Reciprocating mass

[camchain]

mmm physics, a favourite subject now but pity I never paid proper attention during block courses when younger. Without any proper thought, Reciprocating mass = weight & volume moving back and forth.

[takitimu]

Nice explanation from Motu ( bling given ).

I always thought that the KLX450R was a bit of handful because of that engine mass cornering, but I'd never considered it in terms of acceleration & everything clicked into place explained the way you did.

Put it this way doing a figure 8 in the KLX was "fun", in 1st sudden change in throttle & it'd stand up or drop down ( in the rut ) or step out ( exiting ).
Also the 250's ( 2/ & 4/ ) seem to "float" over dirt where the 450 would dig in, that kind of puzzled me & I reckon I have a way better understanding now why that is.

Danger won't like me for saying it, but roll on a 350 4/ I reckon .

[B0000M]

or, if you want even less reciprocating mass, simply get a 2 stroke, more power and less weight!

[Danger]

OK thanks for the reply's, some interesting answers and my understanding is as Noobi's (and some others but Noobi was first and to the point) that it is a mass that needs to accelerate, slowdown, stop and move back in the opposite direction, like a piston.
The context it was used in was in the case of a big cruiser, that had lighter wheels, in so doing it had reduced the reciprocating mass. This had me scratching my head??? WTF?
Now I guess in the context of suspension going up and down over bumps I could understand that reciprocating mass had been reduced if the bike was a dirt bike, but this was a big road bike. Taken to the enth degree I guess reciprocating mass had been reduced even on the big road bike, although I did not get the impression this is what was meant, and its more the bike going up and down rather than the wheel on the road and as I had heard some dubious statements from the same source in the past I questioned what I was hearing. I believe they were actually referring to rotational mass.

Robert Taylor might pipe in and give us some thoughts on the reduction of reciprocal mass as it relates to wheels on big cruiser bikes with any luck.

Thanks for taking the time to answer everyone. My eyes may have been opened a little more.

[Danger]

Nice explanation from Motu ( bling given ).

I always thought that the KLX450R was a bit of handful because of that engine mass cornering, but I'd never considered it in terms of acceleration & everything clicked into place explained the way you did.

Put it this way doing a figure 8 in the KLX was "fun", in 1st sudden change in throttle & it'd stand up or drop down ( in the rut ) or step out ( exiting ).
Also the 250's ( 2/ & 4/ ) seem to "float" over dirt where the 450 would dig in, that kind of puzzled me & I reckon I have a way better understanding now why that is.

Danger won't like me for saying it, but roll on a 350 4/ I reckon .

Not at all, they just are not my thing but a 350 would be a good idea.
Often a four stroke will feel better in the suspension action than a two stroke with the same components, the engine mass pushing on the front end often makes the four strokes feel more plush and forgiving, the two strokes more prone to being a little light up front and deflecting more.

[bogan]

reciprocating mass seems an odd term to use with respect to wheel weights, whats wrong with calling it unsprung mass? Benifits from lighter rims are three-fold, less unsprung mass makes for better suspension, and it also greatly reduces the weels rotational inertia, which allows quicker acceleration, and less gyroscopic forces mean the bike can be turned quicker.

Engine reciprocating mass i would call the piston rings etc, keep as low as possible for better 'balance' which allows higher/quicker reving. Also low reciprocating mass means the forces on con-rod etc are much lower.

[Danger]

Thats what I thought, so its not just me lol!

[BMWST?]

reciprocating mass as i understand it is the mass of pistons and valves.The whole valve train has a huge reciprocating mass,and absorbs a lot of energy (thats another discussion).
Talk of reciprocating mass re wheels and suspension (to me anyway) is unsprung vs sprung weights.The wheels,fork,swingarms,brake calipers rotors tyres and tubes are all unsprung weight.The rest of the bike (including rider) is sprung weight.Not omly is it important to keep unsprung weight as low as possible,the ratio of sprung to unsprung weight is also important.A relativey light bike with a low unsprung weight will have a much nicer ride and suspension response than the same weight bike with heavier unsprung weight.
A much heavier bike will have a nice ride than a lighter bike but actual suspension response will be poor if the unsprung weight is relatively high.

[B0000M]

Thats what I thought, so its not just me lol!

i also agree with what bogan just said

[bogan]

reciprocating mass as i understand it is the mass of pistons and valves.The whole valve train has a huge reciprocating mass,and absorbs a lot of energy (thats another discussion).
Talk of reciprocating mass re wheels and suspension (to me anyway) is unsprung vs sprung weights.The wheels,fork,swingarms,brake calipers rotors tyres and tubes are all unsprung weight.The rest of the bike (including rider) is sprung weight.Not omly is it important to keep unsprung weight as low as possible,the ratio of sprung to unsprung weight is also important.A relativey light bike with a low unsprung weight will have a much nicer ride and suspension response than the same weight bike with heavier unsprung weight.
A much heavier bike will have a nice ride than a lighter bike but actual suspension response will be poor if the unsprung weight is relatively high.

weellll, technically the swingarm is a semi sprung mass, but im just nitpicking there! And i agree that the ratio is very important.

Yeh the valve train absorbes a lot of energy, something like 10% of an engines power. I wonder if the pnuematic valved engines suffer from more or less energy loss in the valvetrain.

[xwhatsit]

I don't understand what wheels (or flywheels) have got to do with reciprocating mass -- they're not reciprocating mass are they, they're rotating mass, surely? Unless of course the wheels are unbalanced (e.g. your crank, the counterweight is rotating but it's unbalanced so it sets up a reciprocating force).

[CookMySock]

reciprocating mass seems an odd term to use with respect to wheel weights, whats wrong with calling it unsprung mass?

Nuthin. It's just the technically correct term to use. "Reciprocating mass" refers to the entire suspension mechanism, not just the wheel weight.

Take for example, a rear suspension with twin shocks mounted at or near the rear axle.

Contrast this to a modern link suspension, and even if all the components have the same weight, overall the system has less reciprocating mass since the link ratio is much different. The shocks move at a much lower speed as they are geared down from the swingarm - therefore less force is required to set them in motion or reverse the direction of motion. The link ratio also helps with other nasties, like stiction, as the effect of it is multiplied by the link ratio.


Steve

[Ocean1]

Now I guess in the context of suspension going up and down over bumps I could understand that reciprocating mass had been reduced if the bike was a dirt bike, but this was a big road bike.

I don't understand what wheels (or flywheels) have got to do with reciprocating mass -- they're not reciprocating mass are they, they're rotating mass, surely?

Yes, seems likely they're either talking 1) unsprung weight, 2) a branch of mechanics we've never encountered, or 3) utter shit.

[bogan]

Nuthin. It's just the technically correct term to use. "Reciprocating mass" refers to the entire suspension mechanism, not just the wheel weight.

unsprung and semi sprung terms also refer to the entire suspension mechanism i thort

[kezzafish]

yeah wow interesting stuff. Got that one wrong lol. so what has more effect on handling would be interesting to know... recipricating mass off piston/conrod/valve train or the spinning flywheel weight? and what do we call the spinning flywheel weight 'cos it ain't unsprung weight....?