Can someone help me calculate Nexus 8spd gearing

N

nukezero

10 kW
Joined
Jul 2, 2013
Messages
560
Location
Los Angeles, CA
Hi I would like to compare cadence/wheel speed for the following:

16" wheel 52T chain ring and 14T rear cog
versus
16" wheel 52T chain ring and nexus 8 speed shimano hub (tallest gear)

how big of a difference are we talking about?
 
The late Sheldon Brown might be able to help there...

This is his gear calculator...

http://www.sheldonbrown.com/gears/

But you still need to specify the rear teeth count
 
Well according to shimano: http://www.shimano.com/publish/content/global_cycle/en/us/index/products/0/nexus/product.-code-SG-8C31.-type-..html

The smallest sprocket you can get seems to be 16T

So it would be 52:16 = 3,25:1 instead of 52:14 = 3,714

This is approx. 14% ( 3,714 / 3,25 = 1,14) of increase in pedal rpm to have the small sprocket turn as fast as it did before.
 
Exodia said:
Well according to shimano: http://www.shimano.com/publish/content/global_cycle/en/us/index/products/0/nexus/product.-code-SG-8C31.-type-..html

The smallest sprocket you can get seems to be 16T

So it would be 52:16 = 3,25:1 instead of 52:14 = 3,714

This is approx. 14% ( 3,714 / 3,25 = 1,14) of increase in pedal rpm to have the small sprocket turn as fast as it did before.
Click to expand...

So you're telling me that, that the smallest sprocket in the nexus 8speed bafang is a 16T. And if i were to convert/switch to the nexus 8spd, it would be disadvantageous for me since my 14T is a taller gear for top speed?

or wait, i'm not understanding this. If the smallest sprocket is 16T, and the internal ratios are:

Gear Ratio 1 0.527
Gear Ratio 2 0.644
Gear Ratio 3 0.748
Gear Ratio 4 0.851
Gear Ratio 5 1.0
Gear Ratio 6 1.223
Gear Ratio 7 1.419
Gear Ratio 8 1.615

then by definition, the "smallest" cog that I'd be comparing to is: 0.527 * 16T = 8.432T

So comparing apples to apples, 8.432T is a lot taller than the freewheel 14T. So if I were to use a Nexus 8Spd hub with 16T sprocket, I'd get a big jump in top end? Yes no?
Thanks
 
I think you mis understand

You have the gear ratio from front to rear no matter what.

So your cassette will give you a 52:14.

If the snakes cog the nexus accepts is a 16, you are starting off with a 52:16 ratio before you apply the hub gearing
 
CmdrBond said:
I think you mis understand

You have the gear ratio from front to rear no matter what.

So your cassette will give you a 52:14.

If the snakes cog the nexus accepts is a 16, you are starting off with a 52:16 ratio before you apply the hub gearing
Click to expand...

Ok, that's right. I start out with 52:14. Throw in the nexus 8spd, then i am now back at 52:16.

Now, I can apply the gear ratios which will reduce those gears further.
CorrecT?
 
nukezero said:
Ok, that's right. I start out with 52:14. Throw in the nexus 8spd, then i am now back at 52:16.

Now, I can apply the gear ratios which will reduce those gears further.
CorrecT?
Click to expand...

If there is another gearing stage inside the hub (missed that one), then of course you would calculate with the tooth count of the last gear.
In that case, your assumption is correct, and you should get twice the top end speed, on paper, of course.
Double check if shimano is really saying that it gears inside the hub with these ratios.

And I am not sure why you are mentioning "bafang" all of a sudden. Bafang 8Fun != Shimano Nexus 8 Speed (just in case)
 
Exodia said:
nukezero said:
Ok, that's right. I start out with 52:14. Throw in the nexus 8spd, then i am now back at 52:16.

Now, I can apply the gear ratios which will reduce those gears further.
CorrecT?
Click to expand...

