Lightened Flywheel

tlangman4 · Jan 12, 2019

Hi All -

I have previously owned three airheads to include 2 R75s and an R100RS. All three had their flywheel lightened in an effort to improve shifting. It did seem an improvement, but not totally sure. My latest project is an ‘81 R65. I am not familiar with this shorter stroke engine, and have not heard whether a lighten flywheel is beneficial for this engine for the same reasons. I am thinking that perhaps the reasons it applied to the 247 engine don’t apply to the 248/1 due mainly to the shorter stroke. Just curious.

Anyone care to offer an opinion on this topic?

Thanks!

Tom

20774 · Jan 12, 2019

Tom -

Were the other two bikes pre 1981 bikes? Prior to 1981, the flywheel was a big heavy mass and it was felt that lightening it would improve shifting. The downside was that irregular power pulses might be more noticeable since the mass momentum was gone. The engine might spin up more quickly but it will spin down quickly as well...sometimes having that heavy mass can assist with shifting but one has to get used to it. The heavy flywheel was good for a sidecar rig.

Beginning in 1981, the flywheel along with the clutch was changed to a "clutch carrier" setup which eliminated the big mass flywheel. There is really nothing to lighten from that point on.

http://bmwmotorcycletech.info/lightened-flywheel.htm

gator · Jan 12, 2019

A Properly assembled Tranny shifts like Butter. Not everyone Knows the secrets to Proper shimming and no I am not one of them but I do know Someone who is.
Gator

barryg · Jan 12, 2019

Another upgrade in the shifting department in this time frame for airheads was the kinematic shifter. The shifter had the linkage that allowed the shifting to have a mechanical advantage of being faster, which allowed for smoother shifts. I think I'm correct on this.

Na Cl K9 · Jan 13, 2019

Hi Tom... Your 81 R65 will have the stamped steel clutch carrier from the factory. That clutch is from production date 9/80 on (per Max BMW Parts catalog). It represents the lightest 'flywheel' arrangement available for the airheads. Cant get much lighter or faster revving unless you remove the weights from the crankshaft. Your R75's and R100RS heavy flywheel bikes with flywheel lightened should have provided quicker revving engine and presented a slightly different shift point and procedure - neither of which are difficult to master. Honestly, the heavy FW bikes can be shifted as smooth as silk - the light FW is all about performance. I have had several R100's lightened by 28% (see drawing) and have enjoyed the resulting performance gains especially the one in my dirt bike! The linkage mentioned earlier appeared in 1978 and improved mechanical advantage making shifting a bit more reliable (for some), and in 1980 the dampened drive shaft appeared and softened the blow to the drive train when a rider messed up shifting the new light clutch carrier bikes. The best part of that arrangement is a stronger swingarm (better handling). The trick to lightening the heavy wheels was not to compromise the integrity by making it look like a piece of Swiss cheese. The timing marks in the example drawing are replaced after machine work is completed.

tlangman4 · Jan 13, 2019

20774 said:
Tom -

Were the other two bikes pre 1981 bikes? Prior to 1981, the flywheel was a big heavy mass and it was felt that lightening it would improve shifting. The downside was that irregular power pulses might be more noticeable since the mass momentum was gone. The engine might spin up more quickly but it will spin down quickly as well...sometimes having that heavy mass can assist with shifting but one has to get used to it. The heavy flywheel was good for a sidecar rig.

Beginning in 1981, the flywheel along with the clutch was changed to a "clutch carrier" setup which eliminated the big mass flywheel. There is really nothing to lighten from that point on.

http://bmwmotorcycletech.info/lightened-flywheel.htm

Thanks for this information, very helpful! Yes, all three privois bikes were pre-‘81, so this makes sense.

tlangman4 · Jan 13, 2019

Na Cl K9 said:
Hi Tom... Your 81 R65 will have the stamped steel clutch carrier from the factory. That clutch is from production date 9/80 on (per Max BMW Parts catalog). It represents the lightest 'flywheel' arrangement available for the airheads. Cant get much lighter or faster revving unless you remove the weights from the crankshaft. Your R75's and R100RS heavy flywheel bikes with flywheel lightened should have provided quicker revving engine and presented a slightly different shift point and procedure - neither of which are difficult to master. Honestly, the heavy FW bikes can be shifted as smooth as silk - the light FW is all about performance. I have had several R100's lightened by 28% (see drawing) and have enjoyed the resulting performance gains especially the one in my dirt bike! The linkage mentioned earlier appeared in 1978 and improved mechanical advantage making shifting a bit more reliable (for some), and in 1980 the dampened drive shaft appeared and softened the blow to the drive train when a rider messed up shifting the new light clutch carrier bikes. The best part of that arrangement is a stronger swingarm (better handling). The trick to lightening the heavy wheels was not to compromise the integrity by making it look like a piece of Swiss cheese. The timing marks in the example drawing are replaced after machine work is completed.

View attachment 72274

Thanks John! Great information!

tlangman4 · Jan 13, 2019

gator said:
A Properly assembled Tranny shifts like Butter. Not everyone Knows the secrets to Proper shimming and no I am not one of them but I do know Someone who is.
Gator

Thanks Gator. I would like to know who that would be, as I will need the tranny serviced.

20774 · Jan 13, 2019

Tom Cutter at Rubber Chicken Racing Garage first comes to mind when thinking about transmissions. I think he might be on limited duty these days for medical reasons, so not sure what he can take on.

What part of the country do you live in? Wisconsin maybe? The Chicago Region BMW Club has some great people associated with it...many were taught by Oak Okleshen in years past. There might be some skilled people who can work on transmissions around there.

Friedle · Jan 13, 2019

20774 said:
Tom Cutter at Rubber Chicken Racing Garage first comes to mind when thinking about transmissions. I think he might be on limited duty these days for medical reasons, so not sure what he can take on.

What part of the country do you live in? Wisconsin maybe? The Chicago Region BMW Club has some great people associated with it...many were taught by Oak Okleshen in years past. There might be some skilled people who can work on transmissions around there.

Tom Cutter is back at work selling parts and doing individual component repair, though still recovering from major surgery. He is no longer doing complete restorations. Do get in touch in your transmission needs attention.

His wonderful sense of humor and gentle nature were unaffected by his medical issues.

Travel fast Tom on the road to full recovery. :wave

Friedle

PAS · Jan 13, 2019

Friedle said:
His wonderful sense of humor and gentle nature were unaffected by his medical issues. Travel fast Tom on the road to full recovery.

Friedle

Funny!!

pglaves · Jan 13, 2019

PAS said:
Funny!!

Not if you just had multiple bypass open heart surgery. Tom needs our support.

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