stronger BMX frame- cheap or expensive?

[keatonx]

Hi, I'm making a motored bike with a sachs 160cc rotary/wankel engine that I'm building from the engine block up (was missing almost all the parts, except for the main housing, side plates, rotor, etc. I'm modding an ignition from a weed wacker to fit, making a fan for cooling, cutting down mazda RX7 apex seals to fit it, making an intake manifold, etc. 90% of the parts are missing tons and tons of work to get it working).

Anyways it makes 8hp STOCK and will rev to 11 000rpm ungoverned wit no other mods. So it will be an insanely powerful and fast bike, and the last thing I want is this engines torque twisting the frame apart at high speeds. I've decided to put it on a BMX bike, because they seem alot sturdier built, with stronger frames, more spokes, etc. (and they also look awesome LOOL). The bikes I'm looking at on kijiji/craigslist are a diamondback BMX, a (probably cheap) "razor" BMX, a schwinn "havoc", and maybe a Dyno brand BMX.

What I'm wondering is if the cheap bikes (razor, dyno, schwinn) are built with stronger frames, as they are heavier than expensive bikes (daimondback). Because what I'm afraid of is buying the daimondback (my gut reaction), and it being weaker than the cheap bikes because it's likely made from thinner materials to conserve weight. And I don't care at all for weight, I just want the absolute strongest frame.

Here's the link to the daimondback ad-

http://toronto.kijiji.ca/c-buy-and-sell-bikes-bmx-Good-condition-bmx-W0QQAdIdZ515438958

Here's the link to the razor (my second choice)

http://toronto.kijiji.ca/c-buy-and-sell-bikes-bmx-bmx-for-sale-W0QQAdIdZ517231708

This is my second choice, because the frame tubes look thicker. However it has a really steep headtube angle, which would make high speed wobble more likely.

I can't find the dyno ad anymore, maybe it's been sold?

Here's the schwinn havoc-

http://toronto.kijiji.ca/c-buy-and-sell-bikes-bmx-SCHWINN-HAVOC-W0QQAdIdZ519596372

I know I'm being really safety conscious, but I'm going to be investing alot of work/time into building this engine/bike and want to make it last, and preferably not send me to the hospital if it breaks haha!!

 

[keatonx]

I'm worried because I've already had to add a reinforcing bar to the frame of my lawnmower bike, because the seat post was bending slightly!

 

[keatonx]

So you think the diamondback would be better?

 

[keatonx]

Basically it doesn't need to withstand HUGE shocks like casing a jump, etc, just needs to not get metal fatigue from vibrations etc., because I plan on having/riding this bike for a LONG time. So m thinking is a frame made from harder metal (diamondback) would be better because it's flexing less with each vibration than the soft frame, and the vibrations won't exceed the metal's tensile strength like a hard landing would. Does this sound right?

 

[IbedaYank]

your going to need lot stronger wheels then just a standard bicycle coaster hub.
To use 11,000 rpm your going to have to make a seriously large jackshaft to get the motor into its powerband for a useable amount of time and use a clutch that can handle that amount of rpm... standard gokart will not hold up. Now if you get the deathtrap up to speed how do you plan on stopping it without the use of a large tree?

 

[FurryOnTheInside]

Hehe reading the OP again, that does sound a bit powerful Standard 14mm solid cro-mo BMX axle should be okay though.
Might have some stability issues though at the speeds you might reach.. Freestyle BMX (21" toptube length) bikes are made for doing spins and flips, not straight line stability.. but longer BMX frames are all lightweight racing frames.
Perhaps a mountainbike with 24" wheels would suit you better, though if you think that Diamondback is expensive maybe not! or even one of the hardcore BMX 24"-wheel bikes from early part of the century.. hmm I've a nice a Federal that'd do the job even though it's 10mm axles.. now you're giving me ideas! Specialized did a hardcore 24"-wheel BMX with hollow 14mm cro-mo axles front and rear, but you need 10mm front so you can fit a hydraulic disk brake.. could swap out the fork for a Pitchfork or a mountainbike suspension fork.

 

[keatonx]

I'm assuming the diamondback is made of cro-mo rather than carbon steel, right?

I'll probably be getting the diamondback then.

Would it likely be heat treated? Because if it is then I can't weld anything to the frame :/
Nuts and bolts and homemade brackets, my favourite!

 

[FurryOnTheInside]

Well I know BMX bikes; but only the hardcore stuff. Diamondback is a budget brand; but it will say on the frame, just ask for a photo from the guy selling it of the sticker on the seat tube.
I can tell you the construction method is nothing special, that's why I wanted you to see the back end of a 1990s Hoffman frame, as they're basically the standard to judge strong BMX frames by.
I doubt the Diamondback is heat treated, it's only a Diamondback.. but the sticker on the seat tube will tell you what it is. I expect it just says "Cro-mo" not "4130 Cro-mo". I wouldn't weld to a Hoffman, just out of respect for a classic (sentimentality really).
I think your project has some other issues.. a tiny wheelbase and steep head angle + riding it very fast just does not seem sensible, I know what my BMX bikes are like riding downhill on smooth roads.. they get VERY unstable at speed. What about acceleration with a centre of gravity so far back? 14.5" chainstays and 21" toptube won't it do wheelies? I'm scared for you!! Look into getting something longer?

 

[keatonx]

your going to need lot stronger wheels then just a standard bicycle coaster hub.
To use 11,000 rpm your going to have to make a seriously large jackshaft to get the motor into its powerband for a useable amount of time and use a clutch that can handle that amount of rpm... standard gokart will not hold up. Now if you get the deathtrap up to speed how do you plan on stopping it without the use of a large tree?

I've had alot of problems with the freewheel bearing on my 2 stroke lawnmower bike that I built earlier tis year-it would stop catching within 45 mins of riding! So to fix it I welded the freewheel cassette right to the hub, and spent around 4 months perfecting a system that keeps the top of the chain tight on deceleration, and prevents the derailleur from getting thrown into the sprockets when I let off the throttle (I went through 7 derailleurs before I got it all dialed in).

But on this bike, I'm welding a sprocket on the crankshaft of a small motocross engine (and getting rid of it's counter-weighting). The chain will go out of the case and to a sprocket on my engine shaft. This gives me gears and a clutch from the gearbox.

From there I'll have a left side drive to a sprocket that's welded to the rear hub (I'm going to use a large diameter hub on a 48 spoke wheel for extra strength).

Btw this motor runs at 3000-5000 rpm stock, it will only reach 11000 ungoverned.
I'm still thinking about brakes, maybe dual rim brakes on each wheel?

Either way I don't and won't cruise on any of my bikes at any dangerous speeds, but I really love having the extra power, just in case

 

[keatonx]

Well I know BMX bikes; but only the hardcore stuff. Diamondback is a budget brand; but it will say on the frame, just ask for a photo from the guy selling it of the sticker on the seat tube.
I can tell you the construction method is nothing special, that's why I wanted you to see the back end of a 1990s Hoffman frame, as they're basically the standard to judge strong BMX frames by.
I doubt the Diamondback is heat treated, it's only a Diamondback.. but the sticker on the seat tube will tell you what it is. I expect it just says "Cro-mo" not "4130 Cro-mo". I wouldn't weld to a Hoffman, just out of respect for a classic (sentimentality really).
I think your project has some other issues.. a tiny wheelbase and steep head angle + riding it very fast just does not seem sensible, I know what my BMX bikes are like riding downhill on smooth roads.. they get VERY unstable at speed. What about acceleration with a centre of gravity so far back? 14.5" chainstays and 21" toptube won't it do wheelies? I'm scared for you!! Look into getting something longer?

Thanks, I'll ask the guy who's selling it about the sticker. If it doesn't have one or just says CRO-MO then I won't weld to it, just to be safe.

I was also preferring the diamondback because it looks like it has less steep of a headtube angle than the others. But I'm going to buy/make a steering damper for it, to minimize the risk of speed-wobble.

For weight distribution I'm honestly not sure how it will work out, but my engine will be pretty close to the front. Not that it makes a huge difference, this rotary engine isn't even close to as heavy as a briggs or tecumseh 8 horse would be (think of it this way, I carried it home in my backpack!)

I don't think the bike will be too small for me, I'm super small! (5'4", 115 lbs)