School me on scrub radius

So I've read a few articles on it, I've watched the (always awesome) Jason Fenske on EE explain it on youtube. But I am left with a few questions that people smarter, and with more experience, than me could hopefully answer.

Stock Austins leave us with 0mm scrub radius. I have stock suspension right now. I have been tracking with using 235/40R18s on an 18x8 rim w/ et35. So, using online scrub calculators, they say that my wheel set-up increases my scrub radius to a positive 13mm. I think I feel an increase in steering effort with the track wheels on (vs my winter tires on Ausitns) but it may just be the stickier tires...but again, I don't know enough...so

1) How will a positive 13mm effect the car's handling?

2) Think it will effect turn in feel notably?

3) Is 0mm scrub radius the ultimate goal for track driving set-ups?

4) How can I determine how 034 camber plates (-1.5ish...its still my DD...not going to full adjustable set-ups) would effect my scrub radius with the track wheels?

5) I have had lots of lowered cars, but never thought about scrub before. If I lower it modestly (.75-1" max) is there a way to figure out how much negative scrub that will add?

And finally (feel free to skip all the above questions); am I just overthinking the whole dang thing for a DD that gets ~4-6 track events per year?

I have 15mm spacers on my front hubs so that the Austins will clear my Brembos. Plus the SuperPro LCAs, plus the CSS spindles, plus lowered ~1". Not sure how all that adds up. I might be ham-fisted because I can't feel anything that seems bad. Then again, I don't track this combo, so I'm not pushing the performance envelope.

I haven’t measured mine yet but I assume that VW tunes the feel with a bit of negative scrub. Negative scrub will make the steering feel tight while on the gas, so if you go too positive it may feel floaty and darty. It probably won’t affect turn-in and stability but your steering may feel lighter.

You can counter the drastic positive scrub change by going with a taller tire.

Most modern front drive car have negative scrub, the intent is to keep the steering straight under longitudial drive/brake torque from the reaction torque at the contact patch. Stock wheel is definitely not 0-scrub.



Scrub radius is the distance "3" in that picture, in the case with negative scrub it means the axis formed by the top of the strut to the lower ball joint intersects the contract patch outside of the wheel centerline(as opposed to what the pic show here being inside of the wheel center). The quantity of scrub for most cars, especially FWD cars won't be a big number mainly because the larger it is(negative or positive) the amount of force imparted to the steering system will increase, and since drive torque is transmitted through front wheel, the likelihood of torque steer is increased. And strut suspension is harder to control scrub based on packaging, you want to stick the lower ball joint further into the wheel to let that axis fall closer to the center of the wheel but that compromises control arm design and also brake packaging, and if you have a wide wheel it makes it harder still, with the additional issue of the more angled that axis is(KPI), the more positive camber the wheel will gain when steered(and this is the reason why they make things like Hi-Per Strut or Revoknuckle because it can move the actual steer axis away from the strut axis to provide a more precise control of steering geometry).

If you take out the negative scrub or actually add to the positive scrub(12mm change in offset would probably do that) you might lose some of that on-center stability and the car may feel more likely to wander going straight especially if there are bumps or road camber involved. Also possibility of more torque steer and darting under braking. Bottomline though its not necessarily a trait makes or breaks handling, it just changes how the driver may have to drive the car. OEM tune cars with a lot more compromises on usability for a wide variety of users. For enthusiasts the priority may well be different. I definitely notice this when I run my autoX wheel setup(I am only allowed to change the offset by no more than 7mm).

Thanks all - this helps a ton!

Thanks all - this helps a ton!

I realize now that the online calculator I was using (wheel-size.com) starts with the assumption that your basepoint (unless you alter it manually) is 0mm scrub (hence why I assumed that is how the cars come from factory). Everyones input was much appreciated. I definitely don't think the car feels 'bad' in any way while commuting or on the track. I get some (manageable) darting while braking, maybe the 034 camber plates will help to minimize it. We shall see. So I think i will just forget about worrying about scrub for now and (especially since it is just an HPDE and DD car) and just go drive it.

RacingManiac said:

I definitely notice this when I run my autoX wheel setup(I am only allowed to change the offset by no more than 7mm).

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So for the wheels that you run on road courses, how much does your offset vary vs stock, and do you notice any difference there too?

jmason, seems like your car is the MK7 "let's go road racing" starter pack! Go drive on a track already!

Huntermike said:

You can counter the drastic positive scrub change by going with a taller tire.

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I never even thought of that. That's super interesting to think about. I do want to run 17's for track wheels...but I just got a good deal on some brembo calipers that should fit on stock 340mm rotors, so that adds a whole new dimension to wheel buying.

reverend_sean said:

jmason, seems like your car is the MK7 "let's go road racing" starter pack! Go drive on a track already!

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Agreed! But it's quite manageable (and comfortable) on the road. I've tried to approach the build as a DD first, HPDE car second. After all, I spend 99.99% of the time on the road commuting or errand-running, where I drive (mostly) like an old lady on the way to church. Not enough time on the track though!

RacingManiac's explanation above is really good! Here's another link which breaks down the pros & cons of negative, zero, and positive scrub radius fairly well:

https://www.racingaspirations.com/scrub-radius/

As an aside, one of the newer Civic Type R's traits of is a very close to zero scrub radius, accomplished with a knuckle-design that creates a steering axis closer to vertical, intersecting very close with the wheel center-line. One of the many articles explaining that here along with the cited YT EngineeringExplained video which breaks it down nicely:

https://www.roadandtrack.com/new-cars/a13527683/honda-civic-type-r-torque-steer-elimination/

jmason said:

Agreed! But it's quite manageable (and comfortable) on the road. I've tried to approach the build as a DD first, HPDE car second. After all, I spend 99.99% of the time on the road commuting or errand-running, where I drive (mostly) like an old lady on the way to church. Not enough time on the track though!

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Do you have a different set of track wheels for the occasion?

reverend_sean said:

Do you have a different set of track wheels for the occasion?

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I use 8.5x17 for the track. 17" tires are less expensive than 18"! And the combination of wheel/tire is lighter I suppose. Currently using Hankook RS4. They're pretty good when warmed up. Not so much when cold. Will try Toyo Proxie R888R next, which is an R-comp tire.

When sizing my tires, I look for tread width that is close to the wheel width. That's the important consideration, not section width. It just so happens that for the tires I just mentioned, 235 is the correct size.

reverend_sean said:

So for the wheels that you run on road courses, how much does your offset vary vs stock, and do you notice any difference there too?

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I don't have different wheels for road course use because my GTI was primarily for autoX. Basically you will probably notice it if you end up varying from stock dimension. As you said you can feel it dart more but it shouldn't be a huge issue for most. If it really becomes a problem you can probably play with toe to get some stability back. My main consideration would just be high speed braking stability.