Transmission Skips/Jerks When Under Load

[tvpierce]

The bike is a '96 R1100RT with 76K miles. I've owned it for a few years. Always meticulously maintained by the previous owner and myself.

 

When I'm riding along at cruising speed, no problems. When I go to pass someone (in 3rd or 4th gear) I grab a handful of throttle, and get a quick skip/hesitation that almost feels like a momentary ignition dropout. It does this just once as I'm accelerating, then all is fine for the rest of the pass.

 

What's kinda odd is that this never happened -- not once -- before two weeks ago. But once it started, it's done it every single time I pass or accelerate hard. So it was certainly not a gradual thing.

 

Did a little research, and found this on Anton's site: http://www.largiader.com/tech/oiltrans/

 

My bike 'skips' in third gear...

 

...almost like the ignition cuts out momentarily, or some gear is skipping a tooth." You need transmission work. This can happen to any gear, but usually second or third suffers first. People will try to tell you the shock coupling in the transmission is slipping from one position to the next... hogwash. It's not possible. What's happened is that the shift dogs have rounded off and a shift fork is bent. The gear is only engaged by the rounded part of the shift dogs, which let the gear slip off and bear against the fork. That in turn springs the gear back into engagement the next time the dogs line up a fraction of a revolution later. The problem can start with a bent shift fork (from dropping the bike on the shift lever, etc) or with rounded dogs (from too many missed shifts or half-shifts). It can also start from poor initial assembly of the transmission, whereby there was never enough shift dog engagement in a particular gear.

 

You can be certain you have this problem if the oil is an irridescent gold color when you drain it. The gold is the surface of the shift fork wearing away; normally there is absolutely no wear on the shift forks regardless of mileage. Thin slivers of metal that you find will be from the edges of the shift dogs.

 

Bottom line is, it needs to be repaired. The problem will get worse and you will damage the shift drum. A good rebuild brings M93 and M94 transmissions nearly to M97 spec. Nearly, meaning that the engagement dogs are straight and five out of six bearings are 'clean.' There will still be one open bearing and an M94 will still have O-rings, but neither difference is significant.

 

Sounds just like my problem. I haven't checked my oil since this started happening, but I changed it about 700 miles ago, and there were no signs of metal residue in the oil. (and I thoroughly checked it in a clean white container after letting it settle overnight -- as it my normal ritual when changing the tranny or FD fluid)

 

Any further thoughts from those with more experience/knowledge than I are very much appreciated.

 

Thanks in advance!

 

[dirtrider]

Evening Jeff

 

There is a BMW service bulletin covering what sounds like your issue:

 

Complaint: Brief power flow interruptions observed in various gears.

 

Cause: Gears are not correctly and fully engaged. The edges of the shift dogs and shift gears are rounded.

 

Solution: Beginning with the 1997 model year, a modified set of gears has been introduced in production.

 

Service Remedy: Installation of tolerance-optimized gears and shift forks is now possible.

 

It really sounds like you have worn shift dogs, that MIGHT include worn shift forks & might not.

 

There were also some new bearings (clean bearings) updates to help with internal bearing wear & shaft movement.

 

You can put a LOT of money into your trans once you open it up so finding a good used later 1100 trans is sometimes more cost effective.

 

You can also remove your gears & sliders, & if found to be in good condition, send them out to have the dogs back cut for more positive engagement under load.

 

The other slight possibility is slipping input shaft/clutch disk splines but those usually keep slipping under load not just one hiccup then back to normal.

 

 

[smiller]

Jeff,

 

This is a common issue on the M94 model transmission (used from 1994-1996.) This model had straight shift dogs which (depending on manufacturing tolerances) can allow the gear to press against the shift fork which eventually causes wear of the shift fork. When this happens the worn shift fork will not completely engage the gear dog upon shifting, so when power is applied the partially-engaged dog unlocks and then catches at the next opportunity, causing what feels like a skip in power. If this goes on too long it can also round the shift dogs, exacerbating the problem.

 

Assuming the dogs themselves aren't too worn (and they're probably not if it just started doing it) then you can probably get away with just replacing the worn shift fork(s). They aren't very expensive, and if that is all you do then you don't need to worry about re-shimming the shafts. But this doesn't address the root cause of the problem and eventually it will likely return (although if it takes another 75k miles you might not care.) The two best (permanent) fixes are to either have the gear dogs undercut (some specialty shops will do this such as Bruno in Canada, but it isn't inexpensive.) Another option is to obtain an M97 transmission (used from 1997 -2002) which is a drop-in replacement. The M97 has undercut shift dogs from the factory, presumably to cure this problem although BMW never acknowledged it beyond the hint in the service bulletin that DR noted.

 

If you are capable of doing the transmission work yourself then you can do as described above, but if you have to pay a dealer to do the work then it will likely be cost-prohibitive. In that case and considering that some of the internal parts for the M94 have gone obsolete the best option is probably to find a clean used M97. They are out there, and you have some time because your current transmission will do the 'skip' thing for some time and it won't strand you, so it isn't urgent. But the problem will only grow worse with time so eventually you'll have to address it.

