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Yamaha XS650 GS - Project Plan
The thought behind this project is that the Yamaha
XS650 is a suitable core vehicle for an All=Road Motorcycle or, perhaps Fire Road Touring Motorcycle. The suffix GS is acronym for
the Germanic phrase Gelansde-Strasse or trail and street, and most
commonly attributed to BMW’s series of twin cylinder dual sport
motorcycles. We use the suffix GS to refer to any multicylinder motorcycle
intended to fill a similar role.
The XS650 is notable for the robustness of its engine
and its exceptionally long production history, some15 years in the US
market, perhaps longer elsewhere. It that time span changes to the engine
design were minimal, meaning there is essentially interchangeability
across the XS’s entire production history. No other component of the
motorcycle in any of its iterations is particularly desirable, except
perhaps the front hub. The frame is sufficiently robust for most any use,
primarily because of its 55 lb. heft. Lots of thick or double walled
tubing, although not in
a particularly well designed configuration. I suspect it is sufficient to
hold the wheels together under moderately severe use so the main frame is
tolerable, if uninspiring.
The engine is the key element for a GS built on the XS650 platform. The engine has a stellar reputation for reliability, even in highly modified form. Parts are readily available both as new production and via a vast nationwide, if not worldwide, stockpile of dismantled bikes. The engine is reasonably compact although a heavy lump, probably around 150 lbs or so. Consequently, a complete bike built with a custom frame is probably constrained to scale in at a minimum of about 50 lbs more than a 600 class four stroke single trail bike, whose engine typically runs about 100 lbs or so. Our stock framed project is probably doomed to exceed 400 lbs., although we hope not by much.
The XS650 engine is an air cooled vertical twin with
wet clutch, highly accessible screw/nut valve tappet adjusters and a
manually adjustable cam chain tensioner - about as amenable to field
maintenance as a motorcycle engine could be, short of hydraulic pushrod
valve operation - boy, would that be sweet ! This engine has a relatively
unsophisticated oil filtration system and a fairly low sump capacity, but
this is probably mitigated by the engines all roller bearing
configuration, which should be relatively tolerant of contaminants in the
oil. There is at least one aftermarket conversion that incorporates
a modern paper cartridge oil filter in the original primary cover filter location.
Methodology for modifying the
engine to incorporate an oil cooler is well documented. The stock engine components are well capable of handling significant rev limit and power
increases without apparent reliability issues, so moderate increases in
displacement alone should be well tolerated. Perhaps the only fundamental
drawback of the engine deign that it is a 180 deg firing order and is not
counterbalanced. Consequently, engine vibration can be intrusive in
certain rpm ranges, especially toward the upper limit of useable rpm. This
is perhaps aggravated with increases in displacement. However, conversion
of the engine to a 270 deg firing interval is well documented and
indications are that this modification significantly reduces the level of vibration, a very desirable improvement.
We are strongly considering this modification for our next XS GS project. The only other drawback for use as
dirty bike engine is that the counter shaft sprocket centerline is farther from
the swingarm pivot than would be ideal. this may require a fairly elaborate
solution for chain guiding and tensioning
mechanism, necessity and complexity increasing commensurate with increases in rear
Initial design investigation suggest that suspension
travel in the range of 8 to
10” would be adequate for the intended usage parameters. Seat height
will be somewhat of a guide here. Operator ease with a bike of this weight
would be greatly improved by the ability to touch the ground with both
feet, flat footed being most desirable but toes down perhaps a minimum
requirement. Fully laden weight will probably be in the neighborhood of
450 lb, a considerable lump to control on uneven terrain.
One other limitation will probably be fuel capacity.
For an Adventure Bike it is highly desirable to pack as much fuel on board
as possible, perhaps even in compromise of rider fit and ease of
maneuverability. Forks of sufficient travel to meet the design spec are,
by default, almost certainly to be of a leading axle configuration. It is
likely that the fitment of leading axle forks will cause an interference
fit with the stock fuel tank at full steering lock. It is likely that the
stock fuel tank will require modification ( think big hammer ) to allow
unrestricted lock to lock fork movement. Initial research indicates that
the most commonly fitted candidates for larger capacity fuel tanks will
probably aggravate this interference fit. Short of a custom fabricated
fuel tank, the solution for extended range may well be found in fitting
auxiliary fuel containers to the bike.
