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      03-10-2012, 05:57 AM   #243
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Quote:
Originally Posted by tomprest View Post
I need help

Can I use them on my Z4MC 2007

If I have to use spacers What size?

19x8.5 ET35
19x9.5 ET45
20mm spacers in the back
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      03-10-2012, 10:07 AM   #244
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Quote:
Originally Posted by BIMMERGUY91 View Post
do you guys think 18x9.5 et35 with a 245 up front and 18x10 et25 with a 275 in the rear will work. I am dropped pretty low on H&R coilovers and I know I will atleast need 2.5 degrees camber out of some camber plates up front.
FYI - I just put 18x9.5 et35 Apex ARC-8 wheels (all 4 corners) with 265 Star Specs with OE suspension and factory alignment. Only thing I needed to do was add 5mm spacers on the fronts so the tire's rim protector would not brush the strut. Installed wheel studs. Have driven ~75 street/highway miles with no problems.

This is while I wait for AST 4150s to become available at Vorshlag at which time I will get camber plates as well.
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      03-10-2012, 11:53 AM   #245
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Good to know. I doubt I will be so lucky since I am lowered
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      03-25-2012, 10:38 AM   #246
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Any issues with running 19x8.5 +30 with 245/35's in the front?

TIA
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      03-25-2012, 10:49 AM   #247
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Quote:
Originally Posted by _MM View Post
Any issues with running 19x8.5 +30 with 245/35's in the front?
I have the same except +32. No issues for me, but I've heard others with slight rubbing issues. It may come down to the shape of the tire. I have Hankook Ventus V12.
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      04-25-2012, 05:45 PM   #248
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What's the highest ET people have run in the front with 8.5" rims, 245 tires, without rubbing? From reading the thread it looks like 35 is a good candidate. I want to affect my scrub radius the least, while running .5" wider tires than stock.
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      04-26-2012, 10:29 AM   #249
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Quote:
Originally Posted by beta View Post
What's the highest ET people have run in the front with 8.5" rims, 245 tires, without rubbing? From reading the thread it looks like 35 is a good candidate. I want to affect my scrub radius the least, while running .5" wider tires than stock.
in order for the scrub radius to stay the same, the offset cannot change when changing rim width.

for example...

stock = 8.0 et42
same SR = 8.5 et42
same SR = 9.0 et42

this keeps the center-line of the wheel and tire combo in the same spot ONLY IF the overall diameter is the same as stock. if the car has been lowered, or if you change the overall diameter, the scrub radius will move.
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      04-26-2012, 01:25 PM   #250
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Quote:
Originally Posted by v3.2mc View Post
in order for the scrub radius to stay the same, the offset cannot change when changing rim width.

for example...

stock = 8.0 et42
same SR = 8.5 et42
same SR = 9.0 et42

this keeps the center-line of the wheel and tire combo in the same spot ONLY IF the overall diameter is the same as stock. if the car has been lowered, or if you change the overall diameter, the scrub radius will move.
Does this reflect the info in paintpro's thread? I know his comments were regarding 17s but 255/40-17 are still same diameter as the 18 in 255/35...
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      04-26-2012, 02:55 PM   #251
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Quote:
Originally Posted by Dubbedown View Post
Does this reflect the info in paintpro's thread? I know his comments were regarding 17s but 255/40-17 are still same diameter as the 18 in 255/35...
a 255/40r17 is only 0.26mm different in diameter from a 255/35r18, so the question of diameter doesn't apply to that part of your setup.

BUT, and i'm sure you know this already, going from a 8.0 et47 to a 9.5 et35 will definitely alter the SR. i understand you plan on lowering the car, and you'll need to add neg camber... and HACK is probably better at explaining this than i... you'll have a positive scrub radius of ~ 31mm, which is not great for cars like ours with a macpherson system. but you'll have boat-loads of grip!!

here's my favorite resource for info on SR..

http://www.hrsprings.com/technical/scrub_radius

EDIT!

the 31mm above is for the the external movement of the wheel face in relation to a stock ZSP wheel, NOT the scrub radius. SR really only moves 7.05mm positive from stock, as long as the suspension remains stock as well. adding neg camber complicates things a bit, as the center line of the tire's contact patch moves in, in relationship to the center line of the wheel. by my logic, that means that the SR "point" may move depending on tire compression and spring compression.

here is the math behind the SR edit...

8.00in = 203.20mm
divide by 2 = centerline = 101.60
plus 42.00mm offset = 148.60

9.50in = 241.30mm
divide by 2 = centerline = 120.65
plus 35.00mm offset = 155.65

155.65mm - 148.60mm = 7.05mm (positive)
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Last edited by v3.2mc; 04-26-2012 at 04:12 PM.. Reason: more thinking
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      04-26-2012, 03:43 PM   #252
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Quote:
Originally Posted by v3.2mc View Post
a 255/40r17 is only 0.26mm different in diameter from a 255/35r18, so the question of diameter doesn't apply to that part of your setup.

