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rack and pinion

Posted by cavell (My Page) on
Wed, Sep 12, 12 at 16:15

can anyone identify this rack and pinion? it is on a streetrod. probably 15yrs old. i think it is from a mustang 2 application. it has a block off attached to the fluid port. i assume it is a power rack but it is not boosted now. it works but the steering is very stiff. no boost. car has been in heated garage from day 1. probably 15k total miles in 15 yrs. i assume seals are dried out and that is why it is so stiff. and no boost. i figure change rack with new version and install power steering pump. car has chevy small block for your info.


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RE: rack and pinion

2nd pic


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RE: rack and pinion

Power steerring racks have a higher gear ratio than an un-powered, manual only rack. [ the number of steering wheel turns from lock-to-lock is more for a manual rack than a power assisted one.] This could explain the high steering effort. Also, normally, one does not turn the steering wheel of a manual gear with the car standing still. it places undue strain on the gear. There should be some motion for turning the steering wheel.

If your rack was intended for use in power steering and is now applied as a manual unit, this is a gross misapplication. Power steering racks depend on hydraulic assist to supply the bulk of force required to steer. The gears are not designed for continuous use without power assist, and will wear prematurely. In an emergency if the power assist fails, one can get home with manual only, but he should consider this a temporary situation and remedy it as quickly as possible.


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RE: rack and pinion

There are no external fluid lines on rack. Rack has been somehow altered for streetrod. No clue as to internal bits. Shop I talked to said it may have been shortened. Need to check spacing between mounting bolts. The steering has no self centering. It requires constant correction to go straight. Quite unnerving to drive car.


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RE: rack and pinion

"Self centering" is not a function of the rack and pinion, but is determined by the geometry of the pivot points of the wheel and location of the tire's contact patch with respect to this geometry. It is a rather messy problem of descriptive geometry to compute or solve graphically, but there is a simple test one can do to check 'centering'. (This test works for bicycles and motorcycles also. First, try it on a bicycle to see the effect since this is very easy to do.)

When the wheel is turned to left or right from center, the car may rise or fall a small amount. If there is no change in elevation, the setup is neutral and has no prefential state. If the car has it lowest elevation with the wheels straight forward and it rises a bit when the wheels are turned either left or right, it has positive centering. In this case, the preferential state is for the car to seek its lowest elevation and that corresponds to "wheels straight ahead".

If, on the other hand, the car reduces in elevation when turning it either left or right, it has negative centering and will be a bearcat to keep it tracking straight. Its preferential state is to turn. You can not let go of the steering wheel. If you do, it will turn. This is a bad situation.

The steering geometry can be correct and yet the car may tend to wander requring constant correction. This can be caused by differences in sideways stiffness of the tire between the front and back set. Once I had a car that was fine for tracking, but when snow tires were put on the rear, it required constant correction to keep it on the road. Through experimenting with various tire combinations, this problem was pinned to differences in sideways stiffness between the front and rear tires. The snow tires with blocky treads plus diffeences in sidewall stiffness were 'walking' sideways.

There is yet another factor for tracking on center while at speed, and that is differences in the sideway stiffness of the suspension system between front and rear. Tire sideway flexibility is part of this system. An auto when cruising down the road at speed also responds to side forces notable caused by air (wind) pressure. When a car recieves a blast of wind from its side, it may make a self correction if the rear end of the car moves sideways a bit more than its front. Moving the tail end over tends to point the front wheels toward the blast. If this is overdone, oscillation and difficulty in control can happen.

These are some of the factors that affect tracking straight. Since your vehicle is a street rod, likely, its steering geometry was not 'enginnered' and contains a design fault. Its camber and caster may be suspect. Also, the location of the tire's contact patch with relation to the pivot axis is critical.


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