Utilizing the derailleur gear system?

[86BG]

The bike I'm picking up tomorrow is 44 crank, 14 sprocket, and 26 wheel. That's 185 RPM needed at the crank to achieve 45 MPH, correct?

I don't understand why you squared your division. Is it because it's being geared UP isntead of down? Can you help me understand it using these new numbers as an example?

Also, I was just thinking about something -

The electric starter is mounted permanently ont he spinning shaft, so I think it doubles as an alternator since there was this little box I ignored that was in the battery compartment that could be a voltage regulator. Freakin cool!

 

[duivendyk]

OK I figured out that the rearwheel rpm was 650 rpm (I work in metric units so you'd find that hard to understand,your speed calculator does the same thing in inches,feet&miles which drives me nuts,but the calculations are OK. Your calculation figured out the front sprocket speed as if the front one was 14 and the rear one 29. The front sprocket speed you computed was
29/14* S.,instead you should have figured it at 14/29*S .The ratio is (29/14)/ (14/29) which is (29/14)* (29/14) or 29/14 squared.
(S is the rear wheel speed),that's is around 4.3, that was the error in the calculation.There is some discrepancy between your speed calculator and my values, the real life wheel diameter is about 3.5% smaller than the official (unloaded) numbers,I use the actual values.I would say that your gearing seems rather high (low reduction),what is the max rear sprocket size and the engine displacement?I hope things are a bit clearer now.

 

[86BG]

49cc, the math we are doing is for the last gear, so the gear system will generate loads of torque

 

[duivendyk]

You were calculating for the top speed condition,I think these rear deraileurs have 28/32 teeth max or about 2 to 1.These engines can rev up to about 7k,but you'll prob have run out of torque by then,do you know anything about it's HP rating?.I would be inclined to use a 5.0 reduction or even higher.The best thing would be to make it easy to change the drive to the jackshaft.That would still leave you with an overall reduction of 5*5* (14/29)=12.and a top speed of 42 mph at 7k.But you can play around, if your top speed turns out to be the same or higher in a lower gear, you have overdone it All these engines have similar torque characteristics,but the higher performance ones maintain torque better at high rpm.

 

[duivendyk]

My computations were for a 24" wheel not a 26",your road speed will be about 8% higher, all the more reason to go for a larger reduction.

 

[86BG]

I'm leaving to go get the bike now

 

[86BG]

welded a piece of tubing between the frame bars and put engine mount on top of it and welded it down as well.

engine is mounted and ready to go.

 

[86BG]

So with the new Tire size of 26, top gear rear sprocket of 14 on the rear and 44 on the crank the calculator says it'd take me 185 RPM at the crank to go 45 MPH.

6000 / 185 = 32.4

So if my engine has 5:1 reduction gearbox and I run a chain from an 11t sprocket on the engine to a 30t sprocket on the jackshaft I have 2.72:1 reduction. Then if I run another chain from the jackshaft on the left side down to the crank from a 11t to a 26t sprocket I get a 2.36:1 reduction.

So if I multiply them all up to the crank

5 x 2.72 x 2.36 = 32.09

6000RPM / 32.09 = 187 RPM

Is my math correct?


http://www.diygokarts.com/speed-calculator.html

 

[Bigwheel]

Your math is better than your idea to want to go 45 mph on a bicycle not designed with that type of speed in mind, especially with the extra weight of the engine, jackshaft and welding stuff on there without any heat treating is not a good idea. A motor cycle or a scooter is a much better way to get that type of speed safely.

The law in most states says 30 mph max for a gas motor bike and the more folks out there abusing that will make changes for the worse via big brother come along instead of for the better. Apologies if you plan to ride your creation strictly offroad.

 

[duivendyk]

You math is OK ,but I think you should use two 30 t sprockets for an overall reduction of 5x2.72x 2.72=37 and a topspeed of 39mph.This corresponds to an overall reduction from engine crank shaft to rear wheel of 22 which is about right.Most people don't seem to realize how dramatically power requirements increase with speed due to aerodynamic drag.To go from 35 t0 40 mph requires an 50% increase in power,and to go from 30 to 40 at least a 100 % increase.You are likely to run out of torque before you get above 40mph.Besides I think any speed above 30 mph is very risky.Do you have life&disability insurance?