Modifying a BMW engine to drop into a Dnepr or Ural

[fencermatt]

[quote="mattcfish"
Why can't the Airhead tranny be used with a modified or custom built drive shaft?[/quote]

Because the transmission output from the BMW tranny spins the opposite direction from the Ural/Dnepr one. If you hook a BMW transmission to a Dnepr final drive you end up with 5 reverse gears. If you want to use a BMW transmission you also have to use a BMW final drive.

[fencermatt]

Hi Mike,

Thank you very much for the advice, I hadn't considered what issues could come up with the Dnepr pushrod. I actually read a lot of your old posts from when you were doing your conversion while I was getting ready for this project, and I will definitely get in touch if/when I run into problems.

Thanks!

[Major Softie]

Is the Ural Tranny bolt pattern the same as the Dnepr? I've got a friend with A Ural that has a crapped out motor. This could be the conversion for him.

No, it isn't. I'm having a welder move the bolt holes on the transmission to match the BMW's pattern, but some other people have made adapter plates. With the adapter plate you don't need to mess with welding up the transmission case, but you do need to lengthen the input shaft and shorten the drive-shaft. I'm not planning on switching back to the Dnepr engine, so I decided to modify the tranny.

Why can't the Airhead tranny be used with a modified or custom built drive shaft?

You will note in his original post that retaining reverse was one of the things he liked about retaining the Dnepr tranny.

Edit: Yeah, I guess that the rotation direction might just be important too.

[mattcfish]

[quote="mattcfish"
Why can't the Airhead tranny be used with a modified or custom built drive shaft?

Because the transmission output from the BMW tranny spins the opposite direction from the Ural/Dnepr one. If you hook a BMW transmission to a Dnepr final drive you end up with 5 reverse gears. If you want to use a BMW transmission you also have to use a BMW final drive.[/quote]

That's a good point.

[Motorhead]

Install american bearings and you'll get a faster and better tranny

[chasbmw]

Running an ex soviet sidecar rig, I doubt if you will the notice the relative smoothness of a R80 but you might wellmiss the extra torque of the R100. Good luck with it all

[Steveff69]

Hi Matt any chance I could see the photos you have of your conversion please. I’ve have too got a R80 to match with my dnepr and don’t know where to start as regards transmission

Steve flavell

[jackonz]

Response to Major softie's comment about "why an R80"...The R80 is "supposed' to be the best of the airhead engines as far as longivity and smooth running. I'm not too sure of the electronics, but the mechanical parts are reputed to be a lot sturdier than the R100 FOR THE DISPLACEMENT.
Around here, making an "RS" out of an R80 was considered the "cats pajamas" and there are/were quite a lot of them runnning around the Atlanta (GA) area in the 1980's.

I have a R80/7 1978 model and during the rebuild did consider upping it to 1000cc but with the speed limits in NZ and our twisty roads I decided to leave it as is, having finished it I am really happy with my decision it will easily break the 100kph speed limit and enough torque to pull up most inclines without having to down change gears, NZ roads in the South Island are motorcycle paradise with the west coast and further south being my favourite.

Phil J

[Rob Frankham]

Is the Ural Tranny bolt pattern the same as the Dnepr? I've got a friend with A Ural that has a crapped out motor. This could be the conversion for him.

No, it isn't. I'm having a welder move the bolt holes on the transmission to match the BMW's pattern, but some other people have made adapter plates. With the adapter plate you don't need to mess with welding up the transmission case, but you do need to lengthen the input shaft and shorten the drive-shaft. I'm not planning on switching back to the Dnepr engine, so I decided to modify the tranny.

Why can't the Airhead tranny be used with a modified or custom built drive shaft?

Because the driveshaft on Ural/Dniepr models (and the earlier BMW models that formed their basis) rotates in the opposite direction to that on a '69 on airhead. If you look at exploded diagrams of the final drives of both types, you will see that the crown gear on the 'ural' type drive is on the side of the pinion away from the wheel while that on the BMW drive is on the wheel side. To pouit it another way, the bike would go backwards...

I suppose it would be possible to modify the frame to take the later BMW drive but that would be quite a task and, in any case, you'd lose the drive to the sidecar wheel.

Rob