Board index Flathead Power-Technical Questions, Answers, and Suggestions K-Models lightened flywheels

lightened flywheels

Moderators: Curt!, Pa

Post Thu Apr 23, 2009 6:59 am

Posts: 139
Location: USA
In building my k model with a 4 5/8'' stroke,big intake valves I am contemplating the possibility of lightening my flywheels,I would like to know if French Fowl or Frankenstien would build their bikes different a second time around with lightened flywheels.This bike will be a solo rider and the cam will probably be a KHK .Any thoughts would appreciated,Thanks. :D :D

Post Thu Apr 23, 2009 1:49 pm

Posts: 49
I would ask Paul Osborn at Truett & Osborn. That's how they started out, hot rodding K's and Sportsters. Give 'em a call or shoot them an email.

info@truettandosborn.com

mike

Post Fri Apr 24, 2009 4:20 am

Posts: 587
Location: Crewe, Great Britain

Hi Celtic,

Never ran lightened cranks on my Ks, but did it on my defunct 1959 XLCH in the past, had the std wheels skimmed by about 3mm on the radius, 1/4" on the diameter. Anymore skimming above that would probably hurt the oil scavenging from crankcases to timing cover. I did not increase the size of the cast-in scrapper... It ran OK!!!

Patrick

Post Fri Apr 24, 2009 8:00 am
panic


Post Fri Apr 24, 2009 3:05 pm

Posts: 1659
Location: Interlaken, NY USA

My personal experience was that the Stroker S&S wheels were no problem on Frankenstein. However, the bike weighs 350lbs, a little less than a K model. Also, I'm using the KHK cams which appear to have more overlap than other grinds available. However, I wasn't happy with S&S wheels in my 84" pan with swingarm, bags. A 650lb bike. You miss the stored energy when under heavy load like climbing a hill, the engine appears to vibrate more. You feel each power pulse more strongly. Also, as Panic states, I had to modify the bike from original intent to get it to start reliably. I.E., went with a higher overlap cam so that the energy stored from kicking was enough to carry through the reduced static compression so the damn thing would start. To wit, I ended up using an Andrews "B" instead of my preference an "A".
DD

Post Fri Apr 24, 2009 3:55 pm

Posts: 2683
Location: Los Angeles, CA
What about lightning flywheels by cutting a knife edge as some KR, XR models have?

Post Sat Apr 25, 2009 6:18 am

Posts: 139
Location: USA
Guys,Thanks for the input and Panic that was an outstanding essay you wrote about ''Lightened Flywheels''.Many years past when I had my shop I had taken shovelhead flywheels ,(which I know is an entirly different animal),narrowed the heavy side to the same width as the right flywheel and then cut a 1'' 45 degree cut on the outer edge on both flywheels,used a sifton cam,higher compression pistons,(had everything balanced)and the motor was pretty much like a big sportster.It was snappy and I do recall some idling differences.I was unaware of any oiling problems but that alone is a grave concern.Hopefully the 4 5/8'' stroke on the K Model will enable the motor to be more responsive when I crack the throttle and actually do something.Again,Thanks to all for sharing your thoughts and experience. :D :D

Post Sun Apr 26, 2009 8:27 am
panic

The added size will speed up the response.
However, if you look at a late Buell motor, the 883cc flywheels (3.00" × 3.8125") are almost dinner plates - far less weight than I would have thought possible, yet they pull fairly heavy bikes with tall gearing, big power and high compression around in traffic very well.
What's different?
They're 5 speeds and electric start.
If the rest of the driveline matched, a KH-sized motor could use even lighter wheels. The problem is that if you guess wrong it's horrible to drive (stalls, lugging, poor idle quality, hard starting, wheel locks on closed throttle, clutch slipping), and no way to cure it except to start over.

Post Tue Apr 28, 2009 6:51 pm

Posts: 66
To say nothing of computer-controlled electronic ignition and injection.

panic wrote:
What's different?
They're 5 speeds and electric start.

Post Tue Apr 28, 2009 8:50 pm
panic

..and tubeless tires, and alternators, and halogen bulbs, and disc brakes, and cast wheels, and...

Post Wed Apr 29, 2009 5:55 pm

Posts: 66
Actually, no. Timing and fuel delivery, like flywheel design and gearbox design, have a direct effect on tractability which tires and bulbs don't (electric starters you can argue them either way). The precise control they allow can make up for radical measures elsewhere, letting a designer use extremes of flywheel balanc... design, compression, etc without making the machine unrideable.

panic wrote:
..and tubeless tires, and alternators, and halogen bulbs, and disc brakes, and cast wheels, and...

Post Wed Apr 29, 2009 6:50 pm

Posts: 1659
Location: Interlaken, NY USA

I'd leave electric starters in the mix because flywheel inertia definitely plays a part in kick starting :lol:

Post Thu Apr 30, 2009 7:20 am
panic

"Actually, no."

Actually, yes.

Your turn.

Post Fri May 01, 2009 12:05 am

Posts: 587
Location: Crewe, Great Britain

Panic,

Would I be right in saying they probably lightened the Evo Sporty crank because they added the alternator rotor weight to the equation?

Patrick

Post Fri May 01, 2009 8:52 am
panic

That has to be a factor, but the weight change occurred well after the alternator was added in 1991. The new wheels are also far lighter than the added alternator weight, plus the alternator isn't nearly the same diameter as the flywheel.
The 1984-90 alternator behind the clutch is also inertia, but only rotating at 58% (?) engine speed, not the same effect.

I'd like to say that a 4-5/8" 45 with a strong 1st gear (like 2.70:1 or more), a 4 or 5 speed, a reasonable high gear (3.50:1 or more) and light chassis weight could use wheels as light as 15 lbs. but I'd hate to be the first person to try it.
The 1st gear acceleration would be dramatically improved since the amount of power wasted speedup up the crank is a straight function of mass, diameter and RPM - not related to engine power, so a mild motor like a 40 hp 45 will get a far greater proportionate improvement than a 90 hp Buell.

Post Fri May 01, 2009 8:56 am
panic

IMHO: yes, the factory knew this is 1941. The WR light wheels were probably made that specific weight to keep as much traction as needed, and they may have made and tested sets in 1 lb. increments between 10 and 20 before settling on that weight as providing the best control and lap time, rather than the most power to the wheel.
For pavement road racing, where (almost) perfect traction is presumed, lighter wheels will always help, but the effect at high speed and high gear will be much, much less.
This allows a single choice of weight to work fairly well for both racing venues: any lighter will reduce traction on dirt, and any heavier will reduce acceleration on both tracks.

Post Fri May 01, 2009 9:05 am

Posts: 587
Location: Crewe, Great Britain

Thanks for the insights, Panic

... The only experience I've got with low flywheel weight was an early Munch Mammuth 1000 from a friend, unrideable!!!

Patrick


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