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Tank Slapper Causes

jhall

jhall

BMWMOA #24809
In this month's ABC Airmail, a writer told of a recent tank slapper, on an 86 R80GS, that instantly put him down hard. He is slender, was riding solo, with bags and rear rack fully packed. Later, he mentioned a, "dangerous combination" Oak had warned of years ago: light weight solo rider, weight behind the rear wheel, and handlebar mounted windshield.

I experienced a mild tank slapper on a Harley in the 1970s, soon after installing a handlebar mounted windshield. I tossed the windshield and never had one again. I bought an R100/7 with Luftmeister fairing in 1981, and have only owned frame mounted fairings since. In 1992, I rode an 82 R100RT cross country, Kentucky to California and back. On I-70, riding through the Utah mountains, I zipped along at 80-85. Truck ruts and crosswinds made handling a bit squirrelly but I still had fun, though MPG dropped and reserve came on early. I stopped at a gas station, and kicked tires awhile with three Harley riders. The subject of MPG came up and I said mine had dropped to mid-30s, but only due to running 80-85MPH. They looked at each other as if I was lying. Finally one said they could only run 55-60 through those mountains due to severe instability and concern about crashing. I then noticed they all had handlebar mounted fairings. I have only ridden BMWs with frame mounted fairings/windshields, and have never had even a hint of tank slapper. But the old Harley quickly introduced me to the phenomenon, and apparently the three Harley riders in Utah were aware also. I rode many miles with large Luftmeister tail trunks on the 79RT, 82 RT and R100/7, and often loaded them far more than the recommended 11 pounds. GoldWings, have a huge tail trunk, and I even have a trailer hitch mounted, "Pakit Rak" for mine, which I have used several times, that weighs 30-40 pounds when loaded. Never had a tank slapper on a GW either.

IMHO, the Airmail writer's tank slapper was most likely due to the handlebar mounted windshield. Still, I am now having second thoughts on mounting a larger trunk on the current 95 RT. I had even considered a trailer hitch to mount the Pakit Rak, but don't think I am gonna bother now.

Does anyone else here have experience with tank slappers? If so, in retrospect, what do you believe cause it?
Thanks
 
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IMO a steering damper is just a bandaid. The problem should be fixed and/or not create the problem in the first place.
 
I had a tank slapper on my 1981 R100RT. This occurred after riding from NC to Colorado. After that one event, it never happened again. The bike was not loaded as all my gear was in a tent in Paonia. I continued my trip on to the National and back home to Raleigh. I serviced my steering head bearings after I got home but never did have another tank slapper. Maybe it was crosswinds?

Doug
 
20774 said:
IMO a steering damper is just a bandaid. The problem should be fixed and/or not create the problem in the first place.
Click to expand...

I guess you could look at it that way, but if that's the case why do many bikes come from the factory with them and they are common on race machines? Is adding an Earles type fork to a side hack equipped machine an effective way to make it handle properly or a band aid? (Not trying to be abrupt or sarcastic, just trying to understand the difference.)
 
Steering head bearings

Loose or worn steering head bearings can cause instability. Improper air pressure can also be a cause. Improper loading too much weight to far back or behind the rear wheel can cause the problem.

I had a handle bar mounted windscreen on my R75/7. It was one of the reasons I traded the bike for my R80RT. I couldn't stand the look of the after market fairings and the handle bar windscreen could be a pain in adverse wind conditions.

To be honest, I have never had a tank slapper, I keep things maintained and loaded right. St.
 
ridesolo I understand your point. IMO addition of steering damper for a street bike tends to be more of a selling point. The thing with race bikes is they are hauling butt with tremendous front end forces on heavy braking with the rear tire lifting off the ground...you really do need something to stabilize the front end.

As for /2 machines, both of my vintage bikes have steering dampers...they are off 100% of the time. Sure, if I had a sidecar, the steering geometry changes as the rake is changed so yes, a steering damper is probably needed. A damper can be of great benefit when going slow over rough terrain to keep the bars from being jerked from your hands.

But I don't think one should look to use a steering damper to make it OK to ride fast with a handlebar windscreen and lots of lot aft of the rear axle. Maybe ride slower or something. A damper has its place as I highlight a few above...but not to mask an existing problem.
 
jhall said:
Does anyone else here have experience with tank slappers? If so, in retrospect, what do you believe cause it?Thanks
Click to expand...

