• Welcome, Guest! We hope you enjoy the excellent technical knowledge, event information and discussions that the BMW MOA forum provides. Some forum content will be hidden from you if you remain logged out. If you want to view all content, please click the 'Log in' button above and enter your BMW MOA username and password.

    If you are not an MOA member, why not take the time to join the club, so you can enjoy posting on the forum, the BMW Owners News magazine, and all of the discounts and benefits the BMW MOA offers?

1973 BMW R75/5 SWB versus LWB

One question in that there is a second red wire that has to be hooked up to the battery as well with the starter lead which ends up at the key switch. I don't see it in any wiring drawings thus can some one enlighten me?

Ahhh, the curse of the PO shows its head!! The Haynes diagram only shows one battery red wire going to the starter relay. So, you'll have to trace the wire to the other end...you said key switch...and figure out what's going on.

In my limited understanding, as I said, a red wire goes to the starter relay from the battery. From the starter relay, it shows two other red wires, one to the diode board and one to the ignition switch. So, I'm wondering if there's a problem with the red wire from the relay to the ignition switch and the PO decided to "fix" that problem by just running a separate red wire?? :dunno
 
High Speed Wobble??

Well if you say so...but in about '7i or 72' I bought my first bmw, a '70 swb r75/5. Rode it all over the States and Mexico in the seventies and never a wobble I knew of, but I was young and always had a girl back there holding it down. Best bike ever and one that you could kick start; how sweet it was.
 
I have to tell this story. After Voni rode her R60/5 for a few years, she got an R65 and I took her R65/5.

We were headed to Seattle or Olympia, Washington, or some such from Kansas. In Idaho or Montana, on US 2 we kept seeing signs for fisherman's access to some silly river. I told Voni I would stop for coffee next time I saw a good pull off. I saw a sign and took the gravel drive. I was just getting the thermos out of a side case as she roared by. She missed the fact that I had pulled off. I stowed the coffee and tried to catch her. I had an R60/5 SWB trying to catch an R65. trying to catch me. I had a top box. I had a bag on the seat. I had bags strapped atop the Wixom saddle bags. She accelerated to try to catch me even though I was actually behind her. I sped up to try to catch her. I could reach a certain speed - about 85 mph - where the bag across the seat and the bags lashed atop the Wixom bags caused the whole bike to twitch. The back end was dancing and the front not quite tank slapping. Eventually she gave up and slowed down and I came up behind her, much to her surprise. I don't blame the bike, but for any bike - you need to figure out how it handles before you need to know how it really handles.
 
Tony Foale's book on Motorcycle handling and chassis design and the /5 short frame.

Foale's book is a technical discussion of motorcycle frame and suspension design and includes a section which concerns the BMW /5 chassis and a phenomenon known as "Squat" or the tendency of the shaft drive to cause the rear wheel to move away from the frame under acceleration. We recognize this as the rise and fall of the motorcycle as we throttle on and off. The /5 is a "high squat" design by calculation so that when the throttle is snapped open, the bike rises to the top of its front and rear suspension which in the case of the /5 the bike is nearly level. When the rear suspension 'tops out', it becomes in effect a solid rod. At the same time, the force of torque on the swing arm overcomes the weight of the bike/rider/passenger/gear and then loads the rear tire causing it to compress (this is 'squat') - tires do offer some dampening. This sudden tire compression and a suspension now reduced to a solid rod, is the precursor of wheel hop and that is the principle problem with /5 handling. Wheel hop and the subsequent uncontrolled movement of the rear wheel imparts inertia to the frame and causes the head shake phenomenon - among other things.

When the load you add to the frame is biased to the rear, it exacerbates the problem by adding a 'pendulum' to the bike which swings one way or the other as the bike hops over one bump and bounces off the next due to suspension packing. Each time the pendulum swings it imparts a moment of inertia longitudinally to the frame and this induces the head shake. When BMW lengthened the swing arm, "Squat" was reduced by %85 as calculated by Foale and that reduced the tendency to produce head shake. As most later /6 and /7 drivers will know, it didn't completely get rid of head shake however.

