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mic
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MarJay
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Knightsy
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UnspeedySam
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Noxious89123
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binge
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Noxious89123
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binge
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 Posted: 23:59 - 21 Oct 2012  Post subject: |
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I've been poking around at down-sizing for ages. But the R1 does everything I need it to, all of the time.
Where as an R6, will only do what I need it to, some of the time.
Plus, after having a good 3 year stretch of owning V-twins, oil boilers, and big single thumpers, I do feel guilty revving the fuck out of an IL4.  |
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pinkyfloyd
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Casper
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mic
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Teflon-Mike
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| Posted: 06:34 - 22 Oct 2012 Post subject: Re: 600 vs 1000 (older models) |
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| mic wrote: | Hi
Just surfing ebay having look as you do. Have noticed something like a mid 90s CBR600 is 100bhp and averages 50mpg yet a 1999 Firestorm VTR 1000 is only 97bhp and does 35mpg.
I wouldve thought a 1000cc bike would have more power than a 600cc? Both are sports bikes? |
yeah, its about the bigger picture and peak power vs 'useable' power.
Power is a bit of a strange and actually entirely 'imaginary' commodity, its a rate like the interest rate on your credit card; when you spend £50 and have to pay 30% you dont ever 'see' something called 30% you see five pound notes..... and 'max-power' or 'quoted' power, or 'rated' power, is like the limit on your credit card, if you have a £1500 limit, doesn't mean you always have £1500 to spend! Its just the maximum you can spend in one go, IF you haven't got any outstanding payements on it!
Base definition of Power is Rate of Energy Transfer.
Take that and some A-Level Newtonian physics and a bit of algebra on it, you can get to this formula:
Power = Cylinder Capacity x Cylinder Pressure x Engine revs
give of take a couple of constants of proportionality.....
Torque is an oft used and frequently even less understood concept; and to a greater or lesser degree an even more confounding imaginary irrelevence.
Strictly Torque - Force x Lever-Length.
And if we bung that into the power equation above:
Pressure is force x area, so if we take the cylinder pressure and multiply it by the piston area, we get the force on the piston, we then need a lever, which in this case is the crank-pin offset, or half the stroke..... and multiply the force by that we get torque, and we get the oft quoted equation: Power = Torque x Revs.
BUT more useful if we take the fact that piston area x stroke gives us capacity, and ignore that one.....
SO... Power is proportional to engine revs; means that for a fixed capacity, and fixed cylinder pressure, power will increase through the rev range.....
Cylinder capacity ENT going to change.... not on a conventional engine any way.... there are some funny engines, I think they are marine diesels that have funny variable length rockers between the piston & con-rod that can change the displacement while running, but we're talking motorbikes......
Cylinder Pressure, unfortunately WONT stay constant, and how it varies accross the rev-range is described by the 'torque curve' for the engine.... torque = cylinder pressure x Engine capacity, in effect, remember.
Peak torque denotes where maximum cylinder pressure is made, where you get the 'best' cylinder fill from the valve timing, and the best burn from it to get optimum pressure, and this can be quite low in the rev-range, especially on engines in lower states of tune, but even as torque is falling, power may still increase with revs, due to the multiplier.
BUT and this is the important bit; power & torque traces are showing the 'max' power at any engine speed, the throttle wide open, and as much charge sucked into the engine as the cyclinders can suck to give max cylinder pressure at those revs.
REAL WORLD we ride on 'part throttle' more often than we hold the thing against the stops.
With the throttle half open we are choking teh flow of charge into the motor, and limiting the cyclinder pressure the engine can make, hence the power.
SO, real world we RARELY will achieve 'max-power' from the engine, as working for a living, rather than running in a test lab on a brake force dyno, it wont be turning very often at max power Rpm, or at max power rpm with the throttle wide open.
Power in = Power out
Remember its a rate of energy transfer.... MPG is then actually an expression of 'power' in so far as its a measure of the rate you turn
the 'potential energy' contained as chemical potential in the fuel, into heat, and thence into motion in the engine....
THIS is why its very hard to get more power AND more mog at the same time.....
Power out.... where does it go? Well it goes to the rear wheel, and Keiths comment about 'thrust' is explained.
At the piston tops; turning the pressure created by burning fuel in air we get the useful formula
Power = Cylinder Capacity x Cylinder Pressure x Engine revs
But, once we have got that energy, on the crank-shaft, shifting it around the transmission....
Power = Torque x Revs
Often called 'shaft-power' is far more useful.
Crank-shaft is turning something between 1200 revs at tickover and I dont know, maybe 12,000 revs at the red line.
17" wheel with 18 section tyre on it, rolling diameter of what, 550mm? circumference is Pi x Diameter, lets call it 2m for easy numbers. One rev of the back wheel then covers 2m.
150mph/240Kmh is 240/60= 4Km per minute? 4000m, or 2000 wheel revs, so if we want to gear the bike for 150mph at the 12000rpm red line we want, a transmission that 'reduces' 12,000 revs a minute to 2,000 revs a minute, or 6:1 reduction.
