/5 /6 and /7 Airhead Design Blunders
Posted: Wed Mar 20, 2013 10:31 am
I reckon that title got your attention!
I don’t really know what might have prompted this post. But these thoughts have been brewing for quite a while now. For the record I regard airheads as fine motorcycles, certainly better than their contemporaries. But even fine motorcycles, perhaps like fine women (and men) have their flaws. See what you think about my impressions.
Caveat: some of my comments may only serve to reveal my ignorance, or simply lack of experience. Plus I was using Haynes and Clymer manuals which weren’t very good. So some of my criticisms may be totally unfounded. Responses to this post may be another learning process for me. But I reckon I will mirror frustrations y’all may have had dealing with the machines we like.
Rear Wheel Bearing Stack
The first time I had to replace a bearing in the rear wheel I paled when I saw what had to be done to change out and preload the bearing(s). Heating up the hub and pulling the stack was daunting. Plus, I imagined a lot of wear and tear on the hub. But then using a threaded rod and fabricated disks to hold the stack together so you can select the correct “wedding band” spacer by feeling how tight the outer drum was seemed at best not very reliable. I had little confidence that I had gotten it right after making my decision. But I did my best and hoped for the best. And I never had a failure.
I have to wonder, though, how many BMW mechanics were really adept replacing the rear wheel bearings.
(Note, since my first experiences I have learned a lot more about the bearing system. And the stack has to be pulled only if the drive side bearing is faulty or a suspect. And that bearing is in a better environment than the “car-wash” environment of the other side. Duane Ausherman’s writings were a bit of an epiphany for me. I learned that you can do a fine job of evaluating the preload of the bearings without pulling the stack along with other things.)
I guess the thing that bugs me the most about the “stack” is that with the advent of the snowflake rear wheel BMW decided that a stack wasn’t needed after all. And, of course, a stack never was needed for the front wheels.
Rear Wheel Splines
The splines mating the rear wheel with the final drive have vexed me ever since I changed my first rear tire. The main concern here is that the splines, both on the final drive and the rear wheel hub do wear. And renewing the splines on the final drive is an expensive proposition. The splines in the hub can be replaced with attention to what you’re doing. But why were there splines at all? Why would lug bolts not have worked well? They seem to work well enough on the monolevers.
I’m guessing the reason why splines were used in the first place was for competition. If you need to replace a wheel fast, that is in competition, splines are the thing to have. BMW was involved in racing and ISDT competitions.
7 Inch Headlight
I’m probably being picky on this one. I seem to remember in some of their airhead flyers BMW bragging about their 7 inch headlight. But is it better? The size of the reflector doesn’t seem all that important to me. It’s the bulb wattage, the quality of the reflective surface, and the quality of the lens that would seem most important. Plus a sealed beam 6" headlight would only require replacing, as opposed to replacing a dulled reflector. It would be so simple to just pop in a new unit.
Side Stands
I’m not harboring any grudge on the /5, /6, and /7 side stands. They work well for me. But I’m tall and long legged, so I can even deploy the side stand from the saddle (except for the last inch of travel.) But I think a rider with a shorter inseam has a problem. He must hold the bike up as he dismounts and then deploy the sidestand And if he’s riding a heavily packed bike, such as when Dwerbil came to the first rally on his /7, he has to hold the laden bike up as he struggles to get his right leg on the left side of the bike past the load. And then he can deploy the side stand.
(Getting along with the side stand on my ‘91RT Monolever was another story. I couldn’t get my toe on the damned tab from the saddle. It’s now wearing a Brown sidestand.)
Side stands do have a reputation for excessive wearing, but that hasn’t been an issue for me.
Perhaps the /5, /6, and /7 sidestand “issues” are why their center stands are so often worn through. That and riders carelessly “throwing” their bike onto the center stand, skidding it across the concrete.
Under The Tank Master Cylinder
I’ve never understood the under the tank front brake master cylinder. One of the main advantages of a hydraulic braking system is somewhat nullified by operating it with a cable. I’ve read speculation that BMW was concerned about passers by screwing around with the master cylinder. That’s as good of a reason as I can think of. But the long term issues with the under tank master cylinder can be dire. Out of sight is out of mind and most /6 and /7 bikes I’ve seen have had the master cylinder leak at one time (or more). That’s bad enough but the leaking brake fluid has a tendency to wick into the starter relay connector on the L/H side of the upper frame tube. On my /7 the engine and all electrics died. Luckily it was at my front gate rather than miles from home on some lonely Oklahoma byway. The problem was a corroded/mucky connection between the starter relay and it’s mating receptacle. As has been noted, the 12V positive red wire from the starter solenoid goes to one terminal of the starter relay connector, then thru the relay itself, and then from another terminal on the same connector to the headlight shell. (I’ve heard that BMW did it that way in order to build a less complicated wiring harness and the relay connector (with appropriate relay) was a good way to connect two sub harnesses together.)
