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Alpinas & GSs Design Philosophy Projects Library Gallery
Vaporware Wish List - Gas Gas side valve 350cc four stroke engine in a custom Alpina chassis
Suzuki DR350 Yamaha XT225 KTM RFS
Air cooled or water cooled. One of the difficult decisions in choosing a platform
for an Alpina project is the issue of water cooled vs air cooled engines.
Water provides much better control of engine temperature, especially in severe
conditions, so power output remains more constant than an air cooled
engine in similar circumstances. Typically, a water cooled engine can be
operated at higher power outputs without fewer reliability issues compared
to an air cooled version. The more precise temperature control also
permits tighter internal engine clearances and reduced fluctuation in
piston to cylinder bore clearances. One can reasonably expect longer
engine longevity from the watercooled version.
Unfortunately, water cooling exacts a price. Although inconsequential for many types of riding, we consider the compromises significant for our application. The Alpina concept desires extended range, maneuverability in difficult terrain, and high reliability. Our context for reliability here is the likelihood of completing a ride without mechanical failure.
With the watercooled platform, it is typically very difficult to fit a very large capacity fuel tank without severely compromising riding position and therefore maneuverability. It's them damn radiators !! The forward edge of the fuel tank must wrap around the radiators and mounting system so A tank of what we would consider even moderate capacity tends to be exceedingly wide. This interferes with the rider 's ability to use extreme body English when navigating over difficult terrain. The tank configuration also places much of the fuel far away from the centerline of the motorcycle, so the handling deteriorates as well.
Because of the different perspectives this is not entirely a fair comparison but it will give you an idea of what the rider is facing. Both fuel tanks are Acerbis 6.6 gallon. On the left is a water cooled KTM RFS and on the right is a air cooled Suzuki DR650 .
As a rider explores the
limits in this type of riding, low-speed tip overs in nasty terrain are
going to be fairly common. Radiators and hoses are. perhaps, the
components on the bike most subject to ride ending damage.
Suzuki DR 350 and Yamaha XT
225 We think these two air cooled bikes are particularly good
candidates, in part because they share some
common characteristics which are very suitable for the design concept we
are pursuing here.
A third candidate might be the Kawasaki KL250 Super
Sherpa, although we are not fond of its shim valve adjustment, which
violates the easy to maintain requirement.
A third candidate might be the Kawasaki KL250 Super Sherpa, although we are not fond of its shim valve adjustment, which violates the easy to maintain requirement.
- wide ratio six speed gearboxes
- broad power bands oriented towards low-speed torque
- large capacity, aftermarket fuel tanks are available
- exceptionally easy to maintain
- long production histories so there are many available in the used
Neither bike would be considered a lightweight for its displacement,
although the XT is about 50 lbs. lighter than the DR. Nor could either be
accused of having sophisticated suspension systems. However, it is far
simpler to modify one of these platforms with upgraded suspension then to
retrofit a wide ratio six speed gearbox to state-of-the-art dirtbike that
doesn't have one.
are generally biased towards air cooled bikes for this application, we are
highly impressed by KTMís RFS series. This model line has these
desirable characteristics, not found together in any other top line
dirt bike that we know of.:
- 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
- can be fitted with large capacity aftermarket fuel tanks
- 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
It is only because of the KTMís many exceptional design characteristics
and overall package 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 fading away beneath you under those conditions. So, although
high-speed desert work is not within our near term design goals (yet) we do envision the
real possibility of being compelled to wade through deep sand to get to the
kind of riding the Alpina is intended for ( think mountain trails in Baja ). One
of the crew here at Kestrel Design has serious experience at
building and riding bikes in expert class desert racing, 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 one killer ride. The KTM has some
characteristics that are
immensely appealing for a mountain Alpina. A class leading low weight,
wide ratio gearbox 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. 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, less work and more fun in
the tight stuff.
It is only because of the KTMís many exceptional design characteristics and overall package 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 fading away beneath you under those conditions. So, although high-speed desert work is not within our near term design goals (yet) we do envision the real possibility of being compelled to wade through deep sand to get to the kind of riding the Alpina is intended for ( think mountain trails in Baja ). One of the crew here at Kestrel Design has serious experience at building and riding bikes in expert class desert racing, 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 one killer ride. The KTM has some characteristics that are immensely appealing for a mountain Alpina. A class leading low weight, wide ratio gearbox 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. 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, less work and more fun in the tight stuff.
