What Determines "Speed"

[Francis]

Hey, guys. I'm new to mechanics and such so obviously not an engineer. Anyways, what factors determine a motorcycle's speed (acceleration).

I've been reading some posts and articles and I've noticed some people don't recommend the CBR 600 as a "first bike" but recommend the SV650 even though it has a little more cc.

I doubt the weight differences of the bike matter too much. Am I wrong?

Also, any good books out there that I can read that can help me understand my 250r more (and motorcycles in general). Example questions:

- what do carburetors do
- compression ratio
- difference of four stroke and two stroke
- would I benefit with more tooth in sprocket or not

I've done some Googling but it would be nice to have a book. The things I've Googled still puzzles the crap outta me.

Thanks o.O

[dyeforever]

the cbr 600 is a inline 4 while the sv650 is a 650 v twin. much difference in power and delivery.

[Francis]

I'm still lost there, mate.

That's like telling a 5 year old 9 - x = 25

Even if he knew how to add and subtract, he wouldn't know what to do. haha.

[setasai]

If your question is why the two bikes even though is 600cc+ but one is ok for beginners and the other is not recommended is like dyeforever said. The engines are made different and results in the delivery of power differently. It has to do with the sensitivity of the throttle. In a CBR600 a slight twitch of the throttle accelerates you very quickly because of the nature of the engine. In a SV650 the engine is different and so the power delivery is more gradual and thus easier for a beginner to control.

Both bikes have much more power than our ninjette obviously but the delivery is different. That's not to say a beginner cant start on a CBR. There are lots of people that do but it might just be more difficult.

Hope I explained it right and I hope it helps. Please correct me if i'm wrong. I'm still a beginner as well.

http://www.bestbeginnermotorcycles.c...r-bike-updated

See if this helps.

[Francis]

I get that, guys. But I was hoping someone could recommend me a book that could help me figure out and understand what's difference about them.

Like I said, most things online that I've read is still pretty hard to understand. I hope it's because I have no mechanical background and not because I'm dumb hahaha.

I was Googling the difference of 2 stroke/4 stroke but I still couldn't really understand what it was talking about lol

[Cuongism]

SV650 has a V twin engine (2 cylinders at an angle) whereas a CBR600 has a inline 4 engine (4 cylinders in straight line). The difference is the I4 creates a lot more power; over 100hp vs SV's 75ish hp. 250's have about 25-30hp.

So the 650 may have more engine size, but it creates less hp so a little more beginner friendly. The other "friendliness" of the SV is that it's torque is fairly flat across the rpm range. It's powerband is around 7k whereas the CBR's powerband is around 11k and it really kicks in.

Do you want to know how engines work or just the differences in them? Two stroke engines don't really exist on the street anymore either, but I can break down the differences if you want.

[sombo]

Some advice. There are some things you are asking that quite honestly are not really needed to know for basic motorcycling and basic mechanics. Some of your questions are a little closer to engineer level which means it would be harder for someone here to explain in a way that would be understandable.

Now as far as a good book for all your questions, I can't help you. There are books on engine design, then there are books on motorcycle riding (every will be chiming in to recommend Twist of the Wrist one and two sooner or later), but not really one for everything.

Now to try and help with some of your questions.

What determines speed (acceleration):

-- well that comes from the amount of power the engine of the bike puts out which is measured in horsepower and torque. The higher both of those ratings are the more the engine is capable of doing and the faster you will go.
-- weight also comes into play in that the lighter a bike and rider are the less the engine has push to achieve the same goal.
-- wind resistance also plays a major roll in that the more streamlined you and your bike are the less wind resistance you have.

What do carburetors do:

-- they combine the fuel and the air to deliver the mixture to your engine. This controls how much fuel/air gets inside and how well it's mixed. Too much of one or the other and you won't have as good of a reaction as is possible.

