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Interesting discussion on Up/Down Travel, bumps, and jumps

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Old 11-11-2011, 07:46 PM   #1
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Interesting discussion on Up/Down Travel, bumps, and jumps

There's a summary and an even shorter summary in the next post if you don't feel like reading a bunch of half-math and geek speak.

I was reading this enormous thread on bent raptor frames and found some really interesting talk about hitting bump jumps versus ramped jumps. A steep bump will wreck your truck harder than a smooth launch several feet in the air (of course a bad landing...)


Quote:
Originally Posted by Bad company View Post
Ok, I ran some numbers, and they are eye opening.

Assumptions:
The "kicker" or "dirt speed bump" is one Raptor tire diameter accross, and is shaped like two ramps joined together (like a flattened triangle).

Case #1:
Kicker height is 18" (flattened to 12" by your tires)
Truck speed is 60MPH (88 fps)
The bottom of the tire has to move up 12" in .0166 seconds. this is an average speed of 60.24 ft/s
To generate this suspension speed, the truck would have to free fall 56 FEET!!!!! That's an impressive number, but what does it mean?

It means that the truck is recieving an impact equavalent to the first 12" of a 56 FOOT DROP WITH THE SUSPENSION ALREADY HALF COMPRESSED! My best guess is that the suspension acounts for about 6" of that, the tires for maybe another 3-4", the axle for some fraction of an inch, the truck vertical motion for another small fraction of an inch (no time to respond), and the rest? It all goes into deforming the frame by 1"-2".

This also tells me that even if the frame were boxed in that area, or if it was 3 times thicker, it would still bend. Even if the frame was reinfored enough to hold, the next likely point of failure is the axle, which would bend, stranding you.

Case #2:
Kicker Height is 12" (flattened to 9" by the tires)
Truck speed is 45 MPH (66ft/s)
The bottom of the tire has to move up 9" in .0221 seconds, an average speed of 33.94 ft/s vertical.

To generate this suspension speed, the truck would have to free fall about 18 feet! This is a MUCH less severe impact! More importantly, the impact is only 9" long (insert obvious joke here). Additionally the truck may be able to absorb it without requiring metal to deform.

The suspension can absorb 6", the tires their 3", the axle a tiny bit, the truck can move upwards a bit more, and all you probably have is tortured bump stops.

So those of you who have jumped their Raptors (and the most vertical air I've seen is maybe 8-9 feet) and said they don't have bent frames? This is because hitting an 18" kicker at 60MPH is about 7 TIMES as severe. PLUS your suspension is fully extended when you jump, cushioning the fall better, unlike the speed bump example.

Conclusion? If you hit something like a speed bump or "kicker" taller than your available suspension travel, plus tire "squash", at HIGH speed, you will bend metal.

The ONLY way to adress this issue is increase suspension travel to be greator than the intended acceptable "bump". Or god forbid (*sniff), slow down. Reinforcing the frame will either fail to work (bend anyway), or cause something else to fail (like the axle).

Nobody designs trucks to withstand those forces (like the first foot of a 56 foot drop with already compressed suspension), that would be incredibly impractical (and they'd sell one a year at $300k). I do not believe there is a design flaw, I believe that somebody found a situation that exceeds its strength.

I would love to join in the next Raptor Run, jumps are fine, short gullies are fine, just look out for the big kickers when you are burnin' up the dirt!
Quote:
Bad company, when you compare the kickers to jumps, is that comparing the suspension compressed cycle speed? Are the forces also equal at that point?

Quote:
Originally Posted by Bad company View Post
What I was trying to quantify is the energy in the impulse when the axle bottoms out against the frame rails.

There is a ton going on here, and you have to make some assumptions. However at high speed, when you hit a kicker AND it is big enough to bottom out the suspension, you create an impact between the axle (moving up) and the frame (holding steady more or less due to its own inertia).

What is equal in the two cases is the relative velocity between the frame and the axle. The reason why you can compare forces is because in both cases the axle is accelerating the frame upwards, and acting on the same mass (the truck itself) with the SAME relative motion forced by the ground moving upwards relative to the truck. In one case the truck's relative motion is caused by a fall (acceleration due to gravity), in the other the relative motion is caused by the tires being forced upwards by a bump.

Where the two comparisons begin to differ is that after the kicker is cleared, the suspension recovers, however, on a 56' drop it does not. So the comparison is valid only in this case for the first 12" (or height of compacted kicker) of relative displacement, which is plenty to deform the frame. In other words, the forces are equal for the first 12" of displacement. The mass being accelerated is the same (the truck minus the suspension and tires), the acceleration is the same (defined by the suspension geometry and materials), therefore the forces are equal.

