55MPH Cruiser Engine?

I think this is a bad idea. Unless you have a lot of skill in fabrication, technical know-how and deep pockets don't even think about this. To be successful and live you have to build a motorcycle with the best components you can find, yet you don't want a motorcycle. Lower your speed expectations to around forty (which is still crazy fast) and you have something more doable. 30 mph on a motored bicycle can feel plenty fast.
SB

55mph and reliability don't go hand in hand on a bike.

First off go to your local moped dealer and take a 150cc one out for a test drive. Those things will do 65 easily so give it a shot. Then decide what you REALLY want from a bike that doesn't have rear suspension, adequate frame or other parts. A micargi royal is a BAD IDEA because it is a junky frame that MIGHT be strong enough IF you modify it correctly. I'm not talking the prettiness of the welds or thickness of the tubing, I'm talking about workmanship and the fact that frames from that mfg have cracked- weld penetration etc.

Follow scottos builds and use a felt slant frame. Quality american frame.

But seriously take a moped out for 55mph. I did it once in shorts and flip flops and it was awesome, but that bike have front and rear suspension, good brakes, etc.

Not exactly the best idea, but not totally impossible either...
You'll need some really good wheels like from a downhill bike or lacing in small motorcycle hubs to your 26" rims etc..
The frame would need to be stripped and EVERY weld checked out, gussets added to the head tube area, the bottom bracket, and rear triangle as well as re welding any weak looking connections.
The front end would have to come off a down hill bike or a small motorcycle or moped as I wouldn't trust a regular bicycle type fork at those speeds.
There are tires available that can go that fast, but be ready to open up your wallet...

For the engine, a Pred 212 would get you there or you could go 2 stroke and use a DiNardis engine or the like. Other good 2 smokers that'll get you there are Morini, KTM, Cobra Racing, and their clones... These engines range from 50 to 65cc and put out between 10 and 15hp in stock trim give or take a pony or 3 each way.

If I was going to build a bicycle that could cruise at 55mph I'd go with the liquid cooled 65cc DiNardis engine since it has the power and torque to cruise at 55 or so, it's compact and lightweight, and it's reliable. Top speed with an engine like this would be in the 75 to 80mph range depending on gearing so it could cruise at the desired speed but have a little head room for passing or getting out of harm's way etc.

If you are set on 4 stroke engines, people have successfully grafted the Honda horizontal cylinder clones into bicycles with a lot of fab work to make them fit and these engines range in size from 50cc up to about 150cc and they have 4 speed semi automatic transmissions or either a 4 or 5 speed manual transmission. You would want something in the 110 to 150cc range to cruise at these speeds. The 70 or 90cc size would max out at around 50 to 55mph. The Lifan brand engines seem to be the best choice for quality and reliability here.

I concur on the Felt frame... it's strong enough and very well built.

As long as you build it safely it'll be safe to ride at high speeds... check out Culvercityclassic's builds in here, he's built some really nice ones that are more than capable of the speeds you're looking for and they're definitely built for the task at hand. If you can get some pointers from him on frame construction and modifications as well as how to mount the engine and build the wheels it could help you along immensely with a project like this.
Check out this thread... this may give you an idea of the work involved in making a really fast bicycle.... http://motorbicycling.com/showthread.php?t=55783

If you truly intend to ride regulary at that speed, I would advise against ANY frame designed for a bicycle. For safety, road bike engineers design for stresses 10x the loaded weight of the vehicle. That's in the ballpark of 3000 lbs of stress. Sure, you won't see that going around the block but when a huge, unexpected, unavoidable pot hole comes at you at 60 mph, its a bad time to find out your frame or wheels aren't up to spec .

That said, yes it can be done. I've done it. You're going to need a 150cc motorcycle engine. A 212cc predator won't cut it, it has more displacement but less than half the horsepower. You're going to need a well equipped shop, and a budget of at least $3000.

If everything goes right you may end up with a machine that can safely cruise at 60 mph, but reliable? Best carry a bag with plenty of tools and spare nuts. You'll have a unique, one-of-a kind bike that you can be proud of, but think about the nice used motorcycle you can get with that money.

There is a lot of expense involved in making a bicycle, (designed to go 20mph), cruise and stop safely from 55mph. The engine is just a part of it.
Go build a 55mph bicycle if that is what you want to do. However, they sell naked Chinese scooters on the internet these days that are cheap and much easier to maintain and mod than a full body scooter. The Chinese scoots are also safe and legal at 55mph.

Sportmanflyer.com

$3500 professional quality board track style motorbike. Even if you only spend $2000 in parts on your build, the extra 1500 is totally worth it for all the work and internet research you don't need to do. My own bike cost almost 1500 to build including free machine shop and weld work, but had there been a builder I would have gladly paid 2000-2500 for a pro to build it.

Here are a few cheap, non-bicycle ideas for going 55 mph. I don't seem to be able to post img links today.
http://www.ebay.com/sch/Scooters-Mopeds-/6720/i.html?_sop=10
http://www.superiorpowersports.com/150cc_gas_scooter_p/rk150sc-22y.htm

So you want this done cheaply? Not trying to be mean here but this is going to be extensive to do reliably.you basically have to upgrade literally everything on the bike. My bike will go almost fifty, but I doubt I have anything less than $1,500 in it. And that's what's on the bike not including repairs and trial and error.

I can do it for the money you're talking.

racie35 said:

I can do it for the money you're talking.

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Exactly what I was thinking.... But realistically I'd make a bike that can go say 40mph then work your way up to whatever speeds you want to go. What I enjoy personally about this is there is so many things you can do to get where you want. Heck a 212CC predator with a cam, some more compression, and a big carb can get ya where ya wanna be with the right gearing...

Your asking a lot for a bicycle to do an average 55 mph, gear a 212cc for that and you will be buying clutches all the time. If you want to do it...use this one:http://www.pccmotor.com/li205spenmoc.html

I used one in a cb100 and it really moved.

That's one way for sure. Not many bikes will take that like the cb honda did though. Brings back memories,my first motorcycle was a cb100 I traded a Batavus moped for. Really liked that honda.