rodneyj123
New member
First, I apologies for the length.
I don't know how open minded you people are but I have a few things to say about engines and fuel economy. This is just my opinion.
In all of the following the reliability of the engines is keep constant. This means that if you take an engine with a 4" stroke and rev it to 6,000 rpms and then compare it with the same engine reving to 8,000 rpms it's an invalid comparison because the engine, with the same length stroke, reving higher is less reliable because the piston speed has increased causing a greater load on the connecting rod and crank. So an engine with a 4" stroke and a 6,000 rpm redline has the same reliability as an engine with a 2" stroke and a 12,000 rpm redline because their piston speeds are the same. I don't want to complicate things with connecting rod length to stroke ratio and other minor enginerring stuff. We'll just keep things simple for now.
There are two things that are important to me about engines: the amount of power and the power bandwidth (the low end power divided by the top end power).
The two components of the engine that affect these things are the piston area (not the piston displacement) determines the power. The piston stroke determines the power bandwidth (and also the rpm limit).
In order to increase the power of an engine the piston area must be increased. If you reduce the piston area you will reduce the power output of the engine.
In order to increase the power bandwidth (meaning the low end power) you must reduce the upper rpm limit which is done by increasing the piston stroke.
As a street rider I like big power bandwidths with lots of low end power. Most of my Japanese bikes never had this with their 10,000 to 16,000 rpm redlines. My Yamaha 920 cc V-Twin had lots of low end power and only a 7,000 rpm redline. My Harley Street Glide had lots of low end power with only a 5,500 rpm redline. Low end power is inversely proportional to redline. Higher the redline, lower the low end power. Lower the redline, higher the low end power. Since rpm limit is inversely proportional to stroke, low end power (or a bigger power bandwidth) is proportional to stroke. Long stroke, more low end power. Short stroke, less low end power.
If I were building a mildly tuned engine for street use I would use a 4" piston (about 50 hp per cylinder) to 4.5" piston (about 65 hp per cylinder). I would use a 4" stroke (6,000 rpm redline) to a 4.5" stroke (5,400 rpm redline). In a twin cylinder engine the 4" pistons would give about 100 hp and the 4.5" would give about 130 hp. Plenty for the street. With the 5,400 to 6,000 redline the engine would have lots of low end power.
If you take a 100 hp engine and increase or decrease it's stroke, your new engine will still produce 100 hp (beause the piston area hasn't changed) but it's redline will decrease or increase with the change in stroke. Therefor it's power bandwidth (or low end power) will increase or decrease depending on the new stroke.
If you take a 100 hp engine and increase or decrease it's bore, your new engine will have the same power bandwidth but will have more or less power depending on it's new bore. Since the power bandwidth is the same, increasing the power will also increase the low end power, decreasing the power would decrease the low end power.
Power bandwidth determines the spacing between gears in the transmission. With a wide power bandwidth the ratios in the transmission can be farther apart. This is good for the street since a very low first gear can be used giving good take off from a stop and a tall top gear can be used, giving a relaxing ride and better fuel economy. With a narrow power bandwidth the ratios in the transmission must be closer together so that when one up shifts one doesn't lower the rpms too much and fall out of the availably power bandwidth and loose acceleration. With narrow gear ratios and a top gear set for top speed, first gear will be very tall. This is bad on the street, since it ruins clutches and can cause stalling of the engine when taking off from a stop.
Fuel economy is determined by wind (and rolling) resistance and engine load. The higher the wind resistance, the lower the mpg. The higher the engine load, the higher the mpg. So, the Spyder RT has more wind resistance the the RS because it has a taller windshield. And the wind resistance is greater when the windshield is in it's highest position than it's lowest position (assuming the rider isn't a really tall giant). Also the RT rider sits up taller than the RS rider. The other factor, engine load, is determined by how tall the gearing is. You get better fuel economy in 5th gear than 3rd gear becasue the engine is under a higher load in 5th.
Unfortunately most motorcycle engines (except cruiser engines), like the Rotax, are built with motorcycle road racing in mind. This means more hp per cc (since the classes are cc dependent). Since hp is increased with bigger bore, the stroke has to be short to keep the cc's within the road racing rules. The short stroke means high rpms and poor low end power. No problem for Aprilia (or Honda, Yamaha, etc.) but a problem for the Spyder RTs.
Let me tell a story about the perfect car (I own one so I'm prejudice). I own a 2004 Chevrolet Corvette Z06 with a 400+ hp engine with a 6,500 rpm redline, a 6 speed transmission, and it gets 28 mpg. It goes from 0-60 in 4.0 seconds and turns the 1/4 mile in 12.7 @ 115 mph. The transmission is what makes it so good for street driving. First gear is very low and good for only 55 mph. From a dead stop I can let the clutch out and not touch the throttle and the car will simply drive off, never stalling. Fifth gear is good for 173 mph but if shifted into 6th gear it would slow down to 145 mph. It can't "pull that tall of a gear". However, at only 75 mph it "pulls" 6th gear just fine and gets 28 mpg because 6th gears is what we call an overdrive (a gear so tall that top speed can't be achieved in it). If you took 2 cylinders from my engine you'd have a 100+ hp v-twin and if hooked to a like transmission, with a low first gear and an overdrive 6th gear you'd have a very nice engine-transmission for a Spyder RT. Lots of low end power for pulling a trailer and plenty of speed when you want it plus good fuel economy.
Unfortunately we will never see this from motorcycle builders. So, until the engineers at the Chevrolet Corvette department start building 3-wheelers, we'll just have to live with what Can-Am gives us. Let's hope they wise up in the near future (but I honestly doubt it).
