Thanksgiving Dinner Debate About GoPed Gearing

[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?