Re: considering changing gearing for a few more mph

[Steve Swan]

I'm considering changing gearing to get little more top speed. Right now i am running standard 74 ci solo gearing; 16/43 on the front and 28/44 on the rear; the engine likes to run at road speeds between 35-45 mph. i'd like to be able to run between 45-55 mph if i don't sacrifice too much when taking off from a stop in low gear. If it's a good choice, i'd prefer to change out the engine sprocket instead of the countershaft sprocket. how much difference would going to an 18t or a 17t from a 16t make?

[Steve Slocombe]

Dear Steve, it's easy to work out. A 17t sprocket would increase top speed by 17/16 or 6% and 18t likewise by 11%. On the VLs the standard solo drive sprocket was 23t, bizarrely reduced to 22t in 1934 when the more powerful motor came out. I like 25t on the VL drive sprocket, and several of my bikes have the 100 mph gearing with the 27t drive sprocket, which does make them tricky round town. It was dirt roads in the 1920s and 30s, and I think the bikes were undergeared compared with what they will do on our new-fangled paved roads. Check with the Cannonball riders to see what gearing they were using on the JDs, but I'm guessing it was a lot taller than you have at present.

[sswaney]

For what it's worth I'm running as tall as I believe you can go on a J Model.
18 Motor Sprocket
43 Clutch Sprocket (Thanks Mark Masa!)
28 Countershaft
40 on the Rear Sprocket. I think Slocombe's point regarding dirt roads and resulting slower speeds explains the scarcity of original 40 tooth sprockets floating around at swap meets. Not many were ever in use.
With the large diameter flywheels on a J the taller gearing really doesn't present a problem getting under way from a stop.
Further it gives you more use of Low gear rather than up shifting immediately.

[Steve Swan]

Gentleman, thank you for your replies! And to "sswaney" for listing your sprocket sizes! I am going to start out with going to an 18t engine sprocket, see what happens, and then go from there. Considering the scarcity of rear wheel sprockets smaller than 44t, my going with an 18t engine sprocket might be as good as it gets.

[Steve Swan]

For what it's worth I'm running as tall as I believe you can go on a J Model.
18 Motor Sprocket
43 Clutch Sprocket (Thanks Mark Masa!)
28 Countershaft
40 on the Rear Sprocket. I think Slocombe's point regarding dirt roads and resulting slower speeds explains the scarcity of original 40 tooth sprockets floating around at swap meets. Not many were ever in use.
With the large diameter flywheels on a J the taller gearing really doesn't present a problem getting under way from a stop.
Further it gives you more use of Low gear rather than up shifting immediately.

sswaney, considering your currnet fron and rear gearing, what average road speed does you engine enjoy?

[Steve Swan]

Dear Steve, it's easy to work out. A 17t sprocket would increase top speed by 17/16 or 6% and 18t likewise by 11%. On the VLs the standard solo drive sprocket was 23t, bizarrely reduced to 22t in 1934 when the more powerful motor came out. I like 25t on the VL drive sprocket, and several of my bikes have the 100 mph gearing with the 27t drive sprocket, which does make them tricky round town. It was dirt roads in the 1920s and 30s, and I think the bikes were undergeared compared with what they will do on our new-fangled paved roads. Check with the Cannonball riders to see what gearing they were using on the JDs, but I'm guessing it was a lot taller than you have at present.

Steve, i've been looking on the internet for a way to figure these percent speed increases without any apparent luck. Can you tell me where i can find them or do you have a table you can share? i am purely guessing, but i would guess since there is about a 5% increase from a 17t to 18t, than a 19t would be another 5% or 16% increase and a 20t would be a 21% increase in top speed.....?

[Rubone]

To gain higher overall speeds one needs to not only increase gearing,. but also increase the ability to utilize that gearing. Overgearing is one of the most common mistakes made in function of powered vehicles. Often taller gears can result in a reduction in top speed., not a gain, along with increased heat. poor fuel economy, etc. More speed means more power needed., gearing is only a small par of the overall equation. Then there is altitude. What works at Sea Level can fall on its face at 6000 feet. Friction loss increases exponentially at speed.
So don't be fooled by percentages based on pure numbers. That isn't the real world.

[Steve Swan]

To gain higher overall speeds one needs to not only increase gearing,. but also increase the ability to utilize that gearing. Overgearing is one of the most common mistakes made in function of powered vehicles. Often taller gears can result in a reduction in top speed., not a gain, along with increased heat. poor fuel economy, etc. More speed means more power needed., gearing is only a small par of the overall equation. Then there is altitude. What works at Sea Level can fall on its face at 6000 feet. Friction loss increases exponentially at speed.
So don't be fooled by percentages based on pure numbers. That isn't the real world.

Robbie, thank you for sharing that bit of wisdom. i am at 5,000 feet. What are your thoughts on going from a 16t engine sprocket to an 18t? My engine's "sweet spot" is around 40 mph; if i could raise it to a shade above 50, i'd be happy, but i am thinking that only a change in a couple teeth on the engine sprocket won't do that. and i am thinking to let well enough alone and enjoy my JD for what it is. (i am also thinking i'll give Jon Neuman a ring to see what the JD Cannonball guys are running for ratios and see now those ratios work from crossing varying altitudes.) Because of the insane driving habits in Fort Collins, my riding the JD is pretty much confined to a couple different routes, but those routes let me enjoy all the potential the old IoE engine has to offer.

[Steve Slocombe]

Dear Steve, sitting here with a calculator, 19/16 = 1.19 or a 19% increase, and 20/16 = 1.25 or a potential 25% increase in top speed. Rubone's point is well made, you need the power to pull the higher gearing. At the Wauseon races the bikes with tall gearing are at the back, not the front.

[sswaney]

I can comfortably run 50-55 without pushing it and have run those speeds consistently all day.
When I first got the bike up and started running it I thought, this thing will never hold together at these speeds.
No issues and the motor seems to like it. I have ran it much harder for streches without a problem but it seems comfortable at those speeds. 80 MPH once on the 2011 Sandhills Road Ride where there was enough road to get it there. I am running a slightly upgraded inlet setup. JDH inlet rockers, which I believe are the same lift ratio as a JD, but I'm only running the outer spring not the inner so my inlet valve spring pressure is greater than a J but less than JDH. I was concerned both springs would put too much stress on the lifter arm rollers. 15/16" venturi in a Schebler DLX45.
When I first started building my first J, all my AMCA riding buddies wanted to know why I was wasting my time on something I couldn't ride on our Chapter road rides. After seeing what a J model can do I would describe their responces as shocked and surprised.

[sswaney]

Actually I was a little shocked and surprised myself having no experience with these great machines.