We are providing these build notes to assist anyone interested in converting an old air cooled mono-shock bike into a twin-shock. The ridiculous amount of money required to build a competitive Brit twin-shock nowadays pretty much rules out “P65” competition for many, and a properly modified original or replica version of a competitive twin-shock can easily cost well over £5k, so a cheap twin-mono makes a lot of sense.
A reasonably well prepared twin-mono is certainly more competitive than a poorly prepared original twin-shock, but the same can be said for any machine! The only real advantage the converted mono-shock bikes have over the more competitive original TS bikes is the longer swinging arm, and weight bias directed more toward the front wheel.
This is nothing new though, and is a modification that can be carried out on most original TS bikes, and was used by Bultaco on various works supported machines back in the early 80s! Increased weighting on the front wheel obviously provides improved steering, while longer swinging arm gives better wheel grip, and improved suspension action.
Firstly it must be born in mind that while it is quite possible to convert any mono-shock machine to TS, that some are much easier to do than others, and that a few event organisers may not accept these bikes, even though Brit twin-shocks being ridden in their P65 classes may well be just fine!
Perhaps the very best choice for the basis of a twin-mono is the bulletproof and still easily available TY250 Yamaha Mono. These bikes still have very reasonable spares availability, and scruffy but mechanically sound examples can often be found on Ebay for around £1k.
The frame and swinging arm are both steel, so alterations are easily made and there is no need for TIG welding aluminuim. Shorter type suspension units mean that the top mounts of the units are less intrusive, but the frame needs to be braced as top shock mount is situated around about the middle of an unsupported frame tube.
We would recommend longer TLR Honda type suspension units, and have provided specifications to Falcon Shock Absorbers, who will be able to provide exactly what is required. The longer shocks mean the top mounts are more noticeable but suspension action is improved, and chassis modifications are more easily carried out.
Other than the obvious fitting of the mounts for twin-shock suspension units, the TY250 benefits from alterations to the steering angle and footrest position, but other than minor modifications to some exhaust back boxes to clear the suspension unit, everything else remains standard!
As soon as we have time available we will be picturing the process of properly converting a TY250 frame, and hope this will assist anyone wanting to become involved with TS competition get started without any need to spend a great deal of money, or put up with riding something that's uncompetitive, unreliable, heavy, and simply not much fun to ride!
Finally please email if you have any questions about converting your mono-shock machine to twin, as we are always pleased to help, and feel that reducing costs and increasing access in relation to competing on older machines, can only be a good thing.