Vibration at high speed/rpms in '90 K75S

Hi All-

Just picked up a new to me 1990 K75S and rode it home 200 miles from PA to DC yesterday. The bike has 55k miles on it and lots of major maintenance done. That being said, there was a crazy amount of vibration above 45 mph or so. Is there a common place to start when looking to fix this issue? I'm sure it could be a lot of different things, but any help for a K75 rookie like myself would be appreciated.

Welcome to K-land, cavisconage. The K75 is a great bike, I had a K75S from 1988 to 1997.

The vibration you feel is definitely not normal; normal is really smooth at all rpms and speeds. As what you feel is speed-related, you should start with wheels and tires:

Are the wheels straight and unbent? Put the bike on its centerstand, and spin each wheel slowly while holding a pencil or small piece of wood next to the rim. There should be little or no runout in either the radial or lateral direction. If there is, the wheel(s) should be straightened or replaced.

Same issue for the tires. First, how old are they? Numbers molded into the sidewall will disclose the month and year of manufacture. The rubber compounds in tires degrade with age, and will generate less traction even if there's tread remaining. And there will be tread wear indicators molded into the tires, showing how much treadwear has occurred. If the tires are five years old or more, or are worn close to the wear bars, replace the tires.

If the tires seem OK, make sure they're correctly inflated and inspect them carefully for flaws. Any significant out-of-roundness (checked in the same way you checked the wheels) or a sidewall bulge is immediately disqualifying; if either is found, replace the tire(s).

Finally, each tire-and-wheel assembly must be balanced. An out-of-balance condition could easily generate the vibration you feel.

Do some checking, and let us know what you discover.

Welcome to the forum!

A K75S should have virtually NO vibration at virtually any speed. It is. One of the smoothest motors ever built.

The first place to look would be the tires and wheels.

Check for bent wheels.
Checked for out of round or damaged tires.
If neither of the above, have the tires rebalanced and ask the tech to double check for bent wheel or tire runout.

Let us know the results. If that doesn't fix it, we can suggest some less likely and more involved possibilities.







PS: I guess since David lives in Santa Cruz, just a quick ride from here, we both get up and check the Forum about the same time. I was typing while he was hitting send. But the advice is the same. It's most likely something wrong in the wheel/ tire area. These bikes don't vibrate. When the K75 first came out, one of the complaints from traditionalists was "These bikes don't have any soul" (read "vibration"). And, like David said, old hard tires are also possibly the problem. It's amazing how much smoother an already smooth bike feels with brand new tires.

cavisconage - I just got my ‘94 this Spring in Arlington, VA, somewhere near you!
Yes, mine is smooth. In addition to checking tires and wheels you might want to check the bearings in each wheel. Are the forks straight? Anything going on in the steering head bearings? Any notches there?
Fortunately for me, the previous owner was pretty maintainence savvy.
Good luck getting yours smoothed out.

what they all said. K75's are super smooth at all speeds.

welcome to the DC K75S crowd!

cavisconage Can you tell if the vibration is related to road speed or engine RPM?


Regards,


Richard Flood.

richo2006 said:

cavisconage Can you tell if the vibration is related to road speed or engine RPM?


Regards,


Richard Flood.

Click to expand...

(Which you can do by running the bike up to speed and then pulling in the clutch... obviously done in a safe area)

Or, just a different gear.

Have a similar issue with an '86 K75C. Vibrates terribly at about 5000 RPM regardless of what gear I'm in (follows RPM, not speed).

I replaced the clutch a few years ago without using 'official' alignment tools. Has been ridden by my son since then and he never complained (likely didn't know any better). I suspect it is the clutch being slightly misaligned that doesn't manifest itself until the higher speeds.

Will be working on it this winter since the transmission needs seals anyway.

Other suggestions of possible causes is welcome.

J

2000rsv said:

Have a similar issue with an '86 K75C. Vibrates terribly at about 5000 RPM regardless of what gear I'm in (follows RPM, not speed).

I replaced the clutch a few years ago without using 'official' alignment tools. Has been ridden by my son since then and he never complained (likely didn't know any better). I suspect it is the clutch being slightly misaligned that doesn't manifest itself until the higher speeds.

