Caster Change Implications and Side Effects

[260DET]

Caster change side effects seem to be usually thought of in regard to steering geometry but if the change is made by way of adjustable tension rods then the car's wheelbase and front/rear weight distribution is changed too. Wonder if such changes could be used as a chassis tuning method? If the car has adjustable strut tops too then there is a choice as to which to use for a different side effect.

 

Another possible chassis tuning effect concerns the custom S14 based front struts on my S130, don't switch off S30 owners, this could concern you These struts attach by two bolts to the stub axle assembly as is usual but incorporate a camber adjustment in the attachment bolts. This in effect 'bends' the strut to change camber, just like a S30 strut can be bent to change camber. I can't take this any further at the moment but this 'bending' certainly changes the geometry when the wheels are turned. Anyone been there?

[tube80z]

Caster change side effects seem to be usually thought of in regard to steering geometry but if the change is made by way of adjustable tension rods then the car's wheelbase and front/rear weight distribution is changed too. Wonder if such changes could be used as a chassis tuning method? If the car has adjustable strut tops too then there is a choice as to which to use for a different side effect.

 

I used to have a severely bent race car and it always pushed in one direction and was loose in the other. When I measured the car I found the wheelbase was about 1.5 inches shorter on one side. Squaring the car made it work the same both ways so I would say this can be used. On a friends car pushing the wheels forward for more caster move the balance close to 1% to the rear. But you have to be careful with rules as a lot of sanctioning bodies require a specific wheel base.

 

Cary

[JMortensen]

I just became aware of caster's affect on bumpsteer, sounds like Richard already had a handle on this issue. As the caster increases the angle of the bottom of the strut and also the steer knuckle increases. This means that as the caster changes the outer tie rod moves up or down in relation to the ball joint, changing bumpsteer. I know I've related my bumpsteer adjusting experience here quite a few times, where I moved the LCA pivot up while measuring with dial indicators on the front and back of the rotor and jacked the car up and down 3 or 4 inches and had no measurable bumpsteer. I realize now that's basically impossible. I guess I did my testing wrong, could have been that the jack was putting weird loads on the chassis which was messing things up, I can't really say. I was able to minimize the bumpsteer and got it to the point where I couldn't feel it at all and was doing a lot less sawing on the wheel on bumpy sweepers, but a more accurate gauge or better tester I think would have shown that I didn't eliminate it completely because that isn't possible.

 

Johnc has also noted on previous occasions that shorter, adjustable aftermarket TC rods like AZC or TTT or Mike Kelly's old units mess with the caster curve. That's true, and that also messes with the bumpsteer, potentially making it a lot worse at the extreme ends of the travel (I can only imagine how bad this might be on some of these stupid low Zs). I think it is likely that my short TC rods were exacerbating the bumpsteer issue that I was having where my car was doing an emergency lane change maneuver without any input on the steering wheel on one particular bump at the track. So for minimal bumpsteer you want the angle of the TC rod level or slightly down at rest, and preferably on a race car you want to go with the old "make the suspension work by not letting it move" thing.

 

Richard, first thing that jumps out about the camber adjusting bolts is that you could use that to affect scrub radius.

Edited September 9, 2010 by JMortensen

[johnc]

Some notes on caster:

 

Caster does not directly cause the steering self-centering that many people attribute to it. That self-centering comes from Geometric Trail which is the distance from where the steering axis hits the pavement and where the vertical wheel center line hits the pavement.

 

Radial tires also have something called Pneumatic Trail where the lateral forces on the tire focus on a point behind the nominal center of the contact patch. That point varies with speed and the forces involved but its generally about 25mm behind the nominal contact patch center. Pneumatic Trail increases self-centering.

 

The cater jacking effect when turning the wheels increased the load on the inside rear wheel in a corner.

 

Positive caster increases a car's resistance to side winds and increases high speed stability.

