Vincent Related David Tompkins Vincent at Daytona

[Keith Martin]

I have had the pleasure of looking over David's Comet and seeing it in action. The bike is very well set up with attention paid to every detail. I really like it. David also rides hard!
The Comet is in AHRMA Class C category. The cut off year is 1951 for Class C bikes. 4 speed gearboxes only. Limited carb sizes for twin cylinder bikes. No featherbed framed Nortons. The Comet is one of the only bikes with a swing frame that fits into Class C. A nice advantage especially when the suspension is dialed in like David's bike. Alex is riding a plunger frame Norton that goes well but the plunger rear is not the best with modern tires.
We won the Class C championship a few times running a rigid frame Triumph T100 pre unit. Worked over the front end to help handling. We ran later unit yokes that do not have the rake that the pre unit yokes have. Moved the steering head angle back bit. This loaded the front tire more and quickened the steering. It would cook a front tire in one weekend of racing. We used a 5 speed cluster inside the gearbox case with 1st gear blocked off. The shifting on a Triumph 5 speed is so much better than the 4 speed box. We just started the race in 2nd gear and downshifting into turns so much smoother making for better lap times.
Nourish crankshaft, Thunder Engineering rods and Newby belt drive. Sold it a few years ago to a racer in the UK. We named the bike "Stiffy" as the rear suspension was always on the stiff side.

 

[Peter Holmes]

Was your gearbox trick with a blanked off first gear actually legal in racing terminology, I realise you only had 4 selectable gears, but with an obvious advantage gained otherwise you would have not done it, were the scrutineers aware. I am not moralising BTW, just interested.

 

[Keith Martin]

Yes it is legal per the AHRMA rules and the scrutineers were aware. Neutral was all the way up so shifting was 4 down.
We raced a Hinckley Triumph Thruxton in AHRMA. It is basically a spec bike class with limited mods. My son won a race at Road America by 12 seconds and then ARHMA had a surprise post race inspection on all bikes in the class. Of the 12 bikes running that day we were the only bike to pass the post race inspection all other bikes failed. That was a fun day.

 

[medat727]

I have had the pleasure of looking over David's Comet and seeing it in action. The bike is very well set up with attention paid to every detail. I really like it. David also rides hard!
The Comet is in AHRMA Class C category. The cut off year is 1951 for Class C bikes. 4 speed gearboxes only. Limited carb sizes for twin cylinder bikes. No featherbed framed Nortons. The Comet is one of the only bikes with a swing frame that fits into Class C. A nice advantage especially when the suspension is dialed in like David's bike. Alex is riding a plunger frame Norton that goes well but the plunger rear is not the best with modern tires.
We won the Class C championship a few times running a rigid frame Triumph T100 pre unit. Worked over the front end to help handling. We ran later unit yokes that do not have the rake that the pre unit yokes have. Moved the steering head angle back bit. This loaded the front tire more and quickened the steering. It would cook a front tire in one weekend of racing. We used a 5 speed cluster inside the gearbox case with 1st gear blocked off. The shifting on a Triumph 5 speed is so much better than the 4 speed box. We just started the race in 2nd gear and downshifting into turns so much smoother making for better lap times.
Nourish crankshaft, Thunder Engineering rods and Newby belt drive. Sold it a few years ago to a racer in the UK. We named the bike "Stiffy" as the rear suspension was always on the stiff side.

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Hi Keith, thanks for the kind words. I’m in the process of building up a 650 Triton for this season’s Classic Sixties 650 class using a unit Triumph motor, I too will be running a five speed with first blocked off, I was wondering if you might share the best was to go about disabling first?
Thanks,
David

 

[Keith Martin]

We welded a threaded boss on the front of the gearbox housing that lined up with a stop welded on the back of the gearbox cam plate when in neutral. A bolt was screwed in to threaded boss to hit the stop on camplate which would not let it shift into 1st. For the club races that we ran that allowed 5 speed gearboxes all we had to do was unscrew the bolt and it would shift into 1st.

 

[medat727]

We welded a threaded boss on the front of the gearbox housing that lined up with a stop welded on the back of the gearbox cam plate when in neutral. A bolt was screwed in to threaded boss to hit the stop on camplate which would not let it shift into 1st. For the club races that we ran that allowed 5 speed gearboxes all we had to do was unscrew the bolt and it would shift into 1st.

Very clever, you ran a pre-unit box, do you feel this can be accomplished with a unit box?

 

[Keith Martin]

Maybe if you welded bung to top of gearbox on case. Bolt would go down to hit stop. I will look at a case when back at shop later today.