Calculate Top Speed of Car Using Gear Ratio, Torque, Power & RPM

Click For Summary

Discussion Overview

The discussion revolves around calculating the top speed of a car using gear ratios, torque, power, and engine RPM. Participants are exploring the relationships between these variables, as well as how to derive acceleration and tractive effort at different gears and RPMs. The context includes specifications for a vehicle with relatively low horsepower, prompting questions about the nature of the vehicle and the calculations involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Karan seeks assistance in calculating the top speed of a car based on given specifications, including maximum torque and power at specific RPMs.
  • Participants discuss the tire specifications and how to calculate the effective tire diameter from the provided measurements.
  • Some participants suggest multiplying the initial and final gear ratios by each gear ratio to find effective ratios for speed calculations.
  • There is a correction regarding the peak power and torque RPMs, with a suggestion that the power peak should occur at a higher RPM than the torque peak.
  • Participants emphasize the importance of considering aerodynamic drag and rolling resistance when calculating top speeds.
  • Karan clarifies the correct values for power and torque after receiving feedback.
  • Questions arise about how to calculate acceleration at different gears and RPMs, with references to tractive force and mass of the vehicle.
  • Participants discuss the need for the effective rolling radius of the tires and the mass of the car to calculate tractive force and acceleration.

Areas of Agreement / Disagreement

Participants express varying levels of agreement on the calculations and assumptions involved. There is no consensus on the correct approach to finding acceleration or the implications of the vehicle's specifications, indicating multiple competing views and unresolved questions.

Contextual Notes

Limitations include the lack of clarity on certain assumptions, such as the effective rolling radius of the tires and the impact of aerodynamic drag and rolling resistance on calculations. The discussion also highlights potential discrepancies in the provided power and torque data.

Who May Find This Useful

This discussion may be useful for individuals interested in automotive engineering, vehicle dynamics, or those working on similar projects involving performance calculations for low-power vehicles.

Karan Dua
Messages
4
Reaction score
0
Hey guys,

I am a bit confused while calculating top speed of the car using gear ratio, Torque, Power and Engine Speed(RPM). Can someone help me out with this?

Specifications are given as follows:

Maximum Torque : 19.2 N-m @ 3800 RPM
Maximum Power : 10 Hp @ 2800 RPM

Tire Specifications: 25x8 - R12 or 254/65 - R12

Gear Ratios:
Initial Input Reduction - 2.869
First Gear Ratio - 4.6
Second Gear Ratio - 2.733
Third Gear Ratio - 1.667
Fourth Gear Ratio - 1.115
Final Forward Reduction - 2.38
Final Backward Reduction - 4.17

I am looking for top speed at each gear at different RPM, as well as acceleration, Torque and Tractive Effort.

Your answers will be really helpful.

Thanks and regards

Karan Dua!
 
Engineering news on Phys.org
Wheel diameter is 12 inches? Tire width = 254mm = 10 inches. Tread wall height = 65% x 10 inches = 6.5 inches. Tire diameter = 12 + 6.5 + 6.5 = 25 inches. Multiply initial and final gear ratios (6.82822) by each gear ratio: 1st = 31.41, 2nd = 18.66, 3rd = 11.38, 4th = 7.613 . See if you can continue from here.
 
Karan Dua said:
Hey guys,

I am a bit confused while calculating top speed of the car using gear ratio, Torque, Power and Engine Speed(RPM). Can someone help me out with this?

Specifications are given as follows:

Maximum Torque : 19.2 N-m @ 3800 RPM
Maximum Power : 10 Hp @ 2800 RPM

Tire Specifications: 25x8 - R12 or 254/65 - R12

Gear Ratios:
Initial Input Reduction - 2.869
First Gear Ratio - 4.6
Second Gear Ratio - 2.733
Third Gear Ratio - 1.667
Fourth Gear Ratio - 1.115
Final Forward Reduction - 2.38
Final Backward Reduction - 4.17

I am looking for top speed at each gear at different RPM, as well as acceleration, Torque and Tractive Effort.

Your answers will be really helpful.

Thanks and regards

Karan Dua!

Welcome to the PF.

Can you give us the context of the question? Is is a homework problem, or a schoolwork project, or a personal project? What kind of car is this that is running on only 10hp? Some kind of a go-kart? :smile:
 
berkeman said:
Welcome to the PF.

