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1973 BMW R75/5 SWB versus LWB

b25bsaboy

b25bsaboy

New member
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.
 
Depends on your intended use of it. If you are going to load the back end of it and travel long distances, you might prefer a long wheelbase for increased stability. The parts are readily available and include the swingarm, driveshaft, rear sub-frame, side covers, and seat. If you are using it for lightly loaded sport and recreation riding, I personally think it is more fun with the SWB. Most of the horror stories about the handling are either highly aft-loaded bikes, or the front end is not properly set up. It takes some work to do a rigorous front end setup on one, but it is nothing but time and the details are widely available for how to do it. I test rode one and the steering shook strongly when off-throttle. The seller told me that all SWB's did that. He is wrong.

For contrast, there is a fairly famous SWB in the museum at Bob's BMW with over 600,000 miles on it - all of them loaded to the gills for travel.
 
Rick, just to help identify what you have ("... so I'm told."), here's a picture (thank you Google) of the difference - look for the "extension" weld 2" forward of the flange where it bolts up to the rear drive. I thought that '73s all had the longer arm & larger battery, maybe not (mine did).
newvsold.jpg
 
Not 100% sure, but I think that all long wheel base /5's came with the small battery box set-up. I've converted a couple of long frame /5's to the large battery box set up, back in the day. :)
 
Pauls1150 said:
Rick, just to help identify what you have ("... so I'm told."), here's a picture (thank you Google) of the difference - look for the "extension" weld 2" forward of the flange where it bolts up to the rear drive. I thought that '73s all had the longer arm & larger battery, maybe not (mine did).
View attachment 69967
Click to expand...

That picture shows a /2 swing arm modified to use a later model rear drive and doesn't actually portray the difference between a short and long wheelbase /5 swing arm.
I don't know where the myth about LWB 73 models coming with a larger battery came from but it is just that, a myth. All /5s came with the same battery brackets and the smaller battery!

So if you must change from short to long then you'll need the long swing arm with driveshaft, a long sub frame that can either be a /6 or /7 as well as the correct /5 item. The side covers are the same for both but the holding tabs on the sub frame are in a different location on the /6 and later models and they do not line up correctly with the covers. If you look in the parts book you will se a pair of longer bolts and positioning tubes at the bottom front of the fender.
An original /5 LWB sub frame can be distinguished from a /6 and later by looking at the cross braces at the front of the sub frame. The /6 and later will have a stamped flat plate cross brace. Using this will also require a /6 or later tool tray that will be long enough to span the gap. The actual /5 piece will have two tubes across the sub frame. The rear tube will have the tabs for the rear fender bolts. The forward tube will be in position to support the /5 tool tray that is a bit shorter than the later models.
 
I had a SWB R75/5 in the early 70's and used it for fully loaded long distance travel at high speeds on the Autobahn
and never had any stability problems. Solid as a rock.
On curvy roads too, significantly better than Jap bikes at that time.
I now have a LWB R75/5 but can not comment on high speed. No Autobahn here.
I would not go through the trouble of conversion.
 
b25bsaboy said:
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.
Click to expand...

Good evening everyone,

Well got the R75/5 back to the shop and this Queen B has done well after being in storage for 20 some years. I was surprised to see the internals of the fuel tank looking none worse for wear. The frame, fenders, side panels and fuel thank look respectable for an old gale of 45 years.

Check the tell tail sig of the rear of the drive shaft and yes indeed I have a SW. Will start checking what I have to see if I can in fact convert to a LW or if I really want too!

All in all I am tickled to have another project to work on.
 
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.
 
Na Cl K9 said:
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.
Click to expand...

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!:banghead:thumb
 
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
 
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.
 
b25bsaboy said:
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.
Click to expand...

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. :twirl:dance:dance:dance

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!
 
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