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Newbie
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Discussion Starter · #1 ·
Long story short, I am building a frame out of the front half of a 1990 DR650 and the rear half of a GT250. I have a few reasons for doing this and I know that the frame would have ended up lighter/simpler if I had started from scratch, but that discussion is not what this thread is about.

What I would like from you kind people is ideas for lightest/simplest/strongest way to connect the two halves together. The tubing used for this is 1" in diameter. Feel free to add or subtract to what is already there. The steering head is in it's final position but I am thinking I am going to move the swing arm pivot forward 3/4 inch. This will be a twin shock bike, so the "subframe" has to stay fairly beefy. I have been so far up my own ass with this thing, I may be having trouble coming up with the best way. Nothing is really taboo, so have at it.

Here I some ideas that i doodled up in MSpaint;


(the green line is only for reference)


and here is a blank;


I am leaning toward the orange one. I really look forward to any suggestions and thanks for checking this out.
 

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GURU
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8,827 Posts
First you will need a cradle to go under the engine. No getting around it. If you dont connect the bottom of the frame together you will hurt the engine.

As far as connecting the frame. The yellow and green(first two pics) are over complicated.

Ill have to stare at this thing for awhile later(dont have the time at the moment) but one thing you need to do is slide solid bar inside your tubes(at least an 1" into each side) drill holes in your frames so you can weld these tubes in. This will help keep some forces off your weld joints. In no case should you just butt a tube up against what you have there. All the frame loads will go right to your welds and the structure wont matter.


do you have any other angles of what you are working with on that subframe. I know what a dr frame looks like but im not sure about the gt250 sub frame. Mainly how far apart those tubes are in the pic.
 

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Newbie
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Discussion Starter · #3 ·
None of the tubes will be just butted up, at each intersection there will be a perpendicular tube running to the structure on the other side. You will be able to see the open end of the cross tubes if that makes sense.

As far as hanging the engine, I have seen XR engines and the like hung in such a manner. If I need a cradle I am going to need to rethink everything.

I'll get some better pics up in a bit to give a better idea of what we are working with.

Thanks for the advice.
 

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Dumpster Diver
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Those little lines will never support the whole bike...


I like the first one.
 
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******* Dumbass
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Shift1313 is right. If the engine wasn't designed to be a stressed member, it's going to fail. You need to support it, or find an engine that is already a stressed member setup.
 

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Discussion Starter · #6 ·
In my sketches, the engine is not meant to be a stressed member, all of the chassis loads are carried by the new "backbone" of the frame. The only forces that the engine mounts will have to withstand are the weight of the engine, the 40 odd lbft of torque the dr mill puts out and whatever flex isn't handled by the backbone. Just to clarify, in the sketch with the orange lines the green line does not represent a tube, it was only for reference.

Here are a few pics that may help with an questions;



The gt frame;



The dr frame;


Thanks for all of the advice so far. Keep the suggestions coming in.
 

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GURU
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I like what crash_duc drew but i would make one slight modification. on the gt frame down around the swingarm pivot there is a small tube. I would remove this and put a larger tube there then bring the dr frame down to that point. I would then take the "subframe" tubes from the gt frame around this "spine" and connect them up farther on the frame. the closer to the neck you can get, the better it will be. Then i would take some plate and join all three of these together in a few spots and add a little more bracing.

here are some quick paintbrush drawings.
 

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******* Dumbass
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I'm not sure how you're seeing that. I guess it depends a lot on the swingarm mounting you have in mind.
 

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GURU
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I'm not sure how you're seeing that. I guess it depends a lot on the swingarm mounting you have in mind.
Do you mean my drawing? I was trying to keep the subframe separated from the main frame to a certain extent. Where i drew the two lines coming from the dr frame thats a single tube. It would T into the swingarm pivot and also be gusseted out to the "Spar" of the frame at the swingarm pivot. This will help transfer any torsional load from the swingarm pivot up the spine. I would tie the subframe tubes in just like you drew only extend them past that point farther up the back bone. An example would be similar to my frame. It doesnt exactly correlate but you can see how the two tubes connect to the "spine" but also go up to the steering neck. This will help out with the torsion as well. If you take them all into one point you may end up with some twist in the frame.
 

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Discussion Starter · #11 ·
^^^ That's how the GT frame looked stock in that area.

Here is an attempt at some 3D;


I am trying to avoid having a cradle under the engine. With this sketch, all of the loads are transferred directly from the steering head to the area around the swingarm pivot (which will be boxed). I see no reason why the engine would be pulled upon by the frame with this setup.
 

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Newbie
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Discussion Starter · #12 ·
Thanks for the proposals so far. I do think that they would be simpler and lighter, but having the main cradle in a single plain I feel will allow a certain amount of twist. The back bone is not that large, and I will be running moderately sticky tires with stiff suspension so I would like to beef up that area as much as possible.
 

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GURU
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Even with what you have drawn above you still need to go under the motor. Right above that front motor mount is the balance shaft in that motor which is gear driven off the crank, if your case gets twisted at all that is going to be the weak point and may cause binding between the crank/balance shaft gears. The result will be bad.

You also have to remember both these frames stock had a little bit of flex designed into them. If you want to keep it simple, take the sub frame tubes straight forward all the way to the neck. Bring the single back bone down into a tube that spans between them, then bring two tubes out from there down to around your swingarm pivot. Keep it simple for the main structure then brace it. Having all 3 points come together right in front of a trapezoid isnt a good idea. You want to connect the backbone down to the swingarm pivot as fast and simple as possible without going through multiple links. i know it might seem silly but it will really be the most sturdy design.

I think the design is getting better it just needs a little more refinement.
 

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Newbie
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Discussion Starter · #14 ·
I changed it up a bit. I took a bit of your advice and also because I wasn't happy with the chain clearance, engine offset, and swing arm situation with the other set up.




I think this is much cleaner and I am much happier with the result. Let me know what you think.
 

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******* Dumbass
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They all show frame members attached to the pivot point of the swingarm, and I don't know what engine was used to start with, but most of the engines shown in that I believe are partial stressed members anyway. Using the engine to bear forces it wasn't intended to is always a bad idea. dukerollo was showing the engine bearing some of the forces of the bike/rider, and that engine is designed to be cradled.
 

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OLD GUY's RULE
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If he made a brace that went from just above the swing arm pivot around the motor to the front motor mounting point that would help.
 

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Fucking Newb...Oh wait
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What is so bad about curves? Jeebus. Shift what do you think of this? Sorry about the look of the tubing mspaint doesn't give too many options.

 
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