Can you give us the context of the question? Is is a homework problem, or a schoolwork project, or a personal project? What kind of car is this that is running on only 10hp? Some kind of a go-kart? :smile:
Just a project!
 
Karan, my first comment is that your horsepower and torque peak speeds can't be right. The power peak is (necessarily) always at a higher speed than the torque peak. You may have a typo, or got bad data.
As for top speeds in each gear, it depends on the aerodynamic drag Fd and rolling resistance Fr of the car. At each vehicle speed, calculate the amount of power it takes to propel it: (Fd+Fr)*v. Then pick a gear and calculate the engine speed omega at that vehicle speed. If the engine can make more than (Fd+Fr)*v, then the car can go faster. Keep trying higher vehicle speeds until the engine can't produce enough power to propel it, or until it hits redline.
 
Randy Beikmann said:
Karan, my first comment is that your horsepower and torque peak speeds can't be right. The power peak is (necessarily) always at a higher speed than the torque peak. You may have a typo, or got bad data.
As for top speeds in each gear, it depends on the aerodynamic drag Fd and rolling resistance Fr of the car. At each vehicle speed, calculate the amount of power it takes to propel it: (Fd+Fr)*v. Then pick a gear and calculate the engine speed omega at that vehicle speed. If the engine can make more than (Fd+Fr)*v, then the car can go faster. Keep trying higher vehicle speeds until the engine can't produce enough power to propel it, or until it hits redline.

Hey Randy,

Thanx a lot for your reply.
I just want to correct myself.

Power : 10 Hp @ 3800 RPM
Torque: 19.2 N-m @ 2800 RPM

How do I find acceleration at each gear @ different RPM of engine.

For example: How do i find acceleration @ 1st gear with gear ratio of 31.45:1 @ 2800 RPM?
 
Karan, I couldn't tell what the tire radius rTire was from your tire spec's above, and didn't see the car's mass.
You'd need to calculate the tractive force F, which is (19.2 N-m)*(31.45)/rTire. Then it's just a=F/m.
 
  • Like
Likes   Reactions: Karan Dua
Randy Beikmann said:
Karan, I couldn't tell what the tire radius rTire was from your tire spec's above, and didn't see the car's mass.
You'd need to calculate the tractive force F, which is (19.2 N-m)*(31.45)/rTire. Then it's just a=F/m.

The tires which we are using will be 25x8-R12. And the r will be effective rolling radius if i am not wrong? And mass of the car is 400 kgs. And what aabout acceleration on slope? How to calculate that? And do we calculate torque at each gear @ different rpm? If so, how?
 
  • #10
Karan Dua said:
How do i find acceleration @ 1st gear with gear ratio of 31.45:1 @ 2800 RPM?

The acceleration is found with equation 1b on this page. The force Ft in that equation can be found with http://blog.mechguru.com/vehicle-design/tractive-force-calculation-for-a-vehicle/ (TE = Ft).
 
Last edited by a moderator:

Similar threads

Gear Ratios and Constant Power/Torque: find max velocity?
  • · Replies 3 ·
Replies
3
Views
4K
Calculating top speed in each gear given the gear ratios and
  • · Replies 17 ·
Replies
17
Views
21K
What Gear Ratio Is Needed for a 12-Foot Wheel to Achieve 60 RPM?
  • · Replies 6 ·
Replies
6
Views
2K
How to show how speed varies with incline, and with Gear ratio
  • · Replies 10 ·
Replies
10
Views
3K
Gearing Down A High Speed Input (>10K RPM)
  • · Replies 1 ·
Replies
1
Views
2K
Why Does Angular Speed Not Increase Indefinitely in Rotating Systems?
  • · Replies 4 ·
Replies
4
Views
5K
Calculating the rpms at which gear changes must occur: The Power Band
  • · Replies 14 ·
Replies
14
Views
26K
Automotive Torque/hp philosophy (infinant gear ratio)
  • · Replies 7 ·
Replies
7
Views
5K
Gear ratio and power output in a car
  • · Replies 20 ·
Replies
20
Views
47K
What Gear Ratio Should I Use for Maximum Speed on My Model Car?
  • · Replies 23 ·
Replies
23
Views
12K