Final Drive

[schrader7032]

Rob -

You mention R69...do you mean R69S?? I wonder if some of the issue is with the transmission as well. I believe that you can couple a final drive with the proper tranny to get things right for running a sidecar. Seems to me that with these bikes, if you run a sidecar with the appropriate tranny/final drive and then decide to take the sidecar off, I suspect the bike wouldn't be much fun. I'm not that knowledgeable on that stuff but I'm thinking you can't have your cake and eat it too.

Duane has an owner's manual on his website at http://w6rec.com/duane/bmw/2man/index.htm. Look on page 28. You'll see some information which is more or less the standards as far as tranny ratios and final drive ratios. I can't really help with whether a later /5 final drive would work or not.

Kurt in S.A.

[Allan.Atherton]

... .Does anyone know if a later mdl. final drive, [/5 or?] would fit on a /2? ... I'm looking for a lower ratio for sidecar use,... had a 26/6 which I tried for a bit but it seemed way too low for general use... I don't think a /5 final drive will work on a /2. When a /2 is converted to /5 engine and transmission, the /2 swing arm and final drive are retained.

Perhaps you are asking if a /5 final drive gearset will work in a /2 final drive. Again, I don't think so because I have put together two /2 sidecar outfits over the past 20 years and I never heard of that option.

You don't say what sidecar you have, or what you carry, or what your roads are like. The 6/26 is the factory gearset for your R69S to pull a large sidecar with windshield and passenger in hilly country. If you feel it was too low, then the alternative is the 7/27 which was the factory gearset for the R60/2 pulling a large sidecar.

It always seemed to me that the R69S engine was less tractable than the R60/2, so while the R60/2 could handle a small sidecar without passenger using solo gearing, the R69S could not. I had both bikes, and both large and small sidecars.

[EuroIron]

aren't some of the /2 conversion bikes done with the /5 final drive upgrade?

pretty sure I've seen some

I am thinking Carlo, or one of his friends has one with a /5 final drive, they are stronger

however, you can change the internal ratios of the transmission

4.17 / 2.73 / 1.94 / 1.54

these are likely what you have for ratios

these next ratios are easily possible and will result in a "deep reduction" or "granny low" first gear

which will go a long way towards getting your heavy rig rolling from a stop


5.33 / 3.02 / 2.04 / 1.54

bigger jumps between the gears and does tend to make the machine less sporting when solo

but it will make moving out from a stop much easier and since top gear is the same, you won't have the engine spinning up 7K at 60 mph, and I'm just speaking in general terms about that last part which can easily happen if you change the final drive to one having a very low ratio

[tricyclerob]

I have a R69, not the"S" model. I think the only difference between the R69 and the R69S is lower compression,[and less horsepwer]. I pull a Ural sidecar which is on the heavy side, about 1/2 the time with a passenger.
I have a 25/8 or a 3.13 ratio in it now and it seems to lug a bit on hills. I think something a little lower would work well.
The 27/7[3.86] was used w/sidecars, but i've really been looking for a 27/8 [3.375] or a 32/9 [3.55] but you do not see those ratios often.
I see ratios in the later mdl's that would suit, and they look so dagone similar! rj

[schrader7032]

In one of my vintage motorcycle magazines, there's a list of bikes with typical final drive ratios...not definitive but good information. For the R69 solo bike, it lists the 25:8. For the R69 with sidecar, it lists the 25:6. Seems like you're running around with the solo FD...does look like you need a different ratio with the sidecar. The speedo unit with the 25:8 is supposed to be W=1.2 and 1.6 with the sidecar final drive.

Kurt in S.A.

[tricyclerob]

With the 26/6 that I had, it felt like I was reving the heck out of the engine even at 50 or so. I think that rear and the 25/6 was meant for riding in the Alps, with a passenger, and luggage, and a kid on the pillon...
The car seems set-up well as it tracks nicely and is comfortable at 50-55, even 60. I hate to lose that capability by going too low, as here in the N.E. on a lot of roads, you need to run along at a 50ish pace to not get run over.
The 25/6 is a 4.16, which I think is probably a little lower than I need. It's not terrible w/the solo drive, but a long hill and w/a pass. you can really tell it needs a bit lower gear.
The s/c trans w/a lower 1st sounds great, but I do ride the bike solo as well.
I think the 27/8 might be just enough to be a good compromise, although the 32/9 [3.55] might be better. I just don't want to lose too much on top. rj
P.S. Anyone have any final drives gathering dust on the shelf???? Please PM me

[schrader7032]

Rob -

Try using Anton's graphical calculator to get an idea of RPMs, etc., for different final drives. http://www.largiader.com/articles/gearing/

Not sure if it's got all the ratios you're interested it; plus you can't really change the tranny ratios. Follow some of the links on his site...maybe you can use the math on Snowbum's site and create your own best combination.

Kurt in S.A.

[tricyclerob]

Kurt, thanks for that link. Maybe another dumb question, but having never had a tach on my r69, what is a "crusing" RPM these motors are comfortable with? rj

[schrader7032]

Rob -

I also don't have a tach on my R69S. But I used the information on Snowbum's site on gearing to set up a spreadsheet for computing RPM at various speeds. My bike feels pretty comforable around 60-65 mph range. The spreadsheet gives me around the 4000 to 4200 at those speeds.

Kurt in S.A.

[Allan.Atherton]

I have a R69... the only difference between the R69 and the R69S is... and less horsepwer]. I pull a Ural sidecar which is on the heavy side, about 1/2 the time with a passenger. I have a 25/8... now and it seems to lug a bit on hills. I think something a little lower would work well. The 27/7[3.86] was used w/sidecars, but i've really been looking for a 27/8 [3.375] or a 32/9 [3.55] but you do not see those ratios often....
I had a 6/26 in an R69S/Ural with 42 HP.
http://home.insightbb.com/~aatherton/R69S/R69S.html
It is one of the heaviest sidecars, and I think you will be sorry if you don't use the 6/26 or at least the 27/7 with your less-powerful R69.

By "lug" I think you just mean "bog down" -- lose rpms and speed so that you have to downshift. You are using the 25/8 pre-1968 solo ratio now, which cannot give you enough power to use 4th gear much. The 27/8 is the solo ratio for all the 1968-69 bikes, but it gives only a little more advantage. I have used them all.