**Negative** Caster

[RidingOnRailz]

Was browsing a large directory of vehicle alignment specs and noticed a lot of 1950s & 60s domestics had specs for Caster that were up to 1 degree negative! At the same time they possessed respectable SAI(5-10 degrees.)

Question#1 - Was negative caster a product of the composition/characteristics of then prevalent bias ply tires?

#2 - SAI aside, how did negative caster contribute to straightline stability and steering wheel return?

[MagicRat]

I think there were 3 factors at work.

Many of these older cars had manual steering, or at least power steering was an option. I expect the negative caster helped lighten the steering effort, especially with the big heavy cast-iron lumps they used as engines.

I have driven plenty of cars from that era which had extremely light steering, noticeably Fords and especially Chryslers. Many of those old Mopars felt as if they had a radio-controlled steering, with virtually no self-centering and no road feel. Very light steering seems to have been a desirable trait, which would be reduced with positive caster.

Finally, I am no expert here, but I recall the bias-ply tires of the day had relatively long, narrow contact patches which helped self-centering and lessened the need for positive caster. Modern radial tires have shorter, wider patches, which, in my experience, are more apt to wander when used with vintage alignment specs. With modern tires, some positive caster seems to be advisable.

[snshddog]

The heavier the car is the more caster it needed to return effectively. Positive caster will still help return but will tend to wander at highway speeds. Negative caster will help selfcenter while moving. There is more to it but thats the simple version I was taught.

[RidingOnRailz]

Originally Posted by snshddog

The heavier the car is the more caster it needed to return effectively. Positive caster will still help return but will tend to wander at highway speeds. Negative caster will help selfcenter while moving. There is more to it but thats the simple version I was taught.

REALLY?!! You owe me a new keyboard because I just upchucked coffee and oatmeal into the one I have.... PSYCH!! But seriously,

That flies in the face of what every alignment website says about caster(the neg vs pos part), at least in this century!

As far as the weight of the car goes, instinct tells me that more weight is beneficial to the SAI factor(hence SAI specs in the teens for today's lighter compact cars vs single digits for the 60s land yachts). Caster works more upon the pneumatic trail principle(similar to rake on manual or powered bikes).

Now a broader question, and my final one concerning negative caster on older cars:

Is there any factor(other than absence of a way to adjust caster on that 1 in million make/model) precluding the inversion of the caster on, say, my dream vintage car? That is, having a shop set the caster from a negative to a positive value? If I do plus-size the tires I will try to maintain the factory width but go with taller rims for shorter sidewalls(14 to 15" or go 15" to 16" depending on original rim size)

I really did think it had something to do with the composition/deflection of the material in bias ply tires of that era. Correck me if I'm wrong.

[RidingOnRailz]

Originally Posted by MagicRat

Many of those old Mopars felt as if they had a radio-controlled steering, with virtually no self-centering and no road feel. .

And a lock-to-lock in scientific notation on top of that. LOL! Must've been a handful downtown with left & right turns everywhere.

Sadly, Rat, what you described is creeping back into the lexicon of many respected automotive reviewers/magazines when critiquing today's rides. Hence my previous rant posts regarding electric and other FBW control systems being put in cars nowadays.

But back to the issue of neg caster.

[maxwedge]

Increased caster is the first thing I do when when installing radials on older cars in my resto shop, directional stability is enhanced and that twitchiness when making slight corrections is improved. Many older cars do not have sufficient range of adjustment to reach much more than .5 plus, but is better than 1 minus. Post 3 is way off the mark as noted.

[MagicRat]

Originally Posted by snshddog

Positive caster will still help return but will tend to wander at highway speeds. Negative caster will help selfcenter while moving.

As noted above, I just think you have reversed the 'positive' and 'negative' concepts. Change them around and you've got the basics.

Originally Posted by RidingOnRailz

And a lock-to-lock in scientific notation on top of that. LOL! Must've been a handful downtown with left & right turns everywhere.

Haha. I remember my father-in-law driving his manual-steer '73 Dodge Dart, with about 8 turns lock-to-lock. He had to spin that thing like the wheel on an old schooner.

Originally Posted by RidingOnRailz

Sadly, Rat, what you described is creeping back into the lexicon of many respected automotive reviewers/magazines when critiquing today's rides. Hence my previous rant posts regarding electric and other FBW control systems being put in cars nowadays.

Agreed. I am not impressed with the steering of my '97 Cadillac. The car has decent low-profile tires, rack-and-pinion steering and FWD, all of which should help handling. But they engineered all the road feel and progressive steering effort out of it. It's just dreadful, a throw-back to the bad old days, just because that's what GM thought their senior-citizen customers wanted. No wonder they were losing customers to the imports.... but I hear they've improved a lot.

