Author Topic: Gear ratio 18-48 or 17-48 (Read 10527 times)

benny365 · « **on:** August 12, 2006, 12:44:04 PM »

73' 750k

currently run a 18-48 and I'm looking at 4000 RPM at 60ish M.P.H. in 5th gear

What are you guys getting for RPM with a 17-48 at 60 M.P.H.

I'm trying to squeeze out a few more miles per gallon.

Thanks Guys and Gals

Gordon · « **Reply #1 on:** August 12, 2006, 12:56:24 PM »

You're going the wrong direction to decrease the rpm's for a given speed. You would need to increase the front sprocket size or decrease the rear sprocket to lower the rpm. Keep in mind these engines need to rev high to make their power. 4K rpm is pretty low.

What mpg are you currently getting with the stock K3 setup?

benny365 · « **Reply #2 on:** August 12, 2006, 01:10:34 PM »

40 to 43 mpg

Possibly a 43 in the rear?

Gordon · « **Reply #3 on:** August 12, 2006, 01:16:22 PM »

I guess the real question is how much rear-wheel torque are you willing to sacrifice? IMO, the K3's are already geared about as high as the 750 should be geared. Any more, and you'll be downshifting just to get up the slightest hill on the highway.

Gordon · « **Reply #4 on:** August 12, 2006, 01:19:45 PM »

Quote from: benny365 on August 12, 2006, 01:10:34 PM

40 to 43 mpg

If that's what you're getting on the highway, then your bike could probably benefit much more from a thorough tune-up than from a change in gearing. I'm getting 40/50 (city/hwy) on my K1 with stock 17/48 gears.

GroovieGhoulie · « **Reply #5 on:** August 12, 2006, 01:20:52 PM »

Gordon is right. The 18/48 is real tall for those bikes, but Honda went to that for the later K2 onward. With those gears, supposedly they won't pull redline in top gear. The original ratios were 16/45, which was too small of a front sprocket as chains routinely broke, however, the bike would pull strong, even in top gear and see redline, which meant 125mph, so claimed Honda.

The sprockets were changed to 17/48, which maintains an identical ratio to 16/45, and I think is the ideal ratio for the 750.

Since economy seems to be the goal here, smaller rear or bigger front is the way to go I think, at the expense of "pep"

angeldeville · « **Reply #6 on:** August 12, 2006, 01:21:44 PM »

I usually go with a 17-48. or a 17-45, if you must, you really aren't going to save enuff $$ at the pump to pay for the sprockets.

Edit: I try to keep the front sproket as small as possible to give me as much space as possible in the already tight area between the sprocket and engine case.

Clyde · « **Reply #7 on:** August 13, 2006, 03:39:50 AM »

The original sprockets were 16/45, but at engine number cb750E-1003527 Honda changed to 17/45.
The 18/48 was introduced on the K1 (engine number CB750E-1044806) and stayed the same then through to the K6.
It is very common to change to a 17 front and pick up a bit of acceleration
The F1 used 17/48 but it had a different 5th gear and the motor has a higher red zone.
I use the 18/48 combination on my F1 as I find it reduces vibration at cruising speed (70-80mph) a little and also helps the fuel consumption a little. The main reason though is not about consumption, more about vibration and relaxed cruising.
I use 17/45 on my K0 and find it good around town.
The best combination depends on what sort of riding you do. The dollars you save are not great

I would not go any less than the 48, as you would make top gear an overdrive and be changing back to 4th for every hill.
Regds Clyde

cbjunkie · « **Reply #8 on:** August 16, 2006, 11:24:54 PM »

okay well as i was re-spoking tonight, which turned out to be a LOT easier than i had been imagining, i discovered that my K1 has a 45 rear - it is running an 18 front...(geez, i THINK - it has been a while since i had that cover off...)

is this likely a PO mod of some kind? i guess the rear looks pretty new - it still has plenty of shoulder to the teeth and i can still see the tiny machining marks across the flat tops of each tooth...

what does this setup - 18/45 - do to my torque? (sorry can somebody please explain the simple mechanics behind this for me?)

this is contrary to what i was expecting - i was expecting a larger rear...why? because i can never get even to the 2nd mark when adjusting my chain...? i thought that maybe he had used a larger sproket and that it was eating up chain legth, but now i guess that isn't right.

is 1.5-6 mark a normal read on chain tension?

thanks all
junkie out...

angeldeville · « **Reply #9 on:** August 16, 2006, 11:49:51 PM »

losing one tooth on the counter shaft is like adding 2.5 on the rear or something like that, I get confused with the math part of it

Gordon · « **Reply #10 on:** August 17, 2006, 06:42:07 AM »

Quote from: cbjunkie on August 16, 2006, 11:24:54 PM

what does this setup - 18/45 - do to my torque? (sorry can somebody please explain the simple mechanics behind this for me?)

It means you're not having as much fun on your bike as you could be!

This is a K1, right? Switching to a 17/48 ratio will give you back the torque that your current set-up is robbing you of.

ProTeal55 · « **Reply #11 on:** August 17, 2006, 06:50:35 AM »

I wanna get in this topic.

I have noticed that I need alittle more RPM around tollway speed, to keep up with the newer
rocket guys and such. I don't have a speedo , but I am guessing my max cruising speed is right around 60-65mph , at about 4500-5000 grand on the tach.

All I know at the moment is that I have a 48 on the rear , and am guessing that the front is stock (whatever that might be). I also am running a 16'' rim in the rear with a 130mm tire, so I am unsure if that has/did effect my top end "charge".

