Cut frame still strong enough?

Discussion in 'Bicycle Repair' started by go you good thing, Jul 5, 2008.

  1. Hi all,

    I have an aluminum cruiser frame and I will need to cut the lower top tube under the tank to mount a new Honda 4


    The motor will be about 50mm higher than the existing Happy Time that just fits now.

    I may be able to just cut 3/4 of tube leaving just enough to maintain its strength but obviously greatly reduced.

    I wont know how much extra room I need until the installation kit arrives this week but I suspect the entire length of tube will have to be removed.

    This frame is the opposite of most frames in that it has the thicker bar underneath the 2 smaller bars (see


    Does anyone here with any engineering skills know if complete removal of the lower bar will be ok strength wise?

    The frame is good quality but I don't want to risk cracking it.

    I guess I could maybe weld another top bar in but would rather not and I don't think it would look that good not to

    mention having to repaint it.

    Any help or suggestions would be great.


    Attached Files:

  2. lennyharp

    lennyharp Member

    It does not sound like a good idea to cut into the frame as it weakens it. I don't know how much but you already are stressing the frame more just by adding the motor and then to cut strength out is not good I think.
  3. darwin

    darwin Well-Known Member

    If your motor is too tall can you relocate the gas tank, if not I would get a motor that fits. Some of these 35cc motors are really compact and might solve your problem. My bike has an aluminum frame [aluminum ALLOY] and theres no way in heck Id be willing to mess with its integrity. Theres a couple of posts here where frames have cracked and its nothing short of a disaster if your going 20 mph+. Just my 2 cents.
  4. A fix

    Thanks for your comments.

    I didn't think it a good idea but I guess I was in denial!!! :???:

    So what I am looking for is a fix that will work.

    Anyone done something like this before?

  5. Hi,

    Depending on your welding skills, I have seen people add gussets to reinforce things...Maybe it could be strong enough if you reinforce the frame after removing the tube (or maybe put another tube in its place but out of the way of the engine)...It would be important to be able to get good welds tho since poor welding can damage / weaken the parent material ....I have seen plenty of bad welds break....and that wouldn't be good... especially if things fell apart while at speed...

  6. Most bicycle frames usually only have a Top Tube and a Down Tube, your Top Tube (form the look of the pics) splits into 2 and is one piece with the Seat Stays, so that middle tube probably (don't quote me) was put there for "a look" more than strength.

    You could probably remove it safely but you would more than likely be left with a hole in the Seat Tube and a hole in the Down Tube.

    But saying that you wrote "The frame is good quality but I don't want to risk cracking it."

    So why even think about cutting it?
  7. Tube

    Thanks guys.

    I have since inquired via email to an aluminium fabricator to cut and weld the bigger bar. When I get a response I will let you know how I go.

    Phil the reason for cutting the bar is so I can fit the taller Honda 4 stroke in the already tight space.