Jumping 2 strokes

[ryan_cin]

Anyone else think it’s hard to jump a 2 stroke? On a 4t you can just give it gas at the face of the jump. If you do that on a 2 stroke you will a. Hit the powerband and loop out or b. Not hit the powerband and nose dive.

[mlatour]

On the powerband but holding the throttle steady on the face of the jump.

Edited October 2, 2017 by mlatour

[Wessmen]

What worked for me is to get used to the powerband, how it feels and be sure to hit it on the way to the jump and on the face of the jump, let off. You dont want to "jerk" it and have the front end coming up.

[motoxvet]

I do think it's a little harder in that it takes more practice to have your "muscle memory" remember that sweet spot in the throttle to do a proper jump and not  give it too much & end up in the trees somewhere.  If you're a slow learner (me), a G2 slower throttle helps.

Edited October 2, 2017 by motoxvet

[ryan_cin]

What worked for me is to get used to the powerband, how it feels and be sure to hit it on the way to the jump and on the face of the jump, let off. You dont want to "jerk" it and have the front end coming up.

That’s my problem I’m so used to being able to do that on a 4t I try on on the 2t

[turbo dan]

1 hour ago, Wessmen said:

What worked for me is to get used to the powerband, how it feels and be sure to hit it on the way to the jump and on the face of the jump, let off. You dont want to "jerk" it and have the front end coming up.

Wouldn't recommend that.  Letting off on the face will transfer weight forward and likely send you nose down.

Just stay on the power.  Its just that simple.

[Wessmen]

1 hour ago, turbo dan said:

Wouldn't recommend that.  Letting off on the face will transfer weight forward and likely send you nose down.

Just stay on the power.  Its just that simple.

Not really seeing how it transfer weight forward if you are on the gas until going up. There is no enginebraking and no drag if non o-ring chain. If he holds on like on a 4 stroke he will most likely send the nose high.

[ryan_cin]

I’ve also heard people say “just lean back and pull the bars up” but unless I’m jumping like 2 feet that doesn’t really work. I still end up nosediving. And I’m not gonna lie, I’ve done that a couple times and when I landed back wheel first I whiskey throttled and almost looped out

[Wessmen]

This is what Im talking about, before he leaves the ground he rolls of the gas. 

 

[ryan_cin]

This is what Im talking about, before he leaves the ground he rolls of the gas. 
 

How does he keep the front end up?

[turbo dan]

Those are very low jumps.  No real kick.  You could probably coast right over if you had the speed.

Here are some examples of what I would recommend you do:

 

 

[GoneDirtBikeN]

I only rode a two stroke once in my life, recently at a KTM demo day, at the Hangtown track no less. A buddy convince me to try the 2 strokes (like I would actually pass up the opportunity to try one). So first I went through all the 4-t's then I tried the 150. It was really fun but I needed/wanted a bit more power. I rode the 250 and it was an blast. Even though I knew about the no engine braking the fist corner was like "oh crap, that's right, where is that rear brake NOW!".

That being said thinking about the mechanics here, the lack of effect by engine braking means there's less effect happening going off the jump. So that means you don't have as much of that control the bike. The rest seems like it should stay the same. A good approach, consistent use of throttle through/off the face, etc. You simply have less ability to control the attitude of the bike in the air via throttle as lack of engine braking and 2-t reving out quickly. I still wouldn't want to chop the throttle; although less effect I would imagine it still causes some.

Is there something I don't understand that the above is not true?

BTY:So I decided riding the 125 at the demo day was a bad idea as it was just too little power to take me around that track. My buddy traded me my 125 ride for one of his 2nd "bonus ride, any bike" - so I used it to get another wag at the the 250sx! I now understand "the power band". Man, when that one hit, wow.

[shift1313]

The mechanics of the jump have so many variables its not even funny!  For a smooth takeoff that has a lot of ramp you can roll off/over.  The issue is when you have either steep ramps or short ramps because your suspension timing is off.  Basically if you don't stay on the gas off these short ramps then your rear end rebounds when your front is in the air already and sends you over the bars.  You can offset this by throwing your weight waaaay back.  The rear end rebounding is how you seat bounce a jump too but you have to drive hard on a jump and basically wheelie and then unload the rear at the right time to pop the bike up higher.

 

In general i would say on any bike to either keep a steady throttle or to accelerate off the face of a jump.  When you get more comfortable then you can play around with different ways to scrub off speed.  You don't have to be in the power band but you want to make sure that you are controlling the load on the suspension.  Many people let off the gas on 4t bikes but its not as simple as that.  You have to make sure the rear doesn't rebound up and make sure it scrubs out to the side.

[SlowDinoDog]

How does he keep the front end up?

He doesn't really look to be keeping the front end up much, to me? The front wheel drops down a foot or two below the rear wheel, in the short time of the jump...that's good for the slope he's hitting....for a longer jump, the front would drop even more, maybe too much, and you may want the engine at a constant rpm when you leave the face.

 

[suzuki2t666]

Its more about body position than throttle. If you are jumping nose low learn to keep your weight back. If you are nose high be more forward. Use the throttle to control the size/height of the jump.

[SlowDinoDog]

I'm a Novice, and can't jump like that, so I admit I'm speaking out my arse...

But, I have had a couple of physics classes...once a bike leaves the jump...it's essentially on a ballistic trajectory...the height and distance of the jump is dependent on the angle of the jump and the velocity of the bike when it takes off...I don't think it matters whether you were at a constant velocity, accelerated to that velocity, or decelerated to that velocity.

However, the rotation of a bike after it takes off is dependent on both whether you move your body CG forward or behind the bike CG and the rotational inertia effects of the bike.

When you chop the throttle, all the rotational masses, the engine, wheels, chain, etc, decelerate and there is an inertial reaction that causes the bike chassis to rotate forward.

When you are accelerating those masses with the throttle on the face of the jump, the chassis reaction is to rotate back and raise the front.

When those masses are at a constant rotational velocity when leaving the face of the jump, there is no rotational reaction.

Just theory, for me...

 

[shift1313]

29 minutes ago, Sofiedog said:

I'm a Novice, and can't jump like that, so I admit I'm speaking out my arse...

But, I have had a couple of physics classes...once a bike leaves the jump...it's essentially on a ballistic trajectory...the height and distance of the jump is dependent on the angle of the jump and the velocity of the bike when it takes off...I don't think it matters whether you were at a constant velocity, accelerated to that velocity, or decelerated to that velocity.

However, the rotation of a bike after it takes off is dependent on both whether you move your body CG forward or behind the bike CG and the rotational inertia effects of the bike.

When you chop the throttle, all the rotational masses, the engine, wheels, chain, etc, decelerate and there is an inertial reaction that causes the bike chassis to rotate forward.

When you are accelerating those masses with the throttle on the face of the jump, the chassis reaction is to rotate back and raise the front.

When those masses are at a constant rotational velocity when leaving the face of the jump, there is no rotational reaction.

Just theory, for me...

 

That is true with a few additional bits/exceptions.  Conservation of momentum is what you are talking about with the rotating masses.  But the other factor to all this is the suspension.  If your forks rebound after they are off the jump and they rebound down, the mass of the spinning front wheel wants to cause the chassis to rotate.  If the front wheel rebounds in the air and the rear rebounds on the jump its even worse.  This is why you want to accelerate hard on a short lip jump to keep the front light/uncompressed and the rear loaded up until you are in the air.

 

If you just shot a rigid object into the air at an angle with a velocity and no inertial bodies like the wheels/engine the physics is much easier.  

 

A fast rider can scrub off speed by sliding off the jump and letting the suspension rebound out to the side rather than adding height to the jump.  This lets you stay lower and hit the jump faster.  Absorbing the jump by unloading the suspension and absorbing the rebound with your body is another way.  You see racers do this on singles.  Either scrub or soak up the jump.

 

Not sure if you have ever seen the youtube channel "smarterEveryDay".  Smart guy that explains cool phenomena and he did a video on this topic.  

