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CRG Research Report

1967 Camaro Radius Rods

© 1998-2011, Camaro Research Group

Primary Author - , updated by Kurt Sonen
Reviewed by the CRG
Last Edit: 13-Oct-2011
Previous Edits: 23-Jan-2010, 21-May-2004, 08-Aug-2000, 18-Aug-1999, 23-Dec-1998
Original Release: 24-Sep-1998

Index

Introduction

In mid-1967 model year, high-performance 1967 Camaros received a passenger-side traction bar, known as a "radius rod." There were two factory designs - the first design used a round rod while the rod in the second design was rectangular with increased stiffness. Additionally, later in the year, a rectangular rod service retrofit kit was made available to dealers to address wheel-hop complaints. The service kit could be applied either early vehicles with no rod at all or to vehicles with the round rod.

First Design - Round Radius Rod
Round Radius Rod      Round Radius Rod
Round Radius Rod

 

Second Design - Rectangular Radius Rod
Rectangular Radius Rod      Rectangular Radius Rod
Rectangular Radius Rod

The question of which rod was applied to what model, and when, has often been a subject of much confusion. This report defines, as clearly as possible given the known data, the applications of each rod, and also discusses specific areas where data and information is lacking or weak. We encourage all 1967 Camaro owners with installed radius rods or rod brackets to please see the CRG Forum research post to contribute information about their vehicle, that we might continue to improve this report.

 

Summary

The overly-simplified summary of radius rod application is that starting mid-December 1966, cars with a 12-bolt axle received the round rod. After February 1967, cars with automatic transmissions and 12-bolt axles (L48/M35 and L35/M40) continued to receive a round rod, while cars with manual transmissions and 12-bolt axles received a rectangular rod. This is overly simple because there are two caveats:

  1. The rectangular rod was not introduced to all manual transmission models at the same time, therefore some manual transmission cars continued to receive the round rod after other models had changed to the second design. The start date of the rectangular rod for each model is discussed below.

  2. The L48/M35 car, while fitted with round rod brackets, did not always have the rod itself actually installed at the factory. Data to date indicates that only about one-third of this model have installed rods. No specific patterns of installation have emerged.

Table 1 illustrates, in a different format, these findings to date, model by model.

Table 1: 1967 Camaro Radius Rod Conclusions To-Date
Application Early Use/Type
mid-Dec to Mar-Apr
Late Use/Type
Mar-Apr to End-of-Model
L30 4-speed 1, 2 round   (proven) rectangular   (proven)
L48 3-speed round   (proven) rectangular   (assumed)
L48 4-speed 2 round   (proven) rectangular   (proven)
L48 Powerglide 3 round   (sporadic) round   (sporadic)
L35 3-speed rectangular   (assumed) rectangular   (assumed)
L35 4-speed rectangular   (proven) rectangular   (proven)
L35 TH400 round   (proven) round   (proven)
L78 4-speed N/A rectangular   (proven)
Z28 4-speed 4 round   (proven) rectangular   (proven)
  1. Introduction of the radius rod into the L30/M20 apparently lagged behind the L48 by about a month, not appearing until January/February, when the 12-bolt axle was introduced to the L30/M20 (see the L30/M20 CRG Research Report). Note that it appears that both the 10- and 12-bolt axles were used in L30/M20's in January.

  2. The L30/M20 and L48/M20 transition to the rectangular rod appears to vary by plant. Norwood transtioned to the rectangular rods about 04A. There isn't enough data on Los Angeles cars to be certain, but it appears that the transition occurred later in April.

  3. About one-third of L48/M35 cars have been noted with the round radius rod installed, while the remainder have the rod brackets but are absent the rod. No definite installation pattern has emerged.

  4. Z28s used the rectangular bar starting in early March.

 

Design Problem

High-performance 1967 Camaros under heavy initial acceleration experienced a rear suspension problem called "wheel-hop" (or "wind-up"). This problem was accentuated by a combination of insufficient suspension stiffness about the axle centerline and insufficient suspension damping in the monoleaf spring/vertical shock system. High engine torque applied to the differential rotated the entire differential assembly about the axle transverse centerline toward the front of the vehicle and twisted the monoleaf springs into an "S" shape, until the torque broke the tires loose. With the tires off the ground the engine torque no longer applied a load to the deformed springs - with the result that the springs violently rebounded and forced the tires back on the ground, starting the cycle over again. This shuddering, traction-losing, U-joint-breaking, behavior repeated at high frequency until the driver backed-off the accelerator.

The temporary fix implemented by Chevrolet engineering during the middle of the 1967 model year (until the 1968 model year suspension changes were implemented) was a form of traction bar, known as a "radius rod." The radius rod was a single, end-hinged, rigid link between the passenger side of the rear axle and the chassis.

