Sickbikeparts 4stroke shift kit shimano nexus 8 hub gear ratio

UtiliD

Member
May 11, 2018
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Clearwater, Fl
So I'm wrapping up a build and need to figure out where to make adjustments for optimal gear ratio
Here are the pertinent specs

26" wheels

Huasheng 49cc 4 stroke tapered shaft with
4g belt drive tranny 20t - 100t pulleys(5 to 1) to final 9t sprocket

9t sprocket to 17t SICK Bike parts left side jack shaft sprocket

Other side of Jackshaft 9t to 48t sprocket on outer side of front freewheel

Inside sprocket on front freewheel 30t sprocket to sprocket on Shimano nexus 8 hub (I've tried 18-22t)

I've only been able to use gears 4-8 and that's not going to fly.

Goal: be able to use all 8 gears and have a top speed of atleast 40-45

What do I need to change?

Will do a full build thread once it's worked out.
 

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Sidewinder Jerry

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Dec 19, 2011
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Let's start with the simple stuff first what is the total amount of weight (you, bike) you're trying to move? At what rpm is max horse power reached? What percent grade hills do you have to contend with?
 

Sidewinder Jerry

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Dec 19, 2011
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Ok, I looked up the stats on a Huasheng 49cc 4 stroke engine and max hp is reached at 6800 rpm which is 1.6 hp. That's the same as my RS 35 engine and I weigh 230 lbs. At best I'm able to get 30 mph on level ground using 26" wheels.

So what I'm saying is in order for you to get the kind of speeds you're seeking with your engine, you and your bike will have to be around 180 lbs or less.
 

Sidewinder Jerry

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Dec 19, 2011
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Here's your hubs ratios:

Ratios: (1) 0.527, (2) 0.644, (3) 0.748, (4) 0.851, (5) 1.000, (6) 1.223, (7) 1.419, (8) 1.615.

With the information you gave using the 18t rear sprocket gear (8) you'd have a total reduction of 18.71:1. This should give you a speed of 28 mph at 6800 rpm. Which is about right for your engine. Trying to lower the reduction any lower on an 8 speed IGH is probably going to damage it. To get 40 mph you'd have to be in the 13:1 reduction range and 11:1 for 45 mph.

Get a tachometer if you can't reach 6800 rpm in a gear then you need to down shift.
 
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UtiliD

Member
May 11, 2018
51
4
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Clearwater, Fl
No hills
I'm about 180
bike probably weighs 40-45lbs maybe 50
It's a huasheng 49cc 4 stroke you guys have the HP and rpm
The nexus 8 is has a 307% gear range and
These are the relative gear ratios for just the hub if I use a 20t sprocket on the hub
1st .53
2nd .64
3rd .75
4th .85
5th 1.0
6th 1.22
7th 1.42
8th 1.61
 

Sidewinder Jerry

Well-Known Member
Dec 19, 2011
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No hills
I'm about 180
bike probably weighs 40-45lbs maybe 50
It's a huasheng 49cc 4 stroke you guys have the HP and rpm
The nexus 8 is has a 307% gear range and
These are the relative gear ratios for just the hub if I use a 20t sprocket on the hub
1st .53
2nd .64
3rd .75
4th .85
5th 1.0
6th 1.22
7th 1.42
8th 1.61
Reread my last response.
 

UtiliD

Member
May 11, 2018
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Clearwater, Fl
I was hoping someone that had done a SBP shift kit build with a Nexus 8(or a shims no aline 8 or even a sturmey 8 speed would chime in). I believe the fact that 1 to 1 gearing isn't until 5th gear is why people have a hard time with this.


I'm getting nothing out of the 1st 3 really 4 gears. It's geared to low. The bike doesn't even move. The hub works and is hooked up perfectly. I used top notch outer and inner cables. The hubs yellow dots are always lined up perfect in 4th gear. I have the Stanton clutch that's the best for these engines (in my experience anyway). I'm almost positive I need to change the 30t chaining to a 36 and I'll be golden. From there I should be able to dial it in with the cog on the hub I have every size nexus/alfine cog .

