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Author Topic: "Doing it Right" or "How to Build a Functional Café Racer"  (Read 116183 times)

Offline stroker crazy

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #10 on: Aug 02, 2012, 21:43:03 »
A great thread; sometimes we need to step back to focus on the full picture!

I will be reading this primer on the café racer with great interest.  Are there any plans for strokers to get a look-in?

Crazy


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Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #11 on: Aug 03, 2012, 02:47:45 »
I don't disagree with Sonrier's treatise in general, but I'd add that the way to improve performance is first and foremost to reduce lost energy.  That's free performance and it's there to be taken.  Second is to improve volumetric efficiency and means making it breathe easier.  It can include cams and porting, but that's not always necessary or desirable - it all depends on your goals.

You can't beat cubes was an old saying that holds true to this day, a bigger engine will usually make more power than a smaller one, but the costs increase even faster.

let's take some examples.  I have say a CB360 and I decide I want more power and less weight. How much more power do I want and what is the path to that power?  Do I need bigger pistons custom made to my spec to go into new carbide coated aluminum sleeves and a lightened crank and Titanium rods etc.  And at the end of the day if I created the ultimate CB360 how much power would it make and would I have been better off just buying a more modern bike for less cash?

For me, the starting point was always to reduce weight because that's like free power - as long as I don't take that to extremes and have things break.

Then I can make sure that the wheels spin freely and brakes don't drag and there's more free performance. After that I can carefully fit thinner wheels for less rolling resistance or fatter tires for more side grip and I end up with stock sized tires in a soft sticky compound for the best of both worlds. 

Then I upgrade suspension at both ends to improve handling so it goes around bends faster or at least more securely and safely.

At that point I can think about modifying pistons, lightening cranks, porting the head, increasing compression and after that I'll think about how much more I want to spend of this sweet riding motorcycle. I'm an engine guy at heart but most of my performance improvement comes from attention to detail and a logical approach and critical thinking.

In the race world, as in the custom world, most modified bikes are slower than stock until the rider starts to put the pieces together and  gets the details right.  How many threads are about jetting and oil leaks and ignition timing and how many are about cast versus forged pistons or ways to reduce pumping losses? Make the most of what you have with the resources you have available. It's an optimization exercise and not a maximization trip.

Motorcycles are systems and the components don't exist on their own. Everything in life is a balance, and soit is with bikes.

Offline ronnie

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #12 on: Aug 03, 2012, 03:15:54 »
Great information, keep it going Sonrier..
HOLD FAST.

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Offline 808shadow

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #13 on: Aug 03, 2012, 03:49:34 »
Awesome post. Were actually learning about auto cycles right now in my thermodynamics class and I find it fascinating. Even more so now that I've seen a literal application of these theories and math now in the forum. Reading to reality makes all the difference similar to what was said in the original post. I was wondering if someone could calculate out the increase of a small improvement such as a velocity stack. I would love to see how much of a difference they make mathematically if someone is able?  Thanks for the posts and keep em coming!

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808 all day!

Offline stroker crazy

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #14 on: Aug 03, 2012, 04:43:24 »
808shadow - if you're interested in some of the mathematics for intake bellmouth design I can PM  to you the pdf file of a research paper by Gordon P. Blair and W. Melvin Cahoon,

Crazy
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Offline Sonreir™

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #15 on: Aug 03, 2012, 11:36:48 »
Just a slight caveat to my post about displacement.  I tend to get caught up quite a bit in the minutiae of things and perhaps didn't make my conclusion strong enough in that post.

A friend read over what I wrote and had this to say:
Quote
The longstroke and shortstroke stuff is largely irrevelent in the modern era.  A CX 500 has a very short stroke and produces good power up and down in the rev range.  Rod length/stroke ratio is a very important determiner of power production and output.

And I do agree.  There's usually not much of a bad way to go about getting more displacement.  For you newer guys, any way you can get more CCs without adversely affecting your engine, that's what you want to do.  While not all displacement is created equal, stroke and bore are not leagues apart, either.

