E46 Scrub Radius vs Handling

[cobra]

I've been reading up on scrub radius and how it affects steering feel/handling. I don't personally have any experience changing wheel widths/offsets and seeing how it feels.

Does anyone have knowledge on this topic specific to the E46 M3? How should that affect our wheel width/offset/spacer selection?


This came to mind when I was considering spacing out my front wheels to look more 'flush' but potentially there are drawbacks?

[01SG]

I switched back to stock wheels from running square et35 Apex rims when I last changed tires. I am not sure how much of the change in feel is from width of wheel and tire as opposed to offset, but in the end I have come around to preferring stock on the street. The steering feel is much better, both in terms of that granular feeling of the road through the wheel and feedback on what the tires are doing in a turn. The car is easier, more pleasant, and more fun to drive this way.

In all the modding I've done, I have only found it is very hard to improve on what the factory put out.

I would not put spacers on just to get a flush look.

[9kracing]

Both lowering the car, and running wider wheels, increases positive scrub radius.

I would imagine all of us that aren't stock are in the positive range.

What's factory wheel & tire size on these cars?

[BBRTuning]

The thing about scrub radius is with factory suspension geometry, in order to keep the scrub radius correct and/or closer to factory, you're generally going to be sacrificing wheel and tire width. In a racing environment especially with factory or close to factory geometry, fitting more tire and a wider front track takes precedent over minding a proper scrub radius. It will also matter less say for road racing where you have higher speed higher radius turns compared to autocross or something.

That said, if you are not running a lot of wheel/tire in the front, and your choices are to have a lower/more factory negative scrub radius, or to run lower offset wheels or spacers for the sake of looks, then of course you'll be trading off some steering feel, bump steer, wheel bearing wear for a gain in track width and looks.

[bigjae46]

The handling balance with square wheel setups can be returned closer to stock with a 12mm spacer in the rear. If you get the wheel/tire flush in the front, then a 12mm spacer will usually guarantee that its flush in the rear.

I find a wider front tire doesn't feel as sharp but, for a road course, I don't think there's much of a downside. In fact, I think these cars really come alive if you can dial out the understeer with a driver can handle oversteering a little around the apex. That way you can stay in the power band without an extra shift which is very costly since it will come earlier on the straight.

[cobra]

What I'm most interested in is numbers for the E46 specifically. Like... completely stock what is the scrub radius and how do we calculate how that changes with different mods?

Bry5on ?

[oceansize]

This article seems to imply that if scrub radius for our M3 was zero from the factory then adding a 10mm spacer to a stock suspension/wheel/tire setup would be increasing scrub radius (positively) 10mm. Author states that -15mm to +15mm is normal range.

https://www.theautopian.com/our-susp...wheel-spacers/

This guy gives a pretty technical overview the how increasing/decreasing wheel offsets change the cars feel under acceleration, cornering, and braking. https://www.youtube.com/watch?v=SUDMEd1bMZI

So really all you need if the above is correct is to know the factory scrub radius for our cars and then take into account any suspension changes you've made plus wheel offset changes. I'm not sure its going to be easy to calculate that without taking some measurements yourself (or if someone has done that work).

[Bry5on]

What I'm most interested in is numbers for the E46 specifically. Like... completely stock what is the scrub radius and how do we calculate how that changes with different mods?

Bry5on ?

Stock is about +12mm scrub radius by my math. Suspension model: https://cad.onshape.com/documents/a1...5d9001c1313b00
edit: you can copy this model and add your mods to answer your mod question. The model is parametric and fully constrained.

Ferraris have been known to run a close to 0 scrub radius, which helps to provide that talkative front end. A high scrub radius will dull your feedback and in extremely high cases, cause a feedback almost like binding or going ‘over center’ - I went this far on my old e31 and it was really unpleasant. Ultimately I dropped 30mm and 1” of tire and wheel just to get the feedback/scrub to a reasonable level on that car. Hope that helps.

I run 46mm offset, 13mm scrub by the math in my e46 with a 245 tire on a 9” wheel. In practice it’s a bit lower than that statically due to some extra front negative camber. Super reasonable setup and nice feedback. I wouldn’t go below 40mm offset for a car that’s majority street personally.

