shortening a '86 swingarm?

[Bigbill]

I been thinking of using a shortened '86 swing arm with dual shocks. I'm thinking of a z structural cut and rewelding it in the area of the twin shock mounts. I haven't figured out how much to shorten it yet. This is one bike that's going to be a twin shocker, left kicker hybrid butt kicker. In the past I did install a single shock swing arm in a 82 cr frame. It worked but I did extend the wish bone by cutting it and drilling and tapping it so I could adjust it to the proper length till the swing arm arc'd correctly. Once the swing arm had the proper degree of motion I locked the jam nuts with forever loctite.
I regret now 15++ years later not recording what I did. Have pics I just found.

My plan is to use a 420cc cylinder on a 430 crank on a 430cr newer case. A '87 cr500 front end with disc brake. The rear '86 swing arm. On a '81 250cr frame.

After the holidays ill look for a TIG and a MIG welder I can piggy back off my gas engine 200amp/100% duty cycle Hobart portable welder.

 

[fran...k.]

Bill I have carved up these things and some brand x for hillclimbing. These things are more complicated than might origionally seem.
At to the welder, I remember something like you were a military tank service person so perhaps you already know. At a minimum you will need an ac welder with high frequncy. AC=alternating current but a knob to adjust how much goes one way vs the other is the first upgrade from ac stick. I wouldn't bother with mig, (gmaw,fcaw) mig as a term is kind of vague. Maybe with the most modern mig stuff but if you are concerned about husky parts prices I don't think you are going there. With the tig you can see what is going on, hopefully, How are your eyes? a lot of welding is being able to see what is going on.

I have just butt welded and plated the bottom as that was in tension for what I did. Husky is just a rectangular tube, kawasaki of that vintage has another web inside, so much for husky being superior in every way.

1981 cr frame, chances are you will end up putting the shocks on top of the swingarm and then you will need move the mounts up and perhaps for or aft on the frame which complicates the side panels. Guys have posted pictures of banana shaped aluminum swingarm to get around this.

The aluminum swingarms have 14mm swingarm pivot bolt the steel ones have 12. I am almost positive the part of the swingarm the pivot goes throuh is shorter on the aluminum.

Some aluminum swingarms have two cross pieces in the area of the linkage attachment some have one. That is the area you will have to shorten unless you use a smaller wheel. I would recomend having a two cross pieces, acutally put in the forward cross piece (It will pull so that needs dealt with) before cutting section out.

A machine table with T slots is helpful, put the axle and the swingarm pivot bolt on blocks clamped to the table that way they end up in the same plane. Make bars with holes for axle and pivot rod to keep right left length same.

420 71mm stroke 430 74mm stroke you are really taking on a project.

Not sure what you are describing from 15 years ago likely you put a smaller tube inside or larger over with a good deal of slop as if wishbone is the front end the distance between the two swingarm ends changes if length changes straight with those pipes/tubes doesn't it?

 

[troy deck]

rather than trash a good swingarm get the right arm ,start from scratch or call Karl landrus i can see one of Karls coke bottle arms on that badboy

 

[86 400 XC]

85/86 had some nasty mig welds and 87s got nice tig welds. Pivot axles are not the same like Fran..k says.

Open up the windows, too many paint or glue fumes?? They can mess with ones brain. lol joking, not worth it IMO

 

[Kartwheel68]

The stock steel arms dont flex much so I doubt it is worth the trouble.

On the 420 cylinder with the 430 bottom end, I've asked this several times. Why? The 420 cylinder is identical to the 430 with less deck height, there is no point in using a 420 cylinder.

 

[fran...k.]

The stock steel arms dont flex much so I doubt it is worth the trouble.

On the 420 cylinder with the 430 bottom end, I've asked this several times. Why? The 420 cylinder is identical to the 430 with less deck height, there is no point in using a 420 cylinder.

Sorry if I come across argumentative.

Surely the 85-88 swingarms are more rigid in the up down plane. I suspect the 2015 Husqvarna bikes (exclude minis if available) have cast aluminum alloy swingarms tuned for up down vs right left deflection ratio.

I have read on here lately the 390 cylinder, overbored=420 cylinder. To me it appears not all 420 cylinders have the same planed off the top and bottom to come out with what I see. It isn't like I worked in a shop at the time or bought them new unmodified. Not sure what deck height means, I compared distance (measured) down to top of exhaust and how far from cast last fin to Iron liner by eye. Again just from memory.

