame (through the bumpstop), and frame damage.

For a 10' jump example:
Vertical velocity on landing is -25.4 ft/s.
The suspension has 12" of travel to reduce vertical velocity to zero or there will be crashing (suspension includes the use of the stops).
Vf = 0
Vo = -25.4 ft/s
Xf = -1 (feet)
Xo = 0
a = constant
t = constant (solve for a and t)
two equations: Vf = Vo + at AND Xf = Xo + Vo * t + 1/2 a * t^2
yields t = .078 s
a = 321.5 ft/s^s or about 10gs
So,
for the 10' jump example, to not have a crash, the suspension has to "push up" on the frame an average of 30,000 pounds over a foot of travel (possible).

For the kicker, the suspension has to push up an average of 170,000 pounds to avoid a crash between the axle (bump stop is squashed flat) and frame.

Now see why the kicker is so nasty? The truth is that the force during the kicker will exceed that 170,000 pounds due to the fact that there is metal on metal impact. 85+ TONS of force will bend frames (pretty much any frame).