# Drivetrain Power Loss

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#### Doran Jaffas

##### Well-Known Member
I think there was some discussion of it befoe
Thanks. I'll look it up so I don't waste any more space on this thread with it.

#### Vigilant1

##### Well-Known Member
On another note but still along the same lines here...has anyone converted a Honda Goldwing powerplant ?
Yes, it was done at least once, a long time ago. It was written up in Contact magazine. The guy used a VW Rabbit CV joint as the prop bearing, IIRC. All the Honda motorcycle flat 4s and 6s are pretty heavy for their output. Not a problem on a big heavy cruising bike, not so great for an airplane.

#### Doran Jaffas

##### Well-Known Member
Yes, it was done at least once, a long time ago. It was written up in Contact magazine. The guy used a VW Rabbit CV joint as the prop bearing, IIRC. All the Honda motorcycle flat 4s and 6s are pretty heavy for their output. Not a problem on a big heavy cruising bike, not so great for an airplane.
I've owned three gold wings and I was wondering about that. That heavy engine keeps their center of gravity low and makes him a good cruiser. but I never really thought it would make a good airplane engine. It would have to shed too much weight. Thanks for the replies.

#### jedi

##### Well-Known Member
On another note but still along the same lines here...has anyone converted a Honda Goldwing powerplant ?
More than one person has done it. One left the gear box on and just removed the unused parts retaining the PSRU ratio desired.

Post #42 - "All the Honda motorcycle flat 4s and 6s are pretty heavy for their output. Not a problem on a big heavy cruising bike, not so great for an airplane."

But not so bad for a PPC cruiser.

#### DangerZone

##### Well-Known Member
HBA Supporter
Lol the term Crotch Rocket was coined by “Cycle” magazine in a review of the XS1100.
Sorry but in 79 if you didn’t have a XS1100 you didn’t have a crotch rocket .

I enjoyed pointing out to my son-in-law when he bought a new crotch rocket a couple years ago that that granddad old machine of mine was the original.
Both he and my son who is a Harley guy made fun of that machine but I noticed it takes them about 15 to 20 miles to move it from one side of the driveway to the other must be more fun than you would think !
In the 1950s and 1960s the Birmingham Small Arms (BSA) company produced bikes with the name Rocket. Bikes like the BSA Super Rocket up to the BSA Rocket 3 were often called crotch rockets. Some called the old Vincent Black Lightning a crotch rocket, so it is a possible source of the BSA name trend. During the 1970 Isle of Man TT, a 750cc class bike was often called a crotch rocket. Not only BSA, but also Triumph, Norton, Ducatis and BMWs. In the early 1970s this name was often related to Rickman Engineering frames and swingarms with a Japanese engine inside.

The XS1100 is/was an ok bike, but it is unclear why would anyone consider this the original crotch rocket if many bikes bearing that nickname (like the BSA Road Rocket and the BSA Rocket 3) preceded it. Your XS1100 and my Z1000A3 are just plain and simple oldtimers nowadays, beautiful relics of the past.

#### DangerZone

##### Well-Known Member
HBA Supporter
This particular claim seems to be in direct conflict with known laws of thermodynamics. If shaft drives have “huge losses”, and these losses don“t turn to heat, exactly where does that energy go?? The 22hp difference is an enormous amount of energy, so exactly where does it go (because, as we know, it cannot disappear into thin air; it must be converted from the kinetic energy we have at the crankshaft to some other form).

Science isn’t really complicated at all. I’m not a physicist, but this much I remember from the 8th grade physics classes: energy cannot come from nothing, nor can it disappear into nothing; for a closed system, it remains constant (the law of conservation of energy). In our example, some of that energy is converted to sound (a few thousands of a horsepower), but where does the rest (almost 20kw) go? If it is heat, it is massive amount of heat, enough to get the entire motorcycle to red heat. Clearly, something is profoundly wrong with the idea that a simple shaft drive would lose 22hp.
See #30.

#### BJC

##### Well-Known Member
HBA Supporter
Work.

And other stuff, like I already wrote. It's simple physics.
Why is that different from a chain drive?

BJC

#### DangerZone

##### Well-Known Member
HBA Supporter
Why is that different from a chain drive?

BJC
Chain drives are much lighter and the transmission is linear.
Shaft drives are heavier and the transmission is angular at more places.
The shaft drive efficiency depends on the efficiency of bearings, gears, clutches, dampers and differential. The chain drive does not have those elements, has less mass (Newton) and has a simple motion.

