Technical Pos. Caster vs neg caster

Will one or the other make easier steering? Less force to turn wheel.

 

The less negative caster you have the easier the steering. More negative caster is better for down the road stability. Never seen a car with positive caster....

 

Whoops

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Increasing positive caster
•increases high speed stability
•Increases self centering return ( with in reason )
• increase steering effort but only immeasurably.

The least caster I've seen speced was +1.75. So I've not yet seen a car with negative caster.


Barring any worn parts, Most of the time high effort steering is because of the mechanical ratios between the parts. The first is Steering wheel size to box ratio, that ratio to pitman arm length, pitman to steering arm length. Throw vs travel.

Highly doubt you'll lower steering effort enough thru caster change.

 

I used to set more + caster on older folks cars when I was doing alignments. Also the same on stock cars or they tended to have erratic steering on the straightaways.

 

I have recently increased the 1.5 deg to 3 deg castor on the truck . Purpose was to improve stability when cruising with the camper on. Worked great .
Added some noticeable load to the steering when parking, .
So not much change can have a significant effect.

 

Shopping trolleys/carts have negative caster, ever noticed how the wheels wobble when you push them? imagine that happening in your car.

 

Look at a bicycle lots of pos caster so it will go straight turn the front wheel around and the stability is lost.

 

............................................Wow, That puts it into terms that anyone should understand. Nice ****ogy.

 

"casters", those little wheels on shopping carts, tool boxes etc., etc., are called "casters" precisely
because that is the principle on which they work. They are NOT called "negative casters".

The geometry of a "caster" is obvious....the pivot point leads the wheel. That is positive caster. On a bicycle, motorcycle, car or truck, the pivot point (fork head, king pin or ball joints centerline) leads the wheel, not as extremely as the utilitarian "caster", but the physical principle still applies.

Ray

 

The only car I know of with negative caster was the (Australian, GM).. 1971 Holden HQ and HX models from memory, all had negative caster.. Whereas the HZ model had a revised suspension called RTS (radial tuned suspension) not sure what the settings were for them..
Wheel alignment details for HQ - HZ
Caster deg...........1-2 neg.
Camber deg..........0-1 pos.
Toe-in...................0--3.17mm

 

I usually dial in more than commonly done. My 27 is about at 10 degrees and goes right down the road. I can't see how negative caster would even work on a car.

Don

 

Probably better explained to the unknowing by stating that a line drawn through the pivot axis (king pins, ball joints etc) should meet the ground plane ahead of the contact patch of the tire. The further ahead the pivot axis is located the more self centering action occurs, thus dirt bikes have the forks nearly vertical so that they turn quickly with little effort and choppers have the forks raked way out for highway stability. Similarly regular street cars (especially those without power steering) have minimal caster so that they are easy to park and dragsters have lots because parking is not part of their job description. Early front engine dragsters ran huge amounts of caster (as much as 40 degrees) and that is why they lay the front wheels over when making turns in the pits. Even with zero scrub radius (not very common in the hot rod world) excessive caster will increase steering effort as when you are coming off the turn you are effectively lifting the front of the car at the outer wheel as you straighten the vehicle path.
The scrub radius scenario can also be a major contributor to steering effort with higher caster numbers.The further out from the king pin axis the tire contact patch is located in the lateral plane the higher the steering effort as, again, it increases the jacking effect when turning. In other words those skinny little no back space Halibrands with the narrow tires may be as heavy to steer as a wider tire/rim combo if they are hanging way outboard on a set of Buick drums.

Roo

 

Wow-senior moment-sorry. Everyone else is right ,,

 

Don't pick up a 50s or 60s shop manual or you will find plenty.

 

Hi Roo. Long time no see.....used to be with the seals-it gang. Sure had a ton of fun. Thanks for the reply. Jim

 

Too much negative caster can break your arm when hitting a dip in the road. In the 60s some cars had a spec a little negative (maybe up to .5 deg) usually with manual steering, power steering was typically 1 to 3 degrees more positive.

 

While I agree with your explanations before and after this paragraph, I have to wonder if you intended to say what you did here. In fact, the caster " lifts the weight of the car" when turning the wheels FROM straight ahead to either left or right.......not when returning to straight ahead. The positive caster, combined with king pin inclination, causes the raising effect when the spindle is turned and, conversely, gravity tends to force the the spindles to straight ahead, reducing, not increasing, steering effort when coming off a turn.

Ray

 

While the statement is true only for one side the other side is actually dropping.

 

another factor to add to that is wheel offset. Too much positive offset will lower the wheel when it is turned outwards [ the opposite to positive caster ]

If a car had zero castor ,the kingpin inclination will induce positive camber when the wheel is turned either side of straight ahead.
Positive caster is to correct that camber on the wheel that is turning inwards. The side effect is the steering will also try to self align.

Cars with negative caster usually had a lot of negative scrub radius /or positive offset wheels.

 

Shopping trolleys have positive caster ,where the wheel contact patch trails behind the point where the kingpin intersects the ground.

I got this from an expert

 

Yes, I thought about that after I posted my comments, but did not edit my post. Thank you for clarifying the point.

Ray

 

If I remember correctly the first-generation Olds Toronado and fwd Cadillac Eldorado ran negative caster.

It seems to go with front-end weight. By contrast light, rear-engined cars tended to run huge positive caster. I seem to recall the Fiat 850 Spyder running something like +10-11°.

 

some of the manual steered CDP A bodies used small amounts of negitive caster if equipped with Bias tires , radials were zero to small positive my 72 demons with 6 bangers were negitive raked , but the 340 was positive raked , and when you put the 6 bangers positive the wheel was harder to turn and snapped back straight when rolling real quick .

 

Pos caster seemed to drop as tires got wider A wide tire will roll in a straight line better than a narrow one.

 

Once those A bodies got big meats, air shocks and long shackles the caster went forward a bunch

 

Aaah!! Someone else old enough to remember the 70's.

Roo

 

I've seen some alignment specs (older cars) with real small + caster values. In the end, we did what we had to do not to send a car out with a pull condition. Which would be towards the side with the leading upper ball joint (or most - caster).

 

The other factor to consider is that while turning (say) right the weight of the vehicle will tend to be transferred to the left side and that is the side that is allowing the ch***is to drop as the wheel is moving up due to the positive caster/negative wheel offset (the most common set up in our world). The right wheel is moving down for the same reason but is not as heavily loaded due to the g forces and that is why all of this works on the street at normal speeds. Parking, not so much, if the caster/neg offset numbers are high.

Roo

 

Must be camber changes too when turning, perhaps have an effect on steering effort.