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Old 09-09-2011, 10:29 AM   #1
oohhh ya
KK273's Avatar
Join Date: Feb 2009
Location: San Diego, CA
Posts: 11,189
I ride: really, I did ride.
Default Twin-cylinder mx bikes

Some1 mentioned this yesterday? My initial thought was of an old CZ, but ran across this wild machine a minute ago.

“This radical motocrosser from 1980 was the result of the Honda factory’s open chequebook—and a desperate desire to see off its rivals. Born in response to Gilera’s twin cylinder motocross bike, the Honda RC125M Twin was nothing short of a beautifully engineered technological marvel. Producing a staggering 35 horespower at 13,000 rpm from a reed-valved and watercooled, 124.99cc motor, the red rocket brought a new meaning to the word ’screamer’. That radical ‘Ribi’ front end was seen as a worthwhile experiment after Roger DeCoster achieved some success with the design aboard a factory Suzuki RN in 1978; when DeCoster switched teams, he introduced Valentino Ribi to Honda and they bought the rights, which they own to this day. The benefit of the system was a flex- and friction-free operation, coupled to a highly adjustable and variable rate of progression—similar in effect to modern-day rising rate rear suspension systems. The downside was the staggering cost of the 19 separate components, the difficulty in setting it up, the weight, and the fact that visually, it resembles a techno praying mantis. The bike debuted at Suzuka in 1980 with Kenji Sato aboard, and had further outings in 1981 before the FIM stepped in and banned twin-cylinder bikes from international competition. A single cylinder version of this bike—once more with a Ribi front end—won the 1981 All Japan 125cc Motocross Championship in the hands of Yasuo”


the cz...

pictures at

1983 CZ 125cc Twin Moto-cross Prototype
In the early 1980's a few companies thought the twin cylinder engine was the future of 125 moto-cross. Gilera campaigned a twin cylinder 125 in 1982, in the hands of Gaston Rahier. Honda also developed a twin 125 MX'er, but that bike was never raced outside of Japan. CZ also developed a twin cylinder 125 Moto-cross, in 1983. But before it could be raced in the GP's the FIM banned twin cylinder engines in the 125 class. When that happened the 125 twin went directly to the CZ Museum.

The CZ "Twin" had some unusual features. The bike featured twin carburetors feeding into a rotary valve induction system. The single rotary valve was gear driven and sat behind the cylinders, so the carbs were in the traditional location. This motorcycle also featured a countershaft sprocket that shared the same axes as the swingarm pivot. This being accomplished with an extra shaft a the back of the transmission.

The CZ Twin MX was recently on display at the National Technical Museum in Prague, Czech Republic. These great pictures come courtesy of Martin Kratky, and Jiri Starec.
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