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PostPosted: March 26, 2021, 5:01 pm 
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First let me preface I do not know much about steering or suspension geometry, this is all just a side hobby, but I'm willing to learn. After building my locost, test driving and doing some learnin on the online, I have come to the conclusion that my scrub radius is all out of whack. My car feel like its got loose lugnuts going over bumps at speed and the steering is a bear at slow speed.

Thus I come seeking advice.

The parts I am currently using are:
stock Mustang 2 spindles
Granada brakes
Aero wheels 15x8 3"backspace (what I had on hand)

My wheel alignment is:
4 degrees caster.
1.5 degrees camber
and 1/16 inward toe. (car drives perfectly straight at speed and steering self centers like a champ)

I understand the effects that a large scrub radius can have on a car, akin to bumpsteer. In order to negate it you can either change your KPI or Wheel offset.
I prefer to change wheel offset.
After looking at a selection of wheels I only found a few that might work all with varying offsets.

First question. looking at the picture, What wheel offset would you recommend? The current wheel is a 15x8 3" backspace. My thoughts are I need something with at least a 6" BS? to get me close to 1" + Scrub area (yes i know its totally the wrong offset lol)

Attachment:
use this wheel.jpg


Second: What is a preferred amount of scrub radius for a locost? I've heard anywhere from 0.5" + to 1.5"+ is acceptable (0 preferred on miatas)but I'm not sure.

Third: Should I try and ditch the 8" wide (245) front wheels and try and go with a 7" (205)? will this decrease steering effort noticeably?

This is not a competitive car. I Just want a toy to rip around and steer NOT like a sack of potatoes . Any help or knowledge is greatly appreciated!


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PostPosted: March 28, 2021, 11:34 am 
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1st, welcome. I am no expert either. I do see the potential for your bump steer being off. If you do some searching here, you'll find many ways to resolve this. Fixing bump steer should fix your "feel like its got loose lug nuts going over bumps at speed"

As for scrub radius, smaller tire/ wheel "sh0ould" help the low speed steering effort. I ran 205/50R15 tires on 15-6JJ wheels. Although the wheels were a bit small for that tire sizes, it served me well. My scrub radius was virtually zero. The tire width will affect the steering effort as will wheel backspace/offset. FWIW, I prefer to use the offset measurement for evaluating scrub radius. Scrub radius calculation does not change when only changing the wheel ̶o̶f̶f̶s̶e̶t̶ width, where changing the backspace does affect the calculations.

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Visit my ongoing MGB Rustoration log: over HERE

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Last edited by rx7locost on March 28, 2021, 3:17 pm, edited 1 time in total.

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PostPosted: March 28, 2021, 12:35 pm 
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With 15x8 and 4 inch back space, you have zero offset (distance from wheel mating surface to the wheel centerline). For this type of steel racing wheel in similar widths, the most offset you can get is a 15x7 with 4 inch BS (7/2=3.5. 4-3.5= 0.5”x25.4= +12.7mm offset). You could use adapters to fit fwd type high offset wheels but the adapter is going to add 25mm in the wrong direction. However, since you have 4x4.25 (108mm), a first gen Ford Focus wheel has the same pattern, is 15x6 in steel, and has a 47.5mm offset. The normal tire size is 195-60-15 which would also help and this assembly would reduce the unsprung weight significantly. The problem(s) may be caliper clearance to the inside corner of the rim, the center bore (63.3m) may not be large enough to clear the hub center, and the focus wheel is hub centric versus the M2 which is lug centric. Lot of options out there for focus rims if you want something between minimal scrub and appearance.

Rotors are available with 5x4.5 and 5x4.75 bc, so there are many options but the most locost imho is keep the stock rotors and go with used focus rims. Dirt cheap.

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Ford IFS viewtopic.php?f=5&t=13225&p=134742
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PostPosted: March 28, 2021, 3:02 pm 
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Location: Raleigh NC
rx7locost wrote:
1st, welcome. I am no expert either. I do see the potential for your bump steer being off. If you do some searching here, you'll find many ways to resolve this. Fixing bump steer should fix your "feel like its got loose lug nuts going over bumps at speed"

As for scrub radius, smaller tire/ wheel "sh0ould" help the low speed steering effort. I ran 205/50R15 tires on 15-6JJ wheels. Although the wheels were a bit small for that tire sizes, it served me well. My scrub radius was virtually zero. The tire width will affect the steering effort as will wheel backspace/offset. FWIW, I prefer to use the offset measurement for evaluating scrub radius. Scrub radius calculation does not change when changing the wheel offset where changing the backspace does affect the calculations.
Doesn't it though? Please correct me if I am wrong, but changing the offset moves the centerline of the tire in or out relative to the suspension (i.e. to where the steering axis intersects the ground), and isn't that how the scrub radius is defined?

