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Author Topic: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT  (Read 18230 times)

Offline BillyGoat4130

  • Posts: 167
    • Live Oak FL Real Estate
I should have thought that through a bit more :)

The way I have the thing positioned on the table right now, I have it blocked from under the frame. I hung the front end on the front with the 250R steering neck a few days ago just to see what it would look like and noticed that the front axle was about 2 inches closer to the ground than the back one, but I didn't have a wheel on it and it wasn't sitting on the ground either.  Good Call.
The question isn't who is going to let me, it's who is going to stop me.
You can't reason with the unreasonable.
Too dumb to quit.

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Offline jpmobius

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You may wish to consider a bit more than the height of the neck.  Rake angle will be all important.  Likely starting with the rake used in the original application of the front forks you are using will be a good number, but the wheel diameter will impact this if different from the original.  Consider the total suspension travel and resulting percentage sag you will have to determine your ride height.  This will create the actual ride height and pitch of the frame which will determine the actual rake angle and height of the neck.  Make a stand for the front and rear axles (or fit the wheels) to hold them in the right place.  Determine the suspension total travel at both front and back and mechanically hold them compressed 30% (or whatever sag target you like) to get the the forks and frame at the right height and you can mock up the steering neck.  Keep in mind that the geometry used in high performance bikes with USD forks is often quite steep, largely made possible by the very stiff frame.  Your frame almost certainly will be much less stiff, and you may wish to consider adding a bit more rake than originally used to compensate.  On the plus side, your bike will likely be MUCH lighter, making up for some of the reduced stiffness.
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline BillyGoat4130

  • Posts: 167
    • Live Oak FL Real Estate
Not a huge amount of progress, but a bit more. Like I mentioned in my last message, wound up deciding to use the stock rear motor mount on the 250R frame and that required me cutting a section out of the front of the SV650 swingarm, and also made me some more engine clearance. Turns out I still have a small spot I need to grind down on the arm to make enough room, but it'll be easy to do.

Dad worked this week while I was working the real job on some bushings and parts for the front pivot to utilize the original sleeve bearings, and also bush the inside of the arm where I cut that section out so that the pivot bolt is properly support the whole around through and across. The kind of work that has to get done but doesn't really show up as a big accomplishment from the outside. I will say though the swing arm and everything in there is *very* nice and snug now and I will not ever have to worry about anything loose or rattely up in that quadrant.

The question isn't who is going to let me, it's who is going to stop me.
You can't reason with the unreasonable.
Too dumb to quit.

Insta: https://www.instagram.com/hondaatc/

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Real Job: http://landsofnorthflorida.com

Offline BillyGoat4130

  • Posts: 167
    • Live Oak FL Real Estate
So here is how it sits now with the rear suspension final welded, completed pivot bushing/bearing swaps, motor mounts and engine all in, etc. etc.

Also some humble beginnings on our new steering neck tube and bearing holders. Those chunks of iron are already machined internally for the tapered bearing races, and the tapered bearings are fit in there. Measurements look like a piece of tubing about 4.750 long will go in between those and be welded to each end to form my new steering neck. Will be slightly longer to allow for bearing tensioning. Going for 2" OD, .180 wall chromoly for material. Those heavy pieces will have shoulders machine onto them to fit the 2in tubing so we have a nice precision fit. Most likely will use the tensioner nut you see there in the one picture to squeeze everything together and hold it square when I go to weld everything together at finality.

3rd picture a demonstration of a 25 degree steering neck with the protractor, over what the existing rake is on the frame.
The question isn't who is going to let me, it's who is going to stop me.
You can't reason with the unreasonable.
Too dumb to quit.

Insta: https://www.instagram.com/hondaatc/

Photography: http://wgolightly.smugmug.com

Real Job: http://landsofnorthflorida.com

Offline teazer

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Step away from the bike and no one will get hurt....

That surely isn't the swingarm droop angle you plan to run , is it?  Presumably, it's just how the bike was sitting when the picture was taken. Ideally you are looking for somewhere in the 8 - 10 degrees of droop.  What would the seat height be with the swingarm the way it is? 34" or more I would guess.

Offline BillyGoat4130

  • Posts: 167
    • Live Oak FL Real Estate
That's full extension with no weight on it so yes it is pretty aggressive looking angle wise.  I was hoping for a bit less but with sag it is going to change quite a little.

No idea on seat height yet. Sub frame will be removed and a new one built.  It'll be more leveled out and probably lower than that one.

Sent from my SM-N910V using Tapatalk

The question isn't who is going to let me, it's who is going to stop me.
You can't reason with the unreasonable.
Too dumb to quit.

