A-body reduced scrub radius options?

[Blackarrow222]

I'm working on getting my dormant '66 GTO back on the road and one thing I want to address is the lousy front suspension, which is currently stock save for stiffer springs and a TA sway.

I know the standard formulas that are now tried and true. Tall ball joints on the stock spindles with new upper a-arms. Or AFX spindles and whatnot.

But none of these options addresses what I see as a significant failing of the suspension, which is that it has a ton of scrub radius. I think it's like 2.5"? I used to wonder why, when I would turn the wheels, I always had to worry about the tire contacting the very bottom point of the wheel cutouts in the fenders and now I understand that it's because the massive scrub radius swings that wheel around like a sack of potatoes. And it also makes the steering stupidly heavy.

So.. are there any solutions to this floating around out there? It seems that the basic solution is slightly longer a-arms at top and bottom coupled with greater negative offset wheels. Like a C4 Corvette, for instance. It seems this would be a wonderful improvement to the system, now that the basic geometry has been ironed out by the aftermarket.

[Hotwire]

You do realize you're getting a ton of positive camber when your suspension compresses? My tires were rubbing with stock suspension, corrected geometry and the wheel tucks into the well when turning. But as you've noted I'm running 4th gen offset fbody wheels with spacers.

https://flic.kr/p/2njnZ6d

[Blackarrow222]

Yeah, I'm well aware of the positive camber gain of the stock setup. While that certainly is its own problem, it's also somewhat separate from what I'm taking about.

The example I gave about my tires hitting the bottom corners of the wheel cutouts is illustrated in the attached images. Here I've shown 15 degrees of turn on a tire with zero scrub radius and then one with 3.25" of scrub radius (I think that value is actually closer to what A-bodies have than what I sadi in my original post.) You can see how on the latter, the outside edge of the tire hits the sheetmetal. That's exactly how it worked on my car and it's because the tire isn't merely pivoting, it's being swung through an arc (scrubbed!) in the wheelwell and banging into anything that happens to stand in its way.


Incidentally, when I first got the car, the wheels and tires on it were spaced far enough out that I did also have to worry about them hitting the fender lips at the top of the opening on big bumps. Can't tell you how many times I had to reach under and bend the lip back up into position...

[Olds.PhD]

So to answer the question, an easy way to reduce your positive scrub radius is to run wider wheels/tires with more backspacing. For example, going from an original 7" wide wheel with 4" of backspacing to a 9" wide wheel with 6" of backspacing. That would require tubular arms, maybe a sway bar with extra clearance, and 17"+ tall wheels. But that would move your scrub radius inboard. Do note that zero scrub radius is generally considered bad and pretty much no car ever runs it. So you want to be sure you still have some positive scrub radius.

The question is why? It seems like your two biggest complaints are steering effort and tires hitting the fenders. Neither makes much sense to me. These cars tend to have very light and floaty steering. All of the good aftermarket A-arms increase caster to increase steering effort and feel. One of the most popular upgrades is going from the original low effort steering boxes to a fast ratio box. So if you have very heavy steering, I have to think something else is wrong here. Or it is a manual steering car with a small diameter steering wheel?

For the tires hitting, they're too tall or you've got damage pushing things very far out of whack. I've got 26.6" tall 245s up front on factory wheels and I don't come anywhere near the fenders - top or bottom. I have measured for wider wheels and its really obvious how taller tires start hitting things, including the bottom of the fenders. I'm likely going with a shorter tire when I go wider, just to be sure I'm clearing everything.

[Blackarrow222]

The limits of scrub reduction via that wheel reproportioning are unfortunately far below what would be required to get it down to a reasonable amount. Your specific example would yield about 1" of reduction, leaving about 2" to go. But it is a legitimate method so thank you for offering it. Best suited to fine tuning at the end of the process.

I think the target of 0.5" positive scrub is a good one.

The reason is that it's the technically proper thing to do. None of us, if designing a "mid-size performance V-8 passenger car" would give it 3.5" of scrub radius just for kicks. It's a wholly undesirable characteristic. And yet I see $30K custom ladder frames that have seemingly perpetuated it, on high-tech life support. Along with the bindy 4-link rear but that's another topic.