Make sense?

I calculated the speed at which the suspension is being compressed, and determined from what height the truck would have to fall to generate that relative velocity.

Quote:
Originally Posted by Bad company View Post
The 6,000 pounds (maybe less than half on the rear axle) is in play for both cases. In one case (the drop) the suspension is trying to act on the truck's mass by decelerating it from ~60 ft/s downward to zero (at rest).

In the other case (the kicker), the suspension is trying to accelerate the truck frame from 0 to 60 ft/s upward.

The masses are equivalent, the accelerations are equivalent, therfore the forces are equal. This is valid for the entire height of the bump.
Quote:
Originally Posted by Bad company View Post
For the case in point:

12" high compacted bump
6" of remaining suspension travel
3" of tire compression

The only way to avoid a crash in your suspension (DAMAGE) is to move the frame rails up 3" druing the bump rise.

So you need to move half the truck ~3,000# UP 3" in .0166 seconds. The most efficient way is with an even force, therefore acceleration would be constant.

x = 1/2 * a * t^2

x = .25 (corrected)

t = .0166

a = 21773 ft/s^2

so... acceleration required is 56g

Force required by the suspension to move the truck enough to not crash the frame into the axle? 170,000 pounds or force (was 2M, my unit conversion fail).

Still, Good luck with a stronger frame.
Quote:
Originally Posted by Bad company View Post
Remember I am talking about a very specific condition. A condition where the the bump is 3" taller than the tire flex and remaining suspension can absorb. I ignore the springs and dampers for this reason:

At 60MPH, for there not to be a catastrophic crash between the axle and frame, the frame MUST move up 3" over .0166 seconds. So you need the suspension to supply enough force to move the back of the truck UP 3" VERY QUICKLY. The minimum force required to sucesfully move 3,000 pounds 3" in .0166 seconds? About 170,000 pounds of force! [EDITED to correct unit conversion fail] No change to conclusions.

Building a frame or suspension to withstand bumps larger than available travel at HIGH SPEEDS is pointless due to the loads involved for that reason. NO resonable sized rubber bump stop will serve, nor will any frame reinforcement solve the problem. Just slow down.

The same bump at 20MPH? The average force required to move the truck up 3" is only 18,600 pounds. One ninth the force.
Quote:
Originally Posted by Bad company View Post
The case in point is so extreme that even a perfectly designed bump stop would have to transmit 170,000 pounds of force over the entire range of travel (pretty much impossible) to the frame in order to prevent a catastrophic collision. (Was 2M, unit conversion failure, my apologies).

In other situations regarding the suspension bottoming out (even jumps) I am sure that more sophisticated bump stops and even localized frame reinforcement could make a big difference.

A solid 12" bump is KILLER at 60 MPH. Anything solid less than probably 9" (the raptor's up travel at ride height) will be absorbed by the suspension. Anything taller will probably cause damage to your truck if hit at high speed, regardless of stops or reinforcement.
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Old 11-11-2011, 07:47 PM   #2
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SUMMARY
Quote:
Originally Posted by Bad company View Post
I am a Mechanical Engineer with a ton of experience designing shock isolation systems, and I own a 2010 Raptor. There are a few threads about the frame bending that happened on the Raptor Run, and talk of a weak point on the Raptor frame. I did an analysis on the stated conditions: 60 MPH, 18" tall "kicker" or dirt "speed bump" as a cause for this failure.

Below is my line of thought:



Ok, I ran some numbers, and they are eye opening.

Assumptions:
The "kicker" or "dirt speed bump" is one Raptor tire diameter accross, and is shaped like two ramps joined together (like a flattened triangle).

Case #1:
Kicker height is 18" (flattened to 12" by your tires)
Truck speed is 60MPH (88 fps)
The bottom of the tire has to move up 12" in .0166 seconds. this is an average speed of 60.24 ft/s
To generate this suspension speed, the truck would have to free fall 56 FEET!!!!! That's an impressive number, but what does it mean?

It means that the truck is recieving an impact equavalent to the first 12" of a 56 FOOT DROP WITH THE SUSPENSION ALREADY HALF COMPRESSED! My best guess is that the suspension acounts for about 6" of that, the tires for maybe another 3-4", the axle for some fraction of an inch, the truck vertical motion for another small fraction of an inch (no time to respond), and the rest? It all goes into deforming the frame by 1"-2".