Husqvarna TE610 - This is another bike we think highly of, although it also water cooled. It has a very wide ratio 6-speed gearbox, e-start, and can be fitted with a 5+ gal aftermarket fuel tank. By all accounts it is a fine ride, relaxed highway speed with minimal vibration, good grunt off road and much better handling than the other bikes in the 300 lb + weight and 500cc + displacement class. One of our crew with long experience in desert racing says that competitors running the Husky 610 motor found it an exceptionally reliable piece. It is a good bit heavier than than KTM RFS which remains a much more nimble off-roader. That is the main reason we would choose the KTM as a first water cooled project, but if your bias is more towards higher speed long distance and less technical off road, this might well be a superior choice.
Honda XR650R - This is the last denizen of the water cooled world we will mention. It is noteworthy because it has very good suspension and handling, and has a reputation for being relaxed at high speed running over very brutal terrain ( think Baja1000 ). Large to humungous fuel tanks are available. The gearbox has fairly close set ratios so it may be somewhat compromised if your plans include both high speed roadwork and technical trail. The XR's saving grace is that 650cc may well pull this off where the sub 500cc motors with close ratio 5-speeds would have much more difficulty getting away with it. The XR has a phenomenal reputation for reliability, forged during Honda's long Baja history with the bike. Falls somewhere between the KTM and the Husky in technical off road ability, and about halfway between them in weight, but keep in mind it has no e-start or street legal bits. Seems like it has the makings of a very good adventure bike platform.
Yamaha XS 650 GS BMW Airhead GS Convert anything Sidecars
This class of motorcycle has always fascinated me. When I first discovered
motorcycles and just about everything was interesting, the big four stroke
twin Desert Sleds really caught my attention. Considering we lived in
Minnesota at the time this did seem kind of odd. I think maybe as much as
anything the idea of taking a street motorcycle, stripping it of all the
non-essential bits, and outfitting it for dirt was particularly appealing.
Even though my financial wherewithal at the time limited me to saving up
for a Kawasaki G4TR 100cc trailbike, my favorite magazine article by far
was about converting its 90cc predecessor into a no holds barred desert
racer. The urge to tinker was in full flourish before I even got my first
bike. That urge is still alive and well today, and makes me spend way too
much time on eBay.
Just about every time I take a spin on the BMW roadster in my stable, I
always end up wistfully eyeballing the entrances to all of the fire roads
I pass by during the trip, wondering just where they lead, and wishing I
was on a more suitable bike to go exploring up those enticing dirt
It is very possible to build a bike in this class that is capable of some
very serious dirt riding. At one time, twin cylinder bikes dominated the
premier class in the Paris Dakar race and there are some racing venues
around the world that have twin cylinder motocross and enduro competition
classes. Achieving this high level of dirtability is extremely expensive
and, competent as they are, the inherent weight and bulk of these bikes
really makes them unsuitable, even painful at times, on difficult
singletrack terrain. Hence, our two bikes in every garage solution. This
frees us to focus on those parts of the riding spectrum where a
multi-cylinder bike is a better all-around choice, and then selectively
experiment with modifications that expand the comfort zone into more
If your budget will accommodate a newer bike we think the KTM Super Enduro is the best candidate for a twin cylinder All-Road bike that is truly capable, off-the-shelf, of handling reasonably difficult single track. Our only objections are the water cooling and the very user unfriendly shim valve adjustment. The BMW HP2, only in production for a couple of years is also a decent platform. In the homebrew category, we think the Hinckley Triumph twins look really interesting. We don't think it would take too many modifications to make a really fine fire roadster and would like to take a crack at making it single track capable as well.
If you would prefer an older bike, or air cooling, or if owner friendly
maintenance is high on your list, then an easy choice is to just buy a BMW G/S or GS and be done with
it. There are scads of accessories and a huge store of knowledge for
modifying these bikes. Solutions have been found to just about any problem
you can envision or will encounter. The early airhead G/Ss, in many ways
still the best example of the breed, have fallen into near cult status and
command exceptionally high prices. The later GS model line has evolved
from the original, air cooled boxer twins to an entirely different
motorcycle which, except for the model name and general intent, bears
little resemblance to the original. We believe a project built from an
airhead roadster is an excellent candidate for a project, far closer to
the GS concept as originally conceived and, for many people, preferable in
most ways to the newer models offered by BMW, KTM, etc.
However, for us there is another bike that is even more compelling as a first
conversion candidate for one major reason and a number of minor ones. With
a GS, you will inevitably spend a lot of time on gravel roads which
seems like an ideal time and place to practice your dirt track skills.
