Tank Slapper Causes

[alancoles]

There are plenty of good possible reasons provided here, but one that didn't show up, for some strange reason, is spokes

That bike, unless modified, would have had spoked wheels. Checking spoke tension (properly) is a much over-looked regular maintenance issue and will easily lead to a highspeed wabble. I'm not saying that was it, far too little info to identify the culprit, but it is one possibility.

I think one of the most common culprits, especially on a dual-sport bike is the front tire pressure being set (or let get) too low. Some folks will reduce front tire pressure while riding on dirt roads down to 24-26 psi to get better traction/bite on the dirt. Too many don't re-inflate their tires when going back on the tarmac. That is another wabble waiting to happen.

 

[na cl k9]

Maintenance, tires and tire pressure, loading, component wear and accessories.

My factory service manual has all the factory “Service Bulletins” for /6 - late /7 bikes. A lot of important information is published in the SB’s about stability. Here are some of the things mentioned: Properly lubricated and tensioned steering head bearings / swing arm bearings and wheel bearings. Subframe and shock mounting bolts tight. Wheel spokes properly tensioned. Correct tire sizes and pressures F&R (with air not visible through the cords). “Live” springs in the forks with correct grade and quantity of fluid with damper assemblies clean and in good condition. Accessories cases properly fitted and loaded to prevent a weight biased to the rear. Functional steering damper hydraulic unit and adjustment mechanism clean and lubricated. Gross vehicle weight within factory limits. Brakes properly adjusted and disc’s with minimum runout.
BMW made a handlebar mounted windscreen for a while so the effects that design had on handling were calculated to limit its input. Most similarly designed and mounted handlebar mounted windscreens should pose no risk of instability given attention to the above notes. Now, when someone wants to know why they got into a “tank-slapper”, the answer is somewhere in that list and most of those items are part of the 5 & 10 K service. The rest is common sense. My only “tank slapper” was the result of low tire pressure and a heavy fairing (Hannigan with full pockets if you must know). I never forgot as it was on a winding downhill mountain road and I had to open the throttle to escape it. I got real close to the edge of the world that day.

 

[gregfeeler]

Modern bikes, say post 1980 or so, come with steering dampers as part of a design solution. The rake has been reduced significantly to get them to turn quicker. That design requires a steering damper to deal with the inherent instability of steep rakes. An R75/5 has 27 degrees of rake, a Honda CBR 959 has 23.8. Although I have nearly zero experience with choppers, they can have up to at lest 40 degrees -- they don't need steering dampers but then they don't steer very well.

+1!

 

[braddog]

When I first got my '77 R100RS, it had a very noticeable wobble at certain speeds, but thankfully, I never experienced a full blown tank slapper.

After getting a set of new tires, new front fork springs and new rear shocks (can't remember if we did steering head bearings or not), the thing road like wheels on a rail, at all speeds. The difference was amazing.

 

[stevenrankin]

Seems like

It seems like the common thread in the cause of tank slappers with BMW bikes is plain and simple maintenance and owner related. By this I mean, keep the tires at the proper pressure, Don't overload the bike or shift weight too far aft of the rear axle, keep the steering bearings adjusted and greased, keep the fork oil changed and full.

Nothing to do with the design of the bike itself and the damper added on by the factory is there just as an aid for unusual conditions, not to be used all the time.

So, keep in mind the proper maintenance/loading and ride without worry. St.

 

[alancoles]

+1

 

[snowbum1]

Tank slappers

There are lots of opinions. Been that way, regarding tank slappers and other instabilities, since nearly forever, it seems. Few have done serious testing.

A real honest-to-gosh tank slapper is actually quite rare. Some freakily nasty instabilities are only somewhat rare. Some types of instabilities are pretty common.
Here's my article that deals directly with it all. It covers instabilities in general, and specifically covers weaving, wallowing, tank-slappers, ETC.

https://bmwmotorcycletech.info/instability.htm

Snowbum

 

[wbrownell9]

The only instability I ever noticed on my 2016 R1200 GSA was accelerating uphill to pass, it was probably the only time I ever got to triple digits. I have no idea what was going through my head that I needed to pass, under those conditions, that badly, but I guess it seemed like a good idea at the time. To recap the situation: loaded for travel (full panniers, other stuff strapped on the pillion seat, so all cargo weight was pretty much directly over the rear axle, total ~80 lb); uphill (weight biased rearward), accelerating (front wheel lighter than usual).

So when it started to weave I had a moment to think about what to do before things got out of hand. What I did was pull in the clutch and let it find its way and slow down on its own - problem solved. I remember an old British video that dealt with weaving (low frequency) and wobbling (high frequency). The former was addressed by leaning forward - weighting the front wheel. Turns out cutting power does the same thing. Certainly, front brake is unlikely to make things better.

 

[nickrides]

This should fix it:https://www.bing.com/videos/search?...7561905BCC67A00F20B67561905BCC67&&FORM=VRDGAR

 

[daz]

Dangerous Combination

If you're an Airhead there was an interesting article in Airmail magazine, January 2021 about this same topic . Page nine.

 

[72r60/7]

It has always amazed me to watch bike factory assembly videos that show not more than 5-10 seconds spent to torque the steering head bearings.

Though more time may be spent later to bring to specific specifications.

And that’s for millions and millions of two wheelers each year.

Charlie

 

[wemi]

I wish the current RT had a steering damper, I'd probably engage it when riding on interstate highways. I believe it may help cancel out the back and forth movement before it becomes. problem.

Including BMW, all of the other motorcycles I have owned since 1981 have had frame mounted fairings, including the last Harley (86 FXRD). I have never heard of anyone with a frame mounted fairing, reporting a tank slapper bad enough to cause an accident. I understand Harley has came a long way with aerodynamics in their, "bat wing", but if I were looking to buy another Harley, the only candidates would be ones with frame mounted fairing.

I really believe the fork mounted windshield is the culprit, but just an opinion, as I am certainly not an aerodynamics expert. I wish Tom finch was still with us, as I'd love to pick his brain on this. Most know Tom was, at one time, very much involved with Airheads and using them to tow trailers. Later, he was big in the GoldWing world also. I met him at a GW gathering down in Texas a couple of years ago. R.I.P. Tom

The current RT does have a steering damper as factory standard equipment, as does my 2013 F800GT.