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Why do 85s and Superminis have a jumpy rev before a landing?

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I've never heard an answer for this, but it just came to mind after watching this new vurbmoto video with a rider on a small bore two stroke and noticing the way the engine revs out before they land. I've always noticed it at the track, but never thought too much about it until now. But I don't understand it. I don't think I've ever heard a big bore bike rev like this when either I, or another rider gives it a handful before landing. But yet, almost every small bore two stroke that's being ridden by a fast rider has that jumpy rev!

 

For an example, go to 2:30 when he hits the double. Listen to when he's about to land it:

 

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That's how small bore 2 strokes sound..  Maybe I misread the question?

Sorry, it's hard to describe. But basically the "ring-ring!" sound right before he lands. It sounds like the throttles being twisted twice, but I'm pretty sure they don't do that.

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Initial noise is likely from the intake, noise resonates in the airbox briefly before the engine gets to burn that air. 

same with the 4 strokes, just a different tone. 

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Initial noise is likely from the intake, noise resonates in the airbox briefly before the engine gets to burn that air. 

same with the 4 strokes, just a different tone. 

It doesn't sound like 4 strokes have this sound.. Or 250 2 strokes.

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It doesn't sound like 4 strokes have this sound.. Or 250 2 strokes.

They do, just a different tone and sound frequency.

Next time you have one of your 250 2 strokes warmed up, whack the throttle WFO, you will hear intake noise before the exhaust note changes..

Keep in mind where the airbox is. 

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They do, just a different tone and sound frequency.

Next time you have one of your 250 2 strokes warmed up, whack the throttle WFO, you will hear intake noise before the exhaust note changes..

Keep in mind where the airbox is. 

Idk, maybe I've never listened close enough but I've never heard it on those bikes. Now on 125s, I have.

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Idk, maybe I've never listened close enough but I've never heard it on those bikes. Now on 125s, I have.

Open the hood on your car put your ear next to the airbox and and have some one rev the car a little, you will be able to hear it a lot better.

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I think what your talking about is just a bit of panic rev to bring up the front wheel. They blip the throttle once or twice right before they land.

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I thunk it's just initial rich condition (if you can say so) of a carbureted motor, which is especially visible on smaller displacement engines as they rev a lot higher before landing.

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Alot of the time they back brake the rear wheel in the air so the wheel has to spin up again before landing, so there is probably a slight bog before they get on the gas.

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Revving before you land is the correct way to do it. what this does is make a spinning tire under power pull the bike forward . It keeps the front wheel lighter and makes the landing a bit softer. It propels the bike forward instead of falling out of the sky (gas off).

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Rev with clutch in followed by dumping the clutch keeping the throttle pinned the whole time.

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It's no big deal, he's just keeping the engine from bogging on landing.  The small bikes have such a narrow range of power, the rider has to keep it singing.The guys on the bigger bikes do the same thing, it's just a little lower pitch. If they didn't, they would have a very brief lunge forward from the sudden engine braking from the four strokes. 

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If you are talking about him opening the throttle just before landing, it is done to keep forward momentum on the landing ramp, and also when the tire comes in contact with the ground, the chain is under load when landing a large jump which actually helps the rear shock absorb some of the impact due to the natural arcing pivot of the swing arm..

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In that example you showed I believe he pulled the clutch in, brake tapped, revved the bike full throttle, then released the clutch.. riders at this level have amazing clutch, brake and throttle control so even though it looks like the wheel spun up again, that was just his excellent timing and accuracy, it still wasn't spinning at the engine speed, so when he dropped the clutch (final step) the changing sound is just the engine coming under load again to spin the rear tire up to speed...

The 'clutch in_brake tap_rev_clutch out' thing happens on 4 strokes too, and the effect on the engine is the same... but the sound is obviously different and on smaller 2 strokes it's just more noticable.

There's another thing that causes a similar sound too; most 2 strokes don't have rev limiters, if you watched ronnie mac's vids his cr250 does actually do this, it just happens slower.. it's simply the engine revving to max rpm and then naturally cutting out briefly (because the inertia of the crank forces it to rev higher than the engine/ignition can actually operate).. pretty much all 2 strokes do this, and the equivilant on 4 strokes is valve bouncing, but most 4 strokes have rev limiters so that is what you hear when they go full gas in the air.

That is just what I understand from my experience anyway   :blah:

 

Edited by stevo_259

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