3.3TT Any disadvantages to a square setup for the track?

[Volfy]

I saw an old post of yours "We run 18x8.5 ET38 Enkei TFR with 255/40R18 and they bolt right on no problem. Clearance to the strut was tight, with only a few mm's gap, but they do clear." so that looks like the limit for inset, according to WillTheyFit.com is "10.4mm closer to the suspension strut." vs the factory 225s.

Actually, even the RPF1 18x8.5 ET40 cleared my Riaction coilover, although just barely. Depending on the tire mounted on the wheel, this might be okay. 225 is probably okay. 245 or 255 will have problem. Also, a lot of tires have a protector band that sticks farther out past the rim, and so that can cause it to rub against the coilover. I ran into that problem with Dunlop SportMaxx GT.


We no longer run stock struts, but I would imagine the clearances are similar.

So yeah, you can definitely figure that as the tightest you can possibly get away with.

So taking the ET15 offset +10mm, would a 18x9.5 ET25 rim work? There's no more room to move closer to the strut, but that's still 28.1mm of poke vs factory. And if that's too much, then you just can't fit a 9.5" rim up front without crossing the scrub centerline, right?

Nope. An ET15 wheel with 10mm makes it +5mm effective offset.

 

[ElChanclo]

Actually, even the RPF1 18x8.5 ET40 cleared my Riaction coilover, although just barely. Depending on the tire mounted on the wheel, this might be okay. 225 is probably okay. 245 or 255 will have problem. Also, a lot of tires have a protector band that sticks farther out past the rim, and so that can cause it to rub against the coilover. I ran into that problem with Dunlop SportMaxx GT.
View attachment 86893

We no longer run stock struts, but I would imagine the clearances are similar.

So yeah, you can definitely figure that as the tightest you can possibly get away with.

Nope. An ET15 wheel with 10mm makes it +5mm effective offset.

So what offset would you recommend for a 9.5" rim?

 

[Volfy]

So what offset would you recommend for a 9.5" rim?

Rim width doesn't change what the recommended offset should be, since scrub radius is measured relative to the centerline of wheel/tire.

Stock suspension... for street (and occasional track) 32 to 36mm ideally. For AutoX/track only, maybe down to at most 25mm, but that's pushing it.

We only ventured down to 28-30mm because we adjusted the camber top plates on our Riaction coilovers to get more negative camber, thus canting the top of the struts more inward (towards each other). This changed the strut angle line such that the intersection with ground is a few more mm's farther outward. This, in turn, gave our cars a little more negative scrub radius, so we have more leeway in pushing the offset out.

Study the diagram above to see what that is about.

In short, with stock suspension, unless you understand exactly what you're doing, stick close to stock front offset of 34mm.

 

[ElChanclo]

Rim width doesn't change what the recommended offset should be, since scrub radius is measured relative to the centerline of wheel/tire.

Stock suspension... for street (and occasional track) 32 to 36mm ideally. For AutoX/track only, maybe down to at most 25mm, but that's pushing it.

We only ventured down to 28-30mm because we adjusted the camber top plates on our Riaction coilovers to get more negative camber, thus canting the top of the struts more inward (towards each other). This changed the strut angle line such that the intersection with ground is a few more mm's farther outward. This, in turn, gave our cars a little more negative scrub radius, so we have more leeway in pushing the offset out.

Study the diagram above to see what that is about.

In short, with stock suspension, unless you understand exactly what you're doing, stick close to stock front offset of 34mm.

Right, so if you change your offset too far from the stock ET34 then you're upsetting the scrub radius, and no tire width can compensate for that. I'm slowly wrapping my head around the concept.

So if we go with ET25 then a 9.5" WILL fit, since it's "10.1mm closer to the suspension strut" vs stock, which we've established is about the limit for the rim.

And if we want an even closer scrub radius at ET30, then you'd max out at 9.0", which is "8.7mm closer to the suspension strut" vs stock.

Does that sound about right?

 

[Volfy]

Right, so if you change your offset too far from the stock ET34 then you're upsetting the scrub radius, and no tire width can compensate for that. I'm slowly wrapping my head around the concept.

So if we go with ET25 then a 9.5" WILL fit, since it's "10.1mm closer to the suspension strut" vs stock, which we've established is about the limit for the rim.

And if we want an even closer scrub radius at ET30, then you'd max out at 9.0", which is "8.7mm closer to the suspension strut" vs stock.

Does that sound about right?

