How to select gear/sprocket ratios for vehicle?

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Discussion Overview

The discussion revolves around selecting gear and sprocket ratios for a vehicle, specifically focusing on the calculations necessary for transmission design. Participants explore considerations related to vehicle mass, motor specifications, and wheel dimensions, as well as the implications of these factors on acceleration and performance.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant inquires about general considerations for transmission calculations, specifically for a vehicle with a mass of 150kg and a motor with 12kW and 27Nm torque.
  • Several participants emphasize the need for the radius of the wheel, arguing that the tire specification alone does not provide sufficient information for calculations.
  • One participant provides an example of calculating wheel height from tire specifications, suggesting that the diameter can be derived from the given tire size.
  • A participant calculates the force exerted by the torque at a given wheel radius, discussing how this affects acceleration and the necessary gear ratios for desired performance metrics.
  • Another participant questions the representation of acceleration in the calculations, seeking clarification on the units used.
  • There is a correction regarding the units of acceleration, with participants clarifying that it should be expressed in m/s² rather than m/sec.

Areas of Agreement / Disagreement

Participants demonstrate disagreement regarding the sufficiency of the tire specification for calculating wheel radius, with some asserting that additional information is necessary while others believe the provided specification is adequate. The discussion remains unresolved on several technical points, particularly regarding the calculations and implications of gear ratios.

Contextual Notes

There are limitations in the discussion related to assumptions about the calculations, such as the dependency on accurate wheel radius and the implications of mass on acceleration. Some mathematical steps remain unresolved, particularly in the context of gear ratio selection.

Thathrooban
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The mass of vehicle with driver is 150kg. The specification of motor are 12kw, 27Nm torque, using 110/80 R17 wheels . 1. what is the general consideration to do transmission calculations of bikes 2.How to select gear ratios and sprocket ratio?
 
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"110/80 R17 wheels ." ... This does not help we need to Know the RADIUS of wheel , including tire.
 
oz93666 said:
"110/80 R17 wheels ." ... This does not help we need to Know the RADIUS of wheel , including tire.
110/80 R17 is enough information to calculate the radius/diameter; the 110/80 is a tire specification even though OP said "wheel".
 
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oz93666 said:
"110/80 R17 wheels ." ... This does not help we need to Know the RADIUS of wheel , including tire.
I think the below example will be clear you doubt on calculating radius.
Example: 285/75R16 (285 X 75 / 2540 x 2) + 16 = 32.8 inches tall.
 
I guess 'tall' means dia?? ... In which case we have a radius 0.42 meters

We are told the torque is 27Nm which means if the wheel had a radius of 1 meter , it would push horizontally on the ground with a force of 27 Newtons ...

and this would be the force accelerating the bike and passenger ... but the wheel is smaller and so we get more force ... 27/0.42 = 64.3 Newtons

Since the mass is 150kg , the acceleration is ... F=ma ... 64.3/150 =0.429 m/sec

1MPH = 0.45m/sec

So that's the acceleration with no gears ... an increase of 1MPH every second ... it would take you 20 secs to reach 20MPH !

You need to decide what is an acceptable acceleration 5MPH increase every second would need a 5 to 1 gear... that's 0 to 30MPH in 6 secs

If you want to reach 30MPH in 3 secs , that's an accn, of 10MPH/sec ... a 10 to 1 gear is needed .
 
Last edited:
Since the mass is 150kg , the acceleration is ... F=ma ... 64.3/150 =0.429 m/sec

The 0.429 is acceleration you put it as m/sec how? i can't able to get it.
 
Thathrooban said:
The 0.429 is acceleration you put it as m/sec how?
It's m/s2.
 
A.T. said:
It's m/s2.
yeah that's why I ask him.
 
Thread closed pending moderation.
 

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