Diesel Motorcycles - BMW Conversions / BMW Components

[oilburner]

BMW Conversions
Several BMW motorcycles are known to have been successfully converted to multicylinder diesel power. The builders are to be congratulated as this is no small task, particularly with the 1970 - 1995 aircooled boxer twin models.

Here are links to pictures and information for several BMW conversions on the the German Dieselkrad website:

BMW R45 with Lombardini LDW 602 two cylinder
http://www.dieselkrad.info/index.php?ma ... high&id=37

BMW R80RT with Daihatsu three cylinder
http://www.dieselkrad.info/index.php?ma ... high&id=14

BMW R100 with Yanmar TN72 three cylinder
Note: This bike has a new owner and has been extensively updated; pictures may be found elsewhere on this forum
http://www.dieselkrad.info/index.php?ma ... high&id=16

BMW R80 with Smart car engine
http://www.dieselkrad.info/index.php?ma ... high&id=58

BMW K100 with Daihatsu three cylinder (?) turbo diesel car engine
http://www.dieselkrad.info/index.php?ma ... high&id=12
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BMW Conversion - Construction Considerations - Post 1969 aircooled boxer frames

The stock 1970 - 1995 BMW frame will accommodate engines approximately 11 inches long and 15 inches between the backbone and the frame loop. Engines longer and taller will require major frame surgery. The severe height limitation is due to the unique frame design - the downtubes and backbone attachment to the steering head is reversed compared to most double loop frames; the backbone attaches to the bottom of the steering head. The Daihatsu 953cc normally aspirated industrial engines are 15.5 inches in length, require 15.5 inches between above the bottom of the frame loops, and about 12 inches width between the loops. The picture of Dave Hubbard's Daihatsu conversion (first link) shows the extensive modifications required compared to the stock frame (second link):
http://www.dieselkrad.info/pic.php?bild ... torr%E4der
http://maxbmwmotorcycles.com/fiche/Diag ... 000913.png

Stay away from 1970 - 1975 frames. The 1970 - 1973 frames were especially light and flexed a lot, garnering them the name gummikuh (rubber cow). The 1976 frames are somewhat better, the backbone is double (two tubes, one inside the other. The best frames are the 1977 to approximately 1980. The downtube wall thickness is 3/16 inch, compared to all other frames which have downtubes half as thick - 3/32 inches.

FWIW I think you will find it significantly easier to use a frame from another motorcycle e.g. Honda Goldwing, Dnepr etc. and use BMW components.

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BMW Components

Clutches
1970 to 1980 bikes have an automotive type flywheel. The clutch and diaphragm spring assembly can easily be adapted to the Daihatsu flywheel. 1981 and later bikes have a clutch basket. The clutches are the same diameter, but are different (thickness). The clutch basket requires more extensive flywheel modifications or a custom flywheel. The type of clutch used is important in gearbox selection (see below).

Gearboxes

1970 - 1973 1/2 bikes have a 4-speed gearbox. The stock 4-speed gearbox has an overall ratio of 1.5:1 in top gear. This gearbox can be modified ($$$) to 1:1 in top gear by relocating the countershaft and making a new primary input gear. With this modification, a theoretical top speed of 95.6 mph at 3600 rpm is possible with enough horsepower. The downside to this modification is that all gears are taller. You want to be hanging on when you release the clutch in 1st gear - 11 mph at 1200 rpm (Daihatsu idle speed). A wide choice of final drive ratios makes it possible to match the engine horsepower to the modified gearbox for optimum performance.

1973 1/2 - 1980 bikes have a 5-speed gearbox and the flywheel type clutch. The input shaft of these gearboxes has longer splines than the 1981 and later gearboxes, which have the clutch basket setup. The 5-speed gearboxes also have an overall ratio of 1.5:1 in top gear. This gearbox cannot be modified like the 4-speed due to its different design. However, a taller 5th gear is available from several suppliers. Most of these gears were made by Wolfgang Kayser of Germany who is now retired. They lower the 5th gear ratio to 1.42:1 or an increase of about 4 mph top speed (67.4 at 3600 rpm). At one time there was a 1.35:1 5th gearset (2 gears) which to my knowledge is no longer available (top speed about 71 mph at 3600 rpm). If anyone knows of one available, please let me know! My custom 5th gear was purchased from Sibenrock. The tooth angle on 5th gear changed from 15 to 17.5 degrees during the 1981 model year. The 17.5 degree gears are marked with an X. If your gearbox is from a 1981 bike, you will have to disassemble the gearbox to determine which gear is used in your gearbox before ordering.

The 1970 - 1995 aircooled boxer 4-speed and 5-speed gearboxes can be adapted to the Daihatsu engine via a conventional automotive style bellhousing. Here is a link to a picture of the input shaft side of the 5-speed BMW gearbox (4-speed similar):
http://maxbmwmotorcycles.com/fiche/Diag ... 000498.png


Other BMW Gearboxes

Oilhead bikes - 1993 - Present
Gearboxes used in the 1993 to 2007 oilcooled boxers have an integral bellhousing which means it's likely necessary to machine a custom flywheel. One concern might be the weight (or lack thereof) of the new flywheel which will be smaller than the original Daihatsu flywheel. Note that contrary to what you might expect, the overall ratio of all oilhead 5-speed and 6-speed gearboxes is about 1.47:1 in top gear. The clutch diameter is exactly the same as the 1970 - 1980 aircooled boxers, which means you can use the oilhead gearbox and the early clutch setup with the Daihatsu flywheel. Oilhead gearbox:
http://maxbmwmotorcycles.com/fiche/Diag ... 004572.png

Example - Gearbox.......... 6-speed manual
Primary Transmission ratio. 1.823
Gear Ratios I...............2.277
II......................... 1.583
III.........................1.259
IV......................... 1.033
V.......................... 0.903
VI......................... 0.805 (1.823 X .805 = 1.4675)

K Series - 1984 - 2006
The K bike gearboxes should work with the Daihatsu using a conventional engine mounted bellhousing. The K series clutch is 165mm in diameter, requiring use of the K series clutch basket assembly and likely a custom flywheel. K series gearbox:
http://maxbmwmotorcycles.com/fiche/Diag ... 002145.png

Final Drives

Numerous final drive ratios were used on the aircooled boxers over the years. The 1970 - 1984 final drives use twin shock swing arms, 1985 - 1995 are for single-sided swingarms. When using a stock BMW gearbox, the 2.91 final drive is the practical choice, unless you are pulling a sidecar and a trailer. The available ratios are:
2.91, 3.00, 3.09, 3.20, 3.36, 3.44, 3.56, 3.89, 4.25
Some oilhead and K series bikes use 2.54:1, 2.75:1, and 2.81:1 ratios.These bikes have single-sided swingarms.
Note that each final drive ratio requires a different speedometer or a ratio adapter in the cable.


Mixing / Matching
Mixing the airhead, oilhead, and K bike gearboxes, swing arms, and final drives will require modification and/or adapters. Access to a machine shop is necessary for most all of the modifications discussed above.

FYI

Avery

[andrewaust]

Hi Avery



Thanks for posting all that up. I can see it being an enormous help to anyone thinking of using a BMW mount.

Them final drive ratios would be good to have a play with. Good to see BMW making multiple gear sets, as many other bikes stick to the one ratio. The 2.91 ratio would be good for most applications while the 2.54 would be great in a 3 cyl turbo engined bike. Adding a side car would need a play with the ratios a little more.

It's all fun!



Cheers


Andrew

[MNBill]

Avery,

WOW! Good stuff! Thank you for all of you time and effort!

Best regards,

Bill

[balboa_71]

Avery,
Even though I won't be able use the info you posted, I'd also like to thank you for posting the informative data regarding BMW's and their drive train details.
Most people don't have a clue.....

Cris

[Kenco1955]

Avery and all
Just found this site and had a play on it, seems a good tool and may be of use to all those struggling with gear ratios etc....

locost7.info/gearcalc.php

I like it cos it does all the maths for you (gotta be a good thing!!) and you can key in all the variables, press the go button, and hey presto, instant results!!
Hope it's of use
Cheers, Ken.

[Crazymanneil]

Great post avery, thanks!

Neil