sprocket size question...

Since I have a non-stock rear wheel hub, I can change the rear wheel sprocket for my bike to change my final drive ratio, rather than buy a front sprocket and go through the hassle of tearing down the primary case. I really like the powerful feel the 20/42 tooth combo that I am running now on the local roads, but the hiway speed limits out here are 70mph, so that means cars in the left lane are doing 75mph. On the 20/42 combo, I think that's about 5,500 rpm's (speedo's not working at the moment) and I feel like I am unnecessarlly winding out my engine to do 75- 80mph. I don't mind winding it up to 6,000 in short spurts going through the gears, but running 5,500 rpms regularly while cruising along seems like a recipe for wearing things out quicker.

I figure that the standard sprocket ratio is near 1- 2 between front and back sprockets, so if I make a change of one tooth up front, I would need to make a 2 tooth change in the rear sprocket to achieve a similar ratio. I can get a half step change also by changing an odd number of teeth on the rear sprocket (1, 3, 5... etc) I'm thinking of going to a 20/40 or 20/39 and was looking for opinions.

Except for the minor deviation in ratio, is there any benefit to making the rear sprocket 2 teeth smaller, rather than make the front sprocket 1 tooth bigger? My intuition tells me it shouldn't matter which I change, but I wanted to find out if there's some other factor that I'm not considering. thanks all...

A larger (that is more teeth, or greater pitch diameter) rear sprocket results in less chain tension for a given torque on the rear wheel.

From this it would follow that greater chain life results from less chain tension.

As a practical matter, you would likely not notice any real difference!

If it were me, I would prefer to increase the drive sprocket if I had easy access to it. However, considering the work to pull the primary cases, I could live with changing the rear sprocket.

Slick

Another very important consideration is repeat contact number for the chain and sprockets, it is a function of the sprocket sizes AND the chain length. Some combinations are much better than others and probably most are acceptable....but some are horrible and I learned the hard way running 14/42/102 on a Ducati. Got noise, vibration, very rapid wear, etc. The picture below shows how small changes have a big effect on this,
numbers are for 15/42/102, 14/42/102, 15/44/104. OEM sprocket ratios are generally even/odd to avoid problems, but not always.
The chart below came from gearing commander, a free site where it is very easy to check on these things..

http://www.gearingcommander.com/

Doesn't seem right, Got feeling the rear 42T is standard and 20T front is in the range, if you are getting 70mph at 5,500 revs then something seems not right. I use 19T from and standard MK3 rear and easily get over 70mph at or just under 4,000 revs, and blast past cars from that speed no problem at all. Figures based on mechanical clocks in original condition, not sure how accurate they are.

Madnorton said:

Doesn't seem right, Got feeling the rear 42T is standard and 20T front is in the range, if you are getting 70mph at 5,500 revs then something seems not right. I use 19T from and standard MK3 rear and easily get over 70mph at or just under 4,000 revs, and blast past cars from that speed no problem at all. Figures based on mechanical clocks in original condition, not sure how accurate they are.

Click to expand...

+1
My MKIII manual shows 20T @ 6,000 rpm = 97 mph. That translates into 89 mph @ 5,500 rpm.

Pete

Deets55 said:

+1
My MKIII manual shows 20T @ 6,000 rpm = 97 mph. That translates into 89 mph @ 5,500 rpm.

Pete

Click to expand...

About 16 mph per 1000 rpm is what my gauges say, with a 20 tooth front.

Thank you all! Taking the charts and all your comments into consideration, something in my equation doesn't add up correctly. It seems like there's only 2 possible scenarios. Either I have a smaller front sprocket than I think I have, or my clutch is slipping... I have to do my homework to find out...

bluto, that link is great. Thank you for that.

maylar said:

Deets55 said:

+1
My MKIII manual shows 20T @ 6,000 rpm = 97 mph. That translates into 89 mph @ 5,500 rpm.

Pete

Click to expand...

About 16 mph per 1000 rpm is what my gauges say, with a 20 tooth front.

Click to expand...

Agree with all above! My Atlas does about 17 mph / 1000 rpm. The overall drive ratio in top gear is 4.1, primary drive is 2:1; that would make final drive ratio 2.05:1 .... dunno number of teeth on final sprockets without going to bike, and not where it is right now.

Slick

o0norton0o said:

Since I have a non-stock rear wheel hub, ....

Click to expand...

o0norton0o said:

Thank you all! Taking the charts and all your comments into consideration, something in my equation doesn't add up correctly. It seems like there's only 2 possible scenarios. Either I have a smaller front sprocket than I think I have, or my clutch is slipping... I have to do my homework to find out...

Click to expand...

Any chance that non-stock wheel hub includes a non-stock rear wheel (18 inch wheel as opposed to a 19 inch wheel)?

Dances with Shrapnel said:

Any chance that non-stock wheel hub includes a non-stock rear wheel (18 inch wheel as opposed to a 19 inch wheel)?

Click to expand...

yes, it's an 18" rear wheel, which when calculated into the chart on the webpage bluto posted, still doesn't account for the higher rpm's at 70mph.

I looked through my parts bin and found my original 19 sprocket, so I am sure that the front sprocket on the bike is at least a 20 tooth sprocket, which can only mean the clutch is slipping at speed... I do have an extra steel clutch plate to facilitate a lighter clutch pull, so maybe the taller clutch stack isn't working at speed. (although, it feels like it grabs aggressively when I take off...)

Maybe your tach is way off. I run a calibrated bicycle speedo and it tells me that my Smiths speedo is at least 10% optimistic

tpeever said:

Maybe your tach is way off. I run a calibrated bicycle speedo and it tells me that my Smiths speedo is at least 10% optimistic

Click to expand...

