17" wheels for road racing.

[Maximuscr31]

27mm was chosen to allow you to run up to a 275/40/17 without using spacers plus clear 3kgt brakes or up to a 14" stoptech kit.

Here are pics of my TDR wheels from the group buy on my car with some cheap 275/40/17. Keep in mind the gc coilovers are not on here nor aligned but it gives you a good idea of fitment.

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[eclipsegsx1736]

Thanks for the link. I read through that thread twice and didn't see any real explanation why +27 was chosen, simply "because the offset positions the wheel close to where the factory designed it to be for the suspension geometry of our cars".

From what I've gathered of that discussion as well as others, the +27 offset TD selected was an attempt to correct the scrub radius back to factory spec. Just like lowering a car can negatively affect the roll centre and bump steer, changing the wheel width can affect the scrub radius in a negative way, and the wheel offset should be considered to keep it within spec.

With that said, from my research, the effects of a poor scrub radius do not have nearly as much of an impact as many other critical suspension geometry calculations such as those I mentioned above. The effects of a poor scrub radius, I believe, would mostly be seen during changes in steering angle. The idea, I believe, is that you're excessively "scrubbing" the tire during steering angle changes which can contribute to tire slip which can put a tire past its ideal slip angle faster. To me, this seems almost negligible on anything but an extreme auto-x car. I'd imagine steering inputs just don't change enough even in road racing for this to have an effect.

I'd like to see what JToby has to say on this.

 

[jtmcinder]

I don't have a whole lot to add, really. When you're deciding on wheels to put on a car that already has a suspension, you have a variety of issue to consider, but the two that seem to trump all others are avoiding tire-to-anything contact and track. Scrub is way down the list and rarely changes anyone's mind. Yes, more scrub means more steering effort and stress on steering components; yes, more scrub means that the wheels move forwards and backwards more when you steer; but as long as you can steer, don't break anything, and don't cause the tires to hit anything, you ignore scrub. The extra tread and track is just too good to pass up.

It's a lot like caster. More caster means more "free" camber as you steer. Even better than just adding static camber, caster means more negative on the outside wheel and more positive on the inside, which is exactly what you want. The downside - and you knew there was going to be a downside, right? - is that caster causes jacking in exactly the way you don't want: it raises the outside wheel and lowers the inside, causing a combination of extra body roll and less outside-front force. And, yet, we dial in more caster, because the benefits of more caster outweigh the costs.

Oh, and I completely agree that other geometry issues are waaaay more important. Bump-camber and roll-center height are the top two for me. But scrub is completely separate from these, so I guess we leave that aside.

 

[romeen]

I don't have a whole lot to add, really. When you're deciding on wheels to put on a car that already has a suspension, you have a variety of issue to consider, but the two that seem to trump all others are avoiding tire-to-anything contact and track. Scrub is way down the list and rarely changes anyone's mind. Yes, more scrub means more steering effort and stress on steering components; yes, more scrub means that the wheels move forwards and backwards more when you steer; but as long as you can steer, don't break anything, and don't cause the tires to hit anything, you ignore scrub. The extra tread and track is just too good to pass up.

It's a lot like caster. More caster means more "free" camber as you steer. Even better than just adding static camber, caster means more negative on the outside wheel and more positive on the inside, which is exactly what you want. The downside - and you knew there was going to be a downside, right? - is that caster causes jacking in exactly the way you don't want: it raises the outside wheel and lowers the inside, causing a combination of extra body roll and less outside-front force. And, yet, we dial in more caster, because the benefits of more caster outweigh the costs.

Oh, and I completely agree that other geometry issues are waaaay more important. Bump-camber and roll-center height are the top two for me. But scrub is completely separate from these, so I guess we leave that aside.

Sorry for going off topic, but since it was mentioned: Can you briefly explain how increasing the roll center height of the rear to be higher than the front will help with handling? Just today, based on some old posts of yours, I adjusted it like this. The car has a very noticeable rake now. Just wanting to understand what I'm trying to do.

 

[jtmcinder]

Oh, wow. A real explanation of that is complicated, because we have to talk about dynamic vs steady-state in order for it to make any sense. Let's do this: please start a new thread in handling for this and I'll do what I can when I get a chance. Someone else I'm sure knows this stuff, too, and can get it rolling if I'm too slow. It's cool stuff, so I'd love to have it discussed. It a huge issue in the Evo X world, so it's on my mind quite often.

 

[romeen]

Thanks, I will do that.

 

[delta448]

From what I've gathered of that discussion as well as others, the +27 offset TD selected was an attempt to correct the scrub radius back to factory spec. Just like lowering a car can negatively affect the roll centre and bump steer, changing the wheel width can affect the scrub radius in a negative way, and the wheel offset should be considered to keep it within spec.

That was my initial understanding as well, but after learning more about it, I have to assume there was much more than scrub radius determining the offset choice. I somewhat detailed that in my reply to Eric's recent thread about this.

Since the scrub radius is determined by the intersection of the SAI and the tread's centerline, assuming SAI is not changed and still intersects the ground at the same point, ANY change in wheel offset changes the scrub radius by the same amount, whether the wheel totals 6.5" wide or 9" wide. An offset of 46mm on a 6.5" wheel does not have the same scrub radius as an offset of 27mm on a 9" wheel, the 9" wheel would have to have the exact same offset to have the same scrub radius.

It doesn't really matter anyway since scrub radius is a low priority, but it does further beg the question of how the offset was chosen. Perhaps the real goal was to preserve the relationship between the SAI intersection with ground and the inner edge of the tread?

 

[jtmcinder]

I completely agree with the second paragraph, which is what I meant by talking about a person who already has a suspension and is now choosing a wheel. Assuming that the steering axis hits the ground in the same place - i.e., no changes to the suspension - reducing the offset by X increases the scrub by X.

Note, also, that most of the things that we do to 2Gs, such as shortening the upper arms or moving the upper inboard pick-up points, do not have any effect on scrub. As long as the relationship between the tire and the knuckle is unchanged, where the tire hits the ground vs where the steering axis hits the ground is the same. This contrasts with true struts, especially on a car with eccentric bolts for camber, such as an Evo X. When you tilt the knuckle relative to the steering axis (which is what those eccentric bolts do), then scrub can be dramatically changed. When I flipped the bolts on my Evo X, for example, to gain a degree of front camber, I increased the scrub by a fifth of an inch.*

- - -

* the radius of my wheels is about 12.6". The sine of 1* is 0.017. The product of those two values is about a fifth of an inch