Less engine braking, lighter throttle

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tony.mon
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Re: Less engine braking, lighter throttle

Post by tony.mon »

My long rambling post above was my attempt to explore the science and the factors.
It's interesting that different people report different results from the same experiment, and I wondered why.
I'm still thinking this through.

Maybe an analogy might help. Or a thought experiment:
Upturn a bicycle and a motorbike, spin the rear wheel by hand (with neutral selected on the motorbike) then try to stop the spinning wheel by placing your hand on the tyre.
I'm ignoring the difference in diameter for this exercise.

The bicycle wheel stops quickest, because it has less mass.
You put in less force to stop it, and the rate of change of the perimeter speed is greater.

So what do we mean when we say that engine braking increases or decreases when a lighter flywheel is fitted to an engine ?

Is it the rate at which the engine can decelerate on a closed throttle that is being perceived, which, as all other forces are equal in all cases, means that it's only the rotating part's inertia (is this latent energy? I'm running out of knowledge here) that differs, due to the change in mass.

If there are two effects at work here, one, the engine pumping against a vacuum, and the other, opposing force, being the inertia of the rotating parts trying to continue rotating at the current rate, which one are we feeling, and is one or both affected by the lightening of the flywheel?

As said, I'm no expert, and I'm not dissing anyone's point of view, I'm simply trying to understand the phenomenon.
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Chris58
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Re: Less engine braking, lighter throttle

Post by Chris58 »

tony.mon wrote: Sat Dec 22, 2018 11:04 am My long rambling post above was my attempt to explore the science and the factors.
It's interesting that different people report different results from the same experiment, and I wondered why.
I'm still thinking this through.

Maybe an analogy might help. Or a thought experiment:
Upturn a bicycle and a motorbike, spin the rear wheel by hand (with neutral selected on the motorbike) then try to stop the spinning wheel by placing your hand on the tyre.
I'm ignoring the difference in diameter for this exercise.

The bicycle wheel stops quickest, because it has less mass.
You put in less force to stop it, and the rate of change of the perimeter speed is greater.

So what do we mean when we say that engine braking increases or decreases when a lighter flywheel is fitted to an engine ?

Is it the rate at which the engine can decelerate on a closed throttle that is being perceived, which, as all other forces are equal in all cases, means that it's only the rotating part's inertia (is this latent energy? I'm running out of knowledge here) that differs, due to the change in mass.

If there are two effects at work here, one, the engine pumping against a vacuum, and the other, opposing force, being the inertia of the rotating parts trying to continue rotating at the current rate, which one are we feeling, and is one or both affected by the lightening of the flywheel?

As said, I'm no expert, and I'm not dissing anyone's point of view, I'm simply trying to understand the phenomenon.
Tony, I'm no expert on this one either, you have a good point here. This is definately a confusing matter lol,
I think both of our theory's are actually right.

I'm thinking your theory of decreased engine braking applys when gearing down, it is easier to spin up the lighter mass (but revs also drop very fast with a light flywheel when using clutch).

My theory applys when just letting off the throttle. In that case you don't have as much flywheel inertia to overcome engine vaccum as much.

So my conclusion would be that a light flywheel wheel increases engine braking when letting off, but decreases engine braking when gearing down and matching revs.

What do you think
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Flatline
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Re: Less engine braking, lighter throttle

Post by Flatline »

eamonn wrote: Fri Dec 21, 2018 10:16 pm Rings fluttering !!!
Don't get sent to prison then :(
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fabiostar
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Re: Less engine braking, lighter throttle

Post by fabiostar »

ok try this . i spent a lot of time tuning two stroke years ago. as you know the pumping in a 2t is a whole different kettle of fish :D .

and anytime i lightened or added weight to a 2t the engine braking increased or decreased. this was just because of the flywheel change because the pumping effect wasnt present as the piston movement wasnt pumping the crankcase pressure at all. you are taking the vacum effect out of the numbers unlike on a 4t.

when i had both my storms, one with a lightweight and one standard side by side the one thing that made the biggest difference to engine braking was the flywheel.

so id think the pumping has an effect but less so than the flywheel change??
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Chris58
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Re: Less engine braking, lighter throttle

Post by Chris58 »

fabiostar wrote: Sat Dec 22, 2018 4:13 pm ok try this . i spent a lot of time tuning two stroke years ago. as you know the pumping in a 2t is a whole different kettle of fish :D .

and anytime i lightened or added weight to a 2t the engine braking increased or decreased. this was just because of the flywheel change because the pumping effect wasnt present as the piston movement wasnt pumping the crankcase pressure at all. you are taking the vacum effect out of the numbers unlike on a 4t.