If you want an analogy of mechanical work, think about this. Would your aircraft fly more efficiently with a 50 lbs propeller or a 10 lbs propeller, given the same shape? Or, would you be faster pedalling a 30 lbs bike or a 15 lbs bike? When people convert their shaft drive to a chain drive on a motorcycle this is like ditching the 50lbs prop and installing the 10 lbs prop, or changing the 30 lbs bike for a 15 lbs bike. There are less losses, and the efficiency is improved.

The motorcycle shaft drives are even more complicated than automobile shaft drives, and they go through more abuse concerning forces. Hence their factor of safety must be higher, meaning more mass. More mass means more weight, thicker walls, bearings, the whole nine yards. If you wanna see how much I appreciate what Billski and similar transmission engineers do, take a look at the simplest car drivetrain losses and imagine this three times more complex in motorcycle shaft drives.

Drivetrain losses (efficiency) – x-engineer.org

A drive chain? You have two paralel axles and a chain around them. It is so light and simple you can even go for trial and error, just change sprockets and adjust tension. After 500 hours ditch the chain and sprockets and buy new ones. Of course the losses will be lower, there will be less mass to move around and the engine will be able to rev higher due to less work needed for the same job.

Think about this for a second. Why do prop manufacturers always try to find lighter materials to put inside their blades? Why is less blades more efficient than more blades? Why do racing enthusiast try to change their drive shafts for lighter materials, switching from steel to carbon fibers? Or wheels, why does the same bike perform better with lighter wheels? Or airplanes, why is it more efficient to have a lighter airplane than a heavier one? The answer is work, you get more results for the same work (mechanical work).

These questions were rethoric, of course. I'm out of this, gone to the thread about folding wings.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
For an airplane prop shaft the losses could be as small as the bearings, I think.
Three bearings shown here, but only one additional bearing is needed for direct drive.

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##### Well-Known Member
Chain drives are much lighter and the transmission is linear.
Shaft drives are heavier and the transmission is angular at more places.
The shaft drive efficiency depends on the efficiency of bearings, gears, clutches, dampers and differential. The chain drive does not have those elements, has less mass (Newton) and has a simple motion.

If you want an analogy of mechanical work, think about this. Would your aircraft fly more efficiently with a 50 lbs propeller or a 10 lbs propeller, given the same shape? Or, would you be faster pedalling a 30 lbs bike or a 15 lbs bike? When people convert their shaft drive to a chain drive on a motorcycle this is like ditching the 50lbs prop and installing the 10 lbs prop, or changing the 30 lbs bike for a 15 lbs bike. There are less losses, and the efficiency is improved.

The motorcycle shaft drives are even more complicated than automobile shaft drives, and they go through more abuse concerning forces. Hence their factor of safety must be higher, meaning more mass. More mass means more weight, thicker walls, bearings, the whole nine yards. If you wanna see how much I appreciate what Billski and similar transmission engineers do, take a look at the simplest car drivetrain losses and imagine this three times more complex in motorcycle shaft drives.

Drivetrain losses (efficiency) – x-engineer.org

A drive chain? You have two paralel axles and a chain around them. It is so light and simple you can even go for trial and error, just change sprockets and adjust tension. After 500 hours ditch the chain and sprockets and buy new ones. Of course the losses will be lower, there will be less mass to move around and the engine will be able to rev higher due to less work needed for the same job.

Think about this for a second. Why do prop manufacturers always try to find lighter materials to put inside their blades? Why is less blades more efficient than more blades? Why do racing enthusiast try to change their drive shafts for lighter materials, switching from steel to carbon fibers? Or wheels, why does the same bike perform better with lighter wheels? Or airplanes, why is it more efficient to have a lighter airplane than a heavier one? The answer is work, you get more results for the same work (mechanical work).

These questions were rethoric, of course. I'm out of this, gone to the thread about folding wings.
Read a bit about physics, please, before posting the same stuff here again.
You are confusing weight / inertia effects with friction losses.

#### wrmiles

##### Member
Weight only really matters inasmuch as it effects such things as bearing loads and losses, or if you are accelerating the weight (which is exactly how an inertia dyno works).

#### patrickrio

##### Well-Known Member
More than one person has done it. One left the gear box on and just removed the unused parts retaining the PSRU ratio desired.

Post #42 - "All the Honda motorcycle flat 4s and 6s are pretty heavy for their output. Not a problem on a big heavy cruising bike, not so great for an airplane."