I had several on an R100RS. It was worn steering head bearings.

They're quite terrifying. I was barely able to ride out the last one. And the RS toothpick handlebar didn't help.
 
I had major tank slappers once going about 90 kmh on dirt loaded pretty top heavy. The cause was pretty apparent as all the air left the front tire almost immediately from picking up a nail that pincushioned the front tube. I was watching the ditch expecting to go in but somehow managed to stay upright. A steering damper on would have made a major difference but I didn't have one on that bike at the time. Yes a totally different situation that what you guys are talking about but still pretty scary. All of my street time my bikes have been pretty well behaved, I only run steering dampers on my offroad bikes.
IMG_1347-XL.jpg
 
Damper May Help

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
 
My opinion: If a bike comes OEM with a steering damper the manufacturer is compensating for a design problem. If a bike develops, over time, a tendency to do a tank slapper there is a maintenance issue. Usually steering head bearings but sometimes tire related, bearing related, broken frame related and a few others I could think of if I tried harder.

Other issues are known to be: improper loading and a problem with aerodynamics.
 
Slap (un) happy

STEVENRANKIN said:
Loose or worn steering head bearings can cause instability.

About 12 years ago my '78 R80/7 developed a tremor that you could feel if just touching the handlebars with my fingertips. It got a little worse so that if I took my hands off, they would start to dance and wiggle a bit. It finally devolved into the bars shaking and going divergent in short order. This was when I wised up and started looking for a cure. I cleaned and regreased the steering head bearings and set the bearing preload properly. As per Oak's advice, I tensioned the bearings so that while on the mainstand, the bars would not fall in either direction but would slowly continue if nudged. It is great when a potentially lethal problem can be solved for a couple of bucks. This is not the bike's fault, only my lapse in maintenance. I should mention that this bike is equipped with a Windjammer frame mounted fairing, Krauser bags, and a tail trunk. and is often loaded to the nuts with camping gear. No instability noted, even without a steering damper.

Cheers,
Russ '78 R80/7 '78 R100RS
Click to expand...
 
tank, slapper

Hi years ago i was in a bearing company ordering steering head bearings for my 1976 r90/s when a guy waiting to be served said bmw steering head bearings i said yes got severe bar shakes he said what speed around 45 mph, is your rear tire worn said yes about a thousand miles left on it replace the tire i did and that was that never did it again until the rear tire was worn i have had 5 airheads in the last 45 years and the all did it ,yes tighten the steer head bearings until slightly tight then run over a rough section of road to let it all bed in only replaced one set of bearings in 45 years in around 300.000 miles strip them out clean with very fine emery to remove slight brindleing and grease my last airhead i have almost 400.000 km original bearings and yes shakes her head at around 45 mph worn tire,hope this assists Jimmy:ca hopeing for happy healthy new year so we can back to going to rallies
 
beemeruss said:
STEVENRANKIN said:
Loose or worn steering head bearings can cause instability.

About 12 years ago my '78 R80/7 developed a tremor that you could feel if just touching the handlebars with my fingertips. It got a little worse so that if I took my hands off, they would start to dance and wiggle a bit. It finally devolved into the bars shaking and going divergent in short order. This was when I wised up and started looking for a cure. I cleaned and regreased the steering head bearings and set the bearing preload properly. As per Oak's advice, I tensioned the bearings so that while on the mainstand, the bars would not fall in either direction but would slowly continue if nudged. It is great when a potentially lethal problem can be solved for a couple of bucks. This is not the bike's fault, only my lapse in maintenance. I should mention that this bike is equipped with a Windjammer frame mounted fairing, Krauser bags, and a tail trunk. and is often loaded to the nuts with camping gear. No instability noted, even without a steering damper.

Cheers,
Russ '78 R80/7 '78 R100RS
Click to expand...
When I first got the current 95 R100RT, it would shake its head sometimes, at about 30-45 MPH, if ands were removed from the bars. When I went through the front end, of course cleaned, lubed and and adjusted the steering bearings. I removed the fairing seals and installed gaiters, as I did with the two previous RTs, years ago. I have never tried to adjust head bearings with RT fairing seals in place, and cannot imagine how anyone could. My guess, tights the bearing till it bottoms lightly, then back it off 1/16 to 1/8 turn. Then, after tightening the lock nut, the bearing should be tight enough to provide the results you describe with the forks' turning when pushed lightly. That's how I like them to be, but sometimes it takes 2-3 tries to get it right. I have always disliked the fairing seals anyway.

