Anyone lightened, knifed edge their 4 stroke crank?

[fastchink]

I'm in the process of rebuilding my RMZ450 motor. Trying to building a strong reliable motor that likes to rev, quick and snappy. Just wanted to get feed back from anyone before spending $$$ on a crank job. Of course I can always lightened the flywheel, but for now I since the motor is apart I would rather take the opportunity to modify the crank, then I can decide what to do with the flywheel later since it's so easy to get to.

[blue9red9]

My personal opinion which is not based on much is that you would be playing with fire if you lightened the crankshaft.

[dwnlowx]

i have a shop trying to get me to do that with my crank but i kind of have my mind set on on a hot rod crank they are alot cheaper than the knife rods just because you have to rebalance the crank ,they claim the rod on 450z is the very weak link ,oh yeah they also do something with coating the rod so it moves threw oil faster i think im gonna probably end up doing the hot rod crank they are selling cheap on ebay

[grayracer513]

My personal opinion which is not based on much is that you would be playing with fire if you lightened the crankshaft.

More like dynamite. A rod built as an I Beam should never be knifed. The rod should be made in that form to begin with, if that's what you're after.

[DaveJ]

I'm in the process of rebuilding my RMZ450 motor. Trying to building a strong reliable motor that likes to rev, quick and snappy. Just wanted to get feed back from anyone before spending $$$ on a crank job. Of course I can always lightened the flywheel, but for now I since the motor is apart I would rather take the opportunity to modify the crank, then I can decide what to do with the flywheel later since it's so easy to get to.

This is an interesting subject since I've addressed this a few times with modifications to the flywheels, usually when it seemed that everyone else was adding more weight.

On tight tracks, getting the motor to spin up quick out of sharp corners can mean the difference between making or missing a double or triple. Yes you can rev and dump the clutch, but that's lost time, doesn't make for the smoothest transition and sometimes you just can't get to it, (well at least for me). Besides, I always like less engine braking and just enjoyed the snap for all types of riding, track or trail.

On the 426s, there was a big room for improvement via the flywheel. These could be chopped down about 4 to 6 ounces to a total weight of 1lb. 2.5 ounces.

On the new 450s, you lose this option since the flywheel has nothing to chop but fortunately weights in at only 14 ounces.

Now...the more interesting matter here is not always how fast you can get the motor to rev up, but how much rotational inertia you have to redirect when you want the bike to turn. In other words, the more mass you have spinning about, the harder the bike is to redirect, so keep that in mind when you begin to pull your spec together. Or better put, the pro and cons of reducing drag to that of mass.

You may want to start with the easy part (the flywheel) then decide if you really need to take the next step.

I’m not sure what I’ll do about the newer 450s since a little more snap would be nice, but not so if it means major changes to the motor.

[Murmy26x]

In dirttrack a few people put falicon heavy cranks in their 450 hondas. And I've heard that a stock RMZ crank/rod assy weighs more than the heavy falicon crank/rod assy's. So you should be able to get away with knifing the crank itself. Would definetly spin up quicker, and get whoa'd down quicker.

[grayracer513]

Lightening the crank as an assembly is not necessarily going to benefit you. It may, or it may not, depending on what you start with, and what you're trying to accomplish. For one thing, most MX races don't come down to a question of who has one more horsepower than anyone else, especially not in the big class. Then, too, the power you do have is only useful if you can connect it to the ground. This is most of the challenge with riding 500 class two-strokes, and was a big issue with the early YZ450's, so much so that they're often quicker with extra flywheel weight than without.

Apart from the traction issue, which will vary from bike to bike and from one race situation to another, there is a very real question of whether the bike might actually produce more horsepower with a heavier rotating assembly. I've seen this happen with smaller displacement V-8's, and there was one recent dyno run I saw here of a YZ250 (2T) that turned out bigger numbers with a flywheel weight than without it.

Either way, it's expensive, and a low yield modification for those who truly need every edge they can possibly get, like the example of the flat tracker.

[brucifer]

Just reading the info. I take it that the RMZ has a T-Bone crank. I may be wrong but I under stand you want to reduce weight without reducing the inertia. "Keep the OD and lighten the inside".

If passing through the oil is a problem why not pull a vacuum on the crankcase and pull the oil out of the air. This is sounding a lot like a Drag Discussion.

[DaveJ]

Lightening the crank as an assembly is not necessarily going to benefit you. It may, or it may not, depending on what you start with, and what you're trying to accomplish. For one thing, most MX races don't come down to a question of who has one more horsepower than anyone else, especially not in the big class. Then, too, the power you do have is only useful if you can connect it to the ground. This is most of the challenge with riding 500 class two-strokes, and was a big issue with the early YZ450's, so much so that they're often quicker with extra flywheel weight than without.

Apart from the traction issue, which will vary from bike to bike and from one race situation to another, there is a very real question of whether the bike might actually produce more horsepower with a heavier rotating assembly. I've seen this happen with smaller displacement V-8's, and there was one recent dyno run I saw here of a YZ250 (2T) that turned out bigger numbers with a flywheel weight than without it.

Either way, it's expensive, and a low yield modification for those who truly need every edge they can possibly get, like the example of the flat tracker.

So a couple of things.

First, I don't think you can get real horsepower by reducing rotational mass. In other words, true horsepower only comes from within the combustion chamber, and the only change from there is due to friction or drag. In other words, you cannot gain horsepower outside of the combustion chamber, you can only lose it.

If you are seeing a gain, it's the nature of how the dyno operates or the algorithms that it uses to make the HP calculations.

Therefore I would think that if you used a more traditional dyno, such as a brake, you would not see any measurable difference.

