Bike with afterburners

This go round, we will be taking a look at the controls for the bike, and the afterburners. We have been very fortunate to get some good bars from a new sponsor at www.convertibars.com, and I can tell you that their handlebar kits are one of the coolest products I have seen in a while for bikes. These things literally let you set up your bars just about any way you want them. Plus, they just look really cool!

New front end setup

I wanted to make sure that the front end would be rock solid on the bike, especially since the bike was never really designed for speeds higher than 60 MPH when it was originally built with its little 50cc engine 20 years ago. To start, I picked up a billet to triple clamp from P&H Machining out of Tukwila WA. Since the original top triple didn't actually clamp to the forks at all, this is a step in the right direction to getting a much tighter front end. They were even kind enough to machine Bad Bros. into it for us, which makes it look just that much more trick.

Handle bar setup

The new bars from Convertibars now mount underneath the top triple clamp directly on to the forks. The Cyclops clamps allow the bars to be raised, lowered, turned, and twisted in an almost endless variety of configurations. This really helps in getting the right riding position for a big guy like me. The only downside is that they do not make them in sizes small enough to fit the YSR forks. Gary Richards from www.garysjetjournal.com came to the rescue, and machined a set of custom spacers for me so I could get them wrapped around the forks just right.

Clean setup

The setup is now clean, and strong. It definitely looks much better than the original parts, and since they are aluminum, they are much lighter as well.

Clearance issues

After installing the bars, I did find a few issues with clearance. Since the bars now have an upright post that goes down into the Cyclops clamp on the forks, the post wreaks havoc on the front fairing mount. This is an easy fix though, and once everything is mounted I will simply take a measurement and cut them a bit shorter. No harm, no foul, and they will work just the same. I guess they were never really designed for such a small bike as this, but modifying things is easy. You can always make things work out with just a little tweaking.

Billet throttle

Since I was upgrading the front end, I also added a nice billet aluminum twist throttle finished in anodized red. The throttle has a billet body and tube, so it is really nice and sturdy and should keep from breaking easily like the plastic tube throttles are prone to do.

More room for controls

The new bars are also a bit longer than the originals, so I can get better leverage while driving. It also means that there will be a bit more room to add all of the controls and switches needed to fire the afterburners and everything else on the bike.

Afterburner fabrication

I decided I wanted to use a dual afterburner setup on the bike. The dual cans will give a much larger area in which to slow down the hot exhaust gasses so that we can mix in the fuel and get ignition. They also just look really cool! The dual exhaust is very reminiscent of the dual AB setups on military fighter jets.

Hand fabricated cones

The exhaust cones on the afterburners were first designed in Solidworks to get the proper shape and profile, then a template was made out of posterboard using the dimensions from Solidworks. With a template ready, 16 gauge steel was rough cut and then shaped to the final size with the angle grinder and flap disc method shown in earlier updates. The cones were then mig welded to the outer tubes which form the body of the afterburners. The tubes were supplied by Aaron Morriss, who also cut the vinyl graphics for the bike.

Inlet expansion rings

The incoming exhaust pipe is a 3.5 inch diameter, and the afterburners are 5 inch diameter pipe. To make the transition to the larger afterburner body, stainless steel rings were laser cut and welded to the rear of the main afterburner assemblies. Special thanks to Chris Krug for the laser cutting!

Finish welded

Since the afterburner will be very hot and have high pressures exerted on this junction, a nice thick weld bead was laid down to attach the rings to the main AB bodies.

Ground welds

The welds on the cone ends were finish ground to produce a nice clean appearance. An important feature of the afterburners is the propagation tube, which is visible in this photo. The tube connects the two afterburner sides, essentially making this one large AB unit. It also allows the afterburner ignition to be placed in only one side. When the first side lights, the higher pressure quickly pushes the flame front through the propagation tube into the other side causing ignition there as well.

Propagation tube

If you look carefully inside the left exhaust cone, you can see the opening for the propagation tube which connects both pipes. The tube is 2 inches in diameter, and fitted, or "fishmouthed" to fit flush with the inside walls of the afterburner.

Flame holder

Looking straight down the afterburner, the flame holder is visible. The flame holder is designed to cause the air / fuel mixture to rush around it while traveling past, and make a low pressure area in the back side of the flame holder. It is in the low pressure area that ignition is achieved, and the air velocity slows to the point where a flame front can be kept in position. Without a flame holder, the flame front can be pushed right outside of the afterburner.

Flame holder from entrance side

This afterburner design is a stepped design, and the transition from small exhaust pipe to the larger afterburner pipe also creates a low pressure area which acts as a flame holder. The additional angle iron flame holder in the middle is to make sure that good fuel air atomization takes place and as a back up to the step design.

