How much bhp waste

[sidewinder]

Been thinking to my self work and got wandering.say you have a 100 bhp bike. .how much of that is wasted lugging yours and the bikes weight around and transmission losses?how much useable power would you have left
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"Nitrous is like a cheap hooker, you want to hit it but are scared of the consequences

[Frost]

Weight doesn't reduce power. Obviously it reduces the effectiveness of that power by giving a crap power to weight ratio.

Chain & gearbox tends to take up ~10% of power but obviously that depends on the condition of various parts.

The biggest power thief is the valve springs. The more high performance your engine the faster and more precise the valve timing needs to be. So stiffer springs and steeper profile cams are used which take a fair bit of energy to turn the cam shaft. More pistons and valves wakes the problem worse. It's why motoGP bikes use air actuated valves and ducati don't use valve springs at all.

[Kickstart]

Hi

Depends on the speed.

If you convert it to pounds of thrust (which is basically torque x gearing x a constant), then a rough rule of thumb is that overcoming rolling resistance takes about 1% of the bikes weight as pounds of thrust. So a 400lb bike needs about 4 pounds of thrust to overcome the rolling resistance, compared to something around 50 pounds of thrust to overcome aerodynamic drag at 70mph.

So your weight is of pretty minimal importance when cruising.

But any thrust left after over coming wind resistance and rolling resistance will accelerate the bike, and how quickly it accelerates is down to how much thrust remains and the weight.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[haroman666]

I dunno how to work it all out, and it's probably farrrrr too long to be worth working out unless you're a race team.
But there are frictional losses in any interfacing components such as the cams/valves, bearings, cylinder walls/pistons.
Then there's pumping losses which is basically the energy required to draw the a/f mix from the inlet manifold, and then the energy required to push it out the exhaust.
Fluid losses like pumping oil, coolant and the like.
Lubrication losses.
Losses caused by not having a crank-case breather (Pressure underneath the pistons)
Internal aero-dynamic losses (Yep, some race engines have profiled con-rods to reduces parasitic aerodynamic drag)

And that's just the engine.

Then there's your transmission losses.
Electrical charging losses.
External aero/drag losses.

Etc etc etc.

Not much really
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The mistresses: 2000 Honda CBR600f, 2000 Honda VTR1000

[CaNsA]

/me waits for teffers...

[Teflon-Mike]

Power is a rate, not a pysical commodity. And the power rating is the maximum power the bikes engine may make, at full throttle at peak power rpm... wont make 100bhp anywhere else, and even then has to be at full throttle.

Scientifically, Power = Rate of work Done, or Power = Force x Speed.

Driving you along, engine makes whatever power is required to over come 'drag' at that speed.

So, whatever speed you are going.... there's NO power left over.... if there was you'd be accelerating, becouse it would be making more force than there is drag, and Force = Mass x Acceleration, right?

Question, then, is one of efficiency, and how efficient is a motorcycle drive line?

Very tricky question, and it starts with the problem that efficiency is the % of useful work out, ad a proportion of what you bung in to get it.

OK.. so we bung in petrol.... and thats as good a starting point as any.... that suggests that MPG is a pretty good guide to efficiency.

But, being a bit more scientific, you need to know how much power you have to begin with, or how much energy you are using.

Petrol, then is energy; or at least something that contains 'potential' energy, in so much as if we set fire to the stuff, we can get energy out.

In the lab, you can measure this, in a device called a calorimiter; and get a calorific value for your fuel.

Trouble with this is that a calorimitor burns the fuel in almost 'perfect' conditions, usually in pure oxygen, to get 'complete combustion' and measures the heat energy it releases....

In an engine, burning the stuff in air, not oxygen, under elevated pressure, and rather more quickly than in a calorimeter, you don't get complete combustion... and you dont get all the heat energy you might out the fuel.

So, if you measure the net work out of an engine against the theoretical calorific value of the fuel.... the efficiency is horendousely low.

Mostly though becouse in an engine, the heat energy you get out is at incredibly high temperatire, and delivered very very quickly.

Heat travels at a speed proportional to temperature difference.... so if you have an engine, stabilised at about 90 Deg C, and fuel burning in the pot at perhaps 2000 degrees or more... its going to try and escape rather a lot faster than having a gas fire in your living room burning at perhaps 250 Deg C in an ambient temperature of perhaps 20 Deg C.

So you're engine doesn't get the full calorific value of the fuel as heat, and nor does it capture very much of that heat....

I have a diagram somewhere that reckons that shows something like 40% of the heat released during combustion is dumped straight into the cooling system, another 35% or so straight down the exhaust pipe, and about 10-15% or so lost through conduction through the rest of the engine....