Insta: https://www.instagram.com/hondaatc/

Photography: http://wgolightly.smugmug.com

Real Job: http://landsofnorthflorida.com

Offline jpmobius

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Likely you will find that you won't have any of the geometry even remotely close to being acceptable without getting your frame oriented like it will be when driven.  If you think you are that lucky I suggest lottery tickets - much more of a sure thing.  It really is not so difficult to determine where and how your frame needs to be positioned:
You may wish to consider a bit more than the height of the neck.  Rake angle will be all important.  Likely starting with the rake used in the original application of the front forks you are using will be a good number, but the wheel diameter will impact this if different from the original.  Consider the total suspension travel and resulting percentage sag you will have to determine your ride height.  This will create the actual ride height and pitch of the frame which will determine the actual rake angle and height of the neck.  Make a stand for the front and rear axles (or fit the wheels) to hold them in the right place.  Determine the suspension total travel at both front and back and mechanically hold them compressed 30% (or whatever sag target you like) to get the the forks and frame at the right height and you can mock up the steering neck.
Your swing arm does indeed look to be very low, and you frame looks to be pitched forward quite a lot at least in the pics.  Rotating the frame back will make the swingarm angle that much worse.  Mechanically, the alignment of the sprockets and swing arm pivot drive much of the design.  The three centers should be in alignment at fully loaded sag.  Typically you would still have some droop of the awing arm at this point -more if you design to overcome large obstacles like rocks and less if you expect a smoother surface.  Take the spring off of your shock and mock up what you have so the sprockets and pivot are aligned and then angle whole works so the arm is how you want it while riding.  Your shock will need to be compressed about 30% (or whatever you like for fully loaded sag).  I would guess that you will have to relocate your shock mount(s) to get within reasonable range of this condition.
The notion here is that you are not building from scratch so you have to make choices on how to implement the parts you want to use.  You have to fabricate rather a lot to get it all together so you want to work around the most difficult things to change so you can leave them alone and build or alter the stuff that is easier.  It is easy to get shock mounts in the right place compared to relocating the engine so the kinematics of the chain drive are what they need to be.  It will be very difficult to change the relationship between the sprockets and swing arm pivot, so leave that alone and rotate your whole assembly so it is in the right orientation.  Make all the other stuff fit that.
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0


Offline BillyGoat4130

  • Posts: 167
    • Live Oak FL Real Estate
Likely you will find that you won't have any of the geometry even remotely close to being acceptable without getting your frame oriented like it will be when driven.  If you think you are that lucky I suggest lottery tickets - much more of a sure thing.  It really is not so difficult to determine where and how your frame needs to be positioned:Your swing arm does indeed look to be very low, and you frame looks to be pitched forward quite a lot at least in the pics.  Rotating the frame back will make the swingarm angle that much worse.  Mechanically, the alignment of the sprockets and swing arm pivot drive much of the design.  The three centers should be in alignment at fully loaded sag.  Typically you would still have some droop of the awing arm at this point -more if you design to overcome large obstacles like rocks and less if you expect a smoother surface.  Take the spring off of your shock and mock up what you have so the sprockets and pivot are aligned and then angle whole works so the arm is how you want it while riding.  Your shock will need to be compressed about 30% (or whatever you like for fully loaded sag).  I would guess that you will have to relocate your shock mount(s) to get within reasonable range of this condition.
The notion here is that you are not building from scratch so you have to make choices on how to implement the parts you want to use.  You have to fabricate rather a lot to get it all together so you want to work around the most difficult things to change so you can leave them alone and build or alter the stuff that is easier.  It is easy to get shock mounts in the right place compared to relocating the engine so the kinematics of the chain drive are what they need to be.  It will be very difficult to change the relationship between the sprockets and swing arm pivot, so leave that alone and rotate your whole assembly so it is in the right orientation.  Make all the other stuff fit that.
Further back I mentioned you cannot remove the spring from the shock but I made a strut the length of the shock fully extended minus shaft stroke to set and check things.

Perhaps I am under thinking this,  but it does not seem like rocket science to me?  The entire frames position and geometry is going to be modified once I fit up the front end as that steering neck and everything sitting on the table now has little to do with what it'll be sitting on the ground completed?

I realize i don't want the chain cutting into the arm or an angle abrupt enough to make it squat like a bitch everytime you crack the throttle from it trying to make the swingarm climb.



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The question isn't who is going to let me, it's who is going to stop me.
You can't reason with the unreasonable.
Too dumb to quit.

Insta: https://www.instagram.com/hondaatc/

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Real Job: http://landsofnorthflorida.com

Offline BillyGoat4130

  • Posts: 167
    • Live Oak FL Real Estate
Don't forget wheelbase.  You want to be somewhere around 58in - 59in, and no more than 60in.
Thanks!  I had looked up the Sv650 wheel base and found it to be 57 inches and intended to be in that neighborhood

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The question isn't who is going to let me, it's who is going to stop me.
You can't reason with the unreasonable.
Too dumb to quit.

Insta: https://www.instagram.com/hondaatc/

Photography: http://wgolightly.smugmug.com

Real Job: http://landsofnorthflorida.com