BUT, and i'm sure you know this already, going from a 8.0 et47 to a 9.5 et35 will definitely alter the SR. i understand you plan on lowering the car, and you'll need to add neg camber... and HACK is probably better at explaining this than i... you'll have a positive scrub radius of ~ 31mm, which is not great for cars like ours with a macpherson system. but you'll have boat-loads of grip!!

here's my favorite resource for info on SR..

http://www.hrsprings.com/technical/scrub_radius
Right that was my point, since diameter doesn't apply, you could probably take Paintpro's findings and apply it here. I think of the 3 setups he tested, he showed at least 1 that would fit w/o the need for camber up front.
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      04-26-2012, 04:25 PM   #253
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Quote:
Originally Posted by Dubbedown View Post
Right that was my point, since diameter doesn't apply, you could probably take Paintpro's findings and apply it here. I think of the 3 setups he tested, he showed at least 1 that would fit w/o the need for camber up front.
ah, yes.

the only major difference is that the non-///m cars have slightly different geometry than the ///m cars. but seeing as how they are both macpherson setups, maybe it's not so different that the application info provided by paintpro for one wouldn't work for the other.

it's really down to the individual driver. it could be that a bit of squirliness that's just fine for one racer is completely intollerable for another, and that driver is fine with hard steering feel but not the other way around.

haha! how's that for finishing vague?
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      04-26-2012, 04:37 PM   #254
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Quote:
Originally Posted by v3.2mc View Post
ah, yes.

the only major difference is that the non-///m cars have slightly different geometry than the ///m cars. but seeing as how they are both macpherson setups, maybe it's not so different that the application info provided by paintpro for one wouldn't work for the other.

it's really down to the individual driver. it could be that a bit of squirliness that's just fine for one racer is completely intollerable for another, and that driver is fine with hard steering feel but not the other way around.

haha! how's that for finishing vague?
Nevermind my last 4 posts or so, all this time I thought I was in the non-M thread lol
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      04-26-2012, 04:52 PM   #255
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In stock form:

-1 (aprox) camber
8" rim == 203.2 mm
225/45/R18 tire == 659.7 mm
42 mm offset

What I have:
- 0.5" drop.
-2.5 camber
8.5" rim == 215.9 mm
245/40/R18 tire == 653.2 mm

Is there a formula that would help me estimate what ET would keep scrub radius close to stock with the second set of numbers? I imagine we would have to know other things about the geometry to get some formulas.
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      04-26-2012, 05:02 PM   #256
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Quote:
Originally Posted by beta View Post
In stock form:

-1 (aprox) camber
8" rim == 203.2 mm
225/45/R18 tire == 659.7 mm
42 mm offset

What I have:
- 0.5" drop.
-2.5 camber
8.5" rim == 215.9 mm
245/40/R18 tire == 653.2 mm

Is there a formula that would help me estimate what ET would keep scrub radius close to stock with the second set of numbers? I imagine we would have to know other things about the geometry to get some formulas.
there are a few places offering computer programs that calculate this kind of stuff, but you're correct... you would need to know a few things about the stock geometry to move on that. the program will ask you to account for as many variables as possible.

do you have anything like the turner shim kit installed? if not, i think it's doable to do do this all on paper, but it may take some time. maybe we can come up with our own "general formula"... something to use as a guide, but not to be taken as actual data based on track results.
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      04-26-2012, 07:00 PM   #257
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Quote:
Originally Posted by v3.2mc View Post
do you have anything like the turner shim kit installed?
The camber is just from that 0.5" lowering and pulling pins.

Quote:
Originally Posted by v3.2mc View Post
if not, i think it's doable to do do this all on paper, but it may take some time. maybe we can come up with our own "general formula"... something to use as a guide, but not to be taken as actual data based on track results.
Yeah that would be great. A simplified formula would do (doesn't have to be exact). I'm sure we can dig up the sizes of various components between forum posters and part diagrams.

It would be great to know what ET to aim for to keep the scrub radius close to stock, then people could adjust based on taste/flushness/etc. while estimating how much that changes their SR. I love the stock handling and would prefer not to mess with it too much, while still getting wider and lighter wheels. Between the different ET options and all the spacers, one can pretty much aim for any ET number.
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      04-26-2012, 10:24 PM   #258
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The calculation of scrub radius is fairly straightforward on a macpherson strut suspension. There are a number of dimensions that stay fixed, a number that move an inconsequential amount and two that cause the primary change in the scrub radius:

The z (vertical) dimension for the top strut bearing relative to the LCA outer ball joint stays nominally the same for a given spring length. The z-dimension from the LCA outer ball joint to the ground also stays nominally the same, assuming camber changes less than about 2 degrees.

