Swap Frames on a 1980 R65 with 84?

[jjwithers]

I have a 1980 R65 that the previous owner hacked off the rear muffler hanger and foot peg parts of the frame.  They also did a few other questionable "modifications".  The tank was destroyed by his attempt to weld some sort of pop up gas cap into it.

The rest of the bike is OK (drive train, Engine, forks, swingarm, etc).

I also have a brand new 1984 frame from BMW.  A neighbor bought it as a spare frame and never used it.  It's more for the R80 / R100. 

I know the R65 has the relay mount under the tank but are there any other differences in the frames? Is any of the geometry different?  I am considering taking the triple tree, forks and everything and putting it on this new frame. 

Thanks in advance.

-Josh

[wilcom]

The frame part number  does not change from 1978-1985   

Part 46511240462 was found on the following vehicles:

R45-R65LS..78-85   (01/1978 — 07/1985)

[Tony Smith]

In spite of the P/n not changing BMW played tricks.

You already know about the electrics "tray" on the post 81 frame. This means that only a post 81 fuel tank will fit that frame.

Also the post 81 wiring harness was altered so as to use the "tray". You may be able to improvise the pre 81 sir my harness, but I'd suggest removing the tray and emulating the pre 81 bracketry. Of course you could buy an 81-85 wiring harness.

Either the 78-80 or 81-85 subframe will fit but you will need to use the countersunk hotels to mount asubframe to the post 81 frame.

Everything's else will fit OK.

[Barry]

...are there any other differences in the frames? Is any of the geometry different?  I am considering taking the triple tree, forks and everything and putting it on this new frame. 

Thanks in advance.

-Josh

The only difference in geometry I am aware of is the early bike has a slightly shorter wheelbase, I think due to a difference in the swinging arm length as the part number are different.  I would use the later forks as the damper valve was improved from the earliest models otherwise they are the same..

It's a long time since I produced the attached list of differences between the models which indicates differences in part numbers.  You can easily check them again to be sure.

[jjwithers]

Thanks Barry.

I'm more interested in the differences between R65's vs the other bikes from that era.

I suspected that the frames and forks would fit, but I was worried about safety / performance.   

I didn't know about the different sized swingam.  Is that why the shocks are slightly shorter for a R65 vs R100?

I also thought the triple tree had different geometry/offset as compared to a R100.  Or did I imagine that?

[Tony Smith]

Thanks Barry.

I'm more interested in the differences between R65's vs the other bikes from that era.



I didn't know about the different sized swingam.  Is that why the shocks are slightly shorter for a R65 vs R100?

I also thought the triple tree had different geometry/offset as compared to a R100.  Or did I imagine that?

After 1985 there is no frame difference

Prior to 1985 R65s got round frame tubes whereas the bigger bikes got oval tubes. The jury is out as to whether this means squat. My wife has run an R80 engine in her 1979 R65 for more than 30 years. I have run R100 engines in mine and it now has a healthy 860 fitted.

The difference betweem the up to 1985 front endsis that the R65 has a centrally mounted front wheel axle and the others had leadig axles.

The front stability of the R80/100 up to 1985 is a bit on the marginal side, reducing trail with a central axle might not work out well.

Both the swingarm and subframe of the R65 are shorter than their larger peers.

[Barry]

Sorry I misunderstood and thought you were comparing 80 and 84 frames from an R65.  I think you would have to do your own measurements on the later frame and swinging arm

R65 shocks are shorter by approx 15mm.