But not so bad for a PPC cruiser.
I know several motorcycle engines have been used before, but I have always thought it was strange nobody had tried to use strait six motorcycle engines. I have owned quite a few cars with strait sixes (own one now) and they are just so smooth and can really handle horsepower mods. smoothest engines you can buy really.

Motorcycle strait sixes that have been made are: Benelli Sei 750(747)cc & 900(906)cc carbureted and air cooled, Honda CBX 1047cc carbureted & air cooled, Kawasaki Z-KZ-ZN-ZG 1300(1286)cc carbureted & water cooled. Now BMW has the K1600(1649)cc fuel injected & water cooled.

You used to be able to find the CBX for pretty cheap but not really anymore. I am not in Europe but I suspect the Benelli's are not cheap now either. Probably hard to find cheap BMWs too since they are a new design and cost about $30K to drive out of the dealer. So maybe the Kawasaki??? #### DangerZone ##### Well-Known Member HBA Supporter Read a bit about physics, please, before posting the same stuff here again. You are confusing weight / inertia effects with friction losses. Nonsense. I specifically wrote in previous posts that all losses add up in the end, including heat losses and friction losses, among other losses. If you don't like my writing, fine. I provided a simple engineering link so everyone can see how drivetrain losses are claculated and add up. If you don't like that either, feel free to check the book Fahrzeuggetriebe - Automotive Transmissions, by Harald Naunheimer and others. This might provide basic information how losses are calculated in a PSRU or a shaft drive conversion for aircraft use, and anticipate losses. Or, you could disregard my writing and try to shoot me as the messanger, because nobody likes the bearer of bad news. You could then be counting that an engine with a shaft drive conversion might have only 1% to 2% losses, and then be unhappy when your system has up to 20% power losses. And wonder where did those ponies go away, how come they evaporated into thin air. It wouldn't be the first time people commit such a miscalculation, there are plenty examples in the homebuilt world. Just saying smart people calculate first, but do as you like. Last edited: #### DangerZone ##### Well-Known Member HBA Supporter I know several motorcycle engines have been used before, but I have always thought it was strange nobody had tried to use strait six motorcycle engines. I have owned quite a few cars with strait sixes (own one now) and they are just so smooth and can really handle horsepower mods. smoothest engines you can buy really. Motorcycle strait sixes that have been made are: Benelli Sei 750(747)cc & 900(906)cc carbureted and air cooled, Honda CBX 1047cc carbureted & air cooled, Kawasaki Z-KZ-ZN-ZG 1300(1286)cc carbureted & water cooled. Now BMW has the K1600(1649)cc fuel injected & water cooled. You used to be able to find the CBX for pretty cheap but not really anymore. I am not in Europe but I suspect the Benelli's are not cheap now either. Probably hard to find cheap BMWs too since they are a new design and cost about$30K to drive out of the dealer. So maybe the Kawasaki???
The Benelli is a rare find, and the owners sometimes ask a high price because Marc Bolan rode it in the 1970s. Parts are hard to find, engine is heavy. BMWs sometimes lack reliability (if we disregard the BMW marketing hype) and sometimes produce TV (torsional vibrations) which are quite annoying. The most common motorcycle conversions in EUrope are Suzuki based GSXR engines, Kawasaki ZX/ZZR series and the Yamaha turbocharged five valve case (from the old R1 to newer jetski engine versions). If I remember well, Steve Henry has a turbocharged five valve Yamaha in his SuperStol. All these engines are of for forced induction (turbocharging) and the guys from Finland even offer bolt on turbochargers for a relativvely fair amount of money at around $3k. These three engine cases still have a crankshaft and most internal parts with a safety factor 2.5 ever since the first engines appeared in the 1980s. This means they can withstand prolonged abuse at much higher power regimes they offer in their stock version. They often develop more than 250HP even though they are 100HP to 200HP engines stock. Hence it is fairly easy to find bolt on turbocharger system for the Suzukis and Kawasakis, but not for the Yamaha because of redundancy and reliability issues. You see, if a seller sells a turbocharger for$3k and the engine camshaft sensors go bust, the owner is gonna blame the turbocharger seller - not Yamaha. I've ridden Yamaha R1 and its derivates and this is a very common Yamaha design flaw, their original parts shuld often be wexchanged for higher quality if one wants a reliable ride.

