Jensen
I wasn't suggesting that the big sprocket made the engine run smoother, just the drive to the cam. Operating the valves provides an uneven load so that the drive through the cam chain will also be irregular with changing chain tension as the cam rotates. Using the cam sprocket as an energy absorber will smooth the peaks in the drive through the chain, giving the chain and tensioner an easier life.
I'm really talking torsional vibration here. Just as a big single cylinder bike needs a flywheel to smooth power pulses, a heavier cam sprocket will average out the tension fluctuations in the chain. Until the Dream was produced, I cannot think of many bikes with a long cam chain, except for AJS 7R / Matchless G50 which had long blade tensioners.
The difference between the type 1 and type two engines is the orientation of the two halves of the cam. Maybe the CB72 cam drive has a lower frequency of torsional pulsing because of the uneven spacing between the cams. The Dream cam will have 360 spacing and will perhaps have a greater tendency to set up torsional oscillations. Only an opinion, of course.
Judging from the marks the cam chain has made in the engines I have stripped down, Honda's fears were quite justified. Indeed, Honda never did get the hang of reliably tensioning cam chains. Very few of their race engines ever used chain drive for the cams.
It would be good to see into the minds of those guys developing these engines. They made great technical strides in a short timescale and must therefore have made some interesting compromises to get the bikes to market.
G
cam sprocket question
Hi,
@ conbs : the benefit would be that there are more cam sprockets around if you can use the dream cam sprocket for a CB. And if the function is the same or almost the same , why not ? My C78 runs strong and fast, and actually I don’t “feel” the difference between the dream and CB in acceleration or picking up. Also on the dyno I never noticed this difference between cam sprockets. I think that both advance mechanisms work almost the same, and that hysteresis plays a more important role. However I noticed that the force you need to pull out the advance weights is much lower on the dream cam sprockets then it is on the CB sprockets. This is for two reasons, first of all there is a longer arm, and second of all the 259 spring seems a little weaker. @ G-man : You wrote : If this is true, then it is not logical to have two engines, in the same period, with different cam sprockets (with and without the weights). And if it was for the other reason you mention (360 / 180 crank) it would not have been logical because Honda superseded to the lighter version for a dream Either way, it could have been an ongoing “experiment” than showed that the weight wasn’t necessary, or to drop the costs for the superseded versions. For the other thing, the differences in advance mechanism, it is important to measure it (a function between advance angle and rpm). This measurement should be done in an engine, because the point shaft is part of the mechanism, and to important to leave it out. At the moment I’m building a late C77 engine, and I will use that as an engine to measure the differences between the cam sprockets, Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Jensen
As you say - maybe an experiment. All engineering development requires experiment and Honda were going through a period of explosive growth. The big sprocket was there as an insurance policy. As you say, it seem that Honda were confident to use the smaller one in later engines. I think it is interesting also that Honda chose to use a quicker advance curve on an engine with, theoretically, a better combustion chamber shape. The CB may have had higher compression but the Dream chamber, I think, would be kinder for flame propagation. G '60 C77 '60 C72 '62 C72 Dream '63 CL72
'61 CB72 '64 CB77 '65 CB160 '66 Matchless 350 '67 CL77 '67 S90 '77 CB400F G-Man, I would make the same hypothesis as to the reason behind that heavy sprocket. The C runs typically at lower revs than a CB so maybe there's aa resonance effect that they were trying to damp out on the dream sprocket.
I only race them and lighten the whole assembly further so I have no idea if that makes the cam chain flutter more than with a heavy flywheel on the cam but I do know that the cam chain whips around far more than you might imagine. It is barely controlled with those long unsupported runs. Teazer
I do like Jensen's posts and it is always good to understand what people were thinking when they did things. Engineering is always a compromise and I think that is is always good to remember that you are not looking at perfection but the best solution an Engineer could come up with given the available time / money. G '60 C77 '60 C72 '62 C72 Dream '63 CL72
'61 CB72 '64 CB77 '65 CB160 '66 Matchless 350 '67 CL77 '67 S90 '77 CB400F
Hi,
The dream, in it's time wasn't advertised as a tourer, but as a "sports bike", later on, when the CB's entered the scene, the dreams got their lazy touring role. Look at the CB92, an ultimate sports bike in 1959, one carb, pressed frame, etc. Look at the dyno run from my C78, it's revving just as happily as the CB's will when you open the throttle. Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
The second generation Interceptors (1986 VFR750) incorporated gear drive for the camshafts, as the 83-85 VF750F had chain drive. The 86 camshafts had a relatively large weight on each one, which Honda said provided a damping effect, reducing the shocks of valve openings and closings to the gear drive. This could have been similar thinking as applied earlier to the "heavy" 250-305 drive assemblies. Regards, Chase
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||