Reciprocating mass

[Danger]

Without googling the term I would like to hear what your immediate understanding of reciprocating mass is, or if you even have any idea what it is? Just trying to get an idea of the level of knowledge out there regarding this, so don't be worried if you get it wrong. I will reveal the reason why I ask later.

[noobi]

Without googling the term I would like to hear what your immediate understanding of reciprocating mass is, or if you even have any idea what it is? Just trying to get an idea of the level of knowledge out there regarding this, so don't be worried if you get it wrong. I will reveal the reason why I ask later.

i understand it to be the point in which a mass is forced to go in the opposite direction to which it was going, thus it must overcome the moment of enertia. like a piston has to be forced to go up after getting to the bottom of the stroke? year 12 physics boyo

[diesel_mr2]

An object turning in a circular motion, e.g: a wheel?

[kezzafish]

yeah i thought reciprocating meant spinning/turning and mass is mass. when do we get to know why you ask?

[Sam I Am]

sounds like the counter weight in a thumper to offset the piston and rod ?

[Sam I Am]

ok now I have googled it and I got it wrong Duh

[kezzafish]

"Flywheel" is much a simpler way to say it. But it doesn't have to be a motor it can be any thing which spins and has a mass?? the earth even

[Gravel911]

errrrrrrr ummmmmmmmmm

I think it’s the parts that have to change direction of travel in the engine

i.e. Piston, rings, gudgen pin and probably part of the rod.

I know the Husaberg engines have a little rotating weight that spins in the opposite direction to counter the effect of reciprocating mass

[flyingcr250]

yeah i thought reciprocating meant spinning/turning and mass is mass. when do we get to know why you ask?

a spinning object has less recipricating mass than an object heading in opposite directions , like the difference between a piston engine and a rotary engine. or a sabre saw and a circular saw.

[CookMySock]

A moving mass with a longitudal path that reverses direction upon itself twice for every cycle. Do I win?

Steve

[cheese]

Noobi explained it well, but yes a piston is a perfect example. Man 6th form physics are only just there in the back of my memory...

[oldskool]

i understand it to be the point in which a mass is forced to go in the opposite direction to which it was going, thus it must overcome the moment of enertia. like a piston has to be forced to go up after getting to the bottom of the stroke? year 12 physics boyo

I agree with noobi, a mass that reverses on itself and is the point of failure. Like my brain when I headbang to music.

[flyingcr250]

Noobi explained it well, but yes a piston is a perfect example. Man 6th form physics are only just there in the back of my memory...

oh no rotary bike engines??:Pokey: please god no.

[Motu]

I've given this one a bit of thought these last few years after my experiments with the XT400/500/600/621.The changes in an identical bike with changes in reciprocating mass were very marked....it's not so much an engine capacity difference,it's the reciprocating mass.The 400cc engine,with shorter stroke,lighter flywheel etc was much easier to ride - just flicked through the gears up and down without any effect on the motorcycle itself.Just riding the bigger capacity engine bike,it felt heavier,you could really feel the capacity change through the reciprocating mass.The bigger capacity bike was much more powerful of course,but harder to ride.Coming out of turns there was heaps of wheelspin of course....but more than could be controlled by just the throttle.Most noticeable was coming into turns - the smaller capacity bike was very stable,I was able to set up slides with ease....the bigger engines were almost uncontrollable by contrast.I could just rail the turns on the little engine,and was a total mess with the bigger engine.

So when I wanted a BMW airhead to set up for gravel roads - I went for the small capacity R65.Less reciprocating mass means less wheel spin out of turns,and is much more stable on turn entry.The road surface is able to control engine speed....rather than the engine dictate things.

Also after the DT230,I found lack of engine braking a big advantage....2 strokes have that advantage.MotoGP teams have spent as much effort reducing engine braking as gaining HP.

Dunno if this is what you want to hear - but I want minimal engine braking,and an engine that can react to surface conditions to maintain traction.I'm all for less reciprocating mass.

[Danger]

Excellent post Motu, and sorry I can't give you gas as apparently I've given out too much in the last 24 hours! Everyone that posted above you got gas for answering.

[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!

:headbang:

[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....?

[B0000M]

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....?

flywheel is just rotating mass, which is sprung weight, as its attached to the main section of the bike

[bogan]

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....?

i would say the flywheel has more effect on handling as the rotational inertia stored in it makes it more difficult to change direction. But the effect would probably be largely overshadowed by the same effect generated by the wheels.

