Widebody handling question for gurus..

[Bothari]

There's a question a which has been been bugging me about handling and widebodies, and now that bb2's article mentions this i'm curious:

Ever since i read that turbo-bodied cars came form the factory with spacers, it ocurred to me that it sounded like a rather innefective solution to the problem of widening the track.

Later on i read about all the usual complaints concerning steering sensitivity in turbo's and how much better thin-bodies are...

I have also read 2 tests so far with *real* factory 2.8 RSR's (Excelence and EVO) and they both mention how surprising the steering was, light and sensitive. Both these tests (years apart) mention that the steering looses little in these cars to the "standard" RS.

So, how come? What did the factory do diferent in these cars that they never did in the turbo's? How dificult would it be to do the same to a modern widebody? Was it something similar to what Tyson did in BB2?

BTW, If Tyson reads this, may i humbly state, "you da' man" ? (i think that's how you americans say it )

While we're in the subject, if understand correctly the changes to the spindles in BB2, shouldn't the spindle's angle to the horizontal plane change a lot when the steering turns? or is there a compensation i missed? (btw, it's ok if TRE want's to keep details about this a "trade secret" )

[350HP930]

The main reason for the spacers on most widebodies is for the purpose of tweaking the fitment of large wheels on a small 911/930 chassis.

My first hand experience of why there are complaints about the handling of widebodies has to do with the general problem of having large tires and a rear track that is much wider than the front on uneven road surfaces.

[Bothari]

Yes, i know, but what is the diference in a real RSR? Why does just about everyone who drives one say it has handling comparable with the RS?

[Tyson Schmidt]

The problem with the Turbo's is their positive srcrub-radius. This is the distance between the wheels centerline at the ground, and the steering axis centerline where it meets the ground. The greater the distance, the greater the scrub-radius. This radius acts like a lever-arm on the car's steering, and fights against the driver. It also introduces greater side-loads on the sliding members of the suspension.

The Turbo's had 8" fuchs with one inch spacers up front to fill out the flares and widen the track. The RSR had 9 inch Fuchs up front, with that extra inch being all to the inside, which helps reduce the scrub-radius., giving better steering characteristics despite the increase in width.

So in effect, a Turbo-bodied car can actually improve the steering characteristics by going to a wider wheel up front with greater back-spacing. With Jack's car, I was able to achieve even less scrub radius than a narrow-bodied car with 6" Fuchs, due to the geometry changes. That's why the car can be driven with much less steering effort, and more predictable behavior, particularly on uneven pavement and during hard braking.

Yes, the increase in steering axis inclination does make the wheels move in an upside down arc when turned. This is compensated for by increasing the caster.

Increasing the caster, and the steering axis inclination both increase the straight line stability and self-centering effect of the steering, at the expense of steering effort. But the increase in steering effort is more than compensated for by the reduction in scrub-radius. The end result is a very well-mannered car that seems to know what you are thinking. That "intuitive" handling that has been talked about in regards to the '73 Carrera RS.

[Bothari]

Figured it would be you to answer, tyson

So if i understand you correctly the RSR's *also* use spacers and feel better since the extra inward inch compensates the rubber outside the turning axis? In a strange way that makes sense, though i was expecting something like "super-duper-extra-special" suspension arms

Caster! I have to start thinking in 3d.
Tyson, thanks for the info.

[Jack Olsen]

Mark Donohue raced an RSR, and didn't like the spacers, either. So he came up with an idea for "super-duper-extra-special" (lengthened) suspension arms.

The odd thing is, Tyson came to the same conclusion, even though at the time he hadn't read about what Donohue had done.

Great Porsche minds think alike, sometimes.

One side note: the RSR's were considerably lighter than production Turbos. I'm sure that effects the car's feel, as well.

[Tyson Schmidt]

Originally posted by JackOlsen
One side note: the RSR's were considerably lighter than production Turbos. I'm sure that effects the car's feel, as well.

Good call Jack!

[Bothari]

Cool, thanks to both, something else to archive in the back of my mind.

[k9handler]

very cool, I learn something new every day!

[Tyson Schmidt]

Originally posted by Bothari
Figured it would be you to answer, tyson

So if i understand you correctly the RSR's *also* use spacers and feel better since the extra inward inch compensates the rubber outside the turning axis? In a strange way that makes sense, though i was expecting something like "super-duper-extra-special" suspension arms

Caster! I have to start thinking in 3d.
Tyson, thanks for the info.

Yes, you understand that correctly. But I don't want to confuse you with what I wrote about Jack's car. The increase in caster on Jack's car was simply to work with (offset) the SAI increase. It has nothing to do with the Factory RSR.

