Vibration and Changing sprockets to increase top speed

[Agusts]

So I have left Thailand now and in a land far far away I have a Chinese bog standard 150cc bike, 5 gear manual, with carb. and with a name you have never heard of... 

Before you say why, that's what they have at reasonable price there,  everyone has one and parts and service is cheap and everywhere, so I bought one new a couple of years ago. It's hardly used, occasionally, a few months of the year, now has about 3k km...

It has two problems, at exactly 4200rpm the vibration becomes bad and it shakes the whole front dash and make a buzzing noise, I heard this in some other bikes too, basically I think the vibration frequency becomes exactly the same with chassis and some front body parts that it makes this noise. At higher or lower rpm the noise disappears, if the fuel tank is full, also no noise...!!!?

I have opened the fuel tank and checked all under, the rubbers connecting the tank to chassis all okay and I have tighten up all the dash nuts to no avail. My only other things I want to do is check and adjust valves (to generally reduce vibrations), and see if I can put some rubber washer where the engine is attached to chassis  ( I doubt this is possible,  but I see two points on top it may be possible) , any other ideas...!? 

 

Next problem is at the max trottle  ~8400rpm it only does 90km/hr...! One option to increase top speed I hear is changing the sprockets ratio,  easiest being to reduce back sprocket by 2 or 3 teeth. But I have read somewhere if engine does not have power, this might not do anything, that is after the change the bike can not go passed say 7500rpm, and you end up with the same max speed...!? Any experience with this, anything else can be done...

[Agusts]

No one here changed sprockets for better speed or acceleration ..!? 

[johng]

Its a 150cc  air cooled bike  it won't have much power to start with so 90 KPH is not brilliant but not too bad a smaller back sprocket is very cheap  just try it and see what happens..

also pump the tires up to the recommended pressure  ( higher pressure will give you better speed but less grip/safety) 

On 7/1/2019 at 3:17 AM, Agusts said:

if the fuel tank is full, also no noise...!!!?

I would suspect that some rubber dampers on the petrol tank are missing or perished  over inflated tyres might also case vibration  or bad bearings on the front wheel.

[ktm jeff]

Good idea to check the valves and ensure the carb is "in-tune".   Next , check the tires , VERY carefully , for de-lamination , bulges etc , including inside the tread pattern , for cracks. Then have the wheels balanced.  Check for cables ( throttle / clutch ) and wires , that could be vibrating against the plastic headlamp surround / casing. Are these secure .  Rubber mounting the engine may require new brackets , but if easy enough , may be worth a try.  Also check bars , mirrors , control switches and levers. The levers can wear on their pivots - grease them.  You wont be able to increase the power , so going down 2 or 3 teeth on the rear sprocket wont improve top speed ( as you suspected ) but will lower the revs at a given cruising speed. Out of curiosity , what is the bike and its price. Good luck.

[emptypockets]

Natural frequency of the vibration at that rpm otherwise known as the critical speed. ALL reciprocating motors do it (so do a lot of turbines). Some more than others depending on the engineering expertise applied. 

90kph suggests the bike is gutless and I would have thought maybe 120-130 kph at least at 8400 rpm achievable, perhaps it is under geared already?

What is your normal cruising speed? if 90 kph is 8400 rpm and you re having problems at 4200 rpm that would suggest about 45 kph.

What gear are you in at that speed?

 

Something doesn't sound quite right.

[Agusts]

Bike is just a typical put together of  Chinese pieces with some made up name , Domoto...! I think it was US$850, really cheap.

 

I have checked the fuel tank mounting rubbers, all good and in place, it's the weight of the fuel tank that stops that particular vibration frequency...

 

The bike has no problem at 4200rpm, it just makes that funny noise coming from dash area,  that's all, by the way it does not make the noise when stationary and in no gear at 4200rpm...! Only when riding, and it does it in all gears....So it's hard to see where exactly it's coming from on the road...

 

I usually cruise at 7000rpm, about 80kph or just under in 5th gear, some usual vibration but no noise in that rpm, it's the noise that really annoys me just passing that particular 4200rpm accelerating or decelerating ...

 

I investigate any loose cables and fairings etc. again, nothing obvious, thanks ...

[Toosetinmyways]

Had a speed related vibration that was a bad front wheel bearing.