Achieving the rear wheel travel as specified in the design parameters will require significant reconfiguration of the swingarm and upper shock location. two choices present themselves, 1) a single shock, either centrally or offset mounted or 2) a twin shock arrangement. The single shock has the advantages of less weight, far wider availability and section of shock adsorbers and springs, more centralized concentration of weight in the chassis ( polar moment ) and it frees up the back of the bike for a wider choice of exhaust routings and saddlebag mounting configurations. The simplest; no linkage, configuration is perhaps the most desirable. I suspect a non linkage arrangement will be more than adequate for the intended usage. Drawbacks are that this setup, if centrally fitted, may severely compromise the area available for fitting an airbox and electrical components. Offset mounting might mitigate much of this dilemma but would entail significant revision to the frame’s main cradle to provide an upper shock mount, to the extent that fabricating a new frame might be the only viable solution.
Twin shocks are certainly easier to incorporate using
the existing frame. It would appear that a relatively simple relocation of
the upper shock mounts in combination with a suitable swing arm would
achieve rear suspension travel in the desired range. A twin shock
configuration allows essentially complete freedom in airbox design, an
extremely important consideration. A twin shock configuration would make routing of a high exit
exhaust clunky at best and very difficult at worst. It also compromises
the available options for mounting bags but in some sense the
incorporation of bags and their mounts is better suited to a low mount
exhaust any way. Low routing permits the bag mounts to be tucked in as
close to centerline as possible, while high mounted mufflers invariably
forces at least one bag away from centerline, not at all desirable. High
mount also presents exhaust heat issues. Low mount mufflers can tuck up
under the bag mounts/bags. Although not as well protected as high mounts,
the bags will probably protect the mufflers form most damage in the event
of a tip over. Mitigating damage to exhaust undersides would be largely
dependent on the riders skill at picking a line. Solutions for preventing
bag damage from exhaust heat related issues are certainly less complex.,
The only part of the exhaust routing that might be tricky is the
transition under the footpegs form the headers to the mufflers. The
remaining question would be the relative merits of a 2/1 or 2/2 exhaust
system. Twin shocks/springs may be somewhat more difficult to source that
a single. Clearly though the twin shock configuration is much more time
Headers will probably route along the lower frame
rails, within the confines of the skid plate. Choice of diameter is a
research question. As I recall, the inner pipe of the O.E.M. double wall
header is 1.375” in diam. ( O.D. or I.D. ? ) Aftermarket single wall
headers are typically 1.5” for street rides and up to 1.75” for race
engines. I suspect 1.375” will be just fine and certainly easier to
route, probably look pretty wimpy though.
Wheels will be based on disc brake dirt bike hubs. Most desirable configurations would be two wheel sets, a 17”/19” pair fitted with dual sport rubber and a18”/21” pair fitted with more aggressive dirt rubber. A survey of the available tire patterns and rubber compositions indicates that the widest selection of dual sport rubber is available in the 17”/19” pairing. This is really OK as I suspect pushing a mountain road, either paved or gravel on a 450 l rig would feel pretty sketch on a 21” front wheel, especially with a rotor large enough to be suitable for pavement work. The choice between 17” or 18” rear is less easily defined but I think the much wider range of options available in 17” dual sport rubber makes the decision. The dirt combo of 18”/21” is really a no brainer, just a matter of picking desirable brands/models of rubber.
Some question about the necessity of a cush drive
rear hub. The XS’s clutch is pretty robust, with coil spring shock
adsorbers and our exoected pace on pavement will pretty sedate. We despair of finding
a pair of matching, lightweight, high quality dirt bike cush drive
hubs. KTM LC4 hubs are probably excellent but coming up with a pair at
a reasonable price might be difficult since they are highly sought after for
converting KTM RFS irt models to dual sport. I suspect I can get away with
non-cush dirt bike rear hubs,
and if we tear them up we can bite the bullet and get a cush drive hubs as
replacements. Thing to watch out for in choosing a rear hub is sprocket
capacity. I have some flexibility as I can fit an 18 T c/s sprocket in 520
but the stock sprocket combo is in the neighborhood of 17/34.
