What causes a car to follow ruts while driving?

I already know the short answer: Worn suspension or bad alignment.

I'm looking to know the "Why" though.

I recently got my new Valiant on the road with the old and stock suspension and besides the loose steering the car follows ruts in the road pretty bad.

Why does it do this?


Thanks again Moparites!

My amateur opinion/speculation is that rutted roads are also irregular (meaning, the ruts aren't smooth conduits) so as the tires hit different irregularities the tires take turns being the dominant one.

Not enough positive caster, the simple answer.

Same reason a needle follows a groove in a record. The rut is the low point, so gravity causes stuff to go to the lowest point.
Loose steering means that there is play allowing the tires to follow the ruts easier.

Bias ply tires like to follow ruts.

Great question, however any response I gave would violate "feets" directive in my signature below

This car has 14" radials on it.

I know more caster would definitely help higher speed stability but does it really help when it follows ruts? I was thinking if the alignment was off the toe would be suspect.

I know something is loose in the front end because the steering is somewhat "wandery". Not sure if it's in the steering linkage or box itself.

Hopefully tonight I'll get a chance to get under it again and look for excessively worn stuff and tighten up the manual steering box and add lube if necessary.

Thanks guys!

When a road surface is something other than smooth (i.e. rutted or grooved), the grooves in the tread "want" to match up and follow the irregularities in the road.

a. Bias ply tires do this more than radials, due to their construction.

b. Worn suspension does this because there is more slack before your steering input has any effect.

c. An alignment with negative [or less positive] caster does this because it lacks the inherent stability that positive caster provides. See also: worn suspension that can bring with it bad caster. See also: cars with a significant tail-up stance that bring one more degree of negative caster for every additional degree of rake in the stance.

d. An alignment with significant toe-in OR significant toe-out does this because you can't achieve "straight ahead" with both tires at the same time - one might be straight, but the other is trying to head off in another direction. See also: worn suspension that can bring with it bad toe settings, like a floppy idler arm does.

From your description I think that worn parts are the chief culprit. On a rear drive car a slight amount of toe out can help on an autocross course, but will cause the car to wander or be "darty" in anything like normal driving. You MUST fix or replace any and all worn parts before you can expect decent handling. Lower control arm bushings are a bit of a PITA, but necessary. Strut rod bushings are easier and very important since they are critical to maintaining proper toe. Once this is done set camber to 1/2 degree negative, castor to at least 2 degrees positive [ 3-4 ] is better, and 1/16- 3/32 toe in. Also keep in mind that when using wider wheels and tires that offset [ backspace ] can also affect handling. If the centerline of the tires is farther out [ away from the centerline of the car, basically a wider track ] than stock this will increase the scrub radius. Increasing the scrub radius besides causing the tire to drag slightly in a turn also causes an effect similar, but not identical to bump steer.

Overly wide tires will do it also. My '66 is aligned well with all new suspension and it still follows the ruts somewhat because I am running 245/60s in the front. Much wider than it was designed for. Even my new Infiniti G35 does it because of the wide tires. My old BMW 325i also did it. The suspension on all those cars are tight, but they all run wide tires.

Quote:

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This car has 14" radials on it.

I know more caster would definitely help higher speed stability but does it really help when it follows ruts? I was thinking if the alignment was off the toe would be suspect.

I know something is loose in the front end because the steering is somewhat "wandery". Not sure if it's in the steering linkage or box itself.

Hopefully tonight I'll get a chance to get under it again and look for excessively worn stuff and tighten up the manual steering box and add lube if necessary.

Thanks guys!

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My "road wander" was a combination of severely worn parts, and bad tires. The new parts (poly everywhere) made a huge improvement. Then good tires finished it off. My car is 1000x better than it was before my rebuild.

roe