Before making any mods to my '04 MCS (with Sport Susp.) I thought it would be a good idea to have the alignment checked and to get a baseline on the stock setup. I got the report below which shows Caster of 4.2 degrees on the front left, and 3.7 degrees front right. The shop says the specified range is 4.3 to 5.3.

The difference itself is less than the difference of the specified range, but both values are a bit low. Questions are:

1. Does it matter?
2. Anything I can do about it?

Thanks. (learning more each day....)

Left Front (degrees)Actual Before Specified Range
Camber -0.6 -0.6 -0.9 to -0.1
Caster 4.2* 4.2* 4.3 to 5.3
Toe 0.16 0.18 0.11 to 0.19
SAI 12.3 12.3

Right Front (degrees)Actual Before Specified Range
Camber -0.6 -0.6 -0.9 to -0.1
Caster 3.7* 3.7* 4.3 to 5.3
Toe 0.14 0.03 0.11 to 0.19
SAI 11.5 11.5

Front (degrees)Actual Before Specified Range
Cross Camber 0.0 0.0 -0.5 to 0.5
Cross Caster 0.5 0.5 -0.5 to 0.5
Cross SAI 0.8 0.8
Total Toe 0.30 0.21* 0.22 to 0.38

* not within specification

 

Bring the car to another shop and have those figures verified. I personaly get nervous about caster. Were you in the car when the alignment was done? A yes would explain the difference. Have you hit anything?

A good alignment is only as good as the tech performing the work gnhovis. Very subtle movements can make an alignment look good or bad on paper. The relationship between your settings has to be interpreted by your tech. He/she must be able to evaluate the numbers' relationships in order to make changes or recommendations to you. good luck.

 

Thanks for the advice. No I wasn't in the car and as far as I can recall, haven't hit anything....

 

Ever ride a 10-speed bike?
With the handlebars oriented properly, the front axle is in front of an imaginary line drawn through the gooseneck where the handlebars mount.
In this position, the bike is fairly stable, a little slow to turn, & steering effort is moderate. This is a positive caster situation.
Now, turn the handlebars around backwards. Now the wheel is behind the line imagined above.
Now it'll tun on a dime, has light steering effort, & tends to wander all over the place. This is negative caster situation..
Caster does not affect tire wear, although the car will tend to pull toward the side with the least caster (or most camber, for that matter). I wouldn't fret too much if it's slightly under spec. as long as it's within 0.5 degree or so of the other side.
And I wouldn't trust most folks' equipment to be accurate within 0.1 degree. You're likely just fine, gnhovis..

 

here is another way to explain it: project a line through the steering axis of the wheel (the line between the upper strut mount bearing and the lower ball joint) to the ground. the tire contact patch is behind this with positive caster. when you turn the wheel, the contact patch exerts a restoring force to straighten the wheel. like feathers on an arrow.

another note: if you have too wide tires, or too much offset, the center of the contact patch r/l might not be directly behind the extended axis line, causing tracking problems or tire scrubbing when turning.

 

Hey bodinski,

I race bicycles with a heavily sponsored team...we mean business. I never thought to refer to my bicycle. Even though jlm's description is more car, if you will, it is possible to experience different caster and or trail designs with just a change of fork design. We occasionaly experiment with this kind of stuff for different venues. We're just wasting time; the manufacturer usually gets it right. Still, this experiment will illustrate the difference or the extremes between stable and unresponsive, and, responsive and really unstable. I had a race at Lime Rock race track last May - yes, a bicycle race - in the pouring rain. We ran the course backward. If you want to feel what near negative caster feels like, hit the down hill after the chicane between 35 and 39 mph with 50 other guys and you'll know why some positive caster or trail is a good thing.

 

Meb... where do you live? I have raced Cat 2 for many yrs and now race masters. Haven't been training much anymore ... too busy with the el....

 

bicycles and motorcycles throw a curve ball into the explanation: the steering axis angle to the ground has a more apparant effect. the more laid back is your steering axis, the more the wheel raises the chassis when you turn, producing an additional restoring force trying to straighten out the wheel. they can get away with this while tooling along since chassis lean produces most of the steering; different from a four wheeler.

 

...you've momentarily confused me jlm; you mean to say wheels raise in a car, which in turn - punn - causes an additional self centering action? Yes?

 

I raced all over the southwest when I was in college. I actually packed my bags for Europe to go train and race there - Italy. I have family there. My parents cut me off and I came back. I did very well.

Two years ago I joined a team. As you know, no matter how conditioned you are, you must begin at cat 5. I ran cat 5 for most of the first season and then ran cat 4 for the remainder and all last season. When racing as an independent, I found I could cat-up faster. Team orders preclude me acting on my behalf. I am attempting to cat-up to 3 for this season. I'm building a house, I head a design business, I'm a dad and husband; I fit an extra 25 hours a week into my schedule for training last season, but at a cost to my kids and wife. Very selfish sport at this stage in my life. Love it a lot! Cat 3 will be even more demanding...let one or two training sessions slip and you're at the back of the pack. One huge motivation; cat 4s are all over the place interms of control and ethics. I want out after a broken knee and thumb.