2-stroke vs 4-stroke:

-- in a 4-stroke motor there are 4 strokes (up and down movement of the piston) during a cycle of the motor. Each stroke has a different purpose, intake (bringing in the fuel/air mixture), compression (piston moves to top of cylinder to compress the mixture), power (spark plug fires igniting the mixture causing it to expand and push the piston back down), exhaust (piston moves up to push the spent mixture out). This all takes 2 revolutions of the crankshaft.
-- in a 2-stroke motor there are only 2 strokes per cycle. Here the intake and exhaust are done at the same time. As the motor takes in fuel/air it is expelling it at the same time. This can produce more power then a 4-stroke but at the cost of efficiency in a clean burn. This motor is no longer allowed for street use because it has a much higher pollution rating then 4-strokes.

As for the sprocket issue that is discussed in other threads on these forums with plenty of useful information. I hope I helped answer some of your questions.

[Apex]

Originally Posted by Francis

I get that, guys. But I was hoping someone could recommend me a book that could help me figure out and understand what's difference about them.

Like I said, most things online that I've read is still pretty hard to understand. I hope it's because I have no mechanical background and not because I'm dumb hahaha.

I was Googling the difference of 2 stroke/4 stroke but I still couldn't really understand what it was talking about lol

Link to original page on YouTube.


V-twin engine with the same amount of cc's means there is a longer stroke and larger bore. This means the piston has to move up and down farther in the cylinder (stroke) and it is in a bigger cylinder (bore). Now the physics of the inline 4, due to smaller pistons since the bore is smaller, and a shorter stroke, the inline 4 can spin up faster. In many cases there is a power stroke every revolution. This makes a smooth delivery, that has less pulses and typically has very smooth acceleration. A v-twin, due to the increase in motor mass (piston size and stroke length), tends to pulse more. The motor has more of a low frequency vibe instead of the I-4's high frequency. The lower HP makes accelerating out of a corner easier than an I4.

The longer stroke produces more torque and less HP. The I-4 tends to produce more HP than torque.

I can explain more later when I feel like talking (ie-typing) more.

Best way to learn more....go get a book. I read a book on engine design when I was in junior high....so I just "know" a lot of these things, but I can't regurgitate it on demand easily. hahaha
My recommendation, make a trip to the local library and check out what books they have there. Also Amazon/Barnes&Noble has a nice selection. If you really want to learn it, take the time to read about it from a good book.

[Flashmonkey]

Wow...this thread got insanely complicated haha.

This is a good link for general engine and engine related information:
http://auto.howstuffworks.com/car-en...on-channel.htm

Your typical modern supersports (ie: CBR600, R6, zx6r, etc) are pushing something like 120 hp these days. Your typical SV650 and similar sport oriented 'standard' bikes are pushing, what, 80 hp? Where on the powerband that is being applied at is neither here nor there, but the fact remains that the middle weight supersports (600cc ish range) produce a crap load of power given their scant curb weight. Then there's the whole aggressive steering geometry and touchy brakes thing that I won't get into right now.

But there you have it. One class of bikes was made for outright speed. The other was made with overall liveability in mind. And fun. Sometimes fun is engineered in.

[wyckedflesh]

If you had 2 arms with a bat in each hand, and you could swing each bat 2 times per second you could hurt someone pretty well.

If you had 4 arms with a bat in each hand, and could swing each bat 2 times per second, you would REALLY hurt the hell out of someone pretty well.

That's the difference between the SV (two arms) vs the CBR (4 arms)

Now the SV has big beefy arms, so it can swing alot harder, so each hit is intense. The CBR has smaller arms, but with the two extra arms, it just pummels you more. Like watching a Flyweight fight vs a Heavy Weight fight. The flyweights throw 25 punches for every 5 punches in the heavy weights.

Make sense? Clear as Mud? Hope so!

[Apex]

Ninja 250 RULES!!!

That is all.

[scotty]

Check out, Howstuffworks.com
They have vids that will show you how stuff works and what not. They may not have all what your looking for though.

[Sailariel]

It really comes down to the differences--some are subtile.

I ask people what they would prefer to drive on a 900mi trip? Cadillac or a Miata. Both are "comfortable", but one is more comfortable than another.

In bikes, the same applies. We are looking at how the bike is used.

I had a 2008 Ninja 250R--an absolutely superb machine. I was also comitted to a 500mi one way trip a month. The Ninja did the job OK--but I was not exactly rested when I got there.