5 Speed Transmission Shims, Dogs, and Kickers
I’ve not been inside a 5 speed transmission yet, so most of what I’m going to talk about came to me through osmosis. (Hi Duane) To be sure, overhauling or fixing a 5 speed transmission is a daunting proposition. Plus the 5 speed transmission isn’t very reliable in the first place. The kick start (on the few 5 speeds so equipped) simply isn’t reliable. And the transmission dogs are prone to breakage because they were not machined well. (I was heavy throttle torquing on an entrance ramp of a highway when suddenly my engine quit pulling and revved up. Yup, lost a gear because of broken dogs.) BMW, or rather BMW’s supplier should have done better. And BMW should have changed the specs given to the supplier.
The chief concern about going into the transmission is to get the shim stack right. There are tedious calculations to be made and you never know for sure if you’ve got the shim stacks right. I wonder how many transmissions have failed for that reason. And I doubt that one in a hundred of BMW dealers ever owned up to mis shimming the transmission.
So how should BMW have done it? That’s a good question. I’m thinking that on the back side of the transmission BMW should have had had caps on the back side of the three shafts. The covers could be removed and the bearings re-shimmed as needed. Or (this is even wilder) perhaps an inspection hole to the shim stacks could have been created where removing a screw would reveal the shim stack for evaluation. Neither of my ideas are very good, but the situation as it is, is a bitch.
Valve Recession.
The world wide conversion to unleaded fuel caused a lot of problems for a lot of manufacturers, BMW included. So I can’t really fault BMW for the valve seat recession problems that showed up. But what bugs me is that the fix fixed the valve seat recession and instead now the valve heads were tuliping. That bugs me.
Ba dip, ba dip, ba dip, that's all folks!
Ken, overly critical in OKlahoma
I don’t really know what might have prompted this post. But these thoughts have been brewing for quite a while now. For the record I regard airheads as fine motorcycles, certainly better than their contemporaries. But even fine motorcycles, perhaps like fine women (and men) have their flaws. See what you think about my impressions.
Caveat: some of my comments may only serve to reveal my ignorance, or simply lack of experience. Plus I was using Haynes and Clymer manuals which weren’t very good. So some of my criticisms may be totally unfounded. Responses to this post may be another learning process for me. But I reckon I will mirror frustrations y’all may have had dealing with the machines we like.
Rear Wheel Bearing Stack
The first time I had to replace a bearing in the rear wheel I paled when I saw what had to be done to change out and preload the bearing(s). Heating up the hub and pulling the stack was daunting. Plus, I imagined a lot of wear and tear on the hub. But then using a threaded rod and fabricated disks to hold the stack together so you can select the correct “wedding band” spacer by feeling how tight the outer drum was seemed at best not very reliable. I had little confidence that I had gotten it right after making my decision. But I did my best and hoped for the best. And I never had a failure.
I have to wonder, though, how many BMW mechanics were really adept replacing the rear wheel bearings.
(Note, since my first experiences I have learned a lot more about the bearing system. And the stack has to be pulled only if the drive side bearing is faulty or a suspect. And that bearing is in a better environment than the “car-wash” environment of the other side. Duane Ausherman’s writings were a bit of an epiphany for me. I learned that you can do a fine job of evaluating the preload of the bearings without pulling the stack along with other things.)
I guess the thing that bugs me the most about the “stack” is that with the advent of the snowflake rear wheel BMW decided that a stack wasn’t needed after all. And, of course, a stack never was needed for the front wheels.
Rear Wheel Splines
The splines mating the rear wheel with the final drive have vexed me ever since I changed my first rear tire. The main concern here is that the splines, both on the final drive and the rear wheel hub do wear. And renewing the splines on the final drive is an expensive proposition. The splines in the hub can be replaced with attention to what you’re doing. But why were there splines at all? Why would lug bolts not have worked well? They seem to work well enough on the monolevers.
