'85 K100RS handling question

Hi everybody,
so I just acquired a 1985 K100RS with lotsa miles on it (251k). Bike has a great maintenance history and runs well (except a rattle at cold startup- another topic). Here is what I observed today: at 60 mph I am downshifting from 4th to 3rd and when I briefly let go of the handlebars to see how the bike is balanced a wobble starts in the front wheel and it builds very rapidly. As soon as I hold the bars it disappears. No wobble can be felt in any other condition. Only when bike decelerates in that tempo range and when I let go of the bars. Now you may ask, why do you let go of the bars? I usually don't but none of my other 3 beemers do this. Is this a design issue with the K100RS? Any other ideas? Thanks.

allemande said:

Hi everybody,
so I just acquired a 1985 K100RS with lotsa miles on it (251k). Bike has a great maintenance history and runs well (except a rattle at cold startup- another topic). Here is what I observed today: at 60 mph I am downshifting from 4th to 3rd and when I briefly let go of the handlebars to see how the bike is balanced a wobble starts in the front wheel and it builds very rapidly. As soon as I hold the bars it disappears. No wobble can be felt in any other condition. Only when bike decelerates in that tempo range and when I let go of the bars. Now you may ask, why do you let go of the bars? I usually don't but none of my other 3 beemers do this. Is this a design issue with the K100RS? Any other ideas? Thanks.

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That is an absolute classic and almost certain result of loose steering head bearings. You should adjust them. Then if you feel any notchiness in the straight ahead position of the steering you should replace them or have them replaced.

Tire balance or tire wear could also agrivate the situation.

Suspension

Weak or worn out suspension components could also be a factor here.

Gravity pockets could be a factor.

PGlaves said:

Gravity pockets could be a factor.

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You mean gravity pockets in the head bearings?

allemande said:

You mean gravity pockets in the head bearings?

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I was being factitious.

IMHO Sounds like you've got a worn/ wildly out of balance tire.
This has happened to me several times and new tire always fixed it.
If you don't want to get a new tire a ounce or two of Dyna Beads in there will fix it.
Flame suit on!

Wheels

I had a bike that would shake at exactly 80mph only. Wheel threw a weight.

Head set loose: wobble

My '85 K100RS has 203,000 miles and I once changed my head bearings. Got the wobble. As I loosened the top triple clamp to fork leg bolts and then snugged the head bearing knurled nut some and re-snug fork clamps, the wobble got less. After several knurled rotations of about 10deg that I did with a giant set of channel lock things the wobble went away. No hand driving has now been possible for 100,000 miles or more. Like Paul Glaves said.

I'm with Paul and get it done before you have a real tank slapper event....those are not fun. That can ruin a good pair of underwear.

Steering issue solved on K100RS

The wobble while decelerating on my ‘85 K100RS was solved after tightening the steering head bearings. Thanks for the good advice!

I'm glad to hear it. Usually I say, "Now go ride" but since I don't know how things are where you are I will just say go ride when it make sense to do so.

I do have to tell a story. When we lived in Kansas the Falling Leaf Rally, sometimes called the Frozen Leaf Rally, was an annual must-go-to. One year a good friend was at the rally on his R100 something. When we got up and everybody was leaving the rally his bike would not start. Ruh Ruh. Ruh Ruh. Pretty soon Ruh. One of the host club members got his pickup truck and jumper cables. Ruh Ruh Ruh. So we decided to push start / bump start the bike. That totally failed to work. At pushing speed the bike front wobbled to the point that he couldn't even keep it in his lane in the campground roadway. Several of us tried to ride it being pushed and it turned into a giggle fest.

He had said that he noticed a wobble when he accelerated but he just powered up through it. He said he also noticed it slowing down but just used more brake to make it go away

We finally got it started with jumper cables from the pickup. He had a spare key for the gas tank so rode it from Potosi, Missouri to our house in Lawrence, Kansas without turning it off. The starting issue was a badly worn nose bushing in the starter which caused the armature to drag. The wobble issue was a very loose set of steering head bearings. A few hours in the shop and a new starter drive front bushing and new steering head bearings made the bike east to start and easy to ride once again.

Thank you Paul Glaves

Thanks for helping our K community keep these great bikes running, especially helping the people that recently buy these and need to learn some of the quirks they have to get the great rewards these bikes give.

075038 said:

Thanks for helping our K community keep these great bikes running, especially helping the people that recently buy these and need to learn some of the quirks they have to get the great rewards these bikes give.

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What Paul said

I was a K100RS rider for years. I agree with Paul.
I also say if you upgrade the suspension-- high end rear shock and cartridge emulators for front shocks-- it will help stabilize it and transform that bike.
It should be smooth, stable, solid, and handle well.

dp