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Understanding cab to axle and other chassis dimensions relevant to car carriers

12 May
2021

Categories

When ordering a chassis for your new NRC car carrier, your truck sales rep might ask you “what wheelbase do you need”?  What he needs here is the cab to axle measurement. What about after frame?  

Yes, it can get confusing. There are a few things that need to be taken into account when spec’ing a chassis for a NRC car carrier. It’s pretty simple once you know what to look for.

In this article, we will be reviewing the various parameters, clearly define them and put a set of rules to help determine the right values. 

Let’s start with some definitions, as we know them in North America. Elsewhere, in Europe for example, terms like “wheelbase” may have a different meaning. The same goes for different manufacturers.

  • BOC or BOS = Back of cab or Back of sleeper.

That’s not a dimension,but rather a reference point

  • EOF = End of frame
  • BBC = Bumper to back of cab.

Dimensions from the front bumper to the back of the cab or the back of the sleeper

  • WB = Wheelbase.

The distance between the front steer axle and the rear drive axle, or fully equalized rear axle group (tandem or tri drive)

  • CA = Cab to axle

The distance between the back of the cab or sleeper and the rear drive axle, or fully equalized rear axle group

  • CLA = Cab to last axle

The distance between the back of the cab or sleeper and the last, rearmost axle of the drive axle group. NRC refers to this in all documentation: always the last, rearmost axle

  • LOA = Length overall

The overall distance of the carrier, from the front bumper to the tip of the tow-bar.

  • FC = Frame cut

The distance from the BOC toward the rear of the frame where the rails should be cut prior to mounting the carrier

  • X = Deck overall length:  This is a dimension you will see on the dimensioning charts we will check later

So, where do we start? 

It is true that the most discussed parameter is the cab to (last) axle measurement, because it is a function of the desired deck length. The longer the deck, the longer the CLA should be. Wheelbase is also important to look at.  We often hear “longer wheelbase tows better but doesn’t turn as good”.

The first question to ask yourself when spec’ing a truck for a carrier is: is your truck subject to any length limitation by your country and what is it? 

  • If you are in Canada, you LOA will be limited to 41′ (12.5 m)
  • In most US states, it will be limited to 40′. Some states will allow up to 45ft LOA

One thing is certain, the actual LOA will be the summation of BBC and X dimension defined above. So the length of the cab + the overall length of the deck.

You then have to ask yourself what is your priority: do you need a specific chassis or a specific deck length for the type of work that you do with your carrier?

Refer to your chassis specifications and NRC website, and do the maths!  

 LOA = BBC + X

An example please!

Let’s say you want to build a tandem 40TB and you are limited to a LOA of 40′, which is 480′′. The chassis you would like has a BBC of 123′′.

Knowing this, what is the maximum deck length you can have?  

LOA = 480′′

BBC = 123′′

LOA = BBC + X   thus    X = LOA – BBC

480′′ – 123′′ = 357′′      

Looking at the table Recommended CLA for 40TB tandem and searching down the X columns for the highest number that doesn’t exceed 357′′, you will find that the longest possible carrier is 40TB27.      

Now that we have identified the deck length to be 27ft, we can look at the two other important parameters: CLA and FC

CLA – Cab to last axle

The CLA will dictate the position of the drive group in relation to the back of the cab. It must be positioned so:

  • It does not interfere with the deck tilt cylinders or the rear underlift/stabilizer
  • It optimizes the weight distribution while hauling and towing vehicles

The recommended CLA can also be found in the table above. It is NOT an absolute dimension, but rather a guideline. CLA can be a few inches short or long of the recommended values without any substantial change in performance, but greater differences may cause interference with the subframe or the load distribution.

Also, keep in mind CLA must be clear of ANY protrusion that may exceed the back of the cab, like cab suspension mounting brackets or any exhaust component.    

When you order a brand new truck and build the specs from scratch, you can often require a specific CLA measurement. Some manufacturers will only allow for given increments, so simply pick the closest available CLA.   Sometimes, however, the truck is already built and the CLA is slightly outside the recommended range. What can be done? You can still adjust the FC (Frame Cut) to compensate.

FC – Frame cut

The FC is the distance from the BOC where you will cut the frame to fit and weld the hinge plate of the NRC carrier subframe.  The recommended FC values are given in the same table as X and CLA values.  The position of the tilt cylinder and the underlift is a direct function of the FC.  If the FC is 4” shorter, than the tilt cylinders and underlift will move 4” forward. This can prove very useful when cylinders are interfering with the drive group or an oddly positioned crossmember. But, what are the consequences of altering the frame cut?  It just… moves the subframe in the direction the FC offset. Adjustment is available on the deck extension cylinder and the deck itself will remain in almost same position, as long is not altered more than 8” in either direction. So the deck stay in position, and subframe moves? Yes! And it indeed affects the position, both retracted and extended, of the underlift T-Bar relative to the end of the deck. 

Now that we have covered the basic of proper chassis dimensioning for NRC car carriers, we are ready to tackle the weight distribution matters. Coming soon!