Radius Rod Installation Sketch

Floor mount, under the rear seat
Floor mount
The theory was that such a rod would allow an acceptable amount of vertical translation in the rear suspension, but resist rotation of the suspension about the axle centerline such as occurs when a large torque is applied to the driveshaft during a hard start. In practice the Chevrolet factory rod reduced, but did not eliminate, wheel hop, and therefore neither of the two Camaro versions of the radius rod were fully successful.

Both of the Camaro radius rod designs used a rigid steel rod with one end thru-bolted (hinged) to an axle bracket and the other end attached to a bracket bolted to the body (under the passenger-side rear seat). The first design was a round steel rod with a bushing at each end, part number 3914808. The second design improved upon the first and was a stiffer 1-inch by 1.25-inch rectangular rod, part number 3920234, with a modified axle bracket interface.

In the 1967 Camaro Assembly Manual, sheet B3 for RPO L48, dated 08-Nov-66, describes the first design version used with the round rod. The 07-Apr-67 revised sheet A8 for RPO L78 describes the second design version. The bracket welded to the rear axle was slightly modified for the 2nd design - it was wider with an extra hole added and the foward edge of the bracket was stepped. It is believed that all axle brackets were changed to the second design about the time the rectangular rod was introduced. A supplemental "stop" bracket that included a new rubber bumper was bolted to the bottom of the ears in the second design. For both designs, the body bracket was identical: part number 3914806. The body bracket was bolted to the rear frame rail and to the two studs that protuded through the floor.

First design axle bracket      Second design axle bracket      Sketch of axle bracket
First design axle bracket      Second design axle bracket      Sketch of second design axle bracket

Stop bracket for rectangular rod      Stop bracket for rectangular rod
Stop bracket for second design rod      Stop bracket - installed

In contrast, the 1967 Pontiac Firebird included radius rods for both left and right sides of the axle on their higher performance models. The Firebird rod had an I-beam cross-section design (compare to the round or rectangular bars used on the Camaro). See Firebird rod - view 1 and Firebird rod - view 2 for pictures of the Pontiac-designed radius rod.  

Radius Rod Applications

The first rod design was applied in mid-December 1966 to all models with a 12-bolt axle. This was expanded to the L30 275HP/4-speed model, when the 12-bolt axle was included in the L30/M20 in January / February. The second design rectangular rod was introduced in early February for the L35 big-block manual transmission applications, and was also applied to the L78/M21 from the start of that model in late March.

The replacement of the round rod with the rectangular rod on the Z28 appears to have occurred in March. The L48/M20 and L30/M20 cars transtioned to the rectangular rods about 04A at Norwood and later in April at Los Angeles (though the data on the Los Angeles cars is not definitive). Despite an assembly manual cancellation notice, the round rod remained in use for the rest of the year on L35/M40, as well as (sporadically) on L48/M35. GM documentation was used to derive these applications and actual vehicle data supports that information.

The one area of ambiguity is the L48/M35 model, which has sporadic radius rod application. There is no indication in the assembly manual that the traction bar was not applied to all SS-350 models, but other Chevrolet documentation indicate that it was not applied to the L48/M35 SS350/Powerglide combination. The CRG vehicle database provides insight in the actual usages. Of the approximately 100 L48/M35 cars built 012A or later for which we have rod-related data, 35% are reported with the round rod, and 5% with a rectangular (presumably) service retrofit. The remainder (60%) do not have a rod, but have one or more of the mounting brackets There is no obvious pattern of application. Even cars with optional performance axle ratios do not have the rod.

The rectangular rod documentation in the 1967 Assembly Manual was revised after initial release; CRG has only the revised version of this sheet and does not know what the initial release looked like. The 1967 Assembly Manual UPC L78 Sheet A2 table of contents lists the radius rod sheet in the initial release as for "RADIUS RODS" (note the plural). CRG speculation is that the initial release of this sheet included both round and rectangular rod configurations, and that the April revision to the sheet was for the purpose of eliminating the round rod configuration from this sheet.

Chevrolet Dealer Service Technical Bulletin 67-T-30, dated 12-May-67 (pdf file), indicates the round radius rod was actually implemented in production on 15-Dec-66. This same bulletin indicates that the second design went into production on 27-Mar-67. However, use in the L35/M20 pre-dates the production introduction date in the bulletin, since we have several L35/M20 datapoints from 02A to 03A, all of which have the rectangular rod.

It would appear that the 27-Mar-67 introduction date only applies to the L30 and L48 vehicles, which does match with vehicle data. The scope of Bulletin 67-T-30 is to specify retrofit instructions for the rectangular radius rod for L30 and L48 4-speed complaint vehicles, and therefore it makes sense that the information in the bulletin literally applies only to these vehicles.

