View Full Version : Lightweight Flywheels

10th August 2006, 22:57
Does anyone make these? Is there an advantage to keeping the heavy Sportster flywheels or would a change to Buel parts make for a faster reving motor? I'm told only some year Buel flywheels fit sportsters. In speaking with the NRHS folks about an 88 inch kit - they recommended starting with a 91-99 5 speed model and a change to after market connecting rods. It occurred to me that if we're tearing in to the motor that far - why not a change in flywheels?



10th August 2006, 23:21
A lightened flywheel is one modification to a street bike that will have more negative than positive effects. Here’s why.

An engine makes the least torque at idle and low rpm, especially when cold (when there are more misfires.) So when you release your clutch a little too fast, the engine torque is too small to overcome the bike's inertia, and it stalls. If you have a stock flywheel, the stored momentum augments the engine's torque allowing you to use a lower rpm starting-off. With a lighter flywheel you'd need a higher idle speed, or constantly need to start-off at a higher rpm in order to raise the engine torque output enough to avoid stalling the engine.

As you ride at lower rpm in traffic, you are constantly changing between acceleration and deceleration. Engine torque levels are still fairly low at these speeds, so slack in the drive train needs to be smoothed-out with a flywheel. Otherwise, on-off throttle transitions have a jerky effect, giving a less comfortable ride and causing you to use smaller throttle inputs (which is not always easy to do.)

At higher speeds a flywheel slows the rate at which an engine rpm changes, so cracking the throttle open or closed results in a smoother transition in torque being applied to the drive train and tires. Again, without a the stock flywheel you will need to be more careful with your throttle transitions. The key to faster track times is reduced wheel spin so a light flywheel works against you by making it more difficult to modulate wheel spin, even though it helps lap times by producing more acceleration in the straights. So, for the purposes of racing I think a lightened flywheel may have very positive effects however, using one on the street will prove to have some major shortcomings....

11th August 2006, 00:04
S&S makes a set of lighter weight flywheels which are knife-edged, but they aren't cheap. My plan is to get a set for my Buell. The S&S wheels also prevent the keyway shearing problem that some have had in the later motors...

11th August 2006, 00:06
Ride a Buell. You will then want to shed some of the excess mass for sure. The stock wheels have an over abundance of mass. Shaving 'em down a bit will pose NO drawbacks regarding tractability. The wheels will still be very heavy compared to most modern engines. It really livens up a Sportster engine. The more mass in the rotating assembly, the more power is consumed to accelerate that mass. Reducing it allows the engine to spool up quicker.

You will still have full circle flywheels. Look at almost any modern engine crank assembly: not much there, half circle flywheels or less. If it's taken too far the engine will lose low speed smoothness. it is impossible to remove that much on a Sporty crank. They are HUGE.

For agricultural use, big flywheels are great.

11th August 2006, 00:16
Forgot-- the handling of a bike is influenced by the rotating mass in the engine. The crank is a BIG gyroscope. Reduced mass will allow a bike to turn in and transition quicker. Show me ONE modern racing engine with heavy flywheels. Reduced crank lard is a very old trick and very effective.

Other sources of excess rotating inertia: heavy stock wheels. Harley cast aluminum wheels weigh about 16lbs (!) each,bare. And that is UNSPRUNG weight,the worst kind,next to a big fat chick on the back.

People dump boatloads of money in their motors on H-Ds and seldom give much thought to reducing performance robbing excess weight. Funny.

And do not propose that a Buell is a touchy, bad mannered racer in street trim. It's not. It's a rip snorting XL type motor but still a John Deere compared to the Japanese screamers.

11th August 2006, 00:57
Other sources of excess rotating inertia: heavy stock wheels. Harley cast aluminum wheels weigh about 16lbs (!) each,bare. And that is UNSPRUNG weight,the worst kind,next to a big fat chick on the back.

The unsprung weight of a wheel controls a trade-off between a wheel's bump-following ability and its vibration isolation. For this reason, lighter wheels are often sought for high-performance applications. In contrast, a (heavier) wheel which moves less will transmit less vibrations through the bike's suspension. For this reason, a heavier wheel can have a positive effect on ride quality. If you have ever been on a sportbike with carbon fiber wheels you would have no problem understanding the effects of ultra light wheels and those effects are not always good in terms relative to street machines. I for one would rather have the ride quality.