Now, picture trying to pitch a BMW into a slide. What you see is a
cylinder and head right where your skid foot is supposed to go Ė
harrumph. This same issue also rears its head ( sorry J ) if you are working your way down a narrow piece of singletrack closely
bounded by rocks, fallen tree cuts, and other trail flotsam.
offered a 650 air cooled vertical twin in their model line for about 15
years. It is about as bulletproof an engine as there has ever been, and
although a little bit heavy, it is reasonably compact and very friendly to
maintenance by the owner. The aftermarket support in parts, accessories,
and modification knowledge is outstanding. If you are so inclined, power
output can be boosted significantly with few, if any, reliability issues.
Other than the engine, and perhaps the front hub, none of the other
components are particularly noteworthy. Nonetheless, the basic layout of
the XS 650 is suitable for GS project or, as we might choose to call it, a
fire road touring bike or fire roadster. It wouldnít really take much in
the way of modification to make the XS suitable for a leisurely fire
roading pace - suitable tires, upgraded suspension, the removal of
extraneous bits and the replacement of others with more crash proof items,
plastic fenders and such.
However, for this XS 650 project, we want to stretch the range of riding
situations in which the bike is enjoyable to include hustling down some
single track at a pretty good clip or pulling a long, deep desert sand wash at a
decent speed. The XS 650 is a popular bike for project builds, most
commonly pavement oriented street trackers or Cafť racers. The dirt
projects seem to be mostly vintage MX or sidecar MX. We are aware of only
a handful of XS 650 based dual sport projects with serious dirt
specifications, some very clever and all very custom, usually fitted with
the very latest motocross suspension designs. Our design brief proposes
more radical chassis and suspension changes than the vintage MX
conversions, but tries to maintain a general look and specification that
wouldn't be too out of place in the era during which the XS 650 was
BMW Airhead Our other GS project will use a 1970/80s era BMW airhead roadster as a foundation. While the XS project will be designed to also handle more extreme riding situations, our first airhead project will be geared more towards gravel roads, fire roads, and two track. It should still be capable of handling singletrack of moderate difficulty, but at a relatively sedate pace. Our modifications for this project will include tires, suspension upgrades, handlebar and controls, and replacement of many of the roadster bits with more crash worthy components. When we are finished, (loosely speaking ), we hope to present a package of modifications that is applicable to most any airhead model of that era. There is a huge base of these motorcycles in the used marketplace, many of them so cosmetically challenged that the cost of restoring them to an acceptable level of appearance is prohibitive for many people. A lot of these bikes have been displaced as an ownerís primary road ride by 30 years worth of more technically sophisticated motorcycles.
But, dirt bikes aren't expected to be pretty and we think
that the basic BMW airhead motorcycle is so fundamentally suitable as a
foundation for an all-road ride, that it makes sense to resurrect the
neglected ones as such. The list of highly desirable characteristics
includes; relatively light weight, shaft drive, steering geometry, long
travel front suspension, good engine torque, low center of gravity, the
large capacity stock fuel tank, the availability of humongous aftermarket
fuel tanks, and easy, owner friendly maintenance. This is backed by a huge
reservoir of generously shared knowledge in the airhead community and an
extensive selection of readily available parts and accessories to create
just about any vision of an airhead you might have. Oh, just to note, the
word Frankenbeemer owes its origin to Dr. Frankenstein's creation, which
was stitched together out of parts from a variety of sources. The
Frankenbeemer moniker is an affectionate reference for an airhead project
of this nature, but usually in reference to a conversion that has mostly
dirt in mind.
the first time I came across a photo
fully faired, clip ons and rear sets, street tired, four cylinder,
superbike, stacked to the gills with tank bag, saddlebags and
a huge top box, stuck axle deep in a muddy rut on a long uphill
jungle "road" that was nothing but mud. I
thought to myself, what the hell is he doing there. As I read the caption
I come to understand that he was in the middle of a multiyear round the
world trip. So clearly what constitutes a suitable adventure bike is
pretty much a function of mindset. Now for me personally, I am not
terribly interested in struggling with that kind of a mismatch between
bike and terrain. But midway betwixt a KTM EXC and that expeditionary
street racer lie a whole bunch of interesting possibilities. There are a
lot of motorcycles manufactured in the last 40 years or so that would make
a pretty decent foundation for an all-road bike. Perhaps not suitable for
blitzing along single track, but that leaves an immense network of gravel
roads and two track to explore. Given suitable tires, some suspension
upgrades, general crash proofing and a few dirt oriented modifications,
many road motorcycles can be converted into pretty competent fire
roadsters, while still retaining their suitability for pavement travel.
If you have a bike in your garage, or want to start looking for something
as a foundation for your own fire roadster, send us e-mail describing what
you want to do. Put GS Conversion in the subject block. We would be happy to extend our best advice to get you on
the road ( and the dirt ) to building a homebrew GS of your own.