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Old 08-21-2010, 02:25 AM
Chalo Chalo is offline
Motorized Bicycle Senior Member
Join Date: Aug 2010
Location: Texas
Posts: 78
Default Re: Strengenthing my new cruiser

Originally Posted by scotto- View Post
Since you insist on jacking this guys thread....I'll jump right back in...sorry Greenlake...
OK, your talking to an older guy (me) who raced Mountain Bikes for over ten years on the NORBA circuit ( over ten years )...teach me about the wonders of an MTB that I don't already know...pahleeease.
I've been working intermittently as a bicycle mechanic since 1992, working as a machinist and prototypist almost that long, and building bike frames from tubing and bike parts from billet most of that time. My last job was over five years as a machinist and technician with a private space flight program in Seattle (where Greenlake is). You might know a great deal about riding MTBs, but you have said nothing so far that indicates any particular understanding of the mechanical principles at work.

A bicycle frame is a truss. Trusses do not use curved components-- that would defeat the purpose of a truss, which is to eliminate bending loads and substitute tension and compression loads. To get back the strength lost by making a truss out of curved pieces, you have to make it substantially heavier. Or you can choose to let it be substantially weaker instead. Neither choice especially befits a motorized bike, even if that was the most practical choice available to Whizzer back in the Iron Age.

Now then, both of my personal current builds are as you see, cheap Chinese aluminum framed cruisers, they're not superior to anything except your thinking. The black one cost $150 shipped, the blue one $119 from Toys-R-Us.....superior?
They are superior to frames that have double top tubes or cantilever stays, if your aim is to mount a motor on them. Just to be clear, this is a double top tube frame:

and this is a cantilever frame:

I hope you can see how these frame designs, which are typical of cruiser bikes for decades, place needless restrictions on the physical size and location of a motor installed inside the front triangle. Your bikes' frames do not have these features, but most popular cruiser-style motorized bicycle donor bikes, for instance the execrable Huffy Cranbrook, do. My own pedal chopper is built on a '90s Dyno Glide cantilever frame. I like it but would never consider motorizing it, for a variety of reasons.

By the way, I just looked and found a circa 1990 Specialized Hardrock 21-speed mountain bike in good condition offered for $150 on my local Craigslist. That's a normal diamond frame made of round 4130 chromoly steel tubing, with effective cantilever brakes front and rear, and 21 speed index shifting. Its frame is a truss that isn't compromised by being made of curved tubes. It doesn't need a different fork and it doesn't need to be squashed or drilled to fit motor mounts. It could even use billet bar mounts, since its tubes are round and haven't been ovalized by curving them:

"When I see an adult on a bicycle, I do not despair for the future of the human race."
H.G. Wells
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