1976 BMW R75/6 Transmission Shifting Clunking

[b25bsaboy]

Good evening,

Well it took five years from the time I started the strip down to getting the 76 R75/6 back on the road and I must say it has been an interesting journey. Before I get into my issue, I want to say that of all the machines (British) I've restored, this one is on top of the list for something that is beautiful to ride and handles like a dream. Once I figured out where the engine rpm sweet spot is, that is the range I try and ride at. This machine is smooth



The issue, or maybe this is not an issue, but when I shift gears whether shifting up or down the engagement is clunking and not smooth. Shifting into first get is smooth, and up to fourth gear it sounds and feels like its not engaging smoothly as it should. The clutch was adjusted as per specification between Clymer, Haynes and the BMW shop manual. Of course we have to mention our friend Chris ? who has a number of You Tube presentation both in pictures and colourful vocabulary.

What more can I do to get the shifting smoother than it is? Re adjust the clutch? Slow my throttle time down before engaging the clutch lever and then shift. Open to all suggestions.

Will try and find some pictures to post, and here are two.

 

[PGlaves]

Find a rider with a similar bike and ride his/hers. Compare the shifting. It is probably similar or the same.

Many riders swear by different gear oils etc to smooth shifting. My favorite is Castrol Limited Slip with Guard Dog Moly GD401 moly (MSO2) additive. Others like Red Line Heavy. You might wind up trying somebody's favorite.

I doubt there is anything wrong.

 

[DonTom]

The issue, or maybe this is not an issue, but when I shift gears whether shifting up or down the engagement is clunking and not smooth.

If one couldn't hear my gear box shifting from a quarter mile away, I would wonder what is wrong with my tranny! And downshifting is even louder.

I cannot speak for other bikes, but this is normal for the /5 four speed gear box.

But what do you mean by "not smooth"? Other than the very loud "clunk" it should go right into gear every time without any problem.

-Don- SSF, CA

 

[b25bsaboy]

If one couldn't hear my gear box shifting from a quarter mile away, I would wonder what is wrong with my tranny! And downshifting is even louder.

I cannot speak for other bikes, but this is normal for the /5 four speed gear box.

But what do you mean by "not smooth"? Other than the very loud "clunk" it should go right into gear every time without any problem.

-Don- SSF, CA

To Don and Paul,

The oil that I put into the transmission is BelRay 80/90. May have to explore the slippery slope go gear oils

When I pull the clutch lever in let off the throttle and shift it almost feels like the gears aren't meshing together for a smooth shift between gears.

 

[PGlaves]

To Don and Paul,

The oil that I put into the transmission is BelRay 80/90. May have to explore the slippery slope go gear oils

When I pull the clutch lever in let off the throttle and shift it almost feels like the gears aren't meshing together for a smooth shift between gears.

But I assume it is not popping out of gear.

 

[nickrides]

Hey BSA Boy
That is a great looking BMW!
I would take Mr. Glaves advice and change out your trans oils to his recommendation. That is the combination I use in my 1978 R80/7.
Then, when up shifting lightly "preload" the shift arm with your foot just before shifting up.
When downshifting I always pull in the clutch, blip the throttle to get the engine revs up to what they are going to be when you get in the next lower gear, shift down at the same time and release
the clutch, it is all is faster than it sounds.
Just pulling in the clutch and mashing down to a lower gear is hard on these non-syncromesh gearboxes, splines, clutch etc.
But anyway, as you know, these gear boxes are a bit rough around the edges no matter what oils or techniques you use!
Hope this helps.
Nick

 

[b25bsaboy]

But I assume it is not popping out of gear.

Never pops out of gear!

 

[20774]

Also consider "post" loading the shifter on the way up in gears. In other words, continue to hold up pressure on the foot shifter as you let the hand clutch lever out.

And it is all about matching speeds within the tranny. On my R25/2, I have to forget everything I've known about shifting BMWs and slow my shifting way the heck down. I have to give it a count of 1 or 2 once I've rolled off the throttle before I begin to disengage/engage the clutch.

 

[ccolwell]

My first question is how many miles are on the bike? I never believed the old saw that BMW transmissions break in at around 25k until my R100 went past 25k and started shifting as well as the Honda Hawk I had then. The second question is how is the shift into 5th? Every airhead I had hung up slightly between 4th and 5th. I'd check the clutch adjustment but that's not likely to cause an issue as unlike your Brits it's a dry clutch. I used Castrol and Honda trans lube with good results in the past but after seeing a post by Tom Cutter somewhere else recently, I'd use whatever the BMW recommendation is. In any event, trying a different trans fluid is worth a try. Under 25k, an airhead will never shift like a Brit (or Jap) bike. One of their endearing qualities?

And +1 on preloading the shifter.

 

[dennisdarrow]

Understand that the /5 and /6 generation bikes are carry overs from the engineering of the 40's and 50's. With that in mind consider that refugee engineers from Krupp were who designed the firing mechanisms for the famous 88's artillary pieces. These weapons gave my grandfathers and uncles such problems as they were enjoying their Rhine River cruises while cursing their Zippo tin can Shermans..............When those artillery pieces closed the breech, those mechanisms gave off quite a nice mechanical "crunch" that was music to the Teutonic ear.

After closing ceremonies, these engineers needed employment. Instead of heading off to enjoy the benefits of Russian careers, they decided to stay home and rebuild what their actions had wrought. In the pots and pans research laboratories of the BMW factories they designed the precision mechanisms, of the what would evolve into, our cherished /5 and /6 transmissions. These were brought to life from drawings with burns on the edges and fragments put together having dates of 1922. Thereis no blame for error here. Just stand up straight as one shifts, and enjoy that beer hall thump of steins on tables as one shifts...........God bless these United States......Dennis

 

[b25bsaboy]

Also consider "post" loading the shifter on the way up in gears. In other words, continue to hold up pressure on the foot shifter as you let the hand clutch lever out.

And it is all about matching speeds within the tranny. On my R25/2, I have to forget everything I've known about shifting BMWs and slow my shifting way the heck down. I have to give it a count of 1 or 2 once I've rolled off the throttle before I begin to disengage/engage the clutch.

Good point Kurt, as over the years shifting the R1200RT is a breeze. Will have to think when shifting.

 

[b25bsaboy]

My first question is how many miles are on the bike? I never believed the old saw that BMW transmissions break in at around 25k until my R100 went past 25k and started shifting as well as the Honda Hawk I had then. The second question is how is the shift into 5th? Every airhead I had hung up slightly between 4th and 5th. I'd check the clutch adjustment but that's not likely to cause an issue as unlike your Brits it's a dry clutch. I used Castrol and Honda trans lube with good results in the past but after seeing a post by Tom Cutter somewhere else recently, I'd use whatever the BMW recommendation is. In any event, trying a different trans fluid is worth a try. Under 25k, an airhead will never shift like a Brit (or Jap) bike. One of their endearing qualities?

And +1 on preloading the shifter.

The mileage from new is 48,983. Shifts into 5th fine. Brit bikes do not run dry in the clutch as they run in an oil bath. Starting to look for some different 80/90 gear oil on Paul's earlier suggestion. I have used the BelRay on all my restorations with good results and just assumed that it would be fine for the R75/6. Learning curve

 

[pauls1150]

If it doesn't happen to just have some worn parts (forks or cam), and you're willing to invest the time and money, the tranny can be re-shimmed. I had one done by Jehd Webster (probably retired by now) at Marty's (closed) and the difference was amazing, it was almost Japanese in its action.

Some of the clunking is due to the heavy flywheel. I had one lightened, and the shifting was immediately much easier and quieter, and the bike was quite a bit more responsive too!

 

[DanHoug]

also, do not minimize the effect of having clean, lubed trans input splines. the shifting difference on my 81 R100RT with clean splines and Guard Dog 525 as a lube was dramatic.

-dan

 

[b25bsaboy]

Find a rider with a similar bike and ride his/hers. Compare the shifting. It is probably similar or the same.

Many riders swear by different gear oils etc to smooth shifting. My favorite is Castrol Limited Slip with Guard Dog Moly GD401 moly (MSO2) additive. Others like Red Line Heavy. You might wind up trying somebody's favorite.

