79 38T vs 42T rear sprocket

79 38T vs 42T rear sprocket

Postby CopperCollar » Fri May 06, 2011 12:15 am

Any one running a 38T rear sprocket and if so what do you think of it.

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Re: 79 38T vs 42T rear sprocket

Postby daves79x » Fri May 06, 2011 5:34 am

I'm assuming you are talking about the 530 chain? Some guys run that high of gearing, but you lose a lot on takeoff. Lots of guys drop a couple of teeth with OK results, but Honda got it pretty right with the stock 18/42 combo. These things will run all day at 5-6000 rpm or more without problems. You'll also have to shorten the chain dropping that many teeth.

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Re: 79 38T vs 42T rear sprocket

Postby EMS » Fri May 06, 2011 7:09 am

I am running a 41T rear sprocket from a CB1100R. It came from Honda with the guide ring. Looks original. 8)
Going down to 38 is almost 10%. That's a lot.
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Re: 79 38T vs 42T rear sprocket

Postby Don » Fri May 06, 2011 9:57 am

I suspect that with my 140/80-17 rear tire and the stock 42 tooth sprocket I now have the equivalent of a 43 or 44 tooth rear sprocket - Anyway, I have exactly 5,000 RPMs at 70 MPH and that really works well for me

I don't like riding the freeways so anytime I get up around 80 or 90 MPH (or faster) it's for very brief bursts . . . . straight sections in a twisty road . . . . and that's not usually in 5th gear ;)

If you like freeways and want a little lower RPM's for long rides, a 40 or 41 would be acceptible, I would think. If you like to 'play' then 43 or 44 would be good, but as Dave said, for all around performance, Honda got it about as perfect as you could get with the 18/42. Going down to a 38 rear would take much of the 'fun' out of the bike, IMO . . . . too big a change

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Re: 79 38T vs 42T rear sprocket

Postby CopperCollar » Fri May 06, 2011 3:00 pm

All very resonable answers, She just seems to be taching away on the freeway....I wasn't sure of the 38T as with the stock 42T she doesn't tolerate a sloppy throttle hand or clutch hand. I'll stick with the stock sprocket.

Thanks,

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Re: 79 38T vs 42T rear sprocket

Postby EMS » Sat May 07, 2011 8:49 am

CopperCollar wrote: , She just seems to be taching away on the freeway....I Thanks,

Mike


A lot of this is a perception thing and depends on what other bikes you are used to ride. Remember, the CBX is a high rpm-horsepower bike. It was not designed to make lots of torque at low rpms and thus satisfy the typical American cruising-style rider. The engine likes to be run above 5,000 rpm, a spot where the typical Harley ignition, for example cuts off :roll: On a CBX, you have to make use of the transmission instead of rolling-on the throttle in high gear out of a turn.
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Re: 79 38T vs 42T rear sprocket

Postby rekit » Sun May 08, 2011 1:21 pm

On my 81 I didn't like the "busy" feel doing 70 mph. I went with a 38, and liked it. A year later (last week) I went to a 34. The smallest you can get, and I love it. I am running a big windshield and have plenty of power. With the 38 I was running 61 at 4000, now I'm at 65 at 4000. I forgot what 4000 was stock on an 81. Maybe someone can chime in. I do slip the clutch a little more on takeoff, but a lot of that is my carbs are not perfect on the low speed settings right now. But at 4000 the throttle response is so great it is hard to adjust the speed just 1 or 2 mph. The slightest touch jumps up 4-5 mph. The low gearing helped with that too. On a side note the chain fit both 38 and 34 with no change, but I dropped 4 links when I went to the 38. Go for it!
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Re: 79 38T vs 42T rear sprocket

Postby EMS » Sun May 08, 2011 5:16 pm

4000 rpm with stock sprockets is approx 56 mph in 5th.
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Re: 79 38T vs 42T rear sprocket

Postby Don » Mon May 09, 2011 9:39 am

I'm sure that with a 38 running constant freeway speeds, the engine (and the oil) will run hotter than if you left it with a stock 42 tooth sprocket

When I installed the oil temp gauge on my bike I was really surprised at what it told me. I *expected* that the oil would get hottest in stop and go city traffic and then cool off almost immediately once I got it up to a steady 70 MPH where it has excellent airflow around the engine and through the oil cooler - Not so as it turns out

My oil temps are coolest poking along in the 35 to 45 MPH range and HOTTEST doing a steady 70 MPH down the freeway. City traffic falls somewhere in the middle, unless the 'stop and go' is much more 'stop' than 'go' - After 15 or 20 minutes on the freeway, it takes half an hour or more of 35 to 45 MPH to cool the oil back down

