The Mighty Mouse story is a classic tale of drag racing ingenuity. A skilled rider and engineer who took an idea and gradually developed and improved upon it until he had done everything he could to wring every last ounce of horsepower and performance from it. From the mid to late seventies a bike with, in some cases, a fraction of the capacity of its competitors, showed allcomers the way to the finish line with alarming regularity. Like all Drag Racers Brian Chapman was never quite satisfied with his achievements, always looking for the next increase in performance. Over the years the bike evolved, the nitro content increased and by the time he retired the Mouse in 1980, to make way for his new project, he had shaved nearly seven seconds off its original times.
Brian built the first incarnation of Mighty Mouse in the mid-sixties having spent two years competing in various speed trials and sprints on his road going Vincent Black Shadow, complete with sidecar! Like many he was inspired by the success of the Dragfests and the growing popularity of Drag Racing in the UK and decided that it was the direction he was going to move in next.
Being a big fan of the Vincent he decided to use a 1951 Comet engine as the powerplant for his purpose-built Drag Racer. The Comet was a 500cc single cylinder engine that was effectively half a vee-twin. It was not known for being a high performance engine but Brian had big plans for the inoffensive unit and set about constructing his machine in his garden workshop. He decided that he had to keep the bike as low as possible and fabricated his own tubular frame which allowed him to tilt the motor forwards by 15 degrees. The motor itself was used as a stressed member. The new angle of the powerplant meant that modifications had to be carried out to the oil pick up and rocker gear drainage. The frame was in two parts with the front section being easily removable to allow easier access to the engine. Initially forks from a 98cc Moto Guzzi were used. Weight was kept down by dispensing with the need for an external oil tank. Instead two pints of mineral oil were carried in the timing chest and in a small extension attached to it where the original magneto would have been. A dual ignition system was used featuring two rotating magnet magnetos which were shortened at their contact breaker ends and coupled back to back. The contact breaker points were mounted on an external plate and driven from the remote end of the second magneto. A single carb fed the methanol from the three pint fuel tank mounted above the clip-on handlebars and an open pipe provided an outlet for the exhaust gases. The carb was later replaced by a Wal Phillips injector.
The drive was transmitted to the 18 inch rear wheel via a single row primary chain, a strengthened AMC clutch and an AMC Norton gearbox with the first gear pinions removed. This left three standard ratio gears which were all used on the bikes run up the quarter mile. Final drive was via a secondary chain to CMA cast alloy rear wheel with single Vincent drum brake.
To start with the machine had more than its fair share of teething troubles and Brian recalls that he did more pushing than riding at some meetings. When the motor did choose to run the bike turned in times in the region of fourteen seconds. Transport to meetings initially was achieved by mounting the Mouse in the sidecar of the Black Shadow outfit! Over the next two years the times gradually improved until early in 1969 when, running with 25% nitro in the tank, Brian got down to 12.3 seconds. Feeling that he had done as much as he could with a normally aspirated motor Brian added a Rootes blower in mid '69 and experienced problems almost immediately as it frequently cooked the vee-shaped drive belts. A toothed belt was found to be the answer to this problem. The blower was driven at engine speed and took the fuel mixture from a single ex-Jaguar 1 3/4inch SU carb. A fuel cut-off valve was fitted which could be operated directly or from a trigger on the left handlebar. The motor retained standard sized valves with the exhaust being made from Nimonic 80 material. The original front forks were replaced by shortened Godden Engineering grass track items and a hydraulic steering damper was added.
He had some success with individual final round wins and by the end of 1970 the bike was winning more regularly and had run a best of 11.10/120.
In August of 1971 Brian took the bike to the NDRC meeting at Blackbushe. There he put in a run of 10.77 seconds making Mighty Mouse the quickest single-cylinder dragbike in the world. After that momentous occasion sub-eleven second runs started to come more frequently and Brian began to stick the ten second time slips on the wall of his small workshop.
In March 1973 Brian arrived at a Santa Pod practice day and stunned everyone present by putting in a 10.35 off the trailer. When asked about what modifications he had carried out on the bike over the winter he replied that winter had been spent decorating his house and all he had done to the Mouse was run a feather duster over it! Next month at the Season Opener he ran the quickest bike time of the day at 10.48. In April at the Springnationals his luck ran out as the motor blew itself to pieces. Undeterred Brian took it back to his workshop and began the rebuild. He had been using a lightened crankshaft, so light, it is reported, that lead had to be added to balance the flywheel. With a return to the track as soon as possible on his mind he rebuilt using a standard crank. A move that turned out to be for the better as the bike instantly ran quicker than ever! The addition of a twin lobe Marshall supercharger and an increase in the fuel mixture to 75% nitro with 25% methanol meant that the bike was now producing an estimated 125bhp at 9000rpm and returning a fuel consumption of four miles per gallon. The tradition of putting the ten second timeslips on the workshop wall was getting out of hand!
Chapman was now running in the Top Bike class and at Santa Pods August Nationals he amazed everybody by taking the bike into the nines with a 9.92/139 which was good enough for number two qualifier just behind Norman Hyde. He backed the nine up with a 9.98/141 in the first round, more than enough consolation for not making it to the final.
By the end of 1975 Brian had secured the Top Bike Championship honours for the first time. He continued to run against, and regularly beat, the quickest bikes in the Country through '76 when he ran a seasons best of 9.07 in September. A year later at Santa Pods September International he finally managed to get it into the eights with an 8.96. He was awarded the BDR&HRA Motorcyclist Of The Year trophy in recognition of this feat. He also won the ACU Championship outright in 1977 and the following year recorded the bikes best ever figures of 8.81/157. In 1979 despite still being competitive Brian was getting beaten more regularly by the multi-motored Superbikes of John Hobbs, Henk Vink, John Clift and Jeff Byne who were now knocking on the door of the sevens.
He decided that he had taken the machine to the limits of its performance and set about creating a Vee Twin version, with twin superchargers, to be known as "Super Mouse".....
Mighty Mouse is still in existence today, in exactly the same guise as when it was retired. Brian has taken it out a couple of times since then for a quick blast, notably on the Isle Of Man at the Vincent Owners Club Golden Jubilee celebrations in 1999. The bike can be seen on display at the Enfield And District Veteran Vehicle Trusts museum in Whitewebbs Lane, Enfield, Middlesex.
All photographs on this page by Alan Currans of The Acceleration Archive