Sprocket Question/ Opinion

[BigLakeBob]

Working on my basically stock '62FL. Engine runs fine, +.05 low comp pistons, stock Linkert, stock ex, stock cam, stock clutch, etc. Stock 4 sp trans. 16" wheels. Saddlebags and some weight, solo rider at 165lb. Running 23T engine, chain drive 37T clutch, 22T trans and 51T rear wheel. I am not happy with the engine rpm's above 60 mph. I am looking for more top end lower rpm than quickly accelerating from 1st. My first thought was to change the trans to a 23T.

If I do this how much will it effect my speedo true reading? Is there a formula on different sprocket changes?

Would I be better off, or the same, to just change the engine sprocket to a 24T? Is there a formula?

I see in the manual where changing engine from a 23 to a 24 the high gear ratio changes, of course, what would it be if I changed the trans to a 23 or 24?

Would I be better to change both? Engine 24, trans 23 or 24? My end goal is to cruise at 65 to 70 mph with the same rpm's I have now at 55 mph.

Thanks, any knowledge, advice, and experience is appreciated.

[Rubone]

Put a 24 on the engine Bob. The speedo will read the same as it did. You should be able to pull that just fine. I'm always interested in the perceived need to lower RPMs though. But I don't have issues with spinning stuff up so it is foreign to me. Higher top end and lower RPMs isn't a very effective combo as one generally precludes the other. Unless big inches are involved of course!

[kitabel]

W/r/t Is there a formula on different sprocket changes

New MPH @ same RPM: new sprocket / old sprocket, 24 tooth makes 60 MPH into 65.5 MPH
New RPM @ any MPH: 1 - (old sprocket / new sprocket), all RPM is reduced by 8.3%

Note each tooth added or subtracted has a different effect than the last or next.
One tooth change up from 20 is 5%, next change is 4.8%, etc.

[joe fxs fxr]

As it turns out, there is almost no difference between going up one tooth on the motor and one on the trans. As said above the motor change won't affect your speedometer.

Neither one will give you the RPM drop you want though. Going one tooth bigger on both sprockets is closer but still not what you are trying to achieve. You'd have to go to 24 on the motor and 24 on the trans to get close and now you have to wonder if your motor will like that tall a ratio.

Sprockets vs RPM Chart.png

This assumes a 130/90x16 rear tire.

[BigLakeBob]

For now I've decided to do one movement at a time and change the engine sprocket to a 24 and ride it for a while. The rpms didn't change as much as I thought they would. Thanks for the formulas, and advice.

[kitabel]

For many riders, the reduced noise, vibration etc. at cruising speed with really tall gearing (24/24, up from 22/22) is not worth the annoyance of having much less torque in 1st gear (16.6% less, a 74" engine feels like 62").

Do you have the original transmission ratios, or close ratio?

[BigLakeBob]

Original stock ratios.

[FNG1]

I'm curious. How much would changing the balance factor to smooth out the higher RPMs effect the ride at city street speeds?

[T. Cotten]

If you are open to my opinion from experience, BLB,...

You are asking to 'lug' your engine; These motors are ready to serve well at high R's, and you might not realize it until you've done it.

My '65 (one of them) pulled a hack with 22 on the motor sprocket, and 22 at the tranny, on interstate runs with packs of evos, at 70mph. It achieved an 'aircraft' hum at a sweetspot in its powerband where the motor made it fly effortlessly. And there was still enough throttle to pass!

The only way I can imagine putting a solo to a similar test with standard sprockets would be to do 120 mph from St. Louis to Indianapolis. There's a reason these motors became legendary, but most folks have too much invested to enjoy it.

....Cotten
PS: Sorry FNG1, but motor balancing doesn't consider the rider at all; It is only an applied technique to try to keep the motor from throwing itself apart. Bad vibes are commonly chassis and drivetrain concerns. If the motor really shakes, something's not trued, because these motors will run smooth over a wide, wide range of factors.

[Hack]

Running a belt primary drive and a 23 tooth trans. sprocket is a nice compromise set up, even pushing a windshield. You get instant less noise and vibration and you are not hitting 2nd gear halfway through a crosswalk. It's close to the gearing on early Superglides. If you don't dig it you can always go up to a 24 later.

[kitabel]

The answer, of course, is a 5-speed. The 3.2418:1 is 8%% stronger than the 4-speed's 3.0044:1.
A 5-speed with a 24 sprocket has about the same 1st gear overall as the 4-speed with a 22.

All it takes is time and money.

[BigLakeBob]

Something else maybe someone can educate me on. Why is the compensating sliding cam 40292-55A on a BT one piece and on a Sportster 40297-57 & 40325-57 two piece? The sizes of the two different ones seem interchangeable, why is the one rotatable and the other one solid? Thanks again.

[kitabel]

I wrote an .xls spreadsheet comparing all big twin 4- & 5-speed transmissions: shows speed in MPH for each gear, RPM loss on shifting, overall (starting line) ratios. Sliders allow your own input, automatically calculates changes.
E-mail me for a free copy.

[BigLakeBob]

Changed comp engine sprocket to 24t today. Test ride showed more to my liking. Casual shifting 1st to 2nd about 20mph, 2nd to 3rd 35mph, and 3rd to 4th 50 mph, and the sweet spot in 4th was between 60 and 65, where before it was 55 to 60. Up to a smooth 70 also. Some tire vibration and a windshield chatter need to be addressed, but definitely a move towards what I wanted. My ride in SW Ohio was a little chilly and keeping with the spirit of the holidays, as Clark Griswold would say, it was a little nipply. https://www.youtube.com/watch?v=h_yQhALyAx0

Thanks again for the help.

[kitabel]

W/r/t balance:
IMHO engines with long connecting rods (compared to stroke length) and light pistons suffer less from incomplete/inaccurate balance. The 45 is a good example, the rod is almost twice as long as the stroke (7.4375 ÷ 3.8125 = 1.95:1, also true of K and all iron Sportsters) and the piston is tiny.
In a big twin there is (usually) no room for a longer ($$$ Carillo, etc.) rod in the frame, but a lighter custom piston will help at great expense.