Short Wheel Base R75's are UNSTABLE???

[wchargis]

OK--I am sure thiis has been rehashed a number of times.

But the more folks I speak with the more I keep hearing how "unstable" the short Whel base R75's are.

Oh and it get worse with a fairing---Oh and it gets worse with a top case>

Oh my God! I have put about 500 miles on this bike since I got it last week. No instability--but a bit jumpy at speeds over 80--so What!!

Am I missing something? Or do they turn into the Forked Tail Doctor Killers that the Beechcraft Bonanza was supposed to be in aircraft?

Or am I making light of a real killer out there?

Someone fill me in.

[barryg]

I've got a '71 R50/5 and used to tour on it quite a bit loaded down, I never had a handling problem. I guess I'm just a lucky surviver.

[rdalland]

Never an issue with mine.

[shire2000]

The shorter the wheel base combined with high speed tends to create less stability and much faster handling response.

The short wheel base issues of the R75/5 were not as big an issue as most would think. It was an issue if pushing the bike to very high speeds. If traveling at normal or even higher "legal" speeds, there is no real issue. You may find the bike to be "twitchier" than the long wheel base, if you rode one and then the other right after it. You just have to remember that every bike has it's own personality and you have to respect that. If you don't, it will come back to bite you.

The addition of a "heavy" full fairing and/or a top case did tend to make the problem more pronounced. But that did depend on the fairing. Some fairings actually made it better, if they helped to create more down force on the front wheel at speed. Unfortunately, most of the fairings of the day did the opposite.

Lots of the cafe racer types of the day actually preferred the short wheelbase due to it's faster handling response.

I have had a few of the short wheelbase versions and rather liked them, expecially for around town and short trips. Did not like the ride for long distances, but the combination with a cafe setup did make it hard to stay in the saddle for long periods of time. Clip ons, bar end mirrors, seat that was more for looks than comfort, rear set controls, extreme forward lean to almost laying on the bike. Yup, couldn't stay like that for long hours of touring. But sure was fun in the twisties. Handled like a dream, or at least so I thought at the time.

[FLbmw]

I ride a 1971 R75/5 SWB. I've owned it for over 25 years. The bike has over 250,000 miles on it and I have put over 1/2 of those miles on it over the years. I rode my first 1000 mile day on this ride in 1987. Over the years it's been through a lot a changes. Full fairing, no fairing, top box, no top box. No matter what I've done to her, she's always been a true, faithful soldier. She's certainly not designed for high speed, but she'll cruise comfortably at road speed all day long. In all the years, and all the miles, she has never let me down or left me stranded. I recently did a total restoration with her and brought her back to looking as close to 1971 showroom as I can. Won best BMW at a rally last year. Now, she sits in the garage next to my '07 R1200R.

[mymindsok]

I think that a couple of you might be missing the real story and what appears to have taken place back in the day.

Firstly, lets accept that the SWB bikes handle more quickly than the LWB's. OK? With that out of the way, I'll go on...

In 1970, when the /5's appeared here in the US, they really were considered "a powerfull adult sporting machine" and riders put them to the test. The problem that cropped up was that a significant percentage of these riders were being thrown from their bikes after going into uncontrolable tankslappers and some of them died. (Yes, thats the truth and I did know a very good rider who died on his brand new /5, while riding through an open, downhill, sweeping right hand curve, that he rode through daily. He went off into the woods on the inside of the turn and hit a tree.) Of course, BMW dealers quickly became aware that there was a problem and calls went out to Butler and Smith and Germany but any flaws in the bikes were denied. BMW blamed the riders and riders kept complaining about their beautifull new bikes. Finally, the LWB bikes appeared in 1972 and all was well with the world.

Or was it?

If one is to believe Duane Ausherman (Who was a partner in a BMW dealership in SF at the time.) he and his mechanics discovered that the problem rested in the manufacture and fitting of the Triple clamps. That somehow the subcontractor in Germany wasnt holding them to spec (Or prehaps the specs were wrong to begin with!) and that in some bikes the errors accumulated in such a fashion that the bikes became unstable at speed. That would explain why some bikes tankslapped and others didnt, right? If I understand correctly, what Duane and his cohorts did for their customers was to measure, refit and adjust the front end's parts, untill the fork legs lined up correctly and after that the bikes performed admirably. OTOH, Duane's shop did refrain from sharing their discovery with other Dealers or mechanics, so guys who bought from other sources kept falling, injuring and sometimes killing themselves.

