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Lets Talk Transmission/Final Drive Gear ratios and Power Band

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Ok.....Question 1.

I would love to hear a detail response about the relationship between transmission gear rations and the final drive rations. How do the two ratios co exist?

Often you will hear about a car having short gearing or "long legs" but I'm curious how a race team or car manufacturers combines the two? Many people will change the final drive ratio but how does that affect each gear ratio in the transmission.

Question 2.

What is the proper way to determine transmission gear rations? Should they be set up that you shift as peak torque/hp starts to drop off?

And as you up shift to the next gear you are right at the peak torque/hp rpm( for e.g. 4000rpm) Thus every time you upshift you are starting at beginning of the power band and only upshifting near the top of the rev range as power band reaches the end?

I'm just curious about how this can be optimized? Thanks Great forum

In Australian touring car racing the traditional method of selecting the final drive ratio was to use the lowest numerical ratio to get the highest trap speed on the longest straight of any given race track. The legendary, in Australia anyway, A9X Torana as sold to the public for homoligation purposes had a 2.6:1 final drive to ensure the highest trap speed down the equally legendary Conrod Straight at Bathurst.

I would assume the ratios in the transmission would be selected to provide the best acceleration between gears ensuring the engine is kept within its usable toque band.

I can offer some thoughts but certainly don't have all the answers.

The gap/difference between the gear ratios will determine how much the rpm drops from one gear to another whereas the ratio itself is more a question of how quickly the car will pull through the rpm range in that gear.

I expect the way to do this properly is through tractive effort graphs, you take a dyno run, apply it to each gear and multiply by gear ratios, final drive ratio and rolling radius to calculate torque at the wheels in each gear.

However the part of your question I'm also curious about is the relationship between gear ratios and final drive I. E. Why don't you have a final drive of 1:1 and do the rest with the gearbox?

That link Kanjo posted sums it up extremely well.

I would add something that seems to confuse a LOT of people, even those that should know better.

"Low" gearing is numerically high as the ratio for gears is expressed as the number of turns of the input to one of the output.

eg. if the diff' is designated 39:11, it means input (pinion) has 11 teeth, the output (crown) 39 teeth, the input will turn 39/11 times for one turn of the output, or ~3.5454:1. Some refer to this as a 3.54:1, some 3.55:1. So, 'taller' is numerically lower and 'lower' is numerically higher, before the ":".

It is normal practice to have at least one gear with a prime number for the tooth count - for the reason, I suggest googling "Hunting gears" or "semi-hunting gears".

CRG, I don't quite get your question, but here's my guess for answers.

Q1

The gearbox and differential ratios (and drop gears if a truck or off-road vehicle) are multiplied together to get the overall ratio between the engine and the drive axle (road drive wheels). back in the day, gearboxes normally had a direct (1:1) top and the only way to change the overall gearing, for different applications, was to change the differential gearing - this led to the development of quick change diffs, such as Halibrand's, where an additional pair of gears were used inside the differential, between the input and the pinion, that could be used to change the overall gearing.

In time, gearboxes were developed where their gears could also be easily changed, often with no direct gear, and the differential ratio was less critical.

"Short gearing" is the term used when the gearing is such that the road speed is low compared to the engine rpm - for example, a 4.11:1 ratio is shorter than a 3.33:1. "Long legs" means the opposite, the wheels will rotate more times for the same number of engine revolutions. For example, a 2.75:1 will give 'longer legs' than a 3.33:1 ratio. Either way, there is a trade-off - the first may restrict the top speed, the second will reduce accelleration.

Q2

There are a LOT(!) of opinions on this. My take is - it depends...

I'd suggest starting with what you are doing and figure out what you're first test's overall top gear ratio will be. This is going to depend on what you're doing and the engine's characteristics. The first because, for example, if you're looking for pure top speed (salt flats?) you would gear for that. You're usually better off gearing a little lower than that, though, so you are at peak power longer and gain a little more torque multiplication to get to the slightly lower top speed faster, even though you may run into the engine limiter before the braking point is reached.

The next thing is how tall a first gear can you use? Again, it'll depend on what you're doing and the characteristics of the engine/vehicle - if you're 'drag' racing with a relatively low power car and good traction, you may want a low 1st gear to get moving quickly, but if you're road racing you may be happy compromising a little at the start (perhaps by breaking traction a little or clutch slip) to have a taller 1st gear for good drive out of the tighter corners, perhaps you're looking at rallying where you need drive out of hairpins, etc.

You then need to consider how many gears you have, and how best to use the gearing available to you. The affore mentioned link gives the main points of where to change but, again, the actual gear ratios will depend... If you have a limited number of gears, you may need to gear for the best overal power curves, if you have a good selection of gears, and/or an excess of power, you may prefer to gear for the best drive out of corners so the vehicle isn't upset by gear changes.

As a general rule, change points in the lower gears will be dictated by the engine's rpm limit, depending on the engine's characteristics, sometimes you're better changing a little earlier - you need to learn what the overall package needs.

An example of the importance of the gearing may be what happened to a friend many years ago. He had a rather peaky engine that was fine, until he had to refit a stock gearbox when his close ratio one failed. Because the stock had quite a big drop between 3rd and 4th and it actually dropped the engine rpm to a point where it didn't have enough torque to pull top gear - it would actually slow down. however, if he changed up on a downhill run, less torque was required and it would accelerate and pick up the rpm so it would maintain the speed on the flat.

'Luke, it really comes down to the torque.

If the final drive was 1:1, the drivetrain would be subject to 4 times the torque for the same power and wheel speed, this would mean it would have to be much, MUCH bigger to handle the mechanical loads, which would be a problem with packaging. It would also mean those parts would be much heavier (square rule), which would add to vehicle weight, have a LOT more inertia which would add to parasitic loads under acceleration and braking, and compromise shift quality.

It is much more efficient, overall, to keep the rpm up and the torque low - some off road vehicles even take it a stage further with orbital hub gears (I think the HUMMER used them), or drop gears in portage hubs, such as used by Mercedes off-raod trucks.

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