1973 Honda CB350 - how much power?

It does seem to have eluded you, though.

Bwaaahahahahaha!

Makes *no* difference whatsoever. Power is power, and if you don't have
enough power to pull a taller gear, not even dilithium crystals will
help.

I stand by my previous assessment. You are a stupid sod.

 

<VBG>

No, it doesn't. You're giving us a lot of amusement, though. Please keep
playing.

 

In stock trim? Maybe 75mph. I've had a few dozen CD and CB175s, which
aren't so different in terms of size, weight and power output.

Screens and fairings make precious little difference at that sort of
speed, except to rider (dis)comfort.

 

Nice. I see where one guy got 140+ out of a 100cc motor, and the highest
in the 350cc class of just over 189. I ran a "go cart" race track for a
while, we used 100cc McCulloch racing motors, with 12" tires we could
squeeze out a top speed of about 107...but those motors were finicky and
had to be torn down and rebuilt just about every day.

I bought my CL used and I don't know if it had been bored out previously,
or any other mods of that nature. I just tuned it and changed out the rear
sprocket and the chain. Unfortunately, I crashed that one and the frame
wasn't salvageable. (The Docs didn't think *I* was salvageable either, and
told me I'd never walk again, but I fooled them.)

 

OK, have it your way. Practical and professional experience rebuilding
motors and racing since age 12 counts for nothing and is negated by your
theoretical calculations. Nobody has ever taken a 350/360cc motor to 120+,
if you say it can't be done, then it can't be done. I bow to the awesome
power of your mind which has the ability to cancel reality.

Why don't you tell that to these guys, that the one with the 350 didn't go
to 189, and the other guy with the 100cc motor didn't go to 140:

http://www.scta-bni.org/Bonneville/records/BNIrecs_bikes.htm

(Thanks to Rick Cortese for the link.)

120mph out of a CL360 is not particularly difficult. 189, on the other
hand, takes a bit more doing.

 

Ah, the old 'appeal to authority' argument. Never fails, when you can't
weasel out of some bullshit you tried to pull, the old, 'I've been doing
this for years, why do you doubt me' wheeze. I've got news for you, there
are people on this newsgroup that actually do know what they are talking
about and you are making yourself

Nope. I never said a 350/360cc engine couldn't get to 120mph. What I did
say, was your stock CL360 didn't do it, can't do it, because it was a
stock bike without any sort of engine mods or fairing on it. Changing
the sprockets doesn't make your bike 'custom', if you want to cling to
that 'mod' as somehow elevating your CL360 to non-stock, fine. But it
doesn't change how much power the engine puts out, and if the power isn't
there, it won't get to 120 mph, no matter how much you want it to.

Do you know what those various class designations mean? Somehow I doubt
a stock 100cc bike is going to hit 140, and I will bet my life that the
189 mph 350cc bike was a streamliner. But keep on ignoring the fact that
your CL360 isn't a streamliner, as TOG says, you keep on showing how
little you know about bikes and simple physics.

Go on. Get 120 mph out of your CL360. It *can* be done. But it won't
be done simply by changing the sprockets and giving the bike a tune up.

 

No, it doesn't. Try riding a few bikes, and by a few, I mean 'several
hundred'. Dunno about Mark, but I reckon I've owned something like
250-300 bikes in the last 30+ years, and ridden probably another
couple of hundred. This tends to give one a fairly empirical and
accurate view.

Oh, it can be done. The CB350 is quite keenly sought after by the
classic racing boys here in Europe, and can be tuned to go
surprisingly quickly. What Mark is saying, and what I'm saying, is
that a couple of basic mods will not make one do 120. It just won't.

As a very rough rule of thumb, production road bikes need something
like 28-30bhp in order to reach 100mph. To get to 120, you need about
60bhp. Obviously there are other factors here, like the bike size:
Honda racing 125 two-strokes will hit high speeds on considerably less
than 60bhp, but we're talking road bikes here.

The old 360 twin made a claimed 34bhp, in an era when bikes weren't
routinely dyno tested. Assuming something like 25-28bhp at the rear
wheel gives it a top speed of round about 100 - a good one might do
it, and an average one might not, but let's be generous.

To get 120 out of one, you'd need to raise the rear wheel horsepower
to around 50bhp, given the size/bulk of the old 360. Take the Ducati
750SS in my(other) sig: that has a claimed 64bhp and will do
125-127mph, according to differing road test measurements. Most I have
ever seen on the clock is 125: a racer friend managed 130. I reckon
rear wheel horsepower is about 55-60bhp. So we're talking about a bike
that's twice the capacity, nearly twice the rated horsepower, which
manages a mere 5-10mph faster than you claim for your old shonker.

Sorry, you're living in a dream world, unless you paid for some very,
very expensive surgery. Or had a speedo that was more inaccurate than
usual. Or conceivably, laced a smaller diameter front rim to the hub,
which would cause the speedo to do silly things.

These are pukka race/record breaking bikes, you utter imbecile, not
played-with soggy old Honda road twins.

Yes, it is.

Right, your basic ignorance of physics and other things have been
adequately pointed out. Other matters like faulty insturments have
been flagged up. If you're not prepared to admit you've made a howling
monkey of yourself, not prepared to admit that your speedo might be
lying as much as your President, and not prepared to admit you might
be in error, what does that leave?

Fancy running for President?

 

That motor had the power. And it wasn't a fluke, I did it to two different
CL360s and it didn't take a whole lot of work to do it.

I will respect the other gentleman's high opinion of you and not respond
in kind to your repeated insult, though I would have thought that if you
were truly the sort he claims you would have the wisdom not to maintain
such a stubbornly entrenched position. Just because *you* cannot conceive
how a thing can be done doesn't mean that it can't be done.

If you have taken the time to peruse the link that Mr. Cortese so kindly
provided, you will have seen that my paltry 120mph out of that particular
machine pales in comparison to what others have done.

While you're talking about power, consider that power alone is not the
only factor- there's the power-to-weight ratio.

As far as wind resistance- scootching back on the seat, feet on the rear
set of pegs and literally lying down on top of the fuel tank with elbows
tucked in cuts the drag considerably.

This conversation reminds me of a guy who swore up and down that it was
*impossible* to run a V-8 motor using three 2bbl carbs, even while looking
at the motor in operation.

You can continue calling me a "stupid sod" if you like, but I would
suggest that it may reflect more poorly on your character than mine.

 

No, it didn't.

No, you didn't.

As I said, for dedicated record-breaking or race machines. Different
animals. You've been told this, so I can only assume that you're
trying the point again out of desperation.

God, you are ignorant. Weight hardly affects top speed. It affects
acceleration. This was told you earlier in the thread.[1]

OK, sweetie. Do you know how motorcycle magazines achieve their top
speed figures? They get the smallest tester they can find, and dress
him in skin-tight leathers. Top speed runs are effected by doing
exactly what you say - flat on the tank, and with naked bikes, if you
dare, left hand off the bars and gripping the fork leg. Mirrors are
frequently removed.[2] And nope, no way will you get a CB360 to do 120
with the mods you suggested, unless you're going down a very, very
steep hill with a hurricane tailwind.

If a mag gets an utter max of 100mph, then the average Joe can expect
90+.

Not the same thing at all.

Nope, it's an accurate reflection of your intellect. Erring on the
kind side, if anything.

[1] More than 20 years ago, Bike magazine here in the UK did a very
interesting exercise involving (IIRC) a Honda VF400 and a number of
staff members, ranging from Roland Brown (then a staffer, now an
international freelance) who stood six feet four in his socks and
weighed 160lbs or so, and at the other end, editorial assistant Ione
Holmes (five feet one and 112lbs or so)[3]. Top speed runs,
electronically timed, were within a couple of mph of each other.
Quarter-mile times varied immensely, in favour of the small girl, who
had little experience of quarter-miling. I've got the copy in my
archive.
[2] Guess how I know this?
[3] They married each other, believe it or not.

 

Hmmm. How do the 300km/hr or 186 mph bikes (limited by manufacturer's
agreement, some capable of exceeding 200 mph) get going that fast, with
less than 176 hp?

Doesn't the power required to overcome aerodynamic drag increase as
the cube of the velocity? <cheats, via google> Yep.

This guy (if you squint hard) has a spreadsheet

http://www.msgroup.org/forums/mtt/topic.asp?TOPIC_ID=869

that says for some particular bike & rider, it takes 18.2 hp to go 75, so
to go 120, it takes...

e(3*l(120/75)) * 18.2
74.54719999999999999963

about 75 hp.

With 11 hp, that mythical bike would go about...

75 * (e(l(11/18.2)/3))
63.41154904466300639475

63 mph. You are a genius!

Assuming our favorite bike, the CL360, with an advertised hp of 34 at the
crank, IIRC, has all of that available at the rear wheel, and it's roughly
the same frontal area and coefficient of drag:

75 * (e(l(34/18.2)/3))
92.36989283019552082575

It's going to go about 90 mph tops.

FURTHERMORE...

1/4 the hp to go 1/2 as fast? I think you mean 1/8 the hp to go 1/2 as
fast.

<AOL>

 

Amen.

 

I'd query those figures: a decent 14bhp 125 will hit 75, and the
original Honda CB750 did 120+ on a claimed 67bhp.

Pretty close, though.

