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Author Topic: Short screen  (Read 493 times)

shappers

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Short screen
« on: August 18, 2010, 12:03:27 PM »
Hi y'all,
has anyone fitted a short screen to a C5?
Has the extra downward pressure on the fron wheel reduced the tendancy to weave at speed?
Any other comments?

Shappers.

ScooterBob

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Re: Short screen
« Reply #1 on: August 18, 2010, 12:27:20 PM »
Hi y'all,
has anyone fitted a short screen to a C5?
Has the extra downward pressure on the fron wheel reduced the tendancy to weave at speed?
Any other comments?

Shappers.

So far, putting a windscreen (bar mounted) on the C5 only seems to exacerbate any oscillations problem that it wants to have by uneven loading and unloading in turbulence I've talked to a couple of dealers who have tarted one up with a windscreen, got the wobble, took the screen off and all was well. Bear in mind, though - that I cannot induce a wobble in the one I'M riding - not have I been on one that allegedly does this. All this information is strictly anecdotal at this point in time. .
Spare the pig iron - spoil the part!

Rusty

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Re: Short screen
« Reply #2 on: August 18, 2010, 05:33:26 PM »
Hi y'all,
has anyone fitted a short screen to a C5?
Has the extra downward pressure on the fron wheel reduced the tendancy to weave at speed?
Any other comments?

Yes I've had the Watsonian Squire short screen fitted to mine since day one. It still wobbles but at such a high speed (relatively speaking) that it doesn't bother me too much. I find it very useful at 70mph but will take it off to see if it makes any difference, bar mounted screens generally make things worse so it might be stable without it.

ScooterBob

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Re: Short screen
« Reply #3 on: August 18, 2010, 07:30:10 PM »
Yes I've had the Watsonian Squire short screen fitted to mine since day one. It still wobbles but at such a high speed (relatively speaking) that it doesn't bother me too much. I find it very useful at 70mph but will take it off to see if it makes any difference, bar mounted screens generally make things worse so it might be stable without it.

Shappers - At what frequency do you get the vibration? This is something I've yet to get from anyone on this ..... a higher frequency vibration usually means "front end" or related - a lower frequency vibe (oscillation) is usually a frame caused vibration from what I've read. The reason is that the frame is usually a "bigger" piece as a whole and and such, has a lower resonant frequency ......
Spare the pig iron - spoil the part!

shappers

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Re: Short screen
« Reply #4 on: August 19, 2010, 07:28:39 PM »
Hi ScooterBob, if you mean engine vibes then its around 3,750rpm ie about 64mph real almost 70mph indicated.

But i"m sure that's not what you mean.  In the old days an oscillation that quickly stopped was known as shaking its head where the end of the bars moved through about 2 inches.

A wobble lasted longer and was quite high frequency and was often cured by screwing down the steering damper. A BSA shooter I used to ride was prone to this.

The weave I describe is a low frequency oscillation.  I haven't tried tugging on one bar or another as I feel unsettled enough, reduce grip on the bars allows it to diminish.

I'm starting to give it more throttle now and 60 in fourth gear isn't quite fast enough to induce the weave in an attempt to see if it could be rpm based, which I doubt.

If it is a rocking couple induced vibration then the resulting weave must be a harmonic produced by the frame.  Being as the C5 is a new frame the symptoms it displays are similar to the early Triumph twins nick named Mr Whippy and let go for a whip round on the Triumph.

I can't remember which bike it was where lower frame members could be added to improve stiffness of frame by the swinging arm mountings, but wonder if this might be a quick fruitful line of experimentation.

It might have been XS 650 engines in BSA frames where the bottom frame rails were moved as it was thought the engine would provide enough stiffness, but don't quote me on this the memory is a bit hazy.

Sorry about the long reply, Shappers.

ScooterBob

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Re: Short screen
« Reply #5 on: August 19, 2010, 08:19:55 PM »
Shappers - Correct on all counts there! I am familiar with the "Old Headshake" - and I don't think that these cats are getting that - I DO think it may the "whip" that you talk about - but only on a couple of bike over the broad spectrum of them out there!

Engine vibration can, indeed "excite" the frame at a derivative of the resonant frequency. It'd be pretty darn rare, indeed, for say, a 2000hZ fourth order vibration to cause the frame to resonate at 4Hz ...... It would be like Tesla's "building destroyer" - a tiny hammer set to bang out a derivative of a building's resonant frequency that was SUPPOSE to set up a vibration that would make the whole building crumble ......  Possible - but not too plausible ....
Spare the pig iron - spoil the part!