Bar Ends

Hello all,

Do Bar End Weights serve a purpose (crash protection?) or are they just for show?

http://www.ebay.co.uk/itm/Red-Toka-B...item2ecb9e3ae4

Baz.

[Nickj]

The weights can be protective or decorative but what they are really for is a bit more clever.
When done properly they are a specific mass that helps to damp resonant vibration in the bars so should be specific for a certain bike configuration.

[Mr Gazza]

I had terrible vibration in the handlebars of a 650 AJS once.
I was told that filling the bars with lead would cure that, so that's what I did.

Not a lot of trouble melting a pan full of lead and pouring it in, but it didn't make a bit of difference.
Getting the lead out was a different matter, it took ages to get the bars hot enough to melt it out..!

What science is used to find the right bar end weight to stop resonance?

I helped my neighbour build a Vanns RV8, and he had problems with propeller resonance destroying the intercom, which was a big issue as it was part of the CAA certification.
He cured it by fitting a different type of intercom in a completely different place.

[Nickj]

For bars usually a bit of thick spring works quite well rather than a shed load of lead which also raises issues associated with hot pouring, >> THIS << is better as it is flexible. Worked quite well on a few RD yams and some KTMs.
It works a bit like a steering damper, all the damper is doing is damping resonance that makes the bars twitch, once they start twitching the resonance just amplifies it. That's all tank slappers are, resonance in the frame so adding a damper doesn't get rid of it just hides it or moves it somewhere else.
Like your neighbour found you can get resonance in quite discrete structures, either you damp that bit of the structure or the object attached to it or find an area where it doesn't happen. I'd guess the prop resonance happened at a certain range and he was using a metal prop. Metal props do tend to be bad when you get near their natural resonance frequencies and a common cause is pulse excitation from the gearbox, a tiny bit of lash at certain speeds and have prop resonance at a certain speed/pitch combination. Also there is the motor itself, torque output depends on the pistons position so there are going to be pulses even from a relatively smooth 4cyl. The prop slows and accelerates slightly so the prop bends very slightly giving torsional excitation and resonant whip in the blades. Wood blades on the other hand flex a bit more so tend to be less prone to this.
RC helis see this kind of thing and it can be bad enough to rip the head apart so I read up on this. Usually precise dynamic balancing (same of full size helis and planes), the right materials for blades, an appropriate profile and removing lash in the gearing removes or moves it away from the areas you use.
On planes it's very specific on bikes it is more of a black art (wild ass guess) as to what works precisely.

[Nickj]

Oh and a Aj vibrating thje bars!! LOL The whole thing vibrates so its not really surprising!!
Bars with a little flex might have helped but overall it would still feel a bit like riding a large pneumatic drill

If you want to get technical then here's an instance, the lycoming IO-360-A engine and a hartzell prop were rated on one platform. A motor was rebuilt with 10:1 pistons rather than the standard 8.7:1's which was documented as OK for the engine type. The motors torsional counterweighting wasn't changed, the engine certiification said it was OK and the prop self destructed.
Bikes have the equivalent of torsional pulse suppression in the rubber cushes, ride say a dirt bike which doesn't have them and you may notice at a certain speed the chain feels like it's trying to rip the sprocket off ... resonant frequency

Originally Posted by Nickj

Loads of technical stuff

Hmmm very interesting. Not sure if I get all of it though

[Mr Gazza]

Yes I love aeroplane science and technology. If there is an area that could possibly present an unknown, it is explored to exhaustion. Models are a fun way to find out how many different ways there are to destroy a plane..!!
With prop rpm's sometimes as much as 40,000 or more, balance is critical, and with the tips going supersonic, they often fray and very quickly go out of balance. This often means that a part of the airframe may resonate to the destructive frequency and fail bringing the model down.
With such high prop rpm, sometimes the forces are so much and so rapid that catastrophic failure of the airframe results in a mid break up that looks for all the world like the model just crashed into an invisible brick wall....All that comes down is confetti..

It's reasuring to know that vast amounts of time and money are spent studying these phenomena in the ones that carry people.

It's easy to laugh at the vibes on the old bikes like the Ajay twins, but they were the state of the art in thier day. To be honest mine were quite tolarable most of the time. I think that I had a chair on when the "lead bars" issue came up. Maybe that particular Winter that I put it on, I got it bolted a bit differently, and hence some different resonances..?

Interestingly I decided not to attempt the recent Boxhill run after a mere 70 mile run gave me the worst white finger I have ever had. I put it down to some slightly lower temperatures and thought maybe my gloves were getting a bit tight..But...am I getting some bar vibes I wonder.? Perhaps I will experiment with some different bar weights next year.

[Nickj]

Simply put for handlebars ... Stuff wobbles, hook something that's springy with a hint of jelly to a wobbly thing and it sucks up the vibration