The Ninja is a sport bike which has great ergonomics. It can be ridden farther than most sport bikes. It can be used as a sort of sport touring bike if fitted with luggage.

I bought a Honda NT-700V It is a sport touring bike. It is heavy and comfortable. It is not as "Twitchey" as the Ninja--in a sport bike you want twitchey, flickability. and the ability to lean extreme angles. Even though my new bike is a sport touring machine with a 700cc engine, on a curved course the Ninja would blow me away--ditto at a traffic light.

So what is the difference? The 700 can go all day at 80mph in total comfort. The Ninja can also do that but the rider would not be as comfortable.

I hope this sheds some light on motorcycle types.

[FrugalNinja250]

Yeah, it's an interesting subject. Speed and acceleration are two different things, for one. Top speed is usually limited by power and wind resistance. The power needed to overcome aerodynamic drag increases as the cube of the velocity. Doubling the speed takes four times the power.

Power is a function of torque and RPM. You can't have one without the other except at zero RPM. The horsepower formula is torque (in foot-pounds) X RPM then divided by 5252. So a motor running at full output producing 50 ft-lbs of torque at 11,000 RPM would be producing 104.72 horsepower. Note: Power is not something that's always there and you're only using some of it. Power is the measure of the actual work being done at any given moment. For instance, that same motor at 7,000 RPM and only putting out 15 ft-lbs of torque in order to overcome air and mechanical friction just enough to maintain a constant road speed of, say, 75 mph, will only be putting out 19.99 horsepower. At that moment, that's all the power that motor will be generating.

Torque is the turning force of something like a crank or a bolt. If you take a 12" long wrench and put it on a bolt then apply 50 pounds of force at the other end, the torque being applied to that bolt will be 50 ft-lbs. The power you're applying will be zero as long as the bolt does not turn. Once the bolt starts turning then you're generating power, and you're still exerting torque. If your wrench is only 6" long then the torque for the same 50 lbs of force applied will be 25 ft-lbs. (50 lbs X .5 feet).

In an engine, torque is produced when air (primarily nitrogen) is heated in the cylinder by burning fuel with the oxygen in that air, causing it to expand and push on a piston. The piston pushes on a rod that is connected to a crank at a point offset from the rotational centerline of the crank. If the distance from the crank center to the end of the rod is 2", and the piston is being pushed down with a force of 100 pounds, then the torque on the crank will be 16.67 ft-lbs. The amount of torque varies with the distance from the rod to the crank center, the angle between the crank and the rod, the piston area, and many, many other variables.

Piston area is important because the more area there is the greater the push on the rod for a given cylinder pressure. For instance, a 2" piston with 150psi of pressure on it will produce 471.24 lbs of force on the connecting rod. A 3" piston will produce 1,060.29 lbs of force.

Stroke is the total distance the piston moves in the cylinder bore, and it is twice the distance from the crank center to the rod end. To get longer stroke you lengthen that crank/rod distance.

Cylinder displacement is the area of the piston X the stroke. A 2" piston and 2" stroke would produce 6.2832 cubic inches of displacement. To keep the displacement the same you would need to decrease stroke when you increase piston area, and vice-versa. Random factoid: A motor whose stroke is less than its piston diameter is considered to be an "oversquare" motor. Oversquare designs rev higher, losing torque at the low end but gaining power at the top end.

The number of cylinders is important because generally the more cylinders you have for a given engine size the less mass you have flying around in the engine such as rods and pistons so therefor the faster you can spin the engine. Your stroke is generally shorter which also increases rev potential. Remember, increasing RPMs increases power

The geometry of an engine is a fairly complex subject involving lots of trade-offs. For instance, you can increase the stroke by making the crank to rod distance longer, but when you do that the piston slides up and down much faster for a given RPM. This creates problem with piston ring friction and wear, and structural problems because when the piston and rod assembly comes to a complete stop at each end of its travel substantial forces want to break and tear metal. The result is that generally when you increase stroke you increase torque but reduce maximum RPM capability, which decreases overall horsepower because remember power is a function of torque and RPM.

