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Aspiring Mad Scientist
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Discussion Starter #1 (Edited)
Alright, I am sick and tired of answering trivial questions regarding what wheels fit our cars, what offsets, spacers, tires to run, and how to be hellaflush (...seriously?), so I'm going to try and teach the entire process from start to finish, right here, in this thread. If you:

  • Want to space out your stock wheels,
  • Want to run wider wheels,
  • Want to run wide, sticky tires,
  • Want to look "hellaflush",
  • Want to look "meaty", OR
  • Want to understand how the Gen2 Mazda3 suspension and body works to allow you to fit wheels,
Then this is your thread. I will do my best to provide a simple "how it works" explanation so you can get started figuring out wheel specs and car specs that will complement each other. If there's any part of this guide that's poorly written or explained please let me know so I can improve it! I want this to be THE guide to use and clear as day without needing me to cover for the holes I've probably left accidentally.

Let this be your warning: Wheel fitment requires math. I will simplify it as much as possible because I hate math too, but if you can't use a ruler and a simple calculator, close this page and go read a geometry textbook before coming back.

Warning #2: Aggressive wheel fitment requires time and effort. If you want to stretch and poke, run 9.5" wheels, or 255-wide tires, expect to spend a lot of time with your car adjusting the settings and working the fenders to clear the wheels and tires. A difficult fitment could easily take 24+ man-hours. Don't wanna? Close this page.

Note to Gen1 owners: For the most part, this guide is applicable to you as well because I focus hard on the suspension and chassis aspects, and Gen1 and Gen2 cars have the same suspension geometry. Where I can't help you is with exact clearances to the fender lips and wheel wells, so grab your tape measures and good luck.

Note to Gen3 owners: You guys are kinda left out by this guide, sorry! I don't have a Gen3 and don't plan to get one, but the basic workplan -- measure -> compare to stock -> calculate -> adjust -- still applies.

Note to Speed3 owners: The front fenders on the Speed3 are ~10mm wider per side, so with stock fenders you have that much more space to play with. The rear fitment is more or less the same.

Note in General: Between the Gen2 Mazda3 variants (2.0, 2.5, Sky, Speed) I am told there are some differences in the inside of the rear right wheel well. Skyactiv cars have an extra liner panel there that reduces inside clearance with meaty tires. If you want to completely max out your wheel wells and don't have a Skyactiv, shoot me some pictures of the inside of your rear right wheel well I'll be able to help.

Disclaimer: I'm about to write about a lot of things that can void warranty on specific parts of your car; I'm about to write about a lot of things that insurance will view as "damage" and cannot help you with if you get into an accident later on. So please, understand the risks that come with entering this facet of the hobby, and please note that I can't be held liable for anything you mess up via following my advice. If you screw something up that bad, I can't really help you but shoot me a PM and I'll look at it.

With that all aside, let's begin!

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Part 1: Wheel-to-Suspension Clearance

Before we can do any serious wheel fitment, first we must have some basic understanding of the Mazda3's suspension. The Gen1 and 2 Mazda3 are built off of Ford's C1 platform, encompassing a McPherson Strut front suspension and a "Control Blade" rear suspension (just a different type of multi-link).

Here's a rendering (front 3/4 view) of the front suspension. The steering knuckle (green) solidly clamps around the strut assembly (yellow), the top of which bolts solidly to the body. The knuckle also attaches to the lower control arm (red), or "LCA", via a ball joint, and then the LCA attaches to and pivots at the subframe (blue -- it's all one piece).


Here's a crude diagram of the front left strut geometry, looking at it from the front:


Since the LCA (red) isn't going to move that wildly when the suspension compresses, the strut (yellow) won't move all that much either, and thus you can roughly simplify the system to a wheel bolted to the knuckle (green), moving up and down parallel to the strut. So the more fender clearance you want, the more you should move the strut inwards; moving the strut in happens to increase camber, which is why people say more camber gives more clearance. (I'll go over how much camber = how much clearance in part 2.)

The one upside to the front strut is that you can camber all you want in the front and the wheel-to-strut clearance doesn't change. Strut clearance is entirely dependent on wheel/tire width and offset.

Here's a rendering (front 3/4 view) of the rear. The Control Blade suspension is essentially a flexible trailing arm that is forced to work like a true multi-link rear -- we can treat it much like a normal double-wishbone suspension. The trailing arm (red) IS the knuckle (still red) as it's all one piece of steel. From there, the camber arm (green), toe link (purple) and rear LCA (hidden behind the camber arm) force the trailing arm/knuckle combo to travel with them as the suspension compresses. The shock (yellow) doesn't do anything geometry-wise as it is mounted on a bushing to the trailing arm.


