[benbsb29]

While the thread is referring more to SUVs/MUVs, i have noticed the same on my Ikon as well.

[download2live]

Same observation on Punto as well. The 1st gear will make it potter along gently. But reverse gear is scary. Till now I have never reversed it with clutch fully released. It goes back way too fast if I do that.

[Shan2nu]

A short distance can burn clutch. For example at a hill station the hotel parking had a very steep parking lot. An innova started reversing, and could climb on low rpm. I guess reverse is not so high in innova.
Next was a scorpio.
It kept stalling.
Then driver did half clutch to get her up, and after that we could smell the fried clutch. Probably shortened the life by 10,000kms in 1 minute.

I've seen so many people (including my mom) stall the vtec while reversing, where as i never had a prob, so i don't think this example is decisive enough.

Maybe i'l test this on my cousin's Scorpio and see how it compares to the Innova. That would give me a clear picture....

But atleast from what i've seen on the Dicor, a reverse gear which is just 8.5% taller than first, should not really be a prob, since the load on the rear wheels is also less when reversing.

Shan2nu

[Sutripta]

Hi,
Interesting question. Let me turn it on its head.

If you were the drivetrain designer, how would you decide on the ratios for first and reverse?

Regards
Sutripta

[SPIKE ARRESTOR]

Originally Posted by im_srini

I'm guessing it's because reverse gears cannot be constantly in-mesh as the forward gears are. The reverse gears are apart & also have to provide enough space between them for accomodating the idler. Hence, for a given gearbox casing size, the reverse gears would be smaller than their 1st gear counterparts (by atleast the height of their gear teeth).

Yes, pretty much correct. There are two reasons which I can think of:-

1. Design and packaging requirements.

The gear N1 is moved towards the right to engage Reverse, fig (e) represents this situation. Pinion N1 does not mesh with pinion J of the layshaft as a reverse idler gear Q comes in between. N1 meshes with Q which is mounted on a shaft on which it is free to revolve. The shaft is fixed in the gearbox casing and the idler Q is in constant mesh with the pinion J of the layshaft. A different option will be making a bigger pinion (J) turn the smaller pinion (N1), which will ultimately effect the speed and torque reaching the wheels. Although, this can be done but it is not preferred due to reason 2 mentioned below.

2. The mode and nature of usage. The 1st and Reverse gears are used only when the vehicle is starting from a standstill, this condition imposes the maximum amount of road loads on a vehicle, in order to negotiate this the gearing is selected. It has to be mentioned that the reverse gear sometimes acts as a line of defense (specially in case of 2wds) when the vehicle gets stuck and reversing->powering through the obstacle happens to be the only option.

Spike

Pic courtesy "The Motor Vehicle"

[Sutripta]

^^^^
Was wondering when you'd pipe in!

Your sketch shows cogs, rather than dogs moving. Same dog selecting three different ratios.... ?

@ TSK, Spike, other gurus:
What about epicyclics? Has there been a similar shift in ratios over the years?

Regards
Sutripta

[pjbiju]

One more plausible reason other than design constraints could be the following:

You have 4-6 forward gears to be selected depending upon the load, road conditions and speed required to move forward.
There is generally only one reverse gear in most of the vehicles. Now imagine that you need to travel in reverse for a longer distance than your normal "reverse-gear-usage-situations". You would need a balance between torque and speed. So a taller ratio than the first gear lets you travel relatively faster without sacrificing much in terms of torque.

That's my take. I could be completely wrong though.

-Biju

[SPIKE ARRESTOR]

Originally Posted by Sutripta

^^^^
Was wondering when you'd pipe in!

Your sketch shows cogs, rather than dogs moving. Same dog selecting three different ratios.... ?

The point in focus was the right bottom portion of the pic which showed the idler gear arrangement, hence so.

Spike

[Sutripta]

^^^^^
In the diagram, by its very nature, reverse will be lower than 1st. Opposite of topic under discussion.

My question about planetaries, and the thought process behind choice of 1st and reverse ratio remains. (I am not talking about the progression of ratios)

Regards
Sutripta

[SPIKE ARRESTOR]

^^ Arey baba, the figure was just to show how the idler gear comes in between the output shaft and the layshaft, it had nothing to depict ratios. Kya dada, clear??

Spike

[Mpower]

Originally Posted by tsk1979

So why tall reverse gear, any logic?

I can think of 2

1. People slip the clutch anyway during reversing out of a parking spot(just by nature)
2. For a guy like me, I prefer driving for short distance in reverse ...at a good clip rather than making a U-turn and going back. This means the engine is going to be revvvvving and gears whining. So I would prefer a taller ratio

But for any vehicle with off-road or load carrying duty..short ratio is preferred.

Originally Posted by SPIKE ARRESTOR

The gear N1 is moved towards the right to engage Reverse, fig (e) represents this situation. Pinion N1 does not mesh with pinion J of the layshaft as a reverse idler gear Q comes in between. N1 meshes with Q which is mounted on a shaft on which it is free to revolve. The shaft is fixed in the gearbox casing and the idler Q is in constant mesh with the pinion J of the layshaft. A different option will be making a bigger pinion (J) turn the smaller pinion (N1), which will ultimately effect the speed and torque reaching the wheels. Although, this can be done but it is not preferred due to reason 2 mentioned below.

Sounds fine and dandy but I guess when you have to live with an old design Kia gearbox you have to take what you get ya?

[Sutripta]

Originally Posted by SPIKE ARRESTOR

Kya dada, clear??

Spike

Should be 'Ki Dada, bujlen'

Bujlam.

Regards
Sutripta