Does "tank-slapping" happen on high-speed ebikes?

[BahamasBiker]

If you've experienced tank-slapping (steering head shake) on your electric ride, please share the story, thanks.

This is not the same problem as the infamous "Harley Wobble" (aka Harley Davidson Death Wobble)

https://www.youtube.com/watch?v=TpeNWxHF6uI

https://www.youtube.com/watch?v=LmHU-gtlOKo

https://www.youtube.com/watch?v=LZ1srcQMa_0

 

[d8veh]

You can get a tank-slapper on any two-wheeled vehicle. Not all result in crashes. The worst thing is when you get too much torque at the back wheel, you lose traction and the back comes round to overtake the front, it bucks, and you go straight over the handlebars in a classic high-side. Make sure that you have enough traction from the back tyre to avoid that.

 

[MadRhino]

...And make sure your bike is straight and stiff, wheels true, suspension tuned, proper tire PSI...
The speed and acceleration that a bike can safely handle relies on geometry and stiffness, maintenance and tuning. This is even more important with our ebikes because they are shorter and lighter than motorcycles.

 

[Jonathan in Hiram]

Tires can make a big difference in handling qualities.. A long time ago I had a 750 Honda with the stock tires and riding across certain bridges in particular was a scary experience because the bike would squirm, slide and weave, it was also bad on freeway rain grooves but not as much as on the bridges. Put a set of Metzler Laser tires on it and it was a different machine, barely noticed the bridge effect and rain grooves were gone, couldn't feel them at all.

Tank slappers involve a lot of things, steering geometry, load, tire type, wear pattern and inflation, maybe your head bearings are a bit loose or your swingarm bushings are shot and so on, change one thing and the behavior changes, maybe for the better and maybe for the worse.

 

[The fingers]

Maybe since there's no fuel tank, we could call it "battery-slapping". :| My rear rack with the SLA flexes some while riding, not really noticeable but thankfully not serious causing wobble.

 

[John in CR]

I felt a slight speed wobble once at about 60mph. I had gotten my CG too forward trying for an aero tuck with my head past the bars. The rear suspension unloaded enough and front compressed enough that the resulting geometry combine with my CG position created a speed wobble on a bike otherwise set up with a high speed geometry. Luckily I had read enough about speed wobbles to instantly recognize it, and instinctively popped my head up and lowered my ass to immediately stop it.

Big travel bike suspensions are compliant enough that it's not hard to create the conditions on a very fast ebike, but rider mass is such a big portion of overall mass that corrective action is easy. The larger diameter front wheel I run now probably makes one impossible on my bike now due to the geometric change, but I'm not putting my chin out past the bars again to find out. Those types of adrenaline shots don't interest me at all.

John

 

[rider95]

Tank slap?? wow a flash back from the 80s my yz465 2 stroke would go in to a vicious one , but for E bikes under 2kw like mine its more likely you have just hit a bump or ricochet off a tree root (which is why I run a 20mm axle with a doblcrown fork ) handling and rider skill plays a big part . Anyway if your doing tank slappers and crashing you sound like my kinda of rider we should hook up!!

 

[John in CR]

Anyway if your doing tank slappers and crashing you sound like my kinda of rider we should hook up!!

We're at opposites on that spectrum. I don't want to be anywhere near anyone crashing or getting speed wobbles. I don't want to see the aftermath, nor do I want to get tangled up in it.

 

[Drunkskunk]

My first Ebike was a department store frame. it was fine for the first 1000 miles, but after that the frame got more flexible, and would start to suffer death wobble more and more frequently. That's partly why that bike got retired.

My monster bike would go into death wobble with the original fork under some extreme conditions. I haven't had that issue since changing forks to a longer set and a few other geometry tweaks.

 

[dingoEsride]

Tank slap?? wow a flash back from the 80s my yz465 2 stroke

That could be a Yamaha thing, my old XJ 900 required a fair bit of tightening of the knee's at about 200kmh, not sure why I wanted to go to that speed I think my brother was abad influence on hiz honda, and found the problem to be a stretching of the forks along with a bad bush.(fork flexing) so yeah anyloose bush, head set bearings, wheel bearing bad weight distribution tire pressure etc can cause wobbles

 

[motomech]

