Affect of wheel width and offset on stock suspension feel

[Tokay444]

Originally Posted by TommyW

Stickier tires cause more body roll so it’s advisable to lower the car

You think lowering the car reduces body roll?

[NoHaveMSG]

Originally Posted by Goingnowherefast

My guy, what are you talking about?

Are the T4 road racing rules different then TT?

I'm reading this off their build thread.

The 17x7" wheels required by the rules are too narrow for 245 tires. I should have started with 225 or 235s. I just ordered a set of 225/40/17 Hoosiers today which I'm hoping to find time on. I'm comfortable and experienced on HoHos and I should have just bought them from the start. I may test some 235 BFG's in a couple weekends but it's likely that i'm going back to Purple Crack.

[Goingnowherefast]

Originally Posted by NoHaveMSG

Are the T4 road racing rules different then TT?

I'm reading this off their build thread.

Yes. The road racing rules are different than the TT rules. The proof is in the pudding. The road racing rules are setup so that multiple cars can be balanced to the same potential times which creates quality racing. Let's break it down (talking about road racing rules now):

-T3 is a faster class than T4 by definition
-BRZ is required to run 17x7 w/ 225 to slow it down to fit in T4
-The faster T3 class allows BRZ's to run 17x9 w/ 245's

Result: It is nationally recognized that 245's on a 9 inch is faster than a 225 on a smaller wheel. There is really no debate here.

Source: https://www.scca.com/pages/cars-and-rules

[NoHaveMSG]

Originally Posted by Goingnowherefast

Yes. The road racing rules are different than the TT rules. The proof is in the pudding. The road racing rules are setup so that multiple cars can be balanced to the same potential times which creates quality racing. Let's break it down (talking about road racing rules now):

-T3 is a faster class than T4 by definition
-BRZ is required to run 17x7 w/ 225 to slow it down to fit in T4
-The faster T3 class allows BRZ's to run 17x9 w/ 245's

Result: It is nationally recognized that 245's on a 9 inch is faster than a 225 on a smaller wheel. There is really no debate here.

Source: https://www.scca.com/pages/cars-and-rules

Well there we have it. I'll shutup now.

But probably not

[TommyW]

Originally Posted by Tokay444

You think lowering the car reduces body roll?

it does. You can do other things also however stock suspension with good tires and you’ll get more roll. I drove one like that and I can attest to it.

[RuyGuy2]

Originally Posted by TommyW

it does. You can do other things also however stock suspension with good tires and you’ll get more roll. I drove one like that and I can attest to it.

Lowering your car Lowers your "roll center". This will cause a greater distance from your "center of gravity" to the roll center causing more body roll in our cars.

There is a simple fix for your roll center. https://www.ft86speedfactory.com/bud...l#.Xp3oFchKjcs

Here is a good video explaining what i'm talking about.

[RuyGuy2]

Originally Posted by Ohio Enthusiast

I'm planing on buying lightweight wheels to use with summer tires, and have narrowed down the choice to the following:

1. Enkei RPF1 17x7.5 ET48
2. Enkei RPF1 17x8 ET45
3. 949 Racing 6UL 17x8 ET40
4. Konig Hypergram 17x8 ET40

Weight and price wise they are all in the same ballpark, and I'll be okay with either. The plan is to use these with stock tire size (215/45R17), probably Michelin Pilot 4S or Continental Extremecontact Sport..........

I run 16x7 with 225/50 tires with a 35 offset. On a household digital scale it says I'm 9 pounds lighter holding the new rim and tire vs. the old rim and tire. I noticed a significant performance increase in braking almost 10 feet drop from 60-0. I also dropped about .2 from 5 to 30 mph acceleration.

Here is some food for thought when you're looking at offset.

[Tokay444]

Originally Posted by TommyW

it does. You can do other things also however stock suspension with good tires and you’ll get more roll. I drove one like that and I can attest to it.

Lol. What happens to the roll centre?

[Ohio Enthusiast]

Originally Posted by Goingnowherefast

Generally a slight stretch will give you better initial turn-in and steering response at the small expense of a marginally harsher ride.