After owning a couple of /5's, /6's and later bikes, heres what I think:

The /5 bikes were a huge evolutionary step ahead for BMW but they also had some design weaknesses that could, under the right conditions, get you killed. Now, Lets also accept Mr Ausherman at his word; that the /5 bikes had less than steller parts inspection and that out-of-spec tripple clamps reached the assemble floor on occasion. That in and of itself could cause catastrophic instabillity under some conditions but are there any other contributing factors that we need to take into account?

Some of the SWB/5's had faulty top tripple clamps that led to the front ends being out of alignment, straight from the factory. Plus, the swing arms and the frames they were attached to were never up to the forces generated by the new bikes when ridden at high speeds. All that one would need in order to meet disaster would for two, three or all of these factors to meet in the middle of a fast corner and.... You know the rest.


It's my opinion that some of the /5's did arrive with flaky front ends and when they met up whatever was going on in the rather bendy Featherbed frames, anything could happen and sometimes did. My opinion has been further strengthened by my examination or /5, /6/ /7 and late model R-bike parts which tells me that with every iteration of the bikes, BMW beefed them up, using stronger swing arms, stronger subframes, heavier and reshaped frame tubes, a doubled backbone, and increased headstock plating. All of this before the hot rodders and racers began welding in heavier gussetng, caging the swing arms and adding side tubular braces, etc, etc. BMW knew that the bikes wernt allways up to snuff and they continually worked to solve handling issues, right through the end of the series.

If I still owned my R-75/5 I would definately do a top-down on the front end and probably install an R-65 swing arm as well. After that I would try to curb my exuberance while under way and enjoy vintage motoring at it's best while leaving touring and sport riding chores to my other Airheads.

Otherwise, do what I did. Make it your business to ride every iteration of the 247's from 1970 through the last of the twin shocks or monos and then come back and tell us which ones handled best . I can tell you right now that it'll be the latest model and thats why I upgrade all of my "keepers" with welded in gusseting, big tube swing arms and fork braces.

[rdalland]

when ridden at high speeds.

What do you consider high speed on an short wheel base R75/5?

[wchargis]

Thanks for all you responses.

The alaingment of the front end seems to be the big issue. Also taking the bike into high speed and low speed turns (if Duane Austerman is correct).

Jsut got the bike and have been through a number of speeds on the bike and nothing resembling a "wooble" as of yet.

I was going to take the bike to Johnson City Rally. Looks like I should have the front end looked at and aligned before I go.

Is a top case really not the thing to do? if not what are others doing to tour on this bike? where are you putting your gear?

Cheers

[jforgo]

Is a top case really not the thing to do? if not what are others doing to tour on this bike? where are you putting your gear?

Cheers

IMHO, top cases are to be avoided. If you have saddlebags, and they are full, and your tank bag is stuffed, and you already have the back seat filled with gear or passenger, and you have already put your gear on a diet, then, only then, would I consider a top case. It is the worst place to put a load, or to get crosswind pushes.

[mmmalmberg]

I loved mine. I remember when they came out, my riding buddy bought one a couple years later - he did a lot of roadracing and found a SWB because of the quicker handling. I used to ride behind him on my new 1st bike, a '75 Yam. RD350. Great first bike for an 18-year-old:)

I think the LWB probably would have also helped reduced the business about the rear end jacking up under acceleration...

[vanzen]

a SWB R75/5 will be the quickest / best handling 2-shock airhead you've ever ridden
for just as fast as it will possibly go
IF
the forks are certain to be in alignment and all chassis components are in good order and well maintained
... but do leave the handlebar mounted fairing & trunk at home

[mymindsok]

I was going to take the bike to Johnson City Rally. Looks like I should have the front end looked at and aligned before I go.

Is a top case really not the thing to do? if not what are others doing to tour on this bike? where are you putting your gear?