Have fun,
Rod
I don't know how open minded you people are but I have a few things to say about engines and fuel economy. This is just my opinion.
In all of the following the reliability of the engines is keep constant. This means that if you take an engine with a 4" stroke and rev it to 6,000 rpms and then compare it with the same engine reving to 8,000 rpms it's an invalid comparison because the engine, with the same length stroke, reving higher is less reliable because the piston speed has increased causing a greater load on the connecting rod and crank. So an engine with a 4" stroke and a 6,000 rpm redline has the same reliability as an engine with a 2" stroke and a 12,000 rpm redline because their piston speeds are the same. I don't want to complicate things with connecting rod length to stroke ratio and other minor enginerring stuff. We'll just keep things simple for now.
There are two things that are important to me about engines: the amount of power and the power bandwidth (the low end power divided by the top end power).
The two components of the engine that affect these things are the piston area (not the piston displacement) determines the power. The piston stroke determines the power bandwidth (and also the rpm limit).
In order to increase the power of an engine the piston area must be increased. If you reduce the piston area you will reduce the power output of the engine.
In order to increase the power bandwidth (meaning the low end power) you must reduce the upper rpm limit which is done by increasing the piston stroke.
As a street rider I like big power bandwidths with lots of low end power. Most of my Japanese bikes never had this with their 10,000 to 16,000 rpm redlines. My Yamaha 920 cc V-Twin had lots of low end power and only a 7,000 rpm redline. My Harley Street Glide had lots of low end power with only a 5,500 rpm redline. Low end power is inversely proportional to redline. Higher the redline, lower the low end power. Lower the redline, higher the low end power. Since rpm limit is inversely proportional to stroke, low end power (or a bigger power bandwidth) is proportional to stroke. Long stroke, more low end power. Short stroke, less low end power.
If I were building a mildly tuned engine for street use I would use a 4" piston (about 50 hp per cylinder) to 4.5" piston (about 65 hp per cylinder). I would use a 4" stroke (6,000 rpm redline) to a 4.5" stroke (5,400 rpm redline). In a twin cylinder engine the 4" pistons would give about 100 hp and the 4.5" would give about 130 hp. Plenty for the street. With the 5,400 to 6,000 redline the engine would have lots of low end power.
If you take a 100 hp engine and increase or decrease it's stroke, your new engine will still produce 100 hp (beause the piston area hasn't changed) but it's redline will decrease or increase with the change in stroke. Therefor it's power bandwidth (or low end power) will increase or decrease depending on the new stroke.
If you take a 100 hp engine and increase or decrease it's bore, your new engine will have the same power bandwidth but will have more or less power depending on it's new bore. Since the power bandwidth is the same, increasing the power will also increase the low end power, decreasing the power would decrease the low end power.
Power bandwidth determines the spacing between gears in the transmission. With a wide power bandwidth the ratios in the transmission can be farther apart. This is good for the street since a very low first gear can be used giving good take off from a stop and a tall top gear can be used, giving a relaxing ride and better fuel economy. With a narrow power bandwidth the ratios in the transmission must be closer together so that when one up shifts one doesn't lower the rpms too much and fall out of the availably power bandwidth and loose acceleration. With narrow gear ratios and a top gear set for top speed, first gear will be very tall. This is bad on the street, since it ruins clutches and can cause stalling of the engine when taking off from a stop.
Fuel economy is determined by wind (and rolling) resistance and engine load. The higher the wind resistance, the lower the mpg. The higher the engine load, the higher the mpg. So, the Spyder RT has more wind resistance the the RS because it has a taller windshield. And the wind resistance is greater when the windshield is in it's highest position than it's lowest position (assuming the rider isn't a really tall giant). Also the RT rider sits up taller than the RS rider. The other factor, engine load, is determined by how tall the gearing is. You get better fuel economy in 5th gear than 3rd gear becasue the engine is under a higher load in 5th.
Unfortunately most motorcycle engines (except cruiser engines), like the Rotax, are built with motorcycle road racing in mind. This means more hp per cc (since the classes are cc dependent). Since hp is increased with bigger bore, the stroke has to be short to keep the cc's within the road racing rules. The short stroke means high rpms and poor low end power. No problem for Aprilia (or Honda, Yamaha, etc.) but a problem for the Spyder RTs.
Let me tell a story about the perfect car (I own one so I'm prejudice). I own a 2004 Chevrolet Corvette Z06 with a 400+ hp engine with a 6,500 rpm redline, a 6 speed transmission, and it gets 28 mpg. It goes from 0-60 in 4.0 seconds and turns the 1/4 mile in 12.7 @ 115 mph. The transmission is what makes it so good for street driving. First gear is very low and good for only 55 mph. From a dead stop I can let the clutch out and not touch the throttle and the car will simply drive off, never stalling. Fifth gear is good for 173 mph but if shifted into 6th gear it would slow down to 145 mph. It can't "pull that tall of a gear". However, at only 75 mph it "pulls" 6th gear just fine and gets 28 mpg because 6th gears is what we call an overdrive (a gear so tall that top speed can't be achieved in it). If you took 2 cylinders from my engine you'd have a 100+ hp v-twin and if hooked to a like transmission, with a low first gear and an overdrive 6th gear you'd have a very nice engine-transmission for a Spyder RT. Lots of low end power for pulling a trailer and plenty of speed when you want it plus good fuel economy.
Unfortunately we will never see this from motorcycle builders. So, until the engineers at the Chevrolet Corvette department start building 3-wheelers, we'll just have to live with what Can-Am gives us. Let's hope they wise up in the near future (but I honestly doubt it).
Have fun,
Rod
hyea:
