Heavy Vibrations at Mid-range of Revs

[Falco]

My bike is a CG125 ES4.

At the mid-range of the revs; around 22/3-27/8mph in 2nd gear or 32/3-37/8mph in 3rd - roughly around the centre of the rev range (it may be present in 4th but its not really noticeable) the vibrations in the engine get really intense. So intense that I can feel them through the foot pegs and bash guard on the right-hand side.

Once I move above (or below) these speeds, the vibrations smooth out into the more normal levels of vibration for the bike.

This has only started in the last couple of days, is it something I should be worried about?

[MCN]

Is the chain tension OK?
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[ADSrox0r]

Have a good look for a stiff link.
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Current bikes: '08 VFR 800 VTEC(yo) , '07 ZZR1400 Winter hack: '95 Aprilia 650 Pegaso Currently lusting after: RC30

[Silver_Fox]

Let us know your findings. I have that on my CG, but just assumed it was the nature of a single cylinder engine having not ridden one before
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CBR600 F3 FV
CG125

[Falco]

Chain tension is good (chain was cleaned, lubed and adjusted on Monday. I set it to 15mm as per the instructions on the chain guard, though looking at the haynes now, it specifies 20mm, so I might loosen it a little.

I didn't notice any stiff links, but I'll have a good look tomorrow (it's pissing it down and I am about to go to work so the chain will be due another clean ASAP ).

I haven't noticed that sort of really crazy vibration before, but it's entirely possible that I only noticed it recently. Generally the CG is a pretty noisy/vibey bike though.

[mauzo]

When was the oil last changed?
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'04 CG125, '07 CBF500

[Falco]

[mauzo]

[Falco]

[spottedtango]

Engines usually have a natural frequency that they vibrate the most at. It used to be around 5500rpm for older 2 valve 4 cylinder air cooled engines.

Pure speculation but you could have a loose engine bolt that needs tightening up, when you're reaching the natural frequency of your engine the vibration is becoming more pronounced.

The cg doesn't have rubber mounts for the engine? I'd check their condition if you do.

[Falco]

[spottedtango]

The natural frequency will be a bit different for a single cylinder, here's how.

The natural frequency is dependent on the mass and stiffness of a system.

So due to a single cylinder having a smaller mass from a old air cooled inline four and possibly a different value of stiffness due to different materials, alloys used. It will be different although by coincidence it could end up the same value of RPM.

At least that's my understanding. If any fellow engineers see any holes in my logic feel free to correct me.

[Falco]

Just finished adjusting the chain to 20mm (weeeell it's actual 17-18mm but after tightening the rear axle 3 times I was sick of the whole business and decided that was close enough).

The mounting bolts all seemed tight (as in they didn't move at all when I tested them with the socket). I'll report back if it's still doing it after the ride to work.

Failing everything else, I will try to change the oil this weekend since I want to change the throttle cable and lube the clutch cable anyway, so it'll need a test run.

EDIT: Forgot to add, checked the chain for stiff links, all fine (thankfully, less than 1000 miles on the chain and sprockets, stiff links would not be appreciated.)

[Falco]

Well, after a couple of trips, the vibrations seem to have damped down a little bit. Not sure why a tight chain would cause vibrations only in the mid range of the Revs. Will loosen it to 20mm this weekend and see if that gets rid of it completely.

Cheers for the advice guys.