Seat bounce

FWYT
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6/12/2020 5:18pm Edited Date/Time 6/12/2020 5:20pm
davis224 wrote:
I'd just like to note that this has remained somewhat civil with nobody taking it personally yet. Let's keep it that way and just have a...
I'd just like to note that this has remained somewhat civil with nobody taking it personally yet. Let's keep it that way and just have a healthy debate where we make points and present evidence!

Now, who has a good slomo video of someone seat bouncing?
FWYT wrote:
It would be awesome if one of those "physics explained" type of YouTube channel people did a thing
on seat bouncing.
TeamGreen wrote:
Science of Supercross with Adam doing the riding & MC explaining it (360 global view frozen/stop action and MC walks into the frame & explains what’s...
Science of Supercross with Adam doing the riding & MC explaining it (360 global view frozen/stop action and MC walks into the frame & explains what’s goin’ on at different points in the action: set-up, squat/compress, ramp to launch...& take off).

It needs to be one of those turns that’s right at the exit of a 180 SX “bowl turn” at a ATL or Dallas SX type track.

Not that I have an opinion on the matter...
Laughing
Yes! Something like that! But done with some physics/science PhD types. MC can explain what we, the riders,
are doing but the PhDs explain the how and whatfor's. That'd be fun!!!

However, someone would have to warn the PhD's that no matter what they say, they are just going to RAKED over the coals! hahahhahahahha
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6/12/2020 9:05pm
abn166 wrote:
The small amount of rebound that has occurred since the rider reached the face of the jump will not affect the height or distance of the...
The small amount of rebound that has occurred since the rider reached the face of the jump will not affect the height or distance of the jump. below is the rider 5 feet into his jump. As you can see now he is standing and the suspension has rebounded while the bike was in the air.
Ahh, yes it will. Things are happening quickly and the amount of rebound is not a direct tell tale if the force is very high as energy is proportional to force x distance.
But i still think that the force response / rebound from the rear shock is not directly what matters - its what it enables. And that is, once the rider has sat into the bottom of the jump and compressed the suspension to increase the upwards trajectory (key first factor which also reduces the jump absorbing energy from the bike), then by carrying the mass of the rider on the rear wheel, the rider can use the rebound of the front suspension + pivoting their body mass to also lift the front + the engines torque to pivot the bike around the rear wheel and also raise the front wheel - all of which increases the overall upwards velocity of the overall mass. You can tell because the bikes engine is higher off the lip. And if done well, the rider is also higher above the lip when their mass is leaving the lip.

As a side note rebound has to contribute a little and probably from both wheels - but i am not sure how much.
You just need to realise that you don't usually leave a jump with the suspension fully extended. In the fully extended state the spring force is so low that your damping slows the rebound speed right down. Rebound speed in the botttom part and mid stroke is much faster as the spring force is higher and damping is linked to velocity. And if the jump is under the wheels at the time the suspension is unloaded even a little it will be pushing up against it. The speed it does this will mostly be determined by spring rate, amount of compression and damping settings. Oh and the mass it has to lift.
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rjg
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6/12/2020 9:40pm Edited Date/Time 6/12/2020 9:42pm
Spring pogoing is a real thing. Look at trials. Graham Jarvis uses it to his advantage.
1
6/12/2020 11:41pm
Ride your full suspension mtb over a bump and the wheels don’t leave the ground because the suspension absorbed the impact. Do it on ya bmx and see what happens. In physics it’s called impulse. It means change in direction/velocity due to transfer of energy (sort of). Compress the suspension on a mx bike harder on a jump face and there is more impulse because the suspension doesn’t absorb the energy as muck. Bounce a ball bearing or a golf ball on concrete for want of better explanation.
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The Shop

CPR
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6/13/2020 12:14am
This quad seat bounce is impressive.


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Pink
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6/13/2020 12:51am
CPR wrote:
This quad seat bounce is impressive. [img]https://p.vitalmx.com/photos/forums/2020/06/13/432295/s1200_7AEEB92E_A313_4C8C_A116_F86D781519DD.jpg[/img]
This quad seat bounce is impressive.


She gota ass like a Montana mule.!
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CPR
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6/13/2020 2:11am
CPR wrote:
This quad seat bounce is impressive. [img]https://p.vitalmx.com/photos/forums/2020/06/13/432295/s1200_7AEEB92E_A313_4C8C_A116_F86D781519DD.jpg[/img]
This quad seat bounce is impressive.


Pink wrote:
She gota ass like a Montana mule.!
Yeah but if you bounce her does she change her trajectory? 😂
689
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6/13/2020 3:35am
Anyone who think's the spring doesn't give a 'bounce' could you please be so kind to seat bounce a jump and keep the revs constant
davis224
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6/13/2020 3:51am
689 wrote:
Anyone who think's the spring doesn't give a 'bounce' could you please be so kind to seat bounce a jump and keep the revs constant
And why would that prove you pogo? I would say that is more momentum in the right direction.

