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Cage mounts...SOLID or SOFT?

PATCO

Well-Known Member
First I would like to thank everyone for the wealth of info. Should cage mounts be solid or soft? My plan was to build an inside cage with 4" base plates sandwiching cab floor between inside and outside baseplates, Then run supports from lower plates to urethane bushed mounts on framerails and replace cab bushings with urethane. I know this is "old school' design, but a lot of trucks I see have cages that weld solid to the frame but retain rubber cab mounts. They all seem to be cracking at various points. (especially at upper cab!) This makes no sense to me. All attachment points should be solid or soft. correct? What are the advantages/disadvantages? Also, How much strength does the cab have, seems like frame rails are very weak with nothing attached. A post from Bob Sheaves would be great! (sorry about the long post)

P.S. The truck is a street driven play/prerunner.
 

FABRICATOR

Well-Known Member
It's not a question of whether solid or flexible joints are good or bad, it's about what you want out of it. You're correct in not wanting to put a solidly mounted cage in a flex mounted cab. What your planning on doing is very acceptable if you follow some basic rules. It's kind of tricky to explain, but basically each mount must be strong against twisting and bending without the help of the cage tubing itself. It is more work and money to mount everything on flexible mounts. This is why a lot of people skip it. Stock frames are not weak, but they are very flexible. You can add a lot of rigidity to the OEM chassis by a properly designed cage and corresponding braces. Once you start harnessing the chassis you must consider not only up and down bending but also twisting. Otherwise your cage will have problems. You might save yourself some grief by looking at some race trucks or good pre-runners. A cab does not have much strength and should not be used as part of the system. A properly designed and built system would get the job done with or without the cab. It should not be hard to get the design you need.
 

PATCO

Well-Known Member
FABRICATOR,

Thanks for the input. Most high dollar prerunners seem to be tube chassis or a lot I've seen with rails didn't seem to put a lot of time into the cab floor area and are cracking. I am going to use 1.75" bushing assemblies mounted parallel to the rails and possibly tied into the cab mounts. I was thinking about bolting the mount to the frame versus welding it and creating a stress zone in the frame. What do you think?
 

FABRICATOR

Well-Known Member
Sounds like you want to do it right and are willing to do the extra work. It may be extra work to tie into the existing mounts too. If you want to stay "technically correct" it might be easier just to align the mounts so everything is moving more or less in the same directions to avoid binding and compounding of movement distances. It does not have to be perfect though because you are using flexible mounts. It's o.k. to weld on the frame if you avoid a couple of things. One is to never weld on the flanges at all. The other is to not concentrate a load in too small of an area. I would not make it too rigid without tying in the front end too. On some trucks, stiffining up the rear and mid section of the chassis causes increased flexing and movement of the front end of the chassis and the sheet metal. But this goes back to what you are going to use it for. Have Fun!
 

rdc

- users no longer part of the rdc family -
I'd have to say run everything solid, that is if you are building a front to rear cage because then the whole truck will have uniform
rigidity. When you fab your cab cage attach the cage to the cab in as many points as possible, this will make the cage one with the
cab and if your cage goes through the cab to the frame, you could theoretically eliminate the stock rubber mounts because the cage
is mounting the cab. If you are going to do a solid rear and cab cage then you really should go through the firewall as well because
if you don't the rear and cab will flex at a different rate than the front causing things like rattles, cracks, insightful handling... personally
I don't think making the cage bolt to the frame is necessary, alot of extra work and welding it to the frame with doubler plates gets
the job done just fine for a prerunner(make sure the doubler plate catches the top and side of the frame). Sorry for rambling, hope it helps, late
 

Bob_Sheaves

Well-Known Member
Hi Patco,
Sorry for this being so late a reply- I just returned to Japan from a "vacation" in the US (I tried to stay away from the computer-my wife gets testy when I spend more time playing with it than her....)

"Fabricator" covered all the main points very well IMHO! The only additions I would make is to watch out for the construction of the existing frame under the vehicle. Currently, there are single piece stamped side rails (Ford and Toyota), multi-piece rails (Chevy and Dodge), and tubular (true tubular or boxed- either fully front to rear or partially boxed). The design of the OE rail set and any modifications to those rails and crossmembers needs to be considered in the design of your cage and it's mounting.

As expansion on the comment by "Fabricator" -

"... but basically each mount must be strong against twisting and bending without the help of the cage tubing itself."

- I would add that the flexible joint (urethane or other non-rigid mounting) should be as close to the framerail as possible to minimize the effects of "'leverage" working on the mounting material when the rails transition from bending (left to right or up and down motion) to torsion (twisting effort "Fabricator" mentioned).

NVH (Noise, Vibration, and Harshness) of your street driven vehicle is also affected by the design on the mounting. Simply put, a solid, rigid attachment of the cage to the frame rails will result in an "unacceptable" (to the general population-you must decide this yourself) increase in the noise level of road noise, rattles, and other noise inducing vibrations. The proper (read this as "simulated analysis") location and direction of mounting will tend to minimize this increase. What this means is that the direction of the mounting bolts used to attach the cage (welded to the cab structure or not) should be in the same plane and direction as the existing cab mounting bolts. The biggest exception is in the use of "voided bushings", which is a subject far beyond this discussion.

By mounting the cage in this manner, you have the option to increase the strength of the A-pillars and B-pillars (to keep the doors from failing the hinges/ latch assembly and losing the doors while playing) from the higher frequency in the vibration the cab sees, from the stiffer frame rails. Just something else to consider......

Best regards,

Bob Sheaves
 

PATCO

Well-Known Member
Bob,

Thanks for the advice. When you say that the direction of the mounting bolts used to attach the cage should be in the same plane and direction as the existing cab mounting bolts do you mean the bushing assembly should be perpendicular to the frame rail? Should the vertical bolt(bushing assembly) also be the same distance from the frame as the cab mount bolt, or tucked in close to the rail to minimize
leverage? Come on, what are "voided bushings"?


Thanks, PATCO
 

Bob_Sheaves

Well-Known Member
Hi Patco,

The simple answer to your first question is yes, the bolt centerline should be in the same direction and compression "mode" (meaning the thickness should match the OE assembly, and the material should ALSO match) as the OE bushings. Without doing a modal analysis, I cannot tell you exactly where to locate these new bushings, but generally speaking, they should be as close to the frame rail as possible to minimize torsional loadings on the bolts themselves. Note that this is not necessarilly true on a full race vehicle, but is ONLY true for a "street piece".

The second question on voided bushings is rather complex in application, but the concept is simple. Voided bushings have "voids" (or holes, spaces, reliefs- you may supply your own adjective here) to allow the bushing to react differently depending on the loading. As a practical example, look at some 4x4 Dodge BR (full size Ram), AN (Dodge Dakota) V8, or Jeep inline 6 engine mounts. You will see this is a circle of steel with a centered tube, connected by what appears to be 2 blocks of rubber to control vertical and fore-aft motions and a lack of rubber to provide no restriction on side to side motions.

Best regards,

Bob Sheaves
 

scott

Well-Known Member
The question is, Do you want a cage that sweaks and pings or do you want it fairly quiet??????
 
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