3.31 gears at 40,000 lbs gross.

RoyJ wrote:

blt2ski wrote:
Roy,

Probably a good thing we can't get multiple transmission options, along with axle gears. You'd get some crazy like me, order the 3.31 gear sets, but put a 10 so with a single OD, but have lower gears than the on Ford uses. I would have the same overall low, but at the top end, not have to worry about the DOD and TOD blowing up!
We do have some different folks south of that line!

Marty

I believe that's actually the trend in Class 8s these days: ultra tall rear end, with direct or mild OD in the transmission.

Theoretically it would make slightly more power, as ring and pinion gears of lesser difference have lower parasitic loss.

Think We can agree a point: we can't evaluate the best rear end ratio, without also taking transmission ratios into account.

A lot of the "get 4.1 to tow, 3.55 to cruise" mentality dates back to the Turbo 400 days with emissions choked 170hp 350 SBCs. With modern diesels and 8/10 spd transmissions, a 3.31 can drag a house off a foundation, and sufficient for all but the harshest fleet duties.

This is called dowspeeding and it wreaked havoc on drivelines back when the trend first started for some of the same reasons I stated here. We sold a lot and I do me a lot of clutches during this time. Many of my customers found out that the fuel economy benefits were washed away by driveline repairs. In many cases it even cost more than what they would have saved in fuel.

"Dana claims a downsped engine at cruise speed increases torque in the driveline by 57%. Fortifying the driveline is the only way to protect it against the resulting long-term torque stresses."

Downspeeding can Destroy Driveline

And for those who claim it is only engine torque that goes through the driveline. The max engine torque from these class 8 engines is 2,050 lb-ft and the added torque is way more than that.




This is for class 8 trucks that generally cruised between 1,200- 1,400 rpm.


These are all things we found out many years ago in my industry and is the main reason why I say taller axle ratios put more stress on the driveline.

RoyJ wrote:

ShinerBock wrote:
I never said combust. I just said that their lifespan were diminished in our fleet trucks that were used for towing/hauling on a regular basis, especially when combined with larger/heavier tires, versus our shorter geared trucks. This often negated any fuel savings gained from taller gears and we just stuck with shorter gear setups in these applications. Also, most of the burnt/grenaded clutches (after 100k+ miles) that we have seen in our nine Ford dealerships, were in trucks with taller gears.

I was joking of course. Let's agree on this: it is EASIER to abuse a truck with taller gears. And if a truck is truly a dedicated tow rig, sure, I agree short gears are better. For private trucks, some of us want good empty mpg.

I don't disagree here. I even have tall 3.42 gears in my dieael truck for fuel mileage. You have to know your equipment and its limitations which is why I generally tow in 5th where I likely would tow in 6th if I had 4.10s.

All I did was make a general comment that taller gears put more stress on the trans than shorter gears and some got triggered by it as if I was personally insulting their truck or truck brand.

Roy
I believe you are correct for class 8s. I have not looked into the heavier setups in awhile to see how they are doing things.
I do generally speaking like my direct drive Spicer 7 SP in Navistar. I do wish I had an OD, or 3.42 gears and a lower yet 1st than the 10.08 I have. That so when empty, I can cut rpm's down to 1809-2000 vs the 2400 or so I go at 60. But with a 175/385 non turbo diesel, I need to turn a high amount of R's to motor at 60 at any major gvwr above 18-20k even on a level.

I would agree, unless one take trans, and tire diameter into the equation, rpm peak torque, HP needed, actual best axel ratio is a **** shoot!

Marty

blt2ski wrote:
Roy,

Probably a good thing we can't get multiple transmission options, along with axle gears. You'd get some crazy like me, order the 3.31 gear sets, but put a 10 so with a single OD, but have lower gears than the on Ford uses. I would have the same overall low, but at the top end, not have to worry about the DOD and TOD blowing up!
We do have some different folks south of that line!

Marty

I believe that's actually the trend in Class 8s these days: ultra tall rear end, with direct or mild OD in the transmission.

Theoretically it would make slightly more power, as ring and pinion gears of lesser difference have lower parasitic loss.

Think We can agree a point: we can't evaluate the best rear end ratio, without also taking transmission ratios into account.

A lot of the "get 4.1 to tow, 3.55 to cruise" mentality dates back to the Turbo 400 days with emissions choked 170hp 350 SBCs. With modern diesels and 8/10 spd transmissions, a 3.31 can drag a house off a foundation, and sufficient for all but the harshest fleet duties.

ShinerBock wrote:
I never said combust. I just said that their lifespan were diminished in our fleet trucks that were used for towing/hauling on a regular basis, especially when combined with larger/heavier tires, versus our shorter geared trucks. This often negated any fuel savings gained from taller gears and we just stuck with shorter gear setups in these applications. Also, most of the burnt/grenaded clutches (after 100k+ miles) that we have seen in our nine Ford dealerships, were in trucks with taller gears.

I was joking of course. Let's agree on this: it is EASIER to abuse a truck with taller gears. And if a truck is truly a dedicated tow rig, sure, I agree short gears are better. For private trucks, some of us want good empty mpg.

