H.P.=speed

[Canadian Bird]

does more H.P. always mean faster? All thing being relatively equal. Is the new beamer faster than the Bird.

[The Krypt Keeper]

No and yes..

HP will move you to a point and once you reach this point aerodynamics take over and are in need to go faster..

Example

Busa is down on power to the ZX-12 but still faster as it was breed and designed in a wind tunnel to cut through the air...

[2equis]

does more H.P. always mean faster? All thing being relatively equal. Is the new beamer faster than the Bird.

All "sportbikes" are about as aerodynamic as a barn door. That being said, "faster" comes down to HP and gearing. From what I've seen on this board and with my own XX the average RWHP on an XX is about 138. The shaft HP on the Beemer is about 162. The 20% shaft drive penalty brings the Beemer down to about 130 RWHP. My guess is the XX has a slight edge over the Beemer in top speed.

FWIW

[shovelstrokeed]

I'd put the RWHP of the two bikes at about even. Those artlcles I have read by magazined doing dyno testing on the K12S ususally show about 140 HP, give or take a couple. No way you are going to incurr a 20% penalty in the final drive. Maybe a percentage point or two when compared to a chain, assuming the chain is in good shape and well lubed.

Top speed is so dependant on aerodynamics and, to a pretty good extent, how well the rider can tuck/blend in with the bike's bodywork that it would probably boil down to rider body shape and position. BMW is not exactly backward when it comes to wind tunnel testing and I would imagine the K bike is pretty slippery.

Given equal aero's, even a few mph on the big end would require a pretty good HP advantage.

[2equis]

No way you are going to incurr a 20% penalty in the final drive.

Everything I've read always gave a chain final drive a 10% loss between crankshaft and RW hp and shafties a 20% loss.

[shovelstrokeed]

I'm away from my normal resources but basically, every 90 degree turn in the drive train will incurr about a 5% penalty in drive efficiency due to friction losses in the gears. The BMW design involves two 90 degree turns as two sets of bevel gears are used, one at the output of the gearbox and the second at the final drive on the wheel. So, you incurr a 10% power loss to the rear wheel. It ain't all for free though as a chain drive is only about 97% efficient as well so the difference between the two drive methods works out to about 7%. Those are some pretty rough numbers and the smaller bevel gears up in the front might add a bit more friction.

Bacl om tje early days of shaft drives, a lot of the Japanese mfgs used an intermediate gear box between the engine and drive shaft and used helical gears in there which are a good deal less efficient, although quieter, than the spiral bevel gears used by BMW. That may be where your 20% loss figure comes from.

All the above applies to motors with their crankshaft disposed laterally across the frame. On stuff like Goldwings, ST's and BMW boxer or brick motors, the crankshaft is longitudinal to the frame and only requires a single 90 degree turn and efficiences can approach those of a chain drive, won't get there though as the required universal joints necessary due to suspension movement eat up a bit of power as well.

[2equis]

I'm away from my normal resources but basically, every 90 degree turn in the drive train will incurr about a 5% penalty in drive efficiency due to friction losses in the gears. The BMW design involves two 90 degree turns as two sets of bevel gears are used, one at the output of the gearbox and the second at the final drive on the wheel. So, you incurr a 10% power loss to the rear wheel. It ain't all for free though as a chain drive is only about 97% efficient as well so the difference between the two drive methods works out to about 7%. Those are some pretty rough numbers and the smaller bevel gears up in the front might add a bit more friction.

Bacl om tje early days of shaft drives, a lot of the Japanese mfgs used an intermediate gear box between the engine and drive shaft and used helical gears in there which are a good deal less efficient, although quieter, than the spiral bevel gears used by BMW. That may be where your 20% loss figure comes from.

Is there a value for rear wheel friction or brake drag etc.?

[kf4mtw]

Some people place too much value on a dyno result. What one machine says about a bike is not rellevant to what another machine says about a different bike across the country. Does someone think all dyno's say the same?................ The one I used said my 1st XX with stock pipes had 130 hp, the lowest rating I have ever seen, the next week with the exhaust system on the same dyno it jumped to 142, showed jump in torque and excellent fuel burn. One dyno, one bike, a rellevant exchange of information for my modification. ...............Same dyno shows other bikes in my area have 20+more hp than mine but they can't drive away from me as you would expect them to? :icon_hand:

[tkyler]

does more H.P. always mean faster?

More horsepower allows more energy to work with...what engineers do with that power is what keeps them busy year after year. All other things being equal, more power will accelerate you faster, and it will move you to a higher top speed.....all other things being equal.

[kf4mtw]

What is a Horsepower?... :icon_think: My XX has 142............does that mean I can hook up to 141 horses, have a tug-a-war and be able to drag all them hooves backwards?

[2equis]

What is a Horsepower?...

The term "horsepower" was invented by James Watt to help market his improved steam engine. He had previously agreed to take royalties of one third of the savings in coal from the older Newcomen steam engines[5]. This royalty scheme did not work with customers who did not have existing steam engines but used horses instead. Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute). The wheel was 12 feet in radius, thus in a minute the horse travelled 2.4 × 2π × 12 feet. Watt judged that the horse could pull with a force of 180 pounds (just assuming that the measurements of mass were equivalent to measurements of force in pounds-force, which were not well-defined units at the time). So:

This was rounded to an even 33,000 ft·lbf/min.

Others recount that Watt determined that a pony could lift an average 220 pounds 100 feet (30 m) per minute over a four-hour working shift. Watt then judged a horse was 50% more powerful than a pony and thus arrived at the 33,000 ft·lbf/min figure.

Engineering in History recounts that John Smeaton initially estimated that a horse could produce 22,916 foot-pounds per minute. John Desaguliers increased that to 27,500 foot-pounds per minute. "Watt found by experiment in 1782 that a 'brewery horse' was able to produce 32,400 foot-pounds per minute". James Watt and Matthew Boulton standardized that figure at 33,000 the next year.

Put into perspective, a healthy human can produce about 1.2hp briefly and sustain about 0.1hp indefinitely, and trained athletes can manage up to about 0.3 horsepower for a period of several hours.

Most observers familiar with horses and their capabilities estimate that Watt was either a bit optimistic or intended to underpromise and overdeliver; few horses can maintain that effort for long. Regardless, comparison to a horse proved to be an enduring marketing tool.

[beondwacko]

Let us not forget that also 746 Watts = 1HP. Of course this is in a perfect world with no parasitic losses.