A crude diagram of the rear left suspension, looking at it from the front:


Purple here is the LCA, not the toe link (the toe link is a bit shorter). The camber arm and LCA control the motion of the knuckle (red), so unlike the front end, as the rear suspension compresses, the camber arm (green) starts pulling the wheel inwards, giving you more clearance to the fender (blue). The more you increase camber by shortening the camber arm, the more wheel-to-fender clearance you get.

Now that we've done a basic review of Mazda3 front and rear suspension, let's start throwing some numbers in there! In Part 1 we will only calculate wheel (and tire) to suspension clearance, making sure that all the internal clearances that you can't see are safely handled. Part 2 will handle the actual fender clearance, and Part 3 will deal with what to do when you just don't have enough room in the fender.


Before we discuss how to clear the wheel from the fender (the more difficult question), let's first discuss how to choose a wheel that will safely clear the strut (in the front) and the trailing arm (in the rear). To do this, I must introduce a term from the muscle car guys, "backspacing":



Taken from RobRobinette.com, shamelessly. Backspacing is an extremely useful metric for wheel selection, as you can see it literally measures how much the wheel extends back from the hub mounting pad. You can get different wheels that have the exact same strut/shock clearance, so long as their backspacing measurements are identical. So to clear the suspension bits, all you need to know is the max backspacing that our chassis can sustain, and stay below that max! Straightforward.

Here is my quick and dirty method of knowing the max backspacing we can do on the 3 chassis. With my RPF1 setup, I've discovered that the maximum offsets for an 18x9.5 wheel are +37 front, +47 rear. Note that the front BC (and other "double-adjustable" coilover) struts are thicker than most other struts. With OEM-style struts and KW/H&R coils you can add 3mm front offset (total +40) to your max. But anyways, we can slyly ignore the "Overall Rim Width" dimension and just use the 9.5 nominal width instead.

9.5" = 241.3mm
Inside of wheel to Centerline of wheel = 241.3/2 = 120.7mm
Max Front Backspacing = 120.7mm + 37mm = 157.7mm
Max Rear Backspacing = 120.7mm + 47mm = 167.7mm

(Dont forget to add 3mm to the front backspacing calculation if you're on KW or H&R coilovers or OEM-type struts! These include Konis, Bilsteins, KYB's, Monroes, etc.)

Please remember that this is a hack "backspacing" calculation, not a real one. The numbers that we just calculated are to be used for reference and comparison only -- if you are looking for wheels and don't know whether they'll clear the front strut or not, just run through the above calculation again. If your wheels' "backspacing" is below 157.7mm, then you'll clear the strut! Same deal in the rear: make sure your "backspacing" is less than 167.7mm and you'll clear the trailing arm.

Once you have this general number, you'll have to compensate for the tire you want to mount as well. If you're stretching tires, skip this paragraph -- your wheel is the outermost part of the wheel/tire assembly. If you are NOT stretching tires, then mount the tire to the wheel and measure how much the tire pokes past the edge of the wheel. You will need to measure the amount (usually a few mm) and then subtract this number from the max allowed backspacing. Do this front and rear.

To put this all into an example, here is an 18x9.5 +37 with 157.7mm of "backspacing": it barely clears everything inside, thanks to a mildly stretched tire. I firmly suggest you don't go any closer.



From this baseline you can test-calculate many different wheel combos. Let's take an RX-8 rim as an example.

18x8 +50:
8" = 203.2mm
Inside to Centerline = 203.2/2 = 101.6mm
Front Backspacing = 101.6mm + 50mm = 151.6mm
Rear Backspacing = 101.6mm + 50mm = 151.6mm

Is 151.6 < 157.7? Yep.
Is 151.6 < 167.7? You bet.

Is 18x8 +50 going to clear the front and the rear suspension on the Mazda3? Yep, yes it will. This calculation works for all the usual Mazda3 fitments (try it yourself: 17x7 +42, 18x8.5 +45, 17x9 +45), you'll see that the wheel either clears, or you need a small spacer (in the case of the 17x9 +45) to clear.

Last but not least, let me mention again that even if the rim itself clears all the suspension bits, the tire needs to stay off too. On the same wheels as the last picture, I mounted wider, non-stretched tires, and had to reduce my backspacing (by reducing my offset 2mm) in order to keep the tire off the strut.


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Part 2: Wheel-to-Fender Clearance

Now that we know how to keep the wheel and tire from rubbing the suspension and chassis, the next step in wheel fitment is to ensure that the tires won't rub the fender. I went to the dealership armed with a tape measure to bring you the measurements off a stock 2013 Skyactiv 3:



It's a little difficult to read, but the distance from top of the tire to edge of the fender is 27mm front and 40mm rear. Keep in mind that this is only for this car! Between manufacturing tolerances, poor factory alignments and differences between tires, cars can vary wildly in these two seemingly basic measurements. So measure out your own car and know for sure how much room YOU have to work with.

