Speed Wobble

[The Dude]

As i am only on an RS 125 there's probably no chance of this happening. The question is. how does it happen? And at what speed does it normally happen after? I am planning on getting a bigger bike soon and i would like to know so i can avoid it. Cheers

[instigator]

Erm, I'd guess it'd be something along the lines of the front end getting lighter, so there is less force being pressed down onto the front tyre....the wind takes affect on this and pushes the bugger form side to side

Its usually a manufacturers design fault, happens on maxi scotts quite a lot I hear, along with the big pan europes...

Beats me if thats right

[The Dude]

I thoght it was just an effect that happened over a certain speed if maybe ya hit a bump and the steering got lighter. I seen a real serious one once where the bars just flicked from one side to the other very fast until the dude came off. I just want to understand how it can happen so i can avoid it happening to me.

[instigator]

is that not what they call a tankslapper, where the bars vibrate so badly that they just slam into the tank?

Ride normally and ride with a decent amount of force on the handlebars and I'd have thought that'd be it. Going over something like a cats eye at 100 is obviously going to be a bit unstable....but I've no idea if its the bike or the rider which has an effect on the result. Probably both I'd have thought.

Search google too

https://w6rec.com/duane/bmw/wobbles.htm

[Dom]

What you're talking about avoiding is a tankslapper, which is just what you said - hitting a bump having the wheel come down at a dodgy angle, or basically anything that sends the steering to full lock on the left and then on the right. If it happens it's just a case of holding on as much as you can.

Steering dampers can be fitted to bikes to stop this from happening. They work a bit like footpumps in that you can only move them fairly slowly so if you were in a situation where you'd normally have a tankslapper it'd absorb most of the pressure and it wouldn't happen. One part attaches to the frame and another part to the bars. Once fitted the bars would basically be impossible to move quickly, so it couldn't happen.

Not really worth bothering with on an RS, although I guess you couldn't rule it out happening.

Edit: An example

[california_rookie]

I couldn't tell ya. Been past 100mph a good few times and up to 150 once. Nothing unsettling at all. Then again, I do have a steering damper...
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[The Dude]

I take it steering dampers aren't used in racing? I seen a dude on tv once, i think it was the Isle Of Man TT, he came over a hill, the front wheel started flicking very violently from side to side, as did he, for around 50 yards untill he just lost control. i suppose because ya need to turn quickly in racing, they don't use steering dampers?

[california_rookie]

Agreed. I would suspect that in racing you need every little fraction of a second of reaction time that you can get your hands on, thus a steering damper would be more of a detriment than anything. Also, professional riders are skilled enough to overcome the shakes, for the most part. Most likely, the only way it would work would be if it became a universal requirement.
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[Kickstart]

Hi

A speed weave and a tank slapper are 2 different things really, although they may well be related. On the big scooters the weave is probably due to them having a weak frame which struggles to keep the wheels vaguely in line.

Tank slappers occur when the natural resonant frequencies of the various steering and suspension components coincide, shaking the handlebars very violently.

A steering damper itself can cause a speed weave as you tend to over correct your line constantly, but is otherwise unlikely to help much. A steering damper will help when you get a tank slapper, but realistically few bikes should go into one anyway and you would be better off sorting out the real problem rather than disguising it with the damper.

I have had my RS go lock to lock on me once or twice. A particular corner on my way home that have a nasty dip right at the apex. However going once from lock to lock is not really a tank slapper. A full tank slapper is very violent and quite likely to throw you off the bike.

All the best

Keith
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[california_rookie]

[Kickstart]

Hi

Yep, but then they did launch the TL1000S, a bike with a seriously dodgy rear damper and a frame prone to cracking, and which was very prone to severe stability problems.

The Bimota has one as well. I leave it slackened right off. Only really needed if you change the steering head angle (it is adjustable on the Bimota) to speed the steering up.

All the best

Keith
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[stinkwheel]

It can be aerodynamic as well. When I broke the bellypan on my GPZ500s, I was riding it without for a while. The bike developed a slow weave at about 100mph which got progressivley worse up to about 115 where it was about as much as you could realistically control.

This stopped as soon as I got a replacement belly pan for it.
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[r1_lad]

I have had quite a few tankslappers, mainly on my old gsxr and while i was learning to wheelie (bringing the front wheel down not dead straight). All you can do is accelerate and hope it sorts itself out.
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[california_rookie]

[Frost]

My last tank slapper was front powerbanding my bike whilst overtaking going over raised hatch markings in the middle of the road. The front end was light and it kicked the bars from lock to lock till i was off them.
Similar things can happen when you clip a cats eye in the road

[stinkwheel]

I would make a distinction between a tankslapper and a speed wobble. You can get a tank slapper when moving off from a standstill, a speed wobble, by definition, happens at speed.

In my experience a tank slapper only happens under power, it is mainly (but not entirely) due to flexing of the back tyre when the front end is light. This is why the TL had problems, the combination of an over-wide rear tyre, short wheelbase, the large amounts of power and the dodgy rotary damper, puts far too much load on the tyre and tankslappers occur. You need to keep the throttle open (and therefore the weight off the front end) to get out of this situation. Close the throttle and the front end loads up, probably pointing in a different direction to the rest of the bike and over the handlebars you go.

A speed wobble is more like a slow side to side weave and seems to be mph dependant. I think it is more due to the actual design of the rolling chassis. A combination of an overly flexible frame, slightly out of alignment wheels and unbalanced aerodynamics make the bike veer off to one side. You compensate for this and it goes the other way etc. etc. Gently backing off the throttle will bring this under control. Certain road surfaces seem to make this problem worse.

Bendy framed bikes of the 1970s were notorious for this, noteably the kawasaki triples. You can actually feel the frame, forks and swingarm flexing under you on my dads KH350 once you go over 90mph. The addition of a fork-brace used to cure many bikes of a speed wobble (it is unusual to see a 350LC without a fork brace even though they weren't made with them).
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[Kickstart]

[mchaggis]

https://www.i2roo.com/constructeur/bimota/image/500Vdue.jpg

https://www.motorcycle.com/mo/mcbimota/mcphotos/bimstat.jpg
https://www.motorcycle.com/mo/mcbimota/bimota500.html

That looks like one sweet machine, what's wrong with it? I'd assume the power comes in like the engine is hitting you with a brick in the face, but that's not always a bad thing?

edit: my hotlinking those pictures seems to have broken them, so I put a different one in.

[Kickstart]

Hi

Yes the power band was very violent. On the Bimota mailing list one of the people was the Norwegian importer. When the V Due first came out they had to get it homologation tested locally. The tester came back with a white face having scared himself silly.

One theory for this apparent problem is that the direct fuel injection system relied (to an extent) on squirting fuel in so that it struck the piston to atomize it properly. Trouble is that this only worked well when the piston was in the correct position, which only happened in a fairly narrow rev range.

Think you can have a brand new injected bike for £6000~£7000. Check:-

https://www.bimota500vdue.com/eng/intro.htm

All the best

Keith
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[california_rookie]

I believe the problem with the V Due was that, in most instances, the motor incinerated itsself within the first 500 miles. A shame, really. I've always been interested in the idea of an injected 2-stroke motor in a sport bike. Never seen one in person, though.

On the plus side, if you can call it that, I'm reasonably certain that Bimota recalled each and every one of the initial run of bikes and "fixed" the problem. Haven't heard a thing about whether or not this quick fix was successful in extending the life of the engine, so I'm guessing it was.
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[Kickstart]

Hi

Not heard of any incinerating themselves, but there were gearbox problems and some other mechanical issues sometimes but it is a highly tuned 2 stroke.

Bimota did offer to buy them all back (or swapped them for another bike), but not everyone gave them up.

The fix was to dump the fuel injection system and fit carbs. It is certainly no quick fix (virtually a new engine was fitted) and apparently worked very well. The problem is that the injected version would pass emisions rules anywhere in the world while the carbed version could barely be sold anywhere.

All the best

Keith
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