Fergs Bonneville P4

Bonneville land speed record attempt - Part four

by Mario

Ferg’s Triumph Bonneville T120R has taken great strides in its recreation as a land-speed-record breaking machine. We revisited his workshop to catch up on progress.

Our most recent trip to see Ferg’s 1970 Bonnie revealed something that looked tantalisingly close to a motorcycle. Well, with the engine out and the tank off. But the basic cycle parts are back together and that’s already allowed Ferg to do some thinking about tweaks that might help on the salt flats of Utah.

More power, Igor

I’ll leave it to James May to bore everyone to death with aerodynamic equations on thrust versus drag, but the basic concept is simple. The faster you go, the harder it gets to overcome the drag and increase your speed by any given amount. So, for example, the amount of extra horsepower you need to increase a bike’s top speed from 100mph to 105mph is quite a lot less than you need to increase the speed from 105mph to 110. What’s important about that is Ferg is seeking to break the world land speed record for pushrod 650cc production motorcycles, which currently stands at 106.4mph. The current record holder would have needed significantly less horsepower to overcome the previous record than Ferg is going to need to exceed 106.4.

billet crank in raw state

Therefore, quite a lot of work has been put into the Bonnie’s powerplant. Pulling into his driveway, Ferg was retrieving his new billet crank from the van, looking very shiny but still in a raw state. It needs to be ground for bearings and balanced (by ‘Bob the Balancer’) but that will need the pistons, which is a project in itself. The pistons are going to be custom-made for the combustion chamber by CP Pistons in the USA, based on a mould made by the cylinder head maestros at Landon Motorsport.

Talking of which, the results are in for the gas flow work and it’s good news: the flow rate has increased by over 35%. There’s still some work to do though, to balance the intake efficiency with the exhaust. With a modern overhead-cam engine this would normally be fine-tuned by a series of changes to the cam profiles and timing. With the pushrod Triumph’s camshafts sitting in the bowels of the engine, that's not really an option. So Ferg has asked for the best set-up they can do via bench-testing and that will be that.

Other power tweaks will include boring out the Amal carbs from their standard 30mm to around 33mm, a single-coil ignition and possibly a total loss electrical system, thus avoiding the horsepower drain of a dynamo. A new arrangement for the crank breather is also under consideration.

various reconditioned parts

The frame game

Plans have changed slightly on the ballast front. Adding weight can greatly help traction in the salt, so less power goes to waste in slippage. As a production class entry, this has to be hidden so the appearance is standard, and lead shot has now been added to the swingarm and rear subframe. But as yet nothing has been done to the main frame tube. It’s still a possible option, and others include weight hidden in or under the seat.

Back wheel

As you can see, the frame and wheels are back together, and a fresh new loom is being installed. A brand new, classic pattern Heidenau front tyre has been fitted and heavy-duty motocross inner tubes installed front and rear. During our visit, thoughts turned to the handlebar arrangement. This is complicated by SCTA regulations and the Bonneville’s unusual handlebar clamps. SCTA rules specify that any replacement bars cannot extend more than four inches below or in front of the original set at the bar ends. Unlike a modern bike, the Bonnie’s clamps do not detach at the top. Instead, the bars have to be inserted through the clamps then pinched tight with a single bolt on each. Together, these issues make any radical bar set-up close to impossible. A solution might be to cut the handlebar then weld it back together once in place. Getting the rider as prone as possible is of course key to reducing aerodynamic drag.

Rebuilt front forks

The front forks have been rebuilt by a specialist so, with all new bushes and bearings, they work as good as new. There are still some tweaks to do on things like axle torque to get the wheels running as freely as possible, and some parts to bolt back on. Still, it’s a rolling chassis.

The Triumph is now entering a really exciting phase, one where it truly takes shape as a motorcycle and even begins to run again. We’ll have more on that in our next story. Catch you then.

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