Again I was hoping someone that had done a SBP shift kit build with a Nexus 8 speed IGH would chime in.
 

UtiliD

Member
May 11, 2018
51
4
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40
Clearwater, Fl
Reread my last response.
Read my last one too.
I know guys that run these on electric middrives well over 1000watts as commuters and have no problems with them. I know how to service these hubs and how not to shift them. As long as you let off the throttle between shifts these can last forever as long as your not running a coaster brake or roller brake and keep it from constantly getting drenched.
 

Sidewinder Jerry

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Dec 19, 2011
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Something is very wrong then in 1st gear you'd have a 35.5:1 reduction; which should give you a speed of 14.8 mph at 6800 rpm. The greater the reduction the easier it should be to take off in.
 
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Sidewinder Jerry

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Dec 19, 2011
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As I'm sitting here thinking about it. I really hope you haven't stripped out gears 1-4. Most of the long term builders who like internal geared hubs stick to a 3 speed or might use a 5 speed at the most. The more ratios the more delicate the internal parts are.
 

UtiliD

Member
May 11, 2018
51
4
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Clearwater, Fl
Just rode it again with the motor & figured out that I just wasn't giving enough throttle in the lower gears 1 - 3. They all work they just red line immediately and top speed in 8th gear is in the mid to high 20s. Pointless for a shift kitted bike in my opinion. I'm absolutely sure now the 36t sprocket will put me right where I want to be.
 

UtiliD

Member
May 11, 2018
51
4
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Clearwater, Fl
I appreciate the suggestion. It wouldn't really help because the 2 builds are so different. I wouldn't be able to take anything away from it without sitting down and writing equations that could convert the 2 builds to relevant comporable data and I already had it figured out anyway.

I got the 36t sprocket to replace the 30t sprocket that drives the chain running to the rear hub. I also put a 18t cog on the Shimano Nexus 8 internal geared hub. It worked like a dream.

The issue I was having with figuring the top speed(gearing in general) is that the nexus 8, with a 18t cog, doesn't hit 1 1 gearing until 5th gear.

My new gearing is way more than I need top and bottom. I don't have a speedo but I was clocked by a traffic radar warning sign going 34 and I was going slightly up hill and was nowhere near wot.

If I had it to do again, I would have used the 80t rear pulley on the Grubee 4g tranny for gearing but, also for crank arm clearance. I have 152mm cranks on this bike and...

a Nirve Cruiser I'm wrapping up a very similiar build on hopefully tomorrow(I've got a 3 speed Nexus IGH on that one).


Because the tapered shaft Huasheng 49cc 4 stroke / Grubee 4g combo (I just recently learned) is wider than any combo Sick Bike Parts has on their list of compatible motor/tranny combo drives to mate with their 4 stroke Shift Kit you need a pretty large frame or a lot of luck to make the standard 175mm crank arm clear the rear tbelt pulley of the Grubee 4g tranny (and that's before you put the cover on).

That being said I don't notice the the difference in crank length. I "try to" pedal when 1st shifting into a higher gear to minimize stress on the internal geared hub and when I see someone that may give me a hard time about what I'll call alleged philosophical differences about the bike. Have yet to have ever gotten any flack since I started riding. Wow I don't think there's enough wood in the county for me to knock on to counteract the jinx I just walked myself into...LOL.

Build threads to come.

Jay
 

cannonball2

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Oct 28, 2010
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Colonial Coast USA.
I admit the builds are different was, thinking more along the lines of the final drive ratio. Without reading thru my thread from memory, the SA hub reaches a bit of over drive in third.. You really only need to figure the final ratio for top gear if that’s the one you will use. That’s the issue with so many speeds. If you are building a trail bike then the large gear range would be useful. Would be not much fun on the street, as it would involve a bunch of shifting or gear skipping. Not an issue with an IGH, but not ideal. I have built many bikes using the Nexus three speed. While somewhat fragile, they work fine with an auto clutch as the power is gradually applied. The fastest bike I built used a CAG 49cc and a Nexus 3spd. It’s ability to rev so high allowed a low ratio for pull away from stop, and to reach speed of 45+ turning close to 10k.

the SA fixie hub is robust enough to pedal start the CG engine. It also is a locker so there is engine braking versus free wheeling. I got a bit overboard on the final ratio on it and the bike is a bit faster in second than third as it a bit to heavily loaded to rev out. Still it’s a pleasure to ride.
 

