Thanksgiving Dinner Debate About GoPed Gearing

[schroder]

Danger, I couldn't care less about sayings I don't understand either, so what is turning a moehill into a dolly parton anyway? Look what does my text say in highlighter? "The angular velocity ratio between the gears must remain constant". Don't like angular, just think velocity then, it is only a factor of 2PI anyway. You said your self that you would be changing the gear ratio which is Rinput/Routput and the drive spindle will remain at the same speed. Therefore you would be changing the output velocity as demonstrated in my second equation. The only reason I am so extensive in what I have presented is because what LionTiger presented to me seemed to make sense, I won't disagree it seems logically correct but I was pretty sure there was more to it than that. Upon consulting my machine design textbook I found the fundamental law of gearing and this is what makes sense to me as far as my interpretation of the situation goes.

The fundamental law of gearing proves that LionTiger is correct. To calculate the linear velocity you multiply the angular velocity by the radius, not 2pi. As you can see, this is a self compensating relationship. As the radius increases, the angular velocity must decrease but the product remains constant. You cannot argue with that. It is very simple math!

 

[jaron_denson]

No professor, you are quite mistaken. Linear velocity is equal to the angular velocity multiplied by the radius! Not by 2pi!

Try your calculation again, the correct way this time, and you will see that the linear velocity is unchanged by the different wheel radius. It is not difficult math, as you said.

First of all I am not a professor, I am a student. My professor probably did not fully understand what I was asking him when I asked for advice. And you responded to me while I was responding to danger that is why I did not fix the 2PI. It is simple math. Yes in-fact angular acceleration is measured in cycles/sec so to get to linear you would have to multiply by the 2Pi*radius of the driven wheel. Which would cancel the effect of the output radius of the tire on the right hand side of the equation making the linear velocity only dependent on the radius of the driver wheel itself, my bad didn't catch that. Had I actually worked the problem I would have proven myself wrong once I checked the units. So I guess I am wrong, doesn't surprise me, I have been wrong before, just because I have a degree in physics doesn't mean I know everything or am not capable of a mistake. So Charles the photographer can enjoy this moment as he so pleases because he beat the physics major at his own game!

 

[Danger]

so what is turning a moehill into a dolly parton anyway?

I'm not sure where you live. Here, there is a saying about making mountains out of molehills; ie: making a big deal out of something insignificant. Mountains remind me of Dolly Parton, which is understandable if you've ever seen her.

 

[Ranger Mike]

I agree 100 percent about Dolly P...oh ahhhh

and i was researching the info on conversation of gearing too...but will be able to varify this once and fora all on my tire heat cycle machine..rotates tires under load to heat them to race car load conditions..has constant drive roller, 4 inch dia and can rotate two different size tires..all i need i a hand tachometer i just ordered from Mcmaster Carr
we will see in a day or two

mean while I am dreaming of a Dolly Parton Christmas..

 

[Phrak]

I'm not sure where you live. Here, there is a saying about making mountains out of molehills; ie: making a big deal out of something insignificant. Mountains remind me of Dolly Parton, which is understandable if you've ever seen her.

We can all hope this thread has run its course. On a lighter note, I just saw parts of a loverly British broadcaste entitled "My Big Breast." Significantly, they did not make mountains out of mole hills. Cheers.

 

[Gnosis]

Ok my friend and I are having a disagreement about gearing on a GoPed. I am stating that if you change the tire size of a California GoPed the speed of the GoPed will not change. This is all theory so am not taking into account friction, mass or the like.

For those of you who do not how a GoPed works the tire is NOT driven by the axel as it is on a car or by sprocket like a motorcycle, but is driven by a spindle rotating directly against the rubber of the tire.

I understand that if you double the the size of tire of a car(in theory) the car will go twice as fast. My grounds for this outcome not applying to a GoPed is that if you double the size of the tire will rotate half as fast.

If somepeople could prove to me right or wrong that would be awesome

When employing the use of a drive-spindle to directly drive the circumference of a tire, changes made to the tire’s diameter do NOT alter the vehicle’s speed per a given drive-spindle RPM (assuming no slippage of the drive spindle). So, you stand correct.

For instance:

While the ratio between spindle RPM and tire RPM most definitely changes when doubling the tire's diameter, this ratio is essentially irrelevant. A tire whose circumference has been doubled will by ratio, appear to spin at just half of its previous RPM HOWEVER, it must be remembered that per the tire’s doubled circumference, it now only needs to rotate at half the RPM to yield the identical previous vehicle speed.

Let’s assume the drive-spindle has a 6” circumference therefore, per each of its revolutions it will move the tire’s circumference 6” as well. Regardless of what change is made to the tire’s diameter, it still only moves 6” of the tire’s circumference per drive-spindle revolution therefore the vehicle itself has only moved 6” per drive-spindle revolution. So, in this type of drive mechanism (drive-spindle directly driving tire’s circumference), only changes made to the circumference of the drive-spindle and/or the drive-spindle’s RPM can alter the speed of the vehicle. Again, changes made to the tire’s diameter (therefore, its circumference) are irrelevant.

 

[Ranger Mike]

by no means is this test lab quality but should provide insight to all
Just got my hand held tachometer ( hand contact type)

tire machine consists of 1 x 1 mild steel frame with two idler rollers on bottom, wheel/tire set on these. Top frame has pivot mechanism in the center of the frame. Right side has 5 HP 220v el. motor, pulley v-belt, and connection to pneumatic air cylinder
left side has roller with v-pulley

operation- tire is placed on two idle rollers, motor is turn on thus rotating top drive roller
air cylinder is actuated via regulator valve .i.e air regulator valve is slowly opened , air cylinder rod is pushed out of cylinder and lowers the drive roller to top of the tire wheel. tire /wheel starts to rotate.

top drive roller is 3.5 inch diameter
front tire is 20.625 inch diameter
rear tire is 22.875 inch diameter

the tires have 65 laps on them and all kinds of shredded rubber build up
the hand held tach has a lot of bounce so I used the memory button to capture the readings
I have no way to regulate the air pressure effectively so there will be a variance in tests

drive roller RPM 3850 1280 ft/min
front tire RPM 3858 1288 ft/min

drive roller RPM 3897 1285 ft/min
rear tire RPM 3966 1291 ft/min

I will leave this set up in place for future recommendations from you all..ice storm froze the roads
I am not going any where..

any input greatly appreciated

 

[schroder]

Your linear velocities on the tires of 1288 ft/min and 1291 ft/min are in very close agreement, despite the fact that this is not a lab set-up, as you said. Also I note that you have allowed the rpm to vary on the roller. One of the constraints of the problem set was to keep the rpm constant. The higher rpm for the larger tire will easily account for the slightly higher linear velocity. In the real world keeping the rpm constant is probably not possible because of the increased angular momentum in the larger wheel. But all in all, given the inherent error in the set-up, I think it shows that the linear velocities stay constant with change in tire size. Case closed.

I don’t wish to open another can of worms, but the linear velocities you gave, in feet/min do not agree with the rpm and diameters you provided. Is it possible that the tach is giving you meters/min? Most of them are selectable between feet and meters. It is not really a concern here, as we are only interested in the difference between the readings.

 

[Ranger Mike]

i agree..the rotating speed of tire is the same..but the vehicle speed will change because the larger tire will cover more ground than the smaller tire during the same period of time
D/T=S

tach is Extech and has m/min mode
would it help if i reran the test in m/min mode?

i also can measure circumference of tire..would be more accurate than tape measure readings of diameter



any more recommendations?
anyone?