How does all of this apply to the differences between the two bikes you mention? The 600 has a four cylinder high-revving motor with short stroke geometry, with intake and exhaust passages optimized for flow at high RPMs. This motor's power curve is such that you don't generate much torque in the lower third to half of the RPM band. When the engine gets to the RPM range where power starts to build it builds very, very fast, almost like a light switch. It takes a fair amount of riding experience and good reflexes to use all of the power of this bike safely. Loss of throttle control is a high risk on this for inexperienced riders or riders not used to this kind of power delivery.

On the other hand the 650 has only two cylinders but each cylinder has more area. The engine has a longer stroke which limits RPMs, but the intake/exhaust passages and cams are designed to work best at lower RPMs. Ultimate power is limited by the lower RPMs but the low to midrange power is much higher than in the same RPM range of the 600 due to the higher low end torque. The result is that the motor's power delivery is more linear with throttle and RPM, making it much easier to control by an inexperienced rider.

I wrote all this from memory, and there are a whole lot more things I probably forgot to mention, but I think this covers some basics that are useful to know.

[almost40]

Buy a used 250 (2000-2007) and learn how to ride. They are cheap to buy and maintain, insurance is also alot cheaper. Once you get a grip on riding then consider a larger machine. You may find (as many of us here have) that a 250 is all you need.
You will save yourself the pain of having to repair your $6-7 thousand dollar machine when you drop it only to drop it again.
Drop a $1500 250 and it wont matter. Pick it up and keep on riding. Its a fact that you will drop your bike in the early stages of learning. Its happened to all of us. Learn, get some riding time and then spend your money on a larger machine if you so desire.

[JMcDonald]

The particular vehicles in question aren't necessarily limited by their engine layout. The 650 makes less power than most 600s largely because it was designed to. There are some inherent limitations to the design, like piston weight, piston speed and valve sizes, but twisn can be engineered to be competitive with 4-cyl engines. For example, the Ducati Streetfighter makes both more torque and more power than its 4-cyl competition.

Torque (its higher torque value is primarily due to its displacement, but that torque is actually made later in the powerband than the 4-cylinders):


And thus more horsepower is created. Note that while it does have the advantage of 10% more displacement, the amount of power it makes is almost exactly 10% more than its competitors.


Of course, there are much more powerful 1000cc I4s, but I would argue that the twins are really intentionally just not designed to compete with them. A lot of the low-end grunt people expect from twins would be lost, and people who buy twins buy them largely for that reason. But, even that grunt is largely a matter of design rather than something inherent to twins. So, it is certainly possible to design a ~600cc Twin that makes 100+ HP, but it wouldn't perform like most people expect a Twin to, and it wouldn't sound like most people expect really fast bikes to sound, so few would buy it :P .

Like, look at the power curves of these two 2-cylinder engines, and see how similar it looks compared to other 1000cc suprtsports:

[setasai]

I feel like I'm in school again.

[Flashmonkey]

Originally Posted by setasai

I feel like I'm in school again.

+1...except I can't throw erasers at the cute girls.

[Sailariel]

That's the nice thing about this forum---we learn stuff. I think that Flow is an engineer. He is also the first engineer I have encountered who takes a complicated subject and explains it in a way we mortals can understand without being pedantic---a truly amazing skill, indeed.

[Francis]

Originally Posted by FrugalNinja250

Torque is the turning force of something like a crank or a bolt. If you take a 12" long wrench and put it on a bolt then apply 50 pounds of force at the other end, the torque being applied to that bolt will be 50 ft-lbs. The power you're applying will be zero as long as the bolt does not turn. Once the bolt starts turning then you're generating power, and you're still exerting torque. If your wrench is only 6" long then the torque for the same 50 lbs of force applied will be 25 ft-lbs. (50 lbs X .5 feet).

So let's say that a bolt is really tight and I'm a weakling. I can be trying to loosen it with 10 ft-lbs of torque but it still doesn't mean, it means there's no power? Got it.

Hmm. Is the number for ft and lbs always the same?


