Is this the world’s trickest four-stroke motocross machine? It might be. It is Stefan Everts 2003 Rinaldi Yamaha YZ450FM — the machine he is riding in the new MXGP class (featuring 250cc two-strokes and 450cc four-strokes) against rivals Mickael Pichon of Suzuki and Joel Smetz of KTM.
As you read Yamaha’s summary of the differences between the stock motocrosser and Everts’ machine, note that he retains the stock, steel chassis this year (last year, he raced with a special, aluminum framed four-stroke in the open class).
As four-stroke motocross machines receive more and more attention, it is interesting to watch what Yamaha does with its factory race bikes — perhaps, the most highly-developed four-stroke motocross bikes on the planet at this moment.
Here’s what Yamaha says on its european racing website regarding the development of Everts’ machine:
The all-new YZ450F has a big challenge ahead, not only will it have to carry
forward the fantastic successes of its YZ400F and YZ426F predecessors that
managed to win three World Championships in 1999, 2001 and 2002. But also
the competition has followed four-stroke pioneer Yamaha introducing
challenging four-stroke machines.
In some way the Rinaldi engineers preparing the YZ450F for its maiden world
championship season had an easy and difficult job at the same time. Easy
because the production machine according to Team manager Michele Rinaldi is
already “super good with a very well balanced engine and chassis”. But
because of this it was also not easy, or maybe one should say, not necessary
to raise the competitive level of the machine that much further.
But racing at factory level means that ultimate performance most always be
the goal, so despite the GP level performance of the production machine the
Rinaldi technical team worked as hard as ever before with the clear goal to
produce the very best bike in the newly conceived Motocross GP championship.
The focus was not so much on serious performance enhancement but more on
fine-tuning the engine to suit the teams’ riders. As a result the number of
special parts is relatively ‘modest’ for such an official factory machine,
but nevertheless parts such as piston, cylinder, conrod and crankshaft are
special. One of the most exquisite parts being the factory specifiction KYB
suspension package with massive 50mm diameter front forks.
The engine modifications produce a power curve that shows increased
performance over the entire rev range, mainly achieved by fine-tuning the
cylinder head and carburetor. Also the CDI is ‘home made’ in Italy, with own
mappings to accommodate for an even smoother engine feeling. To match the
revised engine power curve the transmission ratios have been altered to
minimize the need for up and down shifting for the riders, contributing to a
relaxed riding style. A new item is the Rinaldi-developed hydraulic clutch,
very appreciated by the riders for its smooth and precise operation with no
free-play, giving a great benefit at the start, while omitting the necessity
of adjustment during the race.
The reduced engine capacity and increased efficiency, compared to last years
YZ500FM, make it possible to do away with the enlarged 9.5 liters fuel tank
used in the past. Sand tests have proven that the aluminum 8 liters unit
holds enough fuel even for the most severe conditions.
Same as for the engine the chassis mirrors also the production
specification. The team is using the production chassis with unaltered
geometry (apart from a longer swing arm to enable better starts and higher
stability at mid and higher speed), which is a big change compared to the
very special aluminum frame of last years’ YZ500FM. To fit special parts
some brackets have been modified, but besides a machined triple clamp with
modified offset the chassis is quite similar to what can be bought at the
local Yamaha dealer.
Very different though from production are the prototype brakes on the
YZ450FM. Developed in close collaboration with Brembo the bike features
beryllium brake calipers powered by a new master cylinder, whereas front and
rear brake disks from Braking have a new improved specification as well.
It is clear that the efforts of the development team have been focused on
many detail improvements rather than extreme and visual specification
changes. The result is a factory machine with close roots to the production
bike, but with a performance level that makes both the team riders Stefan
Everts and Marnicq Bervoets extremely confident for the challenge to come.