Company Portrait
The company name "HPN" comprises the surname initials of the founders: Klaus Pepperl, Michael Neher and Alfred Halbfeld (from left to right in the photo above) who brought the company to life in 1979. Originally the firm was known for popularizing the "mesh tubed" vehicle frame using the BMW boxer as a basis and used in street racing. Later development of the frame was used by the founder Alfred Halbfeld together with Peter Zettelmeyer for racing, and the frame design also served as subject for Zettelmeyer's engineering diploma thesis. The same frame design was later marketed by the Krauser parts firm in a limited series known as the MKM1000.
The breakthrough in rally circles occurred in 1981 when Hubert Auriol won the Paris-Dakar Rally on a special machine built by HPN under contract from BMW. Additional Paris-Dakar victories as well as others followed, driven by big names such as Gaston Rahier, Eddy Hau, and Richard Schalber.
The main contract source today is still the BMW factory, which provides the five-man shop on the Austrian border with development contracts for both production runs as well as for competition. For example, the first prototypes for the K series frames and the first paralever prototypes were built by HPN. The foremost reason for the awarding of such contracts on outside firms is certainly the know-how of the firm's heads, graduate engineers K. Pepperl and A. Halbfeld, the latter now actively working for BMW Motorrad AG.
A second business branch is the manufacture and marketing of accessories for BMW motorcycles both to aftermarket suppliers as well as directly to customers. HPN can take advantage of the fact that, unlike Japanese manufacturers, BMW motorcycles are built according to the "building block" system. This allows, for example, an oil cooler thermostat developed by HPN to be built into bikes made for the last twenty years since motors and frame systems of the various series have changed little. This situation would be unthinkable for the Japanese competition.
The third and last branch is supporting existing BMW motorcycles - particularly the older G/S models up to 1988 as well as the newer G/S 80 and G/S 100 machines (touring/enduros) for private customers. These are rebuilt according to customer wishes either as competition machines or as extreme touring/enduro bikes. One-time HPN rally motorcycles serve as the inspiration for this work. The range is based totally on customer wishes, beginning with rebuilding of the above-mentioned replacement parts and on to complete rebuilding where, for example, the displacement is increased to 1043 cc., double ignition is added, frames reinforcement is welded in, swingarms are lengthened, etc. A more thorough rebuild can cost from 25,000 to 40,000 DM ($14,000 to $22,000). Impressive is the professionalism and accuracy of the work; this is definitely not to be confused with some casual do-it-yourself tuning. The basis for this difference is found in the enthusiasm with which both company leaders support the firm. Quality has the highest priority; profit stands in second place. Work isn't done as correct as is needed, but rather as correct as is possible. The value of this philosophy can be argued. Fact is nonetheless, customers are satisfied and the workers enjoy the work. This ensures that there are no time pressures and that good, clean workmanship can be foremost.
Modifications
Modifications performed by HPN have many facets and are therefore complex. Apparently "simple" changes such as the addition of larger tanks sometimes also require changes to other parts. Taking the example of the exchange of our production 24-liter tanks with 43-liter ones produced in Italy for HPN, not only the seat had to have a minor change but some models also require that the oil cooler be relocated below the steering head, itself requiring in turn a change to the front shield panel.
At this point I'd like to provide an overview of the changes HPN makes to BMW motorcycles. This should make it clear to the reader the effort and development work involved. This overview can't cover everything, since every rebuild that leaves the shop is effectively unique and custom-built to a customer's wishes. This description starts at the front of the bike and then continues back to the taillight.
1. Front-end Rebuild: The Fork and Everthing Attached to It
Here almost nothing remains as original: Basically, a White Power (manufacturer) upside-down telefork suspension with 29 cm./12 in. spring travel is installed. The spring travel of the stock fork is 22 cm./10 in. The "upside-down" orientation is in principal the reverse of the normal fork. The fixed and movable fork parts are exchanged and the moveable tubes (outer guide tubes) are attached to the fork yoke/bridge. The inner fixed tubes now move up and down. This makes the fork's upper area more stable since the bending forces here are greater. The longer spring travel also naturally means a longer wheelbase, allowing for better straight-line travel. Larger tube diameter and a more stable fork yoke/bridge provide improved torsion resistance.
The BMW single-disc brake system of the GS and particularly the G/S models leave much to be desired. Here, there's a lot of possibilities for optimization. The spectrum ranges from rebuilding with a larger diameter disc to installing a double-disc system, together with a multi-cylinder caliper. In any case, the disc built by BMW up to 1990 should be replaced with a floating disc setup. The builder must pay particular attention to the width of the brake setup together with the fork being used and the possibilities the fork offers for attaching the caliper. In addition, the brake lines are usually replaced with more pressure-resistant flexible steel lines. This results in a more exact pressure point and lower required hand pressure, but makes rebuilding of the brake cylinder necessary.
Wheel quality can also be improved through the use of other spokes and rims. The last touches in the front end are the replacement of, for example, handlebar, quick-response throttle grip, a replacement panel (for example, electronic digital-display speedometer, roadbook), hand protectors, different signal lamps and fender, replacement fairing, larger headlamp...
2. The Frame (Excluding Rear Assembly):
Before rebuilding begins each frame is measured and, if needed, straightened and aligned. The sandblasted frame is then strengthened at critical points (fork head, top rail, swingarm mounts, spring mount) through the welding of additional metal and doubling of existing metal. Mounting points for the oil cooler, tank, integral fairing and spoiler sections are welded to the frame. Depending on the equipment (e.g., single or double swingarms, changed rear assembly), parts such as spring strut mounts are removed and reattached elsewhere. For this, special welding dollies are made to enable precise workmanship. An outside supplier provides the body panels, made according to HPN drawings, fabricated using laser technology for more precise cuts than can be done at HPN.
