Turbo charging is a method of forcing more air into an engine. The turbo charger uses the exhaust gas to spin a turbine which in turn spins a compressor which is used to force air into the engine. You probably knew that though.
Turbo charging Monkey Bikes is not a new idea. It's been done several times in Japan and there are current projects in Germany, Belgium and Holland but to the best of my knowledge, not in The UK. For this reason I wanted to have a go at it myself. Supercharger (belt driven compressor) kits for Monkey Bikes are available off the shelf but that's no fun, I wanted to make my own turbocharged bike. I also wanted to finish it to a very high standard and try to keep untidy component fitment to a minimum
After some research and asking about on The UK Monkey Bike Forum a friendly American forum user pointed me in the direction of the world's smallest turbo charger the Mitsubishi TD-02. A few weeks later I had the tiny unit in my hand after buying it from Turbo Engineering Corp in America
You can clearly see to the left how small it is. I celebrated its smallness with a nice cold can of Scrumpy Jack!
The donor bike
Here then is the lucky bike. It's my 1974 Honda Dax. The engine is in my Mini Trail so what better bike for a major heart transplant?! I've had the bike for about 3 years now and resisted restoring it because it has a really nice patina!
I want to keep this bike very tidy in design. Some of the turbo bikes I've seen pictures of have been messy affairs which look rushed and without much thought for aesthetics.
The air filter normally sits where the turbo is pictured here. It seems the perfect spot to mount the turbo with the exception of possible heat problems. More of that shortly...
Looks wise though it's a great position!
Lack of fuel capacity The Honda Dax fuel tank holds a teeny weeny 2 ½ litres giving a standard 72cc bike a range of about 50 miles. Extra fuel would be needed for my gas-guzzling turbocharged bike! I didn't want to fit an auxiliary fuel tank as this only spoils the look of the Dax.
So where to store the extra fuel? How about making the whole frame the fuel tank? Would it be dangerous to use the entire frame as a fuel tank? I wasn't sure and no one could offer me a plausible answer. After all, the frame is much thicker than a regular motorbike fuel tank.
After many weeks of fabrication and welding I sealed the whole frame into a tank. The three pipes you can see in the bottom of the frame tank are fuel supply, reserve and return. The frame holds a whopping 9 litres of fuel which will increase the range significantly!
The Engine I felt I couldn't just turbocharge a 72cc engine. Imagine if bigger engined Monkey Bikes were faster! Turbocharged engines need a lower compression ratio to avoid detonation and lowering the compression ratio reduces power output. A low compression 72cc engine will be very slow until the turbo kicks in. I want to keep the bike mild mannered and useable in traffic so I needed a fairly big engine. Dragon Racing Parts were touting a 120cc Chinese engine as being very good with an excellent gearbox with nice gear change and ratios. So I bought one!
It's a four speed (one down, three up) secondary 4 plate clutch engine.
Here you can see how neatly the turbocharger sits in that section of the frame above the engine. Turbos give off a lot of heat so the turbo must have a decent air gap between it and the frame. A frame full of fuel.... A good heat shield will also have to be installed between the two. No worries though. Well, not many anyway 
Turbo installation
At the start of 2005 I started fabricating the necessary pipework for the exhaust system to and from the turbo
This whole polished section has been made especially for the turbo as it didn't come with a manifold. It's made from a 12mm stainless steel plate plasma cut and linished into shape.
The 42mm section with the bend was obtained from a local exhaust fabrication specialist. I cut the bend which gave the necessary oval profile to cover the exhaust outlet and waste gate.
The waste gate is operated by the small lever you can see in the foreground on the turbo. A pneumatic actuator opens the wastegate to bypass the turbine and limit boost above a preset level.
With the exhaust outlet section made, the tail section will be fabricated once the bike is on its wheels and shock absorbers in place as the tail section will have to be fabricated around them.
Here's the primary section under construction. This section is made from 32mm OD stainless steel 90° bend sections available from Dragon Racing Parts. The flanges are made from 6mm stainless steel plate.
This really is a very enjoyable part of the build as it's so satisfying fabricating with lovely new stainless steel on the bench.
Here are some views of both pipes fitted and polished
Frame mods for wiring and heat
The Dax houses the wiring loom inside the frame. I'm filling it with fuel so the loom has to take another route. I'm routing it under the neck of the frame on the left hand side away from the heat of the turbo. I added these tabs which cable ties will go through to hold the loom against the frame.
Below: This must surely be a necessity. A heat shield to protect the paint and fuel-filled frame from the heat of the turbo. Seeing this will please many skeptics about my future without burn scars!
Intercooler design
These two pictures on the right show the beginnings of the intercooler mockup and design. The oil cooler will probably be mounted on the front of the forks for good air flow and so as not to disturb air to the intercooler
Just in front of the turbo you can see the dump valve suspended by masking tape roughly where it will be in the system. The intercooler inlet is over at an angle because the intercooler is 9" wide.
This second picture shows the intercooler outlet at even more of an angle. It's pointing towards the proposed downdraught carburettor. The carb is a Solex PICT 28mm from a 1200cc VW transporter. I'm going to have to build up weld around the inlet port and drill and tap some threads to hold the carb on as the spacing between the studs is different on the carb to the regular Honda inlet manifold. Again you can see in the top right of the picture the oil cooler in its proposed position.
Carburation I had been planning to use a VW Solex PICT 28mm (left) for the carb but was going to find it difficult because it's a bulky, ugly carb. A good chap at Southern Carburettors suggested a Weber 28 IMB from a Fiat 500 and another friend suggested I called Ricambio International in Banstead, Surrey. A quick trip there and I had bought the carb (right)
It's a really nice compact unit which amazingly bolts straight on!! Another bonus is the choke pictured below doesn't actually choke the carb, it adds fuel instead which may be handy for an on-boost mixture enrichening device...
Left and above: It's as simple as that! With a heat shield / insulator in place the carb bolts up really nicely leaving plenty of room above for the plenum I guess you'd call it. The air inlet fitting yet to be made....
In the picture to the left you can see the air filter supplied by PedShed with a 45° elbow fits really nicely
One throttle linkage. This is a simple bracket bridging a carb stud to a tappet cover screw. A 'P Clip' tightly grips the cable end. The modified cap head bolt takes the cable end and pivots in the carb throttle arm.
Cable movement is provided by a Daytona quick action throttle from Monkey Bike UK.
I need a vacuum takeoff to operate the dump valve. This needs to be in the inlet manifold..... but my bikes doesn't have an inlet manifold. I'm going to try to drill and tap the heat spacer to take a pipe fitting.
In the pic above you can see the bottom of the fuel tank. This is the view of the cavity between the top of the engine and underside of the tank. The swing arm sits between the circles top and bottom. The three fuel pipes are also visible, as is the surface rust developing by them! Bottom left of the pic is the new high pressure fuel pump mounted on the chain guard mounts. The pump is too big to fit in the cavity. I had hoped to fit it, the fuel pressure regulator and fuel tap in here but the pump's too big so it's living outside on polished stainless steel brackets. Centre is the fuel tap and top is the regulator placed in situ. The rest of the cavity will house wiring, coil, regulator and CDI. Oh, and some heat shielding to protect them from the fireball!
Above you can see the cavity between the fuel tank and top of the crankcase allows for some parts to be concealed here. I don't want everything on show so I'm cramming as much as possible in here. On the left is the fuel tap which protrudes through the left hand side of the frame and will also go through the chain guard. On the right is the fuel pressure regulator. This is needed to reduce the fuel pressure down from 70psi to just a few pounds above boost pressure. The regulator receives boost pressure and increases fuel pressure as boost increases. If boost pressure were to go higher than fuel pressure the fuel would fail to be delivered and the mixture would go lean and risk holeing the piston.
The adjustment screw for the regulator protrudes through the right hand side of the frame which will be needed for setting up.
In this picture you can see the coil, ignition unit (CDI) and voltage regulator can be squeezed in here too, with a little modification
Wanting to hide as much of the gubbins away from view I hid the fuel tap inside the frame. The control for it pokes through the chain guard and sits below the fuel pump. Pictured above is the view from below the chain guard. The original tap handle was cut off. An 8mm bar was notched and slotted over the tap stub. This was drilled and pinned in place. First Build It's time to start thinking about the first build of the bike as a whole. I build my bikes up rough and ride them for a few hundred miles for a shakedown so changes can still be made without having to rechrome or repaint things.