Engines do spool up faster with lighter flywheels however, they tend to hang as well. Engine rpm that you are used to seeing drop off quick during a shift isn't there anymore. For racing, they're very cool but for the street Sporty maybe not so cool.

11th August 2006, 02:15
Axtell Sales probably makes light flywheels. Light wheels are good for a dragster but for the street they make the motor a little hard to contol at low speeds. The throttle will be trickey as the motor will rev up a lot faster. If you are looking for a stoplight to stoplight racer there is a difference.

11th August 2006, 02:45
Thanks for the input guys. I'll ask the folks at NRHS what it will cost to add new flywheels to the new connecting rods.

I'm planning on using spoked aluminum rims for wheels and substituting as many aluminum parts for steel as the pocket book will stand.

Appreciate the help,


11th August 2006, 05:18
I put a 98 Buell S1 crank in my 98 1200s. I believe its about 4lbs lighter. Actually it's the same exact crank with both flywheels being milled down on the inside.

If you look at the XL crank you can see where the flywheels are cut away for connecting rod clearance, the Buell crank took that cutaway all 360 degrees around. I

It's a much better bike now.

11th August 2006, 13:57
I also have a Buell crank in my 1200. No problems with stalling, deceleration, or anything else mentioned above.

11th August 2006, 14:53
I have Buell wheels in my 80" motor. There are no bad side effects that I can perceive at all. Except that the front wheel likes to pull off the ground and the bike is god-awful fast.

Those are bad side effects I can live with!:smoke

11th August 2006, 16:14
I used to hot rod '73 & earlier MG Midgets with the 1275cc motors. The stock flywheel weighed 28 lbs, & the stock wheel/tire combo weighed @ 40 lbs. When I installed a 7 lb. aluminum flywheel ( w/ cam & Weber carb) & some American Racing alloy rims which weighed w/tire @25 lbs , the car would stay with a 5liter Mustang to about 90 mph. The ride quality was also hugely improved. IMHO any way you can remove rotational mass from the drive train is a good thing. I can only see positive results on a Sportster as well. 'Course YMMV

11th August 2006, 18:56
The lighter the flywheel the more gas you will have to give the motor to start from a stop.

I have been thinking about doing this myself, but I do like being able to start from a stop with almost no gas at all. It is a somewhat tough decision. Has anybody compared quarter mile times before and after their flywheel change? I would like to know if it is really worth my time and money. :dunno

12th August 2006, 00:36

I just have a long conversation with NRHS and their feelings were if the bike were to be drag raced - the lighter flywheels are the way to go. For something to be ridden on the street they recommended the standard flywheels. They do recommend aftermarket con rods for something like I have planned: 88 CI with stage three heads. That would dictate a 91-99 bike with re-buildable crankshaft. I can use the flywheel money plenty of other places.


12th August 2006, 05:03
More unsprung weight (heavy wheels/tires) destroys ride quality. The heavier the unsprung weight the greater work the suspension must do. Bikes with very light wheels-like the carbon wheels mentioned- are very often set up for RACING which means a generally stiff suspension. So this should not be confused with the light wheel having poor ride qualities. If heavy wheels absorbed bumps better,then off road motorcycles would have cast iron wheels. And industrial trucks would have a smooth ride with their very heavy wheels. Neither is the case. The heavier a wheel, the greater the energy transfer is to the chassis over bumps.

Unsprung weight is the ENEMY of good handling and ride. Read any number of books about chassis design.

Since flywheels by definition STORE ENERGY engines with light flywheels spool up faster and also LOSE crank speed faster than those with heavy flywheels. It's simple 9th grade physics: an object in motion tends to remain in motion. An object at rest tends to reamain at rest. INERTIA! A heavy object-like a Harley crank- tends to change rotational speed much more slowly wether its an increase or decrease than a lighter crank.

Touch a sportbike or modern dirtbike throttle at idle. The engine will LEAP to a very high rpm almost instantly and spool down just as fast. You can redline it if not carefull. The same throttle blip on a Harley will make a nice sound but the engine will change RPM much more slowly.

Anyway my original point was that H-D's have SO MUCH flywheel mass that some can be removed with only positive results. An exception might be a dresser if you like chugging around at 1500RPM. Ducatis are pretty tractable V-twins with a nice wide power curve. They have tiny cranks compared to H-Ds. Peace out.

12th August 2006, 12:42
I have lightened flywheels in my 88. Its a street bike only.
All that stuff posted in #2 is complete BS. My bike exhibits none of those characteristics.