1969 kawasaki F21m green streak -style vintage mx AHRMA low budget build

Nice start to the build. I have an F9 Bighorn road racer and have also played around with some of the F81M stuff so have an interested in these older Kwaka's. Following along.

Fabulous work, and it's great to watch a genuine bike BUILD from the frame up. I am curious about the 238, as I am pretty sure the first MX bike I ever saw was an American Eagle 238, and it seems like a very underrated engine for the time frame. Not curious enough to get one, mind you, but like to see someone building & racing a good one--

Rick Siemen had a piece on used bikes to seek or avoid, since some earlier bikes were still competitive and reliable enough to be a potential bargain way of getting into racing if you found an example that wasn't worn out. Bikes like sidepipe CZs or early square barrel Maico 400s. He mentions the 238 Greenstreak as a strong engine and says the American Eagle version is a real sleeper if you could find one, surprising power and handling.

Not many other Big Four Japanese bikes made the grade, maybe the Suzuki 185, and the DT-1 because how to improve it was easy to find out & do. The American Eagle 125 & 405 were on the avoid list (shifting & durability issues, respectively). Of course he didn't know the 250 Elsinore was going to arrive that year and pretty much cancel the earlier generation of 250s, but he did single out the AE 250 as still worthy going into 1973.

Great work and I really appreciate all the historical info. Looking forward to more!

I meant to suggest a formula attributed to the late Gil Vaillancourt, who founded Works Performance and built many, many shocks for vintage bikes of different eras. He said that anti-squat characteristics were best if you line up the countershaft center, swingarm pivot, and rear axle in a straight line as your reference: 40% of rear wheel travel should be above this line, 60% below. When the countershaft position is already dictated by existing engine mounts, you will mainly use shock length to dial this in for AHRMA-legal wheel travel; then the Hatta fork should give you a lot of flexibility to work out what rake/trail you like.

IIRC, from Works's start in the late 70s, his shocks did have separate high & low-speed compression damping circuits, even if adjustment required disassembly, so he could use damping to help control rear squat characteristics--we used to just crank in more spring preload in antique shocks to try to fight squatting, which was rougher over stutter bumps coming or going and tucked the front end more entering corners.

The Japanese really struggled with off-road shocks prior to the 1973 Elsinore 250. They seemed to favor spring rates suited to two US-size people riding two-up, with rebound damping that probably wasn't enough for the spring rate for a single small Japanese rider. That relates to why moving the shock mount down helps the stock F4 shock: more leverage on the shock suits the heavy spring, and you add a little wheel travel.

I remember learning to lift the front end on my 1972 Yamaha LT2M 100 and just slam the back end into whatever hole or bump was coming. I didn't know why it worked, but it was that heavy spring, although the rebound could be hairy. Several years later we had at least half a dozen pairs of shock springs around and I hit upon a drop test for rough comparison of spring rate: drop the springs from about waist height on flat concrete, compare how high they bounce The LT-2 spring bounced nearly as high as a 95 lb. Bilstein spring, I'd guess 90 lbs. The common Euro spring rates for short travel I think were 62, 75, and 88 lbs., with the 88s reserved for really big & fast guys, so the LT-2 springs were ridiculously stiff on a bike meant for maybe 100-140 lb. riders.

And I don't recall people back in the day only changing springs on Japanese shocks to a better rate; we used to just pitch the shocks for a better brand like Konis or Girlings. That LT-2M, we found a pair of Spanish Telescos with 55 lb. springs and vastly improved that bike. The damping fluid in Japanese shocks wasn't good either, didn't seem to last well.

Girling shocks were nominally not rebuildable and their fluid also tended to deteriorate, so people in the US used to take a chance on reviving them. You drilled a hole very, very carefully down at the lower end of the body, using magnets & gravity to try to keep shavings out of the interior, then pumped out the old fluid and flushed with brake cleaner or suchlike. Cleaned carefully around the hole and epoxied on a nut, into which a short bolt w/ sealing washer could be threaded. Then you filled it very carefully with the lightest damping fluid we could get using a syringe, maybe we could get what car shocks used, or maybe it was 5w fork oil? At any rate, the goal was to completely fill the body with no air bubbles, or else it would cavitate. Then button up the bolt and repeat with the other damper.

Mainly patience & careful work; IIRC, new Girlings w/ springs or Konis without were about $350-400 in today's dollars, and Girling fluid deteriorated faster than their seals wore out, so people tried to keep them going in 1971. I don't recall hearing of people doing this on Japanese shocks, probably because we knew Girlings worked well to start with and it was a ticklish operation to undertake if you didn't know if just changing the stock fluid would make vintage Japanese shocks work okay.

So you could try that on other old shocks if you dared; at least today more damper fluid variety is available. (EDIT: But NOT on F4 shocks if they're true pressurized deCarbon-type shocks, you'll injure someone, very likely you. My bad, I am actually old enough to remember those shocks being hyped in 1971, but only recalled Hatta forks).

Early vintage shocks just have fixed orifices for damping, and almost zero compression damping, to avoid hydraulic lock at big impacts, springs do all the work on compression. Might be able to vary rebound usefully by changing oil viscosity, but watch for that hydraulic lock. People would experiment with changing orifice size/location on fork damper rods, but most shocks couldn't be taken apart, they were disposable because so low-tech.

Current brands, I don't know what's good and seriously cheap. As wheel travel increased, shock damping often came in light, medium or heavy flavors (short to long travel). Those Progressive shocks are the old S&W brand, which were good, but I think the damping may be too heavy for short travel if you're a lighter person using lighter springs. Obviously a lot of old guys spend $$$ on bespoke boutique shocks, where there's usually some modern speed-sensitive damping and the shock makers know what works for common vintage applications. I like the NJBs my brother has, but they're still $250 or more new.

Last year I ran across something relevant: all the Chinese RFY-type shocks on eBay have about 3" of travel regardless of shock length, which can be enough for a vintage racer. A blogger posted about how they seem to be crude Ohlins copies overall, and that changing & properly filling them and pressurizing the reservoir bladders made them viable for streetbikes. They might be okay for a short-travel dirtbike; vintage Konis & Girlings were streetbike shocks too at the time. RFY shock details This seems more in line with your approach if you want to take that gamble.

Speaking of spring rate, Moto-X Fox had a worthwhile formula as shown below:
It is biased towards fast riders a bit; note their examples below, as Fox elsewhere recommended 60lbs. for Elsinore 125s, 70 lbs. for experts, and the Husky example leaves a little margin over the stock springs:
Most pre-75 bikes had a suspension lever ratio of about 1.2, as seen below. If you lower your top mount, the Montesa VR is probably the most similar on the chart below:
Once you lay the shock down past an equilateral triangle by lowering the top mount, you may gain wheel travel, but resistance to bottoming decreases. It was mechanically easy to do, didn't have to change the swingarm, so a lot of riders and manufacturers did it early on and used dual-rate springs to compensate, but the final generation of twin-shock racers tends to have equilateral shock mounts.

I wanted to pop in and say I really like this project! And it is looking good so far.

It is a lot like my F81M replica, using an F9 as a basis.

https://www.thejunkmanadv.com/kawasaki-f9-project.html



https://www.thejunkmanadv.com/kawasaki-green-streak.html

Well, ca. 1972, shocks were often the big expenditure on a race bike, so you're being authentically old school. Seeing how simple inside those Boges are, it's a mystery to me why China doesn't clone something as simple as that; I guess the cruiser market does demand something heavier-duty just because sheer bike + rider weight makes a shock work hard, and people like or expect reservoirs as a visual. Not sure on the AHRMA rule without looking it up, apart from remote reservoirs not allowed in early classes.

