Yamaha DT3 - Small Garage, large ambition

[TresCuatros]

I keep updating the wheel/fork section with more technical info, so I figured I'd parse out the info a bit. I use this page a lot for recording thoughts/details online.

Electrical:
Considering Rex Speed Shop 12v & CDI conversion
Need Flywheel Foto1971 (74 DT 250) or F1T350 (late 70's 250)
Once the flywheel has been acquired - call back to Rex and order the matching Rmk # CDI kit

Some people have success with the slotted stator and Foto2173 from the DT 360's, but Rex's was saying it's hit or miss based on who's installing it and may require some machine work. Best to avoid that rabbit hole of complexity and source a drop in 250 flywheel.

 

[TresCuatros]

Progress been slower than expected due to the bike being kept together for the California VIN verification (I purchased this bike w/out a title and want to register it for street use).
On Christmas Eve-Eve the lovely CHP officer was able to perform the inspection and now I get my new title issued - A true Christmas Miracle!

Over the past few weeks you have have noticed I've been going back and editing previous posts with new information. This was part of my ongoing research to minimize custom parts and find factory parts from other bikes to fit the build goals.

So here we are - finally getting to the good stuff.
I found that BMW K75 clamps have the offset and width I wanted for 41mm forks, so I put together the front end with a mock up-wheel from my Honda 450 flat track bike.
This is only a mock up since the BMW K75 steering stem needs to be pressed out and turned down still to accommodate the DT3 bearing ID. As luck would have it, the DT3 frame doesn't have many conversion bearing options, so I'm better off modifying the stem to fit.

Next up - find a brake rotor for my DL1000 wheels so I can set the caliper and make the correct wheel spacers.
Looking for a 5x91mm bolt pattern, preferably 296mm OD rotor. No luck yet - so may go with 300mm and pray, or a 296mm rotor and make a wheel adapter. Last option would be go with the 310mm DL1000 rotor and have a caliper bracket made to fit.


Edit:
EBC Brake Catalog / Buyer's Guide: https://ebcbrakes.com/Assets/ecatal...talog-Jan-2020/US-Issue-302-Full-Catalog.html
EBC Front Rotor Dimensions for race bike application: http://www.eurospares.com/graphics/brakes/mc-front-rotor-sizes.pdf

 

[TresCuatros]

Ordered a used 04-05 GSXR Rotor for mock up. If I like it, I'll buy a brand new one for the final product.
300mm OD with matching bolt pattern to the DL1000 wheel (5x91mm). Should be close to the 296 diameter the CB600 fork/caliper was designed for. Big question will be getting the caliper aligned with the rotor since the CB600 has an offset rotor and this DL1000 wheel will not. Fingers are crossed I can slightly modify the caliper bracket spacing to align.

I also figured that since I'm power coating the wheels before final assembly, I didn't want to mess them and the $30 bearings up with pressing in/out a few times. So I also bought a set of cheap 42mm OD, 20mm ID bearings that I'll grind down a little for a mock-up only set. This'll allow me to throw the wheel and brake on to size up the correct spacers. Should be about a week before I can get to it.

DL1000 bearing installation procedure: https://www.vstrom.info/Smf/index.php/topic,4250.0.html
Note: Gap between bearing race and hub acceptable - designed to have longer space than hub seat width

 

[TresCuatros]

This is becoming quite the endeavor!
I'm converting the A. Graham Bell equations into something I can use. Turns out there were some author-freedoms exercised in the equation prints, so I've had to debug some of the details along the way.
They don't take into account exhaust temp, so I'll be modifying the tuned-length equation to ensure it matches the EGT's once I get some HP predictions/estimates and header temps.

 

[Jimbonaut]

^^ needs more ekwayshuns

 

[teazer]

Just a point of data:

EGT is the temperature a short distance from the exhaust port and as such it tends to be higher than the rest of the pipe.