I’m guessing the reason why splines were used in the first place was for competition. If you need to replace a wheel fast, that is in competition, splines are the thing to have. BMW was involved in racing and ISDT competitions.
7 Inch Headlight
I’m probably being picky on this one. I seem to remember in some of their airhead flyers BMW bragging about their 7 inch headlight. But is it better? The size of the reflector doesn’t seem all that important to me. It’s the bulb wattage, the quality of the reflective surface, and the quality of the lens that would seem most important. Plus a sealed beam 6" headlight would only require replacing, as opposed to replacing a dulled reflector. It would be so simple to just pop in a new unit.
Side Stands
I’m not harboring any grudge on the /5, /6, and /7 side stands. They work well for me. But I’m tall and long legged, so I can even deploy the side stand from the saddle (except for the last inch of travel.) But I think a rider with a shorter inseam has a problem. He must hold the bike up as he dismounts and then deploy the sidestand And if he’s riding a heavily packed bike, such as when Dwerbil came to the first rally on his /7, he has to hold the laden bike up as he struggles to get his right leg on the left side of the bike past the load. And then he can deploy the side stand.
(Getting along with the side stand on my ‘91RT Monolever was another story. I couldn’t get my toe on the damned tab from the saddle. It’s now wearing a Brown sidestand.)
Side stands do have a reputation for excessive wearing, but that hasn’t been an issue for me.
Perhaps the /5, /6, and /7 sidestand “issues” are why their center stands are so often worn through. That and riders carelessly “throwing” their bike onto the center stand, skidding it across the concrete.
Under The Tank Master Cylinder
I’ve never understood the under the tank front brake master cylinder. One of the main advantages of a hydraulic braking system is somewhat nullified by operating it with a cable. I’ve read speculation that BMW was concerned about passers by screwing around with the master cylinder. That’s as good of a reason as I can think of. But the long term issues with the under tank master cylinder can be dire. Out of sight is out of mind and most /6 and /7 bikes I’ve seen have had the master cylinder leak at one time (or more). That’s bad enough but the leaking brake fluid has a tendency to wick into the starter relay connector on the L/H side of the upper frame tube. On my /7 the engine and all electrics died. Luckily it was at my front gate rather than miles from home on some lonely Oklahoma byway. The problem was a corroded/mucky connection between the starter relay and it’s mating receptacle. As has been noted, the 12V positive red wire from the starter solenoid goes to one terminal of the starter relay connector, then thru the relay itself, and then from another terminal on the same connector to the headlight shell. (I’ve heard that BMW did it that way in order to build a less complicated wiring harness and the relay connector (with appropriate relay) was a good way to connect two sub harnesses together.)
5 Speed Transmission Shims, Dogs, and Kickers
I’ve not been inside a 5 speed transmission yet, so most of what I’m going to talk about came to me through osmosis. (Hi Duane) To be sure, overhauling or fixing a 5 speed transmission is a daunting proposition. Plus the 5 speed transmission isn’t very reliable in the first place. The kick start (on the few 5 speeds so equipped) simply isn’t reliable. And the transmission dogs are prone to breakage because they were not machined well. (I was heavy throttle torquing on an entrance ramp of a highway when suddenly my engine quit pulling and revved up. Yup, lost a gear because of broken dogs.) BMW, or rather BMW’s supplier should have done better. And BMW should have changed the specs given to the supplier.
The chief concern about going into the transmission is to get the shim stack right. There are tedious calculations to be made and you never know for sure if you’ve got the shim stacks right. I wonder how many transmissions have failed for that reason. And I doubt that one in a hundred of BMW dealers ever owned up to mis shimming the transmission.
So how should BMW have done it? That’s a good question. I’m thinking that on the back side of the transmission BMW should have had had caps on the back side of the three shafts. The covers could be removed and the bearings re-shimmed as needed. Or (this is even wilder) perhaps an inspection hole to the shim stacks could have been created where removing a screw would reveal the shim stack for evaluation. Neither of my ideas are very good, but the situation as it is, is a bitch.
Valve Recession.
The world wide conversion to unleaded fuel caused a lot of problems for a lot of manufacturers, BMW included. So I can’t really fault BMW for the valve seat recession problems that showed up. But what bugs me is that the fix fixed the valve seat recession and instead now the valve heads were tuliping. That bugs me.
Ba dip, ba dip, ba dip, that's all folks!
Ken, overly critical in OKlahoma