When you increase SAI, the wheels go into more positive camber when turned. This is fine on the inside wheel, since that is beneficial, but bad on the outside wheel. Increasing caster does just the opposite. It increases negative camber on the outside wheel when turned. This compensates for the SAI. None of this was changed on the Factory RSR.

[Bothari]

It has nothing to do with the Factory RSR.

Don't worry, i understood that

I assume SAI is the initials for the "official" name for that weird upside-down arc i was expecting from your work on Jack's car (compensated by caster changes). What does it stand for ? I'm a portuguese code monkey so english mechanical terms sometime elude me

[Tyson Schmidt]

SAI stands for Steering Axis Inclination. Back in the days of beam front axles, it was called King Pin Inclination.

On a Mac Pherson Strut front suspension, this axis is the imaginary line drawn from the center of the upper strut mount, down through the lower ball joint, and to the ground. On a double A-arm front suspension, it is drawn from the center of the upper balljoint, and down through the lower ball joint.

It's simply the axis on which the steering pivots. The more tire that you have outside of this pivot point, the greater the scrub-radius, and all the negatives that come with it. Ideally, you want a zero scrub radius, although a little bit of positive scrub radius enhances road feel. (although at the expense of increased kick-back)

[Jack Olsen]

The Schmidt "super-duper-extra-special" front suspension.

[Bothari]

'k , got it. Thanks.

This is like Colombo's egg, I can see why it didn't occur to many so far, but it's pretty ingenious in it's simplicity and tried-and-tested methodology (speling??). Definite kudo's to you, Tyson.

The pic explains the basics of the mod pretty well, Jack, you should use something like that in your (eventual) site on the car. btw, how's that coming along?

[hilandscott]

Tysone wrote: "So in effect, a Turbo-bodied car can actually improve the steering characteristics by going to a wider wheel up front with greater back-spacing. With Jack's car, I was able to achieve even less scrub radius than a narrow-bodied car with 6" Fuchs, due to the geometry changes. That's why the car can be driven with much less steering effort, and more predictable behavior, particularly on uneven pavement and during hard braking."
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I have a turbo bodied early offset SC... should I be trying to fit a 9" wheel up front with a custom offset for no spacers?

Reading the above statement I started to wonder if more spacer and a deeper offset (55et or greater) was better for scrub radius.

Thanks!

[stlrj]

SAI definition

The angle between the centerline of the steering axis and vertical line from center contact area of the tire (as viewed from the front). SAI is typically not adjustable, but deviations from specification can indicate vehicle damage. A maximum variation side to side of ± 1.0° may also indicate vehicle damage. This topic is covered in detailed charts later.

http://aligncraft.com/terms/terms.html#Steering%20Axis%20Inclination

Since SAI is not adjustable on a 911, where are you getting this SIA increase? The only way I can think of changing the SAI on a 911 is to bend the spindle but no mention of this...

What am I missing?

Cheers,

Joe Garcia
86 Carrera
Redwood PCA since 1976

[Chuck Moreland]

The lengthened control arm changed the SAI.

[stlrj]

S.A.I. and Included Angle

* Is a directional control angle
* Is a built In angle

a) * On vehicles that have camber adjusters on bottom of the strut, S.A.I. angle does not change.
* If S.A.I. angle Is wrong on this type of front end. It would indicate a bent shock tower, bent lower control
arm or bent cross member, etc.
b) * On vehicles that have camber adjusters at the upper or lower control arm, or at the top of the struts, S.A.I. will
change, but Included angle will remain the same. If INCLUDED ANGLE Is Incorrect, a bent spindle or MacPherson strut Is Indicated.
http://www.geocities.com/danielmacmillan/alignment/angles.htm

More illustrations here:
http://www.familycar.com/alignment.htm#Steering%20Axis%20Inclination%20(SAI)


My understanding is that by lengthening the control arm you are changing and creating a camber adjustment from below the strut. Therefore: a) * On vehicles that have camber adjusters on bottom of the strut, S.A.I. angle does not change.

As far as I can tell, a longer control arm would only increase negative camber, not SAI.

Joe

[Colby]

hilandscott: I am reading it just the opposite. That if you have an SC turbo body, that you would want custom spacers that are larger than factory and wheels with more positive offset (ala late model 911's) to put more of the wheel on the inside. By adding more negative offset to eliminate the spacers would make the scrub radius worse. Am I correct on this Tyson?

Colby

[Bothari]

Basically, the point about reducing scrub radius is to get the center of the tire closer to the turning axis, so getting wheels with bigger offset should help, but *not* using more spacers. the point is to get the rubber more "in" not "out".

Tyson's mods should do that while keeping a wide track.