How much positive scrub radius until screwing things up?
 

Many was written here about scrub radius change with different ET rims / spacers, but mostly there are fears and questions than answers...

In scenario, where you want to use lets say stock tires but you hate hate hate the tucked in setup, how much you can go until things go badly? I don't want to make the car worse... Will 10-15mm increase cause problems with handling/braking/leverage/power steering/feedback/safety/etc?

Thanks if anyone know

curious about this as well

Negative effects of excessive scrub radius are rarely written about.

Quite a good explanation:

http://www.pistonheads.com/gassing/topic.asp?t=1004532

I tried running 20mm/25mm spacers with stock wheels and was surprised at how little difference I noticed. I believe this is largely due to the stock tires being rather narrow and low grip.

In theory, a substantial change like that should be immediately noticable, but in reality it didn't seem to be the case. One additional concern is bearing load, but with all the aggressive/wide wheel fitment and sticky rubber, and stance etc, bearings don't seem to be a widespread problem.

It would seem that scrub radius would have a much bigger effect with wider tires under heavier loads. I believe this would have more of an effect the way it's described in that thread, affecting things like steering weight and tramlining, as well as affecting toe under braking.

Do we know the OEM scrub radius?

That's because it's typically mental masturbation.

Does it affect things? Sure. Is it particularly noticeable? Not really.

okay

on the street or on the track/autocross

Street, canyons, and track.

My BRZ is my daily driver, mostly driven in town. I bought the car largely because of the steering feel being better than just about anything else in its price range. With some trepidation, I switched from the stock set up (+48 offset) to 225 MPSS on 17x8 +35, so a 13mm difference in offset/scrub along with more grip and more tread width, but not huge differences. I really do notice a difference in steering feel and weight. My sense is that the bulk of the additional steering weight, once up to speed, is from the offset. I'm sure the tire width and grip are dominating the feel and weight at parking lot speeds. (Three point turns are a workout!) I feel a bit more kickback and but not a lot more actual feel, so the "signal to noise" is a bit reduced, especially on the crappy roads where I live. I'd love to try the same tires but with +48 offset rims. Too bad no one makes negative thickness spacers...

Overall, I much prefer the grip and intensity of the MPSS tires over the Primacies on the street and I am willing to live with the slight increase in kickback (and big increase in noise), especially since my particular wheel/tire combination is a few pounds lighter than stock at each corner and didn't cost and arm and a leg.

I personally wouldn't want to go much further outboard with the tire patch on the street. On track might be a different story, but not my area of expertise.

I love you. Seriously.

Your constant snarky questions and condescension whilst never providing any of your own input is awesome.

Yikes, really? 9.5 ET 38 with 255/35 Direzza IIs.

If you're finding parking lot turns a workout, I guess hit the gym? I have no problem using one hand to get around, or are you just exaggerating for effect?

My Miata with a depowered rack on 205 Rcomps was an order of magnitude more difficult to get around the paddock then the BRZ on wide sticky rubber.

I provide plenty of input, you just have selective reading.

and if I had a shop and all manner of testing equipment I'd input even more.

but dismissing this discussion as "mental masturbation" makes you the only snarky marvin around here.

What testing equipment do you need? Are you going to put a string pot on the steering column and start parsing individual foot pounds of torque necessary to make the wheel reach 45° at X speed with different offset wheels?

Stock Miata wheels 14x5.5 ET 45, there was no discernible change in effort at speed going to a 15x8 ET 25 wheel.

My point is, if you can't feel the car behaving differently, pointing out that it changed 1.38%.......what does that get you? What help has it provided?

Maybe just a little! :) Seriously, though, we have a very long, narrow driveway, between our house and a fence, that I don't like backing out of, so I usually end up doing a 5 point turn in the back where it is a bit wider. Cranking the wheel from one stop to the other while stopped with the MPSS on tumbled interlocking brick is a minor workout! If I only turn the wheel when the car is moving, it ends up being a 9 point turn...