So , what rear sprocket do I need to be able to cruise this thing 70-75 MPH at a decent rpm ?
I am willing to loose some fun on the street to be able to cruise easier on the tollway..
My bike doesn't like anything over 5k rpm for too long of a time

OldSchool_IsCool · « **Reply #12 on:** August 17, 2006, 08:17:57 AM »

Imagine the front and back gears/derailers on your ten-speed peddle bike, the same principals apply here. To maximize torque (power/acceleration) a smaller front and larger back is called for. As speed builds, you can forgo torque for speed by increasing the front and/or decreasing the back. The price you pay for speed is limited torque you may need to pull a hill. That's why you may need to down-shift to get back some of that torque.

Our machines behave pretty much the same with the transmission acting as the derailers. We have the added burdon of having to decide on a single front/back tooth ratio for optimal torque in low gear vs speed in higher gears.

cbjunkie · « **Reply #13 on:** August 17, 2006, 08:27:53 AM »

so if i want to match the early bike (KO) i should just switch down the front to a 16 or 17 and keep the 45 rear...

hmm...too bad i just bought a new 18 - anybody want?

Bob Wessner · « **Reply #14 on:** August 17, 2006, 08:57:27 AM »

Though chains are stronger today, I'm not sure I would go with a 16 front sprocket. It was a contributing cause to the infamous "holed case syndrome." It also puts more strain and wear on the chain as the circumference is smaller (i.e., tighter turn). Just food for thought.

ProTeal55 · « **Reply #15 on:** August 17, 2006, 09:09:35 AM »

So if I have a 48 tooth on the rear now , and I went to a 45 , would that give me
more or less top end crusing ability ?

upperlake04 · « **Reply #16 on:** August 17, 2006, 09:16:50 AM »

ProTeal55 · « **Reply #17 on:** August 17, 2006, 09:39:26 AM »

Quote from: upperlake04 on August 17, 2006, 09:16:50 AM

more

Will the lack of 4 teeth really make a difference up top in the RPM's ?
Anyone have one of these (45ers) they wanna part with ?
Maybe swap for my 48 ?

super pasty white guy · « **Reply #18 on:** August 18, 2006, 09:12:03 PM »

Just browsing around and found a worksheet for gear ratios. Want to know what changing sprocket sizes will do? Plug in the numbers. Kinda cool

http://4strokes.com/tech/gearing_calc.xls

dave

HondaMan · « **Reply #19 on:** August 18, 2006, 09:30:13 PM »

Quote from: benny365 on August 12, 2006, 12:44:04 PM

73' 750k

currently run a 18-48 and I'm looking at 4000 RPM at 60ish M.P.H. in 5th gear

What are you guys getting for RPM with a 17-48 at 60 M.P.H.

I'm trying to squeeze out a few more miles per gallon.

Thanks Guys and Gals

18-48 is stock from K2 onward. It is overgeared for most users. To try proving this, take a 30-40 mile trip in high gear at 55 MPH or so, then refill and check MPG. Then, repeat the process in 4th gear the whole way. The results may surprise you.

For example, my K2 gets 39-44 MPG at 60 MPH on interstates in 5th gear. When I'm riding mountainous terrain, I often leave it in 4th gear for the high to get more throttle response. I get 48+ MPG in doing so. As a result, I just switched "back" to 17-48 after my rebuild because I will be mostly in heavy freeway traffic and city surface streets. This sets 60 MPH at 4200 RPM with my tires, and throttle response is better in high. I fully expect the MPG will also improve, as it has shown before.

Remember, the original K0 gearing was 16-45, which is almost exactly the same as 17-48. Honda steepened the gears to reduce warranty losses from excessive chain wear (caused by excessively jerky riders...).

ProTeal55 · « **Reply #20 on:** August 19, 2006, 07:33:41 AM »

Well -
I am guessing my bike has a 17 front , and I am know it has a 48 rear.
I just wanna be able to cruise alittle faster on the top end, and not buzz down the tollway like I do now. Do you think me switching from a 18'' rear tire to a 16'' had anyting to do with this ? I mean , I went down in rim size but up in tire (whatever stock size was to a 130mm now)

GroovieGhoulie · « **Reply #21 on:** August 19, 2006, 10:25:56 AM »

Quote from: ProTeal55 on August 19, 2006, 07:33:41 AM

Well -
I am guessing my bike has a 17 front , and I am know it has a 48 rear.
I just wanna be able to cruise alittle faster on the top end, and not buzz down the tollway like I do now. Do you think me switching from a 18'' rear tire to a 16'' had anyting to do with this ? I mean , I went down in rim size but up in tire (whatever stock size was to a 130mm now)

The smaller diameter rear tire would act like a bigger rear speocket and increase your RPM for a given speed.

ProTeal55 · « **Reply #22 on:** August 19, 2006, 01:58:00 PM »

Quote from: GroovieGhoulie on August 19, 2006, 10:25:56 AM

The smaller diameter rear tire would act like a bigger rear speocket and increase your RPM for a given speed.

So my 48 tooth sprocket is actually acting like say a 52 tooth becasue of the smaller tire ?

GroovieGhoulie · « **Reply #23 on:** August 19, 2006, 03:46:35 PM »

Quote from: ProTeal55 on August 19, 2006, 01:58:00 PM

Quote from: GroovieGhoulie on August 19, 2006, 10:25:56 AM
The smaller diameter rear tire would act like a bigger rear speocket and increase your RPM for a given speed.
So my 48 tooth sprocket is actually acting like say a 52 tooth becasue of the smaller tire ?

Could be.

The smaller tire will turn more times for a given distance than a bigger diameter tire, making your engine spin faster.

K8 Scout · « **Reply #24 on:** August 19, 2006, 06:20:16 PM »

My K8 sprocket on the front is a 15 and on the back is a 41....make any sense??? Oh ya the back tire a 17" rim too...

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Author Topic: Gear ratio 18-48 or 17-48 (Read 10527 times)

benny365

benny365

upperlake04

K8 Scout