OHM Brew

CHRIS WHITWORTH’S YAMADA MERGES YAMAHA POWER WITH HONDA RUNNING GEAR – IN HIS OWN FRAME. HERE’S HOW THIS BRIGHT SPARK ELECTRICAL ENGINEER MADE IT HAPPEN

OHM_Brew_April_18

Chris Whitworth is a retired engineer, although it seems that once engineering is in the blood you can never give it up. He currently has two projects on the go and is regularly called upon by our own Gary Hurd for quick engineering fixes. His approaches to problem solving and ability to integrate concepts and components are equal to those of the most celebrated in the industry.

His creation, which he christened the Yamada Mono S1, takes lateral thinking right to the left-field, opens the gate and wanders into zones of its own. The more you look, the more you see. A fuel tank that doubles as a seat unit? An oil tank that acts as its own radiator while directing air down towards the inlet? If a manufacturer had built this bike, with all their staff and resources, we’d be hailing their genius.

Supermotos weren’t a big deal here when Chris built the Yamada back in the early 1990s. “My first marriage broke up in 1990 and I spent five years in the wilderness,” says Chris. “I got into green laning on a Suzuki TS250 but the two-stroke engine broke traction every time you tried to get power on in the mud. Then I heard about a Bultaco Pursang rolling chassis and was given it for free. I put a Honda CB250RS engine in that. My mate Charlie Broomfield had a winter hack Kawasaki KX500 with a Yamaha XT500 engine and CB250RS road wheels on it. An idea was starting to form. Then I went to Paris on a business trip and heard a single cylinder bike blatting between traffic lights. At that point the idea of a supermoto made sense to me.”

The main elements Chris set to to conjoin for his special were a 1991 Yamaha XT600E engine and a 1989 Honda CR500 frame, the former from a breaker in Coventry and the latter from the reader ads in MCN. They were scored for under £300 each.

Looks SO factory. In fact a lot better than a great many factory Supermotos

Looks SO factory. In fact a lot better than a great many factory Supermotos

CHRIS’ ADVICE

1. PLAN AHEAD AS FAR AS YOU CAN. I LOST COUNT OF THE NUMBER OF TIMES I MADE A VERY INTRICATE BRACKET THEN HAD TO CHANGE IT BECAUSE I FOUND I COULD SUPPORT SOMETHING ELSE IN THERE ON SAME BRACKET. I HAD LOTS OF PROTOTYPE ITEMS LEFT OVER.

2. STICK AT IT. THINK TANGENTIALLY. MY DAD ALWAYS SAID TO ME, ‘WELL IT DIDN’T JUST GROW THAT WAY.’ THAT THOUGHT GETS YOU THINKING ON THE RIGHT LINES.

3. WHEN DESIGNING AND MANUFACTURING ANY PART OF THE PROJECT, YOUR FIRST IDEA IS UNLIKELY TO BE THE BEST PLAN UNLESS YOU’RE BLESSED WITH INNATE DESIGN GENIUS. SO ALWAYS QUESTION WHAT YOU ARE DOING AND DON’T CLOSE YOUR MIND TO THE PROSPECT OF STARTING AGAIN WITH A DIFFERENT APPROACH. THE END RESULT WILL BE WORTH IT.

 

So that’s a YAMAha engine and a HonDA frame, hence ‘Yamada’ – you might note Yamaha’s trademark tuning forks cheekily extending from the feathers on the Honda wing on the CR tank side panels. However it quickly became obvious that engine and frame were not close enough to even be described as uneasy bedfellows – there was no way to get a reasonable chain run and after much chopping about it was concluded that a new chassis would be in order. “Although I always intended to retain as much CR stuff as I could,” says Chris.

So Chris’s next move was to build the jig he would need to make a frame. This comprised a 1/2in steel base with the necessary uprights and crosspieces to site the headstock and swingarm to retain the Honda geometry. The sole survivor from the by-then much hacked about frame was the CR headstock.

Oil temp drops by 10 degrees with tank sited behind headstock

Perfect chain run – after Chris made a new frame

Custom ride-height adjuster at rear

SPECIFICATION: YAMAHA

ENGINE: 595CC, SOHC, 4V, AIR-COOLED SINGLE, MEGACYCLE CAM, HOME-BUILT STAINLESS EXHAUST, VALVE LIFTER

CHASIS: HOME-BUILT FRAME IN ERW TUBING. MODIFIED CR500 SWINGARM, FORKS IN CUSTOM YOKES. STOCK SHOCK AND FORK LEGS REWORKED BY CHRIS TAYLOR. CUSTOM-MADE RIDE-HEIGHT ADJUSTER. CBR600 F2 17-INCH WHEELS. 1 X VFR750 FRONT DISC, FZR1000 EXUP CALIPER AND MASTER CYLINDER, CBR600 REAR DISC AND CR500 CALIPER AND MASTERCYLINDER. HOME-FABRICATED FUEL AND OIL TANKS ON ANTI-VIBRATION MOUNTS. DUCATI 900SS FRONT MUDGUARD, CBR600 REAR HUGGER.

 

“I was inspired to some extent by the Ducati trellis frame and had already decided that the best course was a straight as possible run from headstock to swingarm as the way to go,” says Chris. “As an electrical engineer primarily specialising in rotating electrical motors, primarily AC generators, I’d been introduced to the tubular frames they put around small generator sets. The lightest possible tubing was specified for military use. The stuff I used is called ERW tube, which stands for electrical resistance welded. It’s not quite Reynolds 531 but it’s light and easy to bend. It can’t be electrically welded or it will crack, so I used bronze welding which works its way into the molecular structure of the steel and holds it together. You do have to mitre the joins accurately, however.” Some skip surfing at the generator firm yielded enough tube to make the frame.