Note also that Bulletin 67-T-30 includes a reference to "... vehicles built after December 15, 1966, with provisions for a radius rod." Note that this quote doesn't say "with a radius rod," but says with "provisions" for one - indicating that some vehicles had the brackets installed at the factory (the provisions) but without the rod in place.

A significant complication to the analysis of the radius rod applications is the rectangular radius rod service retrofit kit. This makes it a bit more difficult for us to be sure if a rectangular bar was factory installed or dealer installed. However, by comparison of the service replacement instructions with the original factory assembly instructions, it appears that there are three clues that should distinguish a service replacement rectangular bar from its factory brother:

  1. When the service kit was applied to a car with no previous bar, the body bracket was welded to the body, rather than bolted-on as was done at the factory.

  2. If the axle bracket didn't have holes to mount the stop bracket, the service kit required the bumper stop bracket to be welded to the axle bracket, rather than bolted-on as was done at the factory.

  3. When the service kit was applied to a car with a pre-existing round bar, the round bar axle brackets were not replaced with the new brackets but remained on the axle and were used as-is. The two bracket types have distincly different shapes.
 

Supporting Data

Chronological Data

In chronological order, here is what is recorded in the 1967 Camaro Assembly Manual, 3891775, combined with information from Bulletin 67-T-30, and key datapoints from the CRG database:

Table 2: Chronological Documentation of Radius Rods
     Document                   
 RPO Reference  Status   Date                  Notes
 --- ---------- ------- -------   -----------------------------------

 M40  67 AIM   new dwg  26Aug66   M40 docs released include drawing with
      M40 all                     radius rod ears on the axle indicating
                                  that this change was contemplated even 
                                  at the beginning of production.

 L48  67 AIM   new dwg  08Nov66   L48 docs released for round rod 
      L48 B3                      (1st design).

 Z28  67 AIM   new dwg  09Nov66   Z28 docs released referring to L48 for
      Z28 A2                      round radius rod.

 L35  67 AIM   new dwg  30Nov66   L35 docs released referring to L48 for
      L35 A2                      round radius rod.

 ---  Bulletin  start   15Dec66   Round rod (1st design) usage started in
      67-T-30   prod              production.

 L30  67 AIM   rev dwg  16Dec66   L30 docs revised to add radius rod 
      L30 A2                      reference to L48. This was never revised
                                  to delete the reference.

 ---  CRG database     December   Several mid-December-built L48 cars have a
                                  round radius rod. Usage is not consistent.

 ---  CRG database        01B     01B Z28 car with round radius rod.

 L35  67 AIM   rev dwg  08Feb67   Reference to L78 radius rod added to
      L35 A2                      existing reference of L48 rod.

 Z28  67 AIM   rev dwg  08Feb67   Reference to L48 radius rod changed to
      Z28 A2                      L78.

 ---  CRG database        02A     Several 02A L35/M20s exist with
                                  rectangular rods.

 L78  67 AIM   new dwg  17Feb67   L78 docs released with reference to 
      L78 all                     "RADIUS RODS"- plural.  Only sheets
                                  A1-A6 released on this date. Remainder 
                                  released 23-Feb-67.

 L78  67 AIM   new dwg  23Feb67   Remainder of L78 sheets released, incl.
      L78 A8                      sheet A8 with initial L78 radius rod(s). 
                                  Note that L35 and Z28 have already 
                                  released docs referencing this.

 M40  67 AIM   rev dwg  17Feb67   M40 docs revised to change M40 axle part
      M40 A3                      number from 3910882 to 3920569. If this 
                                  was for the purpose of using the rect-
                                  angular rod axle brackets, it didn't
                                  materialize in actual cars. Done same day
                                  as initial release of rectangular rod.

 ---  CRG database        03B     03B Z28 with rectangular radius rod.

 ---  Bulletin  start   27Mar67   Rectangular rod introduced on assembly
      67-T-30   prod              line. Scope of this bulletin may be
                                  limited to L30 and L48.

 L34  67 AIM   new dwg  30Mar67   L34 docs released referring to L48 or L78 
      L34 A2                      for radius rods. The L34 was not released 
                                  in 1967 despite this documentation.
 
 L35  67 AIM   rev dwg  06Apr67   L35 docs reference to L48 radius rod
      L35 A2                      removed, leaving only the L78 reference.
                                  Despite this, all L35/M40 used the 
                                  round rod.

 L48  67 AIM   cancel   06Apr67   L48 docs cancel round rod design sheet
      L48 B3    dwg               for L48 (and, implicitly, for L30).
                                  However the round rod continues in 
                                  production.
          
 L78  67 AIM   rev dwg  07Apr67   L78 docs for sheet A8 on radius rod 
      L78 A8                      "revised and redrawn". New release
                                  shows only a rectangular rod.

 

 

Parts Manual Data

Supporting data from Master Parts Catalogue 721A, Dec 1971, Section 5.415:

 



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