joseph mitro

i understand the advantages of camber and toe-in/out for handling, but i can't seem to grasp caster. it seems that more positive caster helps produce more negative camber in the outer wheel while turning, and more positive camber in the inner wheel. correct?

also, for those that have 944s, which setting induces more caster? tab down? or tab sideways?

Larry Herman

Caster (or for those who have 356s, King Pin Inclination) is the angle of inclination of the steering axis. As the angle becomes greater, as measured from dead perpendicular to the road, the amount of steering induced negative camber increases. Also, because you are raising the car slightly as you increase the caster, the steering effort increases. Some cars run up to 9 degrees of caster.

To make this easy to visualize, tape a cd to a pencil and pretend it is your front tire. Hold the pencil straight up and down and rotate it like you are steering the front wheels. You will see that the cd remains perfectly perpendicular to the "road", i.e. stays at zero camber. If you lean the pencil back, simulating an increase in caster, and rotate the pencil, you will see that the cd leans, gaining camber the farther that you turn it. It is as simple as that.

As far as your 944 goes, I believe that having the tab down on the camber block bolt keeps the rear of the "A" arm in-line with the caster block. Turning it up towards sideways pushes the rear of the arm out, moving the ball joint forward, increasing caster. You can also increase caster by pulling the top of the strut backwards, if you have adjustable mounts.

Edward

Nice analogy, Larry! From my motorcycle days, it is also easy to describe by visualizing the front wheel and forks. The front tire goes off vertical (changes camber) when you turn the handlebar. This wouldn't be the case if the forks were perpendicular to the ground....the wheel would simple turn without changing it's vertical attidtude (zero camber).

As a side note, more caster also provides more self-centering when the wheel is turned, and tends to add more "weight" to the steering wheel. This latter point aids in transmitting more "road feel," which is a fine balance for designers trying to set the geometry of a front end that responds well for the driver, but not too much the the driver feels "everything."

Edward

RedlineMan

Hi Joe;

Think of any wheeled item, such as a desk chair, shopping cart, etc. What do the wheels do when you push it? They line up and go in the direction to which force is applied. That tendancy exactly illustrates the principle of caster... and is quite why the wheels on those objects are named as they are!



The steering angle is an imaginary line between the upper and lower ball joints in this illustration, or from the strut bearing through the ball joint on a McPherson strut vehicle. This is the tangent that the steering pivots about. As you see in the diagram, the tire contact patch is behind this line. This is positive caster.

Because the contact patch trails behind the steering axis, it naturally wants to straighten the wheels to follow the direction of travel. If you started your shopping cart with the wheel's contact patch in front of the steering axis (negative caster), they will automatically rotate around to trail that axis once you move the cart forward.

This contributes to the self centering effect that most cars have in their steering. The more positive caster you have, the more high speed stability you will have as well. The steering will also be effected less by road irregularities. If you think of the distance between the contact patch and where the steering axis line meets the road as a lever arm, you can see that they will be pulled straight more effectively by this increased "leverage."

On a Series II 944, you are using the eccentric caster block to move the rear pivot point in and out relative to the front pivot. Moving it farthest out is max caster. I think the spec is for 2.8 degrees positive caster. I usually max it, but also then even it side-to-side. The Series I cars work slightly differently, and also do not offer nearly 3 degrees of caster.

911

Man, you guys are good. We could have a tech session right here!

Robert Henriksen

What's the downside of having too much caster? How would one go about determing the sweet spot for a particular car?

Larry Herman

One of the down sides of too much caster is the weight jacking that it produces. As you increase the caster you actually start lifting the front of the car as you turn the wheel. Not a big problem with power steering, but it can be a bear with big tires and manual steering. I also think that there are problems with increasing the scrub radius with wider tires as well. I do not know what the upper limit is, and there are other guys who probably can tell you more in engineering terms with regards to different types of front suspensions, but 9 degrees is as much as I have ever seen.

joseph mitro

thanks for all the responses. the porsche mechanic that did my alignment basically poo-poo'd caster and set the blocks straight down, meaning no caster. however, i have heard others say it means a lot, so i just wanted some more feedback.