I had a can of aerosol paint in the cupboard I used for another project. It turned out to be orange! To the right you can see the threaded bosses welded on to accept the intercooler mounting
Time to chuck the bike together roughly to get a feel for how it's going to look. Everything's finger tight at the moment. The hubs are not properly fitted. They both need modifying for brakes and to fit the rims The rear suspension is too high. I don't like this look but those rear shockers are just cheapies. The seat should be level and the frame go uphill. I still don't have the air intake for the top of the carb or the oil cooler or intercooler....
< Left - View from the seat
Upfront is a fat 120mm tyre on the 3.5" wide 10" rim. It's rather rude and only just fits between the upside down fork legs
Lots more going on here than normal. On the left is the air filter and the gold unit is the turbo's wastegate actuator. On the right is the turbo exhaust.
Next job is getting the front caliper to clear the wheel and mount the disc to the hub.
Some more pics of the intercooler and oil cooler in cardboard I've got a Takegawa headlamp but it's a bit high at the moment
The front end
Now on to the front end. I'm using a Honda Nice disc and caliper with the original Dax hub and 3.5" wide G-Craft wheel. This is no simple bolt-together job. I decided to press ahead using the wider rim because it's difficult! There's no major gain using such a wide rim and in fact some people say it might actually effect handling adversely. We'll see. I just wanted the challenge. The problem is caliper clearance to the wheel. Offering it up it's clear the caliper will have to be ground. This weakens the caliper so replacement material will have to be welded in.
First off the hub was modified and a boss to take the disc was machined - see right
The boss has a lip to locate the disc (above) and is drilled and tapped to bolt the disc to it.
The boss is bolted to the hub from the inside where the drum brake used to be (right)
Here's the disc in place (below right)
The 120mm tyre just fits between the fork legs. There's a tiny gap but a gap's a gap as they say! (below)
Above shows the small clearance gap between the disc and right hand fork leg. This small gap allows as big a gap as possible for the caliper to use. Here's the finished Dax drum plate with the location lugs, brake pivot hole and cable bracket removed and blended away leaving only the speedo drive in place.
These three pictures show the caliper having been modified. I ground away whatever was required to clear the rim replacing any unnecessary grindage with weld. Any extra areas which could be strengthened with more material were welded too giving the large flat area bottom left.
Having removed so much material, strength is always going to come into question. I have significantly beefed up the fingers and their construction to the main body of the caliper.
Fear not for my life, this is plenty strong enough!
To the right is the caliper in place. It's got about 2mm clearance from the rim now. In the picture it has no pistons fitted and the rubber mounts are finished so it is actually resting slightly on the rim but once rebuilt it will clear the rim satisfactorily
A gap's a gap after all!
Frame finishing The final jobs to get the frame finished include fitting of the steering lock. The original early Dax steering lock is incompatible with the new forks. I decided to fit a Jincheng M50 steering lock, mounted to the frame and locking the lower yoke. I made up a bracket from 6mm steel and welded it to the frame then drilled through this into the yoke so the lock could do its thing. See right.
I gave the frame a coat of metallic blue, the shade I'll have the bike painted professionally finished in. I followed this with a couple of coats of lacquer. Halfords aerosol paints go on really nicely. They recommend leaving the lacquer two weeks before flatting and polishing
The last job before the road build will be to slosh seal the frame to make it completely fuel tight. The inside has been prepared with shot blasting and will use top specification sealer.
The Start of the Build And we're off... It's April 2005 and I'm 8 months into the project and only now am I starting the build! I never said it would be quick and I don't get to work on it for long periods (lunchtimes and weekends) but I have started the first road-going build now
Below: Shortly after this picture was taken I realised I'd forgotten to put the coil in there. After some jiggery-pokery I managed to fit it in too (after I introduced it to Mr Hacksaw....) A layer of heat proof lining was laid in here and the engine fitted with everything upturned which helped prevent the coil, CDI and regulator move.
Above and above right: Initial engine fitment. Below: Beginning the fuel tap and fuel pressure regulator plumbing.
The coolers arrive!