As for head shaking at 30-45, Honda GoldWings are infamous for it, and riders have complained for decades. Honda Motor Corp's answer it that it's illegal to ride with both hands off the handlebars, and should not be done anyway. LOL The fix is similar to Airheads: fork brace helps, replacing cupped front tire helps, steering bearing R&R helps, etc..
Click to expand...
 
For me, Oklahoma Highway 66 Between OKC and Yukon. The pavement was that slappity slap uneven concret slabs. Running 75 to 80, just cruising along on a Saturday morning on a crisp fall day going to the bank in Yukon. Was running a full frame mount Avon Fairing on my 72 R75. Was running brand new Dunlop K81 tires that had been put on the previous night. Those tires had a very V shaped profile and had maybe in retrospect a 1/2 inch diameter friction area while upright. It was in the corners that they supposedly shined while leaning the bike over.

Suddenly, no vibration, no dips, do jarring, nothing just began oscillating from side to side with the bars......Instinct took over and old truck driving skills for jack knifing trailers........take the foot off the gas and slow down while putting on the trailer brakes......so roll off the throttle and drag the rear brake........It worked and I lived.....

Those K81's came off immediately and never had the hint of that again..........God is good.....Dennis
 
I had one on my R90S, which caused me to crash. I attributed the cause to the Dunlop K-81 tires I had recently mounted. The front was a 3.60-19 which was a lower profile tire compared to the stock Metzler 3.25-19, thus reducing the trail, leading to instability. I fixed the machine, put on a set of Conti tires and rode to the 1987 MOA rally in Vermont. I still have the machine.
 
Like Paul said, different reasons. For me the first one was because of an improper size tire on the front. The next was from improper preload on the back. It's one thing you want to fix fast!
 
PGlaves said:
My opinion: If a bike comes OEM with a steering damper the manufacturer is compensating for a design problem.
Click to expand...

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.
 
Dampers

I have the factory installed dampers on my 84 R80RT and my 78 R100RS, I have only ever had to use them a hand full of times. Mostly if I am on a poorly surfaced or dirt road.

The biggest thing I find to prevent problems is to keep the steering heads adjusted and lubed properly. Don't mess with tires that are not recommended for the bikes and keep the air pressure at the correct level. I don't have a tail box and never will unless it is forward of the rear wheel axle (the Pillion passenger seat, I don't ride double so putting luggage there is not a problem). I keep all weight as low as possible and as centered on the bike as possible.

One of the customers in my friend's shop told the tale of his father and mother touring Europe on a R26 when they were young. They had loaded the bike for two passengers and the bike was tail heavy to the extreme. While riding up a very steep slope, the front wheel came off the ground and they went over backwards. Now of course on an R26 going up a steep slope, they were not going very fast so neither was hurt nor the bike damaged but I wonder, If they had been able to ride faster, would they have had front end instability due to almost no weight on the front wheel? One thing, I have never heard of a tank slapper happening on an R26.

Speed and light weight on the front tire, two things to think about. St.
 
There are a few reasons, bad bearings, too much weight on the rear of the bike that can cause tank slappers but handlebar windscreens are a potential cause of tank slappers. When I was 16, I had a small Honda with a homemade windscreen and it caused me to dump the bike on the highway. I know on my stock 2000 R1100R with the stock windshield it came with a BMW factory steering damper. You could see how it worked, put the bike on its center stand so the front wheel was off the ground, move the handlebars in a normal fashion and you wouldn't feel any resistance. Then try moving the handlebar quickly and the damper would slow the movement down enough that you had to push fairly hard to get it too move. I'm pretty sure BMW put the damper on because of the handlebar windshield.

My current ride is an old 86 K100 RT that I converted back to a naked bike. I installed a Givi aftermarket windscreen that is a two part screen and directs a lot of air behind the screen. It is fairly small but very effective in providing a quiet air space behind it. I live in an extremely windy part of the world and haven't had any issues with it affecting the handling. I might be tempted to put a damper on the front end, if I can find one that is small and would fit the bike. It can't hurt.
 
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