 

Modern performance cars come with large amounts of positive caster from the factory. My 350Z has +9. My 1986 Mercedes Benz 420SEL had +11. I raced the ROD with anywhere from +6 to +8 positive depending on the track and tire width.

[260DET]

Probably what should be done is to put the car on a wheel aligner and take note of the various angles involved when the steering wheel is turned, as Jon mentioned, scrub radius may be one. When you look at a turned 'bent' strut one wheel takes on more negative camber than usual, the inside one from memory, but its all a bit beyond me as to for what purpose such changes would be applied.

 

Modern strut suspensions like the S14's are so good, I would just love to know their 'secrets'.

[JMortensen]

Caster affects camber when the wheels turn, but I think that effect is totally independent of how the camber is manipulated, eg if you had camber set via plates on the strut top, longer LCA's, or the camber bolts, the camber change would be the same when you turn the wheel, because it is a result of the caster.

[tube80z]

When you look at a turned 'bent' strut one wheel takes on more negative camber than usual

 

Modern strut suspensions like the S14's are so good, I would just love to know their 'secrets'.

 

I think mostly what your seeing is a design that has less SAI (steering axis inclination) and a lot of caster. Where I think the magic is in new cars is the bushings. The can be very compliment in certain directions and very stiff in others.

 

Cary

[Leon]

Some notes on caster:

 

Caster does not directly cause the steering self-centering that many people attribute to it. That self-centering comes from Geometric Trail which is the distance from where the steering axis hits the pavement and where the vertical wheel center line hits the pavement.

 

Radial tires also have something called Pneumatic Trail where the lateral forces on the tire focus on a point behind the nominal center of the contact patch. That point varies with speed and the forces involved but its generally about 25mm behind the nominal contact patch center. Pneumatic Trail increases self-centering.

 

The cater jacking effect when turning the wheels increased the load on the inside rear wheel in a corner.

 

Positive caster increases a car's resistance to side winds and increases high speed stability.

 

Modern performance cars come with large amounts of positive caster from the factory. My 350Z has +9. My 1986 Mercedes Benz 420SEL had +11. I raced the ROD with anywhere from +6 to +8 positive depending on the track and tire width.

 

Great info John. I'd like to add that more positive caster will also increase steering effort and it would decrease feedback to the driver since the mechanical trail would make up a larger fraction of total trail. It's the self-centering characteristic of pneumatic trail that gives feedback through the steering wheel. Just a few more things to keep in mind about changing steering axis inclination. Please correct me if I'm wrong with any of this since this comes from memory and not actual hands-on experience.

Edited September 10, 2010 by Leon

[tube80z]

I'd like to add that more positive caster will also increase steering effort and it would decrease feedback to the driver since the mechanical trail would make up a larger fraction of total trail. It's the self-centering characteristic of pneumatic trail that gives feedback through the steering wheel.

 

Not to be argumentative but you can increase caster and not increase trail. I also think you'll find that pneumatic trail's contribution to self centering is much smaller than that of caster or SIA.

 

Cary

[Leon]

Not to be argumentative but you can increase caster and not increase trail. I also think you'll find that pneumatic trail's contribution to self centering is much smaller than that of caster or SIA.

 

Cary

 

Yes, you are right on the first point. By changing the steering axis offset, you can increase caster and still have zero mechanical trail. I think that you would have to redesign the steering system of the car to achieve that though.

 

I think that what I meant to say didn't come across right on my point about feedback. Mechanical trail and pneumatic trail added together gives total trail, which directly affects the torque required to turn the steering wheel. However, the forces produced by virtue of the pneumatic trail are non-linear and dependent on tire characteristics. The forces produced by virtue of mechanical trail are linear and proportional to lateral force. Therefore as mechanical trail increases, driver feedback from the tires (pneumatic trail) gets drowned out. I am not saying that the actual pneumatic trail decreases, what I'm saying is that the effect from it is lessened since mechanical trail dominates (as caster increases with steering axis offset held constant).

 

Thanks for the correction.

 

Leon