If there is another gearing stage inside the hub (missed that one), then of course you would calculate with the tooth count of the last gear.
In that case, your assumption is correct, and you should get twice the top end speed, on paper, of course.
Double check if shimano is really saying that it gears inside the hub with these ratios.

And I am not sure why you are mentioning "bafang" all of a sudden. Bafang 8Fun != Shimano Nexus 8 Speed (just in case)
Click to expand...

i accidentally mentioned bafang, because i plan to use bbs-02 750w on a 52/14T. but now reconsidering to use 52T/8spd nexus. It seems like the nexus will yield me much, higher top end
 
nukezero said:
CmdrBond said:
CmdrBond said:
So I used Sheldon's calculator I linked above...

View attachment 1

Click to expand...

According to that, the 52×14 will give ~8.9 MPH on 37-305's + 170mm cranks.

Whereas the 52×16+Nexus would yield 52×16×1.62 = 12.6 mph
Click to expand...

If a bafang bbs02 750w runs at 120RPM, then essentially, it would be x2 right? so 12.6mph x 2 = 25.2mph
Click to expand...

That's correct. But I must say I find this calculator a bit confusing, especially the fact that the "high" gearing gives the higher speed. Normally it should be Teeth_output/Teeth_input, right? Which means that a smaller gearing gives faster output speed.
 
Highest gear is slays the biggest÷smallest teeth count.

Lowest gear is smallest÷biggest teeth count
 
You can get a 14t cog for the Nexus8.

Here's a practical comparison of MPH at 100 rpm crank speed:

52t/14t single speed = 14.9 MPH

52t/14t with Nexus 8 = 24.0 MPH, in high gear
 
Avitt said:
You can get a 14t cog for the Nexus8.

Here's a practical comparison of MPH at 100 rpm crank speed:

52t/14t single speed = 14.9 MPH

52t/14t with Nexus 8 = 24.0 MPH, in high gear
Click to expand...

is it easy/typical to swap the cog of a nexus out by yourself? or do you need to take it to a LBS to do the job?
 
If you have basic tools and skills, you can do it yourself. The sprocket is held in place by a split ring, which must be pried off. Here's a nice YouTube video:

[youtube]Dz-3PfboKTw[/youtube]
 
I have to correct myself:

The smallest sprocket for an Alfine/Nexus 8 is 16t, as previously stated.

So my comparison at 100 RPM should be:

52t/14t single speed = 14.9 MPH

52t/16t with Nexus 8 in high gear = 21.0 MPH

.
 
first off props to you ...thank you for starting a discussion that is on so many of our minds here....mid drive chain combo and or hub gearing and can the hub take it etc....

so how did it go for you. I am curious if your nexus 8 could handle the power of your bafang 750 watt. and how many watts actually is full power on that. is it 36 volts at 20 amps?

so then low gear would be what chainring and rear wheel multiplied by your gear.

52/14..front is 52 teeth and back is 14teeth... one rpm produces 3.7 rpm at the wheel.
so I hope I am right here so you take 3.7 times 0.5(low gear) and you get 1.85 rpm for every one rpm you put in at crank.

100 rpm at crank means 185 rpm at wheel. 50 rpm is half of that 92 roughly at wheel.

100 rpm on the back tire on a 26 inch tire give you a little more than 8 mph...
so if you peddle 50 rpm at crank then back wheel gets 92 rpm so your almost at 7 and half miles per hour.
I hope this is correct. if not please help me out....

so a little confused with the math here but if his rear hub high gear gear number 8 is three times higher than first gear. then roughly (7.5 times 3) 22 mph can be accomplished at 50 rpm from the crank.

but doesn't it get more complicated with amount of watts needed to move a bike. I have heard you need 750 watts at 333 rpm on a 26 inch tire to reach 26mph.
also I have heard that 20mph on a mountain bike needs 430 watts....on a bike where you lay down on it you only need 350 watts or power to go 20..