As for rod length:stroke, that is something I neglected to mention too much about.  A topic for a future post, perhaps...

Also, I totally agree with what swan and teazer have added.  Especially the portion about getting it running and learning your bike before you tear into it.  You MUST, MUST, MUST, have a baseline for your modifications.  If you modify a bike you've never ridden or don't know well, how are you to know whether your changes are improvements or detriments?

Finally, I also agree with teazer about tackling losses and volumetric efficiency, first.  VE, especially, is a topic that will take pages to cover and so I'm leaving it for a bit later.  The order in which I choose to address these topics has more to do with getting the "simple" stuff done first rather than implying that this is the order in which things should be done.  Trimming weight and, to a lesser extent, increases in displacement can be done with a minimum of investment.  When you start talking about significant gains in volumetric efficiency is when the money really starts to matter.  A future post, to be sure.
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Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #16 on: Aug 03, 2012, 17:27:23 »
808, I don't have the theoretical data but this empirical data was published by David Vizard and is posted on Michael Moore's Eurospares site

http://www.eurospares.com/graphics/engine/Vizard/

Scroll down until you get to VizardVstack001a.tif and download that image.


You will see that taper helps but the key to good flow is a rolled entry to allow air to enter smoothly from all around the stack.
« Last Edit: Aug 03, 2012, 17:31:06 by teazer »

Offline Sonreir™

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #17 on: Aug 03, 2012, 21:12:59 »
Power Goal #2 - Increased Revs

To first understand why revving an engine produces more power, you must understand that there is a difference between Force and Power.  Force is something that causes an object to undergo a change in direction or movement.  Power is how quickly that force is applied.  Zero force applied over any period of time is always zero power and infinite force applied over any period of time (except zero) is always infinite power.

Since I'm going to mainly talk about horsepower, I'll stick (mostly) with imperial units for this one.  This means we'll be dealing with foot-pounds for our unit of torque and horsepower for our unit of power.  For true correctness, this force unit is actually called a pound-foot, but the two terms are generally interchangeable, so I'll use with ft/lb as it's a bit more common.  A foot-pound is defined as the amount of force, measured at a central pivot point, generated by applying one pound of force at a distance of one foot from that central pivot point.  Because ft/lb is dependent on both direction and magnitude, this makes it a vector unit.  It can also be thought of as a force that "twists" something.  Because torque is a function of both the initial force (one pound) and the initial distance (one foot), it can be increased by either utilizing a larger distance or a larger initial force.  Doubling the distance or doubling the initial force will also double the torque.

Hopefully, that gives a basic idea about torque, because you need to understand torque to understand power.  Power is how much force is applied over time.  The definition of mechanical horsepower (the kind we equate with engines) is 33,000 ft/lb per minute.

If you recall from my first post, horsepower is derived from torque using the following equation:
     HP = (T * rpm) / 5252.

So how does that relate back to 33,000 ft/lb/min?  Well, all that has to do with our inclusion of Revolutions (it's the 'r' part of rpm) into the HP equation.  One revolution is equal to 2pi radians in the math world.  Dividing 33,000 by 2pi gets us 5252 and this is the reason for the use of that constant number in the equation.

Enough math for the moment; the important thing to remember is that without torque, you have no power.  Applying your torque faster gets you more power.  Furthermore, unless you're applying your torque with an appropriate speed, you also have no power.  To get power from an engine it must not only produce force, but apply that force quickly.  While the human body can produce great amounts of torque (especially with mechanical advantages), our inability to apply that torque quickly is what prevents bicycles from breaking the ton.

So... nitty-gritty time.  How does one go about revving an engine faster?  Well, it's not quite as easy as changing gears later in the rev range, though this is a basic gist of it.  There are several main factors in preventing your engine from operating well at high RPMs and each of these issues must be dealt with in order to create an engine lives up to its potential as it approaches five figures.

