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Wheelbase will alter with head angle and swingarm angle as you know. I wouldn't worry too much if it ends up at 54-56 inches.
I hear you about some sag when you sit on it, but the starting point is so far away from useable though. Might I suggest that you take a good look at say a stock SV650 for comparison and maybe say a MotoGP bike which lives at the edge of total instability. The shock mounts will have to be changed or find a shorter shock assembly.
Here's a link to someone looking at the same issues. http://pinkpossum.com/GT750/phattrakka/part4a.htm
edit: I went back a page or two and see where you took a wrong turn that is causing the current issues. That shock is designed and valved and sprung to work with a linkage and the linkage would in turn have allowed a better swingarm angle. If the stock SV linkage mount is still on the lower side of the swingarm, you can probably pick up a linkage arm cheaply and then month the lower end of the link to the lower rear frame rail. I suspect that will bring things back into alignment.
It will also allow the frame to rotate backwards to an angle that is a lot closer for the steering head.
Re: Building my own two-stroke "Mongrel" - Inspired by CharlieT & TedT
teazer said:
Wheelbase will alter with head angle and swingarm angle as you know. I wouldn't worry too much if it ends up at 54-56 inches.
I hear you about some sag when you sit on it, but the starting point is so far away from useable though. Might I suggest that you take a good look at say a stock SV650 for comparison and maybe say a MotoGP bike which lives at the edge of total instability. The shock mounts will have to be changed or find a shorter shock assembly.
Here's a link to someone looking at the same issues. http://pinkpossum.com/GT750/phattrakka/part4a.htm
edit: I went back a page or two and see where you took a wrong turn that is causing the current issues. That shock is designed and valved and sprung to work with a linkage and the linkage would in turn have allowed a better swingarm angle. If the stock SV linkage mount is still on the lower side of the swingarm, you can probably pick up a linkage arm cheaply and then month the lower end of the link to the lower rear frame rail. I suspect that will bring things back into alignment.
It will also allow the frame to rotate backwards to an angle that is a lot closer for the steering head.
I will revisit the rear again at everyone's behest. The Sv linkage set up is not feasible for several reasons - like it's offset to the left where my carb and air filter need to ride (directly in line with the shock), the distance from the bottom of the frame where the linkage mount is to the center pivot is about 2 inches lower on the Sv to allow for the sump of the 4 stroke motor. So I'd need to build a new lower frame section closer so a mount is not hanging out in the wind. The upper shock mount is also too high then and you wind up with a negative angle on the arm under compression in order for the shock to reach it.
I won't say it's impossible because I cut up a snow mobile engine and made it fit the 250r, but I question the feasibility of going through all of that for what it may accomplish.
I may be reading this all wrong, but by all descriptions it appears that's not the right shock. Why not utilize a non-linkage shock (90s Ducati SS, maybe) instead? You'd have more work to do to adapt the swinger, but the way they're setup you'd find even more room for carbs and air.
I may be reading this all wrong, but by all descriptions it appears that's not the right shock. Why not utilize a non-linkage shock (90s Ducati SS, maybe) instead? You'd have more work to do to adapt the swinger, but the way they're setup you'd find even more room for carbs and air.
The problem with most other shocks I have looked at is the attached reservoir, that gives me some of the same clearance problems. I have found a few shocks shorter than the SV, but most have that problem. The Bandit 1200 pre 2000 according to a shock chart is shorter and does not have the reservoir.
I could probably make use of one of those with a stock 250R engine, but the intent is to put the Hybrid in here eventually, and it is a really, really cozy fit with the OEM parts so I just have to be very conscious of what I build this bike with to account for that.
How about a different intake set up to move the carb enough to get the shock where it wants to go? Or dump that swingarm and grab an older GSXR600 arm with centrally mounted shock - say a 2001 on. It has teh same sort of linkage as that SV arm, but mounted dead center.
Can't move the intake and carb any further over to the right or center because than the bell will rub on the side of the engine cases housing the kicker gears and shaft The intake is a custom billet aluminum machined one, and while you could change it slightly, it wouldn't overcome the problem without getting kind of radical and making the thing do a 90 and stick straight out the side of the bike.
That may be an option though...
The picture is during mock up of the hybrid engine several years ago but gives you an idea of the confinement issues I face between everything. PS I did away with that coolant elbow and went elsewhere as that clearly was not going to work, haha.
A few pics from my DSLR - I have posted mostly from my cell but I take pictures with the real camera from time to time for posting and review later on.
2 pics from the "mock up" where I leaned the front end against the frame and laid a tank on the back bone.
My old man sitting in front of the South Bend making some bushings to fit in the steering neck so it will go on my frame fixture I built
Steering neck with bottom bushing welded on. Laying in the lathe bed way so dad can check it with the calipers against his work piece periodically
Frame fixture I built about 5 years ago to be able to replace certain sections of the stock ATC250R frame when changing out engines or replacing damaged sections. It mounts to the swingarm pivot, foot pegs, and 2 engine mounts. The all thread is adjustable and allows me to change the angle and height of the steering neck. It will need a little modification for this project but not too much.
Looking good but still looks really tall. Just out of interest, how high was the steering stem on the donor bike versus how high was the swingarm pivot? I suspect that the larger donor bike frame was much taller and the pivot lower. At least it would explain why it looks so tall.
Looking good but still looks really tall. Just out of interest, how high was the steering stem on the donor bike versus how high was the swingarm pivot? I suspect that the larger donor bike frame was much taller and the pivot lower. At least it would explain why it looks so tall.
Donor bike for the front and rear end, the SV650? Or the ATC250R? The SV engine by itself is ginormous compared to the tiny little ATC250R.
I don't have those dimensions off hand, I keep my SV650 I ride at the office off the dirt roads. But my fork rake angle is right on 25 degrees which is what the SV is. My forks are not slid down in the trees anymore than usual. I'd be very surprised if the height of the steering neck itself was significantly different than this at full extension. Don't really know how it could be?
Exactly. If you think about an SV650 and then fit a smaller motor and then make the engine cradle smaller to fit the steering head, the cradle and rear shock pivot will be much higher than on an SV and that's why the swingarm droop is so significant. What that suggests is that the forks should be "shorter" to drop the engine cradle and rear pivot to more usual levels for a street bike.
Or make a slightly taller frame to drop the pivot and keep the top of the frame where it is. What is the height of the seat frame rails if you add the height of tires to number as it sits now?
That is surprising. Not as high as it looks. I was looking for ways to reduce the swingarm droop to something more workable on the street. I guess it's going to be what it's going to be. Nice home made jig BTW and always good to have a family member to help out.
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The intake is a custom billet aluminum machined one, and while you could change it slightly, it wouldn't overcome the problem without getting kind of radical and making the thing do a 90 and stick straight out the side of the bike. 
