Strengthening Aluminum Frames

Discussion in 'Frame Mounted Engines' started by Phill, Mar 8, 2014.

  1. Phill

    Phill New Member

    Hi all,

    First Post here.

    I've done a little searching but couldn't find anything suitable, so may I please ask a question.

    I am about to embark on my first build.

    80/66c china girl engine kit

    the bike is a 20 inch aluminum mountain bike I've had for years, in it's day it was very expensive, and is still a nice bike.

    I'm reading a disturbing amount about the relative perils of frame mounting motors to aluminum frames, my down tube is 2 inch in diameter, so i'm going to have to fabricate some alternative to the standard mount. this in and of itself is not a problem, don't worry I'm not taking a drill to my nice bike.


    does anyone know of, or have any suggestions for decreasing the fatigue to my frame, I'm not at all convinced that putting rubber between the frame and the motor is going to reduce vibration, I'm no mechanic, but it seems to me that adding more material, particularly flexible material like rubber, only provides more "stuff to wobble".

    I was wondering whether some kind of sleeve arrangement wrapping around down and seat tubes might be an option.

    any wisdom would be most appreciated.

  2. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    Hi Phill, very happy to meet a fellow northerner. I'm from Cheshire originally too. :) You were supposed to post in the introductions section though. ;)

    Like you I joined MBc to read and learn all about MABs, 2-stroke engines and specifically China Girls before getting in over my head (there'll be time for that later). And I'd like to congratulate you on your wise decision to do things the right way around haha. :D;) However please read my following replies in the light of this, and as "for discussion purposes only, not as experience-verified information". I know bicycles enough to comment on your specific question but I'm not suggesting I have experience with Motor Assisted bicycles, I just prefer to comment when I have questions/ideas that I'd like to see discussed rather than to lurk like the hundreds of lurkers reading this right now, lol. :whistling:

    Awesome, which UK distibutor of 66cc China Girl? Cuz I will want to pick your brain later. ;)

    You've had your prized very expensive alloy mountainbike for years? You loved it and rode it daily and kept it nice?
    Yeah they build aeroplanes and helicopter airframes from 4130 cro-moly steel. Steel is real, so is wood (some aeroplanes are wood). AFAIK they don't build aeroplane and helicopter airframes from aluminium alloy, gliders maybe. Aluminum has no fatigue limit, so it will eventually fail even from small stress amplitudes. Steel, OTOH, does have fatigue limits, so repetitive stress cycles with amplitudes below the fatigue limit can be applied an infinite number of times without failure.
    Frame designers know this, and design aluminum frames, and guarantee repeat business. ;)
    Scot Nicol, founder of Ibis Cycles, wrote a great series of articles in the early nineties, Metallurgy for Cyclists (, and they're a must-read for anyone interested in the properties of materials used for building bicycle frames.
    So why would you want to do this to your cherished bike when cro-mo frames are so readily available and 1 year old alloy MTB frames are so cheap second-hand?


    Any undamped spring will, like you say, just wobble. Rubber has some inherent damping, just not a hell of a lot.. I expect it's negligible. I have no experience of using rubber in this application** :goofy:

    Yeah sleeving sounds like an option. But it's welds that crack on old bike frames (in cycling use, anyway) not middle of the tube, so I doubt that would matter if you're not creating a stress riser by drilling your frame tubes. To my mind sleeving would help avoid crushing tubes (which may be an issue with (car exhaust type) clamps) more than anything else. Check the Sick Bike Parts website and see their ideas for mounting to oversized tubes, which you could buy or pinch the idea, then also sleeve it if you decide you want to. :)

    I'm not wise but I hope that was food for thought.
    Last edited: Mar 8, 2014
  3. butre

    butre Well-Known Member

    The more solidly you mount your engine the less it will wobble. The people who are using rubber are using it as a crutch for poor mounting or poor choice of frame. If you mount to rubber the engine only has to vibrate it's own weight, while if you mount it so solidly that it may as well be one object, the engine has to vibrate it's own weight plus the weight of the bike and the weight of the rider. With that information, I'm sure you can figure out how important solid mounting is.

    In the case of an aluminum bike, I'm pretty certain that you'll never overcome the inherent weakness of aluminum no matter how solidly you mount your engine. It's probably best to just go and pick up a steel frame bike somewhere. You can get all sorts of nice bikes for motorizing at big box stores like target or k-mart for dirt cheap because they use a bunch of parts (heavy steel frames, no rear suspension, etc.) that would normally be considered undesirable in the cycling world. You could even pick up a phantom bikes frame for $200 and use your existing wheelset, forks, brakes, and whatever else you want off your current bike.

    Another option is rack mounting the engine, since I've never heard of anyone breaking a frame like that, but it would off the center of gravity pretty badly and make the bike look downright goofy in my opinion. Not to mention you can't use the rack anymore.
  4. Phill

    Phill New Member

    Furry, thanks for the detailed reply and warm welcome, i'll compose a detailed reply when i get a sec
  5. jaguar

    jaguar Well-Known Member

    If you are set on using the aluminum frame then you need to take apart your engine and drill bigger holes in the flywheels for true engine balance. With it balanced there will be minimal vibration and minimal stress on the frame. Click onto my signature link to read about balancing.
  6. Phill

    Phill New Member

    i'm not set on using my aluminum frame, it just would have saved me from buying anything else.
    Jaguar, thats some impressive mechanical knowlegde you have there and waaaay over my head for the time being. but thanks for the input.
  7. Phill

    Phill New Member

    is aluminium alloy (specifically AL6061) subject to the same fatigue problem?

    ive been offered a free frame which I think, will accept all my components.

    this being my first build i think i'll probably use it regardless, im bound to run into a load of trouble with the mb anyway, a cracked frame i got for free likely being the least of them.
  8. FurryOnTheInside

    FurryOnTheInside Well-Known Member

    In that it's alloy, yes. Whether it's a "problem" or just a charactristic of that material is a matter of opinion I think. Suffice to say good Al alloy frames come with a guarantee, but it's not a lifetime guarantee, it's like 7 years at best. Some dude from Trek (probably the CEO but I can't remember now) claimed that when they switched to making frames from Al their number of customers making warranty claims dropped rather than increased But he didn't mention what they did to the length of their warranties!
    IMHO, just use the free frame as long as it accepts the components you want to use. Your cherished old friend/frame is sure to live longer if left unmotorised.
    There's a lot of non-cyclists selling complete bikes which they bought on the "ride to work scheme", they are often in newish condition having spent a year or two parked in the owner's garage, so these could be a good source of components. I just recently bought my second Carrera Subway 2 and there's some very usable parts on there if that's your kind of thing.
    If you read and research I'm sure most problems have already been encountered and fixed by others so much heartache and frustration can be avoided. You joined a forum before blowing up an engine so IMO you're on the right road. :)
  9. jaguar

    jaguar Well-Known Member

  10. IbedaYank

    IbedaYank Member

    4130 steel Cold drawn—normalized[2] tensile strength 85,000–110,000 psi (590–760 MPa)

    6061 aluminum Tensile Yield Strength 40,000 psi
    now what is stronger?

    2x the aluminum to get roughly the same strength