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-   -   Airhead tank slapper (http://www.advrider.com/forums/showthread.php?t=559726)

davemon 03-16-2010 07:37 AM

Airhead tank slapper
 
Just did a 1200 mile ride on my 1977 R100S loaded with gear and camping eq. on the ride down road like a champ. Added my coat and riding pants onto the gear in the back and at 25 MPH the handlebars flapped like the tire was square. Repacked the load and same thing just not so bad. Was able to accelerate through the slap mode and cruised fine at highway speed. Made it home removed the load and same dance at low speed. Had the forks rebuilt about a year ago new rear schocks and new tires. Anyone else had this problem? Whats the fix?
Thanks
Dave

jtwind 03-16-2010 07:55 AM

My r80 with an S fairing did this. Not uncommon on airheads with handlebar or fork mounted fairings and windshields. On mine it was steering bearing adjustment that fixed it. But could as easily be the fork, tires, tire pressure, fairing or windshield attitude, poor shocks or loading/weight issues. I'd just work on making sure each one was right and then move on to the next if it doesn't seem to improve.

If you rode it similarily loaded before you did the forks, shocks and tires and it didn't do this, I'd start there.

Foot dragger 03-16-2010 08:00 AM

The local BMW dealer is an airhead guy,he helped me put new races and bearings in the steering head on a R75/6 I brought back from the dead,he really stressed the adjustment on the steering head bearings. He used a torque measurement device suggested by BMW. He also said my forks would never work rt with out expert assembly and new stock parts.
Evidently the ft end on these older beemers is touchy about adjustment so maybe check all that over closely.

BKMLWR 03-16-2010 09:09 AM

been riding a 77 R100s since I bought it new in 77, over 170k miles never had a tank slapper except for hitting washboard road once, riding includes 2up cross country trips

Check tire pressure and steering head brgs

Bigger Al 03-16-2010 10:20 AM

15 years on an R90/6, and the only time the front wags is when the tire reaches the end of it's life span. Once the shimmy begins, I replace it and things go back to normal. This is irrespective of brand or tread.

Still a good idea to check out the bearings, though.

chasbmw 03-16-2010 01:25 PM

low speed wobble is almost always caused by too loose steering head bearings, move the adjuster nut in small increments until the wobble goes away, but not so much that the bike feels as if you have wound down a friction steering damper.

Charles

dilandau 03-16-2010 01:33 PM

Quote:

Originally Posted by chasbmw
low speed wobble is almost always caused by too loose steering head bearings, move the adjuster nut in small increments until the wobble goes away, but not so much that the bike feels as if you have wound down a friction steering damper.

Charles

what he said - at least that fixed my wobble. the adjuster nut and the top nut sort of work together to make a friction damper so in my experience it was a bit of trial and error, moving the adjuster nut, then tightening the top--- testing how that was, then doing it again and again- maybe 4 or 5 times until it felt firm but the front end moved easy and smooth side to side.

anonny 03-16-2010 08:33 PM

So.... how is your front tire? ..... check real close for cupping.

mphoppe 03-16-2010 10:10 PM

Steering head bearings...this is what fixed mine... follow the directions exactly....
http://www.largiader.com/bearings/

BadBob 03-17-2010 01:37 AM

Tank slapper!
 
Yo Dave, nature of the beast!

Airheads, untill you`ve felt it, you would not believe!

Mainly because of the engines design.

The airheads are VERY intolerant of ;

Tyre wear/adjustment

Headstock/swingarm adjustment

Any loose frame/rack bolts!....one loose bolt on my rack, while touring in France. Tank slapper so bad, thought my frame had broken!

If you look at the position of the headstock relative to the rest of the frame, (with the tank off)

Its no wonder they somtimes shake there heads!

I welded two tabs on the frame, and attached a strut, on each side. (as I`d seen on endurance BMWs)

If you hold a inch dia. tube in position without removing anything.
just under the tank at the front. just above the S/Arm at rear. Its clear where to attach.

The change in handling is astounding!

From BMW to Italian motorcycle!!!

Ride safe, roll on summer!

BB

davorallyfan 03-17-2010 02:21 AM

BMW's like yours are bad handling things and extremely sensitive to front tyres - change out - check pressures - my 90S was a shocker - I used to change out half worn front tyres to make it handle - ME33's.

They came with dampners but..

A new tyre should fix it steering head bearings - yes but its usually front tyre related

slash5swb 03-17-2010 07:51 AM

To get the steering head adjusted right, you have to lift the bike so the front end is hanging free. Starting with the wheel in the straight ahead position, you start to push the bars to one side or the other (doesn't matter which direction.) As you start to push, the bars front wheel should gently and slowly fall the rest of the way to the steering stop. This slow fall represents the right amount of friction from a properly tightened steering head. If the wheel falls quickly and bangs to the stop, it's too loose. if the wheel won't "fall" over on it's own (you have to push it all the way to the stop) it's too tight. It might take several adjustments to get it there, but it works, and it will make the front end very stable. (NOTE: Try to account for cables and wires that impede movement. Test fork in both directions and try to account for this in your estimation.)

charliemik 03-17-2010 11:09 AM

When you're tweaking the steering brg adjustment, you'll know you've gone too tight if your bike starts to do a slow weaving back and forth on the road.

Wirespokes 03-17-2010 11:12 AM

Quote:

Originally Posted by slash5swb
To get the steering head adjusted right, you have to lift the bike so the front end is hanging free. Starting with the wheel in the straight ahead position, you start to push the bars to one side or the other (doesn't matter which direction.) As you start to push, the bars front wheel should gently and slowly fall the rest of the way to the steering stop. This slow fall represents the right amount of friction from a properly tightened steering head. If the wheel falls quickly and bangs to the stop, it's too loose. if the wheel won't "fall" over on it's own (you have to push it all the way to the stop) it's too tight. It might take several adjustments to get it there, but it works, and it will make the front end very stable. (NOTE: Try to account for cables and wires that impede movement. Test fork in both directions and try to account for this in your estimation.)

Exactly! With one hand I hold the front of the fender and the other a handlebar. Then turn the front end slowly and feel for either too much free play or notchiness. Yes, notchiness! Too tight and it'll feel like bad bearings.

P B G 03-17-2010 12:09 PM

Quote:

Originally Posted by slash5swb
To get the steering head adjusted right, you have to lift the bike so the front end is hanging free. Starting with the wheel in the straight ahead position, you start to push the bars to one side or the other (doesn't matter which direction.)

And when you tighten it up the top nut will change things a bit.


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