I have to be careful of being saddled with short gearing. Although my
typical road cruising speed will be relatively sedate, intrusive vibration
might severely compromise the quality of the ride. Gearing for the dirt
wheel should be comparatively easy to fit. Unlike the front, we are not too concerned
about swapping rear wheels/rotors while retaining same rear caliper/pad
set. Less than optimum rear brake performance is probably not
Front hubs. I want a 300mm or larger rotor,
preferably a floater in cast iron, for the 19” wheel. The rotor diameter
for the 21” wheel is less critical. Perhaps wise to try the rotor that
comes with the hub for a while and see if it is adequate. The aftermarket
has oversize rotors for most modern dirt bike front hubs. I suspect the
300mm setup will benefit greatly from a four piston street bike caliper,
Brembo being the no brainer choice. Swapping front master cylinder caliper
and wheel seems like a perfectly reasonable idea, if not preferable to
just swapping wheels. Always thought of a rotor and pads as developing a
nicely matched interface over time, have my doubts about just slapping a
different rotor in there and expecting similar braking performance.
Plastic will be pretty much standard dirt bike stuff,
some thinking to do on relative merits of high or low front fenders. Seat
will most likely be a custom build, me the seat pan, an upholstery shop
for the rest. A decision to make here about a solo or dual saddle. Solo
certainly allows the fitment of more elaborate rear rack configuration (
perhaps a place for additional fuel capacity bur as I have found with the
/7, a dual seat is nor a bad mounting surface for a free form pack and it
always allows the possibility of riding two up, tough to accommodate with
a solo seat and rack for a passenger’s bum.
Airbox will be custom fab, designed first and
foremost to accommodate an easily serviced air filter. have to consider
the relative merits of all foam filter vs a K&N with a foam cozy.
Might be able to also accommodate the battery and assorted electrical
components within the airbox which would be a pretty clean design.
Headlamp will be H4 100/55W. Remainder of lighting
will be LED. Heated vest circuit, probably heated grips as well. There is
documentation about replacing the stock alternator/battery/ coil/points
ignition with a pointless magneto out of an RD/RZ/Banshee 250/350. Have to
weigh the relative merits of this. Crank triggered/ non battery ignition
is appealing in many ways except for the loss of the “ignition system in
a shirt pocket” advantage. Of course there is also the EMP proofness of
the points system to consider J.
The other issue is the output of the RD mag vs the O.E.M. alternator in
stock configuration, and then the possible outputs of modified versions as
Handlebars and controls will be pretty standard stuff. My 28” mini CR bend. Choosing a dirt suitable front brake master cylinder of appropriate bore dia. for the Brembo four piston will take some research. Play handlebar switches by ear. Much as I would prefer a small diameter analog speedo/odo I will probably select a Trail Tech Vapor instead if only to get all of the instrumentation in a compact package, speedo, odo, tach, clock, oil temperature etc. Will mount an A/F ratio meter and a GPS as well. Will custom fab an instrument panel.
XS650 GS - Project Log
Boy this project has been a long time in gestation.
We purchased the core bike back in 1992 or so and have been kicking around
various project concepts on and off since then. Over the last couple of
years though the idea of a gelande/strasse or trail/street ride has
There won't be much of the original bike left when this project is finished. Components retained will be:
- the carburetors
- the frame, although heavily modified
- the front wheel hub
- the fuel tank
That's about it. All of the other components will either come from other motorcycles, accessory suppliers, or be fabricated in-house or out. Since this project involves significant modification of the chassis and suspension, scale drawings in a number of iterations were completed first to crosscheck the preliminary eyeball estimates and calculations. In the scale drawings, the general layout of the XS650 GS and many of its key dimensions are remarkably similar to the KTM950 SE, a 950cc twin cylinder dirtbike, which by most reports is a great deal of fun to ride fast in the dirt even though it scales in at about 420 lbs. dry weight. We can only cross our fingers that the apparent similarities are actually meaningful when it comes to our project’s real live performance in the dirt.
