HAWAIITALKS!

What exactly are the negative effects of a positive scrub radius? All I could recall is that it simulates a toe-in condition as you accelerate/decelerate, increasing steering effort. However, wouldn't it also change the Ackerman angle and effectively alter the suspension geometry? I'm just curious because I'd like to know how this affects my vehicle's handling characteristics, before I buy some hubcentric spacers.

Any suspension gurus care to discuss?

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after you find out, please enlighten me.

wtf is a scrub radius, a ackerman angle and hubcentric spacers?

word.

i drive american. american car go vroom. fast.straight line.

that iz all.

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When you turn the steering wheel, the inner-wheels will travel in a different radius, than the outer-wheels:



Because of this, the inner-wheel needs to turn more than the outer-wheel, to prevent the tires from scuffing/scrubbing (losing grip).

However, in order to achieve this toe-out on turns, you need to integrate the Ackermann steering geometry. In short, it's basically an imaginary "V" from the steering arms to the dead center of the rear axis as seen in the above 2 photos.

That Ackermann angle is engineered into the vehicle with the OE wheel in mind. When you buy a wider wheel, wheel spacers, or something that changes the offset, your Ackermann angle has also changed. The reason why I started this thread is to see if anyone knew if this made any difference. They way I see it on paper, the wheels are now turning at a different radius than the stock setup, and the Ackermann angle hasn't been compensated. Therefore both wheels should be scrubbing (losing grip) which means understeer and reduced vehicle handling.

The scrub radius is basically the centerline of the wheel relative to the steering axis inclination:



When you change the wheel's offset, you change the scrub radius. Apparently the only thing that this effects is the amount of effort required to turn the wheels, and the car's stability during braking/acceleration.

As for hubcentric spacers, everyone knows that the vehicle's weight is supported by the lug nut bolts right? WRONG! The vehicle's weight actually sits on the hub assembly. There's a round protrusion on the hub where the vehicle's weight is supported, and transferred to the wheel:



When you add wheel spacers, you're pushing the wheel off of the hub, and onto the bolts which CANNOT support a 3000+ pound vehicle. This is why you need hubcentric wheel spacers:



They have that same lip that the hub does so that the vehicle's weight is properly supported. The vehicle now rests on the hub again instead of the wheel bolts, which can break.

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Are you doing this for road racing, drifting, or just to get more offset for street driving?

IIRC, increasing scrub radius helps drifters keep the steering angle while sidewards, without much effort because the inner tire is so positive cambered and castered that it follows the momentum of the car.

I don't mind a little more offset for road racing and autoxing, the added track is usually worth it. I have some 15x9 with a +26mm offset coming for the miata, and stock is 14x5.5 with a +40mm offset. Quite a big difference!

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ic..so the hubcentric spacers sit on the normal hub, but they're bolted TO the hub instead of just shimming up the lug bolts... O_o;;

and for the 'scrub radius' deal, if you drop the car//lower it, while getting bigger tires wouldnt that sort of offset the deal? since with a drop the top of the wheels pulls in? (according to the pic?) or is this gonna make it worse.. hmm. making me think twice about droping my car..

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Nah, that just affects camber. If you changed where your shocks bolted to (or upper/lower a-arms if you have a dual a-arm design) then that would affect everything.

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Oh yeah, to answer your main question on the thread, I'm not that meticulous in measuring my Ackerman or scrub.. but I will need to toe out an 1/8 of an inch per side to get the miata to turn-in well once again. Being 14mm farther out, with a 9" rim, and 90mm wider tires (than stock) tends to really confuse little miatas.

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Sort of. As mentioned earlier, the vehicle's weight is supported by that round protrusion on the hub assembly - not the actual lug nut bolts.



Most people don't realize this and they buy spacers that push the wheel OFF of the hub, and onto the lug nut bolts (very dangerous):



Notice how that protrusion is blocked off by the spacer? Yeah, now your 3000 pound car is resting on a few bolts.



Hubcentric spacers (shown on the left) not only shim and move the wheel outward, but they properly support the vehicle's weight. The hubcentric spacer also acts as an extension for the hub, so now the vehicle's weight is properly supported again:

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It's for a daily driven application. A change in steering effort is perfectly fine with me, but I just don't want to ruin the suspension geometry for what's effectively going to be an aesthetic modification.

That's what I figured needed to be done, but none of my textbooks or online information had any relevant information. Thanks for the help

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Well, I didn't know about the ackermann theory . . . heard the term though. I do know that when the steering rack is in front of the front axis, when you turn, the wheels will toe in. Vice-a-versa when the rack is behind the front axis. As far as scrub radius effect on handling; our formula cars have extreme amount of negative scrub radius. The pivoting point is almost at the inner side wall. For a street car (with all the weight) this will cause the steering to be extremely heavy. Also, if your running alot of negative caster your tires will stand up on their side walls.

I've always been fascinated with all the different variables and how they interact with each other to change handling performance.

If you start off without spacers, and your alignment is where it works best...
wouldn't putting spacers on would increase any misalignment?
Kind of like removing stock-offset factory wheels and putting on reversed-offset, or deeper wheels.

