Author Topic: The truth about crank and wheel HP  (Read 518 times)

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rep_movsd

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on: January 08, 2019, 04:40:45 am
This topic has bugged me for almost a decade now....
People in the industry and out of it have this notion of "X HP at the crank" and "Y HP at the wheel"

From a physics standpoint that is actually completely incorrect - Let me explain.

Lets say we have an engine in gear, clutch depressed at peak revs - why does the engine not continue to rev beyond the peak RPM?
Because the energy required by it to suck the air in and push the exhaust out has reached the amount of mechanical power that the combustion is producing.

Now what happens when you progressively add a load to the engine thats revving at peak?
Imagine you engage the transmission and use a CVT to engage more and more mechanical load on it - the engine slows down...
When the engine slows down from the peak RPM, it actually makes more usable power - because the several dozen HP required to make the engine rev to the last 200 RPM or so is now flowing out of the transmission.

Consider our very dear Iron Barrel Bullet 500 which makes a claimed 22 BHP at 5000 odd RPM - what it means is that at 5000 RPM, all the power in the combustion is just enough to keep the bike revved up there.
By definition, if there is a transmission load (including the friction of the gearbox, primary and secondary chain, tires, wind) it will never hit 5000 RPM

So the point is not that you have 22 BHP at the crank that is somehow getting reduced at the wheel - that could never be - the transmission is solid - there is no mechanical "slop" - its like a lever - if the engine is at a certain RPM, the wheel is at a certain RPM and the bike is at a certain speed.
What is actually happening is that the torque demand from the rest of the vehicle has slowed the engine RPM down to the point where it is able to maintain fixed revs but not rise higher.

This means that if your 22 HP @ 5000 RPM bike is only revving to 4200 RPM on the road, the actual power being delivered is what you read off the dyno chart for that RPM (say 16 HP)
The difference in power between the RPM you hit in top gear on the road and the the RPM you hit on an engine dyno doesn't actually mean anything - 6 HP of power is not being converted into heat by the transmission alone.

Rather, all 16 HP from the engine is being converted to aero drag and transmission heat (and gravitational energy if you ride uphill) leaving none for the engine to reach a higher RPM (and hence suck more fuel and make more power)

There is no separate "crank HP" and "wheel HP" - there is only "no load HP" and "on road HP" - You can always "boost" your so called "wheel HP" by making the bike more aerodynamic, and have less friction with a slicker tire, better chain or whatever.

I hope this made sense.


Stanley

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Reply #1 on: January 08, 2019, 10:50:08 am
Dyno measurements of engine power or wheel output power cannot be improved much by aerodynamic improvement because the bike is stationary. Perhaps a disk wheel would decrease turbulence to a tiny degree.
Neither measurement represents "on the road"
conditions. Imagine paying to dyno your own bike if it did!
And so it goes.
« Last Edit: January 08, 2019, 11:00:07 am by Stanley »


oTTo

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Reply #2 on: January 08, 2019, 11:37:07 am
One rather fundamental law of physics states that power = force x speed. Force would be the load, or when it comes to rotational load it would be the torque. Now what would the power (hp) be if the load is zero?


rep_movsd

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Reply #3 on: January 09, 2019, 05:46:33 am
One rather fundamental law of physics states that power = force x speed. Force would be the load, or when it comes to rotational load it would be the torque. Now what would the power (hp) be if the load is zero?

The load without the road is the pumping loss at the peak RPM which eats up every bit of power available and wont let the engine rev more.

The actual peak RPM reduces as we add loads - whether its the transmission, dyno drum, tire friction, or aero drag


Arizoni

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Reply #4 on: January 12, 2019, 04:44:36 pm

There is no separate "crank HP" and "wheel HP" - there is only "no load HP" and "on road HP" - You can always "boost" your so called "wheel HP" by making the bike more aerodynamic, and have less friction with a slicker tire, better chain or whatever.

I hope this made sense.
The Crankshaft horsepower is measured by applying a load directly at the crankshaft and measuring the amount of torque the engine produces with the throttle wide open.

As the crankshaft speed increases from a rather low rpm, the torque value will also continue to increase up to a point.  After this point of maximum torque is reached the torque value will begin to decrease as the engine speed increases.

Because horsepower is a torque X rpm value the horsepower will continue to climb as the speed increases above the maximum torque point but finally a point will be reached when the torque is decreasing faster than the rpm is increasing.  This point results in the maximum horsepower value.
At engine speeds faster than this value are reached the engine is still producing power but the value of the horsepower will continue to decrease.

Testing horsepower at the crankshaft is a very real and useful thing to do.  It gives the value of the maximum power the engine is capable of producing without the losses inherent in the chains and gears that are between the crankshaft and the rear wheel.

These gears and chains can eat up 20 percent of the engines horsepower in a poorly designed system.

Of course, as far as the rider is concerned, the horsepower at the rear wheel is the most important thing.
Jim
2011 G5 Deluxe
1999 Miata 10th Anniversary