Roy,

Probably a good thing we can't get multiple transmission options, along with axle gears. You'd get some crazy like me, order the 3.31 gear sets, but put a 10 so with a single OD, but have lower gears than the on Ford uses. I would have the same overall low, but at the top end, not have to worry about the DOD and TOD blowing up!
We do have some different folks south of that line!

Marty

RoyJ wrote:

Grit dog wrote:
I'm gettin confused....now I need a show of hands...
Whos team 4x and whos team Shiner?
Lets go boys!

Obviously physics change when you cross the 49th parallel :B

Down south transmissions combust when hooked up to 3.31 gears :p

I never said combust. I just said that their lifespan were diminished in our fleet trucks that were used for towing/hauling on a regular basis, especially when combined with larger/heavier tires, versus our shorter geared trucks. This often negated any fuel savings gained from taller gears and we just stuck with shorter gear setups in these applications. Also, most of the burnt/grenaded clutches (after 100k+ miles) that we have seen in our nine Ford dealerships, were in trucks with taller gears.

Grit dog wrote:
I'm gettin confused....now I need a show of hands...
Whos team 4x and whos team Shiner?
Lets go boys!

Obviously physics change when you cross the 49th parallel :B

Down south transmissions combust when hooked up to 3.31 gears :p

philh wrote:

RoyJ wrote:
Wow, physics, hell I'll play :D

For any rigid shaft, torque in = torque out, correct? Basic Physics 101. Unless the shaft deforms, which is a failure...

3.31, 4.30, 4.88 truck, whatever, your max T-input at transmission is always your engine max torque, right? Assuming locked converter.

You axle torque is always dictated by your speed, grade, weight, aero, and instant acceleration. Doesn't matter if you have 3.31/4.30/3.55/4.88/5.13 gears. To pull a 40,000 gvw rig hp 7% grade with no wind at constant speed, you need a FIXED amount of axle torque (thrust), correct?

That means the ONLY difference in stress between a 3.31 and 4.30 truck is the transmission output torque, or driveshaft torque (again, T-in must = T-out). Because on the same hill, a 4.30 truck may be in 6th and the 3.31 truck in 5th.

If you're burning OD clutches, either you've gone above the GVW rating of the tranny, and /or you should've locked out OD.

Again, I don't know anybody pulling a heavy trailer that does WOT Launches. OEMs do extensive testing, including WOT runs on a dyno.

Which is why off the line is the only place the 4.30 truck has an advantage is "lower stress".

If the hill / weight is high enough, a 3.31 truck may require 100% throttle to get going, and the 4.30 truck 77%.

After that, with 10 spds to play with, you can always find a hill where the 3.31 truck in 3rd climbs it at slightly less throttle than a 4.30 truck in 4th. And vice versa.

In fact, if both trucks had perfect CVTs, a 3.31 and 4.30 truck would pull 2 equal hills, equal weight, at absolutely equal throttle %, rpm, and speed. The ONLY difference is driveshaft torque (4.30 truck is 77% less). Axle torque, transmission input shaft, would be IDENTICAL.

4x4ord wrote:

ShinerBock wrote:

dodge guy wrote:


Ughh! Spinning 4.30’s is easier all around compared to spinning 3.30’s. It’s basically the same as removing 4K lbs of weight from the vehicle!

Yes, as if the force my legs need to put on the pedals of my 8-speed(which changes the ratio to the tire) bike in short gear is the same as taller gear and it would have no effect on the friction material if there was a clutch on my pedals rather than a chain.

What if you look at it this way: If I have 4.30 gears and I’m pulling a trailer that puts me at 28500 lbs gvw and say I’m travelling 65 mph and let’s assume the power required to do this is 150 hp I will be in 10 th gear running at 1800 rpm and my Powerstroke will be feeding 438 lbft of torque into my torque converter. Now I come to a hill ... say a 6 percent grade. The power require to maintain 65 mph goes to 446 hp. My foot goes to the floor and transmission drops two gears to 8th gear. Now my Powerstroke is feeding 968 lbft of torque into my transmission through my overdrive clutches.


With 3.31 gears say I only lock out 10 gear and I’m towing that same trailer at 65 mph. Now my Powerstroke is turning 1506 rpm and I’m feeding 523 lbft of torque into the transmission on level ground (and if I was concerned about my overdrive gears I could lock out 9) When I hit the hill and my foot goes to the floor I drop to 7 seventh and my overdrive clutches are out of the equation.

You’re not going to come up with a more realistic scenario where an RVer with 3.31 gears is going to be putting more than 968 lbft of torque through the over drive gears.

All that still doesn't change the fact that a taller gear ratio puts more stress on the transmission clutches gear for gear than a shorter rear gear ratio. If you are compensating for this stress by reducing gears(which can also be done with a 4.30) then that is another subject, but the fact still remains that a 3.31 puts more stress on 10th than a 4.30, a 3.31 puts mores stress on 9th than a 4.30, a 3.31 puts mores stress on 8th than a 3.31, etc.....