JackOlsen

Imagine a chopper with a long front fork: self centering, but it lifts the bike when you turn. Then imagine a shopping cart's wheels. Not self centering, but your milk doesn't spill.

On 911's, we try to max out caster.

timo944

I had a discussion with the people at Appalachian tire about caster and the effects on tire wear. Basically, they advised me not to put too much in because, as Larry says above, it increase scrub on the tires when you turn, and significantly so. The spec is 2.0 +/- 0.1 for the NA 944, and I run mine at 2.0. I doubt that anything different would help turn-in, so I just check it once in a whiel to make sure it hasn't moved.

timo

Professor Helmüt Tester

Aside from the steering 'heavyness' that has already been mentioned, my 'hynie dyno' says that too much caster makes the car feel sluggish and numb (more than can be accounted for by the increase in steering 'heavyness') at turn-in. Some people are very sensitive to this, others aren't. Personal preference here is for less caster and a more 'nervous' car ('nervous' is somewhere below the level of 'schizophrenic', which is always bad in a car).

A few years ago, I took a few laps in a friend's 355 Challenge car at WGI. On the 'out' lap, by the time we'd gotten out of the Bus Stop chicane, I was jabbering something like "How much friggen caster is in this pig ?". He indicated that he'd had it set up with max caster (it was his car, after all) and never had noticed/cared about that piece of the handling 'feel'. Lesson learned. Once you get enough to reap some benefits of increase loading in the inside tire, it's a matter of personal 'feel'.

joseph mitro

what does "scrub" mean? and what is the "scrub radius?"

Larry Herman

Not having access to John's nifty keen pictures, I'll try to explain it in a thousand words. Imagine that your tire pivots directly around it's center. It would have no scrub radius because the same amount of the outside of the tire that pivots is matched by the inside that pivots. This way any road force would act equally on the tire and not cause it to rotate around the "pivot point". Ya with me so far?

Now if we use a really wide wheel with lots of positive offset for our example, you can see that the pivot point is now well inside the centerline of the tire. As the tire pivots, any road force will affect the outside of the tire more, and try to push it back, causing pivot it to pivot outward. No problem for forces that affect both front wheels, but if it affects one, it feels just like bump steer, trying to turn the tire in the direction of the bump. Hang on, we're almost to scrub radius.

Now lets take our first example and add caster to the mix. As you add caster, you move the pivot point forward in the contact patch. Now as you turn the wheel, you cause the tire to pivot in an arc instead of directly around itself, and the measured distance of that arc is called...the scrub radius. Additionally, as you run wheels that have less negative offset (moving in the positive direction) you increase the scrub radius to the point where you are literally wiping the tire across the road as you steer it. I think that this is one of the reasons why late model cars that are designed with high negative offset wheels can run a lot of caster with no ill effects.

Hope this explains things a little. Keep in mind though, I could be totally wrong.

fatbillybob

Maybe I'm a dork but...I like lots of caster in my tail heavy car. 1) if feels like the car tracks more true at high speed. 2) at high speed even with manual steering the steering is not heavy because I'm going fast. 3) I found that if I try to save an "off" I often make it worse. By mistake I went off and did a tank slapper and let my hands off the wheel. The caster helped to selfcenter the car and when pointed in the right direction I grabbed the wheel and stepped on the gas. Gee...that was cool. It looked like I know what I was doing. So now I correct loop by doing less not more and it has saved me a few times. It is all in the caster. Just a few times I try to keep the black side down. Caster is the reason some bicycles are harder or easier to ride with no hands.

RedlineMan

At your Service, Larry Darling!



Scrub Radius - The distance between the extended line of the steering axis, and the centerline of the tire contact patch.

Similar in effect to Toe-in and Caster, some amount of positive scrub radius (Steering axis inside tire centerline) induces the wheel to be rotated backwards (pivoting around the steering axis) by road friction. With both tires being pulled equally, you get a steadying, centering effect. However, too much positive scrub radius becomes problematic when an impediment is encountered by only one wheel. Again, similar in effect to 0 or toe out, or to a lesser extent having very little caster.