Will be working on it this winter since the transmission needs seals anyway.

Other suggestions of possible causes is welcome.

J

Click to expand...

Three parts of the clutch assembly need to be assembled in a certain way so as to spread out the heavy (or light) spots to maintain overall balance. Marks on the flywheel, pressure plate, and clutch cover are supposed to be spread out at 120 degree intervals as closely as possible. Not doing this on assembly would precisely cause the symptoms described.

Paul -

The clutch I put in my /7 a few years ago came from BMW and there were no marks. It was my understanding they come balanced and there's no need to worry about spacing. However, if I had a clutch already installed with no issues, I would certainly mark the existing orientation for reassembly.

This thread is over 1 year old.
The last time the OP visited the site was April this year.

20774 said:

Paul -

The clutch I put in my /7 a few years ago came from BMW and there were no marks. It was my understanding they come balanced and there's no need to worry about spacing. However, if I had a clutch already installed with no issues, I would certainly mark the existing orientation for reassembly.

Click to expand...

Yes but, we don't know in this case what was replaced. If it was just the disk for example and the original orientation of the heavy parts was ignored you get precisely the type of imbalance described. And we don't know exactly when this clutch job happened compared to when BMW improved its balance specifications. When symptoms match a common error I don't assume there must be some other weird cause.

Lee said:

This thread is over 1 year old.
The last time the OP visited the site was April this year.

Click to expand...

Yes but post 12 by a different poster with a similar issue was posted just today - new and fresh.

The "new poster" says he replaced the clutch. True, we don't know what that means, but I take it as the whole shebang was replaced. If a person replaced only the fiber disk, seems like they would go so far as say that. But, it's hard to read all of what is meant by just the typed word in this case.

Even still, if he did just replace the fiber plate, they come these days with no markings as far as I know. Back to what I said, if disassembling the entire clutch pack, and replacing say only one item, it would certainly be best to put the old items back in their original orientation. The replaced item would have no markings so it's just put in and you take what you get.

PGlaves said:

Yes but post 12 by a different poster with a similar issue was posted just today - new and fresh.

Click to expand...

20774 said:

Paul -

The clutch I put in my /7 a few years ago came from BMW and there were no marks. It was my understanding they come balanced and there's no need to worry about spacing. However, if I had a clutch already installed with no issues, I would certainly mark the existing orientation for reassembly.

Click to expand...

The clutch for the /7 probably comes without marks because they are "close enough" for a /7. A good running /7 vibrates more than a poor running K75, so the tolerances would be looser.


If you don't believe me, take a nickel and balance it on edge somewhere on the bike. Reach over and start the bike. Then slowly raise the throttle to 3000 rpm. If you can get near there, your bike MAY be near as smooth as a typical K75.

20774 said:

The "new poster" says he replaced the clutch. True, we don't know what that means, but I take it as the whole shebang was replaced. If a person replaced only the fiber disk, seems like they would go so far as say that. But, it's hard to read all of what is meant by just the typed word in this case.

Even still, if he did just replace the fiber plate, they come these days with no markings as far as I know. Back to what I said, if disassembling the entire clutch pack, and replacing say only one item, it would certainly be best to put the old items back in their original orientation. The replaced item would have no markings so it's just put in and you take what you get.

Click to expand...

I replaced just the clutch fiber disk. The bike had been sitting for a lot of years before I bought it and was slipping on hard acceleration. The fiber disk was cracked and glazed. I did mark everything and reassembled as instructed in the Haynes manual.

Upon further searching on this forum, it's possible it is only the monkey nuts (assume they are original). I had ridden it around a bit before the kids took it over and am sure I would have noticed the vibration. Although most of my riding was back road twisties (or as best we have of those here in WI) and I may not have noticed. I road it back from Phoenix and during the long hours of droning it was really evident.

Thanks all for the replies and advice. Will update again this winter after I get it torn apart and repaired.
I have my GSA to bide me over until then.

J

I had to do some searching to find out what "Monkey nuts" were!

Hopefully your search wasn't on Google. That might return some odd information.