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PostPosted: February 18, 2016, 11:04 am 
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KB58, the almost 3" measurement is the movement of the laser on the wall, not at the wheel. If you take the laser track and subtract the lateral movement, then multiply the result by the ratio of distance as you did, it should give you an accurate figure. Lets say you find that your the point where your laser is mounted moves laterally by 0.5" in 2" of bump. Subtract that from your 2.258, so the laser movement due to rotation is 1.758". The movement at the front end of the rim (relative the the hub centre) is 7.5"/87.5" * 1.758 = 0.151". This results in a bump steer angle of arcsin(0.151/7.5) = 1.15 degrees. While that's not huge, I think it would be noticeable since it's equivalent to turning your steering wheel an additional 15 degrees or so (depending on your steering ratio).

On project binky Nick used a long bar attached to the rotor with a couple of plumb bobs handing down to a piece of paper on the floor to figure out the bump steer. It's a bit easier to account for lateral hub movement with that method I think, since you can just use the difference between the front and rear plumb bobs for your calculation.


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PostPosted: February 18, 2016, 12:51 pm 
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Unfortunately, it isn’t measured at the rim. It wouldn’t be 15” for a 15 inch rim. If the radius to the tire od is 11.5, and the toe change is 2.258” at 87.5” to the hub center, I get 0.296” at the tread.

Of course, we don’t know it is 2.258” because there is no point of reference behind the kpi to compare to, only a rough estimation.

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PostPosted: February 18, 2016, 5:25 pm 
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Miatav8,MstrASE,A&P,F wrote:
only a rough estimation


Yup, just something to do because the tank was empty on the MIG. I think I will try the plumb bob method though just to see how it works. Unless the car gets darty when I start pushing it harder I'm not going to change anything. I'm doing this more out of curiosity than anything else.

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PostPosted: February 20, 2016, 12:04 pm 
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A lot of emphasis on keeping the rack links lined up with the suspension pickups, but in this case the steering arm looks a lot longer than either A-arm, which can't help.

I've never understood why everyone is so quick to dismiss the instructions in the book--keep the steering arms level at static ride height to reduce bump steer. No rack-shortening, no lining up with the pickups. Level steering arms minimize toe changes regardless of arm lengths, at least for the first inch. Worked perfectly for me. Early Locosts might've gotten a bad rap for poor bump steer, but how closely did those builds follow the book?


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PostPosted: February 21, 2016, 9:40 pm 
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Nick, level tie rods aren't always going to minimize bump steer in every case, as it depends on the geometry of the upper and lower control arms as well. Level tie rods will minimize lateral movement of the point where they attach to the hubs, but if the whole hub is moving laterally because the contol arms are not level at ride height, you will have bump steer. in most builds the UCA is angled upward towards the hub which means the top of the hub moves in towards the car in bump. If the steering connection doesn't move in a proportional amount, the result is toe out in bump.


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PostPosted: February 22, 2016, 1:30 am 
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How 'bout two lasers, one mounted forward and one backward? Then you could compare the laser tracks on the front and rear walls. I think it would be easier to do the comparison than to do the math. Tape graph paper to the walls, mark where the lasers light at the start point, take photos with your phone where the light hits the paper at the point of travel you're measuring, transfer the photos to Photoshop or equivalent, make a mirror image of one and make it 50% transparent, and overlay it over the other one with the start points on the photos overlapping each other. The difference of the ending laser positions between the two photos (using the graph paper for scale), divided by (distance between walls / wheel radius), should give you your toe.

Not that I've actually tried this...it just occurred to me that it would work, and this wouldn't be the first time that something occurred to me that wasn't actually valid. I've always found it easier to get light to go straight than to get string or sticks to go straight, so I use lasers when and where I can. Then again, as my grandmother used to say, give a boy a hammer and everything looks like a nail.

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PostPosted: February 22, 2016, 2:54 am 
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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.

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

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PostPosted: February 22, 2016, 5:59 am 
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Excellent! :cheers:

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: February 22, 2016, 9:53 am 
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The mirror and laser look interesting. Since it is posted here I can find it later. I got my tank filled Friday so I was back in business with the welder. I finished the welding cleaned the welds up and prepped for paint. What a pain it is to mask around a frame when modifying a completed car.
Attachment:
IMG_0820.JPG
I started reassembly Saturday night hoping to be done in time to drive it Sunday morning before it started raining. Saturday was sunny and 60s so I had the Mini out doing errands. The rain earlier in the week had washed the salt off the roads so I didn't feel bad about having it out. I was dropping the hood on Sunday when the rain started.
Attachment:
IMG_0822.JPG
No more bump steer measurements got done since I was trying to beat the rain.


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PostPosted: February 22, 2016, 12:19 pm 
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Kartracer47 wrote:
Accurate bump steer is easily accomplished with a laser and a mirror [snip]
Very smart. Please ignore my previous post, the way you're describing seems waaay better.

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PostPosted: February 22, 2016, 5:21 pm 
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Posthumane wrote:
Nick, level tie rods aren't always going to minimize bump steer in every case, as it depends on the geometry of the upper and lower control arms as well. Level tie rods will minimize lateral movement of the point where they attach to the hubs, but if the whole hub is moving laterally because the contol arms are not level at ride height, you will have bump steer. in most builds the UCA is angled upward towards the hub which means the top of the hub moves in towards the car in bump. If the steering connection doesn't move in a proportional amount, the result is toe out in bump.

Post, you're correct if the rack has been shortened. If it hasn't, the tie rods will shorten at about the same rate as the control arms, or even slightly faster, at least for the first inch or so of travel. I believe this is what Champion was thinking, and it worked for me. Measured bump steer at an inch of travel was less than 1/8" toe in, which has turned out to be undetectable on the road. An inch of travel is a lot in a stiffly-sprung Locost.


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PostPosted: February 22, 2016, 7:47 pm 
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FWIW, that makes sense to me too. Every rack has a sweet spot/range.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: February 22, 2016, 8:21 pm 
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Your angles look just about perfect to my untrained eye! I couldn't help but notice - and it's probably inconsequential - but did you know your front tires are backwards of intended rotation direction?


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PostPosted: February 23, 2016, 9:48 am 
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I saw the tires before I took the picture. Since I was taking them back off anyway I just let it go. I knew somebody would catch it. :D

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