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KTM RFS Alpina
KTM RFS Alpina - Project Plan
KTM introduced the RFS series model line in the year
and production ran until about 2008 when the RFS engine was
replaced with the RC4 series.
The newer engine has separate oil reservoirs
for engine and transmission, a desirable change. An unwelcome change is
that the valve clearance adjusters are no longer screw and locknut, but
shim style. This change makes it much more difficult to maintain the bike
in the comfort of your own garage. We understand the technical rational
for using shim style adjustment but are generally dismayed when we see
manufacturers abandon the owner friendly screw and locknut method. This is
one of those areas where we can correctly be accused of “retro grouch”
tendencies. Although we have no reason to suspect the RC4 series to be
anything other than typical KTM in reliability, the RFS has a long,
well-established reputation for just that and there is generous enthusiast
community with a huge body of
knowledge regarding modifications and resolution for the few design related
reliability issues that do exist.
generally acknowledged that KTMs excel in almost any dirt riding
application. The KTM RFS chassis is definitely designed for going very
fast, at expert level, over the nastiest of terrain. Although a
sufficiently skilled rider atop a standard RFS can successfully negotiate
the kinds of tight, technical riding we envision with an Alpina, we would
like to focus our design and fabrication efforts on frame and suspension
modifications that put the RFS engine/gearbox package into something more
along the lines of our idealized Alpina chassis.
This RFS model line has these desirable characteristics:
- equipped at the factory with top grade components throughout
- available with wide ratio six speed gearboxes
- electric start plus a bonus kick starter
- class leading in regards to their weight vs displacement
- exceptionally easy to maintain
- many choices for large capacity fuel tanks, both KTM and aftermarket
- parts from different models can be mixed and matched to customize the
ride for your application
- mechanical reliability is well-established
- utilizes a linkless rear suspension system, which we view as highly
is only because of the KTM’s exceptional design characteristics that we
are willing to live with the water cooling. Our design brief for this
project will include ways to minimize the compromises that water cooling
forces on our design. There is one specific riding situation where we
believe water cooling is highly desirable. Desert riding. Slogging through
deep sand puts extraordinary load on a engine, you can actually feel an
air cooled engine dying beneath you under those conditions. So, although
high-speed desert work is not our design intent (yet) we do envision the
real possibility of being compelled to wade through deep sand to get the
kind of riding the Alpina is intended for ( think mountains in Baja ). One
of the crew here at Kestrel Design has serious expert level desert racing
experience, including a successful run in the Baja 1000. Fortunately, he
also enjoys extremely technical mountain trail riding so we look forward
down the road to blending the requirements of Desert riding and Alpina
riding into a killer ride. Their are two characteristics of the KTM not
commonly found together in the same package but are immensely appealing
for a mountain Alpina. A class leading low weight and readily available
knowledge to build a 600cc + engine with the power band biased towards
low-speed torque, a combination not available in any other dirtbike that
we are aware of. The design parameter that requires new research and
investigation on our part regards the chassis.
We are going to divide this project into two parts. Part one includes any modifications other than shortening the swingarm or modifying the main frame cradle to alter the steering head geometry. We are pretty sure that eventually we'll have to undertake these modifications as well to get to the handling traits we are looking for in the Alpina design concept but we want to see how close we can get with frame in swingarm in standard configuration.
One of the KTM's most appealing characteristics is its lowest-in-class running weight while the KTM components are about as robust as they come. The goal of this project is to make the chassis perform better on the types of trails described in the Alpina design philosophy. The KTM’s chassis is relatively middle-of-the-road compared to the most agile bikes available. The last generation Honda XR 250 ( starting in 1996 ) is probably the best example of the design bias we are looking for. The XR pushes the envelope towards the Alpina end of the trailbike curve, steep steering head, short trail, short swingarm and wheelbase, moderate suspension travel. The critical steering specs are 92mm of trail and head rake set at 24º. These numbers are a huge departure from the numbers typically found in the trail bike class. The swingarm length. at 20.5” is moderately shorter than the norm. This, combined with the tight steering head and appx. 10” suspension travel, yields a relatively short wheelbase of 55.1”. If that XR chassis came with a 400 - 500cc e-start engine with a wide ratio six speed gear box we might be in the ballpark. An interesting and welcome discovery, even if very surprising, is that the distance from the swingarm pivot to the steering head centerline is significantly shorter on the DR than on the XR. This might imply that a reconfigured DR has the potential to be an even more compact chassis package than the XR - encouraging
Although this image is of a chassis with an XC4 engine, it is a good illustration of just how simple an excellent dirt bike chassis can be. ( click on the image for much larger view )
Reconfiguring the KTM towards the Alpina end of the
spectrum involves these areas of design focus.
