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...)
Ok, I ran some numbers, and they are eye opening.
The "kicker" or "dirt speed bump" is one Raptor tire diameter accross, and is shaped like two ramps joined together (like a flattened triangle).
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.
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!
|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?|
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.
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.
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.
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.
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.
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.