Is the gearing in a car linear?

[warped head]

what i mean is, is each gear in a car linear or does its ratio change?

I ask this because if im in first at 2000rpm and change to second it drops to 1400rpm, then if i build up to 2000rpm in 2nd and change to third the revs drop down to 1400rpm again, and this applies thoughout the gearing range.

However if i rev first to 6000rpm and change to 2nd it drops down to 4500rpm, then if i build back up to 6k in second and change to third it drops down again to 4500rpm.

Does this mean that each gear is not linear, or is the tachometer reading not linear?

[blank]

When changing from say 2nd to 3rd, the revs will always drop by the same ratio/percentage.

But the percentage drop from 2nd to 3rd won't necessarily be the same as from 3rd to 4th - although they are often similar.

Edited by blank on Sunday 9th May 23:31

[warped head]

blank said:

When changing from say 2nd to 3rd, the revs will always drop by the same ratio/percentage.

But the percentage drop from 2nd to 3rd won't necessarily be the same as from 3rd to 4th - although they are often similar.

Edited by blank on Sunday 9th May 23:31

so does that mean the gears are linear or none linear?

[LD1Racing]

Basically the higher the input speed (engine speed), the higher the output speed (wheel speed) and when you change that ratio (by changing gear) the engine speed falls according to that ratio. and the wheel speed remains the same (assuming constant speed).

Supposing the car only has two gears and their ratios are 1:2 and 1:1.

Imagine the wheel speed is 3000rpm, in first gear the engine speed would be 6000rpm (1:2), as you change into second, the engine speed would drop to 3000rpm (1:0), a drop of 3000rpm

Now Imagine the wheel speed is 6000rpm, in first the engine is doing 12000rpm (1:2), and in second the engine is doing 6000rpm (1:1).

The same ratios but now a drop of 6000rpm.

I know these aren't realistic figures for car gearbox ratios, but I wanted to keep the numbers simple.

[warped head]

no, thats not what i mean.

as said, when i change from 2nd to 3rd at 2000rpm the engine speed drops from 2000rpm to 1400rpm.

But when i change from 2nd - 3rd at 6000rpm the engine speed drops from 6000rpm to 4500 rpm.

that means that from 2nd to 3rd at 6krpm the engine speed falls 1500rpm, but at 2krpm the engine speed falls only 600rpm.

thats what i dont get.

[Strawman]

Depends on the gearbox ratios, some cars have a 'close ratio' box so the intermediate gears are quite close in ratio, so you can change up and down the box without such a big change in revs that a standard gearbox will give. Often the top gear, 5th or 6th if talking manuals, will be a different change in ratio to allow economical higher speed cruising.

[Strawman]

warped head said:

But when i change from 2nd - 3rd at 6000rpm the engine speed drops from 6000rpm to 4500 rpm.

that means that from 2nd to 3rd at 6krpm the engine speed falls 1500rpm, but at 2krpm the engine speed falls only 600rpm.

thats what i dont get.

What is the car doing, maintaing a constant speed, and with equal pressure on the throttle before and after the change?
Both changes are losing (roughly) a 1/4 of the revs for an up change, because the ratio change is constant.

[davepoth]

In a close ratio box, you would expect that to be the case (i.e 6000 to 4500 on each upchange) but most road car boxes are designed to do 2 things. 1 is hit 62mph at maximum revs in a particular gear to get the 0-100 time as low as possible. the other one is to be at the most economical rev range (about 2000rpm or so) in top gear at 56mph, for the european economy tests.

[busta]

Linear. With solid cogs and fixed numbers of teeth it can't be anything but linear. I think what you are seeing is either an inaccuracy in the tacho, which would be greater at lower speeds, or the speed drop whilst changing gear is proportionally greater at lower speed so the revs don't quite match the same as when going quicker.

[cambiker71]

warped head said:

no, thats not what i mean.

as said, when i change from 2nd to 3rd at 2000rpm the engine speed drops from 2000rpm to 1400rpm.

But when i change from 2nd - 3rd at 6000rpm the engine speed drops from 6000rpm to 4500 rpm.

that means that from 2nd to 3rd at 6krpm the engine speed falls 1500rpm, but at 2krpm the engine speed falls only 600rpm.

thats what i dont get.

Yes they're linear, I'd take a guess the rev counter isn't overly accurate, as the above seems nearly the same drop from either 6000rpm or 2000rpm?
2000rpm x3 = 6000rpm
1400rpm x3 = 4200rpm

[busta]

BTW, if you could design a non linear gearsbox (other than CVT type things) that worked effectively in cars you could make a lot of money.

[crackedfinger]

warped head said:

what i mean is, is each gear in a car linear or does its ratio change?

I ask this because if im in first at 2000rpm and change to second it drops to 1400rpm, then if i build up to 2000rpm in 2nd and change to third the revs drop down to 1400rpm again, and this applies thoughout the gearing range.

However if i rev first to 6000rpm and change to 2nd it drops down to 4500rpm, then if i build back up to 6k in second and change to third it drops down again to 4500rpm.

Does this mean that each gear is not linear, or is the tachometer reading not linear?

Ratio remains constant.