Shaft power = Torque x Revs, so ignoring any losses, we reduce the revs by a factor of 6 we increase the torque by a factor of 6, and we get 6x the torque at the rear wheel...... torque is force x leverage, wheel is the lever, so divide delivered rear wheel torque by half teh wheel diameter...... 1/4 m or so, we get a pure force.
This is THRUST or motive force, and THIS is the actual 'cash value' of power, and the closest thing we get to something we can actually see or hold... though even then, still a bit ambigiouse!
That force can do one of two things; provide acceleration, Force = Mass x Acceleration, OR it can over come 'Drag', which is the resistance force to motion.
This is why even thrust is a bit ambigiouse, in terms of what we feel, because what we experience through seat of our pants is speed and acceleration.
But speed is effected by drag, and acceleration by mass, and through the transmission mass of rotary parts being accelerated, and 'lag' effects how rapidly the engine will respond to opening the throttle..... fit big carbs to a bike you can get more power, when they are wide open, but loose response, as they take longer to 'pick up' and start flowing extra air, so it 'feels' like you have less power, simply because the bike doesn't accelerate as hard..... or seem to due to lag.
Used to have a laugh with my trials bike in the field, telling lads it was 'restricted' and letting them have a go, but it had a second fly-wheel to make it more tractable at low speeds, so would wip that off making the engine about ten times more responsive and lads would truly believe that I had found twice the power! Nope, same power, just more response!
BUT back on track!
Energy = Rate of work done, work done = Force x Displacement.... So where its USED pushing a vehicle through air along a road:
Power = Drag x Speed
Remember Mpg is an expression of power; specifically 'power in', and something you can 'sort' of experience, like when you have to fill up and open your wallet to pay for the petrol.
Speed, acceleration, something you can similarly experience, seat of your pants burning that preciouse petrol.
Power, as its made at the pistion tops = cylinder capacity, x cylinder pressure x engine revs..... imaginary and pretty irrelevent
Other than to apreciate that engine size makes bog all odds, and rated power is just ONE index of how much power you might get from an engine so you have some idea what it might do.
Your litre-twin, making 'max-power' almost the same as a 600-four', SUGGEST and it IS only a suggestion, that while having the same MAXIMUM power capacity, bigger engine will be making as much or more power at lower engine speeds... might not, but likely to.... so on the road, where you rarely get to experience 'max' power, the bigger capacity engine is likely to offer more power in the low and mid-range... and be able to deliver a bigger thrust force to the back wheel more often... BUT even that is down to teh actual shape of the power curve and effected by gearing and reciprocating mass and everything else.
Fact that the fuel consumption figure is 30% lower? Hmmm......
Well Power in = Power out, give of take efficiency losses. Bikes are not much different in physical size, so 'drag' ought not be much different, and they aren't that much different in weight, so going the same speed, USING the same power, they ought to return similar Mpg.....
And I suspect that the figures are 'averaged', is they were 'best economy' figures, doing constant 55mph round a test oval, then I suspect the figures OUGHT to be pretty damn close, and the VT ought to have a slight advantage....
But as 'average' figures from real world riding..... fact that the liter twin is burning 30% more juice SUGGESTS that it's 'using' 30% more power.... efficiency losses wont account for THAT much, so reasonable to take that as a measure of how much extra 'performance' is being used, 'on average'.....
In short, bakes have similar statistics, in so far as the peak power and top speed, but that mpg figure, is NOT a measure of 'efficiency' but how much 'fun' they are, because mpg is money per mile, and fun costs!
Think about it, work ent fun which is why they pay you to turn up; if it was fun, like Alton Towers, there would be a turn-style & kiosknot a clock-in point and they'd be charging you!
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?' |
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lilredmachine
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| Posted: 06:41 - 22 Oct 2012 Post subject: |
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Eh?
Anyway, my street triple never managed over 32mpg on the island.
Mega MPG bikes rely on the ecu being able to lean the mixture out on constant throttle openings. If you don't have constant throttle openings (I.E you live on a small Island and ride like a nutcake everywhere) then it won't lean out, and you are left with shit all mpg. Moment I got on the motorway on it, it went up to 52 mpg.
Don't know where Marjay got his figures from, but my street triple redlined at 13600 rpm.
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MarJay
But it's British!
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G
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Bendy
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evoboy
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| Posted: 09:27 - 22 Oct 2012 Post subject: |
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V-twins just seem to make a big fuss about accelerating from low revs. They do need to be revved aswell to get them to 'go'.
My zx7 seems to be more grunty down the bottom of the rev range than a firestorm and it doesnt make a big deal about it.
Bike with the most low down go ive ridden is probably a new model 'Busa. Considering it can still do silly speed with tall gearing, you can still drive it in 'eco' mode and not see it struggling to pick up revs/speed.
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Suzuki GT250 x7------- Fazer 600------CB250RS------Aprilia Rally 70----- Bandit 600
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pinkyfloyd
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G
The Voice of Reason
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| Posted: 09:46 - 22 Oct 2012 Post subject: |
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The firestorm is more 'do it all' than 'sports' - the sportier twins are a bit more lacking in midrange and even jerkier low down.
And still (though I can no longer help you I'm afraid), you need to get a ride on a decent litre 4  . |
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Old Thread Alert!
The last post was made 14 years, 3 days ago. Instead of replying here, would creating a new thread be more useful? |
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