Above and below, intercooler and oil cooler fitted with the engine in a running state
Above is the intercooler as it arrived. Below is the oil cooler as fitted to the bike. It's not up to the standard of the intercooler so it's being remade. When I'm happy with the quality I'll remove this picture, take a new one and tell you who made them!
Getting it on the road If I was to do some proper setting up and running in it would have to be on the road. These pictures show the bike pretty much together and having passed its MOT test. The tune is still very rough. Miles off in fact. It runs and is just about rideable but at the moment I haven't got the carburation close enough for the engine to rev out nicely and generate any noticable boost.
.... and that's as far as the project is as of 17th May 2008. Stay tuned for more updates! - Ferg
This project is on the back-burner at the moment due to the turbo having sprung an oil leak and needing returned to America for rebuild. Oh and it didn't really work anyway!
Turbo charging Monkey Bikes is not a new idea. It's been done several times in Japan and there are current projects in Germany, Belgium and Holland but to the best of my knowledge, not in The UK. For this reason I wanted to have a go at it myself. Supercharger (belt driven compressor) kits for Monkey Bikes are available off the shelf but that's no fun, I wanted to make my own turbocharged bike. I also wanted to finish it to a very high standard and try to keep untidy component fitment to a minimum
You can clearly see to the left how small it is. I celebrated its smallness with a nice cold can of Scrumpy Jack!
The donor bike
The air filter normally sits where the turbo is pictured here. It seems the perfect spot to mount the turbo with the exception of possible heat problems. More of that shortly...
Looks wise though it's a great position!
Lack of fuel capacity The Honda Dax fuel tank holds a teeny weeny 2 ½ litres giving a standard 72cc bike a range of about 50 miles. Extra fuel would be needed for my gas-guzzling turbocharged bike! I didn't want to fit an auxiliary fuel tank as this only spoils the look of the Dax.
So where to store the extra fuel? How about making the whole frame the fuel tank? Would it be dangerous to use the entire frame as a fuel tank? I wasn't sure and no one could offer me a plausible answer. After all, the frame is much thicker than a regular motorbike fuel tank.
The Engine I felt I couldn't just turbocharge a 72cc engine. Imagine if bigger engined Monkey Bikes were faster! Turbocharged engines need a lower compression ratio to avoid detonation and lowering the compression ratio reduces power output. A low compression 72cc engine will be very slow until the turbo kicks in. I want to keep the bike mild mannered and useable in traffic so I needed a fairly big engine. Dragon Racing Parts were touting a 120cc Chinese engine as being very good with an excellent gearbox with nice gear change and ratios. So I bought one!
It's a four speed (one down, three up) secondary 4 plate clutch engine.
This whole polished section has been made especially for the turbo as it didn't come with a manifold. It's made from a 12mm stainless steel plate plasma cut and linished into shape.
The 42mm section with the bend was obtained from a local exhaust fabrication specialist. I cut the bend which gave the necessary oval profile to cover the exhaust outlet and waste gate.
The waste gate is operated by the small lever you can see in the foreground on the turbo. A pneumatic actuator opens the wastegate to bypass the turbine and limit boost above a preset level.
Here's the primary section under construction. This section is made from 32mm OD stainless steel 90° bend sections available from Dragon Racing Parts. The flanges are made from 6mm stainless steel plate.
This really is a very enjoyable part of the build as it's so satisfying fabricating with lovely new stainless steel on the bench.
Here are some views of both pipes fitted and polished
Frame mods for wiring and heat
Below: This must surely be a necessity. A heat shield to protect the paint and fuel-filled frame from the heat of the turbo. Seeing this will please many skeptics about my future without burn scars!
These two pictures on the right show the beginnings of the intercooler mockup and design. The oil cooler will probably be mounted on the front of the forks for good air flow and so as not to disturb air to the intercooler
Just in front of the turbo you can see the dump valve suspended by masking tape roughly where it will be in the system. The intercooler inlet is over at an angle because the intercooler is 9" wide.
This second picture shows the intercooler outlet at even more of an angle. It's pointing towards the proposed downdraught carburettor. The carb is a Solex PICT 28mm from a 1200cc VW transporter. I'm going to have to build up weld around the inlet port and drill and tap some threads to hold the carb on as the spacing between the studs is different on the carb to the regular Honda inlet manifold. Again you can see in the top right of the picture the oil cooler in its proposed position.