Reconfiguring the KTM towards the Alpina end of the spectrum involves these areas of design focus.
1) alter the steering geometry to make the bike easier to maneuver on tight, technical terrain
2) shorten the wheelbase for the same purpose ( limited to axle adjustment in phase 1 )
3) move the footpegs rearward to make it easier to lift the front wheel over obstacles
4) shorten the suspension travel to make it easier to dab in difficult terrain
5) reconfigure the subframe to compliment the new suspension specs, and the goal of #4
6) improve all facets of the suspension to achieve both compliance over obstacles at low speed and resistance to loss of control at high speeds
7) shorten the swingarm to compliment the goals of #1 and #3
8) reposition steering head on the
1) Steering geometry may be the chassis characteristic most difficult to alter significantly. Rake and trail are essentially dictated by the design of two components; rake, which is essentially set the by the steering head’s angle of attachment to the frame and trail, which is essentially set by the combined offset of the fork triple clamps ( steering stem centerline to fork tube centerline ) and the offset of the front axle ( from front axle centerline to fork tube centerline ). The “as built” geometry numbers can be altered in a number of minor ways with adjustments to the moveable bits of the chassis, but those adjustments typically result in compromises to other handling characteristics, not necessarily desirable.
Also, keep in mind that changes made may not yield the results that theory would have you expect. How the rider perceives chassis feedback is ultimately all that matters. This article does a nice job of describing this issue. So, we start by trying to move towards a set of numbers typical of the kind of bikes that have a reputation for the handling characteristics we desire. After that, it's test, test, and test some more.
The most effective, although most radical, solution to changing the rake angle is to cut the steering head off of the frame and reattach it in a new position, a modification we will reserve for phase two if we decide to go there. Typically, the steering stem is concentric with the steering head bore. A less radical alternative then repositioning the steering head is to reset the fork steering stem on a different angle than the head. This can be accomplished in two ways, both of which involve extensive fabrication. The first is to incorporate intermediate cups between steering head bearing races and steering head which are adjustable in a manner that permits the fork steering stem to be set in a position not concentric with the frame’s steering head, effectively changing the effective angle of the forks relative to the frame. In our application this would allow the effective steering head angle to be steeper than stock, promoting quicker steering, partially from the resulting shorter wheelbase but mostly from the steeper steering angle and the reduction in trail. However, unless an adjustable bearing and cup set that fits the steering head is available in the aftermarket, this solution probably requires fabricating and attaching larger head stock bearing race seats and or fabricating a different steering stem. Another method is to fabricate a new set of triple clamps that locate the fork tubes at a steeper angle than the steering stem. Fabricating clamps that kick the forks out at a shallower rake is commonplace in the custom chopper market but we are not familiar with the similar market for pulling the forks in. ( check out dirt track aftermarket )
Given that all of the methods to steepen the effective steering head angle involve difficult fabrication, adjusting the trail would appear to be a comparatively simple pathway. While retaining the stock fork legs, this can be done in one of two ways or via a combination of both. The offset from the steering stem centerline to the fork tube centerline combined with the offset from the fork tube centerline to be axle centerline gives you total offset from steering stem to axle. This cumulative offset is a critical dimension when calculating trail. Trail is, perhaps, the single most important dimension affecting the balance or compromise between turning quickly and high-speed stability. Generally, this dimension is most readily altered by fitting a new set of fork triple clamps with a different offset. Changing the fork tube to axle offset is generally difficult to achieve unless one finds fork legs whose sliders locate the axle in a more suitable position than the stock forks. There is one company we know of that manufactures a kit which allows alternate positions for the axle within the axle clamp bosses on the sliders. Fortunately, they have a kit to fit the WP forks on the RFS.
a) change the offset of the fork tube bore centerline from the steering stem bore centerline. This typically involves purchasing or fabricating a new set of triple clamps. Fortunately, after market triple clamps with varying degrees of offset are available for the KTM.
change the offset of the axle bore on the fork sliders relative to
the fork tube centerline. Again we are fortunate in that an aftermarket
kit is available for the KTM RFS forks. E line
axle kit allows repositioning of the front
axle to plus or minus 2 mm.
2) shorten the wheelbase. Numerous methods, separate or in combination
a) a minor change can be accomplished here by keeping the rear wheel forward in the adjustment slots of the swingarm.
b) minor change by sliding the fork tubes up through the triple clamps
c) shorten the swingarm length ( moderate to extensive fabrication, dependent on swingarm design )
d) reduce the triple clamp offset ( however, this alters trail in the wrong direction for our purposes )
e) steepen the steering head angle ( extensive fabrication frought with perils )
f) move the steering head back on the top tube towards the swingarm pivot ( see “e”, only more so )
Moving the footpegs rearward makes it easier to lift the front
wheel over obstacles. This should be relatively simple project. Most
trials bike design positions the footpegs behind the swingarm pivot. This
alters the peg to shift lever and peg to brake pedal relationships. Need
to assess severity of changes and difficulty in restoring acceptable peg
to control relationships. Unknown yet how this affects the feel when
riding in the standing position.
