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Author Topic: Why the countershaft speedo drive?  (Read 922 times)
Spurlock
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« on: December 17, 2017, 03:55:33 PM »

While waiting for my CB-1 to arrive in a couple weeks I've been reading through the service manual and am just curious, does anyone have an idea why Honda decided to drive the speedometer off the engine? Seems like a bad idea in the event someone wants to change sprocket gearing.

And also, does the speedometer on these seem to be accurate with stock tires and gearing? On my other two 1989 Hondas the speedos read around 3-5 mph high.

-Bill
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1975 Honda CB125S2, 1989 Honda NX250, 1989 Honda GB500, 1989 Honda CB-1
Efreeman55
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« Reply #1 on: December 17, 2017, 04:44:27 PM »

I agree that the engine-driven speedo seems strange considering front wheel drive speedos have worked so well for so many years and are independent of sprocket changes.  Plus, the CB1 drive is prone to problems.  I had a seized drive unit that sheared the plastic coupler when it jammed.  I was able to drill some small holes in the non-repairable drive unit, inject some oil and get the gears to move freely after some working back and forth.  I then cleaned the chamber using syringes and injected grease in there.  So far, it's been working fine.  Sure don't have that problem on any of my other bikes with front wheel speedos though!

I'm running a rear sprocket that's 3 or 4 teeth smaller for slightly less revs at highway speeds and also a 150/60 rear tire that combined give a reasonably accurate speedometer reading.

Eric
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a_morti
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« Reply #2 on: December 18, 2017, 04:00:54 AM »

It sure looks cleaner than a front wheel driven Speedo, and although rear wheel driven is used on a lot of modern bikes, I don't think it'd work with Bowden cables.

I've been around cb's since around 2000/01, and the only problem I know of with the drive mechanism is that if you don't carefully align the plastic drive part with the sprocket bolt, it will crack. They can (could a few years ago anyways) still be bought new so was never really a big deal. There's plenty of ways to brake a front wheel Speedo drive through misalignment so I don't see it as a disadvantage there.

I've had a front wheel Speedo where the cable unwound itself and the inner dropped out, couldn't really happen on a cb-1.
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« Reply #3 on: December 18, 2017, 09:46:04 AM »

i tend to think of the cb-1 as a factory custom kinda bike.  it was clean slate designed to be a naked bike.  as such they took some extra measures to make it look tidy.  theres cosmetic covers with fake bolts just to keep the look they were after.  the countershaft driven sprocket was just another way they went about cleaning up the bikes looks.  no messy cable running down the fork.  take the fairing off a same year cbr600 and you really see how much effort was put into the presentation of the cb-1 when they knew it wasnt just going to be covered by a fairing.  its a great design to this day imho. 
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« Reply #4 on: December 18, 2017, 11:19:17 AM »

Agreed!

Take a look at that sprocket cover and the attached plastic trim, and compare with the cbr600f1 or contemporary cbr400 which have the same engine block. Extra weight sure, but really tidies things up.
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See Bee-Won
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« Reply #5 on: December 18, 2017, 02:59:55 PM »

The speedo reads about 10% high on average. If you trade out the stock 41t rear sprocket for a 37t like many of us have to make highway runs less frantic you will be about perfect on your mph gauge/true speed calibration.
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Spurlock
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« Reply #6 on: December 18, 2017, 03:54:07 PM »

The speedo reads about 10% high on average. If you trade out the stock 41t rear sprocket for a 37t like many of us have to make highway runs less frantic you will be about perfect on your mph gauge/true speed calibration.

That's great info, thanks. By the way I see that JT Sprockets now makes CS sprockets with the rubber dampening rings to match OEM: http://www.jtsprockets.com/catalogue/sprocket/JTF297RB/

-Bill
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1975 Honda CB125S2, 1989 Honda NX250, 1989 Honda GB500, 1989 Honda CB-1
Spurlock
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« Reply #7 on: January 02, 2018, 08:34:28 PM »

The speedo reads about 10% high on average. If you trade out the stock 41t rear sprocket for a 37t like many of us have to make highway runs less frantic you will be about perfect on your mph gauge/true speed calibration.

I've read many posts here about the 10% optimistic speedo, but my sense from riding was that my speedo was pretty close to true. So yesterday I tested with a Garmin GPS device and was not surprised to find that my speedometer reads only 1-2 mph fast. I checked it on a 12 mile ride with speeds between 25 mph and 90 mph and it was always the same - reading only 1-2 mph fast. Since the slight error was linear that means the gear ratio of the speedo drive is correct and the needle is just advanced 1-2 mph too high. My gearing is stock 15/41 and rear tire is a stock size BT45  with about 50% tread left. I suspect if the rear tire was new the speedo would be dead on. Testing the odometer using mile markers along the highway I found it to read very slightly low, i.e.  4 miles actual records as 3.95 miles on the odometer.

So here I am, new to this revvy bike, used to my GB500 and NX250 singles but also quite experienced with four cyl. bikes, feeling the bike is geared much lower than it needs to be at least for my purposes. From the forum posts mentioned above I thought dropping to 37T rear would fix the supposed speedo calibration as well as lowering the revs. But for whatever reason in my case it would cause the speedo to read low and the odometer even lower so now I'm having second thoughts.

I'm not at all worried about the ability of the engine to withstand high revs, but 7,300 rpm at 65 mph just seems unnecessarily buzzy. I'll test the 37T sprocket out on my roads, but in the meantime would appreciate others' experiences with gearing changes.

-Bill

EDIT: I just noticed my rear tire is 140/80-17 rather than the stock 140-70-17, so about 28mm larger diameter. That must be why my speedo reads very close to correct.
« Last Edit: January 04, 2018, 10:15:51 AM by Spurlock » Logged

1975 Honda CB125S2, 1989 Honda NX250, 1989 Honda GB500, 1989 Honda CB-1
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