Cheers

Sometimes it's different strokes for different folks....

If you have never experienced any problems with your /5, then you might be ok but I'd still check the front end out. Measuring the alignment only takes a couple of hours and it's time well spent but don't please pass this job on to anyone else. Do it yourself or it probably won't be done correctly.

I know that in my case, I started doing road trips on my BMW's in the 80's and I ran into the "Rubber cow" once I loaded the bags and strapped a tent, a asleeping bag and a bunch of other crap to the seat. Under some circumstances, the bike started to wobble because of swing arm twist combimed with the marginal shocks. The same thing happened when back road-bombing on that bike and following riders frequently commented on my rear wheel flopping around while I was accelerating out of corners. I got used to it but it wasnt kool and a braced swing arm helps.

Top case? No. No, no, no! No top cases and no handle bar fairings! Get your gear down to where you can pack two saddle bags lightly and maybe throw a 20" dry bag, containing your tent and sleeping bag, across the seat. Any more and you'll probably be overloaded. Weight reccomendations can usually be found inside the saddle bags and even though they appear to be exceedingly low, try to stick to them. It took me a long time to figure that out! :banghead

You might not beleive this but the less you pile onto the bike, the more you'll enjoy it and the less you take to rallys, the more you'll enjoy the rally. I learned that from a buddy, who never packed anything more than would fit into the saddle bags of his "S" bike, while my scooter was allways packed like a mule.

Airheads are about "sport touring" with the accent on "sport"!

PS: for me, "high speeds" begin at 80mph on the highway and on byways, it depends on how you ride the corners. Some of us simply ride harder than others. If you scrub all the nubs off of your new tires within the first couple of days, youre probably riding pretty fast and pretty hard. If the nubs are still there after a month or so and you've had no reason to scream into your helmet, youre normal.

I'm a no-nub kinds guy! :wave

[108625]

The May issue of Classic Motorcycle Mechanic (published in the U.K.) featured a column by an American writer remembering how his swb/5 tried to kill him, at speed on a California freeway.
The headshake phenomena was not the subject of the story though, Duane was.
Here's a guy who claimed to know about a dangerous problem that BMW denied, and have a fix for it as well that could prevent accidents and perhaps save lives.
What did he do with that knowledge?
His conversation with the author was to tell him basically that he knew how to fix it, and he would only fix bikes that he sold out of his shop. Period.
He would not fix the author's bike, bought from another dealer. He would sell him another one, and fix that.
Very interesting to get that feedback about one of the so-called airhead "authorities".
The column was about that one individual you encounter that sticks out in your memory negatively; for the author, Duane was that guy.

[amiles]

My recollection of the short Wheelbase instability situation was that the handlebar mounted fairings of the time were thought to be a primary culprit.

Personally I think that a lot of dangerous handling problems are due to the weight bias of the machine being moved from it's factory design ratio front to rear to a heavy rear wheel, light front loading.

Lots of added knowledge of aerodynamics, improved tires and improved motorcycles since then.

[vanzen]

My recollection of the short Wheelbase instability situation was that the handlebar mounted fairings of the time were thought to be a primary culprit.

Forks misalignment is the cause of the instability.
A handle bar fairing will exascerbate / amplify the effects of a forks misalignment issue on an SWB /5, or for that matter, on any machine.
Although I don't personally like them for a number of reasons, that being one of them,
many motorcycles past and present use handlebar fairings without issue.

Personally I think that a lot of dangerous handling problems are due to the weight bias of the machine being moved from it's factory design ratio front to rear to a heavy rear wheel, light front loading.

A valid point worth some serious thought if considering a top box –
Heavily loaded, a top box can be dangerous in terms of it's affect on combined CoG – esp on a SWB machine.
Even empty, some will introduce aerodynamic instability.
A heavily loaded top box and a passenger ... OUCH !!

signed: a crash victim of the speed wobble in 1973, riding a 1971 R75/5 at the time of the incident.

[20774]

I'm not sure how this relates to the handling issues of a box-stock SWB R75/5 ??? But I will suggest that any rear tire that can physically fit on a /5 will not impact combined CoG significantly.

