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Caster is wrong

2K views 19 replies 6 participants last post by  pokihokie 
#1 ·
Hello!

I have a problem with the wheel alignment which is causing the car to pull to the right. According to the computer the only problem is the caster of the front right wheel. The left has 3.5 and the right 1.6
Could you help me how to solve this problem? Is the only solution moving the subframe?
I don't think bushings are worn at the rightside because that would cause more caster not fewer. Am I right?
 
#2 ·
I'll try to explain, and also offer a troubleshooting option you can use--

I often think of caster this way:
If you have strut suspension, 0 caster means the strut is completely vertical in terms of front-to-rear (we will ignore inside-to-outside tilt for this discussion). The hub is straight down from the strut mount. Positive caster means the strut is leaned like motorcycle forks: The hub and wheel are in front of, or leading, the strut mount. Negative caster means the hub & wheel are trailing behind the strut mount. It's really uncommon to have negative caster on purpose on road cars.

Super-soft or broken bushings alter caster, but often (I'll say usually) more while you're driving than while the car is just sitting there on the alignment rack.

Cross-caster is when the caster is different (more than a tenth or two of a degree) on one side vs. the other. Cross-caster is 'normal' for automakers to specify. They use cross-caster to help the car track straight down a crowned road (most roads are high in the middle, and taper down toward the edges to help drain away rain water. This is called "crown" in the road).

There's something else besides cross-caster and toe problems that makes a car pull to one side: A separating belt or tread in a tire. You can rarely or never see this with a quick visual inspection of the tire when it starts, in my experience.

To give you an example, my pickup started pulling to the left recently. I initially thought maybe my son or my wife ran unto a hole or a curb and knocked it out of alignment. However, before spending the money for a 4-wheel alignment on that pickup (they're expensive here), I swapped the two tires on that side. That cured the problem, so I know the pickup does not need an alignment or any bushings to be replaced. Usually, unless the separation is really advanced, moving a tire with this problem to the rear axle will stop the vehicle from pulling to one side, or at least significantly reduce the pulling. It's a cheap and easy way to troubleshoot alignment vs tire problems.

I will have to be careful. Sometimes a separation like this doesn't get any worse, but sometimes it does and the tire will fail very suddenly. That can cause a crash unless you're an alert, skilled driver (and sometimes you just have to be lucky).

Good luck. Please keep us informed as to what you discover is making the car pull to one side.
 
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#3 · (Edited)
Good explanation...

I'll come to the point presently, but humour me here...

In my downright stellar (cough, cough) stable of cars I have a 1994 Mercury Topaz GS 2 dr, 2.3 litre ohv 4 engine, 5 speed MTX-III (Mazda) transaxle. Long story about how/why I got it (& I won't go into that)... and it's been off the road for some time (but it only has about 84,000 mi.). Putting it back on the road to teach my two adult daughters to drive manual, then probably giving it away to some young person who can't afford to buy a car. Believe it or not it drives reasonably well for what it is, and the pedals are absolutely wonderful to heel/toe with while double-declutching:).

Anyways, the front suspension is rather poor by way of design, and I believe the caster values are equal, right versus left...but the camber is differentially set by the factory i.e. right versus left are different. It has unequal length halfshafts... and despite its low (96 HP/) 126 ft lbs of torque it will torque steer.

I always thought the differential values, in (positive) camber were in place to in part mitigate against even worse torque steer.

BTW I hate the extreme positive camber the chassis assumes when accelerating. UGLY...
 
#9 ·
Anyways, the front suspension is rather poor by way of design, and I believe the caster values are equal, right versus left...but the camber is differentially set by the factory i.e. right versus left are different. It has unequal length halfshafts... and despite its low (96 HP/) 126 ft lbs of torque it will torque steer.

I always thought the differential values, in (positive) camber were in place to in part mitigate against even worse torque steer.

BTW I hate the extreme positive camber the chassis assumes when accelerating. UGLY...
IIRC, cross-camber will also mitigate the effects of road crown. Just drawing imaginary lines in my head, I think it should do so. Maybe that's what they use it for in that model?
I'm unsure about how it will react relative to unequal-length halfshafts. Maybe it's helpful there, too.

...is this a test you already know the answer to? I'm eager to hear the answer, if it is.
(-:
 
#4 ·
Caster helps the steering move back to center, camber helps you turn. Theres no way anyone wants positive camber (where the wheel sits on the outside of the car vs the inside). OP, unless things are physically busted, theres no way you can have that much difference in caster on each side. Are you sure he didnt say "camber'? If it truly is caster, its possible the subframe shifted, but im not 100% sure.
 
#5 ·
Caster. I saw the datas on the monitor. I already switched the tyres, even bough new winter tyres. Today I tried to measure distances under the car, and the lower control arms are the same distance from a hole I found independent from the subframe.
I don't know what to do....

Can the upper control arm bushing cause this? The rightside needs more caster. I don't see how it would cause this, just a guess.
 
#12 ·
Interesting article. No, it's not a test for you that I devised, hehe.

Just curious (and I'll look it up presently) but say for MacPherson strut, is Steering Axis Inclination (SAI) the same as caster? For upper and lower wishbones is caster different from SAI?
 
#13 · (Edited)
No, I guess SAI is inclination as viewed in an elevation view, from the frt of the vehicle or from the back of the vehicle...

For MacStrut, SAI is the tilt (usually) inwards to the vehicle centreline... of the strut, and I believe caster is the tilt angle (usually) backwards of the strut.

I THINK...
 
#14 ·
I have seen that explanation given for steering axis inclination before. Steering axis inclination as I understand it is the inward-outward angle between vertical and the kingpin (steering) axis. Is that angle always or necessarily the same as the inward-outward angle of the strut tower in a Macpherson strut system? I did not think the two had to be the same, but I could be wrong.

 
#20 ·
There's been a good bit of chatter in this thread about camber but the OP said and confirmed that it was caster. So that's what I'm replying to.

I didn't see what year your car is but you did note that it was a double wishbone design. I'm presuming that also means that it has the separate arms at the bottom. IF all that is correct, one thing you should consider is the spring. If the spring on one side is broken or sacked out it allows the strut to be shorter, reducing the height of the suspension. Because of the design of the pickup points, that means that the caster will be higher on the bad side. It could be total nonsense, but the caster was off on my 04 wagon and out of curiosity, I put a pole jack under the subframe and lifted it about half an inch and it came right into spec.

Something to ponder. I hope this helps!
 
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