I have not seen or heard anything about the current Betors. They may be aimed more at trials riders & restoration. Serious Spanish bike racers seemed to use Konis, and later 70s Betor shocks tended to be overdamped. I was intrigued by the current ones, but they're not enough cheaper than NJB Experts, which do have good word of mouth. Seems to me that swingarm angle formula is key--how do the stock length 13.4" shocks come out relative to that, given 4" of wheel travel, including the alternate upper mount location?

One thing that occurred to me: I couldn't find F8 specs, but if your bike is getting to be around 23-25 bhp at the rear wheel now, is what I would guess, then you probably want minimum wheelbase of 54 inches. Short wheelbase is good on a woods bike, but gets hairy outside the woods once you hit a certain horsepower level. Even some Euro bikes like early Husqvarna or sidepipe CZs, hot tip was to lengthen the swingarm about 40mm, not just for straightline stability but for more predictable turning. Too much power for the wheelbase and it'll lose the rear too quickly driving out of turns, or stand up due to too much weight transfer if it hooks up, which all is aggravated if the bike has a high center of gravity.

Early Japanese enduros tend to suffer from all of those, though you may want high ground clearance as a given. The notorious frame kits usually dropped the central frame cradle and lengthened the swingarm, but the latter alone at least calms down the handling, makes the front stick better, and is simpler to do. What were called Z-plates were one way to just move the stock axle slot back 30-40 mm, which doesn't change the point where the countershaft/pivots line up straight, but you jigger your shock length to fit the 40/60 @ 4" parameters. It's really a reverse Z shape with the center bar of the Z vertical. If an F9 swingarm fits and is longer, that's an easy move.

The Hatta fork complicates your wheelbase calculations, so you probably ought to ride it some first to feel out what moving the front axle and sliding forks up & down does, even before you buy shocks. I can't tell from the ad whether the Hattas actually adjusted a damping parameter as well as spring preload.

I'd say it's true that Ceriani-influenced damper rod forks are usually designed for a true 14-weight oil, Type F ATF was the hot tip. Faster riders used lighter oil for less compression lockup and faster rebound. Varying the area ratio and location of the compression and rebound holes on the damper rod was how one adjusted a damper rod for a baseline fluid, but was real trial and error stuff; sometimes a shop or team had worked out a formula, but that was much more common in the late 70s.

My used Kaw 350 triple was too soft in front and pitched too much on both ends, even bottomed in front on braking bumps on asphalt. Putting ex-MX Konis on the back was easy. I had to replace fork seals anyway, so I made air caps and used 35mm Teflon seals on 34mm tubes for less stiction, and maybe 3-4 lbs. of air. The damper rod had identical rebound holes on opposite sides of the rod, so I tried drastic first and epoxied one shut on each rod and tried 5-weight oil. I was super lucky trying all those changes at once, because they steadied the bike right down at any speed I cared to go on the street.

I don't know if I would have had the patience to experiment with drilling the second rebound hole back out at various sizes, but that was an option, along with heavier oil. I didn't want to fool with compression hole size or location if I could avoid it, hoped it would be OK with just oil weights and was lucky. It's like Race Tech emulators, they want you to dial rebound with oil weight and then configure the emulators for that viscosity to handle compression damping.

Contrary to how many old guys make it sound, you can go fast on the old style suspension: of course people used to do it, and as a young person you can handle being beat up more by bumps. Emulators require you to work out a whole new chassis dynamic for corners if the front doesn't settle so much under braking, which is a lot easier for common bikes where people have already worked out rake/trail/ wheelbase adjustments, like CZs or Huskys.

It is looking good!

To answer a question about damping adjustment on the Hatta's. No, they just feature preload adjustment only. However, on the sets I have dis-assembled I have noticed the the oil passage holes were rough from manufacturing. I did an ever so slight chamfer on each hole and then ran a file to take off the interior bur. I don't really know if it helped, but it made me feel better.

I have Emulators in my F9 racer, and like the other poster said there is a bit of setup time and testing, but they work really good.