Topic: Rear sprocket...

[katmol]

I've got a 16" rear wheel on my 750A and it seems like the RPM run a little high. It has a 43 tooth rear sprocket on it now. Would a 40 tooth be to small you think?

[Cqyqte]

No reducing the by 3 teeth on the rear sprocket is like going up 1 more tooth on the front!

[raymond10078]

What tire is on that 16" rim (as in 140/90 or)?

Which year are we talking about?

All years had a 17" tire with a 4.5" profile (26" tall tire).

The 76 had a sprocket ratio of 2.824 ratio (17F, 48R).  The 77/78 had a sprocket ratio of 2.800 (15F, 42R).

Let's say you have a 130/90 tire, that makes your tire height of 25.2" ((130*.9*2/25.4) + 16).  That would alter your ratio slightly (26/25.2 = 1.032) - your engine is running 3.2% faster than stock (at the same speed).  To get that back to a stock value, you'd want to get that drive ratio down to match.  Using the 2.800 example, divide that by the 1.032 number to get 2.713.  That'd be the sprocket ratio you'd want (to get back to stock).  That'd be between a 15/41 or 15/40.

Hope the example helps out.

[salukispeed]

I am hoping to reduce the highway rpm on my 77A this summer If I get better power from the K Carbs and Cam I just purchased. The engine seems to be higher reving than needed for a 57 mile run back and forth to work at 65-70 MPH I was looking for a 630 40 tooth sprocket also. Where did you locate one that size? I realize this will likely erase some of the acceleration improvment from the carbs and cam.

[raymond10078]

Your other option is to install 530 chain/sprockets, which opens up the choices regarding sprocket sizes.  A 530 was used on the 76 bike, and then Honda changed to the 630 on the 77/78 bikes.  Today's 530 chains are much stronger than "back in the day", and will carry the load just fine.

A quick check with Dennis Kirk (not my preferred supplier - the catalog is just handy), 530 front sprockets are available in 15 through 18, and 530 rear sprockets are available is 39, 40, 43. 46, and 48.

JT Sprockets shows 530 rear sprockets in 37 through 40, 43, 45, 46, and 48.

I currently have a 630 chain/sprockets, but will convert to 530 when the time comes (when they wear out).

[katmol]

 Well I've got two new rear sprockets coming, a 41 tooth and a 38 tooth. Also a small digital tach. This will be interesting, to me anyway.
 Anybody know what RPM's a '77"A" with stock sprockets, wheels and tires turns at 60 or 70 MPH?

[katmol]

125 mile trip today, 45+MPG! Combined in town 20-45mph lots of starts and stops, back roads 40-50mph and highway 60-80mph. It still seems like the RPM's are too high. So, I'm curious at what the new sprockets will do.

[raymond10078]

I hesitated to share - from a Cycle World article, they showed 5500 RPM at 60 MPH.  I hesitate because this one data point is from their 1/4 mile test - meaning that I doubt that the bike would need 5500 RPM to hold the bike at a steady 60 MPH.  Later, I'll calculate the RPM assuming a "locked converter" - which I know we don't have, and will share the results later.

[raymond10078]

OK - if we assume that the converter is locked, the engine speed at 60 MPH is:

4686 RPM for a 76; and
4639 RPM for a 77/78.

I have no basis for a guess, but I'll guess that at 60 MPH - crusing along, I'd think that the engine is turning around 4700 to 4800 RPM (some converter slippage - but not the 5500 RPM quoted in the magazine for a full throttle 1/4 mile pass).

[katmol]

Well, that not that bad. Not as bad as I thought it would be. I'll post what I see when I get the digital tach on mine.
What does a '77 "K" model turn at 60mph in 4th and 5th gears?

[raymond10078]

Using the same method, I calculate 3655 RPM at 60 MPH in 5th.

This assumes: a 4.50 tire on a 17" rim, and a 4% reduction from calculated circumference to get to a rolling circumference.

[katmol]

Kandrtech, you are good!
I just got my little digital tach and went for a quick run and at 50mph 4000rpm, 60mph 4600rpm and 70mph was around 4900rpm.
My speedo bounces around some so these are approximate but close enough to tell me what I need.
Not as high as I thought it would be.

[salukispeed]

I showed 48-4,900 at 60 with a garbage digital tach too. Seems excesive will have to see.