Topic: Gear Ratios, Speed, and whatnot

[DHuffman]

Hey guys,
First of all - I finally rode my CB today!!


However, I'm concerned my drivetrain configuration may not be where it should be. I converted it to a 530 chain (17T Drive, 45T Driven), with a 160/60zr17 rear tire.

I used the calculator on GearingCommander to make sure that the Engine Speed at each gear would produce similar output to stock - actually I wanted it to be a little faster gearing (i.e. lower rpm at 55-65 mph).

STOCK:


CURRENT CONFIG:


The bike rides great around town. However, I have to crank it to ~5k in 5th gear to get to 45ish mph. According to the gearing commander site, I should be at around 65 mph. Also get some engine braking when I let off the throttle.

It could be that my Vapor Speedo (wire around spark plug lead for tach pickup) is not getting an accurate reading, but the amount of engine braking I get seems suspicious to me.


This is my first bike, so idk what it should really feel like.


Any thoughts/suggestions or something I missed here?

[frenchy51]

I have no advice for you but nice looking cafe!

[BenelliSEI]

Do you have the original tach and cable? A temporary hook up might help to confirm tach accuracy? To check the speedo, guidance app on your phone should do it. Once to know accurate speed at say 5,000 RPM, let us know what gearing you have and changes you want. Lots of folks here can calculate the sprocket changes required.....

[DHuffman]

Do you have the original tach and cable? A temporary hook up might help to confirm tach accuracy? To check the speedo, guidance app on your phone should do it. Once to know accurate speed at say 5,000 RPM, let us know what gearing you have and changes you want. Lots of folks here can calculate the sprocket changes required.....

Negative on the original tach. It may be perfectly fine and just that I don't know what it is supposed to feel like. Hopefully the brake fluid isnt expanding and causing brake drag. Does it make sense if I ask what a "normal" tach value is for you when you cruise at 55 mph?

[Jerry Rxman Griffin aka MuthaF'er]

17:48 70mph at 5000rpm. 18:48 75mph. I don't think it will go 55 

[DHuffman]

Okay so I put about 150 miles on it this past weekend, and I think its the gearing is fine - I think the tach was just wrong (reading twice the actual rpm).

My vapor unit has a wire that you wrap around the spark plug lead to pick up engine speed. You can set the "pulse per revolution as 0.5,1 (default), or 2. It was originally set to 1. I changed it to 2, and the readings seem accurate.

Don't the SOHC4s, use a wasted spark system, so that the #1 cyl would actually spark twice per crank revolution? Just want to be sure that changing it to 2 had some substance and not just cover up a bigger issue like all 4 cylinders firing all the time or something.

[BenelliSEI]

No. The “wasted spark means t fires every revolution (both intake and exhaust cycles).

[maxheadflow]

No. The “wasted spark means t fires every revolution (both intake and exhaust cycles).

I think you mean compression and intake.

[Tracksnblades1]

No. The “wasted spark means t fires every revolution (both intake and exhaust cycles).

I think you mean compression and intake.

I think you both mean
Compression and Exhaust stroke
Or at every TDC (slightly before).

[Don R]

 Nothing to add to this discussion, I just wanted to reply 5 times in a row.

[jgger]

Yes it is a waste spark system as cylinders 1 & 4 fire at the same time but only 1 cylinder at a time "gains" anything from the spark. So when cylinder 1 fires the spark for cylinder 4 is wasted. At least that is how I understand waste spark systems.

[maxheadflow]

No. The “wasted spark means t fires every revolution (both intake and exhaust cycles).

I think you mean compression and intake.

I think you both mean
Compression and Exhaust stroke
Or at every TDC (slightly before).

Yeah, you are right..   I'll blame old age and the bourbon.

[Tracksnblades1]

No. The “wasted spark means t fires every revolution (both intake and exhaust cycles).

I think you mean compression and intake.

I think you both mean
Compression and Exhaust stroke
Or at every TDC (slightly before).

Yeah, you are right..   I'll blame old age and the bourbon.

I knew what you both meant..  I blame Siri, she don’t receive dictation well.

Scratch the Bourbon and head to the scotch.
The Glenlevit 18 (single malt speyside) and Johnny Walkers best Blue Label (Rednk USA) are both super smooth neat or iced down.

[DHuffman]

No. The “wasted spark means t fires every revolution (both intake and exhaust cycles).

I think you mean compression and intake.

