Machining, lightening, BMW Airhead Motorcycles Flywheel, lightening flywheels

3 May 2015 | Author: | Comments Off on Machining, lightening, BMW Airhead Motorcycles Flywheel, lightening flywheels
BMW R45 (reduced effect)

The ads above are Google-sponsored. clicking on them at every visit, and looking at them helps support this website!

Lightened Flywheels


Section 60, sub-part 2A

© Copyright, 2012, R. Fleischer (except the photo)

Below is a photo of a properly lightened flywheel. Lightened Airhead flywheels are sometimes installed on the early heavy flywheel models (pre-1981). After that date, BMW changed to a totally different type of ‘flywheel’, which is called a Clutch Carrier and is much lighter, and the clutch parts were totally different, and the transmission input shaft spline was shortened to match.

The entire assembly was much lighter. This accounted for some of the lack of smoothness of the later models, as the engine had less stabilizing/smoothing mass at the crankshaft output. This was particularly noticeable at idle rpm, but also noticeable at higher rpm, while actually riding.

The later models generally required quicker shifting, as the rpm drop was faster when the throttle was chopped, not to mention the often sudden effect on the rear wheel upon a throttle chop. The faster shifting is generally regarded as a nicety.

Lightening the flywheels on the early BMW’s will enhance shifting, by enabling faster shifting, because the flywheel has inertia, and with less weight, less inertia, so the flywheel. with engine crank, etc. attached, will slow down at a faster rate during shifting. You will have smoother shifting IF you shift FASTER, before the rpm drops too much (assuming downshifting). The acceleration will be somewhat faster, as less mass is being speeded up.

Engine braking generally is reduced with a lightened flywheel.

There ARE downsides to lightening a flywheel. Engine vibration will increase, but not too excessively. In my opinion the sensitivity to the preciseness of the carburetor synchronization will increase.

Some of the smoothness. some of this is almost intangible. will be lost, but, not excessively. Depending on the year and the transmission and if it has the so-called shift kit, shifting can be LESS positive in actually finding the gear/neutral. You will need to shift just a bit faster, and perhaps spend a bit more effort on matching rpm when shifting.

However this is offset by generally less clunky shifting. The engine may be a bit more prone to stalling if not fully warmed-up; and you will have to slip the clutch a bit more when starting out. this is a reason I do NOT ever recommend lightened flywheels on sidecar rigs.

One final thing needs to be said. It is my belief that a lightened flywheel will promote FASTER WEAR on the transmission input shafts. I can, however, argue both ways on that subject.

In case any engineer-types read this, NO, I have never heard of crankshaft cracking/breaking, from lightening the old flywheels. to explain this, flywheels act as dampeners for crankshaft torsional vibration modes.

The old BMW’s are never going to shift like a Japanese crotch rocket, no matter what you do to the clutch or transmission. You can improve things by fine-tuning the transmission innards. but, just how far DO you want to go? Are you REALLY taking this bike onto a racetrack. or do you like its smooth gentlemanly manners when out for a ride?

You PROBABLY would like the lightened flywheel. I know I always did. and I think most riders of 1970-1980 BMW Airheads WILL like the lightened flywheel effect. Do I recommend it.

YES. but ONLY if you understand and are willing to accept the modest to minor drawbacks, see above.

The photo below is of a properly lightened Airhead flywheel. This photo was furnished by Beemershop at my request, as I did not have any photos from when I lightened flywheels.

The Beemershop (Ted Porter’s) is an independent BMW motorcycle repair center, located in Scotts Valley, California (831) 438-1100.

They have the ability to furnish these flywheels, or machine yours, by numerically controlled, and thus precise machining methods. It is VERY important that flywheel lightening be done properly. There is NO NEED, and I recommend AGAINST, machining off the flywheel outer edge marks (S, OT, F).

BMW R45 (reduced effect)

NOTE CAREFULLY the needed outer meat that was left to support the starter ring, threaded clutch attaching boss places, etc.

HINTs: When installing any Airhead flywheel, have the pistons at TDC, and OT showing in the timing window, otherwise you may install the flywheel in the wrong position onto the crankshaft. Do NOT tighten the 5 flywheel bolts right away, but have them SLIGHTLY loose, and CENTER the flywheel on the bolt shoulders, by trying to move the flywheel circularly clockwise and counterclockwise. they sometimes show a slight amount of play. Progressively, criss-crossedly, ABP, tighten the bolts in stages.

Refer to other pages for the tightening torques, which vary considerably between years and models. Be very careful not to overtorque the early (/5 and earliest /6) bolts, which are 10 mm.

The last of the airhead models used, per BMW, oiled threads, and a very high torque. I prefer a somewhat lower torque, although I have seen no reported problems with the high torque settings. There are two other articles on the flywheel, one being a WARNING article . on this website.

The main article that has all the information on the flywheel torque values, and associated information. is the CLUTCH.htm article. It is Article 60, subsection 9, but you can click that hyperlink.

When the flywheel is lightened on an early BMW Airhead, the engine rpm can and will change much faster on downshifting and upshifting. Because this TIME is quickened, a helpful modification is the SHIFT KIT, which BMW installed on later models. A complete description and details are in TRANSMISSION.

When working on Airheads, you may occasionally need to know the spacing between markings on the flywheel (or clutch carrier). You may need to know the distance per degree on the circumference.

The diameter of the larger engine flywheels are all the same, and there is 2 mm between each degree.

For the R45/R65, it is 1.5 mm between each degree.


NEW PAGE, and released 01/28/2011

01/29/2011: go over, fix minor typos, and clarify a few things.

Interesting articles

Tagged as:

Other articles of the category "BMW":

Our partners
Follow us
Contact us
Our contacts

Born in the USSR


About this site

For all questions about advertising, please contact listed on the site.

Motorcycles catalog with specifications, pictures, ratings, reviews and discusssions about Motorcycles.