48cc Power Curve - RPM Optimum & Tach

Has anyone made or seen the power curve for a 2009 48cc Grubee motor?

Specifically, what RPM is the 'sweet spot' for most power? and though I know an X-chamber increases the whole band it ban be tuned as well, so would it be best to tune the X to the sweet spot, or the weak spot in the power band?

Finally, has anyone put a scope or engine tester on a 48cc to get the full RPM range and frequency of it?
How about making a tach for it with that info?

The reason I ask is I have a brand new NuVinci Developer Kit programmable hub in transit as I type this.

I was going to just build a whole new bike to put it on but can't swing that right now so I am just going to have Gordy's Bike Shop lace it to my Giant Suede with a stock 48cc Grubee.



The trick for optimum performance with fully automatic hub is set enough shift point keep the motor in the 'sweet spot' during acceleration.

Any help and thoughts are appreciated.

Well they do make 2-stroke tachs.

Best deal I could find was direct from China for ~$25
Engine Digital Hour Meter & Tachometer - Tootoo.com

Local stuff is ~$80 to well over $100 but those say 'boat' on them and I sure know what that means. The same thing for twice the price.

Most work by inductance with a clip or by just wrapping a wire around the plug wire, and grounding the other wire.
They all seem to have an engine hour counter as well, I am familiar with those from boats and heavy equipment.

It might be a handy next to your speedo for those with 'inquiring minds' as it where ;-}
What we really need is a little speedo with tach for <$50.

Thanks for the tip BA, I PM'ed 4easy and Pablo at SickBikes too, those guys know these things too.

My hub hit town yesterday so I expect it any time now so I was going over the specs again and the controller does have an engine RPM input but it is currently 'unused'.

Vinci... Is the actual circuit in there for inductance RPM and just not programed?
Or is it just a wire connection and no circuit?

I am still kicking myself to this day for selling my Techtronic Scope several years back, I could have answered all my own questions if I had one and might even be able to make own digital one.

It is far from crunch time, I am in the middle of moving and still need to get the hub laced in before I get to that, but I like to try to get all my ducks in a row.

Spiffy, my hub is here.







The package was 25# and most of that is the rather formidable hub.

I just talked to Gordy's and they can lace it up with new heavy spokes for ~$50 but as excited as I am considering doing it for free just to learn about them and a little promo.
That would be a good thing for me.

I am still going to need a 12VDC power source for the electronics but I have a plan for that too.
Electronics are my actual forte.

Since it is a deluxe kit with the digital readout I should be able to glean some info from it, but knowing what the RPM range and sweet spot is would sure make it easier.

Come to think of it, I just happen to have a 12VDC power source, maybe I'll start by playing with the software and programing before it's even mounted ;-}

foureasy said:

there is no "sweet spot" without an expansion chamber.

Click to expand...

Well that's just silly, all motors have a power bell curve. No sweet spot would mean it puts out as many ft/lbs of torque at idle as 5K RPM.

OK, I did some reverse math to get a ballpark of these motors RPM's, check it for errors please as my result seems low.

The circumference of a 26" wheel is 81.6" or 6.8'
A mile is 5280'
To travel 1 mile the wheel must rotate 776.5 times.
To travel 30 miles it is 23294 rotations.

Divide that by 60 minutes = 388 rotations per minute to travel 30 miles in 1 hour.
Multiply that by the standard 4.4:1 gear ration = 1708 motor RPM to travel 30 MPH.

Seem right?

foureasy said:

Oh yeah, your motor is basically.putting out the same amount of torque at idle and wot. The only thing giving you more power is.rpms. you need an expansion chamber to" supercharge" the cylinder during the "sweet spot".

Click to expand...

I knew that hehehe, I was just poking fun at you ;-}

So the stock 48cc motor sprocket is 11 tooth and not 10?

I have a jackshaft already on the bike, it is just getting the new NuVinci fully programmable, fully automatic hub and I looking for the right numbers to program it with.

This is the first Excel spreadsheet you fill the numbers in for the gear ratio from the pedal sprocket to back sprocket, wheel size and how fast you want to go.



Easy enough.

But then you use another spreadsheet and use the column on the left to enter a Source RPM you copy to the actual shift map and where it gets confusing.



The default is 75 RPM for the pedal sprocket and then it uses the gear ratio you entered on the first worksheet to calculate shift pattern and relative speed.

I figure 75 RPM was pedal speed so I tried 85 and 300 as I can have 2 shift tables I can switch between and it got really confusing.

This is the graph for RPM vs Speed.



But then I looked at shift position vs Speed.



According to that 300 RPM doesn't shift until you hit 25 MPH from it's lowest gear.

(cont below)

{cont from above)

I suppose that makes sense since the hubs gear ratio is .5 to 1.8:1
What I need is the 'magic number' for the final pedal side socket through a Jasckshaft to try to maintain about 4500 motor RPM once it gets up to speed and starts up-shifting.

I used a 10 tooth motor for my calculations.



I'll have to see how an 11 tooth changes the sprocket RPM I get at 4500 motor RPM's.

Easy program though, and I have the little controller on my desk.
It works with just the USB power to it so I can program it.



Spiffy stuff ;-}