At best, you get something like 10-15% of the heat energy, itself perhaps only 60-70% of the fuels calorific value captured at the piston tops and turned into motion.

OK.... this gets us close to your answer.

Dyno-Testing. There's two types of dyno; Brake Dyno's and inertia dyno's Then there's two test rig arrangements; the engine bed and the rolling road.

The inertial dyno is more often couples to a rolling road to test power at the wheels of a built up vehicle. The brake dyno is more often used on a test-bed to measure an engine on its own, in the lab.

Inertial dyno's measure power by how fast it accelerates a big fly wheel. This means its quite good for doing dynamic comparisons, becouse the engine is going through the rev range as its being measured. Its not very accurate though and any lag effects can effect the readings, but it is good for showing 'driveability' charecteristics, like throttle response or flat spots in carburation, and giving comparitive power readings.

Engine Brakes tend to measure power straight off the crank shaft, held at constant rpm against a brake... far more accurate, and not subject to lag effects, but limited for dynamic situations.

OK.... you measure an engine by running it up on a dyno and you get a power figure.

Power = Force x Speed.

You measure it at the rear wheel of a built up bike, that's the 'delivered' power..... you can then compare that to your input energy and come up with an overall efficiency rating.

But that is not really what you want to know... you want to know how much power is lost in the drive line and transmission....

So, chuck the engine on an engine bed, and find out how much power you are getting at the crank-shaft.... difference between the two is your answer.

But.... between piston tops and crank-shaft SOME power is being used to drive valves, generate sparks, charge the battery, that kind of thing...... how much is that?

Well, you can do a thing called back motoring, and drive the engine against a dead spark... like pressing teh starter motor with the kill switch off, and measure how much power it takes to drive the engine to the same RPM.... there's your answer...

And the difference is often the disparity between different people's quoted power figures for the same bike... becouse they use different measurement standards that provide for different back-motoring compensation.

Real world?

100bhp car; transmision depending on type, can sap as little as just 5-10bhp. They tend to be simpler, have a dry clutch, bigger splash lubricated bearings, and top gear can often be a lock-out shaft straight through the gearbox, giving almost no losses through unnecessary gears.

Bikes tend not to be as efficient, I've heard numbers up as high as 25bhp for a big bike box, with wet clutch, but obviousely bike like a CG125 with only 10bhp to start with cant have losses that high! So does depend on the bike..... 10-15% though would be a reasonable ball-park.

Weight? Does not really effect much. Rolling resistance of the tyres is probably more significant, especially on lower powered machines.

So.... typical 600cc middleweight 100bhp; power will be anything from bugger all at tick-over to the 100bhp max; of whats cought at the piston tops you probably have about 0.5-1% load overhead driving cams and stuff keeping the engine working; you then probably have up to 3% variable load applied by the electrical system; then 10-15% loss through the transmission, to get something between 85 & 90bhp at the back wheel... of which you may loose up to another 5% depending on tyre pressure rolling resistance.

Whats left over over comes drag, and thats the useful work you want out the engine.... 'lugging you about', and what you got left is however much twist grip travel you ent used!
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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?'

[CaNsA]

And there you have it.

[Sload]

Jesus wept Tef, I understand that but got tl:dr. Good effort though

Condensed version, about 10-15% bhp to whp.
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Honda Varadero >> Triumph Speed Four >> Honda CBR1100xx

[radicalrabit]

brilliant explanation Thanks.... the thing is what can be done to drag all them horses back into a bike thats old.? my engine has done 30,000 miles over 36 years big V4 998 so not exactly been thrashed was 116 bhp originally so what Can I do to get the power to beras close to its peak as possible...
Piston rings I wouldnt have thought would be all that worn , the gears box feels a bit sloppy so work can be done there, JETS..... new diaphragms etc ? One bike I am selling but my original VF1000 I would like to put back into really good condition and any help from all sources gratefully received
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VF1000 F2F, BLACKBIRD with carbs, 2nd VF1000F2F just need somewhere to go now..

[sidewinder]

Cheers for the replays all.i think I understand it all kind of
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"Nitrous is like a cheap hooker, you want to hit it but are scared of the consequences

[c_dug]

In theory if you restored everything back to showroom condition you would be getting showroom power out of the engine again. You mentioned the gearbox being a bit sloppy, carbs, rings etc...

Time and miles have all kinds of effects. Each worn thing will take more from what you had, personally I would start with a good service and go from there.
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I am a bellend, I am a man of constant sorrow, I am a gummy bear, I am a rock.

[Teflon-Mike]

[Walloper]