# Caculating tractive force (i need a formula)

1. May 11, 2007

### dan_bighit

i ve got the following question from my motor vechile diploma

kerbside weight 1360kg
rolling resistance 140N per 1000kg
frontal area 2m(squared)
drag coefficient 0.025

caculate the total tractive resistance when the car travels at 70 km/h on a level road?

what i need is the formula to work out tractive resistance ive searched for about 3 hours on the net but no luck.

any help appreciated

dan

2. May 11, 2007

### Hootenanny

Staff Emeritus
Firstly, ask yourself what is tractive resistance?

3. May 11, 2007

### dan_bighit

is it traction

4. May 11, 2007

### Hootenanny

Staff Emeritus
Which is what in terms of the forces acting on a vechicle?

5. May 11, 2007

### dan_bighit

drag force

6. May 11, 2007

### Hootenanny

Staff Emeritus
Which would include the rolling resistance of the wheels and the fluid friction from the air, yes? So can you write down an equation for these two forces?

7. May 11, 2007

### dan_bighit

i can work out the rolling resistance and air resistance which is
rolling resistnce = 190.4N
air resistance = 452.76N

what i need is a formula so i can caculate the total tractive force

8. May 11, 2007

### Hootenanny

Staff Emeritus
So, if the tractive force is the total or sum of all the drag forces acting on the vechicle, what do you think you should do now?

9. May 11, 2007

### dan_bighit

10. May 11, 2007

### Hootenanny

Staff Emeritus
Sounds good to me

11. May 11, 2007

### dan_bighit

ok heres the full question

kerbside weight 1360kg
rolling resistance 140N per 1000kg
frontal area 2m(squared)
drag coefficient 0.025

caculate the rolling resistance of the car?
caculate the gradient resistance when the car climbs a gradient of 1/20
caculate the air resistance of the car when it is travelling on a level road at 100 km/h?
caculate the total tractive resistance when the car travels at 70 km/h on a level road?

ive answered the top 3 questions but havent used drag coeeficcient
and i thought that might come in to tractive resistance but obvisily not. so where would that come in to surly they couldent of just told you it for the sake of it.

12. May 11, 2007

### Hootenanny

Staff Emeritus
You should have used the drag coefficient in calculating the air resistance of the car.

13. May 11, 2007

### dan_bighit

i got the formula formula form here http://www.webtec.co.uk/techinfo/equation/eqnfr/eqn01.htm [Broken]

i think i see where ive gone wrong because ive worked out using 0.0462 when it should have been 0.025

if you could just clarfy this ill be happy thanks for your help

Last edited by a moderator: May 2, 2017
14. May 11, 2007

### Hootenanny

Staff Emeritus
My best guess is that you are required to use Stoke's-type formula to calculate the drag;

$$F = -bAv$$

where b is your drag coefficient. And we usually work in SI units, m, m/s2, N etc.

Last edited: May 11, 2007
15. May 11, 2007

### dan_bighit

im abit confused now , what does the v mean.
so that formula on the website is wrong ?

16. May 11, 2007

### Hootenanny

Staff Emeritus
v is the velocity or speed. No, the formula on that website is useful in some cases, but here you are given your own drag coefficient so you should use that. This formula is essentially the same as the one given on the website, with the exception of your own drag coefficient. Note, the correction in my previous post, the addition of cross sectional area A.

17. May 11, 2007

### dan_bighit

is air resistance the same as drag resistance or are they diffrent?

18. May 11, 2007

### Hootenanny

Staff Emeritus
For your purposes they're exactly the same.

19. May 11, 2007

### dan_bighit

so 0.025 x 19.444(m/s) = 0.4861 for stokes formula

0.0462 x 70^2(km/h) x 2 = 452.76N for the websites formula