[bigjae46]

Stock is about +12mm scrub radius by my math. Suspension model: https://cad.onshape.com/documents/a1...5d9001c1313b00
edit: you can copy this model and add your mods to answer your mod question. The model is parametric and fully constrained.

Ferraris have been known to run a close to 0 scrub radius, which helps to provide that talkative front end. A high scrub radius will dull your feedback and in extremely high cases, cause a feedback almost like binding or going ‘over center’ - I went this far on my old e31 and it was really unpleasant. Ultimately I dropped 30mm and 1” of tire and wheel just to get the feedback/scrub to a reasonable level on that car. Hope that helps.

I run 46mm offset, 13mm scrub by the math in my e46 with a 245 tire on a 9” wheel. In practice it’s a bit lower than that statically due to some extra front negative camber. Super reasonable setup and nice feedback. I wouldn’t go below 40mm offset for a car that’s majority street personally.

What would a 275/40R17 on a 17x10 et25 wheel on the front figure to be?

[Bry5on]

What would a 275/40R17 on a 17x10 et25 wheel on the front figure to be?

34mm of scrub, pretty bad. You’d be better off running that setup with a 33mm offset which should clear the strut if your aftermarket springs aren’t in the way. It would feel a lot better, that’d still be 26mm of scrub.

[bigjae46]

34mm of scrub, pretty bad. You’d be better off running that setup with a 33mm offset which should clear the strut if your aftermarket springs aren’t in the way. It would feel a lot better, that’d still be 26mm of scrub.

Thanks for calculating that. So is this an issue with feel or something else?

[Bry5on]

Thanks for calculating that. So is this an issue with feel or something else?

It’s a few things, but the highlights for me:
1) steering feel
2) stability under threshold braking. Higher scrub will make you have to wrestle the steering wheel and will make you slower
3) Steering precision at the limit, the tire will chase around grip and slip and make it harder to drive at the tire’s optimal slip angle

[EthanolTurbo]

Stock is about +12mm scrub radius by my math. Suspension model: https://cad.onshape.com/documents/a1...5d9001c1313b00
edit: you can copy this model and add your mods to answer your mod question. The model is parametric and fully constrained.

Ferraris have been known to run a close to 0 scrub radius, which helps to provide that talkative front end. A high scrub radius will dull your feedback and in extremely high cases, cause a feedback almost like binding or going ‘over center’ - I went this far on my old e31 and it was really unpleasant. Ultimately I dropped 30mm and 1” of tire and wheel just to get the feedback/scrub to a reasonable level on that car. Hope that helps.

I run 46mm offset, 13mm scrub by the math in my e46 with a 245 tire on a 9” wheel. In practice it’s a bit lower than that statically due to some extra front negative camber. Super reasonable setup and nice feedback. I wouldn’t go below 40mm offset for a car that’s majority street personally.

This is really cool info. I'm running a 235 on an 8.5" wheel in the front, +35 with a 15mm spacer. Feels awesome with such a lightweight, narrower front tire.

[slavik1988]

My problem with going with a very positive scrub radius/wider tires is that it tram lines like a mother during street driving on our roads here.

[bigjae46]

It’s a few things, but the highlights for me:
1) steering feel
2) stability under threshold braking. Higher scrub will make you have to wrestle the steering wheel and will make you slower
3) Steering precision at the limit, the tire will chase around grip and slip and make it harder to drive at the tire’s optimal slip angle

Thanks.

Yes, I feel the lack of stability under hard braking but I'm OK with that. Steering feel...I have enough of it or at least I know the car well enough I don't need more. I'm really interested in # 3

My aero complicates things, I've gone to and from an understeer as I've (hopefully) improved the aero. When the car is more neutral, I would describe the car as darty which I'm thinking supports # 3 I typically get the nose turned in and then I lean on the throttle to steer the car which is fantastic for slower corners. Then the aero load is high enough on the front at high speeds above 80mph which makes the turn in great. I can take a late turn in and really load up the outside wheels.

My car's weakness is turning into mid speed turns at around 60-80mph. It shows at one track where most of the turn ins are around 60-80mph.

I wonder how much scrub radius contributes to that and how I can fix that.