A tank-slapper is a potentially dangerous phenomenon that has been haunting riders and motorcycle designers for decades. It’s called a tank-slapper (or wobble) because the handlebars suddenly seem intent on battering the fuel tank into submission as the front wheel thrashes from side to side like an unruly shopping cart.
If you’ve never seen, or experienced one, look here:
http://www.randtclub.com/Video/cedwa...pper_tt99.mpeg
(The slower less-violent fish-tailing effect that occurs at higher speeds is called weave but it’s actually the same thing as wobble.)
What sets it off initially is an uneven road surface. A bump or series of bumps is enough to do it. Further, if you take the same bump at different speeds, or straight up instead of leaned-over, it changes the likelihood of starting a tank-slapper. So if, and when it starts is really outside of the rider’s control.
So why does this happen on some bikes and not others, and become a problem for one rider but not others on a given bike?
To answer this question we need to look at modern motorcycle designs. In order to achieve quick responsive steering the chassis designers have made the bikes less stabile and consequently more susceptible to tank-slappers. A lot of bikes now come equipped with steering dampers because of this.
In particular the sport-bikes now have a more upright angle to the steering head and forks as well as a reduced trail dimension.
An outside influence is needed to spark these phenomena.
A central issue in chassis design is stability. A motorcycle is designed to return its front wheel to a straight-ahead condition after hitting an object or bump in the road that causes the front wheel to deflect slightly to the right or left. In other words, it has to remain stable for a variety of road conditions, and motorcycle stability is foremost a safety issue.
The way dynamic stability is assured is to design a bike with enough distance between the point where the front wheel touches the road and the intersection point between the steering axis and the road. This is called the trail dimension.
A longer trail dimension increases the motorcycle's stability on straights, but will also negatively affect the motorcycle's turning effort, i.e. more rider's strength is required in corners and transitions. However, the more trail, the greater the ability of the bike to self-correct it's steering. It's a longer trail dimension, for example, that allows you to easily ride with no hands on some bicycles, but not others.
A shorter trail dimension, on the other hand, produces a lower opposing force to steering inputs. It's kind of like power steering. So the steering requires less rider strength, but larger handlebar displacements from bumps in the road and corners are fed back to the rider. Said another way, the shorter the trail, the more rider input that is needed to hold a line and the more responsive the bike feels since it is more sensitive to steering inputs.
For example, the two superbike steering angle positions, 23°30' and 24°30' produce trail dimensions of 91mm and 97mm respectively. The wheelbase, also an important factor in stability, remains unaffected when you change it. As a comparison, the Monster steering angle is fixed at 24° and the trail dimension is 94mm. Adjusting the trail dimension on most manufacturer's bikes is not an option.
Note Ducati's warning: "Trail should only be altered after all the other (geometry and suspension) changes have been made and you are comfortable on the bike. If the bike displays any instability problems they need to be sorted out first, as this steering head angle change will magnify these characteristics."
(One reason, for example, is that part of its effect mimics changing the rear ride height.)
The Haynes Service manual goes on to say "Warning: The steering head angle must be set to the road position (longer trail) whenever the bike is used on the road. If the steering angle is set to the race position (shorter trail) ... the handling of the machine could become unpredictable on uneven road surfaces."
So, shortening the trail is considered unwise for street riding (unlike tracks) where bumps in corners, potholes and other road hazards repeatedly challenge your bikes steering stability. Here's a case where inexperienced or uninformed riders who set-up their street bike chassis geometry as race-bikes are just looking for trouble.
Trying to mimic factory race bike set-ups can get you into trouble. It's central to racing that race bikes often need to sacrifice high-speed stability to handling. Riders may initially run the steeper steering head angle, but often, as they get faster, they realize they want more stability, not less.
To get more stability there are two things that Ducati typically changes on their race-bikes: the triple clamps and the swingarm. They use triple clamps with less offset, typically 27mm instead of the stock 36mm, and use a 25mm longer swingarm to increase the wheelbase. These changes to the triple clamps or the swingarm have the effect of moving the center of gravity forward which is the typical starting geometry of the Corsa race bikes. Remember, changing the steering head angle does not, by itself, change the wheelbase or alter the center of gravity.
Engineers have experimented with ways to reduce the extent of the problem. For example, they have found that heavier riders are less susceptible to the problem than lighter ones. They know that the amount of weight on the front wheel is important and that tank-slappers often occur when exiting corners under power when weight transfer off the front wheel occurs.
A common solution is to add a steering damper to kill the rapid oscillation. Unfortunately, while increasing the steering damping reduces the risk of wobble, it increases the chance of weave.
So, what should a rider do if it begins?
First, you need to increase the forces on the tire that are trying to return the front wheel to the straight ahead position. If you’re accelerating get off the gas to allow the bike’s weight to push down on the front wheel to let the trail dimension do it’s job. Also, shift your weight forward on the bike to help as well. The reason sport-bikes have clip-ons is to get more of the rider’s weight forward.
Second, hold on but don’t push on the handle bars. Pushing hard just feeds more energy into the tank-slapper. Steer gently until the steering settles down.
If it happens on a regular basis check for excessive play in the the steering head bearings and add or adjust your steering damper. Change your steering head angle back to the street setting.
The stock steering damper is often not adjustable but provides a fair amount of damping that is suitable for most road and track applications.
Supplied on some models, the Ohlins steering damper adjustment knob has 16 different damping positions. The recommended setting is the midpoint 6-8 clicks from full soft. Full hard is 18 clicks clockwise from full soft.
Track surfaces are smoother than typical roads, so less damping is needed. The steeper steering head angle is less stable (less self-correcting) so more damping should be considered.
As you increase steering damping you gain increased control over steering oscillations that are set up by uneven road surfaces and even side wind gusts. As the side-to-side speed of the steering increases, so does the damping effect. This results in a smooth easy-handling ride at low speeds combined with heavier dampening during extreme situations The trade-off between handling feel and steering stability is a personal choice.
By its very nature a steering damper will spoil your bike’s steering a little. It’s main value is to provide protection against tank-slappers.
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