If I recall the heads don't interchange, it appears so long as it isn't the 1984 500 the air cooled 500 head fits on the air cooled 430 cylinder. The 420 head has less fins or maybe more accurately less fin area than the 430. Even the plug location in the 420 heads I have is not the same.

 

[adam6402]

Sorry if I come across argumentative.

Surely the 85-88 swingarms are more rigid in the up down plane. I suspect the 2015 Husqvarna bikes (exclude minis if available) have cast aluminum alloy swingarms tuned for up down vs right left deflection ratio.

I have read on here lately the 390 cylinder, overbored=420 cylinder. To me it appears not all 420 cylinders have the same planed off the top and bottom to come out with what I see. It isn't like I worked in a shop at the time or bought them new unmodified. Not sure what deck height means, I compared distance (measured) down to top of exhaust and how far from cast last fin to Iron liner by eye. Again just from memory.

If I recall the heads don't interchange, it appears so long as it isn't the 1984 500 the air cooled 500 head fits on the air cooled 430 cylinder. The 420 head has less fins or maybe more accurately less fin area than the 430. Even the plug location in the 420 heads I have is not the same.

So what is the difference between the 83 and 84 500 heads?

 

[fran...k.]

I guess we can hijack this thread

Upon looking at the parts sheets they have the same part number. In 1984 only the cr and xc cylinder have the same part number as 83 where the 84 wre wru and ae have a different part number. I based my comment on an ae cylinder and head as I recall that didn't swap out in my mix and match experimentation. Perhaps some more investigation is in order.

 

[Bigbill]

I have a 420 cylinder that has the head with it that has the compression release already machined into it. It's the later 420 cylinder that fits the newer case stud arrangement. If I space it 1.5mm she might workout ok with the 430 crank in the newer case. I can also match the cylinder transfer ports to the newer case. Or I could save the cylinder for a 390. But maybe there is something special about the 390 having 71mm stroke over the 74mm stroke of the 430?

I have two 430 engines one is complete a wr a.c. and the other bottom end is a cr LC.

If you ever hammered a 390cr you would know why I'm after another one. It's a sick bike to ride stock. Even the '78 & '79 were sick rides. At my age it's the closest to a wild woman as I'll get today. That 390 will clean your clock. To me it's an awesome exceiting ride. I never rode a 430cr or a 500cr wr. I enjoy twisting on the cr and having the hit, hanging on when she comes to life.

Another point is I built 283 & 301 v8 Chevy that ate 327/350 v8's. The smaller cc's turns out to be quicker? I'm thinking out of the box again but if I put the 390 pork chops on the newer case crank studs and used the smaller cr motoplat ignition. There are variables to consider. Can the 390/400/420 be made to preform better.

 

[Kartwheel68]

Fran, the 1980 390 cylinder is what became the 420, the 420 was nothing more than an '80 390 bored to what became the 430 bore size. Husky sold a 420 kit in '80 before they produced any 420s as new bikes. The 420 cylinder, not the head, the cylinder is identical to a 430 other than the deck height, which is the height of the cylinder with the gaskets installed. Some bikes like Penton/KTMs or current Sea Doo watercraft, you use different stacks of different thickness base gaskets to attain the correct deck height (so the piston is at the proper place at TDC). The ONLY difference between a 420 cylinder and a 430 cylinder is the deck height, which is different because of the shorter stroke on the 420.

Of course the boxed aluminum single shock swing arm is stiffer than the round steel twin shock arm, that is not the question. The question is, was the round steel tube swing arm a poor performer than flexed too much? The answer to that is and emphatic no. Now, if bill wants to adapt the single shock arm just to say he did it, OK, but the performance difference is going to be negligible.

Bill, there is nothing special about the 390 or 420, the 390 performs a lot like the 430, just a little less everywhere and it spools up a little quicker, but they are both very smooth easy to ride bikes. When is the last time you actually rode a 390? I recently rode a '77 390 built specifically for two time World MX Champ Trampas Parker to vintage race, it had a ported cylinder and an internal rotor ignition among other things. It was very fast, but it was still very Husky-like and smooth.

 

[Bigbill]

Thank you kart for some facts. I was taught by a welding guru the strength is in the vertical plane. The first aluminum boxed swing arms proved this. My '98 & '99 husky swing arms flexed in turns. The early ones didn't.