 

 

[tvpierce]

Thanks for the quick replies.

 

If I could get another 75K miles out of a new set of shift forks, I'd be ecstatic. That would give me another 18 years to find a replacement M97! :-) I'm a pretty competent wrench, so the thought of replacing them myself doesn't scare me. Certainly though a complete & proper rebuild would be beyond my skill-set.

 

Having only exhibited this problem in the last 200-300 miles, would replacing just shift forks seem like a prudent course of action?

 

Again, thanks in advance.

 

 

 

 

[dirtrider]

clip--

 

Having only exhibited this problem in the last 200-300 miles, would replacing just shift forks seem like a prudent course of action?

 

 

Evening Jeff

 

No, the shift forks are there to move the sliders not hold them in place under load.

 

With rounded shift dogs the shift forks would probably just wear out quickly again as the rounded dogs would be placing extreme loading on them.

 

The real fix is to back cut the shifter dogs so they stay engaged by themselves.

 

My guess is you will also find some bearing wear or even shaft wear once you open it up.

 

 

 

 

[smiller]

I'm going to have to respectfully disagree with DR on that point. I've seen two of these and in both cases the very hard gear dogs were not significantly worn, the problem was that one or more shift forks were extremely worn and not able to properly position the gear, causing partial engagement after the shift. When power is applied the dog releases and then slams into its next engagement point resulting in a complete (if dramatic) engagement, which is why it usually only skips once per shift. Replacing the worn shift fork(s) should fix the problem for at least some period of time, maybe a long time, but again I have to emphasize that it does not cure the root problem. As noted the real fix is undercut shift dogs which will ensure that the gears will stay fully engaged and not bear against the forks, preventing the premature wear. With the flat dogs the shift forks can end up holding the gear in position, and as DR described that's not what's supposed to happen and eventually it destroys the shift fork. How much of this effect depends on production tolerances which is why some M94's exhibit this problem in 35k miles. some 75k miles, and some never. If yours lasted 75k miles that's actually pretty good.

 

So in my opinion you do have the option of replacing the shift forks only. This would be a long weekend job and probably put you back on the road for several years, as long as the gear dogs look OK and I'm guessing they will. No guarantees though.

 

[CoolTouch]

Hello Jeff,

 

Once you get into your M94, you will find other bearings and parts that will need to be replaced, for sure. M94's are usually not worth rebuilding as they had issues where bearings were getting destroyed due to a design flaw. I have the exact same bike and dropped a M97 in it about 40k ago and have not had any problems since. My original M94 had a second gear that sounded like gravel was in the gearbox, though the rest of the gears were perfect. Then it started having the same issue you describe. After extensive research, I chose to get a rebuilt M97 and she is running strong as a daily driver.

[smiller]

M94's are usually not worth rebuilding as they had issues where bearings were getting destroyed due to a design flaw.

I believe that is the M97, and the design flaw is that the helical drive gear on the input shaft bears directly on the rear input shaft bearing, eventually deforming the inner race and causing the bearing to self-destruct. The M94 used tapered roller bearings for the input shaft which actually was a superior design to the 'clean' caged roller bearings used on the M97. The main problem with the M94 is that while it shares most of the same parts with the M97 some of the parts that aren't shared (such as the intermediate shaft bearings) are obsolete. Other than the straight-cut gear dogs and the few obsolete parts there's nothing wrong with the M94. If a good M97 presents itself then that is probably the way to go, but if the OP's M94 is in otherwise good condition then it could still be quite serviceable.

 

[CoolTouch]

M94's are usually not worth rebuilding as they had issues where bearings were getting destroyed due to a design flaw.

I believe that is the M97, and the design flaw is that the helical drive gear on the input shaft bears directly on the rear input shaft bearing, eventually deforming the inner race and causing the bearing to self-destruct. The M94 used tapered roller bearings for the input shaft which actually was a superior design to the 'clean' caged roller bearings used on the M97. The main problem with the M94 is that while it shares most of the same parts with the M97 some of the parts that aren't shared (such as the intermediate shaft bearings) are obsolete. Other than the straight-cut gear dogs and the few obsolete parts there's nothing wrong with the M94. If a good M97 presents itself then that is probably the way to go, but if the OP's M94 is in otherwise good condition then it could still be quite serviceable.

 

It was my understanding that the M97, which is the last generation of 5-speeds on the R1100 had most of the issues solved. Mainly, the bearings were sealed as opposed to open cage. Also, I remember seeing pictures where the bearings had a shoulder on the inner race preventing the outer race from rubbing against the adjacent parts. But, I was mistaken in that I had a M93 on my 96. Are you sure yours is not an M93? That is the one not worth rebuilding... only in my opinion, of course.