The GSXR1100 engine cases (and whole bikes) in good condition are less and less on sales in EUrope. Many use them as conversions, for racing cars, caffe racers or homebuilt aircraft. The Kawasaki ZX/ZZR engines are still pretty easy to find, ranging from the ZZR1100 to ZZR1400 or 150HP to 200HP. If needing a lighter engine, the ZX-10R offers the same pony range but with less weight, at higher rpm. If needing more power or flying to higher altiutudes, bolt on turbochargers from Finland are easy to do and the Fins are often open minded and glad to help. Need a shaft driven bike, the GTR1400 (same engine case as the ZZR1400) is much heavier because it uses the engine case as a bike frame. I've even seen a guy use the GTR1400 engine which drove a chain to a three bearing shaft with no problems in a marine application.

#### Pilot-34

##### Well-Known Member
Chain drives are much lighter and the transmission is linear.
Shaft drives are heavier and the transmission is angular at more places.
The shaft drive efficiency depends on the efficiency of bearings, gears, clutches, dampers and differential. The chain drive does not have those elements, has less mass (Newton) and has a simple motion.

If you want an analogy of mechanical work, think about this. Would your aircraft fly more efficiently with a 50 lbs propeller or a 10 lbs propeller, given the same shape? Or, would you be faster pedalling a 30 lbs bike or a 15 lbs bike? When people convert their shaft drive to a chain drive on a motorcycle this is like ditching the 50lbs prop and installing the 10 lbs prop, or changing the 30 lbs bike for a 15 lbs bike. There are less losses, and the efficiency is improved.

The motorcycle shaft drives are even more complicated than automobile shaft drives, and they go through more abuse concerning forces. Hence their factor of safety must be higher, meaning more mass. More mass means more weight, thicker walls, bearings, the whole nine yards. If you wanna see how much I appreciate what Billski and similar transmission engineers do, take a look at the simplest car drivetrain losses and imagine this three times more complex in motorcycle shaft drives.

Drivetrain losses (efficiency) – x-engineer.org

A drive chain? You have two paralel axles and a chain around them. It is so light and simple you can even go for trial and error, just change sprockets and adjust tension. After 500 hours ditch the chain and sprockets and buy new ones. Of course the losses will be lower, there will be less mass to move around and the engine will be able to rev higher due to less work needed for the same job.

Think about this for a second. Why do prop manufacturers always try to find lighter materials to put inside their blades? Why is less blades more efficient than more blades? Why do racing enthusiast try to change their drive shafts for lighter materials, switching from steel to carbon fibers? Or wheels, why does the same bike perform better with lighter wheels? Or airplanes, why is it more efficient to have a lighter airplane than a heavier one? The answer is work, you get more results for the same work (mechanical work).

These questions were rethoric, of course. I'm out of this, gone to the thread about folding wings.
I think you are on the right track but your thought process is incomplete.
You are right adding 40 pounds to a airplane makes it less efficient but when I load it on a Cherokee 6 I can’t measure the difference in fuel consumption.
On the other hand if I wash it off a dirty Boeing 747 I can........but not because of the weight.
As pointed out above a shaft drive/ extension need only have one additional bearing and one moving part. Hardly likely to get us into the 1% loss range.
How important is that extra weight?
In a semi truck I have driven trips of over 2000 miles with a friend who drives like I do , each in identical trucks.we normally got the same mileage so when we got 10 identical loads we kept track of fuel usage.
With one truck weighing 35,000 pounds and one 130,000 we got within a single gallon of the same fuel use.
Here is the zinger the heavy truck always got BETTER!
In the end we think it was because the heavy truck compressed the springs about a inch and a half creating better driveline angularity and most importantly less air drag from less frontal area.
Moral of the story is sometimes tiny things you don’t see or think of can make a difference!

#### DangerZone

##### Well-Known Member
HBA Supporter
I think you are on the right track but your thought process is incomplete.
You are right adding 40 pounds to a airplane makes it less efficient but when I load it on a Cherokee 6 I can’t measure the difference in fuel consumption.
On the other hand if I wash it off a dirty Boeing 747 I can........but not because of the weight.
You don't have to measure an aircraft fuel consumption and efficiency, Boeing has already done that for you in the 1960s and 1970s. They understood that reducing the aircraft weight improves efficiency and reduces losses. This is not my thought process, this was done by clever aerospace engineers years ago to save fuel and money. They still do it today, calculate the losses to reduce them and improve efficiency.

As pointed out above a shaft drive/ extension need only have one additional bearing and one moving part. Hardly likely to get us into the 1% loss range.
How important is that extra weight?
Bike shaft drives and extensions have a lot of moving parts between the crankshaft and the end wheel. A lot of bearings, gears, differentials, clutches, and all these add up. Look at the linked web page, it is a very realistic approximation and allows school students to calculate the losses and efficiency. There are even specs for every single mechanical piece how much efficiency is lost on each depending on the type.