4) In stock configuration the KTM nominally sports 12+ inches of suspension travel. Stock spring rates are suitable for high speed work where adsorbing impact from obstacles at high speed is the primary goal. At high speed, putting your feet down is not a wise thing to do so as long as your feet reach the pegs, all is good. However, since the preferred design is biased toward slower trail speeds and negotiating obstacles, dabbing occasionally and/or deliberately is part of the game. In our opinion, suspension travel in the range of 8 – 10” will be entirely adequate and should yield significant improvement in the ability to foot for stability. The loss of ground clearance is not desirable but is a trade-off we are willing to live with. We are making a deliberate call here that, for our specialized application, shorter travel, if correctly sprung and dampened is superior to the long travel, stiffly sprung suspension of the stock bike or even long travel when correctly sprung. The tricky part is choosing spring rates/damping characteristics that both keep the chassis up in the travel but is still allow exceptional compliance for low speed work over trail obstacles such as rocks and roots.
Ground clearance. Trials bikes typically combine both very high ground clearance and suspension travel in the neighborhood of 6-7’ or thereabouts. Achieving this particular combination of chassis traits compromise is really only available during original chassis design process. Essentially the steering head would be located lower on the main frame cradle and rear suspension geometry is designed to be complimentary. If one were to bite the bullet and relocated the KTM’s steering head to adjust the rake angle, it would be relatively simple matter, within limits, to lower the steering head at the same time. Barring that major surgery though means that we live with the reduced ground clearance in exchange for the shorter suspension travel and its attendant benefits.
Reducing wheel travel is
generally simple in principal but variable depending on the actual
mechanical components in the front forks and rear shock. The usual method
is to reduce the stoke length of the front fork and rear shock by
inserting travel limiting spacers or shortening internal components that
determine stroke length. If one is lucky an off the shelf fork spring of
suitable length and rate will fit the altered space available. It
shouldn’t be difficult to acquire a softer spring for the rear shock. It
may be possible to find damping rates within the range of adjustment
available on the stock shock absorber that are suitable for the new spring
5) reconfigure the subframe.
Depending on the method of execution, the reduction in suspension travel
may yield unnecessary clearance between the rear tire and inner fender
surface at full compression. It would be beneficial if the subframe was
reconfigured to eliminate some of the excess. This might also yield a seat
at lower height and of flatter profile, both desirable changes.
6) Suspension performance is perhaps the simplest
performance goal to describe but perhaps the most difficult to achieve.
The goal, is to have suspension which is exceptionally compliant over any
trail obstacles but still resistant to bottoming and chassis upset when
pushed to the limit in faster off road terrain. It would seem that the
only way to achieve this is to choose an initial spring rate for
compliance and then adjust the damping characteristics to add resistance
to compliment the spring at the longer end of the suspension travel.
Suspension performance is, in many regards, a highly knowledge and tool
intensive endeavor. We may break down and send the suspension bits off to
someone with both.
7) Shortening the swingarm accomplishes two desirable
changes. It contributes to a shorter wheelbase, which generally
accentuates manuverability in tight terrain. It also shifts the polar
moment toward the rear of the bike which makes it easier to loft the front
end. This change does reduce weighting on the front wheel’s contact
patch, theoretically making it easier to wash out the front wheel when
cornering. This compromise is most likely to be discovered when going fast
and exploring the limits of front tire adhesion, a risk we are willing to
take considering our performance goals.
8) reposition steering head on the mainframe cradle
Let's start with the gearbox first since that is a
no-brainer. The wide ratio six speed out of the EXC model is the most suitable for our project. See, wasn't that
easy? There is one other possibly interesting wrinkle here. The 250 RFS,
available only for a couple of years, has a gearbox with ratios unlike the
rest of the model line. It might be possible to build a hybrid gearbox
which allow an even wider overall spread than the EXC box. The trade-off
is that one of the splits between adjacent gears is much wider than
generally found in any gearbox. It may be possible that a large
displacement engine will pull that gap without significant problem.
