Effect of scrub radius?

Hello,

Simply put, how big of an effect in handling does changing the scrub radius make in our car?

For the more specific questions, I drive a 2018 Civic Hatchback Sport Touring with the stock 18x8 wheels with +50mm offset. As I understand it from reading other sources online, FWD production cars are generally equipped with a slight negative scrub radius because they are more predictable/stabilizing in straining (steering) situations. I was looking to upgrade my wheels and the options I've boiled down to are 18x8.5 wheels with a +45mm or +35mm offset. With the aesthetics (flush/fitment) as a secondary concern (secondary to handling/safety) in mind, I do want to upgrade but I don't want to mess up the original suspension geometry by too much. But in terms of looks, I do like the looks of the +35mm offset wheel a lot more than the +45mm one. With that being said, my questions are:

1. Is our car produced with a negative scrub radius from the factory?

2. How big of an effect would a positive increase of +5mm vs. +15mm in scrub radius make in our car? Would those effects actually pose a hazard in emergency situations?

3. What are some options to negate scrub radius increase or offset the negative handling differences of a positive scrub radius? For example, I would imagine that lowering springs could decrease the scrub radius by sending the KIA line out and could theoretically negate the scrub radius increase of a smaller-offset wheel.

4. If I do go with the +35mm offset wheel (i.e. more positive scrub radius), what are some of the steering/handling characteristics I will need to keep in mine as I push the car?

I’ll comment on this since I just changed my wheels and I’m experiencing the change in scrub radius.

Right now I’m running 18x8.5 wheels with +35 offset on the stock 235/40R18 Sport Hatchback tires. The positive scrub radius has had some benefits and drawbacks to my driving experience.

Pros:
The car feels much more stable at all speeds, but especially at high speed cornering. This is aided by other mods I’ve done, but my shocks and struts are stock. I drive spiritedly on SoCal freeways that curve a lot. Even in excess of 80 mph the car is firmly planted and stable. I’ve yet to find the limit of how fast I can go before I lose confidence. It’s above 100 mph for sure. There’s also less lateral shifting of the car’s weight. She’s pinned down good. Even a night of super high winds we had a couple weeks ago couldn’t shake her like it used to. Traction seems to have improved.


Cons:
The car no longer has the snappy handling it used to. On the stock wheels I was able to lightly turn the wheel and the car went exactly where I wanted it to. Now with more scrub it takes a bit more effort/attention to get the car in the right spot. Sharp cornering has also become slightly tougher as the car sticks into the turns more and understeers a bit. I will say as time has gone on, either the car has settled into the new wheels or I’ve become more adjusted to it. It’s not as dramatic of a difference now as when I first had them installed.

Now this is probably due to my new wheels being 10-11 lbs lighter than the stock wheels, but I can’t “feel” the speed of the car as much anymore. Going 80 mph feels like I’m doing 50 mph. This has robbed the car of a bit of its character. It feels more like I’m driving a BMW or Audi and less like I’m driving a punchy Japanese car. This isn’t necessarily bad, it’s just taking some getting used to.


Overall, how do I feel about it? Pretty good. I have to adjust my driving style a little, but the amount of stability at high speeds and hard corners outweigh losing some of my steering feel, which should be correctable with a strut bar any way. I’m thinking an 18X8.5 wheel with a +40 offset is probably the sweet spot for the hatchbacks. If you value being able to quickly maneuver more than being able to turn at silly speeds, then keep the offset closer to stock.

Feddy_Ace said:

I’ll comment on this since I just changed my wheels and I’m experiencing the change in scrub radius.

Right now I’m running 18x8.5 wheels with +35 offset on the stock 235/40R18 Sport Hatchback tires. The positive scrub radius has had some benefits and drawbacks to my driving experience.

Pros:
The car feels much more stable at all speeds, but especially at high speed cornering. This is aided by other mods I’ve done, but my shocks and struts are stock. I drive spiritedly on SoCal freeways that curve a lot. Even in excess of 80 mph the car is firmly planted and stable. I’ve yet to find the limit of how fast I can go before I lose confidence. It’s above 100 mph for sure. There’s also less lateral shifting of the car’s weight. She’s pinned down good. Even a night of super high winds we had a couple weeks ago couldn’t shake her like it used to. Traction seems to have improved.


Cons:
The car no longer has the snappy handling it used to. On the stock wheels I was able to lightly turn the wheel and the car went exactly where I wanted it to. Now with more scrub it takes a bit more effort/attention to get the car in the right spot. Sharp cornering has also become slightly tougher as the car sticks into the turns more and understeers a bit. I will say as time has gone on, either the car has settled into the new wheels or I’ve become more adjusted to it. It’s not as dramatic of a difference now as when I first had them installed.

Now this is probably due to my new wheels being 10-11 lbs lighter than the stock wheels, but I can’t “feel” the speed of the car as much anymore. Going 80 mph feels like I’m doing 50 mph. This has robbed the car of a bit of its character. It feels more like I’m driving a BMW or Audi and less like I’m driving a punchy Japanese car. This isn’t necessarily bad, it’s just taking some getting used to.


Overall, how do I feel about it? Pretty good. I have to adjust my driving style a little, but the amount of stability at high speeds and hard corners outweigh losing some of my steering feel, which should be correctable with a strut bar any way. I’m thinking an 18X8.5 wheel with a +40 offset is probably the sweet spot for the hatchbacks. If you value being able to quickly maneuver more than being able to turn at silly speeds, then keep the offset closer to stock.

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Thanks! So from your experience, it sounds like the understeer could be offset a bit by a rear sway bar and the dull steering feel could be offset by a front strut bar?

pkxeon said:

Thanks! So from your experience, it sounds like the understeer could be offset a bit by a rear sway bar and the dull steering feel could be offset by a front strut bar?

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I’m running a 22 mm sway bar, so the understeer is there even if you have it. I also have the lower subframe brace from the Type R and it’s tough to say how that affects everything now, but the brace did correct my oversteer before I got new wheels. It’s likely a factor in the current understeer I’m experiencing. If you don’t have this mod, then you’ll be ok with increased offset and RSB combination.

A front strut bar may or may not be necessary. Some people like the tighter steering feel, but some people end up disliking the change and get rid of them. I think I’ll keep driving for a couple months and before I decide on installing one. If my car still rides like a luxury sedan at that point I’ll have to make some changes lol