[CookMySock]

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....?In the engine? Thats just part of the sprung weight of the bike. That it's rotating is not relevant to the suspension.

In the wheel? Thats part of the suspensions reciprocating mass. It's not reciprocating coz its going round and round, it's reciprocating coz its going up and down as part of the suspension. The round and round bit here is not relevant, EXCEPT that the more mass there is in the outer parts of the wheel - the stiffer the steering will be as the wheel acts as a gyroscope.

I think.

Steve

[NDORFN]

To answer your question properly as requested, I have no idea what reciprocating mass is and could only speculate based on the wording that it has something to do with a mass that reciprocates.

[Buddy L]

I had my rotating mass mixed up with my reciprocating mass, but understand alot better now.
The advantages of going to school up north and having limited options. ;)

[BMWST?]

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....?

the spinning flywheel would have a gyroscopic effect,and a tourque reaction(think about teh bmw inline flat twins n fours)

[cheese]

i would say the flywheel has more effect on handling as the rotational inertia stored in it makes it more difficult to change direction. But the effect would probably be largely overshadowed by the same effect generated by the wheels.

Would this be the the gyro effect? I recall seeing recently on discovery channel a bit about some guys trying to build the next bike super road bike. I think (from memory) that they made a 4 cylinder engine (north south) with two pistons spinning one way and two in the opposite direction and then coupled through a set of gears much like a differential (not sure I explained that well or right). Anyway the end result was a bike with very little "gyro effect" (I'm still not sure that is the correct term). The tester that they got to test the bike said that cornering with it was amazing, a normal bike you have to push it hard to lean over and this one would glide around with ease.

[xwhatsit]

Flywheel effect? Well, why do you need a dab of right rudder on your DH.82 when the tail lifts off the ground?

[Reckless]

I've given this one a bit of thought these last few years after my experiments with the XT400/500/600/621.The changes in an identical bike with changes in reciprocating mass were very marked....it's not so much an engine capacity difference,it's the reciprocating mass.The 400cc engine,with shorter stroke,lighter flywheel etc was much easier to ride - just flicked through the gears up and down without any effect on the motorcycle itself.Just riding the bigger capacity engine bike,it felt heavier,you could really feel the capacity change through the reciprocating mass.The bigger capacity bike was much more powerful of course,but harder to ride.Coming out of turns there was heaps of wheelspin of course....but more than could be controlled by just the throttle.Most noticeable was coming into turns - the smaller capacity bike was very stable,I was able to set up slides with ease....the bigger engines were almost uncontrollable by contrast.I could just rail the turns on the little engine,and was a total mess with the bigger engine.

So when I wanted a BMW airhead to set up for gravel roads - I went for the small capacity R65.Less reciprocating mass means less wheel spin out of turns,and is much more stable on turn entry.The road surface is able to control engine speed....rather than the engine dictate things.

Also after the DT230,I found lack of engine braking a big advantage....2 strokes have that advantage.MotoGP teams have spent as much effort reducing engine braking as gaining HP.

Dunno if this is what you want to hear - but I want minimal engine braking,and an engine that can react to surface conditions to maintain traction.I'm all for less reciprocating mass.

Which is why some of us ride ride 200cc 2 smokers instead of those big 450"s LOL!!!

I think we will hear more of this rotating mass theory as time goes on. Bikes are revving very much higher and their will be more investigation into the effects of the engine (well 4 strokes anyway) on the dynamics of bikes (road and off road) bearing in mind engines are now part of the chassis not just rubber mounted and sitting in it anymore.

The latest Husaberg is testing these principles with its new engine layout here is what is being said about it:
That high price does get you a radical motorcycle. The new 70-degree engine configuration (denoting the slope of the nearly horizontal cylinder) is no gimmick. Practically flipping an ordinary layout, the design positions the crankshaft very close to the bike's overall center of gravity. This puts the engine's largest rotating component at a neutral location in the chassis, result being that the big four-stroke motor's mass is less noticeable to the rider.

Like holding a skill saw out while its going and then turning it. You can feel the rotational effects of the spinning blade and motor on the balance of the saw as you turn it over!

Interesting stuff!! I wonder if Ktm will adopt the Husaburg theories for their mainstream bikes???

