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Author Topic: Honda 750 Nighthawk  (Read 9906 times)
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Gimpy Joe
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« on: March 24, 2011, 03:21:51 PM »

I'm new to land speed racing, but I've been welding and fabricating for a long time and I'm obsessed with anything on two wheels. So far I have built three bikes with varying degrees of success. My goal is to get out on the salt in the next year or so. I'm glad this forum exists, since there will obviously be a steep learning curve and I'm going to need all the help I can get.

Anyway, the plan is to start with a 90's 750 Nighthawk. I want this bike to stay street legal since I can't justify spending a lot of time and money on a bike I can only ride a couple times a year. I know I won't be setting any records that way, but that's fine with me. I just want to go fast  cheesy

I will be modifying the frame, adding the required safety equipment, and fabricating my own bodywork. I will also make some relatively minor tweaks to the engine and exhaust and hopefully gain a few horsepower.

Supposedly the Nighthawk will do about 130mph stock. What kind of speed could I realistically expect to achieve by streamlining the bike and changing the final drive ratio?
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dw230
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« Reply #1 on: March 24, 2011, 03:34:35 PM »

Is it really fast(or fast enough) if you don't set a record?

Maybe this question needs a new thread.

Honey do list fell to zero, Joanie went to Nashville this morning. A recent text message has her sitting in a bar with live music, pizza and beer while waiting for her sister's flight.

DW
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« Reply #2 on: March 24, 2011, 03:35:33 PM »

That was her sister?
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Gimpy Joe
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« Reply #3 on: March 24, 2011, 04:02:41 PM »

Is it really fast(or fast enough) if you don't set a record?

DW

I don't care if it is fast, so long as it feels fast. If I don't feel like Wylie Coyote strapping himself to a rocket, then this project has failed.
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Milwaukee Midget
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« Reply #4 on: March 24, 2011, 04:16:59 PM »

I seem to recall the coyote hitting a rock and continuing in his trajectory, sans rocket.

Welcome aboard Joe.  Yep, we pick on newbies.
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« Reply #5 on: March 24, 2011, 04:48:15 PM »

I thought that the ACME bodywork was outlawed??   afro

Mike
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« Reply #6 on: March 25, 2011, 12:39:29 AM »

Joe, don't pay any attention to these characters.  It is the off season and they have too much time on their hands.  What is an honest rear wheel horsepower for that bike and how big are you?  I can look on my handy chart.
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Beairsto Racing
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« Reply #7 on: March 25, 2011, 02:46:56 AM »

Welcome to the addiction Joe!  cheers

Do you plan on running at Bonneville or one of the paved venues?

What do you have in mind for the bodywork, lots of enclosure like a Charlie Toy or a full cafe style fairing? With limited hp, your biggest gains will come from improved aerodynamics.

They made @75hp at the crank with very mild cams and only 9.3:1 compression. You could certainly wake it up with cams, head work, higher compression pistons, bigger carbs etc.. but then you would sacrifice some streetability and you mention that it will still serve as your street bike.

LSR is all about the experience itself...record or not. Your Nighthawk is a good bike to start with, you will learn lots and make some great friends along the way.

Cheers,
Scott
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« Reply #8 on: March 25, 2011, 08:20:39 AM »

Is it really fast(or fast enough) if you don't set a record?

DW

I don't care if it is fast, so long as it feels fast. If I don't feel like Wylie Coyote strapping himself to a rocket, then this project has failed.



 grin grin grin grin


 cheers
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Gimpy Joe
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« Reply #9 on: March 25, 2011, 02:30:32 PM »

Joe, don't pay any attention to these characters.  It is the off season and they have too much time on their hands.  What is an honest rear wheel horsepower for that bike and how big are you?  I can look on my handy chart.

Me a couple of years ago on my first bike, a '76 CB360 that started as a pile of old roadrace parts. These days I'm 5'10", 220 lbs built like a linebacker. I hope that won't hurt top speed too much  embarassed

Welcome to the addiction Joe!  cheers

Do you plan on running at Bonneville or one of the paved venues?

What do you have in mind for the bodywork, lots of enclosure like a Charlie Toy or a full cafe style fairing? With limited hp, your biggest gains will come from improved aerodynamics.

They made @75hp at the crank with very mild cams and only 9.3:1 compression. You could certainly wake it up with cams, head work, higher compression pistons, bigger carbs etc.. but then you would sacrifice some streetability and you mention that it will still serve as your street bike.

