Balancing a engine to match a frame ?

acotrel · Sep 10, 2015

Anything I've said about numbers for crank balance factor have been in reference to a dry crank. I think it is not surprising that filling the hole in the bobweight of an 850 crank with steel gives almost perfect balance factor for a rigidly mounted standard motor revving at 7000 RPM.

jseng1 · Sep 11, 2015

marinatlas said:
Hi Jim , thanks for the pic of your radius crank cheek.....may be more sketches could be usefull for us! I had tried to do it on my Seeley crank , but not sure if I had done it in correct manner?

Here's the way it should be on a new crank.

Below is a heavily modified stock crank with the .090" PTO radius.

lcrken · Sep 11, 2015

And one more example.

Cut in a lathe using a .090" radius carbide tool, then polished with wood dowel and emery paste.

Ken

marinatlas · Sep 11, 2015

Hi , thanks for your pics and advises........will do it , on the next crank!

Rohan · Sep 12, 2015

jseng1 said:
Note that Yves switched to lightweight pistons because his crank broke with the previous heavier pistons.

Thats an interesting statement.

So does that mean that all Commandos might suffer from this ? :mrgreen: :mrgreen:

74ok · Sep 12, 2015

related thread link
featherlastic-commando-t23050.html

Fast Eddie · Sep 13, 2015

Rohan said:
jseng1 said:

Note that Yves switched to lightweight pistons because his crank broke with the previous heavier pistons.

Click to expand...

Thats an interesting statement.

So does that mean that all Commandos might suffer from this ? :mrgreen: :mrgreen:

Yes. If revved high enough...

acotrel · Sep 13, 2015

With heavy pistons, I would have thought a rod would have the little end pulled off before the crank would break. My 850 gets revved to 7000RPM and the internals are fairly standard. It hasn't destroyed itself YET , however the look of the internals is the main reason it didn't get raced for a couple of decades. I couldn't believe it would stay together. I think Jim's long rods and light pistons is a great way to go.

Rohan · Sep 14, 2015

acotrel said:
My 850 gets revved to 7000RPM

7000 being redline for a (stock) road bike engine, we'd should add.
All being in good condition, you'd hope it would all hang together. !

Jagbruno · Sep 15, 2015

I revved mine repeatedly to 7500 on circuits with no problem whatsoever, but floated an exhaust valve at around 7800 at the end of a very enthusiastic run in Dijon where the bike was clocked at about 130Mph with 22 teeth sprocket and standard primary.

My engine is quite willing to rev high because of the PW3 cams, flowed head, seriously lightened crank, Dunstall light rockers and...standard compression ratio.

:mrgreen:

acotrel · Sep 15, 2015

On most 650cc Triumphs the upper rev limit is specified to be 6,300 RPM. Over the years in historic racing plenty have been revved to 8000 RPM. However most have destroyed themselves. Looking at the insides of an 850 Commando engine, I have fear and trepidation about going higher than 7000 RPM. I often see 7,500 RPM on gear changes and I really don't like that. If your motor develops plenty of torque, surely you don't have to rev the tits off it to get acceleration ? Somebody once told me that if you have a torquey motor you don't need a close ratio gearbox. Myths abound in motorcycling.

Bernhard · Sep 15, 2015

You will have to accept that if you continually rev an engine to 7K and beyond you WILL shorten the life of the crank- I did exactly that on one racing bike.

Jagbruno · Sep 15, 2015

acotrel said:
On most 650cc Triumphs the upper rev limit is specified to be 6,300 RPM. Over the years in historic racing plenty have been revved to 8000 RPM. However most have destroyed themselves. Looking at the insides of an 850 Commando engine, I have fear and trepidation about going higher than 7000 RPM. I often see 7,500 RPM on gear changes and I really don't like that. If your motor develops plenty of torque, surely you don't have to rev the tits off it to get acceleration ? Somebody once told me that if you have a torquey motor you don't need a close ratio gearbox. Myths abound in motorcycling.

Of course, you are right. I don't gear change over 7000RPM, ( most of the times around 6800 ) but I did rev it above 7500 in top at the end of the Dijon circuit straight which is slightly climbing. :mrgreen:

Eager engine. :mrgreen:

acotrel · Sep 15, 2015

I haven't raced at Phillip Island in recent years. However the thought of reaching turn one with the Norton on full noise at 7500 RPM after a full lap fills me with apprehension. If it lets go while doing that, it is probably 'bye bye'. When I ride these days, it is usually only ever at Winton which is 3 Km in length, for Phillip Island the motor has to be bullet-proof, any get-off is a really biggie. I love my bike, but I don't get lulled into a false sense of security.

acotrel · Sep 15, 2015

The trouble with this stuff is that the internal loads on engine components are a squared relationship with the accelerations involved. Even if I had Jim's long rods and light pistons, a billet crank and strong cases, I would use the same upper rev limit. The difference would be that I would stick my neck out more while racing. I've had seizures with two strokes while on full noise, however never a total disintegration. Doug MacRae's crash on Daytona should be a warning to everyone.

Balancing a engine to match a frame ?

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