**Negative** Caster

[pitts64]

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?

[maxwedge]

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.

[pitts64]

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....

[maxwedge]

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.

[Black Lotus]

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.

[maxwedge]

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.

[pitts64]

Originally Posted by maxwedge

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..

[RidingOnRailz]

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