Thwacking noise when in neutral

[restinthesun]

Hi all, this may be a wicket noobie question, but thought I'd arsk before something funny happened.

When I'm stopped in neutral the bike makes a pretty good whackity-whackity-whackity sound, but if I put the clutch in it stops. Is it normal to have a funny noise change when clutch not in? without actually hearing it...

She's a '88 K75S...

[Laitch]

When I'm stopped in neutral the bike makes a pretty good whackity-whackity-whackity sound, but if I put the clutch in it stops. Is it normal to have a funny noise change when clutch not in? without actually hearing it...

You should  post an audio file. .mov formats have unexpectedly worked here sometimes. YouTube works. mp4 works.

It could a mummified mouse whose tail is caught in the driveshaft u-joint, or some other condition but I bet it will be identified by somebody once it's heard.

[restinthesun]

It didn't seem so pronounced when I first turned it on but you can hear it build a bit in the recording (starts at about :10). It seems even louder once she warms up. I can get a recording of that if this isn't enough. You can hear I pulled in the clutch at :17 and :26.

Ideas?

[Laitch]

Ideas?

That noise wouldn't bother me. If it started to, I'd just wear earplugs.

[Laitch]

I realize denial isn't a technique that works for everyone, so for elucidation read this epic thread that includes audio.

[restinthesun]

I'm not that concerned with it, but the fact that it stops when I pull in the clutch...I thought someone with more engine know-how might be able to tell me what processes are going on that could possibly make such a noise go away.

Or if, indeed, like the other thread you posted, it's just a quirk of the bike and won't affect anything execpt my sensitive ears at stop signs.

 :dunno

You'd think I wasn't the only one with such a noise...

[Laitch]

I'm not that concerned with it, but the fact that it stops when I pull in the clutch...I thought someone with more engine know-how might be able to tell me what processes are going on that could possibly make such a noise go away.
 
You'd think I wasn't the only one with such a noise...

Let's face this head on. If you weren't concerned with it, you wouldn't have started this thread, :hehehe but it's an important topic that should be aired once in a while and I'm glad you did.

My know-how tells me that when pressure is applied to some rattling assemblies, they stop rattling. When centrifugal force is applied to some rattling assemblies they stop rattling. When attempts to make some rattling assemblies stop rattling, they won't; it's in their nature. When earplugs of sufficient density are applied, rattles aren't heard.

Surely, the one thing to be gotten from the epic thread is that your bike's not the only one with such a noise, but if that thread doesn't convince you, there are others that might—out there somewhere.

[restinthesun]

Ha! Too true. Maybe "concerned" was too strong a word... it was a matter "of interest" to me and possibly the K-bike commune...

First world problems, eh?  :musicboohoo:

I'd happily take other theories and solutions other than the earmuff ideal, but will probably just ignore until winter when I dive into the bastard for other upkeep.

[Jez]

Not an expert  but there is a cush drive on the first motion shaft, it does wear but not a big issue. Jez

[Jonico]

Hello from occasionally sunny Spain!
My 1986 K75F has the same problem as restinthesun mentions. The thing that raises my eyebrows is that, like he says, the thwacking rattle goes away when you pull in the clutch. In the "epic thread" they're careful to point out that in their case pulling in the clutch doesn't change anything. So is this a clutch related problem? Did anyone ever get to the bottom of it and solve it? I'm about as excited at the prospect of getting the clutch replaced as I am about getting a hip replacement.

John

[Laitch]

I'm about as excited at the prospect of getting the clutch replaced as I am about getting a hip replacement.

It could be a couple things. Earplugs and clutch lever usage are the way I would go.
Now that we are in the New Millennium, why not provide a sound file? Make one and paste its URL in the reply box, or use the Attachments and other options line under the box if the file is on your desktop. Your bike might have a sound that has never been heard in recorded history!


When is your hip replacement?

[Martin]

The clutch noise is built in, mine has had it for the last 120,000 K's. You can also get alternator Monkey nut noises and balance shaft noise.
Regards Martin.

[Jonico]

Thanks Laitch and Martin. I'll get an audio clip tomorrow to delight everyone's ears. When it comes to mechanics, I'm as wrenchless as I am useless. I bought this K75 a few years ago, a total frankenbike as its VIM decodes it as a 1985 K75C but it has a police full fairing like an RT, and it has ABS (with the red letters on the cylinders--is that ABS1 or ABS2?). I didn't think you could backfit ABS onto a bike but... it kicks in fine and stops on a dime.


Sent from my iPhone using Tapatalk

[Laitch]

I didn't think you could backfit ABS onto a bike but... it kicks in fine and stops on a dime. . . .with the red letters on the cylinders--is that ABS1 or ABS2?

I didn't think you could install a barbecue on the back of a bike but yours looks great. :giggles  Maybe it is the perspective. Anyway,  that's ABS I with red letters on your bike. Here are the particulars from MB.C's Technical Library.

