Dually wheel question

[Lynx0849]

A couple of questions...

Why are the wheels on dual rear models smaller than those on comparable SRW models? New ones have 17” but the SRW has 18”? Would not larger sizes run cooler and rotate at lower rpm for a given speed?

Second, why is it that the front axels on DRW stick out so much further than the ones on SRW models?

Just curious though I expect to end up with a ‘21 model DRW for 5th wheel towing in several years.

 

[matthewneedham]

lynx0849,
You don't need larger diameter wheels to reduce rpm of a given vehicles' wheels. Simple tire change can do that. taller sidewalls will usually end up having a larger diameter/circumference which will lower the rpm of the axle shafts.
The wheels need to be able to be mounted in the front or on the rear of the truck. In order for that to work they have to be the same wheels all the way around. Fitting a bulged out dually wheel on a front axle requires a longer hub assembly for clearance of suspension parts.
I am sure there is a reason behind the 17 inch wheel on the DRW setup. It is probably something pertaining to COST! seventeen inch DRW's have been around forever and tires are cheap and readily available.
Matt

 

[Kirwin]

Lynx the dual wheels have a deep dish rim to allow the use of the same rim at all wheel positions. Hence the front axle has extensions on it to position the dished wheel "in" to center the tire under the front fender. The other thing that dual wheel trucks must be concerned with is the spacing the rear rims provide for the rear tires. The tires must not touch under any load condition or tire failure will shortly occur. So on the rears, one rim is positioned to dish inward and the other is positioned to dish out ward. The tire size is limited buy the rim "dish" offset that allows tire spacing on the rear set.

 

[Lynx0849]

Thanks. That makes sense. Now I know and so does DW.

 

[KCJackson1]

The wheel diameter has nothing to do with being a dually... 4500/5500 Rams use 19.5" wheels with no issue, as do Ford F-450/550s. There are also direct replacement 20" dually wheels in the aftermarket that use the same bolt circle and hub opening as the OEM 17s.

The use of 17s stem from cost. The standard steel DRW wheels are inexpensive, even though they are custom-drilled for the bolt circle and hub of each manufacturer. When the Ram 3500 started using 17s in 2003, there were very few brands of tires making the 235/80 size - now everyone makes that size and FCA switched from General to the less-expensive Nexen. The three manufacturer went from 16s to 17s for brake clearance, but at this point, the rotors are as large as they need to be. Ford doesn't share the bolt circle (8x200mm) or the tire size (245/75) with Ram or GM, but they stuck with 17s all these years for cost as well. If Ram decides to go to a different size with the redesign in a couple of years, they'll find a wheel manufacturer who will make them, and a tire manufacturer who will meet the necessary load requirements. But since it's not yet necessary and DRW trucks are still very low as far as production percentage, there's no good reason to make the change. The 18s and 20s on SRW trucks are strictly for "curb appeal".