 

Thought about one aother possibility; one tire is slightly less inflated than the other??? If my brain is working, I'll get this out correctly; the tire with less air will in fact posses a smaller diameter reducing the distance from the steering axis to the tire's center of drag or contact patch. The car will pull to this side. Check inflation. Conversely, and this is ONLY a trick; if you cannot find any differences anywhere and the alignment is perfect, slightly over inflate the tire on the same side as the pull.

 

uh, bod man, jlm has it right. it's not the relationship of the axle to the steering axis, but the relationship of the tire contact point to the axis.

check this out on your bike: the front tire contact point will be BEHIND the centerline of the stem tube extended to the ground.

 

I think he is speaking about a bicycle, in which case the axle does indeed fall in front of the imaginary line. The imaginary line runs down the head tube of the bike and thru the center of the fork to a point on the ground, which will be in front of the tire. Speaking bike, the distance from the above imaginary line to the center of the axle is called fork rake. This along with the caster measurement for a bicycle determines how responsive or how slow/stable a bicycle will turn or decend a hill. Ultra responsive bicycles, tend to be a little scary going down hill at speed. My last down hill was thankfully on a bicycle designed with classic European geometry. Otherwise, down hilling Mt. Lemon in Tucson, AZ would have been life threatening...+/- 7,500 feet in 22 miles. The race was up the hill, er mountain. A few of us brought along a gear set change and went a coasting...at about 50mph we figured.

 

yes, the AXLE is in front of the steering axis, but what's relavent to the restoring force is the relationship of the tire contact point to the steering axis. lay a straight edge down the center of the head tube on your bicycle or motorcycle - the tire contact point on the ground will be behind the point where that straight edge meets the ground.

conceptually, the idea is the same whether it's a car front wheel or a bike front wheel: put the tire point behind the axis point on the ground so that moving the tire point off to one side generates a restoring force to bring it back to center.

 

Uhm, that's what I wrote. I copied below.

I think he is speaking about a bicycle, in which case the axle does indeed fall in front of the imaginary line. The imaginary line runs down the head tube of the bike and thru the center of the fork to a point on the ground, which will be in front of the tire.

And so, "be in front of the tire" means the tire and or the tire's center of drag is behind the imaginary line's intersect with the ground - which also = positive caster. Still, the axle is in front of that imaginary line which is of extreme importance in bicycle geometry. Center of drag depicts the tire's contact point defined by deformation in the straight ahead. Biased ply tires deformed so much that manufacturers built in static negative caster. Once underway, the tire would deform creating positive caster with repsect to the 'imaginary' line and the tire's center of drag, or dynamic caster. This also points out why cars designed way back when with neg caster can be very unstable at speed if radials are installed. With virtually no distortion, caster remained negative while underway.

On a bicycle, the restoring force is indeed, as you state, determined by caster, or the degree of caster. Further, fork rake, the distance the center of the axle is from this imaginary line, determines how quickly or the rate at which the restoring force is generated. Increase the fork rake and the restoring force is slowed. A straight fork will be wickedly quick, all else equal. I would not want to drive a bike with a straight fork, very twitchy.

 

Yes, that's right.

I once drove a Formula SAE car (basically the first and only time I've ever experienced accelaration that can only be described as "violent"), which was awesome, but they had designed the car with a TON of caster, despite it having a fully adjustable suspension, which they could have dialed in as much camber as they wanted. The massive caster made the steering effort outrageous. I asked why they used so much caster, and they explained that the LSD was so tight that it would scrub the rear tires around the corners and make the car understeer. The massive caster was unloading the inside rear tire, and allowing inside tire to slip a little. I guess the question that begs to be asked is 'why not loosen the LSD then?' but I think the answer is that when a car weighs 400lbs and does 0-60 in like 3 seconds, you need the LSD that tight.

I don't understand how McStruts work in corners, it gets really confusing. But, having a lot of caster makes the outside wheel gain some negative camber in a corner, which is a good thing. The downside is that the inside front wheel is slightly lifting up that side of the car, increasing body roll.

 

That is what I thought jlm meant, just clarifying thru different words. But, I'm under the impression that SAI (steering angle of inlcination) along with camber is what causes a vehicle to rise thereby creating a self center force. I'm not sure caster is involved here. Caster may contribute, just not sure. Gotta go dig something up...I'll get back with some info soon.

Caster is indeed a very complicated measurement. some folks have employed cast as a way to gain some inefficiencies back to the rear wheels, albeit at the expense of other handling qualities. I understand some of the more basic aspects of caster. There is much, much more to learn, however.