 

[justintendo]

very good description of the 390 and 430 kartwheel. this matches my experience as well.
as far as the swinger flexing my 95 chassis is like a rock, i almost wish it had a bit of flex.
an air cooled head seems much easier to install a compression release on compared to liquid, i would not do a bunch of work to a motor just so i could use a head that had a valve installed personally. if you want a 390, i can understand. but just build a 390...then you can put together a 430!

 

[84scrambler]

Less thinking and more riding. I don't recall ever riding down a trail and thinking to myself hmmm I think my swing arm is flexing ? Riding fast and being a fast rider is more about the rider in my opinion. Just build a factory bike and ride more until your a ass kicker and you can prove it.

 

[Bigbill]

On my two street legal husqvarnas a '99 TE610e dual sport and my 98 wr250 is what I experienced rear swing arm flex on the street in turns. I could feel the flex and the rear tire picking another line. I'm a big man and you lighter guys might not notice it but it's there. Ever wonder why the faster super cross guys go down in turns by themselves?
There front tire is in the carved line while there rear tire is riding on the sides of the carved line trying to climb out of it. The flex is there.

I don't feel any flex in the aluminum swing arms with the 85/87 left kicker style of swing arm with the straight verticle sides. It's a rectangle box.

I been to most of the AWS Courses for welding. Structural design, etc. I was a welder fabricator. I'm thinking I can cut a '86 swing arm like a surgeon and weld it up with the twin shock mounts moulded into the side rails of the swing arm.

To me the twin olin shocks are still the best design rear suspension. The left kickers are very trackable.

 

[Kartwheel68]

SX guys fall in turns by themselves because they run ridiculously stiff front suspension spring and damping rates so they can get through SX whoop sections, which causes poor front wheel traction in corners, its got nothing to do with flex.

Are you going to ride this bike on the street or off road? You arent going to cause flex like that in the dirt. Plus like the front disc brake, this swing arm swap on top of being pointless, makes the bike illegal for vintage racing, there is a specific rule against using single shock arms on a twin shock bike, the Honda guys were the first to do it and as a result a rule was written to prevent it.

 

[Bigbill]

I'm building two bikes. One stock one modified.

 

[fran...k.]

Plus like the front disc brake, this swing arm swap on top of being pointless, makes the bike illegal for vintage racing, there is a specific rule against using single shock arms on a twin shock bike, the Honda guys were the first to do it and as a result a rule was written to prevent it.

I can see on line if someone around here wants to travel to virginia there are quite a few such races and what appears to be lots of classes. Edit, add Unidilla NY. Not sure how I ran across the results from three vintage classes run at a NETRA Junior enduro. Thre was a class that allowed disc brake, four entered it. If there is a rulebook for these local things which may or may not be running the rules you speak of they (rules for local stuff) are not or I failed to find them on line where I looked. A lot of times the rules, the finer points take a protest, and that takes a vicotry. Ever see any minors that could not touch the ground with both feet and start and stop on command?

 

[euro bikes]

I have used a 1985 single shock swingarm on my 1984 frame . I didn't touch the wheel end I cut the welds at the swingarm axle end and modified it there by cutting it to the right length then rewelded it ,the shocks were attached the same way the 84 shocks fit through the swingarm using a solid alloy pin welded then drilled, the shock bolts through that.

 

[fran...k.]

I have used a 1985 single shock swingarm on my 1984 frame . I didn't touch the wheel end I cut the welds at the swingarm axle end and modified it there by cutting it to the right length then rewelded it ,the shocks were attached the same way the 84 shocks fit through the swingarm using a solid alloy pin welded then drilled, the shock bolts through that.

How far down were you able to put that alloy pin? The collar or part that contacts the bottom of the spring seems hit the swingarm more on rectangle vs round. You re welded what was cut off not made something to use the bearings and sleeve similar to the dual shock swingarm? My modifications were to go longer and I used an atk shock, one in the middle on a later frame. That bolt thing that goes through the swingarm is a pretty high grade, I built up a worn one with weld or something like that and it broke, had to go like 10 miles on one shock to get back. I think it cost like $18 somewhere in the 1990's

 

[euro bikes]

I worked it out from the 84 angle of the shock to keep it the same . Take off the spring so you can work out how far down you cut the hole for the shock mount. I will see if I can put a pic on I'm not too computer smart! but i'll try.