Swapping to rack and pinion steering
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Many cars suffer from poor turning radius after an R&P install. There are 2 basic reasons. Either the rack was placed too far left and interferes with the wheel, or no adjustment was made to the steering arms. | Many cars suffer from poor turning radius after an R&P install. There are 2 basic reasons. Either the rack was placed too far left and interferes with the wheel, or no adjustment was made to the steering arms. | ||
| − | On a typical RB (recirculating ball) type steering box the Pittman arm has a “throw” of 7 inches | + | 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. |
| − | ==Bump steer | + | ==Bump steer== |
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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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. | ||
| − | ==Rack height== | + | ==Rack height importance== |
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. | 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 | + | 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 BOTH wheels are doing this, so the toe in (or out) changes 5/16" x 2 = 10/16" or 5/8" total during suspension travel caused by acceleration or braking; all this 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. |
==Control arm to tie rod length== | ==Control arm to tie rod length== | ||
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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: | + | [[Image:Center_take_off_pics2.jpg|thumb|400px|left|Rack with original tie rods/center take off]] <br style="clear:both"/> |
| − | + | Using this combination, the bump steer (toe in) changes less than 1/4" at full suspension travel, and less than 1/8" in normal operating range. It's been said that additional toe in is beneficial during hard acceleration or hard braking, as it tends to stabilize the front end. | |
==Inner bracket/inner tie rod mount== | ==Inner bracket/inner tie rod mount== | ||
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