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Recovering couch patato
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Since I've found out about the internet, no single forum poster taught me as much about bikes just from reading his posts, as Shiggy did.

Now, Shiggy and I seem to have a difference of opinion regarding the dynamics of the endo, or my technical English just can't keep up.

It is my understanding that when you're lifting a wheel due to hard braking or hard accelerating (endo and wheelie), the center of gravity is pivoting around the axle of the wheel stay stays on the ground. Shiggy seems to be of the opinion that COG is pivoting around the contact patch of said wheel with the ground.

I made a crappy sketch to try and understand it more, especially in context of the large vs. small wheel debate. My senses tell me that a 29" bike can be decelleted harder before terminal endo.
Background : I had a 26" bike with 425mm fork, on 2" slicks. I swapped the fork for a 400mm Surly fork, and put in a 700c wheel with similar slick tire. The first time I hit the front brake hard on the 28/26 setup, the rear wheel seemde to be glued to the ground, it just wouldn't lift under what felt like pretty hard decellaration, I really had to hold on tight.

You see a layover of 2 bikes, with differently sized front wheels. I drew the highest point COG will reach in an endo, the point of no return. The larger wheel has the COG climb more vertically before endo.
It is my understanding that the higher you raise the COG, the more forward motion (decelleration) is taken up to convert it. I lack technical lingo for it.

When you would instantly lock the brakes, the larger wheel would also ROLL further before the COG reaches highest point.
The difference in COG path between open en locked brakes seems greater for the larger wheels.

What is your take on it? Shiggy's responses to my views have gotten me doubting, and not seeing things clearly anymore.

Does wheelsize affect endo-ability at all? How does the motion of the endo work? Contact patch or axle as pivot point, or both, or a shifting virtual one?

Geek away, thanks in advance!!
Also looking forward to your views, Shiggy!

Happy trails,

J
 

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elbow gloves
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Seems to make some sense to me

In an endo you must rotate the COG to that point of no return, which is higher on a 26er than a 29er. It seems to me that more energy is required to move the COG that much further. The other thing 29ers have going for them is the relationship of the BB to the axles of the wheels, which is greater on a 29er versus a 26er, effectively lowering your COG on a 29er, which increases the distance and energy required to displace your COG to cause an endo.
I don't know if I actually contributed anything, rather a reiteration?:confused:

Good times, everyone have a great weekend.
 

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rider
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Not a simple answer: If the front wheel was locked the bike would be rotating along the contact patch as it rolls along the ground.

The problem is that you are talking about a dynamic situation. This gives you a virtual and constantly shifting pivot point governed by the actual amount of wheel rotation vs. the earth in relation to the actual amount of bike frame rotation vs. the earth.

The simplistic answer: 29er's are harder to endo. The down side of this is you get launched higher and fall farther, if you do endo on one. Imagine the worst case: you are flying down the trail and run straight into a tall rock, then you panic and fully lock the wheel. At this point your front wheel is going to climb the rock while you are endoing, launching you even higher! Fun!?!

29erchico
 

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kinkcrazy said:
In an endo you must rotate the COG to that point of no return, which is higher on a 26er than a 29er. It seems to me that more energy is required to move the COG that much further. The other thing 29ers have going for them is the relationship of the BB to the axles of the wheels, which is greater on a 29er versus a 26er, effectively lowering your COG on a 29er, which increases the distance and energy required to displace your COG to cause an endo.
I don't know if I actually contributed anything, rather a reiteration?:confused:

Good times, everyone have a great weekend.
I should know better than to get into these analytical threads, but don't forget about the effect of wheelbase. The longer your bike is, the bigger a lever it is. Picture a 20 foot long bike with 10" wheels. Tiny wheels, but good luck getting the front end off the ground.

-Erkl
 

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Harmonius Wrench
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Here's the answer....

29erchico said:
Not a simple answer...............
Yes, I agree.

The problem is that you are talking about a dynamic situation. This gives you a virtual and constantly shifting pivot point governed by the actual amount of wheel rotation vs. the earth in relation to the actual amount of bike frame rotation vs. the earth.
And don't forget lateral rotation around the headset, which often occurs during endos, thus adding another dynamic to the equation

The simplistic answer: 29er's are harder to endo.........

29erchico
Hurrah! What else do we need to know?

Let's all go for a ride now, mmmkay? :thumbsup:
 

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Seems pretty simple to me actually. The lower the COG and BB relative to axle position should be the determinant.

Taken to either extreme - let's say a 4" wheel vs. a 60" wheel (no blather about practical fit issues or overlap please) it would be hard to argue that you could even endo the 60" unless you voluntarily lunged forward while braking.

The 4" wheeled bike, on the other hand, would be the Biker Fox flip special (sorry to bring BF into this). Instant endo.
 

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Hmmm. COG

It was explained to me, and I bought into it, that when seated your COG is about at your belly button. When standing your COG dropped to about hub height as most of the weight was on the bottom bracket.

My interpretation of this leads me to believe that some of the increased stability of a 29er comes from the additional height or distance of the axles in relation to the bottom bracket. I guess from a lever standpoint if you were off the seat and ass-back then your weight would be less likely to pivot over the top of the front wheel as the COG standing would possibly be lower than axle height? Bueller? Anyone?