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PostPosted: March 28, 2021, 3:10 pm 
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Miatav8,MstrASE,A&P,F wrote:
With 15x8 and 4 inch back space, you have zero offset (distance from wheel mating surface to the wheel centerline). For this type of steel racing wheel in similar widths, the most offset you can get is a 15x7 with 4 inch BS (7/2=3.5. 4-3.5= 0.5”x25.4= +12.7mm offset). You could use adapters to fit fwd type high offset wheels but the adapter is going to add 25mm in the wrong direction. However, since you have 4x4.25 (108mm), a first gen Ford Focus wheel has the same pattern, is 15x6 in steel, and has a 47.5mm offset. The normal tire size is 195-60-15 which would also help and this assembly would reduce the unsprung weight significantly. The problem(s) may be caliper clearance to the inside corner of the rim, the center bore (63.3m) may not be large enough to clear the hub center, and the focus wheel is hub centric versus the M2 which is lug centric. Lot of options out there for focus rims if you want something between minimal scrub and appearance.

Rotors are available with 5x4.5 and 5x4.75 bc, so there are many options but the most locost imho is keep the stock rotors and go with used focus rims. Dirt cheap.
IIRC they are close but not the same. I think a wheel's stated width is the width between the beads, but the backspace is measured from the overall outside width, which is usually about an inch wider total or 1/2" per side. For instance, an 8" wheel usually has a total width of about 9".

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PostPosted: March 28, 2021, 3:15 pm 
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JBinMD wrote:
rx7locost wrote:
1st, welcome. I am no expert either. I do see the potential for your bump steer being off. If you do some searching here, you'll find many ways to resolve this. Fixing bump steer should fix your "feel like its got loose lug nuts going over bumps at speed"

As for scrub radius, smaller tire/ wheel "sh0ould" help the low speed steering effort. I ran 205/50R15 tires on 15-6JJ wheels. Although the wheels were a bit small for that tire sizes, it served me well. My scrub radius was virtually zero. The tire width will affect the steering effort as will wheel backspace/offset. FWIW, I prefer to use the offset measurement for evaluating scrub radius. Scrub radius calculation does not change when changing the wheel offset where changing the backspace does affect the calculations.
Doesn't it though? Please correct me if I am wrong, but changing the offset moves the centerline of the tire in or out relative to the suspension (i.e. to where the steering axis intersects the ground), and isn't that how the scrub radius is defined?


You are correct. I meant to write it doesn't change with wheel width using the same offset. My bad. I will correct the post.

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“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766


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PostPosted: March 28, 2021, 3:45 pm 
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First thing to change is tie rod end location! Your bump steer change is way too much. Lift the front end off the ground and visually look at the toe change - it will be very obvious.
The tie rod on the rack looks like it can be installed under the rack instead of above it. Use just enough of a spacer to allow the rod end full movement and reset toe. This alone will greatly improve the bump steer and driveability.
There are a number of articles of how to check bump steer on the forum. I use a mirror and laser to do mine.

Good suggestions from MV8 and Chuck.

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PostPosted: March 28, 2021, 4:34 pm 
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Thanks JB.

Great catch kartracer47. It's obvious looking at the pic that the tierod is in the wrong place. It looks like swapping the spacer to the outboard end would get it close.


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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: March 28, 2021, 5:16 pm 
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Good catch on the tie rod angle! much appreciated. ill be sure to change that around!
I like the idea of high offset fwd wheels. Ill do some diggin on that. maybe find a 5x114.3 rim
I did some messing around the other day and came up with some problems/solutions.
Hopefully someone who chooses pinto/mustang2 spindles will see this post in the future.

Flipping the 15x8 3BS wheels that I have around gives me a theoretical backspace of 5" (obviously not safe to drive on) but shows me some interference points.

problem 1. Any wheel with a bigger backspacing than 5" and a diameter of 15x8" will contact the upper balljoint. As it is now a 5" backspace fits but JUST barely but limits steering angle greatly by contacting the upper control arm mount. 5" gives me a scrub radius of about +2ish" (still not ideal)



Solutions to problem 1. Buy 2 inch drop spindles allowing me to run a greater backspaced wheel without contact of the BJ and control arm but obviously would mess with front ride height. I might be able to get away with it by throwing a 2" longer swegde tube in the suspension. This solution would allow me solve my Scrub radius problem entirely but might start a whole new problem to a degree.

OR

Just let the drop spindle lower the front of the car and lower the back of the car 2 inches, going from a ride height of 6" to 4". lower is always cooler right? 8)

OR

Buy bigger diameter wheels, 16" maybe even 17" Which i just dont even want to think about what that would look like :ack:


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PostPosted: March 28, 2021, 6:51 pm 
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Did you measure the front space before flipping? Place a straight edge across the outer lip, down to the outboard face.

The suspension is usually designed to be optimized with the LCA level at ride height, so you could adjust out an inch of drop from the drop spindles.

The 06 15x6 focus wheel offset from center is 1.87 inches plus half the width for 4.87 plus a half inch for the flange for 5.37 inch BS. An estimate of course.

Speedway sells the 5 lug rotors at a reasonable price.

_________________
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: March 28, 2021, 7:06 pm 
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Miatav8,MstrASE,A&P,F wrote:
Did you measure the front space before flipping? Place a straight edge across the outer lip, down to the outboard face.

The suspension is usually designed to be optimized with the LCA level at ride height, so you could adjust out an inch of drop from the drop spindles.