Sidewinder Jerry

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Dec 19, 2011
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Probably the cheapest thing you can do is change one of the 9t sprockets to a 10t or an 11t. With the 10t sprocket at 6800 rpm you'll have a mathematical speed of 37 mph in 8th gear. With the 11t you'll have a mathematical speed of 41 mph at 6800 rpm in 8th gear.

Realistically consider yourself very fortunate if you can move a total weight of 230 lbs at 37 mph on level ground using 1.6 max hp. Get a tachometer/ hour counter and bicycle computer so you'll know exactly what you're doing. You should be able to get both for less than $30 off eBay
 

cannonball2

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Oct 28, 2010
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Colonial Coast USA.
I agree totally with Sidewinder Jerry. With the HP you have to work with be conservative on top end. 35 is good for your weight and engine combo. With the lower gearing you have getting there will be no problem. I have a HS engine that turns a bit over 7k. So you may get more speed if you don’t over load the top end, allowing the engine to rev out. The suggestion of a tach is critical for tuning. You can use the calculators found on the forum or on line to calculate top speed vs rpms. Before I do any gearing I calculate the top speed based on wheel circumference and projected engine rpm, overall weight, and terrain. My bikes in north Ga required totally different gearing than where I live now in the Ga coastal flats. I can run much higher ratios here. The beauty of an IGH is being able to shift with out respect to gearing down before stopping unlike a derail bike. A five speed may be a better choice for general use, though generally I find the three speed more suited for street riding where I live.
 
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Sidewinder Jerry

Well-Known Member
Dec 19, 2011
1,507
343
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Rockwood, TN
I agree totally with Sidewinder Jerry. With the HP you have to work with be conservative on top end. 35 is good for your weight and engine combo. With the lower gearing you have getting there will be no problem. I have a HS engine that turns a bit over 7k. So you may get more speed if you don’t over load the top end, allowing the engine to rev out. The suggestion of a tach is critical for tuning. You can use the calculators found on the forum or on line to calculate top speed vs rpms. Before I do any gearing I calculate the top speed based on wheel circumference and projected engine rpm, overall weight, and terrain. My bikes in north Ga required totally different gearing than where I live now in the Ga coastal flats. I can run much higher ratios here. The beauty of an IGH is being able to shift with out respect to gearing down before stopping unlike a derail bike. A five speed may be a better choice for general use, though generally I find the three speed more suited for street riding where I live.
My formula for speed in mph is:

Max hp rpm ÷ total gear reduction X wheel diameter X ⚻ ÷1056 = mph.

IGH calculations can be a little tricky. There's two ways to figure mph using them.

Calculate input rpm up to the rear IGH cog then multiply it by the gear multiplier for that particular gear for an output wheel rpm you can then put this number into the speed formula:

Wheel rpm X wheel diameter X ⚻÷1056 = mph.

You can also calculate total gear reduction for a particular IGH gear. Here the principals of inversion must be applied. You'll need to calculate input reduction up to the rear wheel cog then divide it by the gear multiplier for the total gear reduction. Then you can use that number in the speed formula.

Input Reduction ÷ IGH gear multiplier = Total Gear Reduction.

Rpm ÷ TGR X WD X ⚻ ÷1056 = mph.

I personally like to know what the max horse power speed for a particular gear is and the max torque speed for a particular gear is. So at full throttle if you can't reach minimum max torque rpm the gear is too high and you need to down shift. If at full throttle you're going way over max horse power rpm then you're in too low of a gear and need to upshift.

Unfortunately, with some of these gear calculators they don't apply the principals of inversion to get the true total reduction of a particular IGH gear.