MacPherson strut alignment relationships

[Hokiefyd]

I had my tires balanced today at Sears, and they checked the alignment as well. I had already had the Honda dealer check the alignment about two months ago (and they adjusted the front toe), but I figured a free check can only add to my data points.

They say the front toe is out on both front wheels, or at least at the limit. The range is -0.08* to 0.08* on both front wheels, and both are at -0.08* (toe out). Front camber was okay; 0.2* on the left and -0.1* on the right. The front camber spec is 0.0*, +/- 0.5*. Rear toe is perfect (0.13* on both) and rear camber is acceptable (-1.8* on the left and -1.7* on the right).

So I got the car home and thought I'd play around with the inclinometer app on my phone. I did not think the toe reading was correct. I had some odd tire wear BEFORE the Honda dealer adjusted the alignment, and that tire wear has evened out AFTER the Honda dealer adjusted the alignment. Unfortunately, their printer was broken, so I don't have a printout from the Honda dealer.

Anyway, the readings I got on my phone matched Sears' camber angles exactly on the back. But the fronts were wonky. I got 0.7* of camber on the LF, where they had 0.2*, and I got 1.0* of camber on the RF, where they got -0.1*. I was showing significant positive camber (camber out) with my phone app. I drove around some later today and got back home and checked again and this time, my readings matched Sears' readings pretty closely.

The difference? This morning, I BACKED the car into the garage before taking the angles and this afternoon, I pulled FORWARD into the garage before taking the angles.

This is leading me to believe that Sears is correctly reporting negative front toe. Is that a good conclusion? Toe out in the front would want to drag the tires against the road OUTWARD, so when you're moving forward, it's going to make the camber go as negative as the compliance bushings will allow. But when you go in reverse, a toe out condition will want to pull the tires against the road INWARD, which will make the camber want to go as positive as the compliance bushings will allow. Do I have that all right? The readings that Sears is getting seem to correlate with the angles I'm getting with my phone, as long as I factor in the direction of travel.

The "rack" Sears used is simply a set of clip-on wheel hats. He rolled the car forward on the bay floor about two feet. Two posts in front of the car appeared to be "reading" a laser beam from each hat. I was skeptical about the accuracy of that type of system, but maybe it's right. It got the rears bang-on...they visually have a lot of negative camber and some toe in. And I think it got the fronts pretty good also...they're about neutral camber, maybe slightly positive. I can't easily measure toe at the house, but if everything else is accurate, I'd have to believe that toe is also.

 

[CBR.worm]

I thought that both front wheels had to be slightly toe out for proper tracking and on center feel. I understood that if they were at 0.0 the car would wander.

 

[bdcardinal]

Originally Posted By: CBR.worm
I thought that both front wheels had to be slightly toe out for proper tracking and on center feel. I understood that if they were at 0.0 the car would wander.


Depends if the car is FWD or RWD.

 

[yonyon]

That thing with the clip on "hats" as you call them is an excellent system. It's Hunter's solution to problems in older style clamp on "sensor heads" which can frequently go out of calibration due to things bending during use or especially if they're dropped. It isn't using "laser beams" it's using modern digital cameras and very good software interpretation.

If you're one of those hardcore chrisitian types, don't let them hook it up to your car. The cameras have flashing red pentagrams mounted on a crucifix and wired up to a computer with artifical intelligence and if it's got a fancy Hunter lift it just gets worse because the lift has holes for a movable step. The step hooks into holes shaped like an inverted cross and there are probably thirteen of them on each side. I don't have to worry about that stuff because I'm Jewish.

Regardless of the alignment system type, you can expect that camber readings are very sensitive to how level the alignment rack's lift (or floor if a pit is used) is, the weight of any person or cargo in the vehicle, and as you have mentioned, the influence of rolling with a non-zero toe setting. When measured on a properly functioning turnplate, the toe should not factor into the camber measurement. Consider also where your tie rods attach to the knuckle. If they attach at the front, any camber change caused by the toe should bring your toe closer to zero while rolling forward but further from zero while rolling backward. If the tie rods attach to the back of the knuckle then the opposite would take place.

 

[mechtech2]

When actually driving a FWD car, toe out is pulled straight [or at least straighter].
With RWD car, front toe in is splayed/pushed out to zero [hopefully].

You can check camber and toe 5 times and get 6 different readings! The suspension joints and bushings flex and move.

If you can't get a perfectly level spot to check things [almost impossible], then mark there the tires are, and turn the car around so the tires are at the same spots. Average your readings to get the true result.

 

[Hokiefyd]

Thanks guys. I guess I was just wondering if that much variability could be seen in a MacPherson strut setup, because the only real soft mounting point is the LCA bushing. With a dual A-arm design, you have compliance bushings in the upper AND lower arms, so possibly more flex.

Anyway, I don't have any desire at this point to mess with the toe. Dynamic toe change must be enough that toe gets pulled straight because my tire wear is now very even. