Topic: Replacing Sprockets: more low end torque?

[DanG!]

The drive chain and rear sprocket on my '76 CB750F are worn out and need to be replaced. I'd like to gain a little lower end torque and some room at the top end of my RPM's while I'm at it. Currently cruising at 70mph I'm up around 6500 RPM, Redline is 8500. I'd like to get that cruising RPM down to around 5000 if I can. I have the 48 tooth rear sprocket and the 18 tooth front, can I put a 46 tooth rear sprocket on the bike and get the results I want? Are there other effects to doing this that I'm not thinking about? Also, should I just change out the front sprocket while I'm at it, even though it's not nearly as worn as the rear?

Any helpful advice is appreciated, thanks.

-dan

[flybox1]

replace both front/rear sprocket and get a new chain, all at once.

[david 750f]

Dan G,
stock on the 76 750F is 17 front and 48 rear. If you are running 18/48 the bike should be revving lower than stock.

Something doesn't sound right. Either your speedo is reading low or your tach is reading high.. Can you confirm that speedometer reading with a GPS?

6500 in 5th on my bike is around 90mph.. Stock gearing

[Old Scrambler]

I'm not expert on the '76F tranny ratios......but just maybe your tach is reading high or you have a 17-tooth front sprocket.  No problem going down on the rear to 46 or even 44.  I traveled a few days with a CB750K4 rider from the San Francisco area who had a 40-tooth rear sprocket and was pulling a trailer.........Yes, he had to downshift on long grades.  Change both sprockets for best chain-life.

[Gordon]

I'd like to gain a little lower end torque and some room at the top end of my RPM's while I'm at it.

With gearing changes you can only choose one of those two.  Larger rear or smaller front will give you more rear-wheel torque at lower engine speeds while increasing the engine rpm for a given speed.  Smaller rear or larger front will do the opposite. 

[DanG!]

With gearing changes you can only choose one of those two.  Larger rear or smaller front will give you more rear-wheel torque at lower engine speeds while increasing the engine rpm for a given speed.  Smaller rear or larger front will do the opposite. 

Thanks folks, that was a typo. Front Sprocket is in fact the 17 tooth. I'll replace both sprockets with the chain. Looks like I need to try an 18 tooth front or a 46 tooth rear to get the RPM's down, and see what that does to the torque.

-d

[Duke McDukiedook]

^^+1 on what the Gordon said.

[ccmclane]

Try using http://www.gearingcommander.com/.  It allows you to try a number of different combinations and understand the impact on rpm, speed, chain wear, etc.

[BobbyR]

With gearing changes you can only choose one of those two.  Larger rear or smaller front will give you more rear-wheel torque at lower engine speeds while increasing the engine rpm for a given speed.  Smaller rear or larger front will do the opposite. 

Thanks folks, that was a typo. Front Sprocket is in fact the 17 tooth. I'll replace both sprockets with the chain. Looks like I need to try an 18 tooth front or a 46 tooth rear to get the RPM's down, and see what that does to the torque.

-d

Dan, this is what you will find when you change the rear sprocket. You will have to hold it in each gear when you need to get on it. You will find yourself shifting down to go up hills or pass cars. No real hills in Brooklyn so it is not a problem.  Up here in Westchester it would drive me nuts.
You will have reduced engine braking. Honda knew the sweet spot in the motors start about 3500 to 4000. Compared to our cars with their super overdrive, the Honda sounds like it is screaming. It can do that all day. 
 
 

[cougar]

Dan ; I'm currently running  17/43 sprockets with a 140/90-16 rear tire. When I got the bike it had 17/48 with 120/90-18. It seemed a little buzzy to "me" at 50 to 65 mph. When I changed my rear rim to match my front I went to the 43 tooth count. VERY pleased with the results. My bike still has plenty of grunt off the line (won't do wheelies but will roast a tire) and has lots of throttle. I've seen 110 mph with quite a bit of throttle left ! Hope this helps.   ...cougar...

[bjbuchanan]

I don't mean to thread hijack but BobbyR are you the guy i see all the time on the taconic on the blacked out old Honda?

I'm in westchester too, just curious

[Jerry Rxman Griffin aka MuthaF'er]

18/48 on my 75F - 5000 rpm is 75mph and 17/48 is 70mph at 5000 rpm. I like the 17 better for acceleration but the 18 is better on the road. 17/48 it'll do 100mph if you get near the far end of the redline. Just don't miss a shift on that side of the red 

17/48 is stock and provides better acceleration and probably doesn't affect top end since it'll rev better and get more rpms in 5th

[Xnavylfr]

The one thing you need to think about, IF you go to an 18 front sprocket, you have very LITTLE clearence between the chain (on the sprocket) and the engine case. If you ran a 17t you have a LITTLE insurance there in case you sling a chain.
More grunt/get up and go off the line=more teeth in the rear. Less RPM at HI-WAY speeds= less teeth on the rear.
I've run as small as a 43t rear with 17t front and it still had enough pull power to roll away from a stop. NOT a tire scorcher but it will pull itself, and at 75MPH I'm turning between 4.5K and 5k on the tach, still LOTS of room to TON UP and not worry about the redline.

Xnavylfr(CHUCK)

[70CB750]

I had to verify my readings today, but as far as my tacho and speedo can be trusted I am at 70 mph with 4000 rpm in 5th gear.

I have 17/48 on my K0.

[BobbyR]

I don't mean to thread hijack but BobbyR are you the guy i see all the time on the taconic on the blacked out old Honda?

I'm in westchester too, just curious

No, but I have seen the guy also. Mine is bone stock and red old Honda. Pm me, maybe we can take some rides across the River when it gets nice again.