Love Child, an unholy union of CB400t and XR250r

Zweihammer

New Member
Well, I was looking for options for the rear shock or linkage for another project of mine, my unloved CM400;

http://www.dotheton.com/forum/index.php?topic=69506.msg813141#msg813141

When I came across this fine example of '80s neglected carcass on the Craigslist for $40 and thought I had nothing to lose, so I picked it up.
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Looking at my current project, I thought I would rather not change direction on it.

Instead, I drug out the frame from my parts bike, along with its engine (I started it before I tore it down a couple years ago).

So began Love Child, the unholy union of CB400t and XR250r.

Here is the broke back '80 CB400t frame;
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The first cut is the deepest;
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This is all I need from the CB;
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Will it fit?
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Need to make more room here, in the middle;
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You know you want to;
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There, that looks better;
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That'll hold it, ready to ride;
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I'm feeling the love, not sure I'm feeling the seat;
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So far I'm into her for about 50 bucks. Like I need another project!
Cheers, Erick
 
cool project nicely done so far ,but why did you not use the entire xr frame an recradlle it ?
how does the rake.,(the steering head angle ,the steering head location and wheelbase compare to the original as an xr ?
you would have wanted to document that before the neck swap
you dropped the height of the steering head the whole bike is now higher in front than the xr
it is going to be very tricky and dodgy trying to make that basartised cross breeded frame backbone jucture secure and strong enough ,it needs lots of help
you made lots of extra work for yourself but carrion im with you ;)
 
xb33bsa, there are two main reasons I am using the "backbone" of the CB400t frame.

First, I want the VIN from the CB, so if when this basterd moves under its own power, I can register it for the street.

Second, the CB engine was much taller and much longer than the little thumper that was first in there.

I love putting things where they don't belong. When I was 12, back in the '80s, I put a XL250 engine in a Z50 frame. Crude would have been a compliment, but I rode it and had a blast. If Z50s didn't cost so much, I would try to do it again as an adult. Despite knowing how stupid I would look trying to ride it.
 
Zweihammer said:
xb33bsa, there are two main reasons I am using the "backbone" of the CB400t frame.

First, I want the VIN from the CB, so if when this basterd moves under its own power, I can register it for the street.

Second, the CB engine was much taller and much longer than the little thumper that was first in there.

I love putting things where they don't belong. When I was 12, back in the '80s, I put a XL250 engine in a Z50 frame. Crude would have been a compliment, but I rode it and had a blast. If Z50s didn't cost so much, I would try to do it again as an adult. Despite knowing how stupid I would look trying to ride it.

i understand why but you could have just ground off the xr #'s and titled it as a new construction
you need a whole buch of careful stratigic strengthenthing and spreading the loads on that frame joint though
but you dint answer the most important bits, the geometry ,what numbers did you have ? what are you designing for?what do you have now ? what is the wheelbase ? are you using the stock suspension ? it is so easy to do it right and make sure it handles nicely or are you just playing blind man welds ?
 
xb33bsa said:
i understand why but you could have just ground off the xr #'s and titled it as a new construction
you need a whole buch of careful stratigic strengthenthing and spreading the loads on that frame joint though
but you dint answer the most important bits, the geometry ,what numbers did you have ? what are you designing for?what do you have now ? what is the wheelbase ? are you using the stock suspension ? it is so easy to do it right and make sure it handles nicely or are you just playing blind man welds ?

He did use the bark busters you likely made or had a hand in making XB ;D

Hey dood... wanna sell those just above the wrist arm breaking debilitating Bark Bustersz??? $20 and a stamp??
 
Thanks for the insight, xx33bsa, I know from following your efforts to head off other potential train wrecks on here you have the best intentions to get people to see the shortcomings of just slapping crap together.

My plan with this one was to pilfer the unmodified rear suspension, the forks, leaving the position and angle of the CM400 headstock the variable. The CM uses the engine as a stressed member, unlike the XR, though any number of lower end XLs have. I will be bracing the spliced interface between them, I am looking at what brackets I will have to finish mounting the rear structurally. If I can come up with a way to relace or replace the front to match the 17" rear, I will do that for a low rent sumo.

I'm not doing a new construction, as I have witnessed the headache to do so here in California and unless, someday, I am building something that will be worth more than the sum of its parts I'm sticking to inexpensive.

As to the resulting geometry, I don't have a baseline for comparison and I know for my purposes and intended use, something slacker than the motocross XR and quicker than the street CB would make me happy and probably keep me from being killed.

But, you never know.

The challenge is making myself something useful from these castoffs.

And to keep myself from finishing anything for myself, apparently.

Tune-a-fish, the bark busters are pretty thrashed, the throttle side is bent, cracked, and pulled out of the bar end.

Like me.
 
I'm on board.
Big fan of the unfeasible and the improbable.
And the downright unwise.
It has to be done :)
Rock on.
 
Took my next step, supported both wheels and took another degree out, giving me another quarter inch clear at the rear.
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The alignment on the front sprocket is surprisingly close;
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Starting with random bracket, I think came off one of the 400s;
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Made a frame tab for the lowest left side and tacked it on;
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Cut the brake bracket of the CB frame carcass and carved a bracket for the lowest right;
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Now she is supported on her own wheels;
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I put my copious mass on the pegs and bounced, yup, feels like an 80's dirt bike!

I need to space the pegs out on each side as the CB engine is a fat bottom girl compared to the XR. The mounting bosses are integral to the lower frame from the XR. This is going to be a great place to anchor the rear of a cradle/skid plate up to the front engine mount bracket.

Not that any of this is rocket science, but I am happy this is going together so straight forward. Not nearly as sketchy as I expected.
 
I thought that was what was being referred to, good catch.

To be frank, I was just thrilled it fell so close along the z axis and the pivot was within the tangent. I have seen some installations where the slider is the only thing keeping the chain from cutting through the swingarm at ride height.

I'll have to consider my options, as the CB engine itself is taller and the counter sprocket position is taller at the bottom of the cases.

While it seems unlikely I will get off so easily, tonight I'll try to check the chain path when compressed. Perhaps, if it doesn't pass through the bottom when compressed I won't have to cut it apart and I can either make a new slider or space this one up.
 
I put the chain back on and checked the swingarm pivot relation to the counter sprocket. I had to take the block off the lower frame cross member where the lower link pivot mounts just to get a quarter inch clear. To get the counter sprocket any lower I would have to change the linkage. At this point, keeping the suspension relationship stock is the priority as that was what I picked up the XR for to see if I could make it work.
I'm considering whether I want to keep going on two similar 400 projects. It is not too late to use the XR rear parts on the CM frame to finish that project and get it back on the road. I already have that one registered and insured.
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you are going to have a lot of chain slkack variation
you need to take the spring off the shock and work the bike thru full travel
this way you can observe what is happening
the chain will be rubbing on top at full droop that is ok
chain slack needs to be taken up on the bottom run with a tensioner a spring loaded job
you just dont want the bottom run of the chain to be getting tensioned by the underside of the pivot when nearing bottom out
 
seeing is how you are not modding the suspension then just remove the shock to observe what is happening
use something though to stop full droop travel the same as if the shock was in place
 
this method is also extremely imprtant to determine how to adjust the chain,how much slack to leave in the chain it will be a lot of slack ,much more than the xr had because the cs is further from the pivot
i cant overstate how important this is,you must leave the chain so it has correct slack
too much is better than not enough
and when observing all of this do it with the top chain run held tight and straight
just put the bike in low gear and hang a weight off the rear tire
 
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