Xeros

Alright I wa sjust reading up on this thanks to a silvia owner who says i shoudl consider far less offset on the 17x9.5 and 17x10 wheels i wa sthinking about getting this is what he says

"I'd recommend going smaller offsets. the recommended for s-chassis' for example are all in the 30's....but we run single figure or o-negative offsets for flushness"

"I don't want it to be plush and the offsets are recommended by road racers that use 7 and the rest of the community. I don't like having it sit that far out of the well. nor do i want to roll the fenders.



o'rly?
have u heard of scrub radius? look into it and its effect on suspension geometry. u may be talkin to the wrong ppl......and all the 7 owners i know are running just as aggressive as myself

but yeah.....i think u should read about it.....it would be good for you."

I did indeed go ahead and read up on got a little confused and then understood it, but I still don't really get what would be the best. Obvisouly he is telling me stay away from negative scrub radius. And I rea don this forum abotu scrub radius as well and it only explain it for a FF car, Now I know the positive scrub will give you more wheel feed back but it might also cause wheel jerkiness and what not. What I want to know is what kinda of srub radius is preferred with FR cars and why. I mean I kind of understanbd but I can't fully picture it...sorry. better yet are the offsets i provided going to help me keep apositive scrub radius...actually I am guess they wontseeign as hwo it will create more negative scrub radius right?

Black91n/a

The scrub radius will change whenever the offset of the wheels do. The designers probably did a good job in selecting the optimal scrub radius, so it'll be best to stay as close to the stock offset as room will allow.

DamonB

http://www.familycar.com/alignment.htm#Scrub%20Radius

rynberg

WTH are you talking to a 240Z driver about offsets on wheels for FDs? Most of the 240Z drivers I see anymore seem to think that a 235 tire stretched on a 10" wide wheel that sticks 1/2" past the fender = good handling.

maxcooper

With the stock +50 wheels, the scrub radius is supposed to be about 0 on the FD. Getting aftermarket wheels for the FD with close to a +50 offset is a good idea. Note that scrub radius is related to the center of the tread, not to how far the wheel sticks out. A 10" +50 wheel will stick out 1" further than an 8" stock wheel, but the scrub radius will be the same.

-Max

Xeros

Rynberg, this is the reason I brought it up here. I am very much in doubt as to what he says with what they do with their cars. The guy seemed fairly knowledgable but it was questionable as well becasue he is mainly a drifter. Just try to understand I listen to everyone and take into account what they have to say think it over research it ask some more questions about it and learn from it. 240 guy or not. and yes I understand that the scrub radius refers to the center of the wheel, thats why he was telling me what he was. obviously if I put wider wheel in the front the center line changes slightly. Thus so does the scrub radius. and DamonB thats the exact sight i was looking at last night ahaha.

maxcooper

Note that the distance between the center of the treads on the front tires does not change because of the width of the tire. It will change if you change the offset of the wheels.

I suspect that wheel diameter can also change the scrub radius. But I don't know how to calculate the change. The effect is probably not very large.

Different cars have different suspension geometries. The right offset for one car might not be right for another car. Your friends advice to not change the scrub radius much was good advice, but it sounds like he isn't sure what offsets maintain a good scrub radius on the FD.

-Max

Xeros

I know the distance between the center of tires don't change do to wheel width but what I ment was when someone gets wider wheels up front and refers to the offset numbers provided by Jim Lab's table technically you move the scrub radius more to the negative side becasue your pushing the center line of the tires closer to the chasis. (Sorry I'm not always that good with getting my thoughts across in words when it comes to it). Is there a way to calculate what would a be a good scrub radius or something I can at least base my changes on that would allow me to decide a good offset for the FD.

If I go with the provided off set adjusting the camber on the wheels would also allow for some scrub radius adjustments right? If I make the camber of say 3.5 to 4.0 degrees negative up front the center line of the tire will shift outward will it not? Which would cause more positive scrub radius.

But too much positive scrub radius will also cause too much feed back and If I brake hard and the balance in brake sis uneven or something odd happens the wheel could rip right out of my hands is what it says.

It just doesnt seem right at all to go with no off set or run in the single digits. that and I dont want my car to look all foul with my front wheels sitting flush with or beyond the fender.

What woudl you say a good offest is to maintane a good scrub radius on the FD is?

alberto_mg

Someone correct me if I'm wrong, but with a 9.5" wide wheel up front your choice of offsets are very limited. Probably within a couple of millimeters. Hence your worries about scrub radius are somewhat irrelevant.