In reality, the tire you select will have magnitudes larger effect on vehicle dynamics (turn-in, steering response, general NVH) than splitting hairs on 10mm of section width. Just choose a quality tire (the Michelin P4S and mentioned Conti's are the best street centered tire that money can buy) and be done with it.

I'm wondering what has the bigger impact on steering feel (again, not outright performance)? I understand that MP4S or CECS would significantly change the feel of the car vs. the OEM Primacys, but how would the difference between 7.5" and 8" wide wheels have on the same 215/45R17 tires? I.e. if going from Primacys to MP4S give a 5x impact, would 7.5" vs. 8" on the MP4S have a 1.1x impact?

Originally Posted by Tokay444

Wheel offset has a huge impact on scrub radius, which has a huge impact on steering feel.

I've read about scrub radius before, and now read some interesting threads here about it (specifically for the Twins). Looks like it's more complex, as lower wheel offset also widens track with its own impact on steering. Throw in camber adjustment (with a difference between lower alignment (camber bolts) and upper alignment (camber plates)) and my head is spinning

I think I'll trust the Toyota and Subaru engineers and stick with the OEM offset and wheel width (i.e. RPF1s at 17x7.5 ET48), although I understand that the differences with a 17x8 ET40 would be fairly minimal and I'm probably over thinking this.

[RuyGuy2]

Originally Posted by Ohio Enthusiast

I'm wondering what has the bigger impact on steering feel (again, not outright performance)?.

Are tires the only modification you're going to make or have made?

Are you looking to get more steering feel or just maintain the stock steering feel?

The wider your tire is away from the Hub its like having a longer arm on a wrench. Wider tires do to offset=more feedback heavier steering feel.

That being said half a Degree of Caster will effect your steering feel more than wildly modifying your track width with wheel offset.

[churchx]

scrub radius increase - via offset or spacer spaced out wheel&tire center line from wheel turning axis.
If they are somewhat same, there should be no illeffects, the bigger the difference, due now longer "lever" from tire center to turning axle there are heavier forces that do such things as trying to return turned wheels (=heavier steering), wheel position easier to be pushed out of place by road pavement defects, or engine torque turning stronger wheel on FWD cars, resulting in torque steer, and there is higher load (thus wear) on wheel bushings due forces applied from side.
All that sounds not that good, but imho extent of illeffects is not that big of, if you space out/increase scrub radius to reasonable extent, eg. upto OE +10/15mm. Of course, if you have wide body build, insane reduced offset wheels or barrel like spacers, illeffects should be there more then noticeable.
Due that scrub radius being between wheel center line and turning axis, not from wheel outer side, simply wider wheels (but of same offset) don't change scrub radius (but if goal is to run wider wheels & tires, not just space out, at some point there will be clearance/rubbing issues between tire inner side & strut).
With your mentioned wheel sizes, where both OE & that RPF1 offset (thus centerline) is same, 48mm, scrub radius will stay same. So imho scrub radius increase of 13mm for eg. ET35 wheels, while maybe noticeable, but not that bad off to be unusable wheel combo imho. People for flush fitment sake of OE 17x7 ET48 wheels often fit in front eg. 20-25mm spacers. Half of that imho is still reasonable.

[Ohio Enthusiast]

Originally Posted by RuyGuy2

Are tires the only modification you're going to make or have made?

Are you looking to get more steering feel or just maintain the stock steering feel?

The wider your tire is away from the Hub its like having a longer arm on a wrench. Wider tires do to offset=more feedback heavier steering feel.

That being said half a Degree of Caster will effect your steering feel more than wildly modifying your track width with wheel offset.

Currently the car is fully stock. I'm planning on lighter wheels, gripier tires and camber bolts for mild camber (most people get no more than -1.5 with bolts, right?). Future plan might include coilovers, but only if I find a need for them (i.e. if they actually improve feel over stock - I'm trying hard not to go to the trap of aftermarket parts just for the sake of modification). I very much prefer feel and enjoyment of the car to performance - even if I'll track or autox, I'd do it just for the fun of it, not to set records.