 

I didn't look too closely to figure out what sort of bike he was modeling.
Maybe it was a tourer with a big fairing and windshield. Actually,
my ZG1000 does about 127 tops, with 91 hp at the wheel, and using the
mythical 75mph/18.2hp bike as the model:

e(3*l(127.2/75)) * 18.2
88.78690795519999999818

It agrees rather well with their measured dyno reading of 91.0 hp peak,
assuming that the top speed actually occurred at the exact power peak,
which can't be guaranteed of course, due to gearing (I wonder if Mr.
CL360 will jump up and claim he can get my ZG to go another 10 or 20
mph faster if I change out the final drive

That's the point, it's close enough.
When the power required increases as the cube of the speed, and you
know you've got 34 hp at the most to work with, knowing a little bit of
1st year physics[2] won't let you wiggle out with a simple hand-wave.

It's easy enough for anyone who's ridden a few similar bikes up to their
real world top speeds, and knows where to dig up dyno numbers for them,
to conclusively prove that without major modifications to power output
or to the coefficient of drag, and/or frontal area, a similar bike isn't
going to magically be the lone exception to the laws of physics.

[1] 127 and 91 hp,
http://www.aquariumfish.com/mcn/model_eval/Kawasaki Concours, Part 4.pdf

[2] Of course if you don't know physics and can't do simple maths,
you can be untroubled by such concepts, and believe anything you want.

 

Yep.

I've owned *a lot* of small low-powered bikes. I just like playing with
them. In fact, I've just bought a 1974 Honda SL125, which I'm planning
to restore to "rather nice" condition....

Anyway, you don't even need to dig up the dyno numbers. After a while,
you hop on something and have a pretty good idea how it'll perform.

You still get exceptions to the rule - my 1988 Yamaha RD350 power-valve
was one. Christ, that was a fun bike. Claimed 60bhp: easily pulled into
the red in top, at an indicated 120+.

I actually geared it up, and it still pulled it (I'd heard it was
capable of doing so) but the "odd spot" at 6000 rpm (when the
power-valve was making its mind up whether to open or not) moved from
70+ mph to 75+, which made it a real pain, so I put the gearing back to
stock.

Thereby proving, I suppose, that the OE manufacturer usually gets it
right.

 

The aerodynamic drag force increases as the square of the velocity, which
makes the power increase by the cube of V.

Despite Wikipedia's dubious reputation, I am pretty sure they've got
it correct, I didn't explicitly quote them before, but since the
question was raised:

http://en.wikipedia.org/wiki/Drag_(aerodynamics)#Power

(Apologies if the character encoding gets screwed up)

The power required to overcome the aerodynamic drag is given by:

Pd = Fd (dot product) v = -1/2 Ï v^3 A Cd

Fd is the force of drag,
Ï is the density of the fluid (Note that for the Earth's atmosphere,
the density can be found using the barometric formula. It is
1.293 kg/m3 at 0°C and 1 atmosphere.),
v is the velocity of the object relative to the fluid,
A is the reference area,
Cd is the drag coefficient (a dimensionless constant, e.g. 0.25 to 0.45 for a car), and
[v] is the unit vector indicating the direction of the velocity (the
negative sign indicating the drag is opposite to that of velocity).

The important bit is the v^3.

 

They tought me that some 40 years ago.
It still is correct.

Cd relates to the shape of the body. A square block
has a higher Cd-value than a smoothly curved
shape.

btw, the CD-value for a body is simply measured (in a wind tunnel).
They place the body in an airstream and
measure the force needed to keep it
in place.
Cd is then calcultated by
dividing that force by
'half rho v-squared times A' (I cannot type
better than this).

The term 'reference area' sounds
a bit misleading to me.
It is just the area of the cross section in the wind. Like the frontal
area of a car. Or the frontal area of you and
the bike together. In square meters.

Laying flat on the bike thus helps in
two ways. You lower both Cd and A.

It always amuses me when a car manufacturer
proudly states a low Cd for the car, and forgets
to mention anything about
the A.

 

That's quite clear, (except for "times a work" should be "times THE
work") and I see nothing wrong with it.

All I will say is in my experience, cars don't typically burn 4x as much
gas at 60 mph than they do at 30 mph... but it does indeed take 8x as
much power _to overcome aerodynamic drag_ at 60 mph as it does at 30 mph,
_assuming their equation holds true_ at those sorts of speeds.

The critical sentence in that paragraph gives them enough wiggle room
to push an elephant through:

"It should be emphasized here that the drag equation is an
approximation, and does not necessarily give a close approximation
in every instance. Thus one should be careful when making
assumptions using these equations."

I had stayed in Aero Engineering instead of the engineering major I
did pursue, I could talk about this with a bit more certainty, but I
am not in any doubt about the likelihood of a stock engined CL360
attaining 120mph without going downhill and/or pushed by a tailwind.

 

See my remark about removing the mirrors when speed-testing bikes.

 

Oh crikey. How did this twit escape from my killfile?

Did the "tinkering" also include painting new numbers on the speedometer?

-gc

 

Heh. welcome to the party.