Is Re-gearing worth it?

I've decided against gears, although I encourage this conversation to continue to help educate others in a similar position as myself. I'm getting a custom towing tune and will upgrade my transmission cooler. Slow and study is the best way to vacation anyhow.

wnjj wrote:

06Fargo wrote:
Like I said not much experience with light duty trucks but I know there is something to gear reduction occurring closer to the wheels (rear axle) vs dropping gears in the trans - at least in heavy trucks - there is probably a mechanical law somewhere - alls I know if you hook up a high geared truck to a heavy set of trailers, you will be rowin' the stick all day in hilly terrain while the lower rear axle geared truck drives away...

Lower rear axle gearing has the very real effect of spacing the transmission gears closer. That's why there can be a performance difference.

I'll try to explain it here:

The axle gearing divides the wheel speed (relative to the driveline speed) the same amount in every transmission gear.

Let's say you lower the rear axle gearing by 10%. Every gear in the transmission will now have the wheels turning 10% slower than before (e.g. 30mph -> 27mph) and in a higher gear (70mph -> 63mph), etc.

You now have the same number of gears between 0 and 63mph as you used to have between 0 and 70mph. If you were to change the rear by 20% you now space the same gears to 56mph. This means you have better choices about what RPM to run the engine for any given hill/road speed and can make use of yet another gear for speeds above 56mph.

In essence lowering the rear gears acts a bit like adding a gear (or part of one) to the transmission. This is why it is less important with a 6-speed than a 4-speed.

Good post. I will add one thing. If you put short gears in your truck it won't make much difference in how fast you make it up a hill because you are not adding power. It will make some, but not much. The flatter the torque curve of your engine the less difference.

06Fargo wrote:
Like I said not much experience with light duty trucks but I know there is something to gear reduction occurring closer to the wheels (rear axle) vs dropping gears in the trans - at least in heavy trucks - there is probably a mechanical law somewhere - alls I know if you hook up a high geared truck to a heavy set of trailers, you will be rowin' the stick all day in hilly terrain while the lower rear axle geared truck drives away...

Lower rear axle gearing has the very real effect of spacing the transmission gears closer. That's why there can be a performance difference.

I'll try to explain it here:

The axle gearing divides the wheel speed (relative to the driveline speed) the same amount in every transmission gear.

Let's say you lower the rear axle gearing by 10%. Every gear in the transmission will now have the wheels turning 10% slower than before (e.g. 30mph -> 27mph) and in a higher gear (70mph -> 63mph), etc.

You now have the same number of gears between 0 and 63mph as you used to have between 0 and 70mph. If you were to change the rear by 20% you now space the same gears to 56mph. This means you have better choices about what RPM to run the engine for any given hill/road speed and can make use of yet another gear for speeds above 56mph.

In essence lowering the rear gears acts a bit like adding a gear (or part of one) to the transmission. This is why it is less important with a 6-speed than a 4-speed.

I'm starting to see now, eh? :)

Like I said not much experience with light duty trucks but I know there is something to gear reduction occurring closer to the wheels (rear axle) vs dropping gears in the trans - at least in heavy trucks - there is probably a mechanical law somewhere - alls I know if you hook up a high geared truck to a heavy set of trailers, you will be rowin' the stick all day in hilly terrain while the lower rear axle geared truck drives away...

With our little rv trucks you do need to do what blt2ski says - make sure the final overall gearing lands you in a "useable" range for not towing situations.

On small vehicle dynos: so that's why they run the vehicle from idle to the pin and produce those swoopy curves on the graph?

Can you use one to set at 60mph tow speed and then start to load the engine until it is at whatever HP (power) output it can manage at the desired fuel rate? Then produce a result that says "your 450HP engine makes 230HP at cruise gear/speed at X lbs/hr of fuel (XX% fuel rate)... you can therefore pull a XX sq ft XX lbs trailer up a X grade..." "And if you haul with it on the 450HP pin all the time it will give you 5mpg..."

No wait, bad idea...:) eh?

blt2ski wrote:

06Fargo wrote:
So if I understand this right, if jwco5.3 could clone his truck and trailer so everything is identical except the rear ratio on #1 is 3.55 and on #2 clone rig is 4.56, #1 has no trouble keeping up in the hills, because it has the same power?

I guess it could be so if the trailer is so light it never really makes either truck lug down at full power on a grade...