 

 

 

[deathman53]

I jump my 2 stroke atvs, alot. It's just like a dirt bike, keep the power going into the jump and stay on the gas to the top of the jump and use the gas or brake to balance you put. Give it gas if you are nose driving and brake if you too front end high. Smooth is the key to successfully jumping(and landing) a 2 stroke dirt bike or atv. I launch my 3 and 4 wheeled 2 stroke atvs 60+ ft and 20ft up in the air. Remember smooth is the key.

[SlowDinoDog]

That is true with a few additional bits/exceptions.  Conservation of momentum is what you are talking about with the rotating masses.  But the other factor to all this is the suspension.  If your forks rebound after they are off the jump and they rebound down, the mass of the spinning front wheel wants to cause the chassis to rotate.  If the front wheel rebounds in the air and the rear rebounds on the jump its even worse.  This is why you want to accelerate hard on a short lip jump to keep the front light/uncompressed and the rear loaded up until you are in the air.  

If you just shot a rigid object into the air at an angle with a velocity and no inertial bodies like the wheels/engine the physics is much easier.  

 

A fast rider can scrub off speed by sliding off the jump and letting the suspension rebound out to the side rather than adding height to the jump.  This lets you stay lower and hit the jump faster.  Absorbing the jump by unloading the suspension and absorbing the rebound with your body is another way.  You see racers do this on singles.  Either scrub or soak up the jump.

 

Not sure if you have ever seen the youtube channel "smarterEveryDay".  Smart guy that explains cool phenomena and he did a video on this topic.  

 

 

 

 

Good stuff !

Ok...let me think about the effects of suspension...I think we agree on the first two effects...body cg position and rotational inertia.

The third effect is a linear acceleration effect of the suspension components...

If the forks are compressed on the face of the jump, they and the heavy front wheel will accelerate away from the chassis when they leave the face. This causes a linear reaction in the opposite direction at the chassis triple clamps which makes the bike go a bit higher AND is a rotational torque at the chassis CG causing the front end to rotate up and further compresses the rear while it is still on the face.

When the rear wheel leaves the face, the shock and heavy wheel accelerates away from the bike causing a linear acceleration at the chassis shock mount in the opposite direction and the chassis to go higher. There is also a rotational torque at the CG that is in the opposite direction from the torque generated by the forks.

As you said, when you scrub, you change the angle of the linear acceleration vectors of the suspension and a smaller percentage goes in the vertical direction, keeping you a bit lower. I suspect there is a loss of traction/velocity effect, too, when scrubbing?

I guess it all is a conservation of momentum thing along with Issac Newton's "For any action there is an equal and opposite reaction."

 

[shift1313]

1 hour ago, Sofiedog said:

Good stuff !

Ok...let me think about the effects of suspension...I think we agree on the first two effects...body cg position and rotational inertia.

The third effect is a linear acceleration effect of the suspension components...

If the forks are compressed on the face of the jump, they and the heavy front wheel will accelerate away from the chassis when they leave the face. This causes a linear reaction in the opposite direction at the chassis triple clamps which makes the bike go a bit higher AND is a rotational torque at the chassis CG causing the front end to rotate up and further compresses the rear while it is still on the face.

When the rear wheel leaves the face, the shock and heavy wheel accelerates away from the bike causing a linear acceleration at the chassis shock mount in the opposite direction and the chassis to go higher. There is also a rotational torque at the CG that is in the opposite direction from the torque generated by the forks.

As you said, when you scrub, you change the angle of the linear acceleration vectors of the suspension and a smaller percentage goes in the vertical direction, keeping you a bit lower. I suspect there is a loss of traction/velocity effect, too, when scrubbing?

I guess it all is a conservation of momentum thing along with Issac Newton's "For any action there is an equal and opposite reaction."

 

Remember too that if you coast off the jump you have nothing to compress the rear suspension vs accelerating off the jump where you are loading up the rear and unloading the front.  If you watch guys when they seat bounce they lean back sitting on the bike hard on the gas and just before they leave the jump they stand up.  They are unloading their weight.  If you also see guys jumping take their feet off the pegs they are trying not to push back on the bike slowing the momentum down.  It is a crazy complicated equation that makes my head hole hurt ?

remember with body position that you are still taking into account where you are touching the bike(pegs, tank, bars).

 

Lotta JS7 haters on TT but i don't care.  Slowmo scrub videos rock