stronger BMX frame- cheap or expensive?

Discussion in 'Frame Mounted Engines' started by keatonx, Sep 23, 2013.

  1. keatonx

    keatonx Member

    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-

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

    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-

    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!!

  2. keatonx

    keatonx Member

    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!
  3. keatonx

    keatonx Member

    So you think the diamondback would be better?
  4. keatonx

    keatonx Member

    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?
  5. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    At least the Diamondback has a proper 1&1/8" aheadset! So it's an adult bike not just a toy. I'd be looking for an old Hoffman Deebo personally.. damn wish I hadn't sold mine now lol.. it was a TANK. Made for the Condor to learn flips and flairs, early 90s BMX freestyle bike frames were the strongest as that was the fashion back then to go really strong (and overweight) with straight tubing, no double butting etc just THICK tubes, pierced top tube, head tube gusset etc.. now they're getting lighter as that is the fashion. I'd try and find some 40 year old ex-BMX-riders and see if you can source a really hardcore 4130 Cro-mo freestyle bike from "back in the day".
    Hoffman Deebo.jpg
    Tried to drill out the dropouts on the fork once.. no chance that 4130 was harder than the drill bit! :D
  6. IbedaYank

    IbedaYank Member

    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?
  7. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    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! :p 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.
  8. keatonx

    keatonx Member

    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! :(
  9. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    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?
  10. keatonx

    keatonx Member

    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 :)
  11. keatonx

    keatonx Member

    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)
  12. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    Where are you going to mount the motor? I was assuming you'd make a rear rack.. that's why I was thinking seatstay construction is so important. The welds (contact area) on the Diamondback is just the TIPS of the seatstays concentrating the force on a tiny weld, not a proper wishbone with a huge contact area like the H. Deebo in the pic I posted. If your motor is as awesome as it sounds, a serious frame would be an investment and "make" the machine.. and won't cost a lot of $$, just time in finding one.
    I think it'll loop out tho, frame being SO short and you being so light; but that's for you to find out haha. :D Wear a MX helmet PLEASE though cuz I did know a guy who died when he looped out (falling off the back).
  13. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    My Specialized BMX has a both side drive 14mm hollow cro-mo axle rear wheel for people who mostly grind on their right side they can switch it over to a left hand side drivetrain, so that would allow you to replace your freewheel when it wears out.

    A bolt on V-brake plate (Brooklyn Machine Works might do you a nice one) or one either side of the fork (think that's what you mean by "dual") should be good. :)

    I guess you could mount the fuel tank inside the bars?
  14. keatonx

    keatonx Member

    I was going to weld a big triangle of sheet metal to the main triangular section of the frame (the one where a HT goes), then weld my motor mounts to that. The motor will sit up closer to the headtube than to the seat post, so it isn't in the way of my pedals. I guess I'll be bolting the sheet to the frame instead of welding it now.

    There was also an old norco spitfire for 100$, but it's built the same as the diamondback and costs alot more.

    Sorry to hear a out the guy looping it, I wear a motocross helmet because I race motoX so I already have the helmet.
  15. keatonx

    keatonx Member

    My 140cc 2 stroke lawnmower bike, that made 4hp stock and probably makes around 7 now will only wheelie in first gear, on a crappy 20" huffy, so I don't think wheelieing will be too bad. Still possible, but not a big problem lol
  16. keatonx

    keatonx Member

    Not sure about fuel tank location, might just use a HT fuel tank

    But am I correct that BMX frames are stronger than regular bicycle frames?
  17. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    I don't know why you wouldn't weld to the frame. You won't be ruining any existing heat treating if the bike isn't heat treated, and for the money you're looking at it won't be heat treated. I wouldn't want a seatstay weld coming undone and you getting the seatstay shoved up your [leg] by a 7hp motor so I feel it needs a gusset adding, personally. I can't picture what you're going to do about making the motor mount inside the frame.. is it really THAT small? I had a 27cc off a goped(copy) motor, never managed to mount it in my Hoffman because of the difficulty getting a sprocket for it (goped chain was tiny, not a BMX chain to make things simple) but it was never going to fit in the front triangle of the frame. I'd love to see some pics of your build once you get it going, this is turning out to be really interesting! :) :)

    Idk MBs, though I am more familiar than most about BMX and MTB.. only meant to chat with you about "general bicycle questions" as I know BMX bikes so I can actually talk in this part of the forum without talking out my armpit. :p I still say go for a 24"(wheel) frame though, you get longer toptube and seatstays that way, and even if you stick 20" wheels in there could well still have clearance for pedalling as long as you don't try to pedal around corners, lol. A 24" fram could use a (adult) mountainbike fork, suspension fork, disk brake compatible fork (check my album), slackening head angle a little by raising bars but without ruining BB height as much as on a 20"(wheel) BMX. Bottom bracket height affects looping out more than anything when messing about with a BMX frame, a slight increase in BB height means quite a few degrees higher from your rear axle.. hope I put that clearly.. I'll draw up a pic if you like though.

