- #71
Devin-M
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Assume it’s an electric skateboard accelerating from left to right. With no bottle and enough acceleration, the top of the booster topples towards the left.
The force of ground on foot is upward and forward, not purely forward. It is hard to speculate precisely about its line of action -- which will move around during the kick anyway.Devin-M said:How is the force in the wrong direction?
This is better, but it is different from a skateboard propelled by the rider. You're still ignoring the force vector due to the weight of the rider though. The wheels apply a force up against that.Devin-M said:Assume it’s an electric skateboard accelerating from left to right. With no bottle and enough acceleration, the top of the booster topples towards the left.View attachment 329658
Then why have we been talking about a skateboarder propelling himself this whole time?Devin-M said:What I’m suggesting is the bottle held out front would allow someone riding an electric skateboard to accelerate faster (without toppling).
An electric skateboard accelerates the rider via a force, which exerts a backward tipping torque on the rider.Devin-M said:Assume it’s an electric skateboard accelerating from left to right.
No!Devin-M said:Even on a normal skateboard the force propelling the rider is applied below the center of mass, still leading to a backwards toppling torque on the body.
You are forgetting the vertical force component again, which acting on the back foot that is pushing off can counter the torque from the horizontal component.Devin-M said:Because the force propelling the rider on a non electric board is similarly being applied below her/his center of mass, leading to a torque on the body tending to topple them backwards.
When you are recovering from a push, the skateboard's acceleration catches up to the COM's acceleration, and there is no net torque on the body. He is standing on one pivot point. Handing him a water bottle would do nothing he isn't already doing with his arms, that's why they're so close to his body.Devin-M said:If we draw a vertical line through his ankle and look at how much mass is on each side of this line, to my eye he looks very close to tipping over backwards, and if his left knee were bent further he would seem even closer still to tipping backwards...
View attachment 329654
So you lean forward an imperceptible amount. That balances out the acceleration every bit as effectively as a tiny extra weight in front.Devin-M said:Because the force propelling the rider on a non electric board is similarly being applied below her/his center of mass, leading to a torque on the body tending to topple them backwards.
That would be a hideously inefficient posture from which to propel a skateboard. You do not want to maximize the extension of the left leg behind your center of gravity. You want to maximize the horizontal component of thrust (times frequency). Extreme postures work against this goal.Devin-M said:Standing on the floor, if I tried to stretch my left leg as far back as possible while still being able to lift to of said leg off the ground, could I lift it while it’s stretched further back if I had a water bottle in my right forwardly outstretched arm?
Devin-M said:Standing on the floor, if I tried to stretch my left leg (toe touching ground) as far back as possible while still being able to lift the toe of said leg off the ground, could I lift it while it’s stretched further back if I had a water bottle in my right forwardly outstretched arm?
It isn't clear to me what you think that means.Devin-M said:I was able to lift my rear foot's toe off the ground about 10 inches further back while holding a water bottle with the same front foot placement on flat ground...
This is not a valid test, for a couple of reasons.Devin-M said:I was able to lift my rear foot's toe off the ground about 10 inches further back while holding a water bottle with the same front foot placement on flat ground...
I just performed a similar experiment and could extend my leg all the way backwards, toe just off of the ground, until the leg holding me up was so bent that I could no longer get closer to the ground to extend my leg any farther. I used no water bottle or other counterweight, just my own body.Devin-M said:I tried to lift my toe off the ground as far back as possible with front foot in the same location with and without bottle, I found I could lift toe off ground 10 inches further back while holding the bottle outstretched.
I can extend my leg as far back as possible, toe on the ground, and then lift it without any counterweight required. Can you not?Devin-M said:Start with rear toe resting on ground and measure how far back you can possibly lift it, then hold out the water bottle and see if you can lift toe off ground from a farther back position. Essentially the question is can you stretch your leg farther back touching the ground, and still lift the toe off the ground when counterbalanced by the water bottle.
I can without counterweight, but I can stretch it back 10 inches further and lift with the water counterbalance.Drakkith said:I can extend my leg as far back as possible, toe on the ground, and then lift it without any counterweight required. Can you not?
No no, I meant that I can stretch my leg all the way back, as far back as it is physically possible to get, and still lift my toe off the ground without using a counterweight.Devin-M said:I can without counterweight, but I can stretch it back 10 inches further and lift with the water counterbalance.
The distance, as @Drakkith and @DaveC426913 are trying to tell you, is the length of your leg either way. Your leg extends from your hip to your toe, period (if you are flexible enough to bend that far). Moving your hips backwards 10" just moves the 0 point backwards 10", it doesn't change how long your leg is.Devin-M said:If the stroke is longer it’s conceivably force multiplied by longer distance per kick.
"feels like" is far too imprecise/qualitative to be of value. I think what you should do is mark off a 100m course (exact distance not critical) and time yourself in a few sprints in each configuration.Devin-M said:I just took a spin, here’s what I found:
I use the water bottle arm forward in 2 situations:
-already going fast on flat ground, 2 feet on board, slowed down a bit from wind drag, lift right arm rigidly forward with bottle, then take left foot off board, 3 kicks only right foot on board, foot back on board, lower arm back to side, trying to regain maximum possible flat ground speed. It “feels like” the bottle helps me reset my stroke in less time.
-pushing up hill, the arm forward with bottle feels even more useful in this setting than accelerating on flat ground.
Was your front leg's knee straight or bent? When I got the longer rear leg reach holding the water bottle forward, my front knee was bent more and so body position lower while having the extended reach and lifting rear leg toe off ground.Drakkith said:No no, I meant that I can stretch my leg all the way back, as far back as it is physically possible to get, and still lift my toe off the ground without using a counterweight.
That is a semi static situation where the net torque around your CoM must be close to zero all the time.Devin-M said:I tried to lift my toe off the ground as far back as possible with front foot in the same location with and without bottle, I found I could lift toe off ground 10 inches further back while holding the bottle outstretched.
It was bent. This is exactly what I did.Devin-M said:Was your front leg's knee straight or bent? When I got the longer rear leg reach holding the water bottle forward, my front knee was bent more and so body position lower while having the extended reach and lifting rear leg toe off ground.
Possibly. But it could also be that the lower stroke frequency outweighs any gain from the longer stroke.Devin-M said:If the stroke is longer it’s conceivably force multiplied by longer distance per kick.
If you're almost doing the splits, I don't see how you could lift rear toe in this position? I certainly can't lift rear toe with leg as far back as I can make it go.Drakkith said:It was bent. This is exactly what I did.
Reseting the stroke is a different balance situation than the acceleration phase with one foot on the ground, that you were initially theorizing about.Devin-M said:It “feels like” the bottle helps me reset my stroke in less time.
I'm not doing the splits, I'm as far back as I can physically go with one leg under me.Devin-M said:If you're almost doing the splits, I don't see how you could lift rear toe in this position? I certainly can't lift rear toe with leg as far back as I can make it go.