No, it's a precessional force thing.
A shopping cart doesn't have a steering head bearings, swing arm bushings/bearings, tire pressure adjustment, etc., but displays steering whl. wobble in the extreem.
Those things mitigate/aggravate the phenom., but are not the root cause.
Pretty much every road bike of any size up until the late '80's had it, in two flavors.
The Low Speed Wobble-you could calibrate the speedo to it, almost always at 40 mph.
As a long time motorcycle tech, I must have heard this a thousand times:
"When I take my hands off the handlebars, the front tire wobbles".
The pat response was; "Well, don't take your hands off the handlebars.
Unless the rider has a 100 lb.s on the rear rack, this wobble wouldn't increase to the point of spitting the rider off the bike.
But, that could not be said about the;
The High Speed Wobble-Again, very speed specific, "the one at a Ton"appears at 100 mph.
Certain bikes were very evil in this regard and woe be the rider who hesitates there.
Riders of some models, like my big Kawasakis, would develop strong inner thigh(and sphincter) muscles and herein lies one of motorcycling's great mysteries;
Why does it help when one squeezes the tank with one's knees?
High Speed wobbles often precede the dreaded progression, speed wobble, "tank slapper", "swapping ends", the "Flying W" and if the rider is lucky, "ëyes"on the top of the tank;
This badge of honor, two small perfectly formed dents, the result of the rider's knees landing, is often seen around the road race track.
So why did speed wobble go away on motorcycles made after the late '80's/early '90's?
Sort of a guess on my part, but I think the biggest factor is, frames becoming stiff enough as to not impart additional cyclical oscillation and frt. tires with a larger contact patch to dampen the movement.

Back to the original question. While I can imagine a careless Ebike rider "stubbing his toe"(crossing the frt. tire up), it's hard for me to imagine it developing into a true speed wobble.

 

[Punx0r]

It's a resonance problem - where the frequency of some external force matches the natural frequency of the bike + rider. This is why a skilled rider can stop a wobble by shifting their weight or clamping the frame tightly between their legs. I think the cure is normally to increase the natural frequency and bending frames or loose pivots reduce stiffness and the frequency with it.

 

[dogman dan]

I've had high speed wobble try to start, but I've rode too much to let it get out of hand.

Too flexy from side to side is a common cause of high speed wobble, often combined with poor battery placement such as 10 pounds or more mounted high on a rear rack. I've had bikes exhibit tank slapper tendencies at speeds as low as 15 mph. Stiff frames help.

But that is not always the same as, can't let go of the bars and ride. Any misalignment of the frame, or wheels can cause that. Like the wrong dishing of a rear hub motor wheel, or just bent frame. It can happen on a bike that normally rides good, once you load up the bike enough with batteries or cargo too.

And of course, a loose headset can make all of the above worse.

 

[BahamasBiker]

It's a resonance problem - where the frequency of some external force matches the natural frequency of the bike + rider. This is why a skilled rider can stop a wobble by shifting their weight or clamping the frame tightly between their legs. I think the cure is normally to increase the natural frequency and bending frames or loose pivots reduce stiffness and the frequency with it.

In this Vespa scooter forum, there are several apparent causes. The Vespa scooter has a trailing link(?) front suspension, which one of our guy's electric scooter also has. More than one Vespa owner says that sudden release of the brakes, and deacceleration, are the causes. The OP that got the wrist injury, and others, weren't going as fast as motorcycles.

http://modernvespa.com/forum/topic60149



Very low speed motorcycle tank-slapper / speed-wobble -- he says
Uploaded on Jun 9, 2009
"Front tyre worn wrong causes tank slapper when no hands on the bars, or when hard slow down in downhill corners"

https://www.youtube.com/watch?v=fRr92HBHKm4




Very small kid on a wooden bike goes into tank-slapper, at walking speed:

https://www.youtube.com/watch?v=YrlZN5DJCf8&feature=related

 

[Martin A]

A tankslapper happens when the steering angle gets out of sync with the lean angle of the bike (and rider). This can happen on a well maintained bike due to the rider or to a good rider due to a badly maintained (set-up) bike.

This rider was able to sort a tank slapper out due to:
i) low speed enabling him to not get out of sync with his corrections
ii) lack of pedals hindering his feet
iii) the fact that he was sitting low, allowing his legs to prevent excessive lean.

https://www.youtube.com/watch?v=YrlZN5DJCf8&feature=related

If you go faster than him don't expect to stay in sync, if you have pedals don't expect them not to hinder you, if you sit high don't expect to prevent excessive lean.

Slop in the system (front/rear/chassis) rider inexperience or road conditions can cause unwanted steering effects even when the bike is upright. When upright, any slip angle, especially of the front tyre, will lead to an induced lean, which will then pitch the bike in the opposite direction to the direction of steering.

Due to the original problem, any correction is likely to be excessive and so the effect increases. Eventually the steering can bounce back and forth from lock to lock with the forces being so great that the rider is unable to control it.

Road conditions are likely to be momentary and so the situation can possibly be saved, slop or inexperience are likely to be longer lasting resulting in a less likely save.

An e-bike usually has tyres that are do not provide as much increase in grip due to larger slip or lean angles as motorcycle tyres, so a tank slapper is less likely, but still possible. The front wheel is more likely to slide out from under you.

Keep your bike frame and wheels properly aligned, maintain the shocks and bearings, be careful where you go fast, and you shouldn't have any problems.