I think you both mean
Compression and Exhaust stroke
Or at every TDC (slightly before).

Yeah, you are right..   I'll blame old age and the bourbon.

I knew what you both meant..  I blame Siri, she don’t receive dictation well.

Scratch the Bourbon and head to the scotch.
The Glenlevit 18 (single malt speyside) and Johnny Walkers best Blue Label (Rednk USA) are both super smooth neat or iced down.

I discovered the world of Peated scotch last night! Tried some Lagavulin 16 and bought a bottle of Ardbeg 10. Tasty stuff

I also momentarily had a lapse in thought and forgot that with 4 stroke engines, each cylinder enters the power stroke once every other revolution of the crank. So with a wasted spark, it sparks once per rev of the crank.

I checked the wires going from the points to the coils, and see no way that they could be bridged or anything weird, so I'll just chock the double tach reading to the sensitivity of the unit.

[ralt12]

Um, this is probably a dumb question, but where did you source the sprockets?

[DHuffman]

My front sprocket is a 17T offset sprocket from Cognito Moto. Rear is a steel Vortex I got from eBay

[70CB750]

Typical sprocket combination is 17 and 48 or 18 and 48, where 18 is used for travelling economy and 17 for around the town to make it more lively. 

I always run 17/48 but HondaMan changes the front sprocket accordingly.  5 000 rpms gives me 60 mph.


Here is a gear calculator for CB750:

[DHuffman]

Yeah I found out later that website I used loaded the wrong sprocket config for a 77 F2. I'm geared a little toward the bottom end compared to stock, but not that far off.

I've gotten a little more used to it and how the motor range feels now that I've ridden it some.

[Tracksnblades1]

Yeah I found out later that website I used loaded the wrong sprocket config for a 77 F2. I'm geared a little toward the bottom end compared to stock, but not that far off.

I've gotten a little more used to it and how the motor range feels now that I've ridden it some.

What sprocket sizes did you end up running...?

OP you indicated 17-45 530 chain with a shorter fatter tire.

I'm thinking your tach may not be accurate....5k in fifth to go 45.....?

Why are you thinking your geared on the low side...?  17/45 is 2.647 to 1. New  530 chain
                                                                               14/43 is 3.07 to 1. old 630 chain
                                   

[DHuffman]

I think the F models had a 17:48 stock (not 14:43 as the original post said). Having a 17:45, I'm geared a little more torque-y, right? I also do believe the tach was picking up the #2 cyl spark somehow. I changed the setting to 2 sparks per rev in the tach and it seems reasonable now. I think before a combination of having a doubled tach reading, and not having a feel for what the engine RPM range felt like (first time riding it), I thought it was getting a lot more spooled up.

Yeah I found out later that website I used loaded the wrong sprocket config for a 77 F2. I'm geared a little toward the bottom end compared to stock, but not that far off.

I've gotten a little more used to it and how the motor range feels now that I've ridden it some.

What sprocket sizes did you end up running...?

OP you indicated 17-45 530 chain with a shorter fatter tire.

I'm thinking your tach may not be accurate....5k in fifth to go 45.....?

Why are you thinking your geared on the low side...?  17/45 is 2.647 to 1. New  530 chain
                                                                               14/43 is 3.07 to 1. old 630 chain
                                   

[Tracksnblades1]

17/45 is numerically lower but higher geared than 17/48. The 17/48 would require more RPMs to go the same speed as the 17/45 rear tire being the same.

17/45 allows the front sprocket to turn 2.647 times per one revolution of the rear sprocket or tire.
17/48 "".                                         "  2.823 ".                                                                     ".

But your back tire rolls about 4.5"  less per revolution compared to the stock 18" wheel. With out doing the math but thinking  out loud.... I bet if you mark your back tire ( center tread)..then roll it 3 chain pitches ( back sprocket)....you will be close to the difference in tire circumferences.

Which I might add your 17/45 combo would have already done.

You might be really close to stock...

77/78f had 630 chain which was purposely geared lower from Honda.
The 17/48 was the factory F1 standard sprockets.
With your 630  14/43 combo at 3.07 to 1, your 17/45 530 combo will definitely make up the difference for your shorter tire and then some.

[DHuffman]

Okay looks like my initial math checks out then XD. Thanks for the info.

It definitely feels more "right" as Ive ridden it. Just having never ridden a motorcycle (other than some dirt bikes a long time ago), combined with me having built it, I think I was just being overly critical at first.