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-   -   cab to rear axle distance? (https://www.irv2.com/forums/f45/cab-to-rear-axle-distance-267084.html)

riverss 10-17-2015 02:36 PM

cab to rear axle distance?
 
looking at many messages on this and other discussion/forums i read a lot about long bed vs short bed with the issue of 5th wheel to cab clearance. Ford/Chevy/Ram pick up trucks have different locations of their rear axles in relation to the cab of the truck. the statement that "i have no problem with clearance"might be true for some but not true for others with regards to a short bed truck. I have a 2015 Ram 2500 crew cab short bed and the distance from the back of the cab to the rear axle ( goose neck ball) is 38". If i factor in 3"on each side for a total of 6 " for additional clearance the widest the 5th wheel trailer can be is 82". most r/v widths are 96" so i am short 14 "or more. that means that i need a sliding hitch that will slide 14 inches. I think that the longest slide is 12". that leaves me 2 short ". Using the B&W only for an example i would have to mount the 5er hitch so that it is 2" behind the rear axle to obtain the clearance from the cab to the R/V in a full jack knife situation. I would like to hear from a Ford and Chevrolet owner of short bed trucks the distance from the cab to the rear axle.

Gordon Dewald 10-17-2015 02:47 PM

How far is your front cap in front of the pin. That will also have to be considered.

Our 5er has approximately 24" from the pin to the front cap. There is a bit of an angle to the edges which would reduce that factor plus considering the width.

We have no issue as our long bed is a 9' bed.

jporietis 10-17-2015 03:56 PM

Quote:

Originally Posted by riverss (Post 2787696)
looking at many messages on this and other discussion/forums i read a lot about long bed vs short bed with the issue of 5th wheel to cab clearance. Ford/Chevy/Ram pick up trucks have different locations of their rear axles in relation to the cab of the truck. the statement that "i have no problem with clearance"might be true for some but not true for others with regards to a short bed truck. I have a 2015 Ram 2500 crew cab short bed and the distance from the back of the cab to the rear axle ( goose neck ball) is 38". If i factor in 3"on each side for a total of 6 " for additional clearance the widest the 5th wheel trailer can be is 82". most r/v widths are 96" so i am short 14 "or more. that means that i need a sliding hitch that will slide 14 inches. I think that the longest slide is 12". that leaves me 2 short ". Using the B&W only for an example i would have to mount the 5er hitch so that it is 2" behind the rear axle to obtain the clearance from the cab to the R/V in a full jack knife situation. I would like to hear from a Ford and Chevrolet owner of short bed trucks the distance from the cab to the rear axle.

1/2 of 14 or 7 inches

john

SmokeyWren 10-17-2015 04:14 PM

A dozen years ago or more I studied the numbers on cab-to-axle (CA) vs trailer width. At that time the 5er hitch installers were using 54" as the minimum cab to kingpin distance required for a "normal" 8' wide 5er. That allowed the kingpin to be centered 4" in front of the rear axle.

4" for hitch positioning in front of axle
48" for half the width of the 5er
2" clearance between front corner of 5er and rear of cab at full jackknife
----
54" total
===

That worked fine on my '99 F-250 with 56.2" CA.

But with the "luxury" 5ers that were the max of 104" wide, things tightened up a bit.

3" kingpin positioning
52" half the trailer width
1" clearance at full jackknife
__
56" total, leaving only 0.2" slop given the 56.2" CA
===

With only 38" CA, then you'll have to do some serious compromising. For one thing, don't even think about going to a 104"wide 5er. Mount the hitch so the center of the kingpin is only 1" in front the center of the axle.

48" half of the width of the 5er
1" kingpin positioning
1" clearance
--
50"
-12" slider
---
38 CA
===

So it's do-able, but you must measure precisely, and your rig won't handle as good as one with the kingpin properly located 4" in front of the center of the rear axle.

JIMNLIN 10-17-2015 08:06 PM

I have the short bed Dodge truck with a sliding hitch for my 11200 lb 5er. This 5er is a older '97 model with a very flat square corner profile.
To get the full benefit of any sliding hitch movement you will have to place the pin zero over the trucks rear axle.
The farther forward from the rear axle the pin is placed is a loss of sliding ability.

Locating the pin aft of the rear axle center line unloads the trucks front axle. Not a good idea as some have found out.
I've pulled heavy GN trailers with the ball placed zero up to 4" in front of the rear axle. No difference in handling on wet or dry pavement.
However on the road I had to buy another trailer, for one that was taken out of service. The hitch guy misplaced the the ball plate 3" behind the trucks rear axle which unloaded the front axles/steering.
I was 380 miles from home and fought the rig all the back. My home hitch guy relocated the plate 2" forward of the RA where I liked it . That trailer pulled like it was supposed to.