With the RFS engine there are numerous possible combinations of bore, stroke, crank weight, flywheel weight, compression, valve sizes, cam camshaft, cylinder head, and carb choices. Displacement options are 250, 350, 400, and then a variety of displacements from 450 to about about 650cc. Since our local riding conditions include singletrack at 12,000 feet, our preference is to build our first engine near the upper end of the displacement range available. Keep in mind, most of the lessons we learn here are applicable to smaller displacement RFS engines as well. Since most of the engines in excess of 570cc are very expensive custom builds, our target for the first engine is 552cc. This specific displacement is derived from the standard 525 bore of combined with a 78 mm stroker crank. Most of the accounts we have read indicate that this combination yields a power band with exceptionally good low-end torque. Although this crankshaft has been known to fail, the failures are usually attributed to sustained high RPM operation, something our motor is very unlikely to experience. Even at that, causes of and solutions to the failures are well documented. Our most likely challenge is, in fact, rounding up a stroker crankshaft. At one time KTM cataloged a 78 mm stroker crank but that item was discontinued a number of years ago. Fortunately, the KTM RFS world includes some very knowledgeable, very industrious, and very devoted fans. After market stroker cranks are available -- price unknown. If the price is prohibitive, we will drop back to optimizing the stock 525 ( really 510cc ) motor for smooth throttle response from very low rpm.
It is common in the literature to find complaints
that the 525 is too difficult to control, and therefore tiring to ride, in
tight, technical terrain. Most of the complaints seem to concern abrupt
throttle response, which we believe can be mitigated in a number of ways.
There are a number of aftermarket flywheels available in varying weights,
including some that are heavier than stock. There is a wide variety of
camshafts available to fit this engine and although the stock 525 cam is
generally considered the best all-around grind, we believe there are some
aftermarket camshafts from the sidecar MX racing world that are even more
biased towards bottom end torque. There are modifications to the OEM
carburetor that improve intake velocity at low throttle openings and
accelerator pump response at initial throttle opening. Also available are
cammed throttle tubes that make it easier to roll on the throttle more
smoothly, so that engine response is less abrupt.
A potentially interesting line of experimentation we
wish to pursue involves the cylinder head from the 250cc engine. This head
has significantly smaller diameter valves than the rest of the models in
the line. It also has correspondingly smaller diameter ports and
originally came with a carburetor of smaller bore. We believe the 250
engine has the same cylinder stud pattern as the rest of the RFS line, but
need to confirm this. The large displacement engine combined with the 250
head should yield much stronger torque at very low rpm, admittedly at the
cost of performance at the upper end of the rpm range. The only question
is whether the shift of power band improves performance at the bottom end
enough to make the loss at the upper end acceptable. We suppose it is
possible that the combination of small valve head and large displacement
engine just doesn't work very well overall. We will have to be very
careful about excessive compression ratios and pay particular attention to
the piston/cylinder head squish band characteristics. At worst, they may
have to look at custom piston to achieve the characteristics we want but
fortunately, custom pistons are not generally difficult to come by. This
line of research interests us in part because we recall the extensive work
that Honda Motor Corp. did in the 1970s in development of their four
strokes for national and world championship observe trials competition. Reducing valve size,
port size, and carburetor bore were instrumental changes in shifting the
power band and the engine into a range most suitable for tackling the
difficult terrain found in observe trials. Of course, that line of
development had an exceptionally narrow focus and may not be suitable for
a more well-rounded intent. But we really feel compelled to find out
whether this approach might bear fruit. We believe that if we go too far, it
will be a fairly simple matter of backing off to a more suitable overall
performance curve. We think it is valuable to know, however, were exactly
too far lies.
There really is quite a selection of components to play with in our search for the optimum trail bike engine.
Beyond the engine and the fundamental chassis
geometry issues there are lots of other decisions to make regarding
components for our build. In no particular order we have:
- lighting and other electrical
Luggage? How far down this path do we want to go. This is an area ripe for innovation. Generally when trail riding we are content with the tool bag attached to the bike and backpack for water, food, camera, emergency gear, etc. however Luggage and packing options for the GS class of on/off roaders are numerous, with many well-established companies providing options. Although there are a couple of very good players oriented more towards packing on pure dirt bikes, there is not near the variety of options that are available in the GS world. Serious off road poses some significant constraints. Threading your way through obstacles not much farther apart than the width of the bike definitely poses some risk to side mounted saddlebags if they protrude much beyond the width of the rider's legs. For the most part this eliminates rigidly mounted hard bags from the list of choices although, it might be possible to fabricate a set of hard cases sufficiently narrow in width to survive vigorous dirt riding. Not surprisingly, mostly off-road oriented luggage systems are soft bags, which is a fine choice. You just have to be exceptionally careful about what you pack where since being upside down is just part and parcel of trail riding and you don't want anything fragile cushioning the bikes impact with the ground. See this link for an experiment we've been conducting in packing for multi-day journeys on a trail bike, quite a bit different from the usual methods.
KTM RFS Alpina - Project Log
Alas, we haven't saved the money yet to pursue our KTM project, in many ways the one with the most interesting possibilities. Stay tuned, we hope to start this one soon.