Air/Fuel gauge: Chris’s ongoing lean running issues led to him buying and fitting an A/F gauge connected to a lambda sensor in the exhaust. “It reveals some surprising info in different wind and weather conditions,” he says.

Valve lifter: With a 600cc single pot and a 10:1 compression ratio, the starter motor has to work quite hard. Chris made and fitted a valve lifter just in case.

Nose cone: Chris wanted a full-on raptor-style nose but his vision was reined in a little by the withdraw angles dictated by the mould he made from the clay buck he sculpted on the bike. That took two months of spare time all in.

Oil tank/airbox: Fabricated oil tank is shaped to shunt fresh air in the direction of the much modified airbox, while itself being positioned to enjoy a cooling breeze.

Next up were the wheels. Chris was faced with the choice of going wire-spoked or cast. The latter won out and as he had mates into racing Chris soon found some CBR600 seven-spoke ones of roughly 1991 vintage. These of course were too wide for the CR500 yokes and swingarm. Chris sought the advice of the engineering company in Oakham where he got the steel plate for the jig as he knew they were into aluminium welding. “I bought another steel plate off them for a swingarm jig,” says Chris. “We flame cut a hole in the plate right there and then so there would be access to both sides of the cut swingarm. I made the jig and took it all back to them. Inside a week they had done the job.

I’d given them a length of precision rolled 17mm rod to make the longer spindles I required. The swingarm pivot turns on bushes rather than bearings and they got it so right you could turn the spindle with your fingers.

“When it came to the yokes, me and my mate Charlie enrolled on an evening engineering course at Melton Mowbray Technical College which gave us access to all the machinery we needed. I think the course cost us £35 each. Usefully, I also knew the store man so I got the aluminium for the yokes cheap too and these were made to the same angles and offset as the CR ones. I made the front brake caliper support, a ride height adjuster, all sorts for the bike in there.”

As Chris wanted to have a single front disc on the Yamada, he opted for a larger diameter VFR disc over one of the CBR600 ones the wheels came with to get some extra braking advantage to make up for the loss of a disc. His college-made mount carries an EXUP caliper. “I was running an EXUP at the time,” he says, “the mastercylinder is EXUP too and the brake feels a little wooden at standstill as it’s only pumping one caliper. It’s alright on the move, however.” Rear brake is a CBR600 disc with the CR caliper and mastercylinder which Chris admits isn’t really powerful enough.

Not the sort of bike you’d expect the average 70-year-old to be riding

Not the sort of bike you’d expect the average 70-year-old to be riding

The stainless exhaust system is also to Chris’s own design, its lines following the frame rails, once again some reusing and recycling came into play. “It was TIG-welded by a friend who worked at Pedigree Pet Foods at Melton Mowbray, so again there was no end of stainless steel tubing and bends kicking around in the skip,” says Chris. The home-rolled cans have carbon-fibre sleeves. My pipes were angled as they are from some time before Yamaha got around to doing it on their later XTs.

If the engineering weren’t already smart enough some extra-clever thought went into the fuel and oil tanks, both of which are on anti-vibration mounts. The petrol tank is also the tail unit and the filler cap section behind the seat came from a Kawasaki ZZ-R600. Chris had a huge amount of sheet metal work to do to make the rest of the tank and integral battery box, shaping it all into the frame. The tank, baffled and gusseted inside, is shaped to get the 14 litres of fuel and the battery as low and central as possible. A conduit for the rear light and indicator wiring runs through the tank. A solid-state pump takes the fuel to the carbs and the excess comes back to the tank.

The home-fabricated oil tank for the dry-sump XT engine forms a U-section over the frame’s spine and enjoys plenty of cooling air in its position behind the headstock. This meant that Chris didn’t need to find space for a cooler. Chris has measured a temperature drop of 10°C as the oil passes

through the tank on its way back to the engine. The tank is also shaped to funnel air back towards the airbox for the stock XT combined slide/CV carb. The plastic side panels that bolt onto the CR tank also help with this. Singles racers Chris knew stressed the importance of stable airflow at the inlet. So he used the bottom of the standard airbox and added a filter element on top to fit under the hollowed out CR fuel tank.

Early on in the project, Chris took the opportunity to rebuild the engine and fitted a Megacycle cam. For years after completing the project, Chris blamed a lean running problem on carburation, endlessly changing and even making larger jets. Finally he deduced it to be down to the cam timing, and after many hours with dial test indicators and the tuning books of Graham Bell and John Robinson, Chris finally settling on maximum inlet lift at 107° ATDC, slotting the cam pulley to achieve this.

It still looks fresh after ‘only’ 20 years in development

It still looks fresh after ‘only’ 20 years in development

So 20 years after completing the Yamada, Chris can confirm that specials are never quite done – the cam timing issue was evidence of that. And indeed only last winter he made a new lower seat, convinced that now he’s past 70 years old he is in fact shrinking.

“If you wonder what the special you’re building now might look like in 20 years, take a look at mine,” he says. To us it looks as just as fresh and innovative as it must have done way back then and that’s true testament to the engineering genius of Chris. It took at least 2000 hours to build. The real beauty of it is Chris has had many years of enjoyment from it since.