Tuned length is based on the speed of sound on average throughout the chamber.

 

[TresCuatros]

Just a point of data:

EGT is the temperature a short distance from the exhaust port and as such it tends to be higher than the rest of the pipe.

Tuned length is based on the speed of sound on average throughout the chamber.

Ah, thank you.
Do you have any input on what typical temps could range?
I'm using my 70cc scooter as a test bed for turning the equations into real results, but my guess is the pipe scooter temps will be lower since it doesn't put out as much energy as the DT3.

 

[teazer]

I would just use the default value which I think is 500 degrees C but whatever Bell used is going to be in the ball park. There are all sorts of software out there using the Jennings/Bell/Blair equations and they will all get you close to the right answer. They all acknowledge the variables but none that I have used provide a lot of guidance as to where in a range to chose and what the consequences might be of selecting say a first divergent cone angle at the small end of the range and say the higher end for the second divergent cone.

What some of us end up doing is to start with a pipe design and then run simulation software for the base and then for each of dozens of of the available choices. That all takes time to optimize but it's cheaper and quicker than the factory had to do back in the day when they had to fabricate and test dozens of actual steel pipes. That would be a little tiring.

 

[TresCuatros]

I would just use the default value which I think is 500 degrees C but whatever Bell used is going to be in the ball park. There are all sorts of software out there using the Jennings/Bell/Blair equations and they will all get you close to the right answer. They all acknowledge the variables but none that I have used provide a lot of guidance as to where in a range to chose and what the consequences might be of selecting say a first divergent cone angle at the small end of the range and say the higher end for the second divergent cone.

What some of us end up doing is to start with a pipe design and then run simulation software for the base and then for each of dozens of of the available choices. That all takes time to optimize but it's cheaper and quicker than the factory had to do back in the day when they had to fabricate and test dozens of actual steel pipes. That would be a little tiring.

Over the weekend I looked into the affect of assumed air-temp in the chamber and it has pretty drastic affect on overall tuned length.
For instance, assuming 500 deg Celsius @12,000rpm with 200 deg exhaust port timing, I get a tuned exhaust length of 762.8mm
When the exhaust is cooler, say 260 Celsius, the tuned length is 639.1mm.

That's a pretty big difference!
Considering how outside air temp, material thickness, and energy expelled may affect the pipe temp, I'm beginning to understand why 2-strokes are so finnicky while warming up. This also makes me want to research actual exhaust temps for similar builds before I assume something too far off.

Looking back at A. Graham Bell's numbers, it looks like he assumes an air temp around 375 Celsius to get a speed of sound at 510 m/sec. So his version puts the tuned length of the above example at 709mm.

Food for though. Something I'll use the next scooter pipe to sort out I'm sure.

Edit: I think I can also use the stinger exit OD from the chamber as a way to tune temps. I'll have to watch for piston temps too though at that point. Fun fun!

 

[teazer]

Stinger diameter and length do have an impact on pipe temperatures, so try to avoid going too small. Stick with Bell's recommendations. He did the research on different pipes.

There are many other factors that effect pipe temperature. Heat loss is more complex, so I would ignore that until you are up around 200 bhp/liter. At our moderate state of tune, the pipe should not be all that hot (we are talking middle of the pipe here, not surface temp and we are talking mean temp as waves travel down the pipe and back.

For example: The higher the state of tune (high BHP/Liter numbers) the hotter the exhaust will be. Retarded ignition timing means more fuel is burned in the pipe and higher pipe temps. Two strokes are finicky little things for sure and run best when everything is spot on and the motor and pipe are warm. Pipes warm up more or less instantly but the motor not so much.

And the nmore power we squeeze out, the narrower the band over which it operates efficiently and that in turn needs narrower gear ratios, so we need a wide power band on the street unless we add power band stretchers like exhaust valves, programmable ignitions etc. Then we also need larger crankcase volumes and larger reed cages and so it continues.

Keep having fun.