The physics of cars - a question sequence.

[newshound]

I'm giving up and killing the thread now. At least there are a few of us who understand.

:-)

[Roger Mills]

No, I mean thrust. The torque which the drive shaft imparts to the wheel is converted into thrust at the contact patch. That's what accelerates the vehicle - and why it doesn't go quite so well when driving on ice.

You don't normally measure it directly[1] - you calculate it from the known parameters - torque, wheel radius, etc.

[1] Motor manufacturers do, though - by running the vehicle on a rolling road and restraining it via a force measuring system attached to the tow hitch. That's what I did in my Rover Research days, anyway.

[Dave Plowman (News)]

No it doesn't. Acceleration is never uniform from rest to top speed. It will obviously be better in a low gear than high, and also within any one gear range. So it then comes down to when it is best in any single gear. Which started off this discussion.

[Dave Plowman (News)]

And the two are directly related.

Why is it impossible to see this? A gearbox simply multiplies the torque of the engine.

[Dave Plowman (News)]

And that rolling road - same as an engine test bed - measures torque. It then calculates the BHP from that. And with most rolling roads adds on a fiddle factor to give the car owner a nice rosy glow. ;-)

[Clive George]

Now that's where I'm going to disagree with you, and I suspect several others are too.

When people say "car X accelerates faster than car Y", they'll be looking at the acceleration to get from one speed to another, eg 0-60,

50-70, etc, not the single point of maximum acceleration in first gear.

But that's a semantic point, not a physics one.

[Roger Mills]

Did anyone say that acceleration *was* uniform across the speed range - I certainly didn't?!

In general, acceleration is highest at a low road speed and decreases by some function with increasing speed.

Power at the road is thrust x road speed. If you could maintain a constant power at the road, thrust would reduce as the inverse of speed. You try to do that by selecting the most appropriate gear. A continuously variable transmission would be ideal if one could be made to work reliably.

But the fact remains that the more power you can get to the road, the greater the acceleration. And you get that by operating the engine as near to its maximum power point as you can. Can you not *see* that?

[I don't believe that you're stupid - I rather suspect that you realise that you've backed yourself into a corner and find it difficult to retreat with any dignity].

[stvlcnc43]

I spent 23 years in an Engine Laboratory, testing engines for Performance, Economy, Durability and Emissions. There's been quite a few misconceptions been aired in this thread; you guys need to sit down and look at some perfo rmance curves and make your own calcs.. Some of you are confusing Max Power with Max Throttle Opening; to maintain a given road speed at Max Power, th e load on the car must equal the Torque at that engine speed; any reduction in load will cause the car to speed up.

[Roger Mills]

The two *are* indeed directly related - but the actual relationship depends on what gear is selected. At any given road speed you'll get more output torque by using a lower gear and a higher engine speed.

[Vir Campestris]

Nobody else has been.

Admit it, you're just trolling.

[Roger Mills]

The setup I'm talking about had 5' diameter rollers and - in the configuration which I described - measured thrust by means of an A-frame fitted with strain gauges. The non-driven wheels were on air bearings so as not to influence things.

[Dave Plowman (News)]

Decent car mags like Autocar tested 10mph increments in top gear. As a measure of how soon you'd have to change gear on a hill, or to overtake.

[Dave Plowman (News)]

That depends on the torque curve and the gear ratios and the speed. It's perfectly possible to have more torque at the wheels in the higher gear. Not that common, but possible.

[Dave Plowman (News)]

Oddly, I was thinking that of you bringing back the subject.

From the very start of all this nonsense, I said that you'll get maximum acceleration at peak torque in any one gear. Others said at peak BHP. Then started taking about different gears.

I'll make this simple.

Choose a gear where the car can reach peak BHP. Say second gear. In that gear only, log the time taken for each 5mph increment in speed at full throttle.

The time taken for that 5 mph increase will be shortest at peak torque. Not peak BHP.

[Johnny B Good]

I think we can all accept that as "A Given". However, for a given road speed where the selected gearbox ratio matches the peak engine power revs, you will gain a modest increase in torque applied to the driven wheels compared to a ratio selected to allow the engine to run at peak torque revs.

If we assume the theoretical case of a perfect stepless gearbox which can automatically adjust the ratio to hold the engine to its peak torque rpm as the road speed increases, you'll find that the resulting acceleration will be less than one configured to hold the engine to its peak power output rpm (it will operate at a larger reduction ratio in this case which increases the torque applied to the road wheels compared to the ratio range used to keep the engine operating at its maximum torque rpm).

In the case of a manual gearbox, we approximate this mode of operation by choosing max power rpm as the next change up point rather than the lower peak torque rpm point.

[Johnny B Good]

You can gain an even greater acceleration over that particular range of road speeds (let's say 40 to 45 mph) by using a slightly higher gearing (less taller) ratio that results in higher engine rpms which results in a higher accelerating torque being applied to the driven wheels simply as a direct result of the higher reduction ratio magnifying the torque available at the gearbox output shaft.

Although the engine torque will be a little less than at peak torque output revs, the larger reduction ratio will still provide a net gain in torque at the driven wheels.

[Roger Mills]

I couldn't have put it better myself!

In fact, I invited Mr Plowman to consider these two scenarios - with a stepless transmission holding the engine either at its max torque point or at its max power point - but he ruled it invalid because such transmissions don't exist. My likely because it proved a point against which he had no valid argument.

[Dave Plowman (News)]

Of course A gearbox simply multiplies the torque of the engine. Regardless of the engine or road speed. But if it increases the torque at peak BHP, it will also do the same at peak torque, so you'll have more torque at the wheels at peak torque. Therefore better acceleration...

The type of gearbox makes no difference to the principle.

A decent box will then plonk you at or near the peak torque point in the next higher gear - to give you the best acceleration.

[Dave Plowman (News)]

Sigh. Only because the transmission type makes not a scrap of difference to my original statement. If you did want to discuss different types of transmissions I'd be happy to join in.

However, I did have access to a DAF van many years ago. On full belt it went to approximately the peak torque point and stayed there. Not maximum revs.

Was simply trying to stick to the original point. Which despite all this, many still disagree with.

[Roger Mills]

IN THAT GEAR.

But almost certainly a LOWER acceleration than you experienced at the same road speed immediately before changing up.