But I could be wrong, so your opinion on this would be interesting to hear.

Secondly, this issue of wheel spin is understood but I have to see any factual information that shows what configurations provides maximum forward speed. Obviously, there has to be a balance between having some wheel spin, no wheel spin and too much wheel spin.

For me, wheel spin is used to control the bike in the corners and get the motor into a higher RPM range (more power) sooner. That said, I know what it's like to ride or drive on ice or very slippery conditions, in which little to no wheel spin is optimal.

All things considered, when I lightened the flywheel on my earlier motors, the bike was easier to ride and I was able to come out of the corners faster and harder. My downshifts were also easier to manage.

With the new bikes coming in with significantly less flywheel mass than the earlier machines, I would have to assume that either opinions have changed or there is something else within the engine compensating for the decease in flywheel weight.

Thoughts?

[grayracer513]

Here's the dyno results link I was referring to.

https://www.thumpertalk.com/forum/showthread.php?p=4069786#post4069786

Obviously, it wasn't a big difference. The dyno used was a Dynojet, which some may not understand calculates horsepower based on how quickly an engine can accelerate under an artificial load. You are right, I would think, in your contention that a really conventional brake dyno would give a different result. But it might surprise you, too, and it's something else to tinker with and wonder about. It does illustrate how, under certain circumstances, an engine can accelerate better with a little extra rotating mass to carry it from one power stroke to the next. As I said, I've sen that happen with less than 300 cubic inch small block Chevys in full bodied steel cars like early Camaros running modified production classes at the drags. Better times with heavier flywheels.

The wheelspin issue often breaks down into one of control. Most would agree that almost any 450 4-stroke is quicker through a tighter section of track with loose soil than a similar sized two-stroke, and I think that's mostly because they bite better. In side by side tests done with riding buddies of mine to confirm my suspicions about my '03, it accelerates significantly harder against the same motorcycles in the same dirt with an 8 ounce weighted flywheel than it does with any of the lighter ones I have. I believe this can only be attributed to reducing the tendency for breakaway wheel spin in the lower gears.

Of course, what works on an '03 YZ450 isn't necessarily going to tell you anything that can be directly applied to anything else, but it could. Like I said, it depends.

On the point about controlling the engine's acceleration with a lighter flywheel, Yamaha has done this, to the best of my knowledge, by controlling the advance curve. A classic example is a comparison of the '03 450 to the '05. Both have the same cams, cranks, and flywheels, yet behave totally differently.

[DaveJ]

Here's the dyno results link I was referring to.

https://www.thumpertalk.com/forum/showthread.php?p=4069786#post4069786

Obviously, it wasn't a big difference. The dyno used was a Dynojet, which some may not understand calculates horsepower based on how quickly an engine can accelerate under an artificial load. You are right, I would think, in your contention that a really conventional brake dyno would give a different result. But it might surprise you, too, and it's something else to tinker with and wonder about. It does illustrate how, under certain circumstances, an engine can accelerate better with a little extra rotating mass to carry it from one power stroke to the next. As I said, I've sen that happen with less than 300 cubic inch small block Chevys in full bodied steel cars like early Camaros running modified production classes at the drags. Better times with heavier flywheels.

The wheelspin issue often breaks down into one of control. Most would agree that almost any 450 4-stroke is quicker through a tighter section of track with loose soil than a similar sized two-stroke, and I think that's mostly because they bite better. In side by side tests done with riding buddies of mine to confirm my suspicions about my '03, it accelerates significantly harder against the same motorcycles in the same dirt with an 8 ounce weighted flywheel than it does with any of the lighter ones I have. I believe this can only be attributed to reducing the tendency for breakaway wheel spin in the lower gears.

Of course, what works on an '03 YZ450 isn't necessarily going to tell you anything that can be directly applied to anything else, but it could. Like I said, it depends.

On the point about controlling the engine's acceleration with a lighter flywheel, Yamaha has done this, to the best of my knowledge, by controlling the advance curve. A classic example is a comparison of the '03 450 to the '05. Both have the same cams, cranks, and flywheels, yet behave totally differently.

Yea...see...I think dynos like the DynoJet look at acceleration and convert that into a horsepower number, which to a purist, is not "horsepower". Or that's what I think.

That aside, I'm interested in the aspect of whether or not the mass of the flywheel effects the way in which power is generated within the combustion chamber...somehow, or if it's just a matter of horsepower delivery which in turn effects acceleration.

As for Chevy's coming out of the gate, is that due to the extra mass of the flywheel during launching or is there something much deeper?

Last but least, particularly whenever I watch a truck-pull or whole-shot, I'm always curious about the relationship between acceleration and wheel spin.

[grayracer513]

Yea...see...I think dynos like the DynoJet look at acceleration and convert that into a horsepower number, which to a purist, is not "horsepower". Or that's what I think.

The calculations are done on sound enough principals, because horsepower is torque x rpm (very roughly), and torque is what accelerates, but it does make you think.

As for Chevy's coming out of the gate, is that due to the extra mass of the flywheel during launching or is there something much deeper?

One of the car owners made a couple of controlled passes at the final 1/8 mile at Carlsbad Raceway one evening with his 276" mouse motor by crossing the line at 4500 in second and then running full throttle the rest of the way. The car was fastest over that distance with a 60 pound flywheel than the others they had with them. The benefit off the line with so little torque was obvious enough, but there was in fact, more to it, at least for him.

Last but least, particularly whenever I watch a truck-pull or whole-shot, I'm always curious about the relationship between acceleration and wheel spin.

Then you'll enjoy contemplating why four stroke singles, other things being more or less equal, always seem better able to find traction than any other engine configuration.