Under the Knife: Kurtis Roberts’ RS250 Makeover


Editor’s note: Recently we spoke with former Honda crewchief Al Ludington about some of the more interesting street and race bikes he has helped build over the years. While Ludington isn’t a huge fan of the RC30s that he worked on, there was one single-sided swingarm bike he did like.

He spoke in glowing terms of the effort he has part of to make Kurtis Roberts S250 into a better racing machine. When we mentioned in response that it sounded like a good story, he said it was, that a stroy had already been done on the bike. Writer Dan Axton wrote this for a long dead siye called interactivebike.com. Soup’s Gordo’ was able to find much of the story in dark corners of the internet. We have tried to contact my Mr. Axton to speak with him in regards to this story but thus far we have been unable to do so. We present this here as an important story for all racing fans.


Neither races or championships can be won with luck, or by adherence to the status quo or corporate dogma.  They are won through a combination of hard work, the right equipment, the right people, and the right timing.  It is extremely important to have managers, technicians, and riders who can not only recognize the types and extent of areas that need improvement, but who will act on those observations with ruthless honesty.  Early in the 1998 AMA racing season, Erion Racing and Team Honda made a painful admission: the Honda Racing Corporation RS250 simply was not as good a motorcycle as Yamaha’s TZ250.

The next question was what to do about it.  The RS250, while still considered a fine racebike, required the rider to work around handling problems and use horsepower to win races.  The TZ250 had a demonstrably better chassis with equivalent horsepower.  It was clear that something had to be done to at least create parity with the Yamaha.

Chassis optimization is no longer an art, it is a well-established science.  This bit of information is not particularly well-known or
understood in the racing or riding community, however.  The notion that chassis and suspension setup are black magic, practiced in secret by a handful of super-tuners, is still widespread.  But that impression was not one held at Team Honda, which had purchased a precision measuring device from GMD Computrack and had been using it extensively to optimize both their RC45 superbikes and the Erion Formula Extreme CBR900RRs.  Al Ludington, the senior technician for Miguel DuHamel, had been trained to use the equipment.  By the time the issues that surrounded the RS250 were brought to his attention by Kevin Erion, Ludington had already become intimately familiar with the application of the technology, and was prepared to implement a solution.

Erion and Ludington approached Gary Mathers, who directs Team Honda’s North American racing efforts, and received approval to move forward with a radical plan.  The Computrack database clearly showed that, for the RS250, there were certain dimensions that would very likely put the machine directly on the bleeding edge of handling technology.  The problem was that those numbers — called “the sweet numbers” by Computrack technicians — were radically different than those of a stock RS250.  It would be a major undertaking to set the bike up in the way that Ludington and Greg McDonald, the Computrack founder, recommended for optimum handling.  Radical surgery on the RS would be required, along with a redesign or outright replacement of virtually every moving part on the chassis.

Once Mather’s approval was obtained, Ludington contacted Greg McDonald of Computrack, along with engineers from Showa.  Tom Jobe, a master fabricator and chassis expert employed by American Honda, was also asked to participate in the redesign of the RS250.  This project was unique for a number of reasons, not the least of which was that much of the work was done as a labor of love by the people involved.  It also required a good deal of international communication and coordination between McDonald, who was in Australia, and Al Ludington, who was based in Los Angeles.

Kurtis Roberts, the Erion rider who was competing on the RS250, had completed two races on the stock machine when the spare bike was handed to Al Ludington.  Those first two races netted him a tenth and a sixth place, respectively.  There was clearly room for improvement.

Roberts is a fine racer, and while Ludington and company were sawing away at his spare machine, he was sawing away at the competition.  Despite the Yamaha’s superior chassis, Roberts placed on the podium in six of the eight races that were held while the project was underway.  Victory evaded him in all but one contest, though, and he slipped to a fifth place finish at Brainerd.  By late summer, the project was finished and shipped to the Erion Racing headquarters in Anaheim, just a stone’s throw from Disneyland.