LSR is all about the experience itself...record or not. Your Nighthawk is a good bike to start with, you will learn lots and make some great friends along the way.

Cheers,
Scott

I want to go to Bonneville and also go to some of the paved events because they are closer (I'm in Atlanta).

I will use Airtech's  John Cronshaw fairing if it'll fit.

 Other than that I'm going to fabricate the seat, tank, rearsets, etc. myself since that's what I really enjoy. I'm not much of a mechanic, but my brother is. I'll let him fiddle with the engine a bit, but I will probably still end up with under 100hp.
« Last Edit: March 25, 2011, 02:41:50 PM by Gimpy Joe » Logged
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« Reply #10 on: March 25, 2011, 09:07:47 PM »

Welcome to the salt! I am just building my 1st bike and hoping to go slower than you but maybe fast for my class...250cc pushrod.  From what I see by looking at the bikes, reading this board (its great) and relying on experienced advice, I would get your bike running as fast as possible before installing the fairing. They tell me you will lose about 10% of top speed on pavement to salt. One thing to consider is removing your front brake system as you won't need it on the salt and it catches a lot of air. Weight is important but most of the fast bikes have added weight to their swingarms to keep traction. Running on the salt requires the motor to be running at top performance for up to a full minute when you count the run-up to speed before shifting to top gear.  I don't know your bike but my estimate of top speed on the salt with your head on the tank would be 120 in perfect tune and condition.  My goal is 80 mph and I need to lose 20 lbs. Good Luck
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« Reply #11 on: March 25, 2011, 10:51:24 PM »

Welcome aboard Gimpy,
I too am pretty new to this game. I ran RWB last year on a '79 cb750L last year. It had sort of the standard street mods, pod aircleaners, Mac 4 into 1 exhaust, jet kit, Dyna ign. The engine internals were untouched. Slightly taller final drive. Small wind screen.
It dynoed at ~60 rear wheel HP. I ran through the mile at an average of 120 and maxed at 123.
I'm going back in an AMA class this year, hope to see you there!
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« Reply #12 on: March 26, 2011, 01:28:58 PM »

Its about the journey not the destination
if you have zero drag you dont need horsepower and vice versa.
welcome to the site dude always happy to see another honda!!!
Caution its addictive
Oz
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Gimpy Joe
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« Reply #13 on: March 27, 2011, 10:23:50 AM »

I'm going to look at a couple of Nighthawks I found on Craigslist. Wish me luck!

I will also be working on my other project today. It's a 1970 CB100 that I'm turning into a monoshock cafe racer sort of thing.

Back to the garage  smiley
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wobblywalrus
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« Reply #14 on: March 28, 2011, 08:10:56 PM »

Joe, this is part of a reply to your first question "How fast will I go with streamlining?"

There is a formal method that involves estimates of frontal area, drag coefficients, friction losses, the effects of altitude on drag and power, the racing surface, etc.  It is good to know and most of us use it.  There are frontal areas and drag coefficients listed on links to this website for many different vehicles.  There are spreadsheets, too, to help with the calcs.  A person can figure out potential top speed with careful work.

The author, John Bradley in his book "The Racing Motorcycle" (ISBN 0 9512929 2 7 for Volume 1 or ISBN 0 9512929 1 9 for both volumes) discusses this formal method.  He also made a simple graph.  It is a plot of rear wheel horsepower versus top speed from published data on modern road bikes.  Road testers are hired guns that can tuck down very well, the "standard" bikes they ride are typically the pick of the litter, and tests are almost always done near sea level on a paved surface.  The data resembles bikes at Maxton, Loring, etc. more than at B'ville.

The upper line on his graph represents road bikes with more than average resistance to motion.  Resistance is friction and air drag.  The lower line represents bikes with less than typical resistance to motion.  A typical beginner guy or lass out will be at or near the upper line.  A road legal street based bike well tuned with a good rider is somewhere between the lines.  A well streamlined production road bike with a skilled rider is at or near the lower line.  The goal is to be below the lower line.  We can do this with our home built streamlining.

At B'ville the air is thinner and we naturally aspirated bikes lose power.  I use a typical power loss factor for the salt flats.  We race on a natural surface which creates more drag and we push against thinner air which produces less drag.  My assumption is both cancel each other out for a street bike.  I do not use factors for them with Bradley's graphs.  The first post is an example of a little Honda.


* Honda Example.jpg (214.77 KB, 1013x768 - viewed 271 times.)
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