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PostPosted: January 8, 2020, 4:54 pm 
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I poked around and can't quite find my issue/question, so he're a new thread on it..

Due to space constraints, I'm having a hard time placing my steering rack in the straight line between my steering arms (tie rod mounts on the upper control arms) as viewed from above the car..

Here's what is known... the Miata (front steer) steering rack will be shortened to fit- no worries there.. also, no worries with the 'height' of the rack (as viewed from the front of the car); the rack pivots will be on-plane with the upper/lower control arm pivots.... car ride height is about 5"... ackerman is close, considering the shortened WB and track on this tiny car (shortened from the donor Miata platform).. ackerman intersect line is now about 10" behind the rear yolk.. Car will have about 3.5" of suspension travel total..

The challenge is, it would be super helpful to move the rack forward of center by about 2-ish inches.. Not life and death, but it would offer some breathing room..

If I move the rack forward, what problems will I introduce to the car/handling/steering?

*bottom picture rack placement is exaggerated for effect

thanks :cheers:

--ccrunner


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brk st rack 1.jpg
brk st rack 1.jpg [ 147.62 KiB | Viewed 1829 times ]
brk st rack 2.jpg
brk st rack 2.jpg [ 150.19 KiB | Viewed 1829 times ]

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PostPosted: January 8, 2020, 7:13 pm 
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Moving it forward is bad. It is called negative or reverse ackerman because the inside wheel turns less the tighter the circle prescribed by the outer wheel (i.e. toe in when toe out is preferred). I believe Sunbeam Tigers had this issue when fitting the ford smallblock. I prefer to move the rack aft from straight for toe out when turning, but I see you don't have that option.

Consider an outrigger on the rack that slides back and forth on bushings to create a U shape rack. I posted about that a while back. I'll find it.

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PostPosted: January 8, 2020, 7:39 pm 
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I had to repost it from the crash: viewtopic.php?f=10&t=19392&p=246874#p246874

The difference would be you are fitting the tube across the front with a pillow block then bring the flanges straight back past the rack to engage the inner tierod sphericals and instead of flanges, thick wall square tube.

Solid mount the rack and you can just bolt the pillow to the frame.
Let me know if you'd like a drawing.

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PostPosted: January 9, 2020, 6:39 am 
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Here is a slightly more complicated solution that solves many problems.Use good quality heims with tight fitting balls and taper gearing bellcrank or tight fitting uhmw or other high durometer bushing to reduce the "slop"

Deleted my pic. I need to work on that some more. It would be easier if you still had some pics up from the front view.

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PostPosted: January 9, 2020, 8:16 am 
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Here is a huge improvement in bumpsteer over my previous pic of a complicated bellcrank system.
Vertical pinion, blue fab adapter from probably 1.25 tube, dom or pipe welded inserts/anti-crush sleeves to attach to rack and 1/8x2x3" strips for double shear, two sphericals, 1x2 or 3 rack mount cross member on lower rails, no spindle mods. Hardest part is getting the intermediate shaft where it needs to be but mounting it further forward and higher with the blue brkt mounted below the flange helps too.

An alternative is to fab uprights with top mount steering arms using a small fwd car bearing hub assy and brakes but you are so far along, it makes more sense to keep going imho.

I'd start by fitting heims with a 1.5-2 inch pipe/dom spacer to the top of the steering arms. This will allow a higher rack for reduced intermediate joint angles and reduced offset in the blue brkt. You could ream and flip the oem tierods but they would not be adjustable and reaming the opposite side greatly reduces the pin contact area.

Determine the best location for the inner heim to minimize bumpsteer for tierod length and heim height.

Mock up the steering where it needs to be from the steering wheel forward. You can use more than two joints but include room to add a split bush or slide on bush or heim shaft support.

The rack pinion will probably work best with around a 45 deg angle to minimize all the joint angles. While it is possible to offset the pinion toward the drivers side and adjust the travel stops on the rack but it take away from rack travel. Only offset if there is enough travel left to turn the outside wheel far enough for parking and turnign at narrow intersections. Running out of travel when trying to pull out to turn right on to a road can cause you to end up head on in the wrong lane.