With that warning out of the way, let's get to the fun part which is actually calculating fender-tire clearances for non-stock wheels. Revisiting my crude little pictures, here is the clearance of the front end. To keep things organized let's call it "Xn" (X-normal; later on, I'll introduce X-pulled for fender pulls):


As measured up at the start, Xn = 27mm for tonight's Mazda3.

And here is the clearance of the rear end, let's call it "Yn" (Y-normal; later on, I'll introduce Y-rolled for fender rolls):


Please take note the horizontal lip on the rear fender. Although the tape measure said 40mm in the initial picture, you need to take into account that the horizontal lip measures about 10mm and thus reduces your actual clearance to 30mm. Yn = 30mm for this Mazda3.

So with Xn and Yn in hand, let's answer the first, easiest question first: How much spacer can you run to flush up the wheels and tires to the body? Well, the mathematical answer is right in front of you, Xn and Yn (27mm and 30mm) are exactly the numbers that you can get away with. In reality I strongly suggest that you take 10mm off these numbers for a stock-height car, simply because having mathematically flush wheels looks dumb at stock height.

Quick Answers #1: For most stock Mazda3's, a 15mm front spacer and 20mm rear spacer will make the wheels more flush, and make the car look better without being ridiculous.

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Now let's revisit how camber works to give you more tire-to-fender clearance. Here's another sketch:


In the drawing above, you see two dotted lines, the one on the right being 0 camber, the one on the left being tilted by whatever camber angle we would like to investigate, conveniently labeled "a". The distance "h" is the distance between the steering knuckle's ball joint and the top of the tire. Let's use some approximate math to figure out how much clearance C you can gain by running 1 degree camber (a = 1).

Trigonometry time! Looking at this system we can make a simple triangle with one side of length h, one side of length C (along the top of the tire), a right angle between C and h, and one more angle a. So, to figure out C given h and a, all we do is plug numbers into the formula

C = h * sin(a * pi/180) ---> (the pi/180 is just a conversion factor from degrees to more usable units)

h is approximately 17 inches, or 430mm. We've got all the unknowns now, so stuff 'em in there,

C = 430mm * sin(1 * pi/180) = 7.5mm

There you have it, a single degree of camber can pull the top of the tire inwards 7.5mm! In reality, it's not exactly this much because the triangle we drew is technically not right. :stuart: But it's quite close, and if you apply a little safety factor into your estimates you can just use this figure outright without having to do the trigonometry every time.

Quick Answers #2: Every degree of camber gives you 6-7mm more tire-to-fender clearance. The more camber you add, the more the effect tapers off, hence the range 6-7mm.

So now that we know how camber can help us fit wheels, the rest of it is pretty straightforward. Recall this image:



Where we want to look now is the imaginary sibling of "backspacing", "frontspacing". Basically, the distance from the hub mounting pad to the outermost extent of the wheel. Like how we calculated strut clearance by figuring out the maximum backspacing the 3 can handle, we can also calculate fender clearance by figuring out the maximum "frontspacing" the 3 can handle. And before anyone forgets, just like with backspacing, the width of the tire also has to be accounted for when figuring out frontspacing. So we'll do just that.

The stock Mazda3 Skyactiv wheel and tire package is 205/55/16 on a 16x6.5 +50 wheel, and as we measured, this leaves 27mm of room in front, 30mm in rear. Calculating the rim-only frontspacing is easy,

6.5" = 165.1mm
Inside to Centerline = 165.1/2 = 82.6mm
Rim-only Frontspacing = 82.6mm - 50mm = 32.6mm

Which isn't all the much at all; of course, the stock tire is quite a bit wider than the wheel, so to get a meaningful number, we need to figure out what the tire's effective frontspacing is before putting it back into the clearance equation. While not entirely accurate, we can utilize the Tire Rack's specs to figure out what kind of tire clearances we are actually working with. For the 2013 Skyactivs the stock tire is the Bridgestone Turanza EL-400, so off the Tire Rack:


For clearing the tire and the fender, the one we care about is the Tread Width, while for optical flushness, it's actually the Section Width that sticks out further on all non-stretched tires. For now though I only care about physical tire clearance, so let's work with the Tread Width and calculate the "tire frontspacing" that tells us how much physical room we have to go wider.

