question on offset crank

Sorry hobot, I must confess that my interest was geared towards an offset crank in a Triumph twin and my statement on costs were in regards to parts available for Triumphs. Johnson cams said his cam grinder doesn't care whether it was grinding a std. cam or offset, so at the time I asked the price was the same. Pazon has EI for 90* offset cranks and Trispark is easy conversion, so if you're going EI anyway then also no difference. So the biggest outright cost is the crank itself at around $1500.00 to 1800.00 depending on who you buy from. If you use a MAP crank and purchase his pistons and rods, he throws in the balance job...not to shabby.

I talked to Geoff Collins years ago and although a viable option, the pricing isn't such that I wouldn't save a bit longer for the new billet. Geoff was very forthcoming and nice to communicate with and his crank work is worth what he's asking, you can't really do that kind of work any cheaper.

I didn't intend to hijack a Norton thread and turn it into a Triumph post, not my intention. I'm keenly interested in what goes on in the Norton community and keep my eyes open for a Commando 'deal'. I was only hoping that maybe 'beng' would be able to add some personal experience to the offset crank comment as I know that the topic interests more than just me. Thanks for the input ...Mark

OHhhhOK, sure, Offsets are right up ole Truniphs' alley and Kawasaki's too : )
If engine orbital intensity is an issue then everything about offset cranks makes more sense than a 360'. I've only been to a few vintage races but the obvious most powerful fastest vintage I've seen so far was a dang ole Truimph. Double bargin with the off the shelf components. Norton cams must be cut and twisted so why I thot might cost more. Go out a make some nosey fast memories, without much buzzy blurry sense.

[quote="beng")

The sweet spot for vintage British parallel twins is to just put them together to run as good as they can within their mechanical limits, just enough compression to do the job but not bend the crank or overheat it, and optimizing the cam, intake and exhaust tuning to make the most torque under 6500 rpm, where it's design can operate with a minimum of distress.

So just a nicely tuned and optimized, mostly stock engine. Instead of changing the Norton twin to compete with modern machinery, change your mind to fit the Norton twin as it is, and put your time and energy into enjoying what it has to give and has given many for decades, simplicity and competence as daily transportation and sane sport riding.[/quote]

Ben G you have made what I consider to be the definitive statement for what just about everyone on this Forum can understand and appreciate. Well said.

Mick

Beng, thanks for your clarification on why you think offset cranks are a gimmick, I never thought of a crank as a beam whose actual length was long and bendy when unfolded, good stuff. In my mind the vertical sections (webs) if stout enough and with large dia. main and rod journals would also see forces in compression and tension and less in bending than the horizontal pins. I'm not sure if the extra beam length from offset in a billet crank would be more bend prone than a stock 360* bolted together 3 pc. crank. I also understand the reasoning behind getting the flywheel wt. as close to the PTO side of crank as possible, makes perfect sense.

I will gladly admit I am not knowledgeable enough to argue the virtues of the 360* vs. 90* (or other offset) so your take on the subject has added to my knowledge base. However, I do feel that from what I do understand, the halved primary imbalance from splitting the reciprocating wt. and not having both reciprocating masses stopped at TDC and BDC at the same time is a good thing, the fact that the energy from the fastest moving piston at 90* while the other is stopped at either BDC or TDC, helps maintain a more constant crank energy/speed, also a good thing and if the result is a smoother, more enjoyable big twin then yes, I guess I would consider that a win. Will this make a rice rocket beater out of my sows ear? Of course not, but that isn't the reason for the original post is it? The desire to create a better or different anything isn't always based on what would be the most rational reasoning, there are those who love to tinker and the original quest was for improved performance not whether or not "Should I keep my Commando stock?" Once you suggest higher compression or lighter pistons, longer rods etc. you've started down that slippery slope, so keeping them stock or within the original design parameters is for another post.

Remember, some of us have had these bikes before they became vintage and they were rarely ever left stock, I didn't want a stock one then any more than I want a stock one now. I will restore my '66 Bonnie to stock although it never has been since '68 when I bought it, but I think stock would be nice since I have other Twins that aren't stock and never will be. I'm off my soap box now and think that I've learned more about the original offset crank question and appreciate all the input, but in light of this info my poor little brain is thinking O.K., if Jim S.'s lt. wt. pistons and rods are the answer then maybe his stuff plus a 90* crank would be double better...hobot gets it, it's all for fun anyway...Mark