Mechanical Considerations
Though most everything on an engine can be qualified as a mechanical consideration, for this section I mean mechanical failure considerations.  If you hold the throttle open on your bike, in neutral, you will probably destroy your engine because of a mechanical failure.  I say probably, because some engines will fail past that point at which they stop making power and they don't make enough power to get to the immediate point of failure (I'll get to that in a second).  On race bikes, over revving an engine is a common cause of failure.  When you live on the edge, it's easy to fall off.

When a four stroke engine suffers a mechanical failure it almost always occurs at the end of the exhaust stroke / beginning of the intake stroke.  Why this occurs is quite simple and can be reduced to a few key points:

1.)  Almost all metals (including those used in the construction of your conn rods, crank, etc) have a higher strength rating when comparing compressive (pushing) to tensile (pulling) forces.
2.)  The tensile forces on the crank, piston, and conn rods are the greatest at the end of the exhaust stroke for two reasons:  First, the piston is the furthest from the crankshaft centerline at TDC than it is at any other point in the rotation and if rotational speed is held constant, the most force is exerted at the furthest point from the center of rotation.  Second, at the end of the compression stroke (the other time the piston is at TDC), there is less stress because the compression of the fuel/air mix combined with the increasing pressure of the ignition of the mixture reduces the tensile loading of the components and so they are less likely to fail.  In short, the compressed/ignited mixture is acting like a pillow for the piston and helping to cushion it.
3.)  For top end failures (due to valve float, etc), the exhaust stroke remains the most common because the speed at which the piston chases the exhaust valve is greater than the speed at which the intake valve chases the piston.  During the compression stroke, the exhaust valve is closed, and so the end of the exhaust stroke remains the critical time.  If you float a valve, it's going to be the exhaust valve that eats it first.

So how do you prevent mechanical failures?  Think of two words: Stronger and Lighter.  Chromoly steel is the weapon of choice for crankshafts, connecting rods, and rocker arms.  Titanium makes for excellent valves, though stainless steel is also a good option.  For pistons, forged aluminum is generally the best option, though hypereutectic aluminum will usually work for most applications where weight is more important than strength.

Remember, as speeds double, forces quadruple.  Pistons weighing in at 200 grams will produce 10 joules of energy at 10 m/s but 40 joules at 20 m/s.  Dropping the weight of your pistons by five grams will save you a joule based on mean piston speeds (it'll actually save you more on peak stresses, though).  That may not sound like a lot, but on a Honda 360, that's an extra 150 RPM you gain on the redline before a potential bottom end failure.  Again, 150 RPM may not sound like a lot, but lets say you can hold a steady 15ft/lb of torque from 11,850 RPM to 12,000 RPM.  For the only 150 RPMs, you've gained almost half of a horsepower.  Tacking on a thousand RPM instead of just 150 would net you closer to three horsepower, or an 8% increase (assuming that torque holds steady, which it won't).

When it comes to building a maximum effort engine, the little things don't mean a lot.  They mean EVERYTHING.  A gram here, a gram there frees up a joule here or a joule there, which adds a few more revs to your redline, which adds a few more ponies to your bike.

Pumping Losses and Parasitic Forces
Most people (it seems to me, at least) tend to think only of the mechanical failure portion of chasing RPMs, an equal consideration would be pumping losses and frictional losses.  While not sending rods through your crankcase is important, it's also useful to realize that the work your engine must do increases exponentially with the speed at which it operates.

For instance, as the rotational speed of the crankshaft increases, this causes more side-loading on the pistons and the friction between pistons/rings against the cylinder walls increases because of this.  Furthermore, it requires more energy to keep your pistons in motion because they are not only traveling faster through the cylinder, they're stopping and accelerating more often.  This change in direction, called reciprocation, requires more and more energy as rotational speeds increase.  Rocker arms, too, reciprocate and energy is required to move them.

Engines built to withstand high RPMs must also be able to deal with a phenomenon called, "valve float".  Valve float occurs when the valves springs are unable to provide enough force to close the valves in the time required.  To combat valve float, stronger springs are used in the valve train.  Unfortunately, strong springs means the engine has to use more force in opening the springs than previously required.  Force over time is power and so by strengthening the springs for more revs, you now require more power to open them.

