# Cars We Drive!

Discussion in 'Fine Living, Home, Design & Auto' started by Bert1568, Jul 18, 2006.

1. ### KookzSenior member

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Torque is torque, a moment like you said. It can be thought of as rotational force, but that's not 'force'. The equation f=ma does not work with ft-lbs plugged in.

Last edited: Apr 15, 2012

2. ### Arthur PESenior member

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torque (wheel thrust) is what drives the car, at any given rpm
you can speak in terms of power (because P = T x w) but mechanical engineers (at least in my 30 limited years of experience) always deal in torque or moment when calclulating machine displacement, now EE's always deal in power because most electric motors are contant rpm (1180, 1760, etc.)
on a lathe we deal in torque when turning something, not power, it is no different with vehicle propulsion

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3. ### Arthur PESenior member

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it IS a rotational force
think of conservation of momentum, without force added speed will not change (accelerate), same goes for a car

F = ma and T = F x r or F = T/r
substituting
T/r = ma
mass = lb / ft/sec-sec and a = ft/sec-sec, ft/sec-sec cancels leaving lbs (force)

lb-ft/ft = lb (force) it works

lb-ft, not ft-lb

Last edited: Apr 15, 2012

4. ### KookzSenior member

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I think you are missing my point, which is very simple. What matters when it comes to the speed of a car, torque or power?

The answer is very obvious, but people seem to get so confused.

T/r=ma works, but that is different than T=ma.

5. ### Arthur PESenior member

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torque is directly proportional to the rate of acceleration
and its magnitude is proportional to top speed
but since P = T x w, it can be substituted, but I prefer to deal in (as most do) the fundemental quanity of force/torque and not the dervied one, power

T/r = ma is no different than F = ma, both r and m are physical constants of the system under study

we are obviously not going to agree, so let us conceptualize the way we each choose

what type of civil eng do you do?
my firm does a good bit of it: small bridges, a good bit of water/wastewater infrastructure, a lot of gas production/upstream site work

6. ### KookzSenior member

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I do none, I'm in the oil business.

I agree that T/r = ma is no different than F=ma, but simply T=ma is what I took an issue with.

When you say we're not going to agree, it's not really up for debate. You're just ignoring that in a car, the system (engine, transmission, FD and wheels/tires) is what determines torque at the wheel, and thus engine power is all that matters and not torque.

Hence why the Golf GTI is much faster than the otherwise identical TDI, even though it makes only 207 ft-lbs compared to the TDI's 236, or why an E46 M3 and Toyota Prius make almost the same torque, the Prius is 10% lighter, and the M3 is still worlds faster.

7. ### HRoiSenior member

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wait, what just happened in this thread ?!

Sir F, the new wheels are BBS LM in the diamond black finish, if that's what you were asking

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8. ### Arthur PESenior member

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I never said T = ma
you are correct, no debate, torque is what motivates a vehicle (or and rotating machine) and determines it's speed and accel characteristics

you can't have power without torque, and torque determines power
it is crank torque that determines torque at the wheel
wheel thrust = axle torque/tire radius
axle torque = crank torque x gear ratios x loss factor (loss factor varies with load and rotational speed)

HP sells cars
torque wins races
Enzo Ferrari (or Carrol Shelby? probably both, since it's true and they both know alot about engines/racing/cars) was once quoted as saying "Horsepower sells cars, torque wins races". Obviously Mr. Ferrari believed that torque was most important of the two provided that both were present in sufficient amounts. With lots of turns, acceleration, and braking, torque wins hands down...

in the cases you cite it's not absolute torque, it is the gearing, power band and the area under the torque curve (which is not power)

you're arguing with a mechanical engineer, you are correct, it is not debatable
the primary driving force is torque/thrust, power is a derived one to measure the application of said torque or POWER under sustained load, NOT FORCE which is what accelerates the car (F = ma or a = F/m)

what do you actually do 'in the oil business'?

Last edited: Apr 15, 2012

9. ### KookzSenior member

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Ok, it's not debatable and torque is what matters for the speed of a car?

I'm arguing with a mechanical engineer who is too caught up in the trees to see the forest, apparently. Once you add gearing into the equation (and this is a thread about cars, so every one in this thread), the actual torque produced at the engine becomes irrelevant.

A car making 400hp and 300 lb-ft of torque will be faster than the same car making 300hp and 400 lb-ft of torque, agree or disagree?

10. ### Arthur PESenior member

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what is 'speed'?
do you mean velocity or acceleration?
once you add gearing into the mix, torque again is the manipulated variable, as in torque multiplication, not power multiplication
engine makes 400 lb ft at 5252 rpm (so also 400 HP) at the crank
gearing is 10:1 so axle torque is 4000 lb ft (what is axle HP? HP = (axle rpm ~5252/10) x 4000 / 5252, or 400 HP, no change)
say gearing is 6:1, then 2400 lb ft and 400 HP at the axle
which will accelerate faster?
why, because you have MORE torque, yet power has remained the same....
the forest is made up of trees, when looking at the forest you see trees...we are talking engineering/physics, generalities have no place in the discussion, these are facts, not conjecture or ones interpretation. it's not dependent on the observer or his 'perspective', only his understanding of the physics

without seeing the torque curves no way of knowing
at what rpm?
all else being equal, power band width (say 90% of peak torque) the one with more torque

Last edited: Apr 15, 2012

11. ### slyNew Member

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blah blah let's all argue/quote]

Not at all related however, Kookz the same Kookz from H-T?

12. ### KookzSenior member

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The answer in a car is power, every time. There are plenty of identical vehicles to prove this over and over again, I posted the specs of the VW Golf TDI vs. GTI above, or you can look at two even more identical cars, the 335i vs. 335d.

265hp, 425lb-ft of torque for the diesel gets it to 60 in 5.7s. 300hp, 300lb-ft of torque gets the otherwise identical 335i to 60 in only 5.2s.

Why is it quicker? Because, as you clearly DO understand, the wheel torque produced is higher in the faster car. Why is it higher? Because making that 300 lb-ft of torque at higher RPMs (i.e. making more horsepower) allows a lower gear to be run, and thus the product of torque x gearing (wheel torque) is higher.

It's so basic, are you sure you're not arguing against me just to argue?

More wheel torque = faster acceleration, I know you agree with me there. That has nothing to do with the issue at hand, though. If you want to know how fast a car is, knowing the torque it produces will in no way help you.

13. ### KookzSenior member

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But this is exactly my point! Torque without knowing RPM is meaningless, but knowing the RPM really just means you know the power, and thus have information one can actually use to calculate acceleration.

Last edited: Apr 15, 2012

14. ### Arthur PESenior member

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you point is incorrect
you calculate accel with force/thrust, not power (F = ma or a = F/m)

400 HP at the wheels, what is the accel for a 2000 lb car?

15. ### Arthur PESenior member

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which is faster to 100?
e46 M3, 333 HP, 262 lb ft, 3400 lb
335i, 300 HP, 300 lb ft, 3600 lb
lb/HP ~ 20% advantage to the M3
if you add 200 lbs to the M3 (or remove from the 335i)
or bump the boost on the 335i until you get 333 HP
or bump the boost to 333 HP AND remove 200 lbs, both same power, same weight