shaky 5hp flat head

#1
Hello. I got a 5 hp flat head mounted on my Cushman airborne build. Just got the throttle wire and brake working the other day and took the first test drive only to discover a terrible vibration in the handlebars at mid rpm. At idle and higher revs it's fine but the mids are not fun. The briggs is in good shape with new bearings (I/C). Reason for asking here is that these engines are common on minibikes and I can't recall ever seen a minibike with rubber dampers or something like that for engine mounts. just mounted right on the frame. Or am I wrong?

//Jesper

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#2
Usually bolts right to frame your vibrations is probably caused by the rubber mounts the motors isnt bolted down solid to plate jmo...
 
#3
I started out with the motor bolted right on the frame, then mounted rubber washers between motor plate and frame with no difference noticed.
 
#6
I started out with the motor bolted right on the frame, then mounted rubber washers between motor plate and frame with no difference noticed.
As it should not. The hold-down bolts will telegraph the frequency you feel and every one of it's harmonics straight through to the bedplate regardless of rubber washers. There are insulators engineered for a particular frequency that offer the required isolation. They will also raise the motor up beyond what is reasonable.
 
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#8
As it should not. The hold-down bolts will telegraph the frequency you feel and every one of it's harmonics straight through to the bedplate regardless of rubber washers. There are insulators engineered for a particular frequency that offer the required isolation. They will also raise the motor up beyond what is reasonable.
Yepp. I had no higher expectations on that test, but it was what I had at hand. I will try vibration dampers as soon as I get some, to see if it gets any better. I start to suspect the frame construction, mostly consisting of flat bar and L-profile. Maybe it's more prone to vibrate than a tubular one? I don't know..
 
#9
...I start to suspect the frame construction...
Doubtful. Any frame will transmit vibs from an engine until the engine becomes decoupled from the bedplate. The main problem with trying to adapt dampers to your engine will be maintaining alignment of sprockets/pulleys. Not worth the effort IMO. All part of the mini-biking experience!
 
#10
My experience in balancing tells me that your frame may be designed to oscillate perfectly at a particular engine speed. That large mass just aft of the handlebars is probably a wave node. Beyond that, heading towards the front, into the steering assembly is the resultant wave of vibration.
 
#11
...your frame may be designed to oscillate perfectly at a particular engine speed...
Well, that was surely not my intention when I weldet it up:smile:

I mounted vibration dampers under the bedplate today and the they did their job fairly well when it comes to the vibrations. But as you mentioned I got some hassle with alignment. Not that the chain flew of or anything but the engine twists somewhat toward the drive side. I think I'll give it try with rubber bushings mounted horizontaly instead
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