If you like our site, please consider joining our club!
By joining you will help ensure that we can continue to provide this service
JOIN HERE!

Questions regarding lightweight flywheel on R69S

jtaylor
Posts: 20
Joined: Tue Jun 19, 2018 8:04 pm

Questions regarding lightweight flywheel on R69S

Post by jtaylor »

'Hope everyone is staying safe and well these past few weeks...

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!
Attachments
2-034_flywheel.jpg
2-034_flywheel.jpg (1.17 MiB) Viewed 2619 times
Jeff

User avatar
schrader7032
Posts: 9016
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX
Has thanked: 2 times
Been thanked: 29 times

I'm helping a friend

Post by schrader7032 »

I'm helping a friend reassemble his R60/2. We knew that he had a lightened flywheel. Turns out it weighed around 7lbs, so a bit more than yours. He sent the engine to Vech for overhaul. Vech refused to use the lightened flywheel and installed a used stock flywheel. I think that should tell you something.

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.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

User avatar
jwonder
Posts: 784
Joined: Wed Nov 23, 2016 7:50 pm
Location: Long Island, New York
Has thanked: 3 times
Been thanked: 7 times
Contact:

R69S light flywheel

Post by jwonder »

Jeff, the R69S came with a factory lightened flywheel that chopped a bit of weight off. I not sure how much. When I disassembled my 1967 R69S, which I know as a fact was never opened up after the factory built it, I was shocked to find a flywheel that was a bit different and seemed lighter than my others. I did a lot of research and it indeed was correct and I believe (someone correct me if I am wrong) that the R69, R50S and R69S all had this version of the flywheel. Salis lists the R69 and R69S as separate part numbers and my parts book (dated 1.10.1967) does not list one for the S models or the R69.

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.
James Wonder
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York

User avatar
schrader7032
Posts: 9016
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX
Has thanked: 2 times
Been thanked: 29 times

I'm not sure the non-R69S

Post by schrader7032 »

I'm not sure the non-R69S flywheels interchange. I have a post by Vech where it looks like he was comparing weights of R69 and R69S flywheels; he indicated the different between non-R69S and R69S flywheels is weight and diameter. He indicated that the non-R69S flywheels were nearly 13lbs while the R69S flywheels were a little over 9lbs.

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.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

User avatar
wa1nca
Posts: 1213
Joined: Thu Sep 27, 2012 5:15 pm
Location: Ashfield Ma
Has thanked: 6 times
Been thanked: 12 times

Maybe you'd have to scrap the magneto and run electronic ignitio

Post by wa1nca »

The Vape (MZB) ignition system clearly states not to be used with a light weight flywheel and advised against high compression single cylinder engines

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
Tommy Byrnes
54 R51/3, 55 R50/Velorex 560 sidecar, 64 R27, 68 R69US, 75 R75/6
Ashfield, Ma
USA

User avatar
schrader7032
Posts: 9016
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX
Has thanked: 2 times
Been thanked: 29 times

I'd always heard that sidecar

Post by schrader7032 »

I'd always heard that sidecar setups needed a heavier flywheel just to counter the added weight of the sidecar...engine being slowed by the extra drag.

Duane Ausherman has some catalog shots of Bowman products...the flywheel is shown:

https://w6rec.com/bowman-products/
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

jtaylor
Posts: 20
Joined: Tue Jun 19, 2018 8:04 pm

Great info...

Post by jtaylor »

Many thanks for posting these comments - all good information here. Regarding the above statement that Vech indicated the R69S flywheels were 9lbs, I'm curious if there are any other sources out there that would corroborate this specific number; however, it does jive with jambo's statement that these flywheels were at least lighter than standard. For now, I'll work off the premise that a stock R69S should have a flywheel somewhere around this figure...

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.

Jeff

User avatar
jwonder
Posts: 784
Joined: Wed Nov 23, 2016 7:50 pm
Location: Long Island, New York
Has thanked: 3 times
Been thanked: 7 times
Contact:

Flywheels

Post by jwonder »

The one on the left of the first picture is the stock, lightened R69S flywheel. The one on the right is the stock flywheel for all the other twins.

From the weights in the picture, the R69S flywheel is 9 pounds and the regular one is 13.2 pounds.

Thank you!
James Wonder
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York

jtaylor
Posts: 20
Joined: Tue Jun 19, 2018 8:04 pm

Great photos and additional info.

Post by jtaylor »

Kurt, what you said made sense to me, too...overcoming the additional weight of the sidecar would be easier with a little more heft, no?

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!

Jeff

sherman980
Posts: 149
Joined: Fri Oct 27, 2006 3:00 am
Has thanked: 1 time
Been thanked: 9 times

Jeff, To your question about

Post by sherman980 »

Jeff,
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.
Thanks.
Chuck S

Post Reply