The Erion technicians probably felt something like Disney’s Imagineers as they surveyed the bare chassis and boxes of parts that needed to be assembled to resurrect Robert’s spare racebike.  No one, not even Ludington, was absolutely certain whether the bike would perform as expected.  They may have even missed the mark altogether, creating a Frankenbike that would simply wind up in the crusher.

By late August the machine was together and ready to test.  Erion Racing rented the very technical Streets of Willow, a 14 turn, 1.8 mile track ideal for evaluating handling and suspension issues.  By the end of the day, the tale was told.  Although Roberts was not initially overawed by the chopped and channeled RS, and found the seating position far too high because of swingarm angle modifications, he was turning lap times on the modified bike that were a second faster than on his stock racebike.  The Streets is a handling track, not a horsepower venue, and the evidence was convincing.

The final modification, a rear subframe that lowered the seat height by 30 mm, was completed at Roberts behest, and the bike was prepped for professional competition.  The verdict came in over the next two races, and it did not favor the competition.  Roberts, who had only won a single race in the entire season preceding his acquisition of the Computrack/Team Honda/Erion Racing RS250, won the last two events.  At Las Vegas, it appeared he was merely dallying with Chuck Sorenson, who regrettably ran off the track while leading with Roberts breathing down his back.  Sorenson, a tough and talented competitor, picked his machine back up and went on to finish fourth.  The project was finished, and it was declared a success by everyone involved.

The Technical Details

The problems that politely plagued HRC’s RS250 were ones that are more noticeable to the expert rider than to the novice.  While the RS is an expert’s machine, and commands a fairly high price of admission, the handling gremlins were not the kind that made the machine unrideable.  But they were obvious enough to make a difference at the highest level of competition.

One of the main issues was a vagueness in feedback.  This is usually attributed to a lack of adequate trail.  Trail can be increased by simply changing the fork offset, but with the RS, this would have been like putting a band-aid on the wound.  Not only was trail increased, the effective rake was extended as well.  Performing these two modifications gave the bike more stability, instilling a much higher level of rider confidence while providing heightened feedback.

Completing these modifications was no mean feat.  The existing steering head was cut off.  A new one was designed from scratch, and welded in place.  The new head features removable bearing race holders which can be exchanged to modify effective rake as required.  The new steering head also mandated the construction of an entirely new front fairing stay and steering stops.  As the photos show, the steering head is enormous, well-designed, and beautifully executed.

Another significant problem was that of chain pull moment (chain squat) under hard acceleration.  The RS250 produces around 90 horsepower at the rear wheel, and is very light at under 300 pounds in race trim.  Under power, the swing arm would compress significantly.  This increases the rake and trail and moves weight away from the front tire, causing the front to push or drift while exiting a turn under power.  The solution to this problem was to rotate the swingarm downward, extending or increasing the swingarm angle.  This changed the required rate for the rear suspension linkage, but without any previous data, the job of determining the precise linkage design fell to a computer.  The linkage in the photos is a CAD-CAM piece, specially made for this bike.

The RS had another challenge in that it was equipped with an eccentric chain adjuster.  Every time the chain was adjusted with this mechanism, the critical swingarm setup was altered.  To resolve this, a unique and complex linear chain adjustment mechanism was designed and built by Honda technician Tom Jobe.  This mechanism uses two eccentric adjusters.  They are moved together to ensure the rear axle remains as close as possible to the optimal position.  This allows for quick final gearing changes without disrupting the suspension setup.

The front forks on this machine, an inverted design, were replaced with a prototype set from Showa.  These feature both high and low speed compression damping adjustments which are accessed externally.  They also have the standard rebound damping and spring preload adjustments atop the forks.  The shock body is the same as that used on the RC45, with internal damping rates and spring rate selected for use with the RS250.

Other interesting features of this machine, although not chassis related, include additional, electrically controlled jets on the carburetors, a detonation sensor which records the number of “knocks” or occurrences of pre-ignition, an adjuster the rider can use to fine-tune front brake lever free play on the fly, a KLS electric shift kit which allows full-throttle upshifts, and a solenoid-operated valve which switches between a pressurized fuel tank vent and an atmospheric vent when the brake is applied.

 

 

 

 

 

Copyright 1998 by Dan Axton.  All rights reserved.


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