I think he's referring to the weight distribution or ratio front to back. For sure, too much weight in the back, especially aft of the rear axle, isn't good from a stability standpoint. I shudder when I see those H-Ds and wannabe bikes with those luggage systems that mount on the aft side of the sissy bar...I just can't imagine...

[R90S]

The May issue of Classic Motorcycle Mechanic (published in the U.K.) featured a column by an American writer remembering how his swb/5 tried to kill him, at speed on a California freeway.
The headshake phenomena was not the subject of the story though, Duane was.
Here's a guy who claimed to know about a dangerous problem that BMW denied, and have a fix for it as well that could prevent accidents and perhaps save lives.
What did he do with that knowledge?
His conversation with the author was to tell him basically that he knew how to fix it, and he would only fix bikes that he sold out of his shop. Period.
He would not fix the author's bike, bought from another dealer. He would sell him another one, and fix that.
Very interesting to get that feedback about one of the so-called airhead "authorities".
The column was about that one individual you encounter that sticks out in your memory negatively; for the author, Duane was that guy.


Before you twist that knife again, read Duane's side of the story.

On Duane's website: http://w6rec.com/duane/bmw/citybike.htm

[mmmalmberg]

couple of thoughts.. Tires were awful in those days. K81's were the hot tire - they would seem quite scary by today's standards. Second, when I bought my R50/2, in 1975, I heard that they were prone to tank-slappers. In my experience that front end was dead stable and predictable. That bike was my only transportation for 12 years and I rode it across the country and then some. Third, and relevant to the above, we know that various out-of-spec conditions can cause front end wobbles, from fork oil to tire pressure to steering bearings etc. etc. So it's easy enough for one or a few incidents to generate a reputation that could in fact be based on improper set-up or maintenance. My $.02...

[vanzen]

I think he's referring to the weight distribution or ratio front to back. ...


Yup. I read the post, was confused, responded, realized my misunderstanding of it ....
but only after hitting "submit reply" – so edited...

[20774]

Second, when I bought my R50/2, in 1975, I heard that they were prone to tank-slappers. In my experience that front end was dead stable and predictable.

Question...how much stability is offered by the Earles fork versus the late '60s models (US versions with the telescopic forks) and the SWB /5s? Seems to me that the SWB along with the telescopic forks might have made things worse, or more prone to the wobbles. I'm very comfortable on my Earles /2 and even have a, heaven forbid, fork-mounted fairing installed. No problems for me...

[amiles]

As to fork mounted fairings from the early 70's,

in my unofficial observations while out and about, this type of fairing/ windshield today seems to have a much smaller and more aerodynamic design than back in the /5 and /6 days, in fact more like a windshield than fairing.

In olden times these items were bought as an aftermarket accessory and I can't imagine that any wind tunnel tests were done on specific bikes. The installation was handled by owners or motorcycle shops who bolted them on. I doubt that any aerodynamic fine tuning was advised or done. That these would or could exaggerate other handling problems should come as no surprise.

If I recall correctly I read just a few years ago about a NHTSA investigation of the HD handlebar fairing in regards to tank slappers on Police bikes. I don't know the outcome, if any but it seems like deja vu all over again.

I too am amazed at those "systems" that put a bag like a huge suitcase on the back of a "sissy bar".

[vanzen]

amiles:

there is no doubt that a handlebar fairing will influence a forks system.
'When' & 'how much' will be a matter of 'chassis design' & 'chassis maintenance' parameters.

20774:

The pivotal thought to keep in mind, here, is that a short wheelbase and / or telescopic forks can perform successfully [u]without[/i] instability.
My considered opinion is that the forks assemblies supplied to BMW for the early /5s
did not conform to a standard of tolerance that would be necessary to provide a product that could behave consistently and safely ...
and that this fact, whether by oversight, ignorance, or neglect – was the reason for the SWB /5 'speed wobble' handling problems.
Remember that the /2 US models used a quite similar forks system, provided by the same supplier, and also had "issues" ...
and this in spite of being attached to an entirely different "long frame" chassis package !
It should be noted that the longer swing arm introduced a few years later did not solve the handling problem,
but rather rendered it moot by dampening it's (potential) effects.

