Why are there springs on my rear cab mounts?
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Here's another question that Jesse fielded on FC cab mounts.  He has even toyed with air bag setups on his trucks.  Great information here.


Hi Jesse,

 I had a question on mounting my cab. I wanted your input since you probably have the knowledge. The cab is currently mounted solid on front, and the rear has valve springs top and bottom of the bolts so it doesn't twist with the frame. I understand the concept, but there are better ways. My question is how much flex does the cab need. Can it just be rubber mounted, or does it need more room to move than that? I remember seeing the small air bags on your 677, and found a nice short set of 5" round air bags but wondered if that was more give than it really needed. Appreciate any input your willing to give.
Dennis Miller

HI Dennis,

It is complicated, but I will try to explain it in an engineering description.
The front mounts of the truck should have at least a 1" rubber pad, the rear, has a spring above and below the cab mount, under the bottom spring, is another 1" rubber mount, and a lot of washers, that is stock, and this works best.
The frames do twist a lot in the middle on the FC-170, for these reasons:
When empty;
The front axle has the spring shackles at the front, the rear springs hangers are hinged, all of the heavy engine/transmission/transfer case and cab weight, are all balanced on the front axle, then add the bed, that has the front half of it's weight equally balanced on the frame, between the front and rear axles, leaving only the rear half of the bed balancing over the rear axle and springs, that have the shackles to the rear? Now during the empty bed condition, the front of the cab can not rock front to rear, only side to side, so when the frame bounces, it squeezes the front springs, sometimes like a big rut in the road, or one wheel on a curb, where only one spring is squeezed and the other spring is relaxed, as either or both springs are squeezed, they  increase or decrease in length by moving the shackle, remember that the spring shackles are at the front of the frame and at the other end at the rear of the frame are the shackles, so that the springs are hinged at the mid section of the frame. Now because the frame must allow all the four inside hinges to twist, when one spring is squeezed (lengthened) and the other side of the same axle is shorter (relaxed) movement of the shackles, then twists the hinged end of the springs. The cab, should not have any allowance for twisting, therefore the springs allow the up and down movement of side to side movement of the rigid cab on the flexing frame. The improper rear coil spring mounting will not allow for proper rear frame flex and will eventually cause the cab to crack above the rear screen, removal of the screen that is structural, will also cause the cab to crack.
Now add a full load on the bed and now if the entire load is centered over the length of the bed then the frame will flex very little, if the load is over only the rear axle, then it has increased the flex of the frame below the rear of the cab. Now the load will rock side to side and the cab can rock side to side in a different direction, only what is worse is, or the difference is, that the heavy rear load is on the bed that is equally mounted to the frame and it will flex at the same amount as the frame, also making it worse because the frame transition is narrow in front, to allow for turning of the front wheels, then wider at the rear for better stability of the 9' bed. OK, not a pretty picture, but it gets worse, the reason frames crack at the back of the cab, is because the cab rear mount is directly over the rear spring hinges, so the overweight cab and engine, that can not move front to rear, only side to side, is all somewhat balancing on only the front axle equally, when the pick up bed is loaded beyond the GVW or the load is at only the rear, the multiplication of torque is increased when hitting the brakes. When the truck comes to a quick or even an overloaded stop, the load shifts forward, then that load is multiplied double over the distance of the far rear mounted axle, the increase of the shift of torque is applied forward, to that area behind the front spring hinges and cab mounting area, where most of them crack.   
The cab on the M-677 is unique as it weighs almost double of what the civilian cab weighs, typically the M-series also run a body lift mounted spacer to gain an additional inch, the mounting however is the same in front and below the mid section, as it is on the civilian FC-170, with the valve spring coils upper and bottom with the pads, at the mid section mounts, on the original equipment M-677 4 doors, there is a fifth mount, at the middle of the rear of the cab, it is not bolted to the frame it is only bolted to the cab, so the rear of the cab is only floating at the rear, but the center, rear, mounted cab pad, only rests on a unique M-677 only, cross member, made for the rear of the cab, for down support only, because the rear cab  is floating unattached, to the frame, this totally allows for the frame to flex in the middle of the frame, the short m-677 6 foot bed is pretty equally mounted over the rear axle.
Now about the air bags, they are needed on my M-677s, my extended use for my trucks allows them to be off-road or on the highway at freeway speeds, sometimes while pulling a trailer. As you may have noticed, Convectional and COE Semi trucks like KW, PB, FL, and others, also have rear mounted air bags under the cabs or sleepers, a similar situation on the tractors exists on my M-677, if all road surfaces are compared, they are all similar to a washboard dirt road, Off road racers use long travel coil springs and shocks to allow for the light weight vehicle to glide over the uneven surface at higher speeds, some trucks use ten shocks to give high lifted trucks some stability. But like a racing sports car or a semi truck, the M-677, needs only to have a small amount off travel for the springs, the shocks one per wheel are enough, this allows for any road condition a bump or rut, and the spring flexes and the shock keeps it from continued bouncing.
Back to the washboard, when a conventional vehicle, is on a washboard dirt road, you are only able to go as fast as the bouncing allows, what could be worse? Concrete on a freeway, these are separate slabs each is a very stretched washboard dip, high at the edge dips down at the middle, when a semi or my M-677 goes on a highway that has concrete, the 36x12 high ply tires and the tight suspension quickly absorbs the rapid bounce and the shock keeps the frame flat, not so on my truck cab, the rear of the cab at 60+mph would be like a washboard, the body would keep on bouncing on those springs, add a trailer with another vehicle on it and it will bounce on a different bounce, causing a ripple effect. The rear air bags stop the secondary bounce by absorbing the vibration bounce.
Unfortunately the air bags mounted on the short civilian cab would work, but unless you are as high from the frame as Jack's flip-top FC-170, it would not be that noticeable, the smaller the airbag the more air you need, the faster the rebound, the larger bags give better softer absorption at lower pressures.
If you install those small air bags, make sure you also add a limiter, to hold the cab down also.
I hope that helps.



Thanks Jesse,

   That was very informative, and helpful. I did not realize the valve spring set up was original. You have to admit, if you did not know, it doesn't look right. My other 170 doesn't have them, only 1" rubber blocks, but it also has house door hinges holding on the mud flaps, gate latches holding the doors closed, and a pull chain lamp switch for the headlights. So I can't use it for any reference.  I'll take a closer look when I pull the cab, hopefully this weekend. I know the rear mounts on the frame have been broke off, and L brackets scabbed on, so I have to build new mount anyways, that's why I was thinking of just building mounts for the air bags. Thanks for taking the time to answer my question in such depth; it's nice to be able to get answers from somebody who knows, rather than just from a group of random people.

Dennis Miller /Denver