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Power Steering rack from EA82 or Legacy in EA81?

BRAT Brumby Liberty

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10 replies to this topic

#1 Phizinza

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Posted 28 May 2014 - 11:03 PM

Hi, I'm wanting to put power steering in my Brumby. It will have an EJ with EJ P/S pump. My major question is does the Legacy rack have less travel (turning circle) than the EA82 rack? I know the EA82 racks are very much like the EA81 racks for allowing sharper turning circles but the 2000 model Outback rack is very limited with turning circle..

 

Thanks,

Phiz



#2 Gloyale

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Posted 29 May 2014 - 07:19 PM

You need an EA81 rack..

 

I've put EJ and EA82 racks in EA81s......but the problem is when you turn it alters the toe.....since the distance between inner tierod pivots and the A-arm pivots is not equal.

 

Toe it correct for wheels forward.........but then at full lock toe inside wheel goes way to far and toe goes out.  Makes for terrible tire wear.



#3 Phizinza

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Posted 04 June 2014 - 02:53 AM

Interesting.. I fail to see how this is an issue unless suspension travel is also factored in..?

#4 Gloyale

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Posted 07 June 2014 - 12:18 PM

You need a rack that is the same width from inner tierod to tierod as the A-arm pivot to pivot width.

 

If that is correct......all parts of the suspension stay triangulated the same as the suspension travels, and the wheels are turned.......

 

If thoses widths are different......the triangulation changes as you turn the wheel off center.

 

I used a legacy rack in an EA82.....and it worked well....seemingly.......until we started noticing that when you turn the wheels the toe changes....and it scrubbed the tires terribly.



#5 Phizinza

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Posted 07 June 2014 - 06:53 PM

Surely that would only have an effect if the suspension were to travel while turning? I'll need to mock up some suspension and see this

#6 Gloyale

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Posted 08 June 2014 - 12:25 AM

I said "triangulation" earlier....but let me be more accurate....it's actually about keeping parallelograms.

 

You want the tierod link and the control arm to maintain a perfect parallelogram.  Both in the horizontal and vertical planes......No matter how you turn or how the suspension travels.

 

A parallelogram with sides of a given length can have any range of angles between 1 and 179.  But this parallelogram must have 2 sets of EQUAL sides or else it's not a parralellogram.

 

this means some distances must be equal. 

 

rack end to end = A-arm pivot width.

 

Rack postion above a-arm pivot = height from lower balljoint to outer tierod end@knuckle

 

Rack position forward of the axle centerline = horizontal distance axle centerline to outer tierod@knuckle

 

 

Any of these being off will make trapezoids rather than paralellograms as things move.  The angles of a trapezoid cannot be changed without affecting the length (distance) of the sides. 

 

This is why the toe of the wheels relative to each other will change when you use a rack wider than the pivots.



#7 czny

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Posted 08 June 2014 - 12:49 AM

Not MacPherson but you'll get the idea of arm & steering geometry:

https://www.google.c...ls.html;400;225



#8 Phizinza

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Posted 09 June 2014 - 02:18 AM

I get it now. I was ignoring the fact that the steering works on the horizontal plane and only considering the effects of it on the vertical plane.. Thanks

#9 Gloyale

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Posted 09 June 2014 - 11:43 PM

I didn't get it either until I tried to put a Subaru rack into my Trooperu project.........first attmepts yeilded terrible alignment....had to find it out.



#10 86hatchback

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Posted 12 June 2014 - 12:19 AM

So if your inner tie rods pull from the center of the rack like a dodge intrepid, instead of the ends like a Subaru, would you still get negative results or would that make toe change better?

#11 Uberoo

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Posted 13 June 2014 - 02:05 PM

I have no idea how good steering geometry is even achieved with a center steer type rack.The inner tie rods mount near the center and are far away from the control arm mounts.When the wheel is turned the inner tie rods slide from one end to another,but the displacement on each side is the same.It might work only in the horizontal plane but throw in some movement on the vertical plane and it seems like its a recipe for bump steer.

 

A conventional rack that mounts the inner tie rod ends at the end of the rack is sized so with the steering centered the tie rods are at the  pivot point of the control arms.So when the wheel is turned each end move the same distance towards or away from center making each wheel behave like the ends of a parallelogram so the wheels are roughly parallel. In real life throw the various angles of the tie rods to the control arms and the spindles themselves a little bit of difference between inside and outside tire angle is achieved which is good for low speed turning but bad for high speed stability.

 

If the wheels have too much difference between them they scrub at low speed and are very sketchy to drive at high speed because a little movement at one tire corresponds to a large angle at the other tire.

 

On the other hand if the wheels are always parallel then they scrub at at low speed but provide better high speed stability(15 MPH +).If you google "ackerman angle" it will probably explain it better than I can.







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