1992 750 Sprocket Combos

[swalter]

Stock is 15/38.  Has anyone tried the 17/38 or is that too tall?  I'd love to hear any combos you've tried and get your feedback on how it felt.  My singular minded goal is to lower the RPM at 70 MPH or there about.

Thanks for your time,

Scott Walter

[DesignFlaw06]

On my 2002 750 (essentially the same bike), I went to a 16/38 like what is done here:
http://www.pipeline.com/~randyo/16_Tooth_Sprocket_Mod.htm

Since my previous bike (85 Nighthawk 650) had a 6 speed tranny, I was looking for something to lower the RPMs around highway speeds too. My cruising speed is just shy of 80 MPH. With the 16 tooth, I dropped from 6000 to 5000 RPMs and saw some increase in fuel economy, very similar to what the website says. I can say I didn't notice any real loss of power on the bottom end either. 16 was tight, but the chain was still relatively new. I think I do remember seeing somewhere that a guy did 17, but not sure how well that went over.

Good luck to you. And 

[Falcon030]

I wrote a little program to get a idea what sprocket combos would do on the NightHawk.  You can get it here:
http://members.tripod.com/larrymears/down.htm

I'm running 15/39 on mine but I ride 2 up a lot and like the lower gearing.
I ran a 16/39 for a little while and you could get by with 3rd gear in town but it took some of the thrill of acceleration away.  I ordered a 38 tooth but it was out of stock at the time so I just went with the 39 tooth.  I got a another 15 a 16 and a 37 waiting.  I tried a 14 but it was too little with the X ring chain, scrubbed the case.

[swalter]

Falcon,

Cool program idea, I read the notes but it didn't open in Vista.  Said something about not working in full screen mode when I was trying to open in in a window.  Do you have anything Vista compatible.  If not I might try and put the equations in a spreadsheet.  I'm assuming the gear ratios in the 92 are similar but I'll double check.

By loss of acceleration, couldn't you just add more rpm's to overcome that situation or did it bog down in the next gear even when you tached it out?

Great info guys, if others have experience in this arena...come forth hither gentlemen and women!!

[swalter]

I put together this XL Spreadsheet called sprockets...download it and let me know if there are any problems.

Still looking for more feedback from folks who have made changes!

http://www.rockinscotty.com/download.htm

[happycommuter]

A12 - rotation is misspelled
Um, what gear is this in? (using stock gear and final drive ratios here you could go nuts.
Is 80 in really the tire circumference?
using a 140/70-17 tire I get pi*(17+2(.7*140/25.4)) = 77.65 inches.

can't see the formulae and don't have the bike so can't comment on accuracy.

[swalter]

Happy,

I'll correct the spelling and email you the unprotected version.  I'm not sure where I got the 80 and it does make almost 2k of rpm difference.  I'll have to grab my tape measure and double check.  Your formula looks good and is probably correct.

All the ratios aren't much of a concern to me, just the final drive, fifth gear, and the initial drive.  This gives you a pretty solid guideline to play around with different sprocket sizes and even tire sizes to see what the effect will be on the highway. 

Now torque and performance go out the window until you ride for yourself.

Thanks for the input..email on the way shortly!

[Falcon030]

The CB750 only has so much torque and when you gear it for 90mph highway cruising you won't feel the kick when you shift the gears at high rpm as much as stock gearing.  I missed that so I went back to close to stock gearing as I could get with sprockets on hand 15/39.  It helps when you are going up steep mountain roads with 2 people, I can leave it in 3rd all the way up where as with the 16/39 I'd have to down shift to 2nd and lose acceleration too.  I got a Bandit 1200 and it's a good bike but the '01 Nighthawk 750 beats it for 2up comfort.  The Bandit can handle the extra load easy but it isn't as smooth and the riding position isn't as comfortable for me especially with a passenger, as the Nighthawk.  I'm glad I didn't sell my Nighthawk right off. The Bandit is more fun for me when the wife doesn't want to come along, otherwise I ask her which bike she wants to take and it's usually the 750 Nighthawk.  I think it's a shame Honda doesn't still offer a big CB in the US, 750 or 1000 ...I know they got a CB1300 but we can't get it, shame on you Honda.


 

[swalter]

Thanks Falcon.  I was going to try a 16/38 but now I'm not so sure.  I'll keep you posted.  BTW..a 16 inch isn't easy to come by.  Anyone have any retail recommendations??

[Old_Rider_54]

BTW..a 16 inch isn't easy to come by.  Anyone have any retail recommendations??

I just ordered a 16 tooth countershaft (front) sprocket.   After an extensive search on-line, I found only one source: http://www.sprocketspecialists.com/ProductSearch2.aspx

I'll give a review of how I like (or don't like) it in a month.  I still have to get a chain and rear sprocket.

[Falcon030]

I got my sprockets from Sprocket Specialists too.  That program I wrote was in GFA BASIC for MSDOS. I was being lazy, hadn't wrote anything in a long time and that basic is so easy. I don't have Vista, it runs  in Windows XP for me.