[mmmalmberg]

Not that this clarifies anything at all but I had a big old 50's police style windshield on my /2. It was great!

[wchargis]

I am not surprised to hear about the handlebar fairings. But I am amazed at how many i see on the road and in photos.

I fully understand the effects it would have on the bike and its designed handling charactgeristics--but I keep seeing a lot of pics of Vintage BMW OwnerClub bikes and other using the Wixom Ranger Fairing.

The fairing seems to be more areodynamically designed than a staright handlebar mounted windscreen.

I found a site for you mathmatically advanced airheads" I cant make hide-nor-hair of the report other than drag is increased by 25% with the Wixom fairing. I cannot find the diagram of how the monment of the bike is changed with a fairing added--that would be helpful for me.

http://www.galcit.caltech.edu/awt/tenfoot/mcreport/index.html

[20774]

Interesting site...I'll have to look at that some time. I think the trouble you get into with the fork-mounted fairings is due to asymmetry of loading. This might be due to the asymmetry of the fairing itself...it can't be perfectly the same left half versus right half. Also the vector of the wind due to the bike's forward velocity and the direction of the wind can create the asymmetry. If the air should spill off one side in a different way than the other side, that might result in different loading which is pushed into the forks and handlebars. By natural reaction, the rider is going to resist that to some degree. Now the air spills off the other side of the fairing by way of compensation and the rider might subconciously respond again. Often the rider is "behind the power curve" in providing the corrective action, and in some ways, contributing to the oscillation. Now speed this up and you can get a sense how a wobble could start. Also, the bike's suspension, as a system, will have some kind of natural frequency that, when excited, will wobble undamped resulting in a tank slapper. The hope is that this natural frequency is high enough or the damping is sufficient enough to minimize the tendency to wobble and allow the rider to ride out of it to safety.

[108625]

Before you twist that knife again, read Duane's side of the story.

On Duane's website: http://w6rec.com/duane/bmw/citybike.htm

Read it, and I must say his position, and his postings have changed since just a few years ago, when I was looking into headshake issues with my airhead.
What I found then, and I think is still buried somewhere in internet arcives was written about '99 or so, and consistent with the "I know a secret I won't tell" tone.

Twist that knife?
First, it implies someone's been betrayed.
Second, if I remembered this while writing essays for my ethics class last semester, it would have made a great case study.

[racegun]

as most riders know, the physics of a 2 wheel machine are complex indeed. Top boxs ( weight to high above center of mass), Faring ( specially mounted to handle bars) and VERY IMPORTANT ...STEERING HEAD BEARINGS are the most likely culprit. After 35 yrs and 20+ BMW's, i can maake any of them tank slap ( mostly very mildly) but none-the-less.. they feel less stable. Weight, tire pressure, seating position, etc. Everything, even the little things mean a great deal when on 2 wheels at speed.

I thought BMW added 1.5 inches to the swing arm ( on /5's) because the carbs were to close to shins ( as well as helping stability). What do you think?

[vanzen]

... I thought BMW added 1.5 inches to the swing arm ( on /5's) because the carbs were to close to shins ( as well as helping stability). What do you think?


Interesting thought, however, the relationship between the rider's peg mounting and the carb, an attribute of the main-frame, was never changed –
the swingarm being lengthened by @ 2" at the rear where it connects to the FD
would not affect the shin / carb condition.

[vanzen]

Interesting site...I'll have to look at that some time. I think the trouble you get into with the fork-mounted fairings is due to asymmetry of loading. This might be due to the asymmetry of the fairing itself...it can't be perfectly the same left half versus right half. Also the vector of the wind due to the bike's forward velocity and the direction of the wind can create the asymmetry. If the air should spill off one side in a different way than the other side, that might result in different loading which is pushed into the forks and handlebars. By natural reaction, the rider is going to resist that to some degree. Now the air spills off the other side of the fairing by way of compensation and the rider might subconciously respond again. ...

Similar effects also play upon BMW / Krauser / Hepco-Becker side-bags at speed.
Hence the warning sticker on the inside of the cover limiting maximum speed with the bags attached.