Lengthening the Frame - Worth the Effort?

Discussion in 'Bicycle Repair' started by MikeJ, Apr 21, 2010.

  1. MikeJ

    MikeJ Member

    I am looking for advice before I start a project. I have a running 2-cycle bike, and it runs well. Now I want to start Project #2. I would like to place a 50cc 4-cycle engine behind the seat post but in front of the back wheel. This would mean pushing the back wheel backward by 12 - 14 inches or so.

    I have seen photos of a green elongated motorized frame covered with cross-country camping gear. I did not get the name of the owner. So it has been done. I noted he kept the triangle geometry behind the seat post, no doubt for rigidity and strength.

    Setting modification expenses aside, are there other technical considerations to be considered? The project bike is 4130 chromoly steel, so additional frame members will be the same alloy. I will get a professional welder to weld it up.

    I'd like to hear comments from bike builders what to watch out for, and what have you.


  2. professor

    professor Active Member

    Well Mike, I only legnthened the frame of my bike 3 inches and it handles great.
    The longer the frame- the slower it responds.
    I do recall the post you refer to, cool idea.
  3. azbill

    azbill Active Member

    search for 'sabrina 2' (augidog's bike)
  4. MikeJ

    MikeJ Member

    AZBill -

    Augi's bike is the one I am referring to! Thanks for the lead!

  5. Elmo

    Elmo Member

    I added 24 inches and it handles fine. It took that much to get heel clearance while pedaling. the engine is wide, a hf 79cc. Here is a pic of the rough draft.

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  6. MikeJ

    MikeJ Member

    Hi Elmo -

    Thanks for the photo. Keep us posted how well your bike rides over different terrains. I see the engine is in a parallelogram. I'm leery of doing that. I'm sure that given a hard bump, the back wheel will jump up and flex the horizontal tubes a bit. Unless those weld joints are well built up, I expect a couple of them are going to crack over time. I'm thinking that steel truss bridges use lots of triangles for a reason; triangles are the only geometric figure that won't flex at the joints. I hope this setup works for you for a long time.

    I bet that engine gives it some good get-up-and-go!

  7. Esteban

    Esteban Active Member

  8. MikeJ

    MikeJ Member

    Esteban -

    xtracycle .com is a great site for my planned build! Already I spent the last two hours just looking around and actually found some good ideas that will save me time and money! Thoughts are running through my head what I can do to make a motorized long bike. It's keeping me awake.....

    I also did a web search on "long bicycles".... Some more good stuff to be found.

    Thanks for your reply; it will be a great help!

  9. Elmo

    Elmo Member

    I have put 500 miles on it and no cracks, have covered everything from dirt to smooth roads,the only hang up is that bottom clearance is limited due to length going over bumps. The two bottom brackets are connected with two pieces of really heavy wall square tubing and the top tube is heavy wall as well.
    I pulled it down to the bare frame yesterday for painting and closely examined all joints and found no cracks or breaks. I left it without paint just for this reason.
  10. professor

    professor Active Member

    If you are leery of frame cracks- make little gussets (triiangle bits) to web between the angles of the frame. Really, I doubt you will have any issue with stregnth unless you weigh a lot.
    I placed the hf79 up in the frame and with a widened crank, there are no clearance problems. Is there another reason to go longer with the frame?
    Last edited: Apr 23, 2010
  11. MikeJ

    MikeJ Member

    Hi Professor -

    I'm glad you asked. There are two reasons I want to try this....

    First, mounting an engine in the front diamond places the engine attach points a few inches or more from the welds. This feeds engine vibrations into the tubes, which flex ever so little by their nature. I suspect the engine vibrations are not damped as well as they could be if the engine were attached to a (hopefully) more solid base. I want to use a 4-cycle, like Elmo is doing, but place the engine closer forward to the bottom bracket. And personal preference is to have the engine and noise behind me. (That's the real reason.)

    Secondly, I read that a longer wheelbase provides a smoother ride. I want to use shock forks up front, just like Elmo and many riders. I would also like to place a 2.5 inch or maybe a 3 inch wide tire in back and run it a bit underpressure to absorb the shocks caused by highway joints. On one road I would like to ride I think the highway joints were purposely engineered in to produce the "bump-bump" ride for miles on end. I have a wheel adapter and sprocket from Jim of Manic Mechanic on order; I will hopefully be able to move the sprocket outward enough so that the chain will clear the tire.

    I might think of a third reason later today.

    But you use the HF engine with a wide crank? That is another good point to know. I can place that engine almost up to the seat post, minimize wheelbase, and not worry about hitting it with the pedal crank. Thanks!

  12. professor

    professor Active Member

    I actually welded another frame bar horisontally in the frame and the engine is bolted to that. Vibes are pretty low with the HF.
    Good idea having the noise behind you. I have a twin muffler and tail pipe, plus added sound box on the intake to help quiet the engine and this stuff works BUT the general clatter from the engine is still there. Never thought of placing it down in a long frame (great idea from a noise viewpoint).
    Beside the fat tire (or in place of) consider a Cloud9 seat and a suspension seat post- should be very comfy. Seatposts come in a great variety of tube sizes- all metric- measure carefully if you go that direction.
    Yes, a long frame always rides better.
  13. Elmo

    Elmo Member

    Wide pedals hurt my knees. I once put a pair of knee savers on my bike and they hurt. Of course with this bike you only have to pedal when you run out of gas or something breaks.
  14. professor

    professor Active Member

    You guys got me thinking of a tandem!
  15. MikeJ

    MikeJ Member

    Elmo, Professor -

    Do you use a centrifugal clutch? I am assuming Elmo does, based upon his photo. But there is some more to Elmo's power train than what I saw. Maybe Elmo, you can post a photo of the left side of your engine?

    I saw a photo from a very-low-budget bike where a V-belt was simply tightened via a tension idler (no centrifugal clutch) to transmit power between a V-belt sheave on the crankshaft and a belt-driven pulley like that found on a Whizzer. I am not going to rule out the V-belt option, expecially since I saw how Augidog mounted the driven pulley on his long bike's back wheel.

  16. MikeJ

    MikeJ Member

    Tandem Bike? Absolutely!

    I just looked at a relatively-inexpensive tandem bike from Toys R Us (personally, I'd go more upscale). But if you strengthen the horizontal tubes between riders, remove the diagonal tube there, remove the rear crank, place your engine in there, you would have the very same design Elmo has! Shorten that section and replace the front crank as necessary for a custom-length frame.