The most exciting moments I've experienced on a short frame were related to pavement ripples at freeway speeds in clear weather on dry pavement with no saddlebags or top case. After a series of gyrations brought on by the first set of ripples put me a touch sideways at 70mph, I got the bike straightened up by opening the throttle again which usually gets you out of trouble but, sent the suspension to its solid top again and the next bump I hit sent me into the air sideways the other direction for maybe 30 feet. I know I was airborne because the whole bike shook when I touched down. After that, I was still on the highway pointed generally in the right direction, the bike made a couple of light oscillations and that was it.

My first response to you was to suggest that service to the chassis and suspension was important which it is but it will not alleviate the high squat design factor. Only riding style and awareness of the problem will keep you safe on a /5. Those that say they have had no handling problems have but probably didn't realize it because, well, face it - /5's are slugs. Cool slugs but slugs none the less. They are actually the best post apocalypse transportation one could own. No transistors, runs on drip gas, part all over the world. I seriously digress :) my apologies to M. Parkhouse. Mr. /5
 
Addendum -

This is the bike I took out and went sideways in the air with. In the end, I remember distinctly emerging with a smile on my face because through that 100 feet or so I was actually in control as the bike moved beneath me. As I said, riding style. Here is a 1973 1/2 main frame like yours perhaps ( however, with visible and hidden gussets added). The sub frame is from a 1970 /5 as is the short swing arm. The front fork is from a 1978 R100S. The engine and transmission (which happen to be from an 83 R100) represent the same weight and mass as a /5 750. But this combo is a hybrid as I mentioned yours coulld be. Look closely at the rear fender. Note the arc at the top of this trimmed BMW rear fender from tail light to where it intersects just above the center of the /5 battery. A stock /5 or /6 rear fender - which are the same, no longer fit! This one is both quick turning and fast. In addition to chassis/bearing/wheel service they need good tires. I believe the steering head angle of the /6 may be narrower than a /5 which means that this combination would produce the shortest wheel base BMW possible without using a saw. I ran out of room and sold it.
 

Attachments

  • cafe.jpg
    cafe.jpg
    60.5 KB · Views: 139
Well if you say so...but in about '7i or 72' I bought my first bmw, a '70 swb r75/5. Rode it all over the States and Mexico in the seventies and never a wobble I knew of, but I was young and always had a girl back there holding it down. Best bike ever and one that you could kick start; how sweet it was.

I had a '71 R50/5 for 30 years and I could never get it fast enough to go into a hi speed wobble. Oh, well. :dunno:scratch
 
Recollection

My feeble memory suggests that handlebar mounted fairings & heavily loaded rear ends were best avoided.
 
This is the bike I took out and went sideways in the air with. In the end, I remember distinctly emerging with a smile on my face because through that 100 feet or so I was actually in control as the bike moved beneath me. As I said, riding style. Here is a 1973 1/2 main frame like yours perhaps ( however, with visible and hidden gussets added). The sub frame is from a 1970 /5 as is the short swing arm. The front fork is from a 1978 R100S. The engine and transmission (which happen to be from an 83 R100) represent the same weight and mass as a /5 750. But this combo is a hybrid as I mentioned yours coulld be. Look closely at the rear fender. Note the arc at the top of this trimmed BMW rear fender from tail light to where it intersects just above the center of the /5 battery. A stock /5 or /6 rear fender - which are the same, no longer fit! This one is both quick turning and fast. In addition to chassis/bearing/wheel service they need good tires. I believe the steering head angle of the /6 may be narrower than a /5 which means that this combination would produce the shortest wheel base BMW possible without using a saw. I ran out of room and sold it.

That is one cool looking custom BMW. :thumb
 
Hi Rick.. I can photocopy the article later in the week and get it to you if you want to ride out for a coffee..

Thanks Al, I got the photocopied article in the mail the other day. Interesting read and I wish it could somehow be posted here as the comments are invaluable.

After much thought and to what the end game is going to be when I am done with restoring the R 75/5, it looks like I will stay the coarse in leaving the R75/5 a SWB machine. Besides, the more I look at her, the prettier she’s becoming!
 
Back
Top