Tread width = 6.7 = 170.2mm
Inside to Centerline = 170.2/2 = 85.1mm
Top-of-tire Frontspacing = 85.1mm - 50mm = 35.1mm

Applying this figure back to our measured tire-to-fender clearances, we get

Available front-wheel Frontspacing = 35.1 + Xn = 62.1mm
Available rear-wheel Frontspacing = 35.1 + Yn = 65.1mm

If we don't change the camber spec on any wheel of our stock Skyactiv 3. From here, we can increase the available frontspacing on any end by adding ~6mm of frontspacing for every degree of camber that we increase over the stock setting. Suppose that the stock camber settings were 0.0F/2.0R, and we're okay with running 2.0F/3.0R; if each degree of camber is worth 6mm we have

New Available front-wheel Frontspacing = 35.1 + Xn + (6 * 2) = 74.1mm
New Available rear-wheel Frontspacing = 35.1 + Yn + 6 = 71.1mm

As you can see, that's quite a sizable difference and is the primary reason why some 3's can fit wide wheels and tires under stockish-looking fenders. Keep in mind that we haven't even discussed poking wheels and tires yet -- so far, our calculations only go so far as making the tire, wheel and fender mathematically flush. So to put our new numerical tools to the test, let's revisit the three fitment examples from Part 1, working under the assumption of 27mm and 30mm front and rear fender space. Flushness is going to be determined based on whether or not our new wheels' frontspacing falls within the available frontspacing we have -- 62.1mm front and 65.1mm rear. If the frontspacing falls under those figures, then we're good, and if they don't, then we can figure out how much camber we need to force those wheels to sit flush (nevermind for now whether or not the camber is physically possible).

Remember that all these calculations are going to be relevant only to the stock car that I was measuring -- for your own car, you will need your own tape measure!

As an example, let's try to fit 17x9 +45 RPF1:

17x9 +45:
9" = 228.6mm
Inside to Centerline = 228.6/2 = 114.3mm
Rim-only Frontspacing = 114.3mm - 45mm = 69.3mm

Is 69.3 < 62.1? Nope. How much camber do we need? 69.3 - 62.1 = 7.2, so

6 * a = 7.2

we need at least 1.2 degrees more camber to get the rim under the front fender.

Is 69.3 < 65.1? Also nope. How much camber do we need? 69.3 - 65.1 = 4.2, so

6 * a = 4.2

we need at least 0.7 degrees more camber to get the rim safely under the rear fender.

Is 17x9 +45 going to clear the front and the rear fenders on the Mazda3? Yes, but only with some camber adjustments. According to this math here we'd need something like 1.2F/2.7R camber to make them fit. If it is close now, what about with tires? Since this is an AutoX/track fitment let's try putting a track tire on this wheel, the Hankook RS-3, in 255/40/17.

255/40:
Tread width = 9.7 = 246.4mm
Inside to Centerline = 246.4/2 = 123.2mm
Top-of-tire Frontspacing = 123.2mm - 50mm = 78.2mm

From 69.3mm rim-only frontspacing to 78.2mm including the tire is quite a difference, as you might imagine. To fit the 255/40 you're looking at at least another 9mm of frontspacing needed -- 1.5 degrees more camber -- bringing the total camber spec up to 2.7F/4.2R just to fit 255/40/17 RS-3's on your 17x9 +45 wheels.


Now that we've gone through all this math, I hope I've been able to convey that

  • Wheel-to-fender clearance takes a back seat to tire-to-fender clearance for non-stretched tires;
  • Tire-to-fender clearance is determined by tread width and wheel offset;
  • Just like we found a max backspacing for the 3, there is a max frontspacing for each 3, and
  • You have to measure and calculate these values individually for your car if you want a personalized wheel fitment.
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Part 3: Rolling and Pulling Fenders to fit MORE!

It should be plainly obvious by now that something super-aggressive like an 18x9.5 +35 is not going under the stock fenders. Doubly so if you want to run a big tire with it, say a 255/35/18 track tire that runs wide even for its listed size. The only solution for these problems is to widen the fender out to fit the new specs. There are 3 main ways to do it:

1) Roll the inside of the fender (that lip, remember?) flat;
2) Forcibly pull the material of the fender outwards; or
3) Cut the wheel arch out entirely and make or install larger arches ("fender flares").

#3 is a full topic for another thread ( :wink: ).

#1 and 2, however, are pretty easy. For the most part, it's a collaboration between a heat gun, a soft-face hammer, and one of these:


Fender rollers like the one pictured are usually somewhat pricey to buy outright (~$250) but you can rent them from Tire Rack or many other local places for a reasonable weekly rate ($50, last I checked).

There are literally hundreds of guides out there, so read this one:

DIY How to: Roll and Pull Your 240sx Fenders Properly

The take-home message here is that you heat up the metal, and use roller and/or hammer to slowly, gently flatten out the inner lip of the fender to tuck it up and away from the tire. Pulling fenders is simply rolling the fender so hard that the metal begins to stretch and come out more. On the Mazda3, the rear fender is much stiffer than the front -- when you roll the rear fender the lip goes up first, as you intend it to, whereas in front the fender tends to start pulling out immediately. It's just how it is, either way you are gaining clearance for the tire so it's not a huge cause for complaint.

