Effect of aggressive offset on performance
 

I just replaced my oem wheels with aftermarket 18x9 +30 wheels (245/40/18 tires). On my first 2 drives, the first thing I noticed was that the steering felt heavier and the car didn't feel as nimble. Did I sacrifice looks for a slight decrease in performance?

(oem wheel/tire weighed 48#; aftermarket wheel/tire 49# if that matters.)

yes, but ur wheels are also wider and tires are bigger

Yeah!You sacrificed performance, money, and looks. Try again!

Wider tires tend to resist change in direction more than narrower. You also have a bit wider stance with the +30 offset and 9" width.

The downside is more steering effort (less 'nimbleness'?) but the upside is the add'l contact patch providing increased road adhesion for spirited turns.

All design is compromise.

I feel with the wider stance; turns are more stable, and I get less body roll. Also at high speeds above 120mph the car seems more stable.

Scrub radius.

Bingo. Contact patch is further out from the axis the steering rotates on.

^^^this man speaks the truth.
It is possible to have wider wheels and have less steering effort than what you're experiencing right now as long as your scrub radius is the same as stock. Search for scrub radius or kingpin offset.

Yea scrub radius with a wider tire will play a huge roll. I noticed it when I switched wheels and I'm sure I'll notice again with the new setup. Just something to get used to.

thx guys! I read up on "scrub radius" and I kinda understand it.

Took the new setup thru some rolling twisties and it does feel more stable....don't know if I'll be doing any 120mph runs anytime soon, though....

As long as you aren't cone wrangling and your clearance is fine, the difference won't matter a whole lot.

It's funny that people always automatically say it "decreases" performance but that's not really the case.

Want that sensitivity back? Add some toe out; alignment can offset some downsides of scrub radius.

Ditch them monster truck tires and try stretched tires

Another thing you'll notice is your current speed on your speedometer isn't representative of your true speed now. I'm 'slower' by 3 mph now, lol. I first noticed it on those radar units the city puts on the hwys to let you know what your speed is.. I then had a cop friend shoot radar at me coming down the street.

I'd actually like someone with racing experience to chime in. From what I've gathered in the past is the only real affects are more effort in turning the wheels, greater tram-lining effect/more sensitive under load/braking.

However, I've noticed some say it "screws up our suspension geometry." Aside from those things above, what does positive/negative scrub radius really effect?

I can't say from personal experience on this car, but I can on a Formula SAE car. Scrub definitely will affect the way the car feels...more steering effort. Now, as far as adverse handling effects, it can. Can't say for sure on this car, because I don't have a baseline. But, typically the more caster the more the effect of offset will be due to the wheels being further out and "swinging" more along the steering axis.

I got a little lost, are you commenting that it can increase how a car feels due to the greater effort needed for steering, or that there is/are other adverse effects?

I've read and re-read posts/articles for the past year but due to a lack of real, applicable, experience (aka I don't really know what I am doing) I can't come to a conclusion. For those of you who want to read what others have to say, skip google and check these links out ->

http://www.ten-tenths.com/forum/showthread.php?t=104575
http://www.hawaiitalks.net/automotiv...ub-radius.html
http://www.hrsprings.com/technical/scrub_radius

Like I said, from what I gather, the real potential downsides are greater steering input + increased sensitivity on hard braking/accelerating and potentially, if something were to go wrong with one of the tires/contact patches, the wheel being thrown out of one's hand. I guess my post is redundant with my previous one but how does this adversely effect our "suspension geometry"? I believe I recall TopGear8 and EricMeyer, as well as others, claim this, all I want is a proven explanation/clarification :D

edit: Tire poke creating aero issues is a whole other topic, though slightly relevant. That one seems a bit more self explanatory :)

http://www.desertrides.com/reference.../sai-scrub.gif

The longer the scrub radius (lower offset wheels) will add to the steering effort. When steering, the contact patch of the tire will "swing" or rotate around the kingpin axis, which is the reason why there is increased steering effort. The longer the scrub radius, the more the effect is enhanced. We aren't talking about ripping the wheel out of your hands with a 20mm offset change, so don't worry about that.

Scrub is part off the suspension and steering geometry so thats why they are saying it'll change that. Sometimes the increased track (width) is worth the change in scrub, sometimes not.