    Mounting the tank further forward would just help a little with (not) looping out, that's what I was thinking.. gives you a little more knee room too, not to mention would be barely visible hidden inside there so the bike would look really cool. The little bar mounted tanks on the Team Boxer bikes (check the Gallery section) that I've seen look ideally sized for this and shouldn't affect your steering too badly if your bars are mounted "correctly" parallel with the fork leg (not wayy dangerously forward as a lot of kids have them set up).

    But am I correct that BMX frames are stronger than regular bicycle frames?

    Well that's a huge sweeping generalisation. Short answer is no, but they have short tubes so lower leverage forces to deal with, same as with the wheels' stiffness. "stronger BMX frame, cheap or expensive?" was the original question.. I'd say twice as expensive as the Diamondback you were looking at, and you won't find a real BMX freestyle frame browsing classifieds same as you won't find a dirt jumping/downhill MTB in classifieds, only 26" wheel road/shopping "mountainbikes".. You would find it by chatting with older BMX riders, the owner of the local indoor skatepark, or maybe putting a "wanted: strong 1990s BMX frame & fork" poster in your local indoor skatepark.
    The majority of all "stunt" bikes are made for kids to ride on the road and feel like their "stuntman" hero, not actually ride like one. You could just weld gussets onto a Diamondback I expect, since you're not going to need to jump it or pedal it at jumps. Or are you going to jump it?? If your motor is truly something special, then a cheap frame would waste it IMO.
  18. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    haha I can ride reverse on my BMX *lands fakie and rides past you backwards* :D Yes it is very new looking paintwork on the one in that pic, looks barely ridden. I WANT IT! :D
    I only showed it as it's a benchmark of strong BMX fames with no regard to weight, and an example of proper rear stay construction. The deebo wasn't longg, ideal dirt jump frame for 6' riders. I admit mine as a "li'l deebo" which was slightly shorter but I was learning 540s (1 and a half aerial spins) etc on it lol. I couldn't weld to one, it's a classic!!! but they sold them as complete bikes, back in 2000 I got one (used) for £150.. Gack wheelset, Primo cranks.. sorted. :) Idk what they sell for in U.S.$ these days.. not $35 though? I would recommend keatonx buys my Federal cruiser off me for this project but I'm in UK. Anyway he says he doesn't have problems keeping the front wheel down on his other 20"er.

    btw the guy selling the Diamondback (looking at the pic) has been riding with the seat sooo high he probably strained the upper seat tube welds by having insufficient amount of his seatpost in the seat tube, that's if he didn't bend the seat tube itself, and wouldn't surprise me if it's impossible to put the seat down now (or might need whack with a mallet to do so!).

    Back to the NORCO you mentioned but I forgot to comment on.. those are RACING bikes. Some newer ones (I just googled) are stonger, with a proper rear stay construction, but older ones are.. pants. :( All of them have a high bottom bracket, which means a BMX racer can pedal deeper into the corner and win the race.. but it WILL loop out much easier than a freestyle bike with it's low BB. I'd say look for a bike with a BB in line with the axles not above them.. you can weld on braces/gussets to reinforce a frame but you can't alter a frame's BB height.

    example pics: ALL norco spitfires, one strong, one not so strong, one pants. ALL unsuitable BB height.

    norco spitfire pro.jpeg norco RACING spitfire.jpg spitfire_lg.jpg
    Last edited by a moderator: Dec 18, 2015
  19. keatonx

    keatonx Member

    Ok, what would the weak points on a cheap bike frame be? I wouldn't mind welding on a bunch of braces, gussets, lugs, etc. if it would fix most of the problems.
    Why would the seatstay be a target, is it from the added pressure of the tight drive chain pulling the back end in?

    Btw the engine isn't going directl inside the frame, it's hanging out on the right side (not by alot tho, maybe 5, 1/2 inches max. It's a pancake shape- round, but thin
  20. keatonx

    keatonx Member

    Oh yeah, Here's the extra bar I had to weld onto my lawnmower bike's frame- the seat post was bending slightly because the seatstay and top tube were attached to it at different points.
    image (3).jpg

    IbedaYank, here's all that stuff I needed to do to make the derailleur system work without a freewheel. Pretty sketchy, used to make the wheel lock up all the time. That's why I'm using a proper gearbox this time.
    image (2).jpg

    Here's the whole bike
    image (1).jpg