Once again, if you tie your bike down and bottom the suspension out, and then cut the straps, your bike will just rebound up at a normal rate, not push the bike off of the ground. That force doesn't change.
689
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6/13/2020 4:06am
689 wrote:
Anyone who think's the spring doesn't give a 'bounce' could you please be so kind to seat bounce a jump and keep the revs constant
davis224 wrote:
And why would that prove you pogo? I would say that is more momentum in the right direction. Once again, if you tie your bike down...
And why would that prove you pogo? I would say that is more momentum in the right direction.

Once again, if you tie your bike down and bottom the suspension out, and then cut the straps, your bike will just rebound up at a normal rate, not push the bike off of the ground. That force doesn't change.
You would go over the bars, or come close to it.

Rolling the power on lifts the front up and balances out the 'bounce' of the rear spring.

Not following that example of having the bike loaded with straps? Can't see how that is relevant to the difference between sitting and standing going off a jump. The force changes between sitting and standing because the riders weight is getting thrown into the jump face.
3
davis224
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6/13/2020 5:10am
689 wrote:
Anyone who think's the spring doesn't give a 'bounce' could you please be so kind to seat bounce a jump and keep the revs constant
davis224 wrote:
And why would that prove you pogo? I would say that is more momentum in the right direction. Once again, if you tie your bike down...
And why would that prove you pogo? I would say that is more momentum in the right direction.

Once again, if you tie your bike down and bottom the suspension out, and then cut the straps, your bike will just rebound up at a normal rate, not push the bike off of the ground. That force doesn't change.
689 wrote:
You would go over the bars, or come close to it. Rolling the power on lifts the front up and balances out the 'bounce' of the...
You would go over the bars, or come close to it.

Rolling the power on lifts the front up and balances out the 'bounce' of the rear spring.

Not following that example of having the bike loaded with straps? Can't see how that is relevant to the difference between sitting and standing going off a jump. The force changes between sitting and standing because the riders weight is getting thrown into the jump face.
If the shock rebound has any effect by rebounding before it leaves the ramp, I would imagine it would just cause the bike to rotate on its axis, not launch the 400 lbs of rider and bike higher into the air, which is the point of my cutting the strap example, it's not enough to lift the bike off of the ground by itself, your rebound speed is your rebound speed. Also if you are sitting down under a steady throttle and hit a jump, chances are your weight is more centered/forward on the bike, pushing down on the forks, and with less weight over the rear, it follows the ramp trajectory. Like the video posted earlier with the heavy fella going over the bars on the old dual sport on the little bump.
davis224
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6/13/2020 5:15am
Ride your full suspension mtb over a bump and the wheels don’t leave the ground because the suspension absorbed the impact. Do it on ya bmx...
Ride your full suspension mtb over a bump and the wheels don’t leave the ground because the suspension absorbed the impact. Do it on ya bmx and see what happens. In physics it’s called impulse. It means change in direction/velocity due to transfer of energy (sort of). Compress the suspension on a mx bike harder on a jump face and there is more impulse because the suspension doesn’t absorb the energy as muck. Bounce a ball bearing or a golf ball on concrete for want of better explanation.
This. It is more about momentum, your suspension soaking up the jump face keeps your momentum going more forward and less with the angle of the takeoff, the more compressed your suspension is, the more your momentum is directed in the angle of the takeoff. Not a spring shooting you higher.
kkawboy14
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6/13/2020 5:30am
davis224 wrote:
Since it was briefly discussed in the 250 main, dare we delve back into one of the greatest motodrive threads ever? No, your shock does not...
Since it was briefly discussed in the 250 main, dare we delve back into one of the greatest motodrive threads ever?

No, your shock does not "bounce" you higher. Discuss.
I guarantee you that if you Remove the rear shock, you will not bounce. If you add less sag and more compression and more rebound, you will bounce higher than you would without a rear shock!
1
ATKpilot99
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6/13/2020 5:58am
689 wrote:
Anyone who think's the spring doesn't give a 'bounce' could you please be so kind to seat bounce a jump and keep the revs constant
You could execute a "seat bounce" on a bmx bike using body positioning and weight shift although you probably wouldn't be sitting directly on the seat but started in a crouched position. Similar to a bunny hop . Now try pedaling as hard as you could through that same jump as if you were on flat ground and it probably wouldn't end well .
Johnny Depp
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6/13/2020 6:42am
abn166 wrote:
Still, the shock does not rebound until the bike has left the ground. So it cannot act like a pogo and push someone higher into the...
Still, the shock does not rebound until the bike has left the ground. So it cannot act like a pogo and push someone higher into the air. That is not what is happening when someone seat bounces.
Here's perhaps an explanation for that?

The rear wheel is spinning creating centrifugal force. By doing so the wheel itself wants to resist motion, therefore the spring and chassis are able to "push" off of it.

In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis passing through the coordinate system's origin and parallel to the axis of rotation.
bradnelson
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6/13/2020 7:18am
"The face the of the jump does effectively change if the sprung parts of the bike (everything above the wheels) is at a different angle." Anyone...
"The face the of the jump does effectively change if the sprung parts of the bike (everything above the wheels) is at a different angle."