Finally the welded frame is again measured, welding beads are finished, bearing seats newly dressed, etc. The center and side stands must be changed to accept the newly created ground clearance. Last, the frame is painted at an outside supplier according to customer wishes (normal finish) or as better protection from stones, etc., epoxy/powder coated.
Since the BMW GS and G/S frames are of steel, modifications to them are relatively easy. It would be more problematic if, as is the case by the Japanese superbikes, the frames were of lightweight alloy or if — like the newer boxer generation — the motor were part of the bike's frame and other elements are simply bolted to it.
3. Tank:
There are five principal tank variations:
A 19-liter tank for BMW G/S models, a 32-liter tank for the BMW G/S "Paris / Dakar" models, a 24-liter tank for the current BMW GS series, a 35-liter tank for the current GS "Paris / Dakar" models, and a 43-liter HPN tank in polyethlene or nylon.
Because most HPN rebuild work is for extreme use, usually the two extreme tank versions are used: the 19-liter tank for easy-to-handle sport or the 43-liter tank for long rallies or long-distance travel. For the HPN rebuilt frames the tank fasteners are set so that either of the two tanks can be mounted. The 43-liter HPN tank can also be mounted on the regular production machines, making necessary a special mounting setup as well as possible changes to the seat and oil cooler. Unlike the nylon tanks, polyethylene tanks cannot be painted and are available in only two molded-in colors: red or black. The resulting cruising range from around 600 Km./365 mi. can be increased by adding an auxiliary tank, produced by Acerbis/Italy, to either the rear of the frame or behind the transmission.
4. The Motor:
The standard BMW production boxer is equipped with either the 800 cc. (50 bhp.) or 1000 cc. (60 bhp. motor. HPN also offers a third 1040 cc. variation.
The motors' power can be increased in a number of ways. One must decide the manner in which the bike will be used. Experience shows that the motor can produce long-term power of about 70 bhp.; beyond that point, increased power is only interesting for sport purposes since durability of the drive train isn't the primary interest.
The significant points in which the motor can be changed are:
Increase the compression by shaving the cylinder heads, building in double ignition (i.e., two sparkplugs per cylinder and requiring greater changes to the ignition system, which is recommended especially for higher compression and/or poor fuel quality), a "sharper" camshaft (changed timing), lighter crankshaft, installing a changed exhaust system, installing a different type of ignition (Nippon or Denso for running independently of the battery), installing a modified oilpan ring and aluminum slide plate for greater oil capacity and greater engine block stability), saving weight by installing a carbon fibre alternator cover, boring through the cooling fins, etc. Installing an HPN-developed oil cooler thermostat ensures a constant motor temperature. And changing to an externally bolted valve cover makes valve maintenance easier.
It remains to be said that when rebuilding up to the 1043 cc. motor with shorter and lighter cylinders the motor runs noticeably quieter while resulting in a motor that's 26 mm./1 in. narrower (result: parts such as the valve guide tubes, exhaust system, etc. have to be refitted). It's also recommended that the transmission be adapted to the motor's higher power output ("longer" fifth gear).
5. Seat and Rear Section:
The rear frame section attaches at four points to the rest of the frame and also serves as the baggage carrier, rear mounting for the seat, mounting for the side cases, the rear body and fender, and partial exhaust pipe holder. Particularly with two people and luggage on loose terrain the rear section is quite unstable. To raise the stability, extra spanners are welded to both sides. In some cases, a completely new rear frame section made of high-grade steel is made. Depending on the customer's wishes, the standard seat is shortened to a one-person seat with different upholstery. The possibilities are many. All saddlework is done off the premises.
6. The Suspension:
HPN's technical state of the art is still a one-piece suspension. With this comes the disadvantage that with older BMWs the well-known "elevator" effect occurs. I.e., when accelerating the rear wheel springing stiffens and the machine raises. With release of the throttle the bike "collapses". for this reason BMW now builds only the paralever suspension that reduces this effect.
HPN took a different direction: a longer version of the old, one-piece G/S suspension. This means taking the old swingarm apart and welding in a 100-mm./4-in. section. Of course the driveshaft must also be lengthened. This results in a longer, more stable wheelbase for better straight-line stability as well as a longer spring distance. The latter offers significantly better advantages off-road. If wanted, the standard single-arm suspension can be changed to a double-arm setup (significant increase in stability). The advantage of this actually older HPN suspension is the durability when compared to the more modern paralever construction. Added to this are the high-quality spring elements from White Power. Ironically, during rally use the "elevator" effect can be used to increase the ground clearance momentarily by accelerating sharply.
7. A Look to The Future:
The next project is to also include paralever suspension on machines rebuilt by HPN. Right now, the suspension of the new boxer generation (1100 cc. four-valve boxer with telelever fork) is being transplanted into the frames of older models. This is a more stable version of the older paralever with a central spring strut (the earlier version had the strut on the side).
Considering that the factory production of the older boxer models will sooner or later probably be stopped, further HPN involvement in developing hybrid version of the new models is unavoidable. This will be harder since the motor housing also serves as part of the frame. Also, complete rebuilding of the new boxer is more complicated than before: materials are being used that make simple changes impossible (cast steering head, light metal alloys, resin components in place of earlier metal ones, etc.). On motors, four-valve technology with head-mounted camshafts and fuel injection, and a telelever front fork whose A-arm could strike the cylinders if a longer spring strut were installed. Since the old rebuild models are based on the BMW factory rally bikes developed by HPN, one could guess that if BMW lets HPN rebuild rally-type machines based on the new boxer models, one will again be able to profit from it.
Back to Possi's HPN Technical Page.