Making the connection

The afterburner, seen here resting on the rear tire, has to connect to the exhaust or "jet pipe". Before making the Y-pipe to split the exhaust, I wanted to check the fit to avoid any clearance issues.

Size compared to rear tire

The size of the afterburner as compared to the rear tire shows what a beast it is. It is going to hang off the back of the bike quite far!

Extra long

The length of the bike is about to grow! That trailer we have been looking at will probably no longer hold the completed bike. I guess we will need to go ahead and try to pick up a large model which will house the bike without removing the afterburners!

Size in relation to the engine

Overall, the afterburner assembly is about 1 foot wide, 5 inches tall, and 2 feet long! The good news is that since we used lightweight material, it doesn't weigh too much.

The Y-pipe

By cutting and re-welding 2 90 degree exhaust bends, I made a Y-pipe to separate the exhaust gasses coming from the single main jet pipe. This will feed the afterburners, and fuel injectors will be mounted into both sides of the Y-pipe right before the step to the larger AB bodies.

Final fit before welding

The whole assembly was final fit using a laser to align everything before welding the Y-pipe to the main exhaust. The AB assembly sits a bit lower than I had originally intended, but will be in a better position mounted here.

Close up of Y-pipe

The Y-pipe construction can be see a bit better here, and it is very simple. At this point the end of the Y has not been cut to fit the main jet pipe, and is only held here to get measurements. It was cut and welded right after this photo was taken.

Afterburner bracket

With the afterburner welded to the jet pipe, I had to make a nice looking bracket to hold up the afterburner, as well as give it something to push against when it is in operation.

Rigid truss

Although I didn't take any photos of the construction, you can see the end result here. The bracket mounts to the frame right underneath the rear body panel, and also has a bracket to clamp to the jet pipe right before the Y-pipe. This gives the afterburner full support, and the bike can actually be lifted by the afterburner with this bracket installed.

Exit from body panel

The bracket exits from the body panel in the original location of the stock tail light. The plan was to fill in the tail light opening, but this actually worked out to our advantage, and keeps a nice clean look.

Cross braced

The truss is triangular in shape to give it the best strength, and has braces running from side to side, as well as top to bottom.

Needs cleaning

Please excuse the mess, as I haven't finish ground the welds, or even bothered to clean up the smoke spatter from welding with flux cored wire!

Pipe clamp

Looking down from the top, you can see where the truss bolts to the frame. The bracket extends to the point right under the rear seat where there is a large U bolt that goes underneath the jet pipe to secure it in place.

Finished bracket

The finished bracket looks clean, and is fully functional. It holds up the weight of the afterburners beautifully, and is also able to transfer the thrust to the frame as evidenced by our testing with someone pushing me on the bike!

Bike rolled out for photos

Since I had done most of the work in the garage (which is still messy from the recent move!) I thought I would take it out in the driveway for a few better photos.

Still need a battery box

I still have to build a battery box on the rear swingarm to house the numerous batteries that will run the pumps. With the afterburner mounted, I now know how much space I have for the task. I'll be starting on that very soon.

Side profile

I think it may be a little longer now! I tried to measure the bike by myself, and my best measurement I could get was 7 feet 2 inches long! The afterburner may stick out a little when I ride it on the public streets.... well, just a bit. Maybe no one will notice!

Bumper melter

After years of riding bikes, I know that at some point I will get a car that rides up on my rear tire really close at a stop light if I take it out on the street. This would be their view. That is, the view they see right before I melt the bumper off their car! Automobile drivers take note, that the biker in front of you may have a jet engine with an afterburner so stay back! Just kidding, the electric motor mounted on the rear swingarm is actually there to help with take off in traffic if this situation actually presented itself. It provides a safe way to get a little more takeoff momentum without firing the afterburner and making toasted cars.

Throttle valve

I wanted to go ahead and show this here. This is the throttle valve that was machined for me by Gary Richards at www.garysjetjournal.com. It attaches to the throttle cable coming from the new twist throttle and works pretty well. With the throttle in the full off position, it looks like it does above. (sorry about the blurry picture, a new camera is on the way)

Throttle valve

This is the throttle valve at wide open throttle. The cable pulls the lever down which acts on a needle valve. The needle valve has two outputs, and as the valve closes it restricts the flow of fuel to the bypass, or tank return. This forces more fuel out to the injectors. Simple, but complicated at the same time. The only thing that matters is that it works.

There isn't much left to do on the bike at this point. Lots of little brackets to fabricate, and pumps to mount. Then it will be time to get this thing off to sandblast, paint, and powdercoat. I can start to smell the jet fuel burning now, and that wonderful sound of a turbine whine. Please stop back in soon for more updates, and be sure to support the site by visiting our advertisers whose products interest you or by making a direct donation. Your help keeps us going to bring you these interesting projects.

Russ W. Moore
and
The Bad Brothers Racing Build Team

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