Every loss will also depend on speed, torque, temperature and a lot of other parameters. Short story: look at the calculating model. When you'll get a chance to disassemble your bike shaft drive for service or oil change, note all the parts inside. Write them down. Look for their specs and efficiency loss. Add it up and you got the difference between crankshaft power and rear wheel power. Then seek an XS1100 chain drive.

Check out our tough Yamaha XS1100 Chain Drive Conversion

You see, people raced with your bike more than 40 years ago, because they understood the information I wrote about here and the thought of engineers. Some ditched the shaft drive and switched to chain drives to reduce losses and improve efficiency. Extra weight was reduced, even a lighter swingarm could be swapped. This is what a Pitamn's racing XS1100 looked like after the shaft drive to chain conversion:

http://i578.photobucket.com/albums/ss224/aussixs11G/Pitman Yamaha 1981 chain drive race bike/20180322_163757.jpg

#### BJC

##### Well-Known Member
HBA Supporter
You are right adding 40 pounds to a airplane makes it less efficient but when I load it on a Cherokee 6 I can’t measure the difference in fuel consumption.
Adding 40 pounds that moves the CG closer to the neutral point can make an airplane faster at the same power setting.

BJC

#### BJC

##### Well-Known Member
HBA Supporter
Shaft drives on motorcycles have huge losses, and all do not turn to heat as you speculated.
Some Yamaha XS1100 owners made shaft to chain conversions to get more acceleration and power.
They get faster acceleration because the engine has less work to do to accelerate the entire system from one steady state condition to another steady state condition of higher speed. The same reason that race cars minimize flywheel weight; to minimize the power needed to increase the momentum of the drive system during acceleration, as well as the total mass of the vehicle.
Ever wondered why would people go through so much hassle to convert an XS1100 to a chain drive?
For faster acceleration. Not for a significant improvement in drive train efficiency.
My point: a lot of % of power loss - some convert to heat others are different (two figure % losses).
The predominant “energy loss” is not drive train efficiency. What you are ascribing to drive shaft inefficiency is the energy needed to increase the momentum of the drive system components under acceleration.
What you are clearly missing is that in shaft driven bikes, not all losses turn to heat. Besides friction losses, there are drivetrain losses, parasitic losses, slippage losses, idling losses and some others specific to this kind of system. The shaft driven bikes have more loss from work. This is elementary physics, energy-power-work. A system needs more enrgy and power to turn a heavier load due to Newton laws.
A system needs more power to more quickly accelerate a heavier system.
There is no sudden "power increase", it is simple physics, the bike accelerates better because there is less work needed to drive it so the engine runs into higher rpm just like when you accidently hit idle and the engine revs up easy like a rocket.
Yes, it is about acceleration, not steady state efficiency.
MY build will very likely result in a configuration that compared with a Lyc of equal performance will burn at least a 1/3rd less fuel.
That I want to see.
Why is that different from a chain drive?
Chain drives are much lighter
Shaft drives are heavier
Yup. See above.
Would your aircraft fly more efficiently with a 50 lbs propeller or a 10 lbs propeller, given the same shape?
Assuming constant vehicle weight, there would be no difference in efficiency. There would be slower throttle response though, assuming that the same propeller were used.
Think about this for a second. Why do prop manufacturers always try to find lighter materials to put inside their blades?
To reduce gyroscopic and inertial loads on the engine, and to minimize total aircraft weight. And, in some applications, such as extreme aerobatic applications, to improve throttle response, but, even then the primary drivers are to reduce loads on the engine due to maneuvering.

BJC

#### DangerZone

##### Well-Known Member
HBA Supporter
They get faster acceleration because the engine has less work to do to accelerate the entire system from one steady state condition to another steady state condition of higher speed. The same reason that race cars minimize flywheel weight; to minimize the power needed to increase the momentum of the drive system during acceleration, as well as the total mass of the vehicle.
For faster acceleration. Not for a significant improvement in drive train efficiency.

BJC
Why would you asume faster acceleration should exclude better efficiency when reducing drivertrain losses or mass?

Using a lighter prop has been done for years to improve efficiency, among other things, like acceleration and performance.

Check Hartzel for example. And this is a simple system with mostly only mass reduction, without shaft gears, clutches, etc.