Anyways, hopefully during the Summer I can meet some local guys in the motorcycle community and show me the parts. I'm a visual guy. I HAVE to see to learn.

Also, I'll be back on here. Can only read bit by bit haha.

Thanks for the info guys. Really helpful.

[oroboros]

I think we lost Francis somewhere up there in all of this! Francis?... Hello?

[coondog]

Originally Posted by oroboros

I think we lost Francis somewhere up there in all of this! Francis?... Hello?

for sure man. in case he's just fogged out for a moment, francis i've a son started out a bit ago and saw him reading a "motorcycling for idiots" book. a good literature intro before moving on to keith code and plethora of serious ex-racer writers. amazon.com for all, don't get frustrated, you're patience and MSF will help as well.

[Flashmonkey]

Originally Posted by oroboros

I think we lost Francis somewhere up there in all of this! Francis?... Hello?

LOL I think the units got to him. This is why learning this stuff using imperial units is ridiculous....because you need to toss in random multipliers to make everything work out hahaha.

Ahh metric...my sweet sweet metric. I'll take my crappy newton-metres thanks!

[lumi]

Can't help you with the actual question, you already have much better answers from others than what I could give. But if you're looking for something to read besides websites... I got The Essential Guide to Motorcycle Maintenance when I first got my Ninja.

Had absolutely no idea about engines or mechanics, but the book was pretty easy to understand. Staring at the pictures of 2-strokes and 4-strokes for half an hour and trying to imagine them in my head might have also helped.

[MiniCog]

Originally Posted by JMcDonald

Like, look at the power curves of these two 2-cylinder engines, and see how similar it looks compared to other 1000cc suprtsports:

There is a problem with that idea... I can explain if you want... Here's a hint. Did you notice that BOTH bike's torque and horsepower curves intersected at 5252 RPM for each bike?

[JMcDonald]

I don't see a problem. What's your point? Mine was that the shape of the curves were quite similar despite the drastically different engine layouts, supporting my point that said engine layouts are not inherently linked to one type of power curve or another.

[MiniCog]

Originally Posted by JMcDonald

I don't see a problem. What's your point? Mine was that the shape of the curves were quite similar despite the drastically different engine layouts, supporting my point that said engine layouts are not inherently linked to one type of power curve or another.

I don't know my own point, actually. I think I just wanted to point something out, I don't know.
I haven't slept much lately, so my mind may be wondering and I may be imagining crap. Or blabbing.
I think what I was going to say is that if the torque curve is flat, the horsepower curve will not be flat due to the simple formula of horsepower being HP = Ft-Lbs x RPM / 5252. And I have no idea how that relates to anything you said.
I suddenly have this feeling that I just made an ass of myself...

I screwed up and I don't know what I was thinking, so if I came off wrong I'm sorry about that.

[JMcDonald]

It's whatevs. I was just comparing the shapes of the power curves and emphasizing that not all twins have to have humps in their curves low in the rev range that taper off significantly with RPM, as most expect of twins.

[kevlarorc]

My friend bought me "Modern Motorcycle Technology" by Massimo Clarke for my birthday last year and I've found it very informative.

[MiniCog]

Originally Posted by JMcDonald

It's whatevs. I was just comparing the shapes of the power curves and emphasizing that not all twins have to have humps in their curves low in the rev range that taper off significantly with RPM, as most expect of twins.

Awesome, I actually slept last night and I can think straight now...

I agree with that, for an engine to make 120 HP (120 for simplicity) at around 5k RPM (yet again, simple) would take roughly 126 Ft. Pounds of torque... That simply does not happen in an engine as small as a motorcycle's. I think that most people believe that a V-twin's max HP is around 5-7k RPM, I don't know why people think that, but they do. It's all about the engine tuning.
Sometimes I wonder what kinds of drugs people are doing...

And the fact that most Twin's torque curve is relatively flat throughout the rev range, the only way for Horsepower to go is up.

[Francis]

I'm still here, guys. Just been busy with work and stuff.

Last night and a little bit of today I was removing my fairings hahaha. I was expecting my alarm to come in today so I wanted to be prepared so I can start installation.