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Old 02-10-2011, 07:56 PM
buzbikebklyn1 buzbikebklyn1 is offline
Join Date: Jun 2009
Location: Brooklyn N.Y.
Posts: 208
Cool Re: Glad I found you!

Originally Posted by 2door View Post
Quote: (I bet it really cancels out some vibration.)

This idea has been discussed here many times before. Mounting an engine in rubber will not eliminate or cancel out vibration. What it does is, as I said above, transfer the vibration to the fasteners, which will eventually fail. You can not compare the mounting system on our little two stoke engines to a motorcycle or car, both of which use resilient mounts. By virtue of the mount design of the engine they rely on a solid, full contact approach which will cause some vibration in the bike frame.
Soft mounting the engine does not address the source of the vibration, which is the engine itself. Isolating the vibrations to the engine means that the fasteners (studs and nuts) and even the engine case then must carry the brunt of those vibrations, a task they were never designed to do. They will fail. Over tightening them, using thread locking compounds, nylon lock nuts, lock washers or any other method will not remove the stress put on them by soft mounting the engine. This is not just my opinion by the way, it is a mechanical engineering fact that can not be ignored.

Soft, or resilient mounts used in cars and motorcycles are a two sided affair whereby there is rubber or a reslient material between the engine case (block) its fasteners and the frame in which is is mounted. Take a close look at any automotive or industrial engine mount and you'll see a metal-rubber-metal sandwich, not just rubber on one side. If one was to experiment with this type of mount they might find a noticable decrease in the vibrations transmitted to the frame but you will still have to deal with the remaining parts of the engine and those that are connected to it in the way of vibration effects.

I'm not interested in blowing my own horn here; what I want to see is a new comer to the hobby enjoying his bike without the frustrations that will surely come from another unsuccessful attempt to soft mount the engine. Every successful and experienced builder here will agree with me on this. Mount the engine as solid as you can get it and enjoy your bike.
I respectfully disagree 2door...
The term "cancel out" was incorrect, my bad, but they do mask secondary vibration.
I'll be a bit more concise...
A properly thought out and built "soft" mount like that which was described in some what less than technical terms as a "metal/rubber/metal" sandwich will do the trick in dampening out SOME secondary vibrations. Yes, good grade fasteners are a must, but it will work.
The very nature of ANY single cylinder engines does NOT lend its self well to 100% balancing or defeating ALL vibrations.
(I used to ride big displacement thumpers/singles)
The weak cast aluminum cases of your average china girl engine means it can not be used as a stressed member, so the base mount (front frame mount) must not put any torsional or tension or compression loads on the engine cases them selves, in other words a good mounting system "cradles" the engine.

When the great Eric Buell was tasked with designing the soft mounting system for The Harley Davidson Sportster and the legendary FRX he came up with a solid rear mount and a series of floating lateral links and a front lower "biscuit"
(no metal sandwich)mount to keep the secondary vibes to a minimum.
IT WORKED, it canceled out about 70% of the secondary vibes, which is considerably better than a rigid mount.
Ever ridden on a pre '84 rigid mount Sporty or FX? and then a soft mount post '84?
The old bikes were like paint shakers by comparison. It worked so well that the factory incorporated the same type of mounting system in all big twins.
In closing Semi soft mounts work IF they are designed correctly in the first place.
Ya gotta do the home work!
Keeping it shiny side up
(and the family jewels from clacking)

Last edited by buzbikebklyn1; 02-10-2011 at 08:06 PM.
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