3:42 rear axle ratio rated to tow 17500lbs?

[NIsaacs]

The heat generated when towing will be higher than 3:73 or 4:10,

I been looking at this for a while now, on the surface it makes sense, but will it for sure? The higher the gear ratio the larger the pinion, more tooth contact and turns slower... . discussion?



Nick

 

[RustyJC]

It's the old leverage thing. For equivalent axle shaft torque and speed, the driveshaft (and, thus the pinion gear shaft and gear teeth) will be transmitting 16. 6% more torque with the 3. 42s than they would with the 4. 10s ([4. 10 - 3. 42] / 4. 10). More pinion gear torque = higher tooth loading and more frictional heat generated.



Rusty

 

[NIsaacs]

It's the old leverage thing. More pinion gear torque = higher tooth loading and more frictional heat generated.



Rusty

Again, on the surface, I agree. But... . if a 4. 10 has 10 pinion and 41 ring gear teeth v/s 3. 54's with 13 and 46, what is taking place here? The torque is divided up between more teeth so less time spent per tooth? I used a Dana-80 for reference, the AAM specs are hard to find.



Nick

 

[RustyJC]

So, how many gear teeth/minute go through the hypoid gear oil? Wheel speed is the same for both @ 60 MPH (let's assume) since tire revolutions per mile (or per minute @ 60 MPH) equals, let's say, 640.



For the 4. 10:

640 RPM x 41 ring gear teeth = 26,240 ring gear teeth/minute

640 RPM x 4. 10 pinion gear revolutions/ring gear revolution x 10 pinion gear teeth = 26,240 pinion gear teeth/minute (surprise - since the teeth mesh, it must be a 1:1 ratio)

26,240 + 26,240 = 52,480 teeth/minute



For the 3. 54 (3. 5385:1 actual)

640 RPM x 46 ring gear teeth = 29,440 ring gear teeth/minute

640 RPM x 3. 5385 pinion gear revolutions/ring gear revolution x 13 pinion gear teeth = 29,440 pinion gear teeth/minute

29,440 + 29,440 = 58,880 teeth/minute



AND the pinion gear teeth on the 3. 54s are generating more heat per tooth engagement since they're more heavily torque loaded.



More teeth/minute dumping heat in the hypoid gear oil AND more heat generated per tooth engagement means the 3. 54 generates more heat that the hypoid gear oil must handle.



Rusty

 

[NIsaacs]

Your proof keeps getting better, I am almost convinced but... . if the 3. 54 gear set generates say, 185 degrees in general then at 16. 6% less torque the 4. 10 should run at 154 degrees or if the 4. 10 runs at 185 degrees then the 3. 54 should be at 215. 7 degrees or no?



Nick

 

[RustyJC]

It's not quite that simple. You have to factor in the difference in gear tooth face velocities as well since higher velocities will tend to generate more heat, although that's not a HUGE differentiating factor with well designed and manufactured hypoid gears. Since we don't know the coefficients of (lubricated) sliding friction or the precise tooth velocities for the two different gearsets, it would be hard to make a firm computation.



Rusty

 

[Jim W]

I know that this has been an issue on another Cummins web site and a lot of debate has taken place too. Some poster have even stated that they will change out the GEAR SET after buying the truck to use a 4:10 instead of the 3:54 gears that are the only option for the SRW trucks.

I also know that there are some engineers on this site such as Rusty and other's like myself. But I have always felt that the design team that designs a product is usually the best source for design specs and options. My reasoning is they have the design specs with the functional specs and design accordingly to them. That is not to say they are always correct or design the product correctly the first time. Also what was correct for the older generations of Ram trucks may not be correct for the current 2013MY truck.

Now if the truck is not capable of handling the size trailer and weight that the Ram design team has stated. Do you really thing the money managers would allow them to market the truck this way? There are design reviews before a product is built and marketed. The managers would be leaving Chrysler open for all sorts of litigation if the truck cannot perform as advertise. As engineers we also tend to be the conservative type and tend to understate the performance ability to allow for growth of the vehicle; at least at Cat we did.

Just my $0. 02

Jim W.

 

[EricBu12]

hey guys, Just for ***ts and giggles, I went to the ford website and started a build, and looked to see what they have for gears. .

Selected 2013 F-35 Lariat Crew Cab SRW, 4x4. Diesel V-8. and the rear axle ratio is 3. 31 !! So much for Super duty. Now when you select DRW, they have a 4. 30 gear. . so that is better than Dodge. . They also have different Diffs. with electronic lock and stuff... I set it up as I would like it, if I was to get one. . wow. Expensive I stopped at $61K and had a few other things to add.

EDIT.

Now I just looked a the king ranch and even on dually and Diesel and 4x4, was only able to select 3. 73 axle. . If I select the 4. 30, it changes to the 6. 2L Gas engine. what a joke.

 

[RustyJC]

Yep, 3. 73 is as low as they go for the F-350 DRW diesel. 4. 30s are for the gassers (although they used them in the past with diesels on their TowBoss package). I have an RVing friend with a 2013 F-350 DRW with the 3. 73s.



Rusty

 

[RVTRKN]

It takes more pressure to turn the 3:42 which means more heat generated, whats there to argue about?

 

[NIsaacs]

It takes more pressure to turn the 3:42 which means more heat generated, whats there to argue about?

Whos arguing? I made some comments and had some questions so Rusty and Jim answered them.

Nick

 

[AH64ID]

Ford also dropped a direct drive gear, so their 4th is lower which "allows" them to run a higher rear end ratio.

 

[rwitt]

Yep, 3. 73 is as low as they go for the F-350 DRW diesel. 4. 30s are for the gassers (although they used them in the past with diesels on their TowBoss package). I have an RVing friend with a 2013 F-350 DRW with the 3. 73s.



Rusty

The F350 Dually Diesel only comes with 3. 73's, the F450 Pickup only comes in Crew Cab with the Diesel engine and 4. 30 gears. The 450 dually pickup is just like the 350 except for the gears and wide trac front axle.



Rich

 

[RustyJC]

Ummmm..... OK, so where's the error in what I said? I believe I said that the F-350 DRW diesel can't be had with any gearing lower (higher numerically) than the 3. 73s. Isn't that what you said as well?



Rusty

 

[rwitt]

Ummmm..... OK, so where's the error in what I said? I believe I said that the F-350 DRW diesel can't be had with any gearing lower (higher numerically) than the 3. 73s. Isn't that what you said as well?

Rusty

No error on your part Rusty, but you did say 4. 30's were for the gasser trucks(but used in diesels in the past). They still use them in the 450 pickup.

Rich