As far as strut clearances go, here is the way I would look at it:

Assuming my 18x8.5 ET40 was the tightest clearance allowable,

An 9.0" wide wheel would need to be ET34 minimum to clear strut. 40mm - (12.7/2) = 33.7mm. With tire, safer bet would be ET30

An 9.5" wide wheel would need to be ET27 minimum to clear strut. 40mm - (25.4/2) = 27.3mm. With tire, safer bet would be ET25

This was why I said earlier that you could possibly push the front offset as low as ET25 to get a 9.5" wide wheel to fit. Again, from a scrub radius POV, ET25 is outside my comfort level for everyday street driving, but might work for track-only.

So... you are correct that we if play the scrub radius more conservatively, then ET30 is what I think is what you should aim for. That does mean 9.0" wheel is the widest the Stinger/G70 chassis will permit. Although might be tougher to find, I do think it is possible some 18x9 or 19x9 might clear the Stinger Brembos.

 

[ElChanclo]

An 9.0" wide wheel would need to be ET34 minimum to clear strut. 40mm - (12.7/2) = 33.7mm. With tire, safer bet would be ET30

An 9.5" wide wheel would need to be ET27 minimum to clear strut. 40mm - (25.4/2) = 27.3mm. With tire, safer bet would be ET25

Sounds good to me I should stress to anyone reading you NEED to account for how much your tire pokes beyond the back of the rim.


On the topic of Square or Staggered for the track I found a very interesting video:

TLDW: Keep in mind this was tested on a lower HP BMW 320



And some comments on the narrow front factory wheels:

I think the reason why BMW puts narrower tires on the front of the car from the factory has to do less with performance and more with safety. Most road cars are engineered to understeer by design so that they are more manageable in situations that result in loss of traction. Understeer is far less scary than oversteer for the average driver.

225 Front and 255 rear will be less likely to aquaplane than having 255 all round.

 

[ElChanclo]

As far as strut clearances go, here is the way I would look at it:

And what do I need to consider for the rear wheels? There's no scrub radius, but I'm guessing you still want to stick close to factory ET just to keep moment load on the bearings down. Is there a recommend range on that?
If I'm looking to go 18x10.0 on 285/30r18 at the rear, is there anything else I need to watch out for?

EDIT: The rear OEM ET is 46.5 mm which is really odd. Some engineer decided 46 was too little and 47 was too much...

 

[Volfy]

And what do I need to consider for the rear wheels? There's no scrub radius, but I'm guessing you still want to stick close to factory ET just to keep moment load on the bearings down. Is there a recommend range on that?
If I'm looking to go 18x10.0 on 285/30r18 at the rear, is there anything else I need to watch out for?

Scrub radius also applies to the rear wheels. However, since they are not the steering wheels, the effects aren't felt as much as the front. On the Stinger, just about any wheel/tire that fit within the well well will probably work just fine. Now... if you start doing stupid crap like wide-body kit or letting the rear wheels hang out wayyy past the stock fender, then that's no bueno.

For what you are trying to do with the 285 rear, just check out what others have done with wheel offset on similarly sized rear wheel/tire. I haven't done wider tires on this platform, so I have no first hand knowledge. From what I've read, 9.5" wheels with offset around 35-38 seem to be no problem.

FWIW, I am toying around with the idea of getting a couple of 18x9.5 wheels, just to experiment with giving the 2.5T more power handling at the track. But it's not hot priority right now.

EDIT: The rear OEM ET is 46.5 mm which is really odd. Some engineer decided 46 was too little and 47 was too much...

LOL. Dunno why that 0.5mm as necessary. It's not like 5x114.3mm came from 5x4.5".

 

[ElChanclo]

On the topic of how much closer can you get to the front strut:

Just ordered a set of 19x9.5 Niche Misano M116 +35 wheels in Gunmetal. They will be wrapped in 255/35R19 Firehawk Indy 500s
I'm an AWD GT2 at stock ride height and will be adding F&R Eibach sways and ATQ collars as well.

I picked up my rims and tires today. Does not look like the 19x9.5 Niche Misano wheels will work on their own at a +35 offset.
Things spin but I can't get the thickness of cereal box cardboard between the rim and the strut.

That ET35 setup is 20.1mm closer to the suspension strut and barely touching, which is too close, so you'd want ET30 to clear the rim 5mm, then probably another 5mm for the tire lip, so ET25 on 9.5" front rims.

If your tires sat flush or behind the rim edge you could maybe fit 10.0" ET25 rims on the front at 16.4mm closer to the suspension strut.

FWIW, I am toying around with the idea of getting a couple of 18x9.5 wheels, just to experiment with giving the 2.5T more power handling at the track. But it's not hot priority right now.