[smiller]

There were several changes (a more accurate term than 'upgrades' IMO) in the Getrag units used in the oilheads over the years and as usual each change fixed some problems while introducing others. There is nothing inherently superior with the sealed caged roller bearings used in the M97 and in fact the tapered roller bearings used in the earlier designs are a better choice for the application. I don't think anyone knows for sure why the change was made, but setting preload on the tapered roller bearing shafts is a complicated bear and probably a PITA on the production line while the caged roller bearings are comparatively much easier to shim. I have seen it posited that the main reason for the change is that is makes the manufacturing process easier and thus cheaper, but whether that is the actual reason or not is anyone's guess.

 

Bottom line, while it is hardly perfected the M97 is probably the lesser of all the evils and certainly the most plentiful and easiest to get parts for. But as above I would not reject any of the earlier models out-of-hand as if they have a simple problem and are in otherwise good shape they are quite serviceable. As usual the practicality depends entirely on how much of the work you can do yourself because $100/hour labor changes the equation pretty fast and starts to make a $500-$800 used M97 look pretty good.

 

 

 

[Trullion]

I tend to agree with that; having owned oilheads with both an M94 and an M97 in my experience the former has a better change. I believe that the taper roller bearing is actually a better design and that the real problem with the M94 (and M93) is the dogs, which I believe BMW specified in an attempt to reduce "clunking" when changing gear. I don't see why sealed bearings should be any better; those in my old Brit bikes are open and don't suffer undue wear.

 

FWIW my 1996 1100RS skipped in third a couple of times shortly after I bought it 8 years ago with 19,000 miles on the clock. Having read up about this on the various oilhead fora I decided to buy a used M97 for when the M94 needed to be fixed or replaced. I still haven't done this at 34,000 miles. What I did was to change my gear changing technique. When changing gear I now always make sure I keep light pressure on the lever until I have fully released the clutch lever, so that the chances of proper gear engagement are increased. My box hasn't skipped in years, other than occasionally in first, which is a different issue common to all the boxes. I shall carry on like this until I have to do something about it. There has always been the usual "paste" on the drain plug, never hard bits of metal, so I don't feel the need to fix it until it's properly broken.

[tvpierce]

What I did was to change my gear changing technique. When changing gear I now always make sure I keep light pressure on the lever until I have fully released the clutch lever, so that the chances of proper gear engagement are increased. My box hasn't skipped in years, other than occasionally in first, which is a different issue common to all the boxes. I shall carry on like this until I have to do something about it. There has always been the usual "paste" on the drain plug, never hard bits of metal, so I don't feel the need to fix it until it's properly broken.

 

I'll give that a try. It should at least buy me some time.

 

 

[schneeman]

Whatever you do, the skipping will get worse and in the end you will have to address the issue. If you plan to do the work yourself (time vs money dilemma), getting the box out is not very difficult, just time consuming (patience and keeping records of all you do is the key). Dropping a replacement box is easiest option, but you risk not knowing what condition it's really in, plus you might introduce misalignment issues that might result in stripped input shaft splines, something that haunts many RT owners, and the origins of which are widely debated and still largely unknown. New gearbox might not sit as well as the old one and the problem might appear. M97, especially used one, might have the the gear-rubbing-on-bearing issue on the input shaft, and if you are unlucky you might just switch one problem for another.

I was faced with the same dilemma about a year ago, and since i had the time and not the money i decided to do the work myself, having pretty decent wrenching skills but no previous gearbox level experience. What i did is i bought a second hand M97, and as my M94 was a late model, it already had sealed bearing in it, so the shafts were interchangeable with M97. So i ended up building a hybrid box, putting M97 intermediate and output shafts into the M94 box, hence getting rid of both M94 and M97 issues. As all such work, it takes patience and research, you will find tons of info here on the forum, and the members here are super helpful and most knowledgeable people on the planet when it comes to BMWs. Just by asking questions and having the manuals and buying some additional tools, like bearing puller and measuring setup components (to shim the shafts properly) etc i was able to pull off this job by myself successfully (put few tkm on the gearbox since then and all is fine so far). In the end it turned out if i just dropped the replacement M97 from ebay onto the bike i would have issues with it pretty soon, as the intermediate shaft was damaged (input shaft bearing was ok though). But you might be lucky. If it was me i would open the M97 before putting in on the bike just to make sure it's in good condition. It's not so difficult and surely less work than going in there again a few months later if it turns out you were unlucky with the replacement box afterall.

If you have too much time you can check out my ordeal (hope link works)

http://bmwsporttouring.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=836572&page=1

 

Good luck !

[tvpierce]

Thanks all.

 

Turns out keeping pressure on the shift lever until the clutch is released didn't help. Probably further indication that replacing just the shift forks wouldn't yield satisfactory results in my case.

 

For the short term, I guess I'll alter my riding style to work around 3rd gear, and keep my eyes/ears open for a used M97.

 

 

[SinNH]

"get a quick skip/hesitation that almost feels like a momentary ignition dropout."

I don't think slipping gears in the box and an ignition miss/hesitation even vaguely feel alike.