I understand that each person's definition of "fun" with a car are different, and honestly I haven't driven the BRZ enough yet to really consolidate my goals. comparing to the Focus ST I had before, I enjoy medium radius turns the most (i.e. not hairpins you take at 15 MPH, nor sweepers you can take at 100 MPH, but rural/mountain/canyon road curves you take at 40-60 MPH). In the ST I would occasionally reach the limits of the tires (mostly in the wet on turns I'm very familiar with that have plenty of margins), and that had its charms, but I'm not a good enough driver to do that consistently on the public road.

[Capt Spaulding]

Originally Posted by Ohio Enthusiast

Currently the car is fully stock.[COLOR="Red"] I'm planning on lighter wheels, gripier tires and camber bolts for mild camber (most people get no more than -1.5 with bolts, right?). Future plan might include coilovers, but only if I find a need for them (i.e. if they actually improve feel over stock -I'm trying hard not to go to the trap of aftermarket parts just for the sake of modification). I very much prefer feel and enjoyment of the car to performance - even if I'll track or autox, I'd do it just for the fun of it, not to set records.

I understand that each person's definition of "fun" with a car are different, and honestly I haven't driven the BRZ enough yet to really consolidate my goals. comparing to the Focus ST I had before, I enjoy medium radius turns the most (i.e. not hairpins you take at 15 MPH, nor sweepers you can take at 100 MPH, but rural/mountain/canyon road curves you take at 40-60 MPH). In the ST I would occasionally reach the limits of the tires (mostly in the wet on turns I'm very familiar with that have plenty of margins), and that had its charms, but I'm not a good enough driver to do that consistently on the public road.

As we talked about earlier, if you make the changes in red the feel of the car will most definitely change. The steering will be marginally heavier, you should notice substantially less understeer and the maximum grip level will increase as well. It will feel more "serious" and less playful.

A couple of years ago my wife and I drove our stock car from the borderlands of Texas to DC. On the way, did the Cherohala Skyway and the Dragon. On both the stock tires screamed like gutted pigs. I pushed them a little past the scream point, but my wife was leaving finger prints in the door handles and turning an ominous shade of green, so I dialed it back to 6 from 7.

The car as it sits now would have been substantially faster. As to the green changes. I am uncertain. For me, that takes the car away from the direction I want to go. Tracking is fun, but that is not what I bought the car to do and I'm probably too damn old to be doing that anyway. I bought the BRZ as a poor boy's GT. In stock trim it worked well in that role, and in its current state, it works better.

My advice is to figure out what you want first. Then start planning your modifications. It seems to me that's what you're doing here. That's great. It'll help you avoid the blue trap.

[RuyGuy2]

Originally Posted by Ohio Enthusiast

Currently the car is fully stock. I'm planning on lighter wheels, gripier tires.......

That's exactly how I use my car. My preference is a wider Offset rim and wider Tire. Going Wider with less of a sidewall ratio won't effect steering feel that much just increase lateral grip. Also a 215/50r17 and a 235/45r17 Both have a 4.2 Inch sidewall.

I drive 65 miles round trip for work every day. I went down in rim size to a 16" and Up to a 225 tread width and 50 sidewall. I got used to the steering feel in the first 20 minutes not a huge change, to be honest I can't remember what it really changed in the steering feel on the car it was so small.

Cornering, braking, and acceleration all improved quite a bit. I have ST X coil overs which in my opinion ride nicer than the factory shocks. If you lower your car get a roll correction kit. My preference is buddy club, but Whiteline makes a good kit too I'm told. The coil-overs didn't make much difference till I put the roll-correction ball joints in.

With my setup the corners I was taking at 35 I can go around at 45 now and it feels much more stable way less over-steer way more predictable. I know if I had 17" wheels with a 235 tire the car would more lateral grip on a perfectly flat surface. I had a set of 245 width tires on 18" rims on an RX8 and I would loose a lot of traction do to the surface of the roads I drive on. Not to mention in the winter time the additional sidewall is much better in snow.