The y (lateral) dimension for the LCA outer ball joint stays fixed.

Basically the two variable dimensions are the y-dimension for the strut top bearing and the y-dimension of the extrapolated line that runs through the top strut bearing and the outer LCA ball joint (the virtual pivot line), to where it touches the ground.

If we are just looking at scrub radius, we don't need to look at any x (longitudinal) dimensions, but changes in "x" can have an effect on castor and SIA (steering axis inclination) values, thus affecting steering feel.

One way to calculate the point where the virtual pivot line touches the ground is as follows:Y

- Measure the distance between the top bearings on both front struts and divide it by 2, then add approximately 6mm (the bearing centre is approximately 6mm outboard from the centre of the top of the strut). This dimension is 'Ty'.

- Measure the height ot the top of the strut above the ground, then subtract approximately 25mm (the bearing centre is about 25mm below the centre of the top of the strut). This dimension is 'Tz'.

- Measure the distance between the centre of both LCA outer ball joints and divide it by 2. This dimension is 'By'.

- Measure the height of the centre of the LCA outer ball joint above the ground (car on the ground). This dimension is 'Bz'.

- Measure the distance between the centre of the contact patches of the front tyres where they touch the ground and divide by 2. This dimension is 'Cy'.

The scrub radius (S) is expressed as:

S=Cy-By-(By-Ty)*Bz/(Tz-Bz)
For example, if Cy=0.760m, By=0.700m, Ty=0.600m, Bz=0.225m, Tz=0.675m, then S=0.010m, or 10mm

As the camber is increased by moving the top bearing inwards, a given movement of the top bearing (Dy), will cause a change in the virtual pivot line where it hits the ground (Vy) as follows:

Vy=-Dy*Bz/(Tz-Bz)
For example, if Dy=-0.010m, Bz=0.225m, Tz=0.675m, then Vy=0.005m, or 5mm
In this case, scrub radius will change by 5mm if the centre of the contact patch stays the same.

The relationship between camber F(C) and Dy is nominally:

C=arctan(Dy/Tz)
For example, if Dy=-0.010m, Tz=0.675m, then C=-0.85°, i.e. a 10mm inward movement of the strut would cause a 0.85° degree increase in negative camber.

If a different wheel is used, then the ET difference will directly corelate to the movement of the centreline of the wheel. If the tyre size, pressure and camber stay the same, then the centre of the contact patch will move relative to the change in ET. If the camber and/or pressure changes, then the centre of the contact patch may also move independent of the ET change (think about the outside of the tyre lifting off the ground as negative camber increases, displacing the centre of the contact patch away from the centreline of the wheel).

The overall effect of increasing camber is that the virtual pivot line will move further outward where it touches the ground, making scrub radius more negative, but the contact patch will tend to move somewhat inward especially with a stiffer tyre and/or higher pressure, driving the scrub radius to be even more negative.

As I'm not at home at the moment (travelling for the next 10 days), the above dimensions don't represent the Z4M, but are just used for illustration!!!

Last edited by aerobod; 04-27-2012 at 12:08 AM.. Reason: Mistake in direction of scrub radius change due to contact patch movement
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      04-26-2012, 10:47 PM   #259
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Quote:
Originally Posted by aerobod View Post
The calculation of scrub radius is fairly straightforward on a macpherson strut suspension...
...As I'm not at home at the moment (travelling for the next 10 days), the above dimensions don't represent the Z4M, but are just used for illustration!!!
this is excellent information! what a great starting point... i've got some ideas on how to make this data user friendly for the odd onlooker. point taken about the round numbers given; i'll do proper measurements this weekend.

safe journey!
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      04-29-2012, 06:29 PM   #260
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Offsets worked out

http://www.zpost.com/forums/showpost...7&postcount=17
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      04-29-2012, 06:55 PM   #261
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haha! what offsets? more info required!
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      04-29-2012, 06:57 PM   #262
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Quote:
Originally Posted by Caddyshk View Post
ET32
ET25
These
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      04-29-2012, 07:00 PM   #263
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Quote:
Originally Posted by Caddyshk View Post
In my experience and calculations with my car and the GC coilovers with 235/265 Tyres

ET32
ET25

Is a comfortable setup - Much depends on the brand of tyre you choose.
8.5 & 9.5
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      04-29-2012, 08:33 PM   #264
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Quote:
Originally Posted by Caddyshk View Post
8.5 & 9.5
ah! thank you!

the advans are certainly growing on me... aestheticstorm's appearance in the exhaust vid definitely caught my attention.

the car looks great!
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