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Questions regarding lightweight flywheel on R69S
Questions regarding lightweight flywheel on R69S
I have VERY slowly been tearing down the engine on an R69S for rebuild (link to thread here). Though I am still a long ways away from putting things back together, I have questions about the flywheel, or rather the lack of one. I was told by the PO that there was hardly any flywheel on this bike, and that appears to be a very accurate statement! What I have appears to be an aftermarket casting that has been drilled and weighs in at only 5lbs, 2oz. and most of that is relatively inboard toward the crank. The only image I have found that matches what I've got refers to a 'Cycleworks' flywheel on an R27 (link to that forum posting here). I have not found any maker's marks, part numbers, etc. stamped on it, though.
I have a bit of extra runout (over the .004" max that is specified) but I think I can resolve that when it comes time for reassembly. My question centers on whether I should even continue running with this flywheel or not, and if anyone has any specific experience with one of this construction on an R69S. I think I understand the benefits towards increasing/decreasing engine RPM quickly with less angular momentum, and when running the bike shifted well without undue need for additional throttle. However, I have had great difficulty in starting this motorcycle since purchased, and though there are of course a number of factors involved, one thought I had was that the flywheel (or lack of a relatively standard one) may be contributing to the situation. My logic was that due to the decreased angular momentum of the flywheel, the crank would tend to rotate less when starting the engine as there is less stored energy. There have been many times when the engine has kicked back, firing the lever back up under my foot, and though I haven't broken my ankle yet, it seems like I'm tempting fate. It becomes frustrating to the point that the pleasure of going out for a ride quickly drains away after many swift kicks...
Does anyone have experience with a reduced weight/lightened flywheel on an R69S? Does the higher compression of these engines (originally quoted at 9.5:1) cause adverse starting issues with this arrangement over other 'lower' compression /2's? How about other high-compression models with lightened flywheels? I'd be willing to compromise a bit of the rapid engine response and install a heavier unit if it kept me from breaking my foot and allowed for easier starting, if my logic is correct here. If it is a Cycleworks flywheel, does anyone have any input on their quality? Can I assume the timing marks are even accurate, or are there known problems/issues with this manufacturer I should consider?
I know these are a lot of questions, but I'd appreciate your thoughts - thanks!
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- schrader7032
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I'm helping a friend
I would imagine that with the increased power of the R69S, the power pulses through the crankshaft would clearly not be damped by such a lightened flywheel...so running vibrations would be more noticeable. I don't think my friend had issues with starting his bike, but you bring up a good point on the heftier R69S.
I wouldn't want to use that kind of arrangement on any of my bikes.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
- jwonder
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R69S light flywheel
It is no-where as light as yours!!!
That being said, if I ever get back into my R69S engine I will most likely put a R50/2, non-lightened one in. Starting my R69S when hot is a real problem if I do not get the engine up on compression and the kick just right. Its an old-knees and amount of rotation scenario. I believe that the extra flywheel weight would help with the rotation of the crank during hot starts.
Hope this helps.
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York
- schrader7032
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I'm not sure the non-R69S
Other info from Tim Stafford, a notable restorer, posted that the "late" flywheels were about 9lbs while the "early" flywheels were around 13lbs. I haven't looked in the Barrington manual to see if they document this.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
- wa1nca
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Maybe you'd have to scrap the magneto and run electronic ignitio
If you have a kick back then the timing needs to be retarded
I have the original r69s flywheel with the MZB system and removed it because the timing was erratic while starting and removed it and installed the original magneto and easy starting again
Their 12 volt charging system works well
Tommy
54 R51/3, 55 R50/Velorex 560 sidecar, 64 R27, 68 R69US, 75 R75/6
Ashfield, Ma
USA
- schrader7032
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I'd always heard that sidecar
Duane Ausherman has some catalog shots of Bowman products...the flywheel is shown:
https://w6rec.com/bowman-products/
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
Great info...
The Barrington guide makes the statement (page 361, 4th ed.) that "the stock flywheel weighs approximately 12 pounds" in reference to all R50-69US bikes. No differentiation of early vs. late given, either. They further state that a Bowman flywheel weighs about 4.25lbs. Additionally, the Salis listing here for a "sports/aluminum" version on the R69S weighs about the same at 1.95kg (4.3lbs). These are both lighter than what I've got already...how one would reasonably and reliably kick over a high-compression engine with this minimal weight is rather perplexing. Maybe you'd have to scrap the magneto and run electronic ignition? Substantially retard the timing?
One further piece to add in hindsight is that this bike is a sidecar rig. The PO said that the stock rear-end gearing was still in place; I have since taken measurements and it seems to be around 3.13:1. Not sure if any of this matters, though.
- jwonder
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Flywheels
From the weights in the picture, the R69S flywheel is 9 pounds and the regular one is 13.2 pounds.
Thank you!
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York
Great photos and additional info.
Tommy, your point is well taken too. As it happens, I have the VAPE charging setup on this bike already, and that's one thing that was working pretty well before the tear down. There were remains of electronic ignition components still installed under the tank but were disconnected and disabled, as the PO said that the bike was going to full advance on start up when he had it installed; after trying to relate this fact to the folks at Powerdynamo to address the situation and not making any headway, he resorted to going back to the standard mag setup. I have no idea if he mentioned that the bike had a lighter flywheel to them, though. Yet another change on this bike per the ignition system is that it's been modified to run dual plugs per cylinder, but I'll save that issue and my questions regarding it for another post
The flywheel was properly indexed (at least per the machined keys, ways, etc.) upon disassembly, and there was no sign that anything had shifted or spun on the crank. I can't rule out the inaccuracy of the casting/stamping of the wheel itself, however, which was why I was curious if anyone had experience with a 'Cycleworks' flywheel (if that is indeed what I have) and could attest to their accuracy or not. Until then, it's great to see photos of what it originally had and what the standard, heavier/larger unit is by comparison - thanks for posting these pictures, Mal.
It sounds like I'll probably need to be hunting for a 9lb. flywheel in the meantime...I really appreciate everyone's input and thoughts!
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Jeff, To your question about
To your question about experience actually riding a bike with a lightened flywheel, do it NOW! For normal around town riding, it is one of the best improvements you can make to a post war Plunger or Earls fork bike. It will start just fine assuming it is well tuned (you shouldn't notice any difference), it will shift better (the engine will match speeds with the transmission much faster) and it will rev quicker. Been screwing with these things for over 40 years with 100's of thousands of miles on an assortment of 50's and 60's era BMWs. Rode my '69 R69US back and forth to OH from AZ several times back in the 70's with a Bowman installed and no issues at all. Just finished an R50S with a Bowman in it and it is sweet. The only reason I can think of to leave a heavy flywheel wheel in is if you are carrying extremely heavy loads (like a sidecar) where the extra inertia can be helpful for starting from a dead stop. Just my two cents.
Be safe and enjoy the ride.
Chuck S.
Chuck S