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

[Bad company]

Physics of suspension over short ridges at hihg speed = HIGH IMPACTS

Guys,
I own a Raptor, and I am a mechanical engineer who used to design shock isolation systems for shipborne electronics. I do not know the deisgn specifics for the raptor suspension (spring rates, damping rates, exact distributed weights etc.) but I am guessing that hitting an 18" ridge at high speed is one of the most punishing things that can be done to your suspension for the following reasons:

1. Your suspension at rest can only absorb about 6" of deflection before the rear axle hits the steel frame (bump stop totally flattened). the rest of your suspension is "droop", meaning that when you jump the tires move down at least 6" relative to your truck.
2. Moving at high speed means that the suspension does not have time to accelerate your truck up. Basically the tires MUST move up RIGHT NOW, but the bed of your truck does not (unlike say, a gradual ramp).
3. A short ridge (like a big speed bump) moves the tires upwards very quickly; coupled with high speed, and already partially compressed suspension means HUGE forces between the axle and frame when the two crash together.

I suspect that hitting a ridge at high speed is MUCH worse than jumping your trucks (where the suspension can extend to absorb the landing better). It is also worse than short gullies where again the suspension can extend and then absorb the impact.

Overheated shocks and loads in the bed will only make the problem worse. I'm not making a statement about whether this is abuse or not. But this kind of treatment will cause metal to bend. I know this is the last thing you want to hear, but you may want to slow down for ridges in future.

Here are the variables to consider:
- Higher tire pressure [worse] (until you cut the tire with your rim) as the tires "give" less
- Higher speed [worse] (increases speed at which the tire moves upward, and reduces time for suspension to move truck up and regain suspension clearance)
- Taller bump [worse] (anything greator than 6" of hard pack or so could be a problem at very high speeds)
- Harder pack [worse] (loose sand or snow- no problem) hard packed clay or concrete = bad) loose ground will be "cut" by your tires and the suspension will not move much.
- Loaded truck [worse] mass in your bed will directly impact the "hit", also it reduces the amount of suspension travel left by compressing the springs more.
- "Abrupt" bump worse - reduces time for suspension to recover

Because of the masses involved, and the accelerations, I doubt any quick fixes will work. I'm betting the loads are VERY HIGH. I'll run some numbers when I get a bit more time.

After giving this some thought, I doubt ANY current production truck would survive that reatment without damage (60MPH, 18" high, "kicker" ridge as hinted at in earlier posts). I also doubt that it is a defect in design or manufacture.

 

[iceman302]

Now there's an excellent first post! Welcome, Bad Company!

 

[gotSVT]

I also doubt that it is a defect in design or manufacture.

When Ford or anyone else designs something with the weakest point in the structure at the point of highest load, what else can you call it?

Btw, welcome and great analysis!

 

[pirate air]

The guys over on the Toyota sites are getting a lot of mileage outa this...

Talk about about irony.

Glad the Raptor can entertain them while they wait for air pumps.



Super informational post Bad company, Thanks!

 

[Raptizzle]

Bad company!!

Very valid points you make there!

 

[Raptorguy21]

Let em. They just wish they had a truck that was capable of being pushed to the limit of bending the frame.

agreed......they're just hatin'

 

[Bad company]

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!

 

[bstoner59]

well put! Sounds pretty right on to me!

 

[Bad company]

Thanks for the kind replys! Honestly, I hate seeing good designs talked about in a poor light. I also hate feeling like there may be weak points in my truck. I feel much better after realizing just how massive the impact was that those trucks were subjected to. I think these trucks are so capable, that they give you a feeling of invincability, and we continue to push them harder and harder with maybe too much confidence.

It isn't necessarily obvious how vicious a big dirt speed bump can be, compared to 5' of air.

I don't always throw dirt everywhere, but when I do, it's in a Raptor.

 

[Raptorguy21]

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!

You're a mechanical engineer huh? Or you take a WAY higher level math class than I did in college.........