Author Topic: When is too light not enough? OR How much can be removed from a crankshaft?  (Read 2101 times)

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Offline fang

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I am about to send Jay $1300 for one of his super-uber "Ultra Crank" assemblies. (LINK -- item #ULCKH750)

The short version of my question is, "how much weight can be removed from a SOHC CB750's rotating crank mass before it becomes detrimental to performance?"  When is too much too much?   I am thinking about putting an APE Ultra Crank with a Cycle-X exposed (and greatly lightened) charging system together. Will this be too light for a track bike that is tuned for aggressive street use? 

So I have a vision for what I want to do with this engine, and I am not exactly sure what it will take to get there.  In the past I have built some very light rotating mass motors, in particular there was a little Fiat race car.  The throttle response was very "blippy."  We shaved a ton off the flywheel, and it was incredibly responsive.  It would rev to the moon almost instantly, but it always wanted to stall out when going slow or over speed bumps, etc.  It had its own unique set of fun pros and cons.  It was a race car the guys at the shop were using to run errands around town: tons of fun, but not very practical....   And nobody's wife could drive it.  ;D

I have built a number of CB750 engines over the years, but I have never fiddled with crank mass beyond shaving a pound or so off the alternator.  I really just have not taken much off before, and am not sure what to expect; I don't know where the line is for these motors between reaching for untapped performance and ruining a motor. I am concerned about potentially removing too much weight, and more or less ruining my ability to enjoy a very expensive motor.

I am intending to build this particular motor as though for a competitive, fully safety wired, AHARM Formula 750 class bike, although it will be non-qualifying with the big bore kit.  Yet I intend to keep it streetable with the cam and carb choices.  Nevertheless, I am wanting to push the performance envelope toward the sharp edge of aggressive, even at the expense of some street riding comfort. 

In my younger days, as a auto mechanic, I had a mentor who who specialized in vintage exotic cars.  There was a particular 1960's 12 cylinder Ferrari that stands out in my memory -- we were tuning the six down draft carbs.  I was drooling in awe and terror at the twelve chrome velocity stacks as he was explaining how Ferrari's standard for carb tuning/synchronization required that from idle, you should be able to stomp on it, rev it to redline (around seven grand?), and have it drop back to idle, all in something like 2.9 seconds.  Ferrari insisted that the tech was supposed to stand there with a stopwatch for this procedure.  For me, that was a very exciting engine, and that quick revving, high compression, instantaneous responsiveness is exactly what I am hoping to obtain with this CB750 engine.  Not too long ago I worked at a local Porsche performance shop.  We also worked on other nicer German models.  The difference I am looking for in my CB750 is the difference between a sporty Mercedes SL-whatever sports car, and a race-prepped Porsche 911.  Both are fast, but one is smooth and refined, while the other brings out the monster in you with something snarling, gritty and mean.  My hope here is to somehow really tap somewhere DEEP into the whole 'authentic' vintage race bike aesthetic, transcending what was mechanically possible in the early 70's, and create a very aggressive, snarling, instantly responsive race machine, while dancing the balancing line that will keep it on the street.

I driven by a vision of a lightly baffled CR-style, four-into-four race exhaust just barking with an instant scream from idle up to 10,000 rpm then suddenly dropping back to a throbbing, loping high-cam idle.  I am thinking about screwing the throttle the rest of the way back at highway speeds and pulling the front wheel off the ground.  That is what a race engine feel like to me.  None of my CB750s currently do this.  =)  ....But I am confident that with an engine built with all the tricks and some fine tuning, you can have a mean little CB750. 


Does anyone have experience with very light crankshaft assemblies on CB750s?  How much is too much?  When is too light not enough?  What are the pros and cons?  I saw on the Cycle-X website that "...for you serious race folks" they are selling a 'Cycle X Super Crankshaft Assembly.'   Its manufacture includes removing seven pounds from a stock crankshaft.  If I remember correctly, the sprint cars folks would remove a lot from their heavily modified CB750 motors -- does anyone have experience with these engines?

Lets hear it about the ideal crankshaft rotating mass for a race motor.

peace and grease,
« Last Edit: November 16, 2011, 06:46:12 pm by fang »
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Offline MCRider

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You can call it a race motor...but is it really. Assuming roadrace?  Most streets just don't allow the back and forth frequency of a track, no matter how much we wish they would. I love APES stuff and they did my last crank too.

If its a race motor that is ACTUALLY a street motor I think you'll want some weight on the crank. My experience is not huge. I've had 3 motors with lightened cranks. The comments will simply be along the lines of your fiat.

Blippy, rev to the moon, takes some skill off the line. Personally I'm OK with this. And I think that's where it will lie. A personal decision. Sounds like you won't be happy with anything less.

But truthfully, on the street, between stoplights, (not on the Autobahn or Swiss Alps) one should lighten the rotor to the max to relive stress on the end of the crank (they've been known to break by aggressive throttle blipping) and not so much on the crank. Dynoman told me they get better launch times with a CB750 if you leave some weight on.

APE says if your doing left right like on a racetrack, you want max lightness to reduce gyroscopic resistance to the left right. That may be a reach but sounds good. And can you really apply it to your riding environment?  ;D

« Last Edit: November 16, 2011, 06:19:26 pm by MCRider »
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Offline fang

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You can call it a race motor...but is it really. Assuming roadrace?

I think I would prefer to say I am building a 'street tuned race engine.'  My whole build philosophy is as though I had a AHRMA campaigned race bike that is being converted to be street legal. 

Putting that aside, Thanks MCRider for your input. 
In a more general sense, I would like to hear from more folks who have experience running CB750s with various lightened crank assemblies. 

(1) How much was removed?
(2) from Where was the weight removed?
(3) for What application was it tuned?
(4) What was the riding experience like (as pertaining to the lightened crank in the HiPo motor)?

peace and grease,
« Last Edit: November 16, 2011, 10:48:55 pm by fang »
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Offline 754

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Offline NitroHunter

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I had a Falicon Supercrank, with RC Goldenrods, in a hotrod street motor, I used the starter gear deleted RC lightened alternator. Oh boy did it wing right up there with the revs, but when you chopped the throttle the lack of flywheel effect was really noticable. Even so, I once ran a full weight alternator as a test to feel the difference - one ride was all it took, light weight was the winner.
Crankshaft inertia creates much better launches - just slower to spin up out of corners.
FYI the crank in our funnycar weighs about 70 lbs. It's all about the application.
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