Took my car to a Firestone shop to get an alignment today. They did it, but said they could not get the caster in spec on the RH side, and asked if the car was in a wreck at some point. They were able to adjust the toe and camber, but just not the caster on one side.

Some facts:
- I've owned the car since new and it has never been a wreck.
-I've had the alignment checked/adjusted at least once per year since purchased, and was never told of a caster problem until now.
- The car has always steered and tracked well (and still does).
-I installed F&R sway bars and strut tower bar from an Acura about 8 months ago. These were new factory parts and fit on easily.

Do you think:
1) this latest alignment shop was wrong?
2) something happened since the last alignment a year ago (pothole, something worn, etc.)?
3) always been this way and I was never told?
4) Sway and strut bar changes had something to do with it?

I'm leaning towrds 2) or 3). Any thoughts? I'm not losing any sleep over this, but just seems a bit odd.

 

I may be wrong but I didn't think camber/caster was adjustable one Accords without a camber it.... Yours might be different, however I think the shop may have their head up their rear... Is this the forst time you've used them? Did they show you a readout from it or just tell you?

 

And even with a kit, you can't adjust caster. The only thing that allows caster adjustment are pillowball mounts.

 

Assuming the measurement is accurate and the car does indeed have have different caster side to side, in generally and long accepted theorythis will cause a 'pull' toward the side with the least caster (I think that's correct). However, I think this theory does assume a significant amount of scrub radius also being present in the geoemtry, as is typical for rear drive cars. When trying to diagnose a mystery 'pull' in the steering of my CB7 Accord I made substantial changes to caster angle on one side of the car (from less than and to greater than the caster on the other side) and it made no difference at all to the pulling problem I had, though it did change the way the steering felt in right vs left hand turns (lighter and 'woolier' one way and heavier / more precise the other way).

I suspect the reason this is so is that on most front drive cars the scrub radius geometry is at or very close to zero and as a result the caster effect can't act unequally side to side through the scrub radius but is instead creates a 'cumulative' affect that is felt at both front wheels, i.e. the caster affect at say the left front wheel acts at the left wheel and is also transferred through the steering linkage to the right wheel, and the caster affect created at the right front wheel acts at the right wheel and is also transmitted through the linkage to the left front wheel. If significant scrub radius is present then this multipliesthe affect of the caster angles, and if the caster angles are different then there will be more 'self centring' affect on one side and a pull will be felt.

On the other hand if scrub radius is zero then there is no 'mutiplying' effect through the scrub radius and even though a self centreing effect from caster does exist (and is stronger on one side than the other if caster is unequal)it can't be felt unequally side to side and no pull is felt. That's probably clear as mud, but if you can follow my less than clear explanation it might explain why you aren't feeling a pull even with unequal caster angles.

If it's not pulling to one side then I wouldn't be worried, the unequal caster won't cause unequal tyre wear. But, you might (?) notice / feel that with unequal caster that the car may still track straight, but the steering might feel a little different when steered right vs left, and this could be because as you steer the wheels away from the straight ahead youintroduce some 'effective' scrub radius, i.e. as you steer, the centre ofeach contact patch moves awayfrom being directly behind the point at which the steering axis intersects the ground to a point that is to the outside of the steering axis on the outside wheel (relative to steered direction) and to a point to the inside of the steering axis on the inside wheel. In this case the introduced 'effective' scrub radius will be different when steering one way vs the other, and cause the steering to feel different one way to the other way, but have no affect on tracking because when not steered the 'effective' scrub radius disappears.

Sorry but this is hard to explain unless you already have a good knowledge of the various geometries involved in steering, and even then it's a not something that can be explained in a few sentences.

PS The mystery pull turned out to be tyre related, i.e. 'conicity' in at least one front tyre.

 

Yes, he showed me a printout. No I haven't used this shop before. I have troublebelieving that camber can't be adjusted on adouble wishbone suspension system, though.

 

Clear as tar to me John!!!! LOL .... nah, I understand you ..... but I may be one of the only ones.

In simpler terms, the caster is the angle of the suspension from vertical when looking at the car from the side. A minor imballance could be equalized for the most part through the steering. This imballance will likely only be fealt as a slight differance in the feel of a right turn vs. a left turn.

 

It can be adjusted on Hondas, but it's not as simple as twisting some threaded adjuster (unfortunately). You may be able to make a very slight increase in caster by removing a small washer that sits against / next to one of the large washers at the front radius rod bush (the radius rodforming the forward arm of the lower 'wishbone'), or a greater range of possible caster decrease by adding more washers at this point. This involves removing the radius rod in order to fit / delete washers, a pain!

Short of this we're talking modification to either the lower radius rod (shortening to increase caster), or to the upper wishbone (to move the upper ball joint backward to increase caster). Caster can be increased with Mac struts by pulling the top of the strut backward, but the scope isn't large. You'd never want to reduce caster unless to equalise side to side and increasing it to equalise was too big a job. Increasing caster significantly / substantially is a very worthwhile mod (if the car doesn't already have a large caster angle), but if you're making more than a relatively small increase you really do need to know what affects this might have on the relationships between other components as you could introduce mechanical problems.

 

What I'm clumsily trying to explain is that there is an interdependant relationship between scrub radius and caster angle that work together to create a pull if the caster is not equal (or if the scrub radius were not equal, but this is highly unlikely to be significantly different side to side). If scrub radius is zero then the interelationship is effectively zero, but increases as scrub radius increases. Even quite a large difference in side to side caster probably won't cause a pull if scrub radius is zero, but would affect feel left vs right.

Keep in mind that scrub radius represents an effective laterally oriented virtual lever arm through which tyre forces are fed into the steering axis and from there to the steering wheel. If no scrub radius is present then at the straight ahead position the (nominal) centre of the contact patch is directly behind where the steering axis intersects the ground and as a result the geometry has 'pure' trail and the contact patch will always 'want' to stay directly behind the steering axis regardless of how great the trail may be, which happens to be with the steered wheels pointing straight ahead(keep in mind that the trail is more or less defined by the caster angle, i.e. more or less caster = more or less trail).

When we add scrub radius we offset the contact patch centre from being directly behind the steering axis, and the contact patch will now 'want' to 'steer' until it is directly behind the steering axis, and this affect will be stronger the greater the scub radius is, and the greater the trail / caster is, so if caster is unequal then the self centring force will be stronger on one side and felt as a pull because to achieve pure trail the steering has to turn one way and the car will veer. Take scrub radius away and the effect drop to zero because the contact patches are already at pure trail with the steering at the straight ahead, so differences in the side to side self centring strengths are not multipled through scrub radius andare not felt.

Clearer??

 

John .... I understood you, but a lot of the details leave people lost, hence my over-simplification. Is there another term for "scrub radius"?

 

There may be, but scrub radius is what I know it as. The term 'steering axis offset' springs to mind, but I'm not sure whether it's something I'm subconsiously inventing or not!

I am trying not to use too much jargon, but it's difficult not to!