**Negative** Caster

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?

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.

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.

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?!!:eek2::eek2::eek2: 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. :)

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

:bigthumb: 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.

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.

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. :)

:bigthumb: 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.


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.

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.

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??

:runaround:Heeelp!!!! :runaround:

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??

:runaround:Heeelp!!!! :runaround:

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.

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

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.

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!!

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

Right! ... or is it left?

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

I'd like to bring this one back around...

I have a 64 Pontiac Bonnneville and do my own alignments at home... I have around 4 degrees postitive caster using a little cross caster. My car handles excellently around bends especially with a light toe out but I'd like a more positive centering. I have around 1 degree of negative camber with a 1/16" of toe in...

My car has a Lee 12 to 1 box Addco sway bar and is in excellent condition.. I have 1962 factory upper control arms with solid bushings, they are the exact same dimentions as the 64 upper only the 64s has rubber bushings...
For rubber I have 235/70r15 H rated Westlake SU307 tires..

I'm tempted to try the stock settings of 1 degree negative caster just to see what would happen...

Any thoughts?

Well for sure you will loose some directional stability, I don't see whay you would do this except to experiment, plus 4 seems a lot for that year car, what is the stock setting, near zero? Your camber seems too much negative for a normal hway cruiser, should be maybe .5 or so? True the negative camber will allow better cornering planting more rubber, but you will wear the tires. I am surprised you could get that much caster out of the range of adjustments, that may be why your camber is so far in.

Update on my 64 Bonneville..

I removed all the rubber control arm bushings and installed nylon bearings from KRC.. I also lowered the front by cutting one coil off the Moog springs, they had the ride height 1" over factory specs...

I have the car set at 3/2.5 Negative caster, .5 negative camber, 1/32" toe in.. It cruises at 100 with one finger on the wheel... It also has excellent centering on the straightaway and after a turn...

I read all the info on caster trail and all but it drives nice this way..

When I tried positive caster, the car banged through pot holes, with negative you don't know they are there.. On my car I could run up to 4 degrees positive or negative caster..

Shows everything isn't for every car..

Moral, check ride height first! Bump steer is a nightmare.....

Thank you....

The factory manual calls for 1.5 deg. neg. caster, 1/4 deg. pos. camber and 1/8 in toe in. With modern radials pos. caster is always called for if the range of adjustment allows it. IMHO your caster is way too negative.

I know of no requirement that radial tires have such-and-such amounts of caster.
When you are re-engineering a suspension by replacing all the rubber bushings with plastic and radial tires are substituted for bias-ply (and that really is re-engineering), suspension settings are back to a free-for-all status.
The caster, particularly, is very much a setting that can be adjusted to whatever a driver feels comfortable with. If a particular driver like a car with lots of straight line stability, he can add lots of caster. And vice-versa.
Since the tires' footprint itself (pneumatic trail) is a source of much of the stability of the car, and positive caster is *additional* stability, it is obvious that some liberties can be taken to adjust the car to the owners preferences.

However, I've found that reducing the caster on one of my mid-engined sports cars had an interesting side effect.
By reducing it from 3.0* positive to 1.5* positive, it had an interesting tendency to actually "steer into" the corner by itself (as felt thru the steering wheel) while negotiating a corner near the limit of traction.
I tried releasing the pressure on the wheel, and sure enough, the wheel rotated, and the front end steered in, and that was enough to pop the back end out.
My guess was there was insufficient trail to keep the loaded center of the tires' contact patch on the stable side of the steering kingpin axis (aft of it).
So sometimes a little extra caster covers up for some handling and geometry issues.

Agreed, Black Lotus. in my shop we restore/ repair 30-60's cars and we have found when putting radials on these cars they tend to have " twitchy" steering issues ie: you have to constantly make slight corrections to stay in the straightaway, no suspension wear or looseness in the steering, we address that first, getting to even slightly positive caster corrects this. Of course this does not mean every car we work on does this, a good examples are a 55 Chrysler and a 59 Dodge, 32 Ford street rod.

The factory manual calls for 1.5 deg. neg. caster, 1/4 deg. pos. camber and 1/8 in toe in. With modern radials pos. caster is always called for if the range of adjustment allows it. IMHO your caster is way too negative.


Thanks Maxwedge, I had your post in the back of my head and it made me keep going to Positive Caster, then it started getting better and better. The difference is amazing... Much more comfortable and stable...

I can add more caster but my car likes it between 3 and 4 degrees.. I had to add more on the passenger side to remove a slight drift I was getting...

I also had the same effect as Black Lotus when my caster was around zero.. It was weird and not something you would want on Western Pennsylvania roads...

I feel better now that I'm done with all these alignment changes, that was a whole lot of work! I doubt I'll ever sell this car now..

Interesting input, folks!


Whether a 1964 or a 2008 car, I need as much positive caster as allowed:

I'm a naturally drifty driver(can never seem to stay in my lane). I almost think I should just get a manual steering rack installed - for the safety of everyone! LOL