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Thread: 1973 BMW R75/5 SWB versus LWB

  1. #16
    Registered User b25bsaboy's Avatar
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    Quote Originally Posted by Na Cl K9 View Post
    If you haven't ridden a /5 short wheel base bike then you owe it to yourself to take time to experience the bike as it was designed. Important items to service or are: Fork assembly- clean and service including steering bearing service and springs, swing arm bearings, new or rebuilt shocks, 'euro' style low bars or 's' bars -(the bike was designed to use low bars and they do help with overall control and enjoyment of the vehicle), wheel bearing service and good tires. That gives you the basic factory handling and hopefully proper tension on the bearings will provide what stability is available in stock trim.

    The 750 motor delivers some nice acceleration and the steering angle helps hold a true course on the smooth. I've built them light and tight and have really enjoyed the ride around town and on short country runs. These bikes are what you make of them... you can make a long wheel base bike using a box of /6 parts. You'll be stuck with the /5 battery box unless you can weld and the tail light wires will be too short... and it will be more comfortable. This combination is a hybrid because the wheel base of a /5 lengthened in this manner is longer by about an inch than a standard /6 due to the steering head angle of the /5 being kicked out a bit more.

    If your 73 /5 is actually a long wheel base 73-1/2 /5 then, it will have a drive shaft housing pictured above with the welded in spacer to lengthen it as its primary identifier. Curiously, attempting to take a 73-1/2 long wheel base /5 back to a short wheel base model requires the exact sub-frame from that bike. The rear fender will not fit if any other short sub-frame is used.
    John,

    Thank you for your words of wisdom! Was happily going with the thought of doing the SWB, but as life has a rude way hitting one blind sided

    Was out to the dint Doctor yesterday to get some dints taken out of the Toaster tank and stopped at a clients shop as he called as he has several motorcycles his customer needs for insurance appraisals. While talking to him, I saw on one of the bike lifts a pile of BMW RS parts. Asked him about the parts as this came from a bobber project that the shop did a while back. A price was agreed and I couldn’t get to the bank fast enough. The missing items were all there to convert the SWB to a LWB such as the rear frame, fender with all the signal/tail light, battery carrier and a plastic bin full of items including the wiring harness. I have all the other items such as swing arm and final drive from another find several ago.

    Guess I will have now really give this some serious thought as to what avenue I want to go this winter!
    Rick MacPherson
    Success is Not a Destination, But a Journey.
    Accredited Motorcycle Appraiser
    1968 BSA Starfire, 73 R75/5 SWB, 2008 Honda GL-1800 Lehman Trike

  2. #17
    Liaison 20774's Avatar
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    Quote Originally Posted by b25bsaboy View Post
    Thank you for the response and where can one find a copy of Matt Parkhouses article?
    For such an old article, it won't be online. I'd suggest checking with the office to purchase a back issue.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #18
    #4869 DennisDarrow's Avatar
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    Decisions Decisions...……..Such a dilemma...…..

    Being one of those that did have the tank slapping high speed wobble often described and talked about......SCARY to say the least!!! Duane Asherman wrote of this back in the day, on his web sight. It was and is, if you can find it, an in depth article as to how to set up the front end and eliminate this problem. Yes, I had one of those heavy Avon full fairings, saddle bags, and one of the Wixom luggage rack boxes...…..on a concrete highway with segment striping that made for a heavy rhythm, 80 or so MPH and FLIP FLOP, 100 YARDS OF PURE SCARY, and clean out the drawers...………..Oh yes, had just put on a set of Dunlop tires with a very triangular pattern on the front.

    Anyway, do give the SWB a chance for a season. You can always sell the parts for triple of what you have paid.....and off you go...Anyway, good luck and God bless...…..Dennis

  4. #19
    John D'oh
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    If I were to suggest one change to your /5 that would be to do away with the fragile wiring and headlight switch arrangement. Since the 'RS parts stash' that came your way has a wiring harness, perhaps it's connected to a good headlight. That would be a perfect addition and all the important wires from the RS will find homes on your /5. The RS tail light harness will be long enough to reach the tail light too. I might have stretched a few /5's over the years but really, when the long wheel base bikes came to market the /5's were doomed. I haven't worked on anyone's /5 but my own in 30 years now and that's just to make short wheel base cafe and scramblers out of them. The /5 BMW motorcycle has the same wheel base as a Buell Lightening.
    John D'oh

  5. #20
    Registered User b25bsaboy's Avatar
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    Quote Originally Posted by b25bsaboy View Post
    Good evening to you all!

    Well I did it again and purchased a new project and it is a SWB 1973 BMW R75/5 so I’m told. If in fact this is a short wheel base, how difficult is it to make it a LWB or can it be done. It not to say that I would but I have the parts that are needed except for the frame tail piece that I bought several years ago. These parts came from an R 75/6 I believe.

    Just thought I would float the question to see what the wisdom says.

    I am looking forward to diving into this project that has been sitting in a gentleman’s garage for the past twenty years. Will forward some photos once they become available.
    Put a PC680 Oddessy battery in the bike and its alive! Everything works except the thing that goes Beep/Beep! No worries as this is a good sign.

    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?

    My original question was the differences between the LWB and SWB and a number of people have indicated keep the SWB and so says you all. Will start the strip down later this week and let the fun begin!
    Rick MacPherson
    Success is Not a Destination, But a Journey.
    Accredited Motorcycle Appraiser
    1968 BSA Starfire, 73 R75/5 SWB, 2008 Honda GL-1800 Lehman Trike

  6. #21
    Liaison 20774's Avatar
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    Quote Originally Posted by b25bsaboy View Post
    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??
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  7. #22
    rsbeemer 22600's Avatar
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    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.
    1978 R100rs MOA#22600 125cc Kymco , 180cc Kymco Racing King
    The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.

  8. #23
    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.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
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  9. #24
    John D'oh
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    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
    John D'oh

  10. #25
    John D'oh
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    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.
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    John D'oh

  11. #26
    Registered User barryg's Avatar
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    Quote Originally Posted by 22600 View Post
    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.

  12. #27
    Registered User amiles's Avatar
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    Recollection

    My feeble memory suggests that handlebar mounted fairings & heavily loaded rear ends were best avoided.

  13. #28
    Registered User barryg's Avatar
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    Quote Originally Posted by Na Cl K9 View Post
    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.

  14. #29
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    Matt’s article

    Quote Originally Posted by b25bsaboy View Post
    Morning Kurt,

    Thank you for the response and where can one find a copy of Matt Parkhouses article?
    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..
    "Fools that we are, we have gone to war with nature. If we win, we die.". Silver Donald Cameron

  15. #30
    Registered User b25bsaboy's Avatar
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    Quote Originally Posted by 81twins View Post
    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!
    Rick MacPherson
    Success is Not a Destination, But a Journey.
    Accredited Motorcycle Appraiser
    1968 BSA Starfire, 73 R75/5 SWB, 2008 Honda GL-1800 Lehman Trike

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