Jerry, I know little about the offset crank except for what I have read from people who have degrees in such engineering endeavors that concern engine design etc. If you google Yamaha xs650 offset cranks, read the posts there and follow any pertinent links, you will know more than I can remember or try to explain. Phil Irving tried to interest the major Brit bike mfgs. back in the good olds in his 76* offset crank idea but to no avail. He was principle in the design of the Vincent V twin. Since then 90* is generally deemed to be ideal (rod to stroke ratio comes into play here) but please read the site I referenced because I can't explain it as clearly or as intelligently as those that know what they are talking about but I'll try:
In a 360* or 180* twin cylinder crank configuration both pistons are either stopped at TDC or BDC and the inertia in the now stopped pistons/rods is transferred to the crank which speeds up and slows down as this scenario repeats..in an offset such as 90* crank this can't happen because as one piston is slowing the other is reaching max piston speed 90* later and apparently this energy is transferred to the other piston assy. and not into the crank...never are both pistons stopped at the same time. Kevin Cameron has written of this in Cycle World, the how and why of cranks other than 360 or 180.* Ducati goes to a lot of design considerations and frame design compromises to maintain the perfect primary balance that a 90* crank offers. In a 90* parallel twin compared to the 360* version, primary imbalance is halved, secondary imbalance quartered at the small expense of a new but small rocking couple whose severity is determined by distance between crank journals among other factors. The worst rocking couple comes from a 180* crank as in the Honda CB77.
Yes, apparently using an offset crank does have performance advantages, one being the ability to use a lighter crank allowing for quicker acceleration, less abuse to crank bearings and cases and lower vibration levels transmitted to the rider and components.
In the CB77 case, I was mainly interested in doing the offset conversion as an interesting exercise with a smoother less buzzy ride at higher rpm's the main goal, the exhaust note of small V twin secondary goal and perhaps a small performance increase. Having the first 90* cranked CB77 would also be fun especially if little cost was entailed which appears to be the case...Mark (take any of what I've posted above as just my take on the subject, in my case a little knowledge is dangerous)
CB77 offset crank?
Dear Mark and others, This topic can be an absolute can of worms depending on what is looked for as an outcome. Firstly Phil Irvings greatest contribution to us motorcycling people in my opinion is not the Vincent, nor the Repco V8 race engine but his books such as "Tuning for Speed". These books have helped many of us in our various quests for all kinds of outcomes. I think that as a project to re phase the crank would be a wonderful project Mark. The sound that motorcycle engines make is probably the most identifying aspect if one cannot see the machine. I know when one of my Hondas are running just from the sound. It is why I like the 180 degree phasing. It sounds like a Honda 180 degree twin. Now we come to crankshafts and the way they behave. In a Honda twin whether 180 degree or 360 degree we have 4 main bearings and the tendency for the cranks to bend and rock is minimised. I know it is still not ideal but a lot better than a 2 main bearing 360 degree vertical twin where the bending moments can be out of control. There are many out there who believe a lightened crank is a wonderful thing. I personally disagree as I cannot see advantages. I run heavy cranks in the race bikes. One loses Kinetic Energy in the crank which will lose top speed and hamper throttle control. One also loses the cranks ability to absorb some of the imbalance variables. This would apply regardless of the phasing of the crank. As to a buzzy feel in the handlebars that we have also experienced believe it or not cloth pushed in very tightly solved the tingles in the clip ons. I have also heard of lead being put into the ends of handle bars. Lee and Dave Kernich who are friends of mine here in Australia raced a Triumph with a phased crank with great success. However they never convinced me to do the same. All the best Jerry
CrankMark, THANK YOU for starting this thread. It is fascinating reading.
With no disrespect to Jerry, I hope you will proceed with the project as I am sure it will a great learning experience for us all, not matter what results you get.
I agree Jerry with the heavier crank being a positive, Todd Henning among others learned this through much testing and experimentation. But the offset crank does negate some of the benefits of the heavier crank since the conditions that make the hvy. crank desirable perhaps aren't as prevalent in the 90*...at least from my limited knowledge. Anyway, I wasn't advocating changing the Hondas wt., just giving an example of what may be considered a performance plus, possibly. Yes the Honda has more main bearings but with an offset crank they would take less of a beating, the sound thing is something I can sympathize with but maybe one would find the new ex. cadence just as sweet.
Yes, there seem to be quite a few Aussies running offset crank Triumphs (among other makes), some 76*, some 90* and one that I have been in contact with says he wouldn't go back to 360* as do most of the Yam xs650 guys on their forum. I don't propose taking a stock, collectable Honda and messing with it. I just happen to have an extra frame and engines and thought a small, light, big bore, cammed, 90*cranked CB77 would be pretty cool..then I could compare it to my stocker. The opinion here on the forum would seem to indicate that the crank rephasing would be easy enough and cams and ignition also seem manageable. The only thing stopping me from pursuing this project is to many other projects but I'm still inclined to play with the crank and cams as a diversion. I see the biggest cost with this conversion (given the crank can be easily reassembled at any phase as is) is the cost of having the assy. dynamically balanced @ 50%. I don't see a big bang Honda as near as desirable as the offset crank, I'm not sure the CB77's primary drive would appreciate it either. I'm glad some of you have found my post interesting and I wish I was in a position to give the project a go so we could all see the results, maybe someone else will get excited and run with it. I'm still trying to decide wether or not to put a rephased crank in my Triumph project, I've been accused of project paralysis and it's true but then these billet cranks aren't cheap...Mark Dear Mark, As an exercise to create something different I believe you should go ahead and do the offset crank. If I were running a 2 main bearing twin cylinder vertical twin for racing I promise you it would not be a 360 degree set up. One must remember however in defence of the British mtorcycle Industry of the time that nearly all twins ran a single carburetor and therefore 360 degrees was the easy way to go. The only advice I give regarding the exercise you plan to go on is please dont take any short cuts. All the best Jerry
Thanks for input Jerry, I'm glad to have your take on the 360* crank for the 2 main bearing twins. I'm leaning towards ordering the 76* billet for my Triumph but for the same monies do I stay with 82mm stroke or go with the 89mm, that's the question now. Yes 360* crank allowed for easy single carb set up with even firing order, another dilemma as my custom frame is designed around a single carb head.
Well Honda mfgd. 360* and 180* versions of the 305, a 90* version seems like a natural progression and I surmise would be a better engineering answer to the balance of a vertical twin, esp. one with a large distance between crank throws such as the Honda. I am still surprised that, if indeed the crank can just as easily be pressed together at any angle, no one has tried it. The Yamaha xs650 has s splined center pin so the closest they can rassemble to 270 (90/270) with stock parts is 277* which is so close that I doubt one could tell the difference. With the purchase of a custom pin $$$ they can reassemble the xs crank at 270*. Jerry, thanks for encouragement, if I get a chance to pursue this, I will bring it up on the forum again and ask for more advice. Carb intake length and carb type & size is one area of change I would be considering since I don't believe my spare engines have carbbies.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||