[curtis73]

Originally Posted by MagicRat

I think there were 3 factors at work.

Many of these older cars had manual steering, or at least power steering was an option. I expect the negative caster helped lighten the steering effort, especially with the big heavy cast-iron lumps they used as engines.

I have driven plenty of cars from that era which had extremely light steering, noticeably Fords and especially Chryslers. Many of those old Mopars felt as if they had a radio-controlled steering, with virtually no self-centering and no road feel. Very light steering seems to have been a desirable trait, which would be reduced with positive caster.

Finally, I am no expert here, but I recall the bias-ply tires of the day had relatively long, narrow contact patches which helped self-centering and lessened the need for positive caster. Modern radial tires have shorter, wider patches, which, in my experience, are more apt to wander when used with vintage alignment specs. With modern tires, some positive caster seems to be advisable.

You actually kinda ARE an expert here... at least from what you post.

Speeds were slower, steering effort was something that consumers thought was evil, and bias ply tires have very large amounts of pneumatic trail (a function of the friction associated with the contact patch), so caster was set positive in many cars to make the public happy.

Primarily, positive caster was designed into many manual-steering cars. When some cars started getting the power steering option, some were able to adjust the caster on the existing chassis and others didn't have that range of adjustment. A good example - my 66 Pontiac Bonneville. The caster spec was 0.75 negative. Once the B-body was homologated in 1963, there was no way to make positive caster without affecting camber. In 1964 they started making different upper control arms with more rear offset so they could have positive caster without camber issues.

[RidingOnRailz]

Originally Posted by curtis73

You actually kinda ARE an expert here... at least from what you post.

Speeds were slower, steering effort was something that consumers thought was evil, and bias ply tires have very large amounts of pneumatic trail (a function of the friction associated with the contact patch), so caster was set positive in many cars to make the public happy.

Primarily, positive caster was designed into many manual-steering cars. When some cars started getting the power steering option, some were able to adjust the caster on the existing chassis and others didn't have that range of adjustment. A good example - my 66 Pontiac Bonneville. The caster spec was 0.75 negative. Once the B-body was homologated in 1963, there was no way to make positive caster without affecting camber. In 1964 they started making different upper control arms with more rear offset so they could have positive caster without camber issues.

Guys, both post #8 and #3 are really playing with my brain! MagicRat or somebody can you please restore sanity to this thread??

In the sentence beginning "Primarily, positive caster was designed into many ..." was negative the intended term??

Heeelp!!!!

[curtis73]

Originally Posted by RidingOnRailz

Guys, both post #8 and #3 are really playing with my brain! MagicRat or somebody can you please restore sanity to this thread??

In the sentence beginning "Primarily, positive caster was designed into many ..." was negative the intended term??

Heeelp!!!!

Yeah... I got them backwards a couple times. here is the corrected version of what I said:

Speeds were slower, steering effort was something that consumers thought was evil, and bias ply tires have very large amounts of pneumatic trail (a function of the friction associated with the contact patch), so caster was set negative in many cars to make the public happy.

Primarily, negative caster was designed into many manual-steering cars. When some cars started getting the power steering option, some were able to adjust the caster to the positive side on the existing chassis and others didn't have that range of adjustment. A good example - my 66 Pontiac Bonneville. The caster spec was 0.75 negative. Once the B-body was homologated in 1963, there was no way to make positive caster without affecting camber. In 1964 they started making different upper control arms with more rear offset so they could have positive caster without camber issues.

[maxwedge]

Originally Posted by curtis73

Yeah... I got them backwards a couple times. here is the corrected version of what I said:

Geez, I feel insulted, no one paid attention to my advice/post above, we are all finally on track. LOL

[DeltaP]

Originally Posted by maxwedge

Geez, I feel insulted, no one paid attention to my advice/post above, we are all finally on track. LOL

Aw gee, I actually perked up and took a mental note when I read your post.

[RidingOnRailz]

Originally Posted by maxwedge

Geez, I feel insulted, no one paid attention to my advice/post above, we are all finally on track. LOL

Actually your advice was heeded! The direction of adjustment(positive/negative) was getting mixed up that's all.

The whole gist of my post was to find out if cars factory speced with negative caster would be adversely affected if the caster was made positive. Unfortunately dys-Lex-Luther was haunting this thread from beyond the 8th moon of Krypton. LOL!!

[curtis73]

Originally Posted by RidingOnRailz

Unfortunately dys-Lex-Luther was haunting this thread from beyond the 8th moon of Krypton. LOL!!

Right! ... or is it left?

[snshddog]

Sorry about that, I guess I got it reversed. It has been many years since I have needed that information.