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Last edited by Miatav8,MstrASE,A&P,F on January 9, 2020, 9:29 am, edited 1 time in total.
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PostPosted: January 9, 2020, 9:27 am 
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Twist the steering rack 40 to 50 degree so the steering input shaft angle just clear the cylinder head and then move the rack towards the engine and up. The rod end attachment would then be closer to the center line and the linkage rod lower on the under side of the rack, all at the same time. The rack position should be dictated [up-down] by bump steer. Keep the design simple. Scrub radius, bump steer, and camber gain is more important than Ackermann. I said it before, Ackermann is all over the map on OEM designs. In the real world a street car is not going to see a 20 or 30% change in Ackermann. Davew


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PostPosted: January 9, 2020, 11:53 am 
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Thanks guys for the quick, thoughtful ideas.. My battle with the steering rack placement is still a few weeks out as I'm currently fabbing in the motor mounts and need to finish that.. So many elements of this car are fighting for the same real estate... I'm having to build this thing close and tight at every turn.. I haven't cut down the Miata rack yet; I'm optimistic and somewhat confident I can in fact shoehorn it inline where it 'should' go, but in the event that it will not fit, I'm looking for a work around.. I'm not so well versed in suspension theory and application, but instinct told me there would be a price to pay for taking that rack out of the for/aft steering arm plane..

I had totally forgotten about the 'outrigger' (blue) solution to this problem! I've had a few sand cars over the years that have had this center rack setup with the bolt-on dummy shaft fabbed to fit the car's needs.. totally blanked on that :roll:

It's a solution for sure if I can't cram the Miata bits down in where I need them to fit... If I do need to go with the buggy/outrigger setup as shown, I will then struggle with routing the steering/input shaft around the lump, but certainly there's a solution for that too.. I'll cross that bridge when I get there..

Thanks again guys.. super helpful as always :cheers: :cheers: :cheers:

--ccrunner

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My '72 Honda N600 build log (bike engine in a microcar)...
http://www.locostusa.com/forums/viewtop ... 36&t=14452

My '63 Volvo 1800 with a turbo inline 4 build log (LNF Ecotec compliments a Svelte Swede)
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

My '59 Berkeley SE492 build log (bike engine in a microcar)... "A Berkeley With Bite!"


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PostPosted: January 10, 2020, 1:16 pm 
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Hello ccrunner
Here is what I did (copied) to deal with bump steer. I attached a laser level to the brake rotor, and rigged up some blocks under the spindle to set the suspension at ride height, 1" droop, and 2" compression. Then I clamped the steering rack where I thought it might go. I marked the laser points on the wall at all 3 positions, and drew a line between them. The first attempt was far from vertical, so I just moved the rack around until the laser points on the wall were vertical (89.5 degrees, close enough for me) then fabricated my steering rack mounts at that point.
I think there are way too many variable to rely on the math alone.
Good Luck!
Doug

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PostPosted: January 10, 2020, 8:32 pm 
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@blownmiata91
That's a very clever approach. I like it.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: January 10, 2020, 10:13 pm 
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I posted this in 2016 on checking for bump steer ----
Accurate bump steer is easily accomplished with a laser and a mirror. Tape or bungee a mirror on the face of the rotor or wheel. Shine a laser (laser pen is ok) on the mirror at near 90 degrees and it will be projected back near the laser. Let the laser dot fall on a piece of paper. Move the suspension (car level and shock disconnected) and watch the laser dot. Raise or lower the tie rod ends to get the dot to trace a vertical line.
A curved trace indicates the tie-rod length is wrong. An angled straight line shows that height adjustment of the tie-rod need to be adjusted either at the rack or the steering arm.

SIMPLE AND ACCURATE!!!!!!! NO MONEY

Here is the PRO version. You can easily make and use what I described above without purchasing this one:

http://www.advancedracing.com/bump_steer_gauge.php

Regarding rack placement. IF there is room inside the wheel to clear the tie-rods when the wheel is turned to lock, the steering arms can be bent outwards to achieve ackerman. Add the angle that is currently built into the uprights and add the angle of the tie-rods with the rack moved.

Your creations are always stunning!!!!!

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My build log viewtopic.php?f=36&t=10658&start=0 NOW NAMED =The Wycked 7

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