I'd love to hear how that goes.
I'll be keeping an eye out on the second hand market, but not holding my breath. 18x9.5 +25 5x114.4, full set of 4, nice style, good condition, affordable, within driving rage, is a tall order. I'll start throwing change in the track day piggy bank, should have enough to buy exactly what I want brand new by the time all my 19s wear out

 

[ElChanclo]

I just realized, if you go from the stock 225/40r19 tires to 245/35r18 like I'm planning, you're reducing your OD from 26.1" to 24.8" (663mm to 630mm), dropping the suspension 16.5mm closer to the contact patch.
Using CAD, assuming we start with an "imaginary-40mm of scrub" and the steering axis is 22.5deg off vertical, we would lose 6.8mm of scrub, down to an "imaginary -33.2mm".

SO, you would need to offset the wheel 6.8mm OUT to compensate, which means a LOWER ET, right?

Maybe those 18x9.5 +15mm Enkeis could fit the bill?

Unless the steering axis is a lot shallower, then the tire OD would have less effect.

 

[Volfy]

I just realized, if you go from the stock 225/40r19 tires to 245/35r18 like I'm planning, you're reducing your OD from 26.1" to 24.8" (663mm to 630mm), dropping the suspension 16.5mm closer to the contact patch.
Using CAD, assuming we start with an "imaginary-40mm of scrub" and the steering axis is 22.5deg off vertical, we would lose 6.8mm of scrub, down to an "imaginary -33.2mm".

SO, you would need to offset the wheel 6.8mm OUT to compensate, which means a LOWER ET, right?

Maybe those 18x9.5 +15mm Enkeis could fit the bill?

Unless the steering axis is a lot shallower, then the tire OD would have less effect.

I'm glad to see you're starting to grasp that, in suspension tuning, changing one parameter can affect several others. However, your math is off.

The difference between 225/45 and 245/35 is a reduction of rolling radius of about 4.3mm. 225x0.45 - 245x0.35 = 4.25mm

Working a bit of geometry, this means about 2mm less scrub radius. 4.25 x tan(22.5deg) = 1.76mm

This means an effective LOSS of negative scrub radius, the same as if you have added a 2mm spacer. Take any of the 3 diagram above and imagine the tire cross section being 4.3mm shorter.

So, if you have the stock suspension setup, with a 18x8ET34, it is now more like an 18x8ET32. In and of itself, this is a very mild change, but a change nevertheless. Certainly the sum of the other benefits, like sharper turn-in, quicker steering response, lower ride height (lower center of mass) without changing changing roll center, therefore preserving the natural resistance to lateral weight transfer might prove well worth the slightly lesser negative scrub radius.

Besides, it is something easily compensated for. This is yet another reason I generally prefer buying aftermarket wheels with a bit more offset than stock. So ET38 and ET40 instead of ET35. This gives me the latitude to adjust scrub radius by adding/subtracting spacers, as appropriate. Spacers are cheap, and I have a whole lot of them, 3, 5, 8 and 10mm.

In your case, this is why I advised against getting a wheel with too little offset to begin with. I didn't want to keep throwing variables at you, as it might be overwhelming, but you caught on to this one yourself. Nice!

 

[ElChanclo]

Just ordered a set of 19x9.5 Niche Misano M116 +35 wheels in Gunmetal. They will be wrapped in 255/35R19 Firehawk Indy 500s

I picked up my rims and tires today. Does not look like the 19x9.5 Niche Misano wheels will work on their own at a +35 offset.
Things spin but I can't get the thickness of cereal box cardboard between the rim and the strut.

That ET35 setup is 20.1mm closer to the suspension strut and barely touching, which is too close, so you'd want ET30 to clear the rim 5mm, then probably another 5mm for the tire lip, so ET25 on 9.5" front rims.

If your tires sat flush or behind the rim edge you could maybe fit 10.0" ET25 rims on the front at 16.4mm closer to the suspension strut.

Another wrinkle; I just found the specs of the PF01 rims I had:
18x8.5 +48 / Xice 245/40/18, which calculates to "20.4mm closer to the suspension strut"

I recall they rubbed so hard the car wouldn't reverse on idle down a slight driveway gradient without applying some gas. And I remember it left a mark on the rims too, so that "20.1 closer to strut" clearing by less than a business card was probably only spinning because it was unloaded. I bet if it were loaded down they would have scraped the strut.

I have some bare rims that come out to "7.3mm closer" I'm going to test fit next time and get a caliper reading, see how accurate these numbers are and go from there.