[bogan]

Would this be the the gyro effect? I recall seeing recently on discovery channel a bit about some guys trying to build the next bike super road bike. I think (from memory) that they made a 4 cylinder engine (north south) with two pistons spinning one way and two in the opposite direction and then coupled through a set of gears much like a differential (not sure I explained that well or right). Anyway the end result was a bike with very little "gyro effect" (I'm still not sure that is the correct term). The tester that they got to test the bike said that cornering with it was amazing, a normal bike you have to push it hard to lean over and this one would glide around with ease.

You may be refering to the conservation of angular momentum effect? which occurs when a rotating mass is speed up or slowed down. The engine spins up clockwise so the bike trys to spin counterclockwise, but this wont occur if the engine is spinning both clockwise and counter-clockwise with a crank going each way. Conservation of angular momentum is especialy useful when jumping mx bikes, as it allows the landing orientation to be changed while in the air.

Unless there also some sort of gyro cancelation that occurs if there are two masses spining in opposite directions at the same speed?

[camchain]

I reckon BuddyL touched on it nicely Re rotating mass being different to reciprocating mass, (although when you combine parts that do both the effects must become related).

The example Danger refers to (as I see it) is just referring to reduced wheel weight = reduced unsprung weight which 'reciprocates' as suspension moves up and down. The fact the wheel rotates as well is an extra issue not directly related to the point being made (I reckon).

I'm not bagging any of the other interesting stuff being raised - I got my first good example about how a big 4-banger doesn't like turning when I rode a new TT600 when they came out (I was riding 250 2t at the time). I thought the thing changed direction like a bulldozer. (Sorry danger no offence, I think you had one of these beasts!?)

[Danger]

No need to appologise Camchain for expressing your opinion, I won't give bad gas as others have to me for expressing my opinion on how a certain bike handles.

Now I know my KTM200 and my RM250 are a couple of the most nimble bikes out there for the sort of riding I'm doing these days, but back then I was racing YZ250's and the TT600 on MX tracks and I honestly cant recall having any issues with the handling of the TT600 but the YZ's were also known as stable handlers rather than quick handlers like the RM's off the day, so if I was to compare them to RM's back then or if I had a ride on one today I'm certain I would have noticed the lack of handling. The bike was 123kg (and I felt every kg in a wet Riverhead enduro for 5 1/2 hours) as I recall and I had the heaviest YZ490 springs I could find with the rear shock modified to use the YZ spring and I do know it was incredibly stable, controllable and was great for sliding (even on the asphalt) getting holeshots and wheelies and very tractable. In my opinion the big four stroke and its ability to hole shot back then made up a great deal for an average rider and I got some great results on that bike, not so great when racing the YZ250's which took more skill to get the most out off. I have great memory's of that bike other than its problematic electrics that used to kill the bike at times mid race (thankfully never over any jumps). Its my opinion that the new 450's have made many riders look better than what they actually are and if I was racing MX seriously thats what I would be riding today.

Incidentally the remarks on the lighter wheels on the big cruiser reducing reciprocating mass were made on Bike Rider TV last night and I did laugh at the time.

[camchain]

Thanks Danger. My bike testing analysis would probably be on a par with that TV show, vaporous like bad gas. Hopping off the 2t at the local track I just remember finding it ungainly and had trouble with the front end tucking/pushing. Mind you I also had that 600's father, the TT500 and although fun for a while I never bonded with it, just too heavy. Sounds like you got a lot out of your early riding years. Wish I had done a lot more racing. Am same weight I used to be but body doesn't reciprocate as effectively as it used to.

Those new Husabergs look great, although crank seems very high to me. I hear Kawasaki looking at similar engine layout. I recall many years ago Honda tried mounting a gas tank under the engine for super low C of G (road race machine) but they couldn't get it to turn properly.

[Danger]

Interesting stuff!! I wonder if Ktm will adopt the Husaburg theories for their mainstream bikes???

Like this you mean?

[Reckless]

Where'd you get that Danger?
The 2010 models that where on here a short time ago don't show the Husaberg engine layout in the ktm? As attached. Other pics here (http://www.kiwibiker.co.nz/forums/showthread.php?t=101413) if you haven't spotted the thread.

PS Danger I noticed they changed the radiator caps to 1.8bar from 1.4 bar on all 2smokers for more temp stability. Might do that mod on mine as it hisses a bit on Wires and on the big dipper . But wanna see how puting the compression back to standard effects it first?