Carburation I had been planning to use a VW Solex PICT 28mm (left) for the carb but was going to find it difficult because it's a bulky, ugly carb. A good chap at Southern Carburettors suggested a Weber 28 IMB from a Fiat 500 and another friend suggested I called Ricambio International in Banstead, Surrey. A quick trip there and I had bought the carb (right)
It's a really nice compact unit which amazingly bolts straight on!! Another bonus is the choke pictured below doesn't actually choke the carb, it adds fuel instead which may be handy for an on-boost mixture enrichening device...
In the picture to the left you can see the air filter supplied by PedShed with a 45° elbow fits really nicely
One throttle linkage. This is a simple bracket bridging a carb stud to a tappet cover screw. A 'P Clip' tightly grips the cable end. The modified cap head bolt takes the cable end and pivots in the carb throttle arm.
Cable movement is provided by a Daytona quick action throttle from Monkey Bike UK.
I need a vacuum takeoff to operate the dump valve. This needs to be in the inlet manifold..... but my bikes doesn't have an inlet manifold. I'm going to try to drill and tap the heat spacer to take a pipe fitting.
The adjustment screw for the regulator protrudes through the right hand side of the frame which will be needed for setting up.
In this picture you can see the coil, ignition unit (CDI) and voltage regulator can be squeezed in here too, with a little modification
I had a can of aerosol paint in the cupboard I used for another project. It turned out to be orange! To the right you can see the threaded bosses welded on to accept the intercooler mounting
Upfront is a fat 120mm tyre on the 3.5" wide 10" rim. It's rather rude and only just fits between the upside down fork legs
Lots more going on here than normal. On the left is the air filter and the gold unit is the turbo's wastegate actuator. On the right is the turbo exhaust.
Next job is getting the front caliper to clear the wheel and mount the disc to the hub.
Now on to the front end. I'm using a Honda Nice disc and caliper with the original Dax hub and 3.5" wide G-Craft wheel. This is no simple bolt-together job. I decided to press ahead using the wider rim because it's difficult! There's no major gain using such a wide rim and in fact some people say it might actually effect handling adversely. We'll see. I just wanted the challenge. The problem is caliper clearance to the wheel. Offering it up it's clear the caliper will have to be ground. This weakens the caliper so replacement material will have to be welded in.
First off the hub was modified and a boss to take the disc was machined - see right
The boss is bolted to the hub from the inside where the drum brake used to be (right)
Here's the disc in place (below right)
The 120mm tyre just fits between the fork legs. There's a tiny gap but a gap's a gap as they say! (below)
Having removed so much material, strength is always going to come into question. I have significantly beefed up the fingers and their construction to the main body of the caliper.
Fear not for my life, this is plenty strong enough!
A gap's a gap after all!
I gave the frame a coat of metallic blue, the shade I'll have the bike painted professionally finished in. I followed this with a couple of coats of lacquer. Halfords aerosol paints go on really nicely. They recommend leaving the lacquer two weeks before flatting and polishing
The last job before the road build will be to slosh seal the frame to make it completely fuel tight. The inside has been prepared with shot blasting and will use top specification sealer.
Below: Shortly after this picture was taken I realised I'd forgotten to put the coil in there. After some jiggery-pokery I managed to fit it in too (after I introduced it to Mr Hacksaw....) A layer of heat proof lining was laid in here and the engine fitted with everything upturned which helped prevent the coil, CDI and regulator move.
Above and above right: Initial engine fitment. Below: Beginning the fuel tap and fuel pressure regulator plumbing.
Above and below, intercooler and oil cooler fitted with the engine in a running state
Above is the intercooler as it arrived. Below is the oil cooler as fitted to the bike. It's not up to the standard of the intercooler so it's being remade. When I'm happy with the quality I'll remove this picture, take a new one and tell you who made them!
.... and that's as far as the project is as of 17th May 2008. Stay tuned for more updates! - Ferg
This project is on the back-burner at the moment due to the turbo having sprung an oil leak and needing returned to America for rebuild. Oh and it didn't really work anyway!