This also tells me that even if the frame were boxed in that area, or if it was 3 times thicker, it would still bend. Even if the frame was reinfored enough to hold, the next likely point of failure is the axle, which would bend, stranding you.

Case #2:
Kicker Height is 12" (flattened to 9" by the tires)
Truck speed is 45 MPH (66ft/s)
The bottom of the tire has to move up 9" in .0221 seconds, an average speed of 33.94 ft/s vertical.

To generate this suspension speed, the truck would have to free fall about 18 feet! This is a MUCH less severe impact! More importantly, the impact is only 9" long (insert obvious joke here). Additionally the truck may be able to absorb it without requiring metal to deform.

The suspension can absorb 6", the tires their 3", the axle a tiny bit, the truck can move upwards a bit more, and all you probably have is tortured bump stops.

So those of you who have jumped their Raptors (and the most vertical air I've seen is maybe 8-9 feet) and said they don't have bent frames? This is because hitting an 18" kicker at 60MPH is about 7 TIMES as severe. PLUS your suspension is fully extended when you jump, cushioning the fall better, unlike the speed bump example.

Conclusion? If you hit something like a speed bump or "kicker" taller than your available suspension travel, plus tire "squash", at HIGH speed, you will bend metal.

The ONLY way to adress this issue is increase suspension travel to be greator than the intended acceptable "bump". Or god forbid (*sniff), slow down. Reinforcing the frame will either fail to work (bend anyway), or cause something else to fail (like the axle).

Nobody designs trucks to withstand those forces (like the first foot of a 56 foot drop with already compressed suspension), that would be incredibly impractical (and they'd sell one a year at $300k). I do not believe there is a design flaw, I believe that somebody found a situation that exceeds its strength.

I would love to join in the next Raptor Run, jumps are fine, short gullies are fine, just look out for the big kickers when you are burnin' up the dirt!


TL;DR: Height of the bump relative to your up travel is extremely critical. The faster you hit a bump at the less time your springs have to push the truck up and away from the axle trying to smash into it. A gradual rise into a jump gives your suspension time to cycle smoothly and change the trucks overall momentum so it's going up and off the ramp, instead of forward with the wheels struggling to rise fast enough to keep up. And coming off a jump you get the full length of suspension travel to soak up the landing. So if you see any sudden rise in terrain that's bigger than your up travel SLOW THE FUCK DOWN.
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Old 11-11-2011, 08:06 PM   #3
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Another explanation on why a smooth jump is safer than a hard bump.

Quote:
quick question, i read all of your math and it all seems very logical. ive been following this thread for a couple of days, and agree with all of your reasoning. my only question is, when you hit a bump, the suspension compresses and rebounds, sending the rear of the truck into the air once the bumpstop bottoms out, when you drop it off a 56' drop, the truck has no where to go, wouldnt this change the math a little? am i explaining my question right?
Quote:
Great question. I struggled with explaining it clearly.

The comparison is equal if you only allow the drop to compress the suspension 12". (and then somehow stop the fall/truck)

The main issue here is the time the suspension has to work on the truck to "move it up". Lets look at it this way, (which is another way to analyze it):

Basically in .0166 seconds the bottom of the tire moves up 12" (forced to by the ground). This is 1/60th of a second. If you assume that the tire squashes 3", the suspension takes up 6" (the amount it has when loaded), that means to avoid a nasty crash between the steel axle and steel frame, you need the suspension to move the truck up by 3".

Ok but how reasonable is that? Can't the suspension move the truck up 3" in that timeframe? It can't, and here is why:

The mass of the truck on the rear suspension is roughly 3,000 pounds. Lets assume that the force supplied by the suspension to the truck is constant (we know it isn't, but this will give us the SMALLEST force required).

x = .25' (3")

a = constant because force is constant

t = .0166

so knowing that v_initial is zero:

x = 1/2 * at^2

leaves a = 1814.5 ft/s^2

Great now what? Well this acceleration is equal to 56 "g"s. So the suspension must exert 56 times the weight of the truck on the rear axle to move it up 3" in that amount of time OR about 170,000 pounds of force MINIMUM.

There is no way this suspension is exerting more force than the weight of an M1A1 main battle tank on your truck. 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.

Better explanation?
Quote:
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).


edit: haha I got to page 24 in that thread and someone FINALLY brought up the point that those hard kickers should have been marked from the prerunning. Why would you go 50+ down a stretch of road you've never hit before?
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Old 11-11-2011, 10:16 PM   #4
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Good info and good thing you summarized it too much for my lazy ass to read,
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