I'm running a 17x9 up front with a +45 offset using H&R springs and 255/40/17 tires. I know of a few people rubbing with 17x9 +40 offset running the same size tires.

Go ahead and read up on scrub radius to learn what it is, but keep it in context of your overall parameters.

Xeros

Sorry but the problem came up as hand in hand situation. larger offset will reduce rubbing but possibly screw up my srcub radius.

rynberg

No, it won't! The scrub radius is determined by the distance between the SAI and the CENTER OF THE WHEEL. If you have 20" wide wheels with a +50 offset, the scrub radius will be exactly the same as the stock 8" wide wheels with a +50 offset.

Besides fitment issues, it is IDEAL that any aftermarket wheels you use have a +50 offset. This is compromised due to fitment and available wheel choices, but that doesn't change the fact that the closer to +50, the better.

Xeros

Wait a 20in tire will have the contact patch inside the SAI would it not simply due to it's size? The radius is determined by the distance between the center of wheel and the SAI. Okay but if the contact patch is inside the SAI it becomes a negative scrub radius. Thus the 20in wheel would have a huge negative scrub radius if I am reading the diagrams and reading all the facts right. Such as the link DamonB sent me. That wheel technically has a positive scrub radius becasue the contact patch is outside the SAI pivot but your 20in tire would be so far in thanks to its size and offset thus givign it a huge negative scrub radius becasue it is within the SAI pivot....Or am I worng. Becasue that would also mean the two wheels would not have the same scrub radius. UUuuuuggghhhh im confused now, it says on the site that the determining factor if the scrub radius is negative or not is IF the SAI is inside the contact patch or not, and not the distance between the middle of the tire and the SAI. I am confused. Becasue if negaitivity or positivity is determined by that distnace than yes, rynberg is absolutely right with what he said which makes sense to me as well, and thats what I thought at first but if it isn't then wtf....

rynberg

How does a tire have more contact patch to the inside if it is a +50 offset wheel? It would have the same contact patch on both sides of the centerline! Unless I am grossly misunderstanding things, a +50 offset wheel of ANY width will have the same scrub radius as with the stock wheels (barring small variables such as camber, etc).

maxcooper

You are not understanding wheel sizing and offset correctly. The offset determines where the center of the tire is relative to the car. The width has nothing to do with it.

Also, smaller offsets move the center of the tread AWAY from the center of the car.

An 18x10" wheel with a +50 offset puts the center of the tire in the same place as the stock FD wheels (since they are also +50). I have a set of 18x10" wheels with an offset of +50 or +51 for front and rear. They fit fine and the scrub radius is very, very close (within 1mm, I suppose, based on the offset) to stock.

-Max

Xeros

No I understand wheel sizing and offset perfectly we are just saying it two diffrent ways, your saying offset moves the center line of the wheel, but as it does that it also moves the entire rim back when you give it a greater offset, which moves the "hub" of the rim forward and the rim itself back towards the center of the car when the wheel is mounted. like thispicture shows http://www.discounttiredirect.com/im...eel.offset.gif What is messing up my understanding of the scrub radius is this, a direct quote form the site damonB posted that I had been looking at earlier that night and from my understanding it says negativity in scrub radius or positivity is determined where the SAI is touching the tire contact patch. "Positive scrub radius is when the tire contact patch is outside of the SAI pivot, while negative scrub radius is when the contact patch is inboard of the SAI pivot " Now if they mean the center of the wheel is that contact patch then okay everythign is cleared up and you guys were right all along and I understand but if they mean anypart of the tire that which is the contact patch then no....I'm soo sorry you guys. This is driving me a little crazy now because I want a good explination on the subject.

Xeros

Okay I've cleared it up with soem more research and reading, it is indeed referring to the contact point of the centerline of the wheel.

"The scrub radius is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. If these two lines intersect at ground level, then you are said to have zero scrub. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively."

off this H&R site http://64.233.167.104/search?q=cache...adius%22&hl=en

Sorry about all this you guys once again thanks for your help Rynberg and maxcooper

wanklin

iTrader: (2)

A taller tire will decrease your scrub radius. It can be caculated using basic trig but you would need to know the included angle and the increase in tire radius. If someone is bored and knows the SAI they could make a spreadsheet but it's probably not worth anybody's while. I'd say that a couple mm offset per inch height increase is probably what you'd be looking at to keep zero scrub.