Wouldn't adding a slight toe-in (and adding negative camber) compensate for the wheel spacers, on the turns?
On the straight-away it might hurt your top speed though...

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The factory setting probably has alot more variables factor in such as crash test results, costs, wear, etc.

In racing we use toe to increase mechanical grip especially if you don't have the aid of aerodynamic downforce. When the left and right tires fight for direction, the tires tend to dig into the surface more, but of course there are limits to this, one being scrubbing speed on the straights.

Again, that's why I like suspension tuning so much . . . it's an art.

If only we had access to those oscilloscopes that show how the alignment angles deflect as we drive the car. Engineers have it easy

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positive scrub radius amplifies/aggravates tq steer and brake pulls, much like low/negative caster/camber/SAI. i believe the extra distance out from from the SAI line diminishes its leverage on returnability.

ackerman is the effect, not the angle. the angle is toe-out on turns. it's caused by the ackerman effect and set by the relationship of the steering arm to the hub. the only way to change that would be to bend/twist the knuckle/strut because no matter how big the circle, it still has 360*.

also, imo those spacers look dangerous. better to have more threads through the lugs than a lip in the rim. someone wheel-offed at drift session because of that. some rims are hubcentric, some are lugcentric, some fords are neither. some hubcentrics are plastic and don't actually support anything. they all work well enough. either way, the weight is on the bolt. bolts are much stronger than you think but they won't hold if they don't have threads to grab

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Yeah that's all I've been able to gather which is what lead me to start this thread. However I would assume that with a greater negative offset (wider track width), the you end up with a larger turning radius due to the increase in toe-out angles. In other words, there's gotta be more than steering effort that's affected.

No, the Ackerman angle is what creates the Ackerman effect. The angle is an imaginary V that extends from the steering arms, to the center point of the rear axle. This is suspension geometry hence Ackerman angle. This angle causes the resulting toe out on turns, hence Ackerman effect

Ackerman effect and toe-out are the same thing.

Those spacers were merely an example. The ones I had planned on getting did indeed have extended wheel studs (H&R).

In any case, thanks for your help... have some chocobees:

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if the ackerman angle is the angle of the steering arm to the hub, then the spacer can't affect it because it's parallel to the hub. the design criteria may be from steering axis to thrustline, but the physical angle is built into the spindle itself.

as for the ackerman effect, if you think in the theoretical of the original intent of ackerman steering, which is all 4 wheels turning about the axis of a single point to reduce friction, then no, offset can't affect that either, because small and large concentric circles are still concentric. the angle is more relevant to the steering axis than it is the tire and this is because of the effect

turning radius gets larger, in the negligible amount of the spacer width, but the left and right turn radii remain concentric, which is ackerman kosher. nothing happened with toe at all...


toe-out is just toe-out. ackerman is ackerman. ackerman and toe-out on turns are similar, as a cause and effect.

thanks, but i don't eat those.

now that ackerman and its physical composition has been beat to death, the more important question is why. hypothetically, if the rim offset resulted in a handling characteristic change other than scrub radius, could it have a positive effect that would be of enough value, handling-wise, to offset the negative effect of a positive scrub radius? if not, why do it?

the most important question is "how will spacers affect handling?" and i think scrub radius is important, but the answer has more to do with the effects of track-width differences front-rear and the effects of the track-width on the roll-center. i do alignments. i'm no engineer. i can't help you with the calculus, or even the data, for the dynamic moments of those effects. i think the spacer will be mostly cosmetic

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Yeah I'm probably looking too far into the subject but curious minds will inquire Thanks again for the info

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pretty interesting stuff to know

and to know there are drivers on the road right know with aftermarket rims that dont have the right hubcentric rings on them. which means there siting on studs!

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hubcentrics are only really responsible for centering the rim and keeping it there before it is torqued. so assuming the rims are machined true, with no damage to the holes or nuts, an acorn/mag shank will center the rim if you torque in steps and crosses, in the air.

the bolts are always carrying the weight. it seems like the car is resting on the hub when it is standing still but take the lugnuts off and it will probably still fall over. much more complicated when the vehicle is dynamic: in a turn, if you can pull off 1.0 lateral G force (sporty cars will do 0.7-1.0ish; F1 3.0-4.0), you're effectively loading the combination of tires with the full weight of the car, sideways, in a direction which a hubcentric has no load-bearing input on. another direction hubcentrics have no input on is rotationally; think of those damn wheel-bolts (audi, vw, bmw, benz, saturn). take them all out and you can spin the rim around on the hub. all the G forces involved with acceleration and braking are held solely by the lugs. the forces acting on those bolts are probably many times greater than the distributed weight of the car. but thats more evil calculus.

also keep in mind the average grade 8 fastener can sustain upwards of 120,000 PSI and there are at least 4 of them. i think the most dangerous thing about changing rims is the lugnuts- i've seen plenty that only grab 3 or 4 threads and it's usually because of spacers.

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Yeah SCCA requires at least the width of the lug nut to be used. Spacers are scary, I've seen them break WAY too often, and it usually ends up in big damage.

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