+1 ... could be speedo way off as well.

I once took my Atlas to the max rpm on the tach (6800) in top gear and the Smiths chronometric said 132! If tach was right, speed should have been 119 per the gearing and wheel diameter. Or maybe Chronometric was right, and I set a 750 land speed record! (doubt that)

Slick

The plot below should help quantify the speed ranges expected when the stock primary ratio (26/57) is employed along with the stock 42 tooth rear sprocket, and countershaft sprocket size is varied over the range shown.

Those front sprockets are available from 16T to 25T

When I rebuilt my G15 basket case, it had a 16t sprocket on the trans. I went with it and found it way too busy - changed to a 19T. Original spec on a CS is 17T for the year. (Guess that's why they were so quick on the street)

My Ranger has a 19T and feels just right, though I rarely top 70 on it.

The 850 has a 22T, and makes for fairly relaxed highway riding. 50MPH is just below 3000 RPM

My G80 (same gearbox as the others) has a 17T gear, and 16T was the common option. The tire is a 4.00x19 Dunlop Trials, which is taller than the Commando tire by over an inch. No tach though, but its pretty happy at 50-60MPH.

Met a guy from Melbourne (FL, not AU) a few years ago. He has a '72 Interstate he's had since new, and has spun the clock 4 times. He runs a 25T for very relaxed highway riding.

Hi, with my Atlas, I have a belt ( so 36/72, means 2/1 ratio), back I have 46 teeth ( Norvil modified norton rear drum/sprocket ), front is a 21 teeth , so a final ratio of 4.38 ( for example a stock cdo with a 21 t is 4.38 , if my memory is good), I have 18" rear wheel with a BT45 110/90/18 (OD is 658mm), theorically that means 70mph at 4000 rpm , 53 mph at 3000, which is nice for me on twisty roads , sometimes a bit revvy , but .........! can't have both, for that I take my Cdo geared at 4.18 !

WZ507 said:

The plot below should help quantify the speed ranges expected when the stock primary ratio (26/57) is employed along with the stock 42 tooth rear sprocket, and countershaft sprocket size is varied over the range shown.

Click to expand...

Belt drives are employed by some Nortoneers, and depending on the particular drive, may provide a means of accessing additional higher gearing options. I realize belt drives are provided by a variety of vendors, and depending on vendor may or may not provide a primary ratio different than factory gearing. Since I am familiar with the RGM belt drive which does provide a higher primary ratio than stock thought readers might appreciate a couple comparison plots showing the stock and higher gearing options.

Very nice plot, and it falls right where my '74 with 22T front and 4.00-18" rear tire show on the clocks. Rolling diameter between stock 19" and slightly fatter 18" is hardly measurable. Even with the taller 22T, there's no problem with launching from stop lights, even to the point where I would consider going to an even larger front for the greater highway comfort.

Nathan

Well,... I was unsure of how many teeth my front sprocket had, because I switched from the stock 19 tooth sprocket about 20 years ago. The slippage is obvious now that I've counted my gearbox countershaft sprocket (21 teeth!) and compared my RPM's to my speed in 4th gear. I'm surprised that it slips because the clutch grabs so well in the lower gears when I goose the throttle.

The link that bluto posted allowed me to type in all my custom specs (tire size and front sprocket size) to get the projected RPM's for my set up. At 70mph, I should be running at 3999 rpm's by the chart. I know I am running around 5400 rpms, which means my clutch must be slipping.


Without dissassembling my entire primary, I removed the primary cover so I could inspect the plates and the stack height. My stack is flush underneath the groove for the retaining ring, so there's very little concavity in my clutch diaphram. I have to press on the stack with some force to get the retaining ring to seat in the clutch drum. I have an extra steel plate in my stack to lighten the clutch pull. I'm sure I could remove that plate, and have a clutch that does not slip at all, but I bet my clutch lever will be much heavier to pull. I've ordered 2 steel clutch plates from oldbritts in .070 thickness to replace 2 of my .080 plates and get a reduction in stack height of .020 I'm wondering if that will be enough to keep the light lever action and stop the slippage.... Anyone go through this adjustment?? if so, how much stack height did you need to lose to get your clutch to stop slipping??
thanks....

Glad gearing commander was helpful but I'm not so sure about the clutch slippage...when I was still running the stock bronze plates it would become a problem about once a year and was very obvious, gave the bike gas in top gear and the engine speed would increase but the bike speed would not. Gentle acceleration would get it up to cruising speed and it would hold that without slippage. Anyway have had clutch slip problems on a couple of bikes over the years and it was far from subtle, no need to look at the tach or speedo to know what was happening so I suspect something else is happening with your bike like maybe grossly inaccurate gauges, etc...

o0norton0o said:

I have an extra steel plate in my stack to lighten the clutch pull. I'm sure I could remove that plate, and have a clutch that does not slip at all, but I bet my clutch lever will be much heavier to pull. I've ordered 2 steel clutch plates from oldbritts in .070 thickness to replace 2 of my .080 plates and get a reduction in stack height of .020 I'm wondering if that will be enough to keep the light lever action and stop the slippage.... Anyone go through this adjustment?? if so, how much stack height did you need to lose to get your clutch to stop slipping??
thanks....

Click to expand...

I shimmed my clutch stack and found that a .005" difference was noticeable. I ended up with a single extra .065" steel plate, as a compromise between easy pull and slipping.

On the occasions when my clutch would slip, it was obvious in 4th gear under load. Giving it throttle up a hill I could feel and hear the engine rev without the bike accelerating. Whacking third on a hard accel would break it loose too.

If you can goose the throttle on the highway and not see the tach rev up quickly, then your clutch isn't slipping. In that case I would suspect one or both of your gauges lying to you.