We'll start here first because engine modifications are relatively simple compared to our plans for the chassis. The previous owner indicated that he was selling the bike because it wouldn't start anymore and he had generally just lost interest in dealing with it. So, after loading it up on a trailer and dragging it home we did a cursory inspection. The battery was quite dead, which might certainly be a good reason why the bike wouldn't start. We checked valve clearances, which were fine, and then pulled the top end off to inspect the bits for any damage of consequence that might be responsible for the non-start issue. For the most part everything was in good shape, falling in the middle range of the specifications from new to wear limit, perfectly reasonable considering the mileage. The fit between the piston pin and the connecting rod of the left cylinder was looser than ideal. There was more evidence of wear on the pin than on the small eye of the rod, and a test fit with a new piston pin considerably reduced the clearance so we are going to leave it at that for now. ( Somewhere lurking in the back of our brain is the idea of a 270° conversion down the road. At that time we would install new connecting rods along with 750 cc or larger piston assemblies). For now, we will be content with honing the cylinder bores, fitting new piston rings, facing the valves and re-cutting the valve seats.
Although not engine components per se, we think of intake and exhaust as integral parts of the engine system so will speak of them here. Since our design goal for the first engine is general civility rather than power increases, we will retain the original Mikuni 34 mm CV carburetors. We will fabricate the air box from scratch which should allow us to choose a high-quality aftermarket air filter element(s) of a configuration we like. We will design the air box to reflect the requirements of off-road operation such as protecting the element from direct mud and water hits, water drainage, maximum air filter surface area and ease of filter service. Perhaps highest on our list of requirements is the ability to inspect the air filter/air box seal to be assured there are no leaks allowing unfiltered air into the engine.
The OEM headers are double walled, partly to keep the headers from heat blueing, but also perhaps because the correct header diameter for the engine is much smaller than the external tubing you see. Magazine rumor at the time had it that the Yamaha marketing department thought the correctly sized headers didn't look “butch” enough for the bike. In any case, we will fabricate new headers, so we can select the correct diameter for the stock engine and route them in a manner of our choosing. We also intend to fit mufflers from another source. We thought long and hard about the exhaust system design, most of the debate concerning whether to mount the mufflers high as found on most dirt bikes or low similar to most street bikes. If this was a pure dirt bike for high mount option would have been a no-brainer but with an all-road bike, we have to include carrying gear for multi-day or perhaps multiweek journeys as a design consideration. This means saddlebags ( panniers ) and probably a top pack as well. High mounted mufflers cause a number of design issues including forcing the saddlebags out even farther away from the bike center line, which is in no way desirable. Another issue is mitigating heat output from the mufflers, well hidden from a cooling external airflow, which has the potential to damage the saddlebags and contents. The third is crowding the rear tire at full suspension travel. Our preference for a twin shock design aggravated the issues with the high mount option. Although doable with careful routing, it certainly adds a complicating element when designing the exhaust system. After considering all of these issues, we decided to install mufflers low on either side and tucked up as close as possible underneath the frames for the panniers. We still have to consider the issue of protecting luggage from heat but the mufflers are well out into the cooling airflow to start with which will help considerably with the extent of feet protection we will have to design and fabricate.
Given our low mount decision we still have some choices and compromises to juggle here, 2/1 or 2/2 design, finding a compact muffler(s) that will tuck up under the saddlebag mounting frames we intend to fit, and then to round up the quietest mufflers we can find that satisfy these requirements. Unfortunately, this last requirement pretty much leaves out aftermarket mufflers, which is sad because they are generally much more advanced in materials choices, rebuildable design, and in many cases, they are of stunningly low weight compared to the OEM pieces. However, we don't know of any aftermarket mufflers that even come close to the 80 dBA level of OEM mufflers fitted to US market motorcycles. We have found two promising candidates that, fortunately, are frequently available on eBay. The KTM 950/990 model line runs a pair of 80 dBA stainless steel mufflers and the BMW F800 GS runs a single 80 dBA muffler fed by a 2/1 exhaust system. These mufflers are used with engines that have much higher power outputs/ flow requirements than the XS 650 engine so we suspect either choice will be suitable for our project. It is mostly a matter of deciding whether a 2/2 or a 2/1 is preferable - weight, packaging, and complexity of the exhaust header fabrication are all considerations. We will leave this final decision until we have most of the other bits in place.