Throw a TALAS dial-a-travel fork on a 26" bike and do some endo experimentation. Wear a full-face. :thumbsup:
 

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DoktorNo said:
It was explained to me, and I bought into it, that when seated your COG is about at your belly button. When standing your COG dropped to about hub height as most of the weight was on the bottom bracket.
IMHO COG is dictated by where the weight is located, not what the weight is on. By standing, you have shifted a large amount of weight forward and up, which is bad. To improve your COG, you need to shift your weight rearward and down. Your theory is rather dangerous for steep descents. I would definitely get a second opinion, or you might learn the hard way.
 

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DoktorNo said:
It was explained to me, and I bought into it, that when seated your COG is about at your belly button. When standing your COG dropped to about hub height as most of the weight was on the bottom bracket.
For crying out loud, who told you this???
 

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Thats a funny looking endo there..

Nat said:
Here's how I understand it:
I on the other hand have found that on my26" bike id endo due to a lack of momentum and the 5" fork compressing..sending my sorry a$s over da barz..

I havent endoed on the 29er....yet...
 

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jrm said:
I on the other hand have found that on my26" bike id endo due to a lack of momentum and the 5" fork compressing..sending my sorry a$s over da barz..

I havent endoed on the 29er....yet...
That's a "wheelie."

"They" say that 29ers are hard to wheelie and hard to endo. Privately funded testing has shown different results.

This is the mother of all endos:
 

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Nat said:
For crying out loud, who told you this???
I dont know either, but they sound like they know what they are talking about. if most of your weight is on your seat then your COG is going to be higher than if it is on your pedals. cause your pedals are lower on your bike. you can do this by lifting your ass 1/4" you know. i dont think he was talking about standing straight up. i mean come on. who does that?

as for cloxxki's question, if you are rolling and you endo while the bike is moving, you will pivot at the drop out. if you slam on your brakes and your front wheel stops, you will pivot at the ground where the tire makes contact with it as it rolls forward.

sounds like splitting hairs really. its raining in my neck of the woods. what the hell is yawlls excuse?
 

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rryyddeerr said:
I dont know either, but they sound like they know what they are talking about. if most of your weight is on your seat then your COG is going to be higher than if it is on your pedals. cause your pedals are lower on your bike. you can do this by lifting your ass 1/4" you know. i dont think he was talking about standing straight up. i mean come on. who does that?
What is important is where the majority of the mass is on you and your bike, not what the mass is resting on. This is because you and your bike are a unit which, until miliseconds prior you your endo, were in forward motion. Every part of this unit was in forward motion.

Think about it, if you keep using this "resting on" theory, it really doesn't matter whether you are on the seat or the BB, both are "resting on" the hubs.
 

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Quasi said:
What is important is where the majority of the mass is on you and your bike, not what the mass is resting on. This is because you and your bike are a unit which, until miliseconds prior you your endo, were in forward motion. Every part of this unit was in forward motion.

Think about it, if you keep using this "resting on" theory, it really doesn't matter whether you are on the seat or the BB, both are "resting on" the hubs.
There it is.

Also think about it: The "resting on" theory means that if I'm standing up on the floor, since my feet are "resting on" the floor and my COG is on the floor, I can't fall down! Go ahead, push me! I'm a weeble-wobble!

As for the original question, aren't crashes usually too pell-mell to fit into such simplistic models? When I crash, all hell has broken loose. Either the contact patch or the hub can be the rotational center at different times during the crash. What happens when both tires leave the ground? What happens if while you're going over the bars you're scrubbing the brakes yet the front wheel is still rotating forward slowly while the tire is skidding a little on the dirt? Too much going on for a simple sketch to explain.

I bet this will be another inconclusive and pointless exercise. At least I'm taking part in it. Great.
 

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First of all, Nat: Great shot of fork/downtube clearance!

Second, center of gravity, aka center of mass is where the mass of a body is concentrated. This has nothing to do with where force is applied within the body (in this case a system of rider and bike). Your body has a certain center of mass and so does the bike. If you're sitting, the bike's center of mass is in a certain place, and yours is somewhere just above the seat. If you stand up, the bike's center of mass doesn't move, and yours remains near the same place in your body (unless you're hollow inside and the bend at your hips while sitting is the only thing preventing your internal organs from falling into your legs), which is now a few inches higher, thus raising the center of mass of the system. Sure it puts more force into the pedals, but center of mass has nothing to do with forces.

Third, I think it's pretty hard to say where the bike pivots in an endo, but it probably is a combination of both the contact patch and the hub, placing an instant center somewhere in the ground ahead of the bike. Because of this, the center of mass is vertically closer to the instant center on a 29er, so more braking force is required to produce the same moment that would lift the rear wheel on a 26er.
 

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Nat said:
There it is.

Also think about it: The "resting on" theory means that if I'm standing up on the floor, since my feet are "resting on" the floor and my COG is on the floor, I can't fall down! Go ahead, push me! I'm a weeble-wobble!

As for the original question, aren't crashes usually too pell-mell to fit into such simplistic models? When I crash, all hell has broken loose. Either the contact patch or the hub can be the rotational center at different times during the crash. What happens when both tires leave the ground? What happens if while you're going over the bars you're scrubbing the brakes yet the front wheel is still rotating forward slowly while the tire is skidding a little on the dirt? Too much going on for a simple sketch to explain.

I bet this will be another inconclusive and pointless exercise. At least I'm taking part in it. Great.
i was confusing the increased stability of hovering over my seat with the COG argument. duh. good call weeble wobble.
and yes, crashes are usually too fraught with factors to simplify in this way. sounds more like clox cant stand the thought of a guy like shiggy disagreeing with him. it might mean he's wrong. :eekster: holy crap.
 
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