when i had both my storms, one with a lightweight and one standard side by side the one thing that made the biggest difference to engine braking was the flywheel.

so id think the pumping has an effect but less so than the flywheel change??
So on your storms, did the lightened flywheel give more or less engine braking?

for me it seemed like more, but i couldnt compare side by side
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KermitLeFrog
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Re: Less engine braking, lighter throttle

Post by KermitLeFrog »

I'm coming to the conclusion. Moving along and you chop the throttle. The rotational inertia of the engine (crank, flywheel, shafts etc) feeds energy back into the system, trying to stop it slowing down. The energy reduction caused by pumping losses causes a drag in the rear wheel. The rotational inertia of the system is trying to maintain the engine speed. The lighter the flywheel the less rotational inertia. Ergo, a lighter flywheel causes MORE engine braking. Not sure how much though.
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Pete.L
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Re: Less engine braking, lighter throttle

Post by Pete.L »

Nope. It’s actually less engine breaking. But that’s when it’s considered on its own.
You’ve all got the concept of a lighter flywheel being able to spin up faster because it has less mass. It also has less mass or inertia to keep the engine spinning once the power is cut. This does mean it would come to a stop faster if that was the only force involved but it also mean with its lesser mass it also has less effect on the momentum of the other parts ie less engine braking.
That said, other factors can now have a greater affect, like the closed throttle vacuum that’s been mentioned, that will have increased and feel like the engine braking has increase as it has less reciprocal mass to work against.
I don’t know the answer you are looking for guys but if Ian wants to change the the engine drag on a closed throttle on the Ap I would suggest a custom map to dump some fuel in or a Intech valve from Harris if they can still be found like I have on the SP. it allows me to very the backpressure by feeding air into the throttle body instead of fuel by means of a one way valve


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fabiostar
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Re: Less engine braking, lighter throttle

Post by fabiostar »

Chris58 wrote: Sat Dec 22, 2018 5:52 pm
fabiostar wrote: Sat Dec 22, 2018 4:13 pm ok try this . i spent a lot of time tuning two stroke years ago. as you know the pumping in a 2t is a whole different kettle of fish :D .

and anytime i lightened or added weight to a 2t the engine braking increased or decreased. this was just because of the flywheel change because the pumping effect wasnt present as the piston movement wasnt pumping the crankcase pressure at all. you are taking the vacum effect out of the numbers unlike on a 4t.

when i had both my storms, one with a lightweight and one standard side by side the one thing that made the biggest difference to engine braking was the flywheel.

so id think the pumping has an effect but less so than the flywheel change??
So on your storms, did the lightened flywheel give more or less engine braking?

for me it seemed like more, but i couldnt compare side by side

on any storm iv tried the lightweight flywheel has caused an increase in engine braking. quite a lot less when the flywheels very light, as in down below the 5 lbs mark. if this is just because of the weight loss or because it has effected other gains or losses in the engines drive train who knows but the effect felt by the rider is an increase..

push two big balls along the ground, same size but one plastic and one metal. the heavy one takes longer to get going but will still be rolling (when you shut the throttle) when the light one stops? :thumbup:
the older i get,the faster i was :lol:
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KermitLeFrog
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Re: Less engine braking, lighter throttle

Post by KermitLeFrog »

I've got to agree with Fabio on this.

The Intech valve looks interesting. It's just an adjustable pressure relief valve. Opening up an air supply to the intake manifold at low intake manifold pressure. I.e. when the throttles are closed at high revs. Adjusted correctly it shouldn't affect anything else.
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freeridenick
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Re: Less engine braking, lighter throttle

Post by freeridenick »

And I thought The Matrix baked my noodle.
tony.mon
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Re: Less engine braking, lighter throttle

Post by tony.mon »

Pete.L wrote: Sat Dec 22, 2018 10:58 pm
That said, other factors can now have a greater affect, like the closed throttle vacuum that’s been mentioned, that will have increased and feel like the engine braking has increase as it has less reciprocal mass to work against.

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I think that this is likely to be the case, a lighter flywheel reduces the inertia of the rotating parts, which effectively increases (proportionally) the engine braking effect of the engine trying to draw air in when it can't.
The two forces act against each other, reducing one has the effect of increasing the other.

So the net result of fitting a lighter flywheel is a perceived increase in engine braking.

Bloody hell, that took some working out!
Last edited by tony.mon on Sun Dec 23, 2018 9:55 pm, edited 1 time in total.
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fabiostar
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Re: Less engine braking, lighter throttle

Post by fabiostar »

:lol: :lol: :lol: anybody want a lightweight flywheel lmao..
the older i get,the faster i was :lol:
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