Single image compare O.E.M. Muffler with KTM & BMW weight of O.E.M.muffler
Interestingly, the BMW 2/1
can weighs about the same as just one of the KTM cans while only a little
bit larger in overall dimensions. Typically, it's reasonable to assume
that as a muffler gets larger in volume, it is easier to restrict the
sound output without compromising the exhaust gas flow and therefore the
power output. Both the BMW and KTM mufflers are nominally rated at 80 dBA,
a maximum allowable. Let's assume for now that the actual noise level is
similar for both bikes. Since the BMW can must handle 800 cc of engine
displacement and the KTM can just 500 cc of displacement it would be
interesting to find out whether the BMW can is more restrictive to keep
the sound level down or if it's internal baffling design is superior and
flows much better, yielding a comparable sound output with a much smaller
muffler volume. When we get a moment we will try to jury rig up an
attachment to an air compressor and see what kind of pressure drop we get
through each muffler. Both the KTM and BMW mufflers appear to be very high
quality stainless steel fabrications, not surprising perhaps considering
their source, and both are equipped with catalytic converters. Anecdotal
reports with the KTM parts seems to indicate that removing the converter
element significantly reduces the retained heat in the muffler so we will
probably go down that path.
We are considering two other changes for this project’s first, essentially all stock, engine. The XS’s oil filtering system utilizes three stages, a mesh screen and a large pleated element in the crankcase and then a small wire mesh element mounted in a cavity in the primary ( clutch ) cover. This last element, responsible for cleaning the oil headed towards the top end of the engine, is a relatively coarse wire mesh and not a very sophisticated bit of filtering technology.
The second change we will make is to modify the primary cover to accommodate the possible addition of an oil cooler in the future. We don't think a cooler is essential with the stock engine but we are interested in running some back to back tests to see if the oil cooler makes a worthwhile difference. We intend to put the XS650 GS in some riding situations that are much more severe than an XS 650 would typically ever see, slow speed single track trail in the mountains and deep sand washes in the desert. We think these two situations will show any weakness in the stock engine’s cooling systems. We will look at that data and weigh the benefits against the drawbacks of the additional weight and complexity of having part of the oiling system external and potentially vulnerable to damage and worst of all, a ride ending failure.
There is another, much more enticing reason for modifying the primary cover in preparation for an oil cooler. The XS 650 engine responds so well to huge increases in displacement and power output without serious detriment to its reliability that we are always entertaining the idea of a “boosted” engine. The addition of the oil cooler gives us much more flexibility in the variety of engine modifications we could incorporate. Although we don't expect to stretch things so far, XS engines well in excess of 1,000cc displacement are quite common, especially in Europe ( it's those Dutchmen again J ) See the article titled ??????? for an extensive discussion about modifying the XS 650 engine. Lots of food for thought here.
This is the part of the project that really intrigues
us. As we mentioned in the opening paragraph, very little of the stock
chassis is particularly noteworthy, especially for an off-road bike. A
custom frame designed for our GS application would be really nice, but way
beyond our budget. Although very heavy at about 55 pounds, the stock frame
is very robust with double walled tubing in many locations and a boxed,
double walled swingarm pivot area. Although pretty much a guess at this
point we suspect the modified stock frame will handle the rigors of
off-road riding, certainly at the pace we expect to be maintaining most of
the time. Besides being cheap and available the frame comes with basic
geometry that is more or less in the ballpark for what we have in mind and
it looks like it will easily accommodate the changes we need to make to
the rear suspension. The one dimension we are stuck with that doesn't
really please us is the long distance from the counter shaft centerline to
the swingarm pivot center line. Most of this is due to the engine design
and taking out the portion contributed by the frame would require major
redesign and fabrication so we're going to live with it for now. This
longer than desirable dimension mostly means that the drive chain slack
varies considerably over the range of rear suspension travel. If
uncontrolled, you run the risk that a very loose chain derails or saws
through the frame and swingarm. The solution here is a set of chain
rollers/guides perhaps fixed, perhaps spring loaded, to take up excessive
chain slack as the rear suspension cycles through its full range of
For the type
of riding we intend to do, we're looking for wheel travel in the
neighborhood of 8 to 10 inches at both front and rear. The front is a
relatively simple matter of bolting on a set of front forks that are
suitable. The most complex chassis modification we will undertake is a
redesign and fabrication of the rear suspension to get rear wheel travel
into our 8 to 10 inch range. Generally speaking, we don't think a linkage
suspension system offers enough performance potential to offset the
additional complexity and maintenance issues, except perhaps if you are
pushing the limits of a performance envelope in certain specific
applications. However, we very much appreciate the advantages of a
link-less single shock design and thought long and hard about designing
and fabricating such a system for this project. Ultimately though, we
decided that such a design required too many compromises with the design
and fabrication of other components on the bike, primarily the airbox, the
frame and the swingarm. Our plans for future projects include another
XS650 GS, for which we intend to fabricate a frame from scratch and in
that build we will incorporate a no linkage, single shock design. For our
current build we have decided that a forward mounted, twin shock design
will give us the suspension performance we are after and significantly
simplify some other design considerations. An added advantage is that the
bike will retain a more or less vintage appearance, something you might
have seen during the era when the XS 650 was in production. A small thing
perhaps but, being kind of retro-grouch at core, it appeals to our
aesthetic sensibilities. Getting our desired suspension travel in the rear
involves three elements, relocating the lower shock mounts farther back,
relocating the upper shock mounts farther forward, and fitting longer
And in a bit of serendipitous good fortune, we were digging around in our
collection of vintage bits and came across a little used pair of S&W
shocks of a suitable length. We also happen to have an old S&W catalog
that has formulas for calculating shock linkage ratios and spring rates.
We knew that this set of shocks came from a Ducati 450 motocross project
under a 200+ pound rider so we thought there was some small hope that the
spring rate wouldn't be too far under the ideal rate but to our pleasant
surprise the calculations showed that, if anything, the springs that came
on the shocks were a little stiffer than what we needed for our project.
So we think this will give us a perfectly usable starting point from which
to dial in our final suspension settings. There is a fair possibility that
the seals in these unrebuildable shocks have hardened over the years and
will puke oil immediately, so we're already expecting to spring for a new
set of shocks. We will be happy if the S&W's last long enough to allow
us to zero in on a spring rate before they die, anything more than that
will be a bonus.
intent for the remainder of the chassis components is to incorporate bits
from relatively modern dirt bikes. This should yield parts that are much
lighter, but more than strong enough for our project. Racing on a
motocross track or at high-speed across the desert for hours on end
punishes suspension and wheel assemblies. Although we intend some spirited
off-road riding with our project bike we don't expect to even remotely
approach the kind of loads that modern dirt bikes routinely endure. A
while back we came across the carcass of a 1989 Kawasaki KX 125 and some
rough measurements seem to indicate that a number of the suspension bits
might be adaptable to our project bike. Since the suspension and wheels of
the KX 125 are the same parts used on the KX 500 we feel pretty
comfortable about using them for a project. The KX500 of this general
vintage dominated the highest levels of desert racing for a decade.
Although we are taking the KX parts to a much heavier package, the impact
speeds and the total loads placed on the components will be much, much
lower. We are reasonably confident that these components will be
sufficiently strong and durable enough for our application.
suspension. The KX front fork,
a 46mm Kayaba conventional, or right side up, assembly required minor
modification to adapt to the XS 650 frame. Using aftermarket XS650 tapered
roller bearings, the KX triple clamps were a bolt up to the XS steering
head. The remaining tasks are to shorten the fork travel from the original
11.8” and fit a fork spring of a rate suitable for our much heavier
project bike. From our initial search, it looks like even the stiffest
rate of aftermarket fork spring that will fit the KX fork may still be too
soft. When we reduce the fork travel we will need shorter springs as well.