OK - so then a 450hp pickup truck can hook on the same load as a 450hp class 8 Freightliner and drive away with it then, right? Same power. Gearing doesn't matter.

The issue is as pointed out, At a given speed, the engine is turning more rpm, hence the motor is producing more HP at the higher rpm, so in a sense, it is producing more HP. The thing with gearing, be it in the trans, axels, or tire diam, is one has to have the correct HP at the speed you want to go for the total weight, and frontal area wind resistance one has. If you need a total of 300 hp to go 60 mph up a 4% grade, you motor only produces 250hp, you will NEVER go 60 mph up the hill, as you are 50 hp to little.

On the other hand, if the motor produces 350 hp at say 6000 rpm, and 300 at 5000 rpm, you could, WITH the correct gearing, theoretically go 60 mph up the 4% grade, IF, you can get the motor to turn 5000+ rpm in a gear, tire diam combo, that will net you 60 mph! If you have a gear that is too slow ie say a 4.88 vs 4.56, OR fast ie taller than 4.56 for matter, you will not hit 60, you will be slow, if you gear it correctly, then you can go 60 mph.

Clear as mud eh!

marty

I can see just fine! :B

06Fargo wrote:
So if I understand this right, if jwco5.3 could clone his truck and trailer so everything is identical except the rear ratio on #1 is 3.55 and on #2 clone rig is 4.56, #1 has no trouble keeping up in the hills, because it has the same power?

I guess it could be so if the trailer is so light it never really makes either truck lug down at full power on a grade...

OK - so then a 450hp pickup truck can hook on the same load as a 450hp class 8 Freightliner and drive away with it then, right? Same power. Gearing doesn't matter.

The issue is as pointed out, At a given speed, the engine is turning more rpm, hence the motor is producing more HP at the higher rpm, so in a sense, it is producing more HP. The thing with gearing, be it in the trans, axels, or tire diam, is one has to have the correct HP at the speed you want to go for the total weight, and frontal area wind resistance one has. If you need a total of 300 hp to go 60 mph up a 4% grade, you motor only produces 250hp, you will NEVER go 60 mph up the hill, as you are 50 hp to little.

On the other hand, if the motor produces 350 hp at say 6000 rpm, and 300 at 5000 rpm, you could, WITH the correct gearing, theoretically go 60 mph up the 4% grade, IF, you can get the motor to turn 5000+ rpm in a gear, tire diam combo, that will net you 60 mph! If you have a gear that is too slow ie say a 4.88 vs 4.56, OR fast ie taller than 4.56 for matter, you will not hit 60, you will be slow, if you gear it correctly, then you can go 60 mph.

Clear as mud eh!

marty

06Fargo wrote:

APT wrote:

Turtle n Peeps wrote:

WOW! please don`t offer advice on things you are not completely knowledgeable on.

You will never see it because chassis dynamometers calculate wheel power by dividing out the transmission and differential ratios.

Actually don't they measure torque at rpm to calculate power (HP) ?

Actually a chassis dynamometer measures horsepower.

A large heavy drum is accelerated to a given rpm and time is measured to reach that rpm.

We know what the inertia of the drum is. We also know how much horsepower is required to accelerate the drum from one rpm to another in a given time.

Many very small rpm changes are plotted vs horsepower required for each change.

This gives a horsepower vs rpm curve.

From this a torque vs rpm can be calculated.

APT wrote:

Turtle n Peeps wrote:

WOW! please don`t offer advice on things you are not completely knowledgeable on.

You will never see it because chassis dynamometers calculate wheel power by dividing out the transmission and differential ratios.

Actually don't they measure torque at rpm to calculate power (HP) ?

So if I understand this right, if jwco5.3 could clone his truck and trailer so everything is identical except the rear ratio on #1 is 3.55 and on #2 clone rig is 4.56, #1 has no trouble keeping up in the hills, because it has the same power?

I guess it could be so if the trailer is so light it never really makes either truck lug down at full power on a grade...

OK - so then a 450hp pickup truck can hook on the same load as a 450hp class 8 Freightliner and drive away with it then, right? Same power. Gearing doesn't matter.

Turtle n Peeps wrote:

WOW! please don`t offer advice on things you are not completely knowledgeable on.

You will never see it because chassis dynamometers calculate wheel power by dividing out the transmission and differential ratios.