You may ask at this point, how much space can I actually gain by rolling/pulling? To revisit our crude drawing, rolling the rear gives us our new clearance Yr, or Y-rolled:


And pulling the front gives us Xp, or X-pulled:


In my personal experience, I was able to roll the rear about 12mm and pull the front about 18mm. Adding those numbers onto Yn and Xn, Yn = 30mm becomes Yr = 42mm and Xn = 27mm becomes Xp = 45mm, which is a sizable gain all around! So if you're shopping around for a new wheel combo and aren't sure what you can fit "even if you roll and pull", then Yr and Xp are my answers for you. You can redo the calculations in Part 2 using Yr and Xp instead of Yn and Xn, and that's basically what I can (almost...) guarantee will fit your car, as long as you put in enough work.



So, that's about all the math I can handle. Putting everything together now and pushing closer to the limits, let's use my actual car as the final example.


Enkei RPF1, 18x9.5 +45:
9.5" = 241.3mm
Inside to Centerline = 241.3/2 = 120.7mm
Rim-only Frontspacing = 120.7mm - 45mm = 75.7mm
Rim-only Backspacing = 120.7mm + 45mm = 165.7mm

Let's clear the front and rear suspension first, at the wheels:
Is 165.7 < 157.7? Nope. How do I get this wheel off the front strut? Need an 8mm spacer at least.
Is 165.7 < 167.7? Yep. 18x9.5 +45 clears suspension in the rear.

Now factor in tires, Michelin Pilot Super Sport 255/35/18:
Tread width (off TireRack) = 9.5" = 241.3mm
Inside to Centerline = 241.3/2 = 120.7mm
Top-of-tire Frontspacing = 120.7mm - 45mm = 75.7mm
Tire-included Backspacing = 165.7mm + 2mm more (measured) = 167.7mm

With the tire included, is 167.7 < 157.7? Nope. Now I need a 10mm spacer to clear the strut.
Is 167.7 < 167.7? Nope, they're equal. Still clearing, but it's a very slim clearance now.

To clear suspension, the 10mm spacer in front is going to change our effective offset from +45 F/R to +35F/+45R. So now, let's look at how much camber and fender work we're gonna have to do to pull this all off. Remember that our stock, available frontspacing is 62.1mm front, 65.1mm rear, and that's what we'll be comparing the new wheel/tire specs to in order to calculate how much more room we need.

Front tire Frontspacing = 120.7mm - 35mm = 85.7mm
Rear tire Frontspacing = 120.7mm - 45mm = 75.7mm

Tackling the rear first, is 75.7 < 65.1? Nope. How much help do we need? 75.7 - 65.1 = 12.6mm, so to make room, I rolled flat my fender and hammered it out as much as I could, which is about 12mm. While this is pretty close, I added 0.2deg camber for a little more clearance over bumps. My final camber spec in the rear was 2.2 degrees, consistent with all the numbers we've been working so far. :)

In the front, is 85.7 < 62.1? Absolutely not. The deficit is 85.7 - 62.1 = 23.5mm, so you can see I had a lot of fun getting this sucker to fit! Adding 4 degrees of camber to get 24mm is not physically possible on the BC coilover setup, so we are forced into doing fender work. Again, my fender pull is good for about 18mm at most, so that leaves 5.5mm of clearance that I have to tackle via camber. For a safety envelope under suspension bump, instead of adding 1deg camber (which would've done the trick) I added 2deg camber to tuck the tires under the fenders with plenty of room (~6mm) to spare.


(Voila!)



To sum everything up, here's the checklist of what conditions a wheel/tire combo must clear in order to fit on our Mazda3:

  • Wheel backspacing needs to clear suspension (Part 1);
  • Additional backspacing from tire must be compensated for to clear suspension (Part 1);
  • Frontspacing to the top of the tire needs to clear fender (Part 2);
  • If it doesn't clear the fender, we can add camber up to ~3.0 deg front, 7-10deg rear to gain clearance (Part 2); and
  • If you want to keep the camber at sane settings, we can roll and pull fenders up to ~18mm front, ~12mm rear to gain clearance (Part 3).
Depending on how much work you want to put into your wheel fitment, you can use one or all of these tools to help you out. Any wheel/tire combo that has less aggressive frontspacing and backspacing than my examples shown will fit your Mazda3 as long as you are willing to put in the time. Gen 1 owners may have to put in more time into working their fenders, and Speed3 owners can save time working on the front fenders because they already have 10mm more space. But the general process is exactly as I've laid it out here, and I hope that you guys can make use of it, intelligently pick wheels and tires, and fit them with no hassle, no nasty surprises, and no risky hacks so that you can have a safe, enjoyable-to-drive, good-looking Mazda3.

Final note to hellaflush crowd: notice that all of the final fender clearance calculations are done using the top of the tire as the reference point, not the wheel lip; if you want to stretch and poke and dump your car, by all means go ahead, just make sure the top of the tire sits well within the width of the wheel if you want to poke wheel without rubbing your tire tread.



And that pretty much concludes the guide! I know it's a long read, and no I don't have enough pretty pictures, so thanks for sitting through it all and I hope you enjoyed the work I put into writing all this down. Thanks! :wink:
 

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On that college budget
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Thank the lord someone made this haha. If people don't understand after this then just don't buy wheels lol.
 

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zoom freakin' zoom
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Fantastic write-up @colo0, thanks for taking the time to post up a very detailed and informative thread.

I second @kryptic...let's get this stickied!!!!
 

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Stickied! Thanks Brian!
 

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Registered
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Let me know when your ready for part 3 Brian
 

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Aspiring Mad Scientist
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Discussion Starter #13 (Edited)
@Frnchplayr15 Phil, part 3! Thanks! I tried editing it into the OP but I broke the post length limit, so please take care of it for me. :)

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Part 3: Rolling and Pulling Fenders to fit MORE!

Up to this point we've gone over how to 1) calculate wheel specs to safely clear the suspension bits and 2) calculate wheel, tire and camber specs to safely clear stock fenders. I've been avoiding giving straight-up answers because I'd like people to actually run through the simple math at least once, but it should be plainly obvious that something super-aggressive like an 18x9.5 +35 is not going to fit under the stock fenders without breaking the suspension. Doubly so if you want to run a big tire with it, say a 255/35/18 track tire that runs wide even for its listed size. The only solution for these problems is to widen the fender out to fit the new specs. There are 3 main ways to do it:

1) Roll the inside of the fender (that lip, remember?) flat;
2) Forcibly pull the material of the fender outwards; or
3) Cut the wheel arch out entirely and make or install larger arches ("fender flares").

In all honesty I do not know how to safely tackle #3, so unfortunately I will have to omit this point from the guide -- for now. ;)

#1 and 2, however, are pretty easy. For the most part, it's a collaboration between a heat gun, a soft-face hammer, and one of these:


Fender rollers like the one pictured are usually somewhat pricey to buy outright (~$250) but you can rent them from Tire Rack or many other local places for a reasonable weekly rate ($50, last I checked).

The actual instructions for fender rolling, I'm not going to cover here. There are literally hundreds of guides out there, and this one here is better than anything I could write myself, so:

DIY How to: Roll and Pull Your 240sx Fenders Properly

The take-home message here is that you heat up the metal, and use roller and/or hammer to slowly, gently flatten out the inner lip of the fender to tuck it up and away from the tire. Pulling fenders is simply rolling the fender so hard that the metal begins to stretch and come out more. With a roll, you can see the inner lip is tucked away, but the outside appearance stays fairly stock (picture stolen from @mazdaspeedmyk):



And with a pull, you end up past "flattening" and start to reform the metal into a new, wider shape. See the kink where the fender meets the bumper? That's the typical fender pull up front.



Note that Mike's picture is of the rear and my picture is of the front fender. While it's not an apples-to-apples comparison, I picked those images because on the Mazda3, the rear fender is much stiffer than the front -- when you roll the rear fender the lip goes up first, as you intend it to, whereas in front the fender tends to start pulling out immediately. It's just how it is, either way you are gaining clearance for the tire so it's not a huge cause for complaint.

You may ask at this point, how much space can I actually gain by rolling the rears? In general, flattening the rear inner fender lip gives about 8-10mm more room to play with, depending on the quality of work. The lip itself will get tucked in and invariably the fender will start to come out a tiny bit too, so in my experience it was about that much space gained. You can go farther and apply more pressure until the rear fender starts to pull out as well but be careful that you don't crack the paint, crack the lip, or wrinkle the metal The max fender pull I was able to get with my hammer gave about 12mm more usable space. So to revisit our crude drawing, should we do this to a stock-tired car, our new clearance is Yr, or Y-rolled,


Compared to the original Yn = 30mm, an additional 12mm of clearance makes Yr = 42mm, which is quite a bit! This isn't to say that you can't do more -- you can always do more! -- but for the purposes of this guide, I can only safely say 12mm because this is what I've achieved. Good luck for you ambitious folk out there. ;)

And how about pulling the fronts? Here again I can only speak for myself, but with professional help I was able to pull the fender out over 1/2", or 13mm. I did some more personal work after that to get them even a little bit wider, and then I later shimmed and remounted the fenders further outwards, so I estimate that I got about 18mm of extra usable space. To help visualize it again, let me introduce X-pulled, or Xp, versus the original X-normal (Xn),


Which is simply how much space you could gain pulling fenders on a stock-tired car. With an 18mm pull, Xn = 27mm becomes Xp = 45mm which is again a sizable gain, and you can start to see how useful an aggressive fender pull is towards stuffing more wheel under the metal.

Now, for sure I know that the front fenders can be pulled even more, because the metal is so thin that it is easily pulled out even further to take on a "flared" shape. If you decide to go so far as to flare out the front fender you can expect to get up to another 5mm or so, conservatively guessing, giving 23mm. And although this starts to get ugly, you can also slot the mounting holes of the fender to remount and move the whole thing out another few mm. At the end of the day I'm fairly confident that you could gain nearly 25mm = one inch! if you take all the processes -- pull, flare, slot, shim, remount -- and add their effects together. You can tuck a helluva lot of wheel under an extra inch of fender! How does Xp = 52mm sound? That kind of pull alone basically allows room for 50mm more tire than stock (255-section tires), without even changing anything with the suspension or alignment.

Since fender pulls and rolls are completely variable, however, for calculating clearances with a pulled/rolled fender it's better to calculate how far your fenders are from fitting the tire, rather than producing maximum roll/pull all the way to Yr and Xp and then forcing the wheels to sit flush. While the rear fender roll on a 3 is pretty standard, the front fender pull is entirely up to how much you want that metal to come out -- the harder you want it, the more it tends to come out lol. So for the calculations following, I'm not really going to use Yr and Xp and instead focus more on balancing the variables of camber and roll/pull.

So, just like Parts 1 and 2 now, we've got the maximums established for how much room we can potentially gain to work with -- in easy mode, let's say 23mm in front (no slotting) and 12mm in rear. Let's work a couple examples just like last time, and you'll see why certain wheel and tire combos "need a roll", as the forum lingo goes.


Revisiting the RPF1, 17x9 +45:
9" = 228.6mm
Inside to Centerline = 228.6/2 = 114.3mm
Rim-only Frontspacing = 114.3mm - 45mm = 69.3mm

With Hankook RS-3 255/40/17:
Tread width = 9.7 = 246.4mm
Inside to Centerline = 246.4/2 = 123.2mm
Top-of-tire Frontspacing = 123.2mm - 50mm = 78.2mm

Recall that our stock-camber maximum frontspacing values are 62.1mm and 65.1mm front and rear, so:

In the front, is 78.3 < 62.1? Nope. But this time, 78.3 - 62.1 = 16.2mm, which you CAN pull the front fender by. In practice I would split up the clearancing required between pulling fender and adding camber, say like 1.5deg camber (9mm) and a gentle, 7-8mm fender pull, which you could achieve even by the phonebook method. In this particular case, I would also advise shimming the fender corner out like I did to help clear the tire during mid-corner bumps.

In the rear now, is 78.3 < 65.1? Also nope. How much help do we need? 78.3 - 65.1 = 13.2mm, so if you roll flat your fender (10mm) and add 0.5deg camber (3mm) you would be good to go!

Now to get the last part of the wheel fitment correct, we revisit what we learned in Part 1 so that our wheel and tire combo can clear the fenders (done, check) as well as the suspension. For our 17x9 +45, 255/40/17 example, recall that in Part 1 we calculated that the front wheel would actually touch a BC coilover strut -- so we need a 3mm spacer to get that thing safely off. What do we change now to continue keeping the tire under the fender? Well, conveniently, another 0.5deg of camber would get you 3mm more clearance, so instead of 1.5deg front camber we need 2.0deg. It's really that simple. :)


Putting everything together now and pushing closer to the limit, let's use my car as an example.

RPF1, 18x9.5 +45:
9.5" = 241.3mm
Inside to Centerline = 241.3/2 = 120.7mm
Rim-only Frontspacing = 120.7mm - 45mm = 75.7mm
Rim-only Backspacing = 120.7mm + 45mm = 165.7mm

Let's clear the front and rear suspension first, at the wheels:
Is 165.7 < 157.7? Nope. How do I get this wheel off the front strut? Need an 8mm spacer at least.
Is 165.7 < 167.7? Yep. 18x9.5 +45 clears suspension in the rear.

Now factor in tires, Michelin Pilot Super Sport 255/35/18:
Tread width = 9.5 = 241.3mm
Inside to Centerline = 241.3/2 = 120.7mm
Top-of-tire Frontspacing = 120.7mm - 45mm = 75.7mm
Tire-included Backspacing = 165.7mm + 2mm more (measured) = 167.7mm

With the tire included, is 167.7 < 157.7? Nope. Now I need a 10mm spacer to clear the strut.
Is 167.7 < 167.7? Nope, they're equal. Still clearing, but it's a very slim clearance now.

