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Old 01-31-2015, 03:39 AM   #1
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Default Confused about combing parts for best bump steer with a lowered miata

I’m too lazy to try and precisely measure it but I have lots of parts.

1) On an NA Conventional widom seems to be shimming the rack up improves bump steer.
2) 93LE tie rod ends move the knuckle end of the tie rod down. ~3mm as near as I can tell.

These two things seems pretty straight forward both serve to level the angle of the tie rod assuming that is what is really needed for improvement.

Now here is where I get confused the NB front suspension said to have better addressed bump steer.

1) Inner A-arm pivots are moved down 5.7mm. assuming rack is at same level this would be similar effect to shimming the rack up 5.7mm
2) The knuckle raises the outer tie rod end position up 7.1mm however rather than down.

So somhow the NB has better bump steer with the tie rod angled up relative to the A-arm in the opposite direction from what conventional wisdom says to reduce bump steer by 1.4mm.

Now supositly an LE tie rod end on an NB makes or shimming the rack up still makes it better

LE tie rod on an NB . =~3mm down
What about just running an NA knuckle with standard end =7.1mm down
How about an NA knucke with an LE tie rod end =~10.1mm down

What is the sweet spot? I have been running NA knuckles with le tie rod ends and an NB sub frame is that too far?. Has anybody actually measured it?
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Old 01-31-2015, 09:31 AM   #2
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Sub'd for info. I have an NB subframe/front end I'm getting ready to swap into my NA.
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Old 02-01-2015, 04:48 PM   #3
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I'm curious as well. I found LE tie rod ends with shims under the rack (~5mm??) made a noticeable difference but I have no measurements to share. My car is an NA with NB underpinnings aside from the LE tie rod ends.
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Old 02-01-2015, 05:52 PM   #4
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I was under the impression that the NB spindle moved the steering arm down for better bump steer, not up for worse.
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Old 02-01-2015, 05:59 PM   #5
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Subscribing...
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Old 02-02-2015, 12:27 PM   #6
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Quote:
Originally Posted by mr_hyde View Post
I'm curious as well. I found LE tie rod ends with shims under the rack (~5mm??) made a noticeable difference but I have no measurements to share. My car is an NA with NB underpinnings aside from the LE tie rod ends.
That sounds like pretty close to the same thing as running NA-knuckles on with an NB subframe.

I'm pretty sure mine with NB subframe NA knuckles and LE tie rod ends tows out a tad under bump. seems like its too much correction.

I do think tow out under bump is better than tow in. I drove a car with significant tow in in in bump a 323 gtx with 2wd knuckles it was squirrely as hell. Cornering force cranks in more turn angle in that case. It was a bit extreme though. Because the knuckles were like 1.5Ē off. The AWD design has to move the rack up over the transfer case making for quite a bit of difference.
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Old 02-02-2015, 12:51 PM   #7
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Also interested in this.
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Old 02-03-2015, 12:27 AM   #8
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Quote:
Originally Posted by bbundy View Post
That sounds like pretty close to the same thing as running NA-knuckles on with an NB subframe.

I'm pretty sure mine with NB subframe NA knuckles and LE tie rod ends tows out a tad under bump. seems like its too much correction.
Interesting on the NA knuckles. I have 2 or 3 sets of them and would rather not shim the rack if I don't need to.
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Old 02-05-2015, 02:48 AM   #9
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doing some measurments on my car now. kind of shocking really. pretty drastic tow out as it swings from full rebound to full bump. more than 1/2" of total tow out cranked in just as it sweeps from ride height to full bump.

I think this means NA knuckels on an NB subframe is too much. pretty sure the outer ends need to go up to lessen this change in toe. Ill try the NB knuckels in place of the NA ones.
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Old 02-05-2015, 10:00 AM   #10
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I have an NA with NB subframe, knuckes, arms, rack etc and LE tie rod ends and I think this is the perfect setup in terms of bumps steer correction. Previously I ran NA everything with LE rod ends and a shimmed rack and my current setup is a vast improvement.

What sort of ride heights and tires are we running too? I noticed a huge change in the way the car behaved at full bump after going from ra1s to Hoosiers.

Last edited by curly; 02-05-2015 at 02:17 PM.
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Old 02-05-2015, 10:47 AM   #11
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Martin, you didn't shim the rack in that setup, right? It's all NB, but with LE tie rods?
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Old 02-05-2015, 11:15 AM   #12
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Yes, current setup has no shims, all NB except for LE ends
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Old 02-05-2015, 11:22 AM   #13
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Excellent, thanks for the data point.
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Old 02-05-2015, 01:34 PM   #14
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Quote:
Originally Posted by greddygalant View Post
Yes, current setup has no shims, all NB except for LE ends
This is what I’m going to try next.

What I’m finding is small changes can make pretty significant impacts on bump steer.

Also what I’m thinking is bump steer is maybe not the death of handling. My car was pretty dam fast with that much tow out in bump. I think a slight tow out in bump might actually make the car more stable. because as you load up the outside front tire squishing suspension towards the bump stops it dynamically tends to self-compensate to soften the steering angle of the tire preventing over working the tire and pushing or allowing it to grab and hook and having the rear end come around.

Like I said before however in my experience tow in in bump is the opposite of good. I drove a car like that and it was hook and spin or plow and push

Last edited by bbundy; 02-05-2015 at 03:32 PM.
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Old 02-05-2015, 02:15 PM   #15
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Sounds like I need to order some LE ends for the NB.

Thanks for the thread guys, I'm digging the data.
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Old 02-05-2015, 04:41 PM   #16
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Quote:
Originally Posted by bbundy View Post
This is what Iím going to try next.

What Iím finding is small changes can make pretty significant impacts on bump steer.

Also what Iím thinking is bump steer is maybe not the death of handling. My car was pretty dam fast with that much tow out in bump. I think a slight tow out in bump might actually make the car more stable. because as you load up the outside front tire squishing suspension towards the bump stops it dynamically tends to self-compensate to soften the steering angle of the tire preventing over working the tire and pushing or allowing it to grab and hook and having the rear end come around.

Like I said before however in my experience tow in in bump is the opposite of good. I drove a car like that and it was hook and spin or plow and push

sounds suspiciously like ackermann...
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Old 02-05-2015, 05:30 PM   #17
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Quote:
Originally Posted by hi_im_sean View Post
sounds suspiciously like ackermann...
Ackerman is sort of a static concept this is more a dynamic load transfer thing.
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Old 02-05-2015, 05:36 PM   #18
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Quote:
Originally Posted by hi_im_sean View Post
sounds suspiciously like ackermann...
ackermann shouldnt change with with roll/bump. Ackermann and bump steer both effect how much the tire has turned, but ackermann is based on geometry in the plan parallel to the ground and bump steer is based on geometry in the plane perpendicular to the ground.
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Old 02-05-2015, 08:41 PM   #19
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I understand how ackermann works, I was just trying to say the perceived benefits work on the same principle as ackermann. Toe out in a corner to help the car turn in.
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Old 02-05-2015, 08:44 PM   #20
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Quote:
Originally Posted by hi_im_sean View Post
I understand how ackermann works, I was just trying to say the perceived benefits work on the same principle as ackermann. Toe out in a corner to help the car turn in.
Well the opposite. ackermann and static toe out make the car turn in better by having the inside wheel operate at a higher slip angle in the beginning of the turn, this comes back to be a negative once you're established in the turn. All things being equal increasing toe out will make the car turn in better while making it push more in steady state.
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