A coil spring is essentially a torsion bar wound into a cylindrical shape,
and as you shorten a torsion bar the spring rate rises so, if we are
lucky, shortening the springs to match the reduction in fork travel might
bring the spring rate into the ballpark. We'll have a better feel for this
when we make a final decision about the spec. for front fork travel. The
KX forks, although very highly regarded in their day are a few generations
old and only have adjustable compression damping rather than both rebound
and compression adjustability as found in newer fork assemblies.
Consequently, we will probably have to fiddle with fork oil viscosities
and perhaps the size of the rebound damping orifices to achieve effective
rebound control with the stiffer springs.
Rear suspension. The KX
swingarm is a nice piece, a very robust aluminum box section design with
bolt and lock nut axle adjustment blocks. If it becomes necessary, it
looks to be relatively simple to shorten the swingarm. In the scale
drawings, the wheelbase with the full length swingarm sketches out to
approximately 62 inches. Normally this is way off our chart for dirt bikes
but that is precisely the wheelbase of the KTM950 SE so this is part of
where we cross our fingers that our final result will be OK. The swingarm
did require some modification for our project. The lower mount for the
shock linkage was an integral part of the swingarm cross brace, machined
from a solid block of aluminum. With our twin shock design, the old
linkage mount was superfluous so we simply cut it away from the rest of
the cross brace. To accommodate our twin shocks we fabricated lower shock
mounting brackets and welded them to the top side of the swingarm.
We also decided to modify the swingarm to accommodate an aftermarket needle bearing kit intended for an XS 650. It is a very nice upgrade and would also allow us to retain the XS’s swingarm pivot bolt or use one of the upgraded aftermarket replacements. The kit features a hardened pivot bolt sleeve that fits between the pivot bolt and the needle rollers in the caged bearings for the pivot bolt sleeve, and radial thrust needle bearings that fit between the ends of the pivot sleeve and the swingarm bosses in the frame. We had to resize the swingarm pivot bore to accept the larger outside diameter of the pivot bearings. Fortunately the original diameter and our target diameter both fell within the range of an available adjustable reamer. This was a whole lot easier than trying to chuck up the swingarm in a mill and ultimately allowed us to creep up on the finished diameter to get a really nice press fit between the needle bearings and the swingarm bore. A huge thank you to Chris for the loan of the reamer. We also had to narrow the width across the pivot area to match the pivot sleeve, thrust bearings and our addition of O-rings to keep dirt out of the bearings. We also drilled and tapped holes to accept grease zirk fittings, located between the pairs of pivot bearings found on each side of the swingarm. The KX swingarm is a split pivot design, a slip fit around the engine cases and bracket so that when you tighten down the pivot bolt everything is pulled together and held in place. We will fabricate a spacer from round section from either aluminum bar or tubular stock to fill the gap in the KX swingarm. When the swingarm bolt is tightened the spacer will serve the same role as the engine cases and bracket on the KX. We will also mill grooves and fit o-rings in the end faces of the spacer to help keep swingarm pivot grease in and water/dirt out.
Forward mounted twin shocks. When the motocross world first escaped 4’” rear travel limitations, there was a tremendous amount of experimentation over a period of years. A consensus evolved that twin shock design was close to ideal when the distance from swingarm pivot centerline to upper shock mount matched the distance from swingarm pivot centerline to lower shock mount. The eye to eye length of the shock absorbers should be somewhere in the same neighborhood, essentially creating an equilateral triangle. After repeated mockups, the “magic “ number for our project was 16.5”. With the KX swingarm, that yielded about 9.5 inches of rear wheel travel which is in the ballpark of our design intent. Locating lower shock mounts where we wanted them was relatively easy since we were building the lower brackets for the swingarm from scratch. To locate the upper shock mounts in the correct position required moving the existing mount forward approximately 3.5 inches. To achieve this we had to cut, heat and bend the frame tubes that run from the area of the swingarm pivot up to the rear frame loop.