Lengthening the Frame - Worth the Effort?

MikeJ

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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.

MikeJ
 
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.
 
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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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!

MikeJ
 
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!

MikeJ
 
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!

MikeJ

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.
Elmo
 
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?
 
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