please correct me on any math out here or any confusion.....
I am very interested in your project and if I gave out misinformed data please correct me. thank you.
I have been wondering how you use a hub without breaking the gears and making a bike mid drive bike....with lower first gears...bike with a reduction and getting a ratio of about 1 to 4(on highest gear set) .. every 100rpm I get 400 rpm.... or every 50 rpm I get 200 rpm.
 
mountain biker said:
first off props to you ...thank you for starting a discussion that is on so many of our minds here....mid drive chain combo and or hub gearing and can the hub take it etc....

so how did it go for you. I am curious if your nexus 8 could handle the power of your bafang 750 watt. and how many watts actually is full power on that. is it 36 volts at 20 amps?

so then low gear would be what chainring and rear wheel multiplied by your gear.

52/14..front is 52 teeth and back is 14teeth... one rpm produces 3.7 rpm at the wheel.
so I hope I am right here so you take 3.7 times 0.5(low gear) and you get 1.85 rpm for every one rpm you put in at crank.

100 rpm at crank means 185 rpm at wheel. 50 rpm is half of that 92 roughly at wheel.

100 rpm on the back tire on a 26 inch tire give you a little more than 8 mph...
so if you peddle 50 rpm at crank then back wheel gets 92 rpm so your almost at 7 and half miles per hour.
I hope this is correct. if not please help me out....

so a little confused with the math here but if his rear hub high gear gear number 8 is three times higher than first gear. then roughly (7.5 times 3) 22 mph can be accomplished at 50 rpm from the crank.

but doesn't it get more complicated with amount of watts needed to move a bike. I have heard you need 750 watts at 333 rpm on a 26 inch tire to reach 26mph.
also I have heard that 20mph on a mountain bike needs 430 watts....on a bike where you lay down on it you only need 350 watts or power to go 20..


please correct me on any math out here or any confusion.....
I am very interested in your project and if I gave out misinformed data please correct me. thank you.
I have been wondering how you use a hub without breaking the gears and making a bike mid drive bike....with lower first gears...bike with a reduction and getting a ratio of about 1 to 4(on highest gear set) .. every 100rpm I get 400 rpm.... or every 50 rpm I get 200 rpm.
Click to expand...

My build thread is here: https://endless-sphere.com/forums/viewtopic.php?f=28&t=72805

but basically, I am able to hit 27-28 mph on this 16-inch folder at 750watt 48v Hot off the charger with the nexus 8 speed. The gearing gives me a MASSIVE advantage. however, I have since detuned it to 18A, 500watt so that I don't wear out the nexus hub. At 18A, it's a bit smoother and less sudden JOLT off the line. With 25Amps, it has a tendency to pull wheelies. Not to mention, at the higher amperage, there is a lot more vibration since a lot more torque is getting transferred.
 
very nice...sounds like a cool ride....i got a question. then with this nexus 8 spd what is the lowest chainring to sprocket gear ratio can you do within manufactures spec and has anyone gone lower than that. what mid drive kit do you have and how many actual amps volts and watts are you putting into it. and torque.

is it possible to have a reduction with the nexus 8 in low gear for hill climbing.

and thank you for your build.....tuned it down to 500 watts ...how is the nexus 8 doing?
 
You must log in or register to reply here.

Similar threads

S
BBS02 on an old Norco 26" kokanee
Replies
1
Views
789
Slim Parker
S
H
    • Like
Dahon Mu P8 that can pull 200kg up 20% grade
Replies
19
Views
2,505
Guerney
Guerney
N
Newbie NEEDS Help/Suggestions for new build
Replies
12
Views
318
MxusMick
M
amberwolf
    • Like
Amberwolf's Schwinn Trike Rebuild / Conversion to Heavy Cargo Hauler
2
Replies
44
Views
3,418
amberwolf
amberwolf
R
Mixing cogs from different speed cassettes
Replies
5
Views
3,573
richj8990
R
Back
Top