What's the difference?

My 66cc really vibrates at high RPMs.
i dont get any vibration, but it is probably a lot to do with my front engine mount that was hand made, hand machined, as well as other tricks used in building it...Plus i changed out the spoke wheels for mags which also made quite a difference, plus i eliminated the chain tensioner and custom fitted both chains for exact fit...DAMIEN
 

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Classic frame. It says Sears Roebuck and CO. on the head tube. My dad and mom had a pair of these bikes when I was a kid. Now I'm envious. I'm always on the lookout for a good frame.
They're made in the old country way, soldered up with lugs and brazing/flux. The frame is really much lighter than the Nottingham made Raleighs/Hercules/Robin Hoods. Steyr made these with 1030 high carbon seamless steel, almost Reynolds quality steel. it's resilient, a wonderful gravel road bike.
 
Excellent front mount. Is it better to hard mount or soft mount to the frame? I have rubber soft mounts front and rear. I needed to raise the engine up in my frame to clear the front derailleur.
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Just about everywhere in the world measures displacement by cc's, except China way over exaggerates these engines for marketing purposes I'd guess but here's the real deal, most engines marketed as 80cc bicycle engines are actually 65.93 cc's with a 38mm stroke and 47mm bore or 69.4cc's with a 40mm stroke and 47mm bore, the 48cc engines are 38mm stroke and 40mm bore with a 50cc being 40mm stroke and bore, then the 100cc engines are similar in having a 38mm stroke and 50mm bore which is 74.61cc's these are the bt and yt versions and a 40mm stroke with 50mm bore is 78.54cc's, and what difference does this make? In the variants with 2mm of stroke difference not much really and the difference between 48 and 78 cc's well it's pretty obvious more displacement makes more torque and therefor useable power in theory but then herein lies the problems of tuners world wide, what do you want to get from the engine, torque monster, screamer or somewhere in between? while displacement with big bores can improve torque output it's usually at the cost of rpm gains because it's harder to turn more reciprocating mass and not do damage to other internal parts such as the wrist pin it's bearing and the big end bearing along with the crank bearings and even connecting rod itself, an engine that has more bore than stroke is referred to as being over square and over square engines are not considered as good high rpm engines mainly due to the afore mentioned but are great torque monsters over a narrower rpm range. The normal 2 stroke is slightly over square and can be made to still have decently higher rpm but again this comes with a cost of losing torque due to having to highly alter port timing and changing the flow characteristics to better suite the higher rpm range. A square engine like a true 50cc with equal bore and stroke of 40mm and lower reciprocating mass on the same weight crank as the larger 66-69's will rev to the moon and get there quick if not over loaded while lasting much longer doing so. It's all about what you want to get from the engine that should direct your research in getting there.

Since the oversquare of the 48cc is so close to square it's not really an effective example of that regime. It's in the Honda 50 cc range with a mix of fair torque and an ability to rev some. Ducati's 250 bevel was a great example of a smooth, free revving OHC 4 stroke with 74mm bore X 57mm stroke that was a ground breaker for fast 4 strokes of it's time, since BSA and Triumph were both making little undersquare paint mixer thumpers They pulled well from low revs but really suffered when you tried to get more power out of them. The problem that effects all underquare engines of both 2 and 4 stroke, is the lack of effective porting and valve size to make them breath better so you can rev them up.
 
Excellent front mount. Is it better to hard mount or soft mount to the frame?
There are 2 usual options or opinions on that question...The majority say "hard mount" as in metal to metal, The other side says "soft mount" it and they are using thick rubber which can actually increase vibration when the rubber is too thick...ive done something a little in between.

When you take a close look at my front mount, take note that though its a really solid mount, i DO have very thin rubber cut to size, from military aviation grade rubber that was supplied to me for this purpose...its thin enough for the mount to have a solid grip on the bike frame with no "quiver" vibration effect, but just thick enough to not mutilate the metal bike frame that its clamped to, and actually helps with a more assured, no slip grip.

I guess mine can be called the 3rd option or opinion...Ive had it to 47 MPH, just once, with just a little throttle left over and had no vibration...DAMIEN
 
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Excellent front mount. Is it better to hard mount or soft mount to the frame? I have rubber soft mounts front and rear. I needed to raise the engine up in my frame to clear the front derailleur.
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Most here recommend firm or direct mounting, as any movement, even if it's damped and restrained will cause changes in the slackness of the chain under load. My engine fits my frame so tightly that I need to tap it with a deadblow to loosen it.

The engine's frame tube saddles are an exact match to the down and seat-tube angles on my bike, which is what the original bikes these engines were designed for. If you've read their instructions "V- framed bikes are best". What they aren't saying is that the closer you are to this classic Raleigh Roadster seat and down tube angle, the less likely the engine is going to be moving around unnecessarily. Tight mounting also takes advantage of the frame's ability to damp down some higher frequency vibration. I've seen first hand an aluminum cruiser frame fail because of a badly vibrating 66cc engine.
 
@Street Ryderz That was awesome. What's your opinion on high compression heads?
Unfortunately the high compression heads marketed for these engines are again exaggerated and in most cases not really high compression when measured properly. When these engines were first released they came with heads that had a volume of 10cc's that was from 02 -06 then they started to come with 6-8cc volume heads with a slightly improved chamber over the original design, and this was when aftermarket heads started to show up on the market. The aftermarket heads for the most part are an improvement in cooling abilities and some have better chamber's but the biggest part is they don't warp, crack, or need replaced often, as for the compression since the stock engine comes with 6-8 cc volume heads and the aftermarket heads have the same volume they are not higher compression than stock, guy's whom deck the cylinder height to bring the piston crown up flush to the deck or close to it gain some compression but it's still not much and not what an engine builder would consider high compression since they started at around 6:1 and being decked only brings this up to 7 maybe 8:1 where high comp is 10-12:1 or higher requiring the use of higher octane fuels, more ignition retarding and a fully functioning squish area all to prevent detonation and the demise of the engine. 2 strokes do not generally use high compression as it is known in the world of engine building and this is because higher compression is harder to push the piston through to tdc when it's being moved by the inertia of the rotating mass alone (crank weight) and even harder in a single cylinder so the happy medium has to be used to get the best results. Higher compression does help with torque somewhat but again at the cost of higher heat and sacrifices rpm gains again so it's back to the happy medium again to get the most from the engine all around.
 
thin enough for the mount to have a solid grip on the bike frame with no "quiver" vibration
Tight mounting also takes advantage of the frame's ability to damp down some higher frequency vibration
I had a feeling that the rubber mounting to my frame was allowing the engine to vibrate too much. I will have to re-think this, and perhaps remove the front derailleur.
 
I had a feeling that the rubber mounting to my frame was allowing the engine to vibrate too much. I will have to re-think this, and perhaps remove the front derailleur.
Like Damien was saying, you might look into alternative materials. I'm using 1.25" irrigation pipe to better fit the engine saddle to this frame. This piping which is a form of PPE it's firm enough to keep the engine on tightly, I suspect there's a tiny bit of give as it's not a real hard plastic.

Polyurethane in the same shapes might make the buzzy vibrations less. It's popular with owners of rally cars for suspension bushings and engine mounts.
 
Unfortunately the high compression heads marketed for these engines are again exaggerated and in most cases not really high compression when measured properly
That's kind of what I thought. I have a very nice looking CNC head. I had it on the engine for a while, but I didn't notice any improvement in performance. I have fallen for the hype of many upgrades. Like a 'Hall of Shame', I have a box full of stuff that I bought that was supposed to make my bike go faster ;-)
 
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