The cab to axle dimension your looking for has changed over the years in the different year models for each brand. And as was mentioned how long is the extended pin box. I've had them with the pin 30" in front of the trailer to 6" behind the front of the trailer on older 5th wheel units.

GM CA shows a 41.28 on a '15 year model short bed or std bed as GM calls the 6' 6" bed per online ordering guide specs..

The same year Fords show a 40.2 " CA per body builders specs

Ram body builders guide specs shows a 36.7" CA.

riverss 10-17-2015 10:24 PM

Great info thanks i do not have any personal experience comparing the difference in handling but suspected it did make a considerable difference especially in a gas rig.

riverss 10-17-2015 10:29 PM

Continued ... those figures explain why GM sb and Ford sb have less problems.

riverss 10-18-2015 10:05 AM

Gordon how does pin to cap distance factor in at 90 deg?
John 7" is right my bad math and that 7 " is a big deal
Smokey that is good info helps me a lot

Gordon Dewald 10-18-2015 11:01 AM

Sorry - the trailer is at home and we are currently snow birding. Not able to take any measurements.

From memory I recall the front outside edge of the cap is about 12" ahead of the pin.

SmokeyWren 10-18-2015 11:31 AM

Quote:

Originally Posted by riverss (Post 2788670)
... how does pin to cap distance factor in at 90 deg?

I don't know. My 5er has an extended pin box that puts the center of the kingpin almost flush with the front of the trailer.

Trouble with cab to trailer clearance begins well before you reach 90. It begins when the front corner of the 5er nearly reaches the back of the cab, somewhere around 70. With only 1" clearance, the front corner of the 5er barely misses the rear of the cab as the trailer completes it's swing past the cab and out past the other side of the cab. Then you have a tiny bit of breathing room until you get to a full 90 jackknife, when you'll again have only 1" clearance.

Watch the linked video carefuly as the tow vehicle backs into a jackknife.
https://www.youtube.com/watch?v=g-SOv7sEHfQ

Also note on the PullRight website that they recommend a minimum of 3" clearance between cab and trailer during a jackknife turn.
Quote:

Originally Posted by Pullrite website
Trucks with less than 6' beds are usually not recommended for 5th wheel towing, however, with some trailer limitations the 3100 SuperGlide may make towing possible. In cases where there will be less than 3" clearance between the cab and trailer, including a 90 degree turn, the 3100 is not recommended.

Also note that the SuperGlide automatic can move a lot more than the 12" you posted:

Quote:

Originally Posted by Pullrite website
It moves up to 22-" while turning

That makes it comfortable for use in a 6' bed, but PullRite says that's often still not good enough for the super-shorty 5.5' bed.

dayle1 10-20-2015 10:08 AM

Regarding short bed (now called standard) trucks, GM has always had the longest C-A and Ram (Dodge) the shortest. That is why I have always preferred GM. The manufacturers also do not change the numbers by more than a few tenths of an inch from model year to model year. Otherwise it would cause havoc for aftermarket suppliers. I recently moved my RKI utility bed from an '03 GMC to my '13 Chevy, couldn't do that if the numbers were constantly changing.

The important question is do you need a slider? Part of the answer depends on the fifth wheel design, if it is notched like most new units, then max turning angle will be close to 90 degrees. It will vary somewhat depending on hitch placement and C-A. But it will still be sufficient for most situations. I do not use a slider with my setup and while my present fiver has a notched front cap, all previous fivers did not and they never caused a problem. Tight situations may require a little more jockeying back and forth if you can't jackknife a full 90 degrees, but the benefit is less stress on tire, wheels, wheel bearings, etc.

If you want full 90 degree capability, then a better option might be the Reese Sidewinder (Revolution) pin box. It moves the pivot point back under the fiver without moving the king pin load point. It also prevents the back side of the pin box from hitting the truck bed sides as can happen with many extended pin boxes.

monkey 10-21-2015 07:57 AM

One thing I haven't seen mentioned yet,,, unless I missed it,,, is these clearances are dependent on your truck And trailer being on totally flat ground. You'd still have to be very careful if either was on an incline/decline... Like a driveway for instance....

schrod 10-23-2015 06:32 AM

Just give it up and buy a dually long bed like I did. I hashed and hashed if my shortbed crew cab F250 would suffice. I finally gave it up for piece of mind knowing I have the turning room and extra capacity for weight. Now I can really sleep at night.

justafordguy 10-23-2015 10:51 AM

Quote:

Originally Posted by schrod (Post 2795583)
Just give it up and buy a dually long bed like I did. I hashed and hashed if my shortbed crew cab F250 would suffice. I finally gave it up for piece of mind knowing I have the turning room and extra capacity for weight. Now I can really sleep at night.


That sounds like a great idea if you have an extra $70k. :D


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