You've got a fine specimen there, John. Ride it, enjoy it, keep with a regular maintenance schedule and leave the obsessing to us. How many miles does it have on it and how many have you ridden it?

Please post some larger profile photos of the bike here or in the Klassic K-bike section—preferably in the K-bike section so we won't be distracted from the obsessing about your incoming audio file.  :yes

[F14CRAZY]

My bike has done the same thing...Oh well. I've put over 40k on the bike and nothing's getting worse

[Motorhobo]

Well -- Martin says that noise is built in but I'm not so sure. Here's my story.

I had exactly what you are describing for a long, long time. I don't have it any more. Two things happened that made it go away:

First, I replaced my final drive and drive shaft with Bruno-rebuilt ones that I got off a parts bike I purchased just to get that FD and DS.

Second, my clutch disk splines fried, so I pulled the tranny off to discover that the splines on the input shaft had been damaged as well, probably because the clutch parts weren't centered properly when I installed it after the last spline lube. I installed a clutch disk and transmission whose splines are 100%.

So my conclusion is that the whacking is some kind of gear slap, either caused by the play that results from worn splines or from a clutch that's not centered properly. Martin -- I had that noise for many thousands of miles too, but I don't have it any more, and it can only be due to one of the two circumstances above -- I suspect it's the clutch disk splines whacking against the input shaft splines with every engine rotation in neutral, which is why the noise goes away once there's torque on the engine -- the splines lock in and the free-play is neutralized.



Comments welcome :-)

[Martin]

I had a rear seal leak, so I ordered a clutch, seal and an "O" ring in preparation to changing the seal. Found hardly any wear on the clutch plate or splines no discernible wear anywhere. However due to the fact that the clutch plate was non returnable replaced it anyway. Lubed up the splines with optimal paste centred the clutch with the recommended tool. Still have the noise unchanged. :dunno I have not worried about it since.
Regards Martin.

[Motorhobo]

Well I don't worry about either because I never use neutral.

BTW I was wrong -- there's still a mild clanking on my 94 K75 that has the Bruno FD and DS. It didn't go away on my 95 K75 until I got a brand new OEM driveshaft and replaced the clutch disk and transmission.

Have you put a stethoscope on it to figure out where it's coming from? I hate that goddam noise. I know I'm not supposed to worry about it but I still hear it and I know it wasn't there 17 years ago when I got that bike.

[Martin]

Some Bricks seem to be noisier than others, I used to know a guy who was BMW trained, I got the bike on his recommendation. I looked at Guzzi's and  BM's ( moment of madness a Harley) and was pretty much set on an R80 until he told me to look at a K75. The more I read the better they looked, anyway I vaguely remember asking him about the noise and he said nothing to worry about, clutch noise. I thought you might have had the solution with a Bruno shaft, possible closer tolerances. I m getting close to doing another lube I will probably use the Honda moly paste this time as it seems to be preferred over the Optimol. Guard Dog no longer available apparently and never sold in OZ.
Regards Martin.

[Motorhobo]

Well, if you get it on the lift and have it running in neutral, give a listen with a steth or funnel to try to figure out the source. This issue comes up over and over again here and I'd sure like to know how to shut the danged thing up so I can give my clutch lever hand a rest when stopped every once in a while.

[Martin]

You need to use one of Laitches solutions ear plugs.
Regards Martin.

[Jonico]

Well I recorded it on video and have uploaded it to YouTube

It sounds the same as the original poster's audio. I've decided not to worry about it, and my mechanic isn't the least concerned either.
I've had the beastie brick for two years now, am it's 8th owner since 1986. I've put about 30,000 km on it. I have no clue how many km it really has, since the clock said 12,450 when I bought it, which means it was a new instrument cluster. I have no clue who did the engine rebuild and abs and all, or when. But the damn thing runs like a champ!


Sent from my iPhone using Tapatalk

[mystic red]

Sounds like a good, strong brick to me.

[wmax351]

My guess with that sound (mine makes it too, 110k on the transmission, with 10k since I re-did the seals and inspected the internals) is that it is lash in the gears, made noticeable due to the lower RPM and therefore wider spread power pulses. When a cylinder fires, a jolt of toque goes through the transmission, and then after the initial power pulse, the momentum of the gears makes them continue onwards at the peak speed of the engine, until the reach the end of their lash, and are jolted to the slower engine speed. Since it is in neutral, all the gears are still engaged and spinning, but are not locked in their shafts.


Pulling the clutch on a BMW totally unloads the transmission due to the dry clutch: on a wet clutch bike, it would continue to make the same noises. On a BMW, the transmission will be totally stationary.


The noise disappears with load, as the impulse damper (in the transmission) takes up the intermittent torque peaks, and the transmission is loaded with forward acceleration.


If you put it on the center stand, and pop it into first, you should probably have the same sound, but amplified as the engagement of the transmission increases the inertia of the drivetrain.

[wmax351]

Other suggestion: Try riding a Harley Davidson Ironhead Sportster sometime. If it doesn't sound like it's about to explode, something is actually wrong with it.