The 06 15x6 focus wheel offset from center is 1.87 inches plus half the width for 4.87 plus a half inch for the flange for 5.37 inch BS. An estimate of course.

Speedway sells the 5 lug rotors at a reasonable price.



I should clarify that I have the 5x4.5 Ford pattern in the front. IIRC I got the whole brake spindle combo from speedway. Ill double check what the actual BS of the wheels are flipped.
The more I think about it drop spindles and not messing with the control arm angles is the way to go. Sure it might be low but at this point I don't care.

So according to the Scrub radius picture I posted (for a 8" wide wheel) a BS of
5" gets me +2ish" Scrub
5.5" gets +1.5ish" Scrub
6" gets +1ish" Scrub
7" gets 0" Scrub
I Know its somewhat opinionated but what would be a good scrub to shoot for?

Jegs has some aluminum 15x8 5.5BS wheels ive got my eye on. But i dont wanna buy them if they wont give me optimum Scrub radius.


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PostPosted: March 28, 2021, 7:49 pm 
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Zero is optimum imho. Adjusting the coilover preload will bring the LCAs to level at ride height. Probably an allen socket set screw on each coilover to loosen then a big C wrench to rotate the seats to compress the springs more until the LCAs are level, then retighten the screws. Scratch that. Looks like you have some motorcycle coilovers tucked in there horizontally. There may be a little preload adjustment.

_________________
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: March 28, 2021, 8:16 pm 
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Miatav8,MstrASE,A&P,F wrote:
Zero is optimum imho. Adjusting the coilover preload will bring the LCAs to level at ride height. Probably an allen socket set screw on each coilover to loosen then a big C wrench to rotate the seats to compress the springs more until the LCAs are level, then retighten the screws. Scratch that. Looks like you have some motorcycle coilovers tucked in there horizontally. There may be a little preload adjustment.



Sweet. looks like I've got some shopping around to do. Ill do some more measurements but 0 Scrub radius is what ill shoot for (or as close as possible).
Ill also shoot for level LCA. Maybe a combination of swedge tubes and preload can get me there with the new addition of drop spindles. And swap the tie rods around and do an alignment.

Thanks again for everyones help!

Ill try and update how it handles afterward but may be awhile!


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PostPosted: March 29, 2021, 2:52 am 
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JBinMD wrote:
I think a wheel's stated width is the width between the beads, but the backspace is measured from the overall outside width, which is usually about an inch wider total or 1/2" per side. For instance, an 8" wheel usually has a total width of about 9".
You are entirely correct. On a 15x8, 0 offset happens at 4.5" BS, not 4" BS. It is a mistake to not account for the wheel lip when converting between BS and offset... Perhaps not one of dire consequences, but a mistake none the less.


cbaily wrote:
As it is now a 5" backspace fits but JUST barely but limits steering angle greatly by contacting the upper control arm mount.
FYI: They do make narrower angle ball joint holders, that could alleviate some of your steering angle interference @5" BS. [url=https://www.amazon.com/Allstar-Performance-ALL56292-Joint-Holder/dp/B007WLYPTC[/url]


cbaily wrote:
Buy 2 inch drop spindles allowing me to run a greater backspaced wheel without contact of the BJ and control arm but obviously would mess with front ride height. I might be able to get away with it by throwing a 2" longer swegde tube in the suspension.
I like this one with the 1-2 inch longer push rod, assuming you don't have enough adjustment in the rod ends. One thing to consider is that in doing so, you'll actually be making for an even larger scrub radius to overcome.


cbaily wrote:
I Know its somewhat opinionated but what would be a good scrub to shoot for?
Mazda marketeering claimed that the NA Miata had zero scrub radius, but much like 50/50 weight distribution claims, claims like that typically rely on an inherent level of imprecision that waters down the definition and rationale. Then NB Miata, which is effectively the same suspension, went to a 5mm lower offset wheel, giving it a small amount of additional positive static scrub. Pretty much every other 'great handling' RWD car I'm aware of also runs a small amount of positive static scrub. Consider that depending on which direction the lateral forces are occurring, zero static scrub also alternates between positive and negative dynamic scrub when taking corners at speed. This seems to coincide with the nominal positive static scrub radius is generally being used on sports and race cars. So I'd generally keep it as small as reasonable, but for this reason would also tend to err on the positive side. Based on what I noted about the effect of drop spindles on scrub radius, then even the 7" BS should actually end up pretty much there anyway... Unfortunately though, I doubt you'll be able to find 7" BS on 15x8 wheels.

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PostPosted: March 29, 2021, 11:55 am 
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I like this one with the 1-2 inch longer push rod, assuming you don't have enough adjustment in the rod ends. One thing to consider is that in doing so, you'll actually be making for an even larger scrub radius to overcome.


Very true. Did not think about it like that. Hmmmm

Mustang2/Pinto spindles feels like you are trying to win a horse race on a donkey. There's always something you have to sacrifice.

I'm wondering at this point do I cut my losses and redo the suspension for Miata spindles and wheels. More work and $ but will give me what I'm after and better longterm.


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