OT: Brakes seizing on electric cars? (2024 Update)

[Vir Campestris]

If you check the ratios between the gears they aren't the same - the higher gears (except that long distant overdrive top) are further apart than the high ones.

The second number here is the ratio between each gear and the next.

5.9 1.84 3.2 1.45 2.2 1.38 1.6 1.23 1.3 1.30 1

I get something similar for my MR2:

3.23 1.69 1.913 1.52 1.258 1.37 0.918 1.26 0.731

(only 5 gears... it is over 20 years old)

Conveniently the ratios they fitted to mine are almost identical to my wife's little shopping car :) although the engine characteristics are completely different. I suspect mine are:

1st: About right to break traction. 2nd: get to 100kph for the book figures. 5th: Highest top speed. 3rd and 4th put somewhere sensible between.

Andy

[The Natural Philosopher]

no, 4th for highest top speed, 5th for best mpg at a freeway cruise

[Vir Campestris]

This is a sports car. 20MPH/1000 RPM. An overdrive 5th would have been nice, but it doesn't have one. 5th is geared for top speed.

[Bob Eager]

1.30 1

My car has only one gear!

[Xeno]

You would probably find the 5th gear is, in terms of the gearbox, an actual overdrive gear. IOW, 4th gear is likely to be a straight through coupling. The final drive ratio is what will ultimately make 5th a performance or an economy gear ratio.

[Vir Campestris]

I just checked the workshop manual.

All 5 forward gears work the same way - a pinion on the input shaft engages with one on the output shaft. A selector mechanism chooses which of the output gears drives the output shaft.

A small gear to the side provides reverse.

There is a shared gear on the output shaft which transfers the power to the differential.

This is the same on both types of manual gearbox. The auto version uses planetary gears, and does have an overdrive.

This is effectively a FWD power unit, although it's where the back seats are on more conventional cars. It is completely unlike the old front engine-RWD layout, where often top gear didn't have a gear at all, but just connected the input and output shafts which were in line.

Andy

[The Natural Philosopher]

In the end it is just a packaging issue - what size you want the final drive to the wheels to be in the differential. No car has a 1:1 ratio between crankshaft and driven axle.

[Vir Campestris]

I think in the dim distant past cars were built like that. Front engine, RWD, top gear a straight pass through.

In fact I have a diagram of just such a gearbox in the book on my lap. "Motors in a nutshell" by Capt. S Bramley-Moore MC, published by Eagle Star Insurance in 1923.

IIRC an overdrive was a separate little gearbox on the front of the prop shaft - but that's just my unreliable memory.

Andy

[Fredxx]

Yes, it was an addon unit that usually only operated in 3rd and 4th. It usually incorporated a solenoid operated pump that drove cone clutches. And if I recall such you didn't have to change gear when flicking the switch on or off.

It was a complicated unit with sprag one-way clutches:

No wonder a 5th gear was later included as standard in a gearbox!

[Bob Eager]

Yes. I used to own a Humber Hawk (one of the'new' ones) that had overdrive like that. I had to dismantle the gearbox to replace a knackered layshaft.

The overdrive unit was on the output of the gearbox. It was an epicyclic anti-reduction gear, and there was a cone clutch linking the input and output. The switch energised that solenoid, and there was a little pump driven by a cam on the input shaft which was then allowed to pump oile into two diametrically oppposed cylinders that force the clutch cones apart. You could operate it even when accelerating.

I don't recall any sprags, and I took it all apart.

[The Natural Philosopher]

FFS *READ* what I wrote

There would have been a 3.5:1 or 4.5:1 reduction on the differential. Make that a 2:1 and you change the gearbox rations to all less than one

Its all about packaging and cost.

[Fredxx]

I thought the one-way clutch was used so you could go into overdrive without losing drive. The cone clutches would be pushed apart and the sprag clutch took over during overdrive. That also worked when reversing.

I'm not familiar with actual internal workings. The Sprag clutch would have looked like a normal bearing.

[Fredxx]

It was about marketing, where an overdrive could be sold at a premium.

[Max Demian]

Maybe the idea of top gear being straight through the gearbox was to reduce gear wear or noise.

[Fredxx]

Not going through 2 gears like all the other gears meant an increase of efficiency of a few %.

[Xeno]

Sprags would have only been required for overrun coasting. Not needed on an overdrive and not a lot of engine braking in an overdrive gear anyway.

[Xeno]

Yep, that was the original idea, reduce gear whine and gear wear.

[Bob Eager]

No, it was an epicyclic, constantly engaged gear train. I think sun/ planet/ring arrangement. The clutch simply stopped it from working and gave direct drive.

Now I have a gearbox with no clutches at all! And no external clutch either.

[Bob Eager]

Ah, I remember now. There *was* an overrun clutch.

[The Natural Philosopher]

More likely to shave a quid off the cost