Pumping losses are a big concern as well.  At high speeds, up to 20% of the power your engine creates goes directly into overcoming pumping losses (both above and below piston, though losses above the piston tend to accumulate more quickly in the very high RPM range as compared to below piston losses).  And that's even before you factor in friction, decreases in BMEP, or other concerns.

I won't spend too much time on pumping losses as it will be covered on more detail once I start attacking the BMEP section, but I will at least give a definition and a few examples.

Because ICEs are basically air/heat pumps, they all suffer from pumping losses of some sort.  And since air is a fluid, frictional fluid laws apply (Newton's third law of motion, again).  Think of this in the same way you would think of the the wind while you're riding.  There is a bigger difference in wind resistance between 60mph and 90mph than there is between 30mph and 60mph.

Air has inertia (and hence, momentum) and so accelerating it faster to fill a cylinder in less time requires more energy.  As your piston is descending and the intake valve remains open, a low pressure zone is created within the cylinder.  This low pressure is not only pulling air into the cylinder, it's also pulling up on the piston as it's trying to descend.  Likewise, as your piston starts the compression stroke, the mixture within the cylinder must be compressed at a more rapid rate.  The relationship between force and acceleration means that as greater acceleration is required, the greater the force must be.  These losses occur on the exhaust stroke, too, but in reverse.  The spent exhaust gases will initially rush to the lower pressure areas of the headers, but as the pressures within the cylinder and the exhaust system begin to equalize, the force of the piston is required to help expel the gases from the cylinder.  As with other fluid dynamics, the force required from the piston to expel the exhaust gases increases as engine speed increases.

Combatting above piston pumping losses can be achieved in a number of ways, but on older engines our usual plan is the same as increasing BMEP, which I will address in a future post.  It's also important to note that pumping losses are a byproduct of all ICEs.  You will never fully eliminate them.

Below piston pumping losses must be considered as well (especially with two strokes, because the crank cases are not open to the atmosphere).  No matter how well your cylinder is sealed, there will always be some blow by.  Some of the exhaust gases make it past the piston rings and into the crankcase.  This represents an increase in pressure within the crankcase and it is usually vented out through the crank case breather.  Though usually not a major concern for 180° twins or inline four-cylinder bikes, below piston pumping losses start becoming a real problem for other configurations.  This is because the crankcase volume varies depending on the angle of the crankshaft.  For 360° twins, both pistons are at TDC or BDC at the same time.  At TDC, the crankcase volume is the greatest and at BDC it is the smallest.  As the pistons approach TDC, a lower crankcase pressure requires more force to overcome it.  Likewise, as the pistons approach BDC, higher crankcase pressures require more force to overcome it.  Regardless of the engine configuration, however, the variations between the cylinder pressure and the crankcase pressure mean that some level of force is being exerted on the pistons as they travel between BDC and TDC.  Below piston pumping losses would only disappear if the crankcase pressure and the cylinder pressures were the same, always, which isn't possible.

For limiting below piston pumping losses, your best friend is going to be decent crankcase ventilation.

Combustion Speeds
Finally, there is one more major consideration that prevents high RPMs from being achievable, and that is combustion speed.  The gasoline being burned must create pressure at a rate which exceeds the increase in volume of the cylinder as the piston descends.  The reason for this is (again) inertia.  The momentum of the crank will want to keep the pistons reciprocating long after the the engine stops receiving fuel, air, and spark.  In a high RPM situation, the reciprocating nature of the pistons "steals" some of the energy from the combustion event because the piston was going back down whether or not that mixture was ignited.  It's the difference in trying to punch someone standing still or punch something that's running away from you.  You're going to get a much better hit on the guy who's standing there waiting for it, and the same is true for your pistons.  If they're running away from the flame, you're not making power.

