bad vibrations ...........

[porkie77]

hi all just after a bit of advice

when we last took our kart out we noticed a bad vibration that goes right through the chassis when it hits about 4500 rpm
you can feel it coming right through the seat

so we disconnected the chains and checked the axle is straight which it is .......

so started the engines with them disconnected and just revved them up and the vibration is definitely there . like I said at about 4500 it goes right through your whole body

just wondering if this is normal ??? as I know you will obviously get some vibration as the engines are bolted directly to the chassis ..

it is a twin engine gx200 that are tuned as follows

14cc head milled .65 thou
stainless steel vavles
26lb springs
alu keepers and retainers
+ 10% lift reinforced rockers
chromoloy push rods and guide plate
flat top piston with file fit rings
billet conrod
.278 lift with .260 duration camshaft
and aluminium flywheel
timing advanced by 8 degrees
with 26mm mikuni carbs
and straight through exhaust

only thing we left alone was the crankshaft ... so wondering if this is our problem ????

we are thinking about taking them out and having them balanced ...

just wondering what you guys think and if it is worth getting cranks done .... or if you have any other suggestions ??

thanks in advance for any help

 

[itsid]

two questions..

1) timing advanced.. you talk about the inbuild timing advance of the flywheel?
or is that an offset key? if so, make sure you have the non advanced flywheels.

2) twin engine.. both engines modded as stated above I assume..
with a clutch setup, both to a single live axle.... correct?
If not, make sure to synchronize the engines at around 5000 rpm.

at a certain rpm a twin engine system that is NOT perfectly synchronised will vibrate
(it'll ALWAYs vibrate, but you can feel the harmonic resonance at a certain rpm )
So either make sure to have that 'noisy' resonance down low in the rpm band (i.e. synchronisze the best you can up in the rpm band)
or get past that as quick as you can. (i.e. go faster )

I doubt it's the crankshaft to be honest, that is easy to spot though.. run each engine individually and check the vibration level..
if it's as bad it's THAT engine if not, it's resonance

'sid

 

[Half-breeder]

... another thing to think of is it 'could' be 1 or both clutches (guessing cent clutches). Im not sure, but dont the cheaper ones tend to have a rpm recommendation/limit(Im nowhere near a pro at Cent Clutches... pretty good at TAV2's though), and the more expensive ones are variable/adjustable(again... I think). So maybe vibrations are possibly due to a slight weight variance/centripetal momentum (or some 'other' college level math reasoning)...?

Im impressed at the stats of the build though. Wouldnt mind a vid to see/hear it performance.

 

[porkie77]

hi
thanks for you responses

and the timing is advanced by 8 degrees done by getting 4 degrees on the flywheel and 4 degrees from an offset key .

and yes both engines with maxtorque clutches to a single live axle

have ran the engines separately with the clutches removed and seem to get vibration at the same rpm on both engines

if you put your hand on the block of the engine when running them you cant feel any bad vibration at all so I am assuming that it is the engines vibration hitting the resonance of the chassis .

. when we next have it out I will try and get a video to upload ..

we re currently building another twin engine kart which will have same tuning but on gx160s .....

 

[Half-breeder]

if you put your hand on the block of the engine when running them you cant feel any bad vibration at all so I am assuming that it is the engines vibration hitting the resonance of the chassis .

Yes, 'that' would be the case then, resonance. I seem to recollect a similar issue occurring w/ my other build... I believe I added support from the rear to front (long side bar connecting the rear to the frontend)... but not sure if your frame is able to do such a framing...

...by doing so, it moved the resonance to the front region (before the vibrations ended at the rear of the seat's back). Vibrations were diminished, however, weight was added.

 

[fowler]

Might be pinging bad

 

[itsid]

have ran the engines separately with the clutches removed and seem to get vibration at the same rpm on both engines

if you put your hand on the block of the engine when running them you cant feel any bad vibration at all so I am assuming that it is the engines vibration hitting the resonance of the chassis.

So we narrowed it down, that's great.
you can try to use vibration dampers
like these
http://www.astonrubber.com.sg/vibration.html

those will of course lift the engine, so you might need to add another link or two to the chain, but they're pretty reliable otherwise.
A simple DIY silicone puck and a through-bolt might NOT do the job, that might allow the engine to move on a slotted mounting plate.
on a fixed pos plate the through-bolt will transmit some of the vibration, but it'll still reduce the amplitude of the vibrations and thus resonance by a fair amount.

Or you can alter the harmonic resonance of the chassis like Tihm suggested (added bracing) that should shift the resonance to higher rpms.

Or you can offset the engines ignition timing by 180°, so that any vibration will be cancelled out by the opposite engine.
That'd be quite a task to say the least.
The only viable route I can see to do this is by running both engines unclutched to a single jackshaft, having their PTO aligned 180° apart.
Starting the engines will be a pain though -uhm-

Vibration dampers would be the first to try I think

we re currently building another twin engine kart which will have same tuning but on gx160s .....

The new ARC flywheels have 8° build in, but I'm sure you already know that

'sid

[EDIT]
WAIT.. you have the original tanks removed I assume, correct?
see if you can add a stiff cross bracing between the two engines rear (well front in fact) tank mounts,
use both tabs on both engines though!
maybe that's already enough to reduce the resonance.

 

[porkie77]

Thanks for all the advice
we are currently looking at tthe dampening engine mounts

if not think it's gonna be a bit of trial and error trying to change resonance of the chassis . As will be bit awkward to attach some bracing as you can see from the pics .

( sorry bout bad pics but all i have on my phone at the min )

never thought about bracing engine together but definitely going to look into that one as would be quite an easier option .

cheers

 

[iwanr]

I would also like to suggest adding vibration dampers. Or if already installed.. Replace them.
I added vibration dampers to my rebuilt Thunder Kart and it made a HUGE difference.

Good luck.