59mm crank breakage?

[manden]

hey dudes? at what rpm does your 59mm cranks normally break? i have a good offer for an 59mm crank. but if it breaks at my rpm levels then its not worth it anyway.

[Gixer1460]

Who actually knows? It depends on what rpm you are trying to achieve and for how long you want it to live? I've got a full billet EN40B 65mm stroke crank with some mods that should make it stronger - it is designed for turbo use ie. lower peak rpm and I wouldn't want to turn it more than stock redline, it had a twin that went into a sidecar outfit - those guys like rpm but how long it lasted is anyone's guess, maybe a race, maybe a season? Maybe if some steel caps could be built into the cases to minimise any crank flex, reliability at rpm may be improved?

[manden]

I was aiming for 12500rpm. Seems to be what the cams and head likes.

[Gixer1460]

3 hours ago, manden said:

I was aiming for 12500rpm. Seems to be what the cams and head likes.

Sustained or Redline through gears? I'd say its potential life would be short with those rpm's if sustained!

[manden]

Don't think sustained. Top speed in fifth will probably be just below 12. Maybe a little more down hill.

[Oilyspanner]

What have you done to your head ? hard to get the heads to work well at a true 12000 rpm on 1127/1186/1216/1246 etc without a huge amount of work - it was done for some extreme drag engines in the 1990s. Maybe best to get a billet crank if you've got repositioned ports, loads of porting and big valves etc - costs loads to replace. Standard tacho reads about 12000 revs a limiter - shows 10800 revs on dyno then hits the limiter. 

Are you road racing your bike ? sounds an interesting build !

 

[manden]

i bought an unfinished project, and its almost finished. little electrical problems left and charging relay problems. but fully build engine with busa rods and 78mm pistons from an modern endurance racer. fully ported chims head that i thought was on standard cams but found out it had mild race cams. it rolled 111.5kw at the wheel on the lowest pull. that was with 36mm carbs. so hoping for alot more with 40´s or 42´s and mappable ignition. i redlined at 11950rpm according to the dyno. only showed 12300 on the tacho actually. i saw an 750 1989 article where the tacho showed 1000rpm too little. so its probably how well the tacho was made. lucky or not kind of thing

[canamant]

That rpm is a mean piston speed of 4900 fpm which is well into race engine territory and associated component life.

 

[manden]

I know. Pistons and rods are up for it. Just don't know about the crank. Maybe I'll just stay on the 58mm for safety.

[Oilyspanner]

That's good going with a 1109/1127 engine - about 150hp !  Have you got the dyno graph ?  - would be good to see how the engine combo makes it's power. What size valves does your head have ? The only problem with going to larger carbs is losing power below peak and having a 5 speed gearbox - normally oilers go for wide torque band, then you never really fall out of the power. Have you got photos of your machine it sounds a great project Manden.

 

[manden]

I'll see if I can find it. Took a picture when it rolled