Its all based on the application.

I've totally got the concept down, though your explanation is very clear for those who don't. What I was wondering about, though have never posted, is why so many insist on posting that the change in scrub radius "ruins the handling of our car." Many have posted similarly, as the beginning of this thread implies.

I can understand autocross as being affected but otherwise, would it be prudent to say that a larger scrub radius doesn't necessarily have adverse effects on the overall handling of the car itself?

The lower offset moves the contact patch out further when going straight, but dynamically changes the contact patch while steering.

So, imagine looking down at the contact patch while turning left. The right tire's contact patch will "wing" around the steering axis and move towards the front of the car. Vise versa for turning right. That is assuming that most of the width of the tire stays in contact with the road.

Now does it have bad effects? Its all application based. For your application for mainly street driving, it probably won't hurt anything at all.

Its just like when drifters put steering kits in so they can manage camber and scrub radius at full lock.

Unless you are very competitive in Auto-X or Road Racing, you won't notice the 20mm difference, Hell I don't even think you'd notice a 40mm difference(the average person that is)

I think that sometimes when you read that a change "ruins" the handling, they are talking subjectively, not objectively.

Mazda goes to great lengths to make the car feel in a certian way. Scrub radius (on the older Miata, they targeted a zero scrub radius; not sure about the RX-8/NC) is one of those variables they try to optimize in order to get a certain combination of feel and performance.

titlmode43 mentions "more effort in turning the wheels, greater tram-lining effect/more sensitive under load/braking" and too many, screwing up those attributes is tantamount to ruining the handling. To others, it's a small price to pay for looks and/or wider wheels & tires.

Now, wether or not an extra 10 mm of scrub radius/offset, all other things being equal, will result in a slower lap time on a given circiut is something I can't answer. As others have stated it probably depends.

I just installed 18x9 +55 wheels, using 265 Bridgestone RE11s, and on my first autocross noticed an increase in oversteer compared to my older wheels (which I think were +45). Still not sure if this was a function of the surface I was running on, or maybe the tires are not fully broken in, or perhaps the +55 wheels and/or shorter sidewalls are giving quicker turn-in and overloading the rear tires. Need more testing to understand.

Ding ding ding, George, I think you're the first person tht understands what my question was! I'm aware it won't have any obviously noticeable affects given I don't seriously compete and I also am aware that it won't really harm anything (hold wheel bearings!) as long as clearance is fine.

Perhaps I phrased the question incorrectly; I should have said, "Theoretically, will a large gain in scrub radius theoretically change lap times noticeably?". Again, thanks for the input everyone, further discussion/POV are always welcome.

Side note: Fate - you're planning a built re-w swap but you bought rotas? Really? What happened to the CCW?

interesting topic I've been wanting to know more about.. subscribed

In autocross, I do believe a *large* increase in scrub radius (i.e., a wheel with a small positive offset such as +25 or smaller) would adversely affect lap times due simply to the fact that you are more likely to hit cones. There is a strong correlation between slalom speed & vehicle width - the wider the car, the slower you are through the slalom, according to some rather experienced & well published autocross testers. So, pushing the wheels outside of the body can have detrimental affects. Of course, if the extra width comes from wider wheels & tires as opposed to just a small wheel offset, then the trade-off is likely worth it.

On the flip side, weight transfer is inversely proportional to track - so, a narrower car works it's outside tires harder than a wider car, which means that the wider track should pay dividends in sweepers.

Of course, these issues have nothing to do with scrub radius - just width. But it shows how hard it can be to nail down a performance problem to one variable.

I have a theory that too much scrub radius make our car understeer in a tight turn, but it's just a theory. I hope to be able to test that theory this season.

I suspect the biggest problem with a large scrub radius on the track for a RWD car is simply stability at high speeds - if the car isn't easy to drive fast, it probably won't be.

My .02, all subject to question, particularly by somebody who has actually tested the tradeoffs.

I agree absolutely with the autox comments, it only makes sense.

As far as stability and turn in, I'd imagine one could finesse damper stiffness/bias, sway bar stiffness, tire profile, and toe to allow for a crisp neutral turn in while maintaining high speed stability. Then again, maybe I'm just being overly optimistic :D: