Swapping to rack and pinion steering
(Clean up) |
(Minor clean up.) |
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On a typical RB (recirculating ball) type steering box the Pittman arm has a “throw” of 7 inches, side to side. It is usually connected to steering arms effectively measuring 7 inches long (The 7 inches is measured from the center of the steering arm mount, where the king pin or ball joint pivots the spindle, to the center of the outer tie rod where the steering pivots). Typical Cavalier racks have 6 inches of throw and originally connected to the upper strut in a manner that represents about 5 ½ inches from center of the strut to the center of the tie rod. Connecting the rack to the original arms causes a loss of nearly 20% of steering angle. It is strongly recommended that this be dealt with before installing the rack. | On a typical RB (recirculating ball) type steering box the Pittman arm has a “throw” of 7 inches, side to side. It is usually connected to steering arms effectively measuring 7 inches long (The 7 inches is measured from the center of the steering arm mount, where the king pin or ball joint pivots the spindle, to the center of the outer tie rod where the steering pivots). Typical Cavalier racks have 6 inches of throw and originally connected to the upper strut in a manner that represents about 5 ½ inches from center of the strut to the center of the tie rod. Connecting the rack to the original arms causes a loss of nearly 20% of steering angle. It is strongly recommended that this be dealt with before installing the rack. | ||
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| + | ==Bump steer considerations== | ||
Changes to the steering arm can affect the steering geometry and can introduce [http://www.longacreracing.com/articles/art.asp?ARTID=13 '''bump steer''']. So, if you are going to address this issue, (some people don’t) do it before the rack install. | Changes to the steering arm can affect the steering geometry and can introduce [http://www.longacreracing.com/articles/art.asp?ARTID=13 '''bump steer''']. So, if you are going to address this issue, (some people don’t) do it before the rack install. | ||
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[[Image:Chevelle_arm_vs_chevy_arm.jpg|thumb|left|330px|1955-'57 Chevy arm on left; 1965 arm on right]] <br style="clear:both"/> | [[Image:Chevelle_arm_vs_chevy_arm.jpg|thumb|left|330px|1955-'57 Chevy arm on left; 1965 arm on right]] <br style="clear:both"/> | ||
| − | + | ===Bending or replacing steering arms=== | |
There is a difference in mount width between the BOP arms and Chevy arms, so they won’t work on BOP without the Chevy spindle upgrade. If you are unable to find shorter arms for your application, bending the originals is the next option. You will find mixed opinions on this issue. Some will insist that heating and bending steering arms compromises their structural integrity and should never be done. Others warn you to be sure they are forged and not cast arms. Bending forged arms may be OK, bending cast is not. | There is a difference in mount width between the BOP arms and Chevy arms, so they won’t work on BOP without the Chevy spindle upgrade. If you are unable to find shorter arms for your application, bending the originals is the next option. You will find mixed opinions on this issue. Some will insist that heating and bending steering arms compromises their structural integrity and should never be done. Others warn you to be sure they are forged and not cast arms. Bending forged arms may be OK, bending cast is not. | ||
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By putting an "S" bend in the arms they are effectively made one inch shorter, pivot point to pivot point. This also made them one inch lower than the original position. To compensate for this change in geometry, the rack was positioned 1 inch lower than the original center link. A correction could also be made by fabricating the center bracket with a 1 inch drop in the inner tie rod mounting points. However you address it, by doing it ''first'' you will have 3 fixed points to work with: Inner control arm pivot point, lower ball joint pivot point and outer tie rod pivot point. This leaves you with the front/back and up/down location of the inner tie rods to deal with. | By putting an "S" bend in the arms they are effectively made one inch shorter, pivot point to pivot point. This also made them one inch lower than the original position. To compensate for this change in geometry, the rack was positioned 1 inch lower than the original center link. A correction could also be made by fabricating the center bracket with a 1 inch drop in the inner tie rod mounting points. However you address it, by doing it ''first'' you will have 3 fixed points to work with: Inner control arm pivot point, lower ball joint pivot point and outer tie rod pivot point. This leaves you with the front/back and up/down location of the inner tie rods to deal with. | ||
| − | We mocked up everything in | + | ==Rack height== |
| + | We mocked up everything in the fashion described above and then designed our rack takeoff mount (inner tie rod mount) to fit. Once the inner bracket was welded up and mounted to the rack, we installed the tie rods and checked for bump steer. The final minutest adjustments were made by tapping the rack mounts on the frame with a hammer. Remember, at this point the rack was just clamped to the frame. When we found perfection, we tack welded the frame mounts, removed the complete rack assembly and welded the mounts and gussets in place. | ||
To understand how crucial the height location is, follow this mathematical extrapolation: | To understand how crucial the height location is, follow this mathematical extrapolation: | ||
A 1/8" difference in height of the rack, caused a 1/16" difference in the location of the tie rod arc (at full compression or rebound), compared to the ball joint arc. 1/16" at the tie rod (6 inches from the spindle center) becomes almost 3/16" at the rear of the tire (typical 30 inch tire). This causes a reciprocal movement in the ''other'' direction at the front of the tire. Now we are dealing with 5/16". Note that the other wheel is also duplicating this, so the toe in (or out) changes 10/16" 0r 5/8" total during suspension travel caused by acceleration or braking, while you are trying to drive in a straight line. This illustrates how height is the most crucial dimension in locating your inner tie rods. | A 1/8" difference in height of the rack, caused a 1/16" difference in the location of the tie rod arc (at full compression or rebound), compared to the ball joint arc. 1/16" at the tie rod (6 inches from the spindle center) becomes almost 3/16" at the rear of the tire (typical 30 inch tire). This causes a reciprocal movement in the ''other'' direction at the front of the tire. Now we are dealing with 5/16". Note that the other wheel is also duplicating this, so the toe in (or out) changes 10/16" 0r 5/8" total during suspension travel caused by acceleration or braking, while you are trying to drive in a straight line. This illustrates how height is the most crucial dimension in locating your inner tie rods. | ||
| − | Duplicating your lower control arm length with your tie rod length is not nearly as critical. In fact many original setups were unequal length. The trick is to make the tie rod as long, or longer, than the lower control arm. When the tie rod is longer, it will have a flatter arc than the control arm. The flatter arc means the tie rod will move outside the ball joint at the extremes of suspension travel and create additional toe in. Up to | + | ==Control arm to tie rod length== |
| + | Duplicating your lower control arm length with your tie rod length is not nearly as critical. In fact many original setups were unequal length. '''''The trick is to make the tie rod as long, or longer, than the lower control arm'''''. When the tie rod is longer, it will have a flatter arc than the control arm. The flatter arc means the tie rod will move outside the ball joint at the extremes of suspension travel and create additional toe in. Up to 1/4" will not be noticeable in handling or tire wear. If the tie rod is shorter than the control arm, the opposite will happen. 1/8" of toe ''out'' will put you all over the road. | ||
| − | On the 41 Pontiac the lower control arms are 18 inches pivot to pivot. We used the original Cavalier tie rods, which measured 21 inches pivot to pivot. | + | On the '41 Pontiac, the lower control arms are 18 inches pivot to pivot. We used the original Cavalier tie rods, which measured 21 inches pivot to pivot. |
[[Image:Center_take_off_pics.jpg|frame|none|rack with original tie rods/center take off..]] | [[Image:Center_take_off_pics.jpg|frame|none|rack with original tie rods/center take off..]] | ||
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