There are some tried-and-true methods for increasing the combustion speed of your mixture, however.  One common way is ignition advance.  By lighting the mixture sooner, you can actually begin building pressure while the piston is still accelerating toward TDC.  By the time it reaches TDC, you also have a ton (literally) of PSI built up and so it pushes the piston back down with greater force.

The next, is good atomization of the fuel and air mixture.  Smaller droplets of fuel burn faster than bigger droplets and so they release their energy faster.  This occurs naturally as your piston speeds increase, which is why additional ignition advance past 3,000 or 4,000 usually isn't necessary.  Using the correct jets in your carbs, decent squish bands in your combustion chambers, and tumble/swirl from your valves also helps (more on these things in the BMEP section).

Also, increased compression will help combustion speeds.  Due to Brownian motion, an increase of density in the air molecules hammers the fuel droplets into smaller portions and smaller portions means faster burn times.  Increased compression also causes a spike in heat which helps to vaporize the fuel right before ignition.

Finally, it should be noted that low octane fuels burn faster than high octane.  It's a common misconception that octane is some measurement of power.  It's not.  Octane is technically a molecule, but it's commonly used as a word to describe a fuel's resistance to ignition.  That's right; higher octane fuels don't catch fire as well as low octane fuels.  Honda's famous RC166 bike required the use of 87 octane fuel in order to reach its redline of 18,500 RPM.  For more "streetable" motors, it's better to opt with higher compression and higher octane fuel as the increase in torque in the lower RPMs is usually more desirable than a few extra turns of the crankshaft at the redline.

Conclusion
In closing, chasing RPMs is actually a losing game (in the long run).  You have almost everything working against you.  As your RPMs increase, the cost in doing it, while still producing torque, is exponential.  It's OK to add a few extra revs once you've tackled some of the other performance areas, but merely adding components for the sole purpose of allowing more revs is not good value for money (though it can be done).  One thing I didn't mention in detail is that it's very possible to rev an engine past the point of being useful.  As more and more energy is required, that's less and less energy available toward moving you forward.  To add insult to injury, the redline of an engine is usually pretty far past peak torque and so while your power may be increasing (slowly), your torque is quickly dropping off.  The faster your torque drops, the slower your horsepower rises.  A large enough drop in torque and your horsepower begins to fall as well.  Building a high revving motor usually requires a peak torque to occur fairly late in the RPM band.  This may build winning race bikes, but they won't be much use on the street.
« Last Edit: Aug 03, 2012, 23:29:12 by Sonreir »
Sparck Moto - http://www.sparckmoto.com

Audaces fortuna iuvat.

1977 Honda CJ360 - Café SOS - Stage One™, Café SOS - Stage Two™
1976 Puch Maxi - APuchalypse Now
Suzi T500 Cobra Resto

Custom Gauge Graphics
Custom Wiring Harnesses

DTT Red, White, and/or Black 360 Club - Better than those Blue guys

Offline legendary_rider

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #18 on: Aug 04, 2012, 11:42:55 »
 :o I didn't like hearing that last bit. lol. Looks like I'll be building another motor. lol. Thanks for all the input Matt.
http://www.dotheton.com/forum/index.php?topic=39352.msg429000#msg429000
1970 Cb350 x 2
1971 cb350 x 2
1972 cb350 x 2
1973 cb 350
1974 cb 360 x 2
1975 cb360
1977 CJ360T

Offline teazer

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Re: "Doing it Right" or "How to Build a Functional Café Racer"
« Reply #19 on: Aug 04, 2012, 13:23:23 »
To Matt's last point, a full race motor is a complete PIA on the street.  Nothing happens until it's revved hard and getting away from the lights is fun for the first time and gets more miserable as time goes on until the clutch fails from abuse and then it's game over.

Even for the track, full race cams are rarely the way to go except at Daytona or RA. What you need is torque and you need it at reasonable revs. The two ways to get that at low revs are cubes and compression. Big ports, valves or carbs all hurt low to mid range pwoer and torque.

Thnink mild and think efficient.