I owned high performance Corvairs for several years and all of the Corsa models came with a cylinder head temp gauge. I never had an oil temp gauge on one and I don't have a CHT on my X, but you can still draw concusions. On a long uphill grade, the Corvairs would all get hot if you left them in high gear and just pulled the grade. Plenty of power . . . . no need to downshift, even to pass but after a couple miles in a steady grade, the CHT would go from about 300 to 450 (the gauges went to 600). Downshifting and running the same 70 MPH up the grade would cool the engine back down to normal temps. On an aircooled bike, downshifting doesn't get you more cooling air like it did in the car, but the more favorable gear ratio and higher RPM's make the engine work much less and generate less heat. I wouldn't try pulling grades on an X geared 18/34 no matter how much power it feels like you have

It would be really interesting to see the freeway oil temps on a bike geared 18/38 . . . . I suspect the temps on a bike geared 18/34 would be downright scary - Run full synthetic in that bike for sure. I know how hot the oil gets on a stock geared bike . . . . even with an aftermarket 10 row Earls cooler

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Re: 79 38T vs 42T rear sprocket

Postby EMS » Mon May 09, 2011 2:28 pm

Interesting thing is, my 79 shop manual shows a final drive of 15/35, which is exactly the same as a 18/42 :?

But if you crunch some numbers, you will find, that running a 34T rear sprocket with a 18T front sprocket will make your CBX act like you are taking off in 2nd gear all the time:

First gear ratio is 2.438. With a 18/34 (= 1.89) final reduction, you end up with a total of 4.61 ratio (Stock would be 2.438 x 2.333 = 5.69)

The stock bike in second gear is 1.750 x 2.333 = 4.08

I am not sure, if I would want my clutch to do this all the time.
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Re: 79 38T vs 42T rear sprocket

Postby rekit » Thu May 12, 2011 4:28 pm

Interesting. I really never thought about the oil temp. Most bikes are so overpowered. I did a quick test of the oil temp today with my 34t. I came home from a 70 mile ride, and locked it on 70 mph for the last 6 miles in 5th gear. I used a Fluke temp gun on the oil pan and the cooler in/outlets. It showed 230 F at the front of the pan and 211-214 on the cooler with the engine idling. I went out and ran the same 6 miles in 4th gear (5000 at 70 mph), and got the same temps. Air temp was 79F. I know I need to start with a cool motor and go farther next time, but I was glad to see no big change on this quick test.
I like the idea of synthetic, but what about the clutch. I've been using Rotella in everything for a while. Honda Turbo jetski's, a Mean Streak 1600 v-twin, my cars and truck.
I really like the overdrive effect of the 34 and I can always downshift. Where is a good place to install an oil temp gauge, and who makes a good one?
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Re: 79 38T vs 42T rear sprocket

Postby EMS » Thu May 12, 2011 5:11 pm

Mobil 1 4T 10W40 is specifically formulated for Motorcycles with wet clutches.
The place for the oil temp gauge is being debated. Some say close to the hottest spot (cylinder head) some say in the oil pan (at the drain plug) A common spot is the oil gallery behind the cylinders. It is capped with a plug that can be replaced with a sending unit.
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Re: 79 38T vs 42T rear sprocket

Postby George R. Garrison #0857 » Thu May 12, 2011 7:19 pm

Regardless of how hot the oil gets,, its good to read and monitor the temp of the conditioned oil as it returns to the engine.

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Re: 79 38T vs 42T rear sprocket

Postby rekit » Thu May 12, 2011 10:56 pm

Alright, Mobil 1 4T is in with a K&N 401 filter. God , this thing hauls ass. I can't help but wonder what it's like to ride a "Busa...... And there are folks that go to a larger rear sprocket............
BUT-why would the return oil temp ever change-unless you plugged the cooler with screw bugs.
New temp readings coming soon. What are you guys reading when aiming a temp gun at the front of the oil pan? Also my alternater makes whirring noises from time to time. I 've read that at 14000 miles, it needs new brushes and bearings. It read 153F today. How about an exact part # list to do that job? Thanks.
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Re: 79 38T vs 42T rear sprocket

Postby EMS » Fri May 13, 2011 7:21 am

rekit wrote: , it needs new brushes and bearings. ... How about an exact part # list to do that job? Thanks.



Bearings are standard ball bearings 6201 and 6203. Brushes are 31124-422-005 and 31125-422-005 ,alternatively 31124-MA2-005 and 31125-MA2-005.
I believe the 31224-MG5-000 and 31225-MG5-005 (from the CB1100F) work also.
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