Anyone ever try seat bouncing one of these hardtails? Woohoo


Johnny, I tried once.

Then she turned around and looked at me like...... wtf are you doing ?? Laughing
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Jbulz
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6/13/2020 7:22am
My shock is a pogo stick and I loosened my throttle tube and rotating it forward to prevent my elbow from dropping when I twist it.
1
Falcon
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6/13/2020 8:09am
Again, it's all about timing. Time the upward acceleration of the combined masses of the bike and your body differently and you will acheive different results. The shock spring helps you do this.
Falcon
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6/13/2020 8:13am
Here is a better way to explain it: when you stand, you isolate your body's mass from the shock spring. Thus, its effects on you are minimized.
When you sit, you couple your body's mass to the spring and add to the overall mass being moved by the spring. That increases momentum.
BMR179
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6/13/2020 9:28am
Trajectory and acceleration. Simple.
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Spudnut
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6/14/2020 9:43am
It’s been talked about here a few times. I’ve seen it in person at Washougal. He does it to scrub sometimes to lift the bike up...
It’s been talked about here a few times. I’ve seen it in person at Washougal. He does it to scrub sometimes to lift the bike up. Also when seat bouncing I’ve seen it, maybe to save him from going over the bars or to lift the rear end up and leveling the bike?

My friend asked him in the bits and he was pretty chill about it. I think it comes natural without thinking about it for him.
I also see him do it in the air when whipping and have a poster on my wall where he’s got his boot tucked under the peg and pulling the bike over
Spudnut
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6/14/2020 9:43am
Tomac a damn beast idc what anyone says
sf702/410
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6/14/2020 10:41am
I think most of the effect comes from the cantilever effect where the seat effectively becomes a diving board loaded to launch the rider. Its up to you to squeeze the knees and take the bike with you or take the free face dive if not. SEAT BOUNCE ISNT FAKE NEWS
philG
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6/14/2020 11:11am
Every action has an equal and opposite reaction... that is all you need to know.

By sitting down you put your full weight of your body , into the bike, causing the suspension to compress and the CofM of the machine to be lower with the suspension compressed, and the result is an elevated tragectory... scrubbing is the opposite, because you lean the bike up the face, to reduce the load on the bike, and turn the wheel to flatten the bike.. there is little or no rider weight,as the rider is bending his legs to absorb the movement of the bike upwards.. the bike and body dont have the same path. If you think the is wrong, then scrubs and seat bounces dont exist.
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abn166
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6/14/2020 12:11pm
sf702/410 wrote:
I think most of the effect comes from the cantilever effect where the seat effectively becomes a diving board loaded to launch the rider. Its up...
I think most of the effect comes from the cantilever effect where the seat effectively becomes a diving board loaded to launch the rider. Its up to you to squeeze the knees and take the bike with you or take the free face dive if not. SEAT BOUNCE ISNT FAKE NEWS
Definitely 100 percent wrong. A diving board would not work so well if it were completely rigid with no give at all. I know if I hit a jump on a bike with completely rigid suspension I will definitely still jump.
sf702/410
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6/14/2020 12:17pm
sf702/410 wrote:
I think most of the effect comes from the cantilever effect where the seat effectively becomes a diving board loaded to launch the rider. Its up...
I think most of the effect comes from the cantilever effect where the seat effectively becomes a diving board loaded to launch the rider. Its up to you to squeeze the knees and take the bike with you or take the free face dive if not. SEAT BOUNCE ISNT FAKE NEWS
abn166 wrote:
Definitely 100 percent wrong. A diving board would not work so well if it were completely rigid with no give at all. I know if I...
Definitely 100 percent wrong. A diving board would not work so well if it were completely rigid with no give at all. I know if I hit a jump on a bike with completely rigid suspension I will definitely still jump.
What's your point? There's a spring, a seat, tires, and frame that all "give", plus roll.
Gravel
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6/14/2020 12:23pm
What about the Eli’s foot being under the frame/peg? That can’t be just an accident, it looks like he’s pulling up..

Why beat an old, extremely dead horse, when there’s a new one?!?
Gravel
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6/14/2020 12:25pm
Anyone have pics of the top of Eli’s boots after a race? Is there wear that shows he does this often?
-MAVERICK-
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6/14/2020 12:37pm
Gravel wrote:
What about the Eli’s foot being under the frame/peg? That can’t be just an accident, it looks like he’s pulling up.. Why beat an old, extremely...
What about the Eli’s foot being under the frame/peg? That can’t be just an accident, it looks like he’s pulling up..

Why beat an old, extremely dead horse, when there’s a new one?!?
Gravel
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6/14/2020 12:45pm
Thanks Maverick

Is he setting up to forcefully lift the bike?

As an old desert guy, I’m seeing a cure to the classic flying W, if I was coordinated enough to actually do it at the right place and time, lol.. or maybe it’d just let me endo while firmly in the seat..

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