Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

[BillyGoat4130]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

Bushed the pivot on the swingarm for the smaller 15mm swing arm bolt. Chain alignment is pretty sweet. Think I got a game plan to mount shock direct to swing arm. Wanted to take the spring off the shock to check range of motion easily but couldn't figure out how. Service manual says "Rear shock absorber: do not attempt disassembly. Unit is unserviceable / unrebuildable. Do not attempt disassembly." then has 2 pages on how to drill it to drain the oil before "recycling". Suzuki logic I guess... Lol

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[BillyGoat4130]

After sleeping on this thing last night I came up with what should have been my first thought. The problem was i wanted to take the spring off the shock so I could setup the suspension at max compression on the rear, but couldn't get the spring off and the suzuki manual was no help. Decided this morning I'd just measure the over all length, measure the stroke from the top of the exposed chrome shaft to the bottom of the bump stop (about 3.250in) subtract from the overall fully extended length (about 13.300 in) and wind up with a roughly 10.125 overall fully compressed length of the shock. So I'll just make a mock-up strut with the eyelets that length, and that way I can check my setup under full compression and make sure the arm does not rub inside the frame anywhere, or pivot up too high and knock the back out of the engine cases, etc.

When faced with a conundrum, I've found for me its best to walk away and let things sit a bit. The answer is usually very simple, just have to dream it up!

 

[540Nova]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

Good thinking!


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[TedT]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

There you go! Told you the chain would be good to go! Looking forward to seeing the next chapter on this, looks great so far.

On the Mongrel we went without a strut and literally just moved the entire swinger as far up and down as possible. The design of the stock Honda rear mount makes it almost impossible to hit the engine with the swingarm, unless the shock completely departs from the bike. With the silencer I figured that if I started running into problems there with tire clearance, that I'd have much bigger issues than the tire hitting the silencer!


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[BillyGoat4130]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

This seemed like the most sensible way to make a shock mount to weld onto the swig arm.

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[BillyGoat4130]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

There you go! Told you the chain would be good to go! Looking forward to seeing the next chapter on this, looks great so far.

On the Mongrel we went without a strut and literally just moved the entire swinger as far up and down as possible. The design of the stock Honda rear mount makes it almost impossible to hit the engine with the swingarm, unless the shock completely departs from the bike. With the silencer I figured that if I started running into problems there with tire clearance, that I'd have much bigger issues than the tire hitting the silencer!


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The SV swingarm is a lot "thicker" up and down than the 250R one, and at full drop (Extension) the arm currently sits on the bottom chain roller guide off the frame, so that is going to have to get whacked off and re-positioned. To help minimize that I re-drilled the bottom mounting fork and moved the bottom shock bolt eyelet about 3/4 of an inch further up.

If the arm came all the way up in full compression (past where the shock would bottom) it would actually rub on the inside edges of the subframe (Probably getting whacked too) but the front cross member of the swingarm where the pivot area is would also lever into the back motor mount on the engine, and that'd be a really bad scenario, haha. So I just had to account for and check that stuff out.

 

[BillyGoat4130]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

At a stand still now for a few days. I'm kind of undecided about doing the mount this way, not too keen on the shock and swingarm angle but looking at yours Ted seems to be fairly close so maybe I'm just looking at things a bit too much

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[BillyGoat4130]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

Rough idea of how the new mount is going to be. Probably do some triangulated gussets off the sides. Debating on Welding the mount to a piece of 1/4 plate from the bottom and sides, and then Welding the plate as a reinforcement onto the cross member there. The mount seems a little dinky by itself even if I made several passes around it. Would probably be good but..

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[BillyGoat4130]

Well I have the rear shock mount welded on. Not super enthusiastic about how it turned out, will probably wind up with some further modification or replacement at sometime in the future. Probably some triangulated gussets off the sides or back, or something.

The front of the swing arm also needed cut out to clear the motor once I had all the motor mount bolts tightened down. I was contemplating just running it without the rear 250R motor mount (It's really 3 mounts, a middle one that the pivot bolt goes through and then a top and bottom that goes to the cases) but after seeing it was rubbing there in that same area ever so slightly once that bolts were tightened down I just band sawed that section out. It also lets me utilize the standard 250R rear motor mount that way (You'll see in the pictures if you look again how it is absent in the previous mock-up). I will need a second set of front pivot needle bearings and turn some new bushing caps, but now its fixed, and I have no worries about full compression of the arm ever tearing into the back of the motor and breaking something serious off. I'd cry if that happened on a built 250r engine, but if that happened on my Rotax, it'd be like having my dog die...

Once the pivot area is completely buttoned up, I'll whack off the steering neck and work on that area. The SV650 has kind of a unique bearing setup where the top and bottom stem bearings are sized significantly different. What I plan to do is get a piece of tubing similar in diameter to the factory 250R neck, and machine it, and then weld on bushings on each end that have been bored ID to fit the appropriate new bearing races. This way I don't have a piece of 2.5in 1/4 wall hunk o' iron welded on for a neck. I've seen some English guys swedge their tubing to fit the different sized bearings at each ends but I am not sure I am brave enough to do that



Question: When setting up the HEIGHT of the steering neck where it is positioned on the frame, it makes the most sense to have the centerline of the axle for the front and rear an equal amount from the ground, correct? In other words, I should make it where I can lay a level across the the rear axle to the front axle, and have it be level, right?

 

[irk miller]

Question: When setting up the HEIGHT of the steering neck where it is positioned on the frame, it makes the most sense to have the centerline of the axle for the front and rear an equal amount from the ground, correct? In other words, I should make it where I can lay a level across the the rear axle to the front axle, and have it be level, right?

Rim + tire height will determine level of the axles to the ground not steering neck height. You're talking about radii from centerline of each axle.

 

[BillyGoat4130]

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.

 

[jpmobius]

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.

 

[BillyGoat4130]

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.

 

[BillyGoat4130]

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.

 

[teazer]

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.

 

[BillyGoat4130]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

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.

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[jpmobius]

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.

 

[irk miller]

Don't forget wheelbase. You want to be somewhere around 58in - 59in, and no more than 60in.

 

[BillyGoat4130]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

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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[BillyGoat4130]

Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT

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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