4 Reasons to Switch to a Composite Aircraft Propeller - Hartzell Propeller

#### wsimpso1

##### Super Moderator
Staff member
Log Member
No one should view my not responding to the unsupported claims posted on this thread as either conceding defeat nor concluding the author of them is correct. No, just recognizing that "One is not required to attend every argument to which one invited."

First and foremost, I am a proponent of WEIGHT IS THE ENEMY in things aviation. So, whenever possible, we should select light weight systems. This means strategically selecting system designs or whole airplane designs that are lighter overall. To get the lightest schemes, sometimes you will be accepting additional parts or parts that are heavier than others, but that pay for themselves by making a lighter total system possible.

Second we found out about 90 years ago that all of the efficiency measures we are interested in on aviation engines (ratios of power/weight, power/volume, and power/fuel burn) are optimized by turning the engine substantially faster than the propellor. We also found out that this effect is so powerful that even substantial gearing, bearings, shafts, weight, complexity, and development costs still make power at lower weights, smaller packages, and less fuel burn than direct drive. So geared airplane engines are a classic case of optimizing the engine by adding stuff. Despite the weight and drag of the added components, you can get a better scheme.

This is still valid today. You can make a smaller package at lower weight and lower fuel burn rates by gearing the prop speed down while letting the engine rev up than by direct drive… That does not mean it is easily done, but it is possible. Virtually all of the engines of the now famous and infamous WWII tactical aircraft ran geared engines. Brownie points if you can name the few direct drive engines and the airplanes that flew with them in combat in WWII. No PT-, BT-, or L- ships, I am talking dropping ordnance and shooting up targets themselves types of airplanes.

Now back to the assertions that a couple simple bevel gear sets and a CV joint on either end of a shaft costs a bunch of power.

Ground rules are that it uses roller bearings on ground races. Gears are heat treated, ground involute teeth. Points to note are that involute gears roll on each other without sliding, as do rolling element bearings. Rolling coefficient of friction on such gears and bearings wet with an oil film tends to run around 0.002-0.005. Yes, ground steel surfaces rolling on each other are that low drag. That is how trains can be so fuel efficient – even rough steel wheels on rough steel track has really low rolling friction.

I just schemed up a bevel gear drive that might stand a 7500 rpm 100 hp engine for a long safe life and much higher loads for shorter times, sort like a motorcycle operation. 1-3/4” pitch diameter, ¾” shaft diameter, bearings 1” and 4” from the pitch line on the input shaft. Put that horsepower at that speed on the gears and you get 960 pounds at the pitch line, and lose 4.8 pounds to drag (assuming the high end of the rolling drag). That also puts 320 pounds on one bearing and 1280 on the other, giving a total on 1600 pounds load and 8 pounds drag:

(4.8 lb * 0.875 in + 8 lb * 0.375 in) * (1 ft/12 in) = 0.60 ft-lb of drag torque or 0.857 hp at 7500 rpm. Looks like that worst case friction gear set is below 1% friction losses when running 100 hp to me. Done correctly, it can be under 0.4%, and is regularly done in automotive powertrain hardware.

We get similar results in CV joints, parallel shaft straight cut gears, etc.

How does this differ from belts and chains? It is different, but not a huge amount. Belts tend to run a little more transmission loss (compared to gears) due to local slippage in the contact patches, while chains can be better or worse than gears, depending upon condition of the chain and sprockets, and they do get worse as they wear. Parallel shaft spur gearing is about the same as the bevel gears, but the bearing forces are lower, so they can be more efficient than overhung gears, like bevel sets. What else? You end up with about same number of bearings and similar losses in both. Yeah, the chain on bikes requires substantial bearings on the shaft too.

If I was designing a GPMoto bike, yeah, I would go with a chain drive too. It is is a little lighter and a little more efficient, but I can also change final drive ratio to suit slower or faster tracks more easily too. Go design a system for a traveling bike needing long reliable life, and it is oil wet gears and shaft all the way. Yeah, it adds a few pounds, maybe a tiny bit less efficient on energy, but I would not have to monitor chain and sprocket wear nor lube the chain every other day on a trip nor find myself needing a chain and both sprockets on a surprise basis along ways from help. But this forum is about airplanes not motorcycles...

So, do not be freaked out about a geared engine and shaft to get power to the prop. Yes, it might theoretically have been more efficient some other way, but you should judge the engine by how much power you get for the weight, package space, fuel use, dollars to buy, and field reliability already reported. And be extra careful about listening too closely to “internet experts”

Billski