Which Sprocket Combination?

[Charlie1717]

I've done a little searching but haven't found anything conclusive.

I'm wanting to lower my Rpms at interstate speeds (70ish mph).
I have a 1972 cb750 with an 18/48 setup right now.

I understand the concept of a smaller front sprocket reduces and a larger rear sprocket reduces rpms.

So what is the best route to go to lower rpms without losing to much acceleration.

Also, where do you guys buy your sprockets?

[TOOLS1]

I just get mine from the local Honda dealer. There is not much difference between their price, and the online suppliers. Since, you want more top end speed, you need a larger counter-shaft, or a smaller rear sprocket. Since 18 teeth is the largest that, you can normally get (Cycle-X does have a 19, but it is expensive). I would drop down to a 45 tooth in the rear.
TOOLS

[Charlie1717]

Woops I miss-spoke in the original post. I got it backwards! Anyway, good to know about the 18 being the largest (typically). The rear should be easier to change anyway. So a 45 would hypothetically reduce my rpms by around 300 rpms which would be nice.

What is the lowest rear sprocket you can go before you start running into issues?

[LukeM]

Depends on what you consider "issues".  The load induced by a smaller rear sprocket will mean more stress on the clutch, and possibly engine issues with the lugging.  Same thing applies to driving a car by upshifting at very low RPMs: you can get buildup of fuel and carbon, necessitating some high RPM time to burn that off.

This may not be a problem with this type of bike, as high RPMs are de rigeur during normal operations. I also assume the Honda engineers went through that process, changing sprockets and ratios until they achieved a nice balance of performance, economy, and comfort/noise. A conservative method might be to change the ratio by about 10% and see how that goes.  Beware of More's Law:  If enough's good, then more's better.

Luke M

[TOOLS1]

I would not go any lower then a 45 myself, but they do make a 41 tooth sprocket. Would not be much fun in stop, and go traffic. Plus it might take it out of the power band, causing it to struggle at highway speeds.
TOOLS

[Re-run]

The 18/48 setup you have is already loading the engine. Most people change the front to a 17. People have reported issues with highway travel and the engine lugging with that combo and having to stay in 4th.
Really, at 70, you should be running at least 5K rpm. If vibration is the issue, install some bar ends to absorb some of it or get some titty grips or a foamy style grip.
Also, fuel economy suffers with the 18/48 ratio. Probably cause of the lugging.

[Charlie1717]

Yeah I'd say the vibration is the big thing. The engine seems to run well at 5,000 rpms however the vibration makes it get tiring quickly. May have to look at reducing vibration first.

[TOOLS1]

Do a tune up, and sync the carbs. You will be surprised at how much smoother it will be.
TOOLS

[Charlie1717]

Done and done. The bike is running tip top and I've riden it hours on end but at around 65-75 there is a distinct vibration that is tolerable but can become bothersome to my hands. However, I do need to replace the tires (maybe slightly off balance as well), it is possible they could contribute to some of the vibration.

[shinyribs]

I know I have a front tire slightly out of balance and it gives me a rump-rump-rump in the handle bars right at 30-35 mph,but it goes away. At highway speeds it is gone. I really dont experience any vibration anywhere else. For a solid mounted motor I was surprised in the fact that there was no engine vibration at my hands at all. If I had to guess I would say you may be correct in saying the tires are the cause of the vibration. How old are your tires? Can you see any cracks in them. I wonder if maybe you have a bad inner tube? All just thoughts.

[Charlie1717]

I'm not 100% on the age of the tires I acquired them with the rims. They are just worn down and I plan on replacing the spokes anyway so I'll have the rims balanced and tires mounted/balanced  after that. Hopefully that will reduce some vibration. I may switch grips as well along with maybe putting some lead shot in the handlebars if need be.