I doubt there is anything wrong.

Afternoon Paul,

Went to my local speed shop this morning, picked up a litre of Red Line Heavy, changed the BelRay out and put the Red Line in. I then went for a short run for about a mile or so and did some shifting and there is a difference. Will have to wait till it warms up here in Calgary, then I will take it for a real test run. Will up date you when I have the test results, but right now, I can see and hear the difference. Thank you for the information you provided.

 

[nickrides]

Sorry Kurt but I give your "post loading the shifter" idea a thumbs down.
It doesn't make any mechanical sense, the gear change is over, nor does it do anything except cause unneeded wear on the shifting mechanism.
Nick

 

[20774]

Sorry Kurt but I give your "post loading the shifter" idea a thumbs down.
It doesn't make any mechanic sense, the gear change is over, nor does it do anything except cause unneeded wear on the shifting mechanism.
Nick

Maybe....but it works for me. What it does for me is it eliminates false neutrals between 4th and 5th. I was getting them fairly often. I read where Oak Okleshen recommended it. From what I can understand, it helps drive the shifting dogs home and not slip out of the engagement. The slipping would cause damage not to mention the over revving. I figure I'm doing less damage this way. No problems with my transmission to date.

 

[lkchris]

My first Airhead was a '78 R100S and I recall no shifting problems with it.

BMW massively upgraded Airheads with the '81 models and two of the changes were a lighter flywheel and lighter clutch, resulting in much lighter clutch pull and generally easier shifting.

Once I'd owned my '84 R100RS for a while, I got the opportunity to ride a friend's R90S and had a devil of a time shifting it smoothly. I think the ultra hard clutch pull threw my timing off and I was shifting slightly before it was declutched.

Had a similar problem transitioning to my first R1200RT, my first fuel-injected bike. With FI, fuel is mostly completely cut off when you back off the throttle and you'll get lower rpms during a shift than you do with carbs and you'll need to compensate with throttle to maintain same smoothness. That took a little while to perfect. (As an aside, this describes a bit of why Airheads are fuel hogs compared to the new stuff, as carbs never shut off as long as the motor's running)

So, while the Airhead transmission is "heavy duty" I bet some part of your problem is getting your timing correct, especially if you have other bikes you're also currently riding. Yes, probably with the Airhead, "slow" will describe the timing.

 

[20774]

It's not like every time I shift I get a false neutral, but it was happening at the odd moment. I agree that shifting fast is not the best for these transmissions and when I do that, I'll miss shift. I started adding the up pressure in about 1985...doubt I can change my habits now. The shifting on my /7 is very nice, almost noiseless. On my R25/2, as along as I really do wait on it, there's absolutely no sound. My R69S is somewhat more problematic, getting more of a clunk on it. Maybe that's the one I haven't figured out.

And somewhere on the Airhead forum, Paul suggested that '78 model transmissions were bad from the get go. I've heard Tom Cutter say that as well. As I recall, he had to "fix" every '78 /7 transmission at Butler and Smith when he worked there.

 

[b25bsaboy]

My first Airhead was a '78 R100S and I recall no shifting problems with it.

BMW massively upgraded Airheads with the '81 models and two of the changes were a lighter flywheel and lighter clutch, resulting in much lighter clutch pull and generally easier shifting.

Once I'd owned my '84 R100RS for a while, I got the opportunity to ride a friend's R90S and had a devil of a time shifting it smoothly. I think the ultra hard clutch pull threw my timing off and I was shifting slightly before it was declutched.

Had a similar problem transitioning to my first R1200RT, my first fuel-injected bike. With FI, fuel is mostly completely cut off when you back off the throttle and you'll get lower rpms during a shift than you do with carbs and you'll need to compensate with throttle to maintain same smoothness. That took a little while to perfect. (As an aside, this describes a bit of why Airheads are fuel hogs compared to the new stuff, as carbs never shut off as long as the motor's running)

So, while the Airhead transmission is "heavy duty" I bet some part of your problem is getting your timing correct, especially if you have other bikes you're also currently riding. Yes, probably with the Airhead, "slow" will describe the timing.

Kent, I think your right about getting your shifting timing correct.