The symptoms I gave are merely my own experiences with the high scrub. The more salient characteristic, in the context of this forum, is the actual driving feel. And I know that this is where my question comes up against a wall, because there are a lot of ass kicking cars running around and very high speeds with 3.5" scrub radius. So I understand that there will be the argument that "it works well enough."

But the truth is that it could be a lot better. I have a Grand Cherokee box in my car and it works "well enough." But it doesn't feel like a modern car. It feels like a heavy old car with a performance steering box that is power boosted out the ass to overcome the monumental turning resistance of a 3.5" scrub radius and then damped back down with a heavier internal torsion bar to simulate "feel." I could sell a family member to afford that Turn One box and I'd be looking at the same scenario: using a power box to mask the negative effects of a giant scrub radius.

With the engine sitting right over the front axle line, I don't necessarily expect to be able to run fast ratio manual steering even with reduced scrub but it would be nice to be able to use some much less boosted power steering that actually allows some real feel, now that it's freed from the task of having to guard my hands from being ripped off when I hit a pothole and the giant lever of the tires against the spindles is gone.

It seems like a reasonably easy challenge:

Increase the length of the upper and lower control arms about 3" and then adjust the position of the tie rod to elude bump steer. Of course this would also use the improved a-arm placement methodology for camber gain and whatnot.

[Olds.PhD]

My understanding is that the lack of feel you describe is from the abysmal caster in these cars to compensate for manual steering that was standard on pretty much all of them. It has nothing to do with the scrub radius. The "modern car" feel you're looking for comes from more positive caster. Modern cars are generally around 7 degrees of positive caster, with some being significantly higher. The A-body was around 0.5 to 2 degrees of positive caster and the factory arms significantly limited how much you could add. The new UMI arms I just put on mine bolted on with over 8 degrees of positive caster. I had to adjust it down to 6.5.

My general take is that there are far more significant issues with the suspension in these cars than the scrub radius. The effects of slightly too much scrub radius are pretty minimal in the grand scheme of things. Its not like the tire is sitting outside the radius. The effects of poor camber gain, lack of positive caster, poor bump steer characteristics, and the combination of both wet noodle lateral stiffness and simultaneous binding of the rear end are generally where most people running fast A-bodies focus on improving.

But if you're serious about minimizing it, you're probably on your own, designing a custom front suspension. None of the aftermarket companies have much to offer from what I've seen. Just increasing the length of the arms opens an entire other can of worms. Especially by such an extreme amount. Might be better off trying to graft a modern corvette suspension to your frame. Either way, a whole lot of fab work in your future. Best of luck.

[Blackarrow222]

Caster is part of it, yes. But there's a difference between feel as a result of geometry and feel as a result of the numbing effects of a power assist system. Both are important, of course. The caster increase helps with tracking and stability. Meanwhile the huge scrub radius is trying to pull the tires in various directions while the super powered hydraulic steering system is buffering those forces for the driver, thereby killing any true feel of the road.

Anyway, I think you're right that there's nothing out there "off the shelf" to address this. Perhaps I've found my new mission...

I bought a second frame so that I can experiment and build new front and rear suspensions and then swap my car over when it's ironed out. I've looked at grafting in a Corvette IFS and it's not a bad idea but it is a lot of work. It'd just be cool if there was a system that could bolt to the existing A-body pickup points. And maybe allow for some rack and pinion steering to boot.

[79T/Aman]

Your original post was basically asking a way to reduce scrub radius while maintaining the overall stock suspension.
Take a look at this option, 10* KPI will reduce scrub radius.
https://www.pro-touring.com/threads/132566-New-spindles

[Blackarrow222]

Those spindles would reduce it a little bit but it would only be some fraction of an inch.

[79T/Aman]

about 1/2" combine it with with a positive offset wheel and you can reduce the scrub radius as much as possible without custome fabrication and re-engineering the front suspension, I was under the impression this is what you were after.