Regarding sprocket change it depends on your bike usage. Long distance cruising or short trips round town. I had a 30 kilo daily commute so went from 34 to a 24 tooth. Round town requires constant gear changing.

[Denim]

I used to have 2 rear sprockets when I had a 125 Honda. Standard for around town and minus 4 for long trips. Fewer sprockets does not effect acceleration much but gives a little more top end speed.

Actually , I didn't want the extra speed. I just wanted the engine revs bought down when cruising at 90.

[Agusts]

Thanks for the tips, I won't be with my bike for another two months, so will report back later...

[Isaanbiker]

On 7/5/2019 at 7:22 PM, Agusts said:

No one here changed sprockets for better speed or acceleration ..!? 

We did that already on our 50 cc Mopeds that ran around 85 km/h. After a smaller sprocket in the back high speed went up to 95 km/h.

 

   

[johng]

32 minutes ago, Isaanbiker said:

50 cc Mopeds that ran around 85 km/h

2 stroke mopeds ??

[thaiguzzi]

15 minutes ago, johng said:

2 stroke mopeds ??

Yeah, Fizzies 'n Fantics when we were teenagers. Dude.

[johng]

1 minute ago, thaiguzzi said:

Fizzies 'n Fantics when we were teenagers

My "friend" had a Fizzie  for a short while..every time  plod saw him on it  he was stopped and given the third degree and or "a producer"

[Crossy]

17 minutes ago, thaiguzzi said:

Yeah, Fizzies 'n Fantics when we were teenagers. Dude.

 

Yeah, I'm from that generation too.

 

The Fantics really were frantic (this was before the 30mph limiter) but tended to get used up rather quicky. 

 

 

The Fizzers (Yamaha FS1-E) went pretty well,  particularly with 10% STP friction modifier in the 2-stroke.

 

 

 

[thaiguzzi]

I had both.

Started on a metallic gold FS1E, then graduated to a bright Ferrari Red Fantic GT (with white racing stripes!) which was the full on road race rep, did 50 mph straight out of the box, guaranteed.

But that got written off going into the side of a Morris 1300 doing a U turn the other side of a bridge.

My first trip in an ambulance.

Luckily a bus driver and a bobby witnessed it, so my Dad got full recompense.

And as i was approaching my 17th birthday anyway, that money went towards a proper motorcycle which is another story for another day.

Win win.

..... apart from the Fantic.

[CGW]

15 minutes ago, thaiguzzi said:

Started on a metallic gold FS1E, then graduated to a bright Ferrari Red Fantic GT (with white racing stripes!) which was the full on road race rep, did 50 mph straight out of the box, guaranteed.

You young uns were spoilt! my generation had bloody BSA bantams, they never did 50 mph unless you lived in a very hilly area! to be fair they were great for improving your mechanical skills, actually riding them for any distance was a rare occurrence, the expectation of impending doom constant!

[Agusts]

Just a feedback now that I'm with this bike. I was told and shown some of these new Chinese bikes now come without the engine being secured at the top to the chassis at the bottom of the gas tank, I saw a few new imports in the shop like that...

So I disconnected the nuts and bolts on mine and removed the connection to see if it makes any difference to the vibration,  to my surprise, not at all, not one zip... !?!?

 

So I reinstalled it but added some rubber washers between them, still there is no change. Quite odd really, the vibration must come and transferred via other parts...!

 

I have also now replaced the sprockets,  changed the front 15 to 16 (17 would not fit) and also back 38 to 34 (again the 36 the shop had would not fit), so all in all about 16% change in total ratio.

 

So far so good, and I like it better this way,  the gear change makes more sense now and although I have less acceleration power I get to 105 or 110km/h at just 6500rpm, a big improvement if you ask me...happy there. 

 

But the vibration is still there as before, and no change in that deoartment. I heard changing to synthetic and thiner oil like 15w50 can maybe reduce vibration - it's a hot climate here, so might try that. And still maybe adjust the valves, but the bike is under 3000km, so hate to mess with it. 

 

Although I think Chinese quality control must be non-existant, I wonder if the factory has even ever ridden these types of bikes when first designed or developed,  just pick up pieces of other bikes and put together... in this case I think it's the engine that has serious vibration issue, the rest of the bike is not so bad - checked wheels tyres and everything is all okay.