The technical adventures of Dr_C

@Spanner: You're not the only one here capable of some serious CAD!

The forging allows up to 29mm length of the crank pin press fit. That's a humongous increase of the stiffness, to use a scientific term!

bushmechanic said:

these are the calcs to get the BF of the stock crank assy,

interestingly total rod mass is not included in the numbers however it is taken into account as part of the physical im-balancing process.

Little end of rod = 137 gm

bob weight = 148 gm

piston + pin + rings + circlips 369gm (all OEM 2008 "650" parts)

(reciprocating weight) x (balance factor) = (bob weight) + (little end of rod)

(369+137)xBF = 148 + 137

so for a BF of 0.62

(369+137)x0.62 = bob wt + 137

bob wt = 176.72gm

so

Stock 80mm crank at 0.62BF "balances" on knife edges with:

bob wt 176.72gm
big end bearing 45gm
rod 373gm
*OEM pin 347gm*

expressing the imbalance of the 2 crank webs alone for the oem parts at 0.62BF is

941.72gm total x 40mm = 37668.8 gm/mm

using the same process for my 105mm build with a theoretical BF of 0.62 the imbalance required is 977.86 x 40 = 39114.4gm/mm

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Super, Bushie! Million thanks! I will walk the numbers through this weekend. To be able to design the new crank, I need an required unbalance torque figure for crank webs and pin together.

spanner said:

Nice DrC, isometric as well, but it is not on the ubiquitous back of the envelope:

I am of the opinion that any custom part needs to be preferably lighter than the original if it is to be able to survive in the RST engine.

Would a 29mm thick boss around the big end pin area work instead of the whole flywheel being 29mm thick?

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Ah, I'll use a paper napkin next time to "level up"!

I'm not sure the total weight of the crank is so critical for the mains. I t sure plays a role concering the rotating mass and the characteristics of the engine. However the unbalance is most important to get as close as possible. But don't worry! I will try to get this design simulated in Ansys (FEM calculations) to help reduce material wherever it is not needed. The area supporting the pin needs to be 29mm, elsewhere is totally up to unbalance and FEM.

@Bushie: My 61mm tall crank pin weighs 337,3g without alu plugs in the ends. I suppose the new one will be 70mm tall. The new will weigh (337,3/61*70) 387g. Together with plugs (10g for both) I suppose the new crank pin will max weigh 397g. We will figure out if there is some weight saving to be made with heat treatment and/or cone shaped hole (piston pin style).

Ran out of coffee today. Scary sh*t! But I handled it...

Yup. Sounds about right!

Swedish language has evolved during a heavy influence from both german and french language. We spell coffe "kaffe" just as danish and norwegian people do. Our finnish friends language comes from a different origin and is impossible to understand without training. However, the most coffeedrinking people in the world (Finland) spell it "Kahvi", if remember correct.

The dished top of the piston comes from my cylinder head being very flat. With this years pistons I have lowered the dish (to get down to 13,5:1 in comp.ratio) and raised the squish area (to reduce squish from 1,6 to 1,0mm). So now I have no knocking and no noice-clerk crawling up my... exhaust system. [emoji4]

It sure feels strange. It's only Sunday night before a race weekend, and the bike is already together and test started. No problems either... I got a bad feeling about this!

canolman said:

Good luck Doc![emoji14]
As a matter of interest what sprocket sizes are you using?

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Top speed on this track is only around 170-180 km/h, so have geared it 15-36. I have 34, 36 rear and 15, 16 and 17 front, to choose from. This low gearing makes the front wheel light! [emoji4]

spanner said:

Well the celebrations must have been good

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Hm. I love that conclusion! The truth is far from it...

I am a lousy, reckless, clueless mechanic!!!
Let me explain why.
When race prepping a set of cases, there are a lots of plugs, bores, threads that need to be taken care of. The "new" cases I wanted to use had an oil pump installed, but no plastic gear. When it comes to assembling the clutch, I found some signs of wear to my old version hub. I thought I might have a clutch lying around with that nice hollow pin design. So I started digging into my boxes and shelves, and found all washers, pins and stuff to make it complete! Then it was lunch. Back again, happy about my findings, I tightened the nut and the springs. The test start went surprisingly well and we set off to Norway (some 7 hour drive). For the first training session I was VERY cautious about not putting too much load on the engine before coolant temp was well over 60 degC. That took longer time than usual, probably because of my new coolant system (twin parallel coolers). After some 4 laps temperature was close to 65 degC, so I decided to open up the throttle a little. After on straight and a chikane, the power dropped and within the wink of an eye the engine seized and at some 140 km/h the rear wheel stopped! I held it upright and rolled into the pits. The anger and disappointment was at a level unheard of.

I had another 7h drive back home to figuring out why the engine seized. The next day I disassembled the engine to confirm my suspicions. The gear for the oil pump was never mounted...

I know all the golden rules about mechanic, assembling, testing and still I managed to make such a shameful Altzheimer rookie mistake! :furious:

So. New piston, sleeve, new intake valves and guides, refurbished rod and crank, is what this ***** mistake cost me. The cylinder head is strong and was intact and I didn't crash, if trying to see something positive in this black hole.

All efforts are now being done to get the engine back together in time for the races first and second of August. hopefully with some practise before that.

Please comfort me!