[swalter]

Thanks Old Rider, that was a great find.  I'll think I'll give the 16 a shot and see how it goes!!

[Old_Rider_54]

The 16T sprocket is now on my bench waiting for a chain and rear sprocket.  Hopefully I will get motivated and order them this week.
But first I have to dink around with my new windshield and try to get it dialed in 

[Wahrsuul]

Well, dang!  Now I'm confused.  Are the sprocket sizes the same for all the late model (91-03) NightHawks?  I've been looking for a 16t front (thanks for the link BTW), but several of the places I've looked; e.g old bike barn and Dennis Kirk, have different ideas.  DK shows one set 15f and 38r sprockets.  However, Old Bike Barn has several different rears and a couple fronts.  They have a 17t front I was considering till I found where to get a 16t.  I wonder if a 17 would even fit and if it would cause too much slow down?  Would it need a longer chain?  Would a 17f need a slightly smaller rear?

[Old_Rider_54]

Yes, the sprocket sizes will be the same for the 91-03 750s.  The standard factory setup is 15 front / 38 rear.  In my research, I could only find the one supplier (Sprocket Specialists) for the 16 front.   

My simple rules of thumb with sprockets and gearing is:
1) Taller gearing = less RPM at any given speed - usually lower wear, noise, etc.
2) Lower gearing = higher RPM at any given speed - usually faster acceleration
3) Taller gearing means larger front sprocket or smaller rear sprocket (or both).  Lower gearing is the opposite.
4) 1 tooth on front sprocket = about 3 teeth on rear sprocket (I know I'll catch a lot of flack on this one!  Just my approximation -there is a final ratio chart on this forum if you want to be precise)
5) Front sprocket sizes are limited - too large causes clearance issues.  Too small causes more chain wear.
6) Rear sprocket size changes usually involve lengthening or shortening the chain
7) Never use an aluminum sprocket.
(Legal disclaimer - rules of thumb are always subject to special considerations!)

It's all about trade offs.  I elected to go with the 16 because I was looking for a little less RPM (noise, vibration, etc.) at highway speeds without sacrificing and significant acceleration.  There will be a little acceleration loss, but the loss should be minimal, and my hole shot days are over (for the most part).   Most people are reporting that the 16t will reduce RPM by around 500RPM at a given speed.

Going with a 16t front allows me to gear the bike a little taller without having to modify the chain.  I elected not to try a 17t because I don't want to significantly reduce acceleration (and I'm slightly worried about clearance).  Would a 17t require a chain mod?  Not sure, but I'm lazy and don't want to go there.  For me, if I intended to spend most of my time on the freeway, then I would consider gearing even taller, changing out the rear sprocket to a smaller one along with the 16t front mod.  But if I intended to spend most of my time on the freeway, I wouldn't be riding my 750.  To each, their own!

[swalter]

I put in the 16T Front Sprocket and I love it!  Gas mileage better, lower RPMS  at highway speed, reduced noise, reduced vibration.  Torque pull in first gear slightly reduced but still ok pulling passengers.  This is a great fit for this bike as long as your primary intention isn't the 1/4 mile.   

[Twisted_Wrench]

was in your program and looks like with a 15/40 setup i would, if i was insane< be running about 130 mph @ approx. 8900 rpm. so with the 40t i would have a better hole shot but my mpg would go down a bit...correct? this is the first bike i have been able to change anything. help is appreciated thanks.

[Twisted_Wrench]

also old_rider you say NEVER use an aluminum sprocket, any reason why? i have seen them used alot on bikes that are higher HP/TQ then our nighthawks, just asking b/c like said above this is the first bike i have done anything to, all others where drive shaft driven.

[luckylindy]

TW,

My mpg went from 50/stock to 46/w 40/15t setup.  The extra torque is worth the 4mpg less to me.

[cmyers]

The only reason to go with an aluminum sprocket is to save weight, but lets face it your not gaining much by saving the few ounces with an aluminum sprocket. The downside to aluminum is they wear extremely fast compared to steel sprockets.

I know alot of supersport m/c riders that go with aluminum b/c it is the cool thing to do. It started in racing were guys are shaving every ounce they can get and toss the sprockets at the end of a race week anyway. It is another useless-to-street fad like stretching a bike.

[Old_Rider_54]

also old_rider you say NEVER use an aluminum sprocket, any reason why? i have seen them used alot on bikes that are higher HP/TQ then our nighthawks, just asking b/c like said above this is the first bike i have done anything to, all others where drive shaft driven.

Like Cymers says, aluminum sprockets wear a whole lot faster than steel.   Sprockets and chains take a tremendous amount of mechanical abuse in everyday riding - makes no sense to use an aluminum sprocket.

[Twisted_Wrench]

thanks, i have seen alot of the aluminum for a few bucks more then the steel and thought what the hey but maybe i'll reconsider....