To clear suspension, the 10mm spacer in front is going to change our effective offset from +45 F/R to +35F/+45R. So now, let's look at how much camber and fender work we're gonna have to do to pull this all off. Remember that our stock, available frontspacing is 62.1mm front, 65.1mm rear, and that's what we'll be comparing the new wheel/tire specs to in order to calculate how much more room we need.

Front tire Frontspacing = 120.7mm - 35mm = 85.7mm
Rear tire Frontspacing = 120.7mm - 45mm = 75.7mm

Tackling the rear first, is 75.7 < 65.1? Nope. How much help do we need? 75.7 - 65.1 = 12.6mm, so to make room, I rolled flat my fender and hammered it out as much as I could, which is about 12mm. While this is pretty close, I added 0.2deg camber for a little more clearance over bumps. My final camber spec in the rear was 2.2 degrees, consistent with all the numbers we've been working so far. :)

In the front, is 85.7 < 62.1? Absolutely not. The deficit is 85.7 - 62.1 = 23.5mm, so you can see I had a lot of fun getting this sucker to fit! Adding 4 degrees of camber to get 24mm is not physically possible on the BC coilover setup, so we are forced into doing fender work. Again, my fender pull is good for about 18mm at most, so that leaves 5.5mm of clearance that I have to tackle via camber. For a safety envelope under suspension bump, instead of adding 1deg camber (which would've done the trick) I added 2deg camber to tuck the tires under the fenders with plenty of room (~6mm) to spare.





To sum everything up, here's the checklist of what conditions a wheel/tire combo must clear in order to fit on our Mazda3:

  • Wheel backspacing clears suspension (Part 1);
  • Additional backspacing from tire is compensated for to clear suspension (Part 1);
  • Frontspacing to the top of the tire clears fender (Part 2);
  • If it doesn't clear the fender, we can add camber up to ~3.0 deg front, 7-10deg rear to gain clearance (Part 2); and
  • If you want to keep the camber at sane settings, we can roll and pull fenders up to ~23mm front, ~13mm rear to gain clearance (Part 3).
Depending on how much work you want to put into your wheel fitment, you can use one or all of these tools to help you out. Any wheel/tire combo that has less aggressive frontspacing and backspacing than my examples shown will fit your Mazda3 as long as you are willing to put in the time. Gen 1 owners may have to put in more time into working their fenders, and Speed3 owners can save time working on the front fenders because they already have 10mm more space. But the general process is exactly as I've laid it out here, and I hope that you guys can make use of it, intelligently pick wheels and tires, and fit them with no hassle, no nasty surprises, and no risky hacks so that you can have a safe, enjoyable-to-drive, good-looking Mazda3.

Note to hellaflush crowd: notice that all of the final fender clearance calculations are done using the top of the tire as the reference point, not the wheel lip; if you want to stretch and poke and dump your car, by all means go ahead, just make sure the top of the tire sits well within the width of the wheel if you want to poke wheel without rubbing your tire tread.



And that pretty much concludes the guide! I know it's a long read, and no I don't have enough pretty pictures, so thanks for sitting through it all and I hope you enjoyed the work I put into writing all this down. Thanks! :wink:
 

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Aspiring Mad Scientist
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Discussion Starter #14
I've been editing these posts on and off to make them more readable, so if anyone has any confusion points where it doesn't make sense, let me know! All suggestions will be incorporated. :)
 

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405 Posts
Excuse my ignorance but if I want to get new wheels and I go to discount tire's website and enter the model of my car will all the wheels listed for my car fit without a problem?
I hear about offsets and the wheels I am interested in (Maxxim Verse) have an offset of .38.
I also wonder if a weight difference of 2 lbs is meaningful? The others I'm considering are the MB Wheels Twist (20 lbs) and Liquid Metal Static (21 lbs)
I have a Mazda 3 i Sport Sedan with 16 inch tires.
Again, thank you and please excuse my newbieness
 

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Celest Zoom-Zoom
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4,919 Posts
Lol thought you said you like the hub caps. :D

Yes all the ones they have listed will work for you car. If you aren't sure give discount tire a call and ask. But a +38 offset is fine.
 

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Lol thought you said you like the hub caps. :D

Yes all the ones they have listed will work for you car. If you aren't sure give discount tire a call and ask. But a +38 offset is fine.
Thanks,

Hah, I know I said that but after getting a close up look at the fitment of the tire and steel wheel with the hubcap laying on top I immediately got the urge for an alloy. They just look so much smoother.
 

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Aspiring Mad Scientist
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1,716 Posts
Discussion Starter #19
Be sure to confirm the width too. A 6.5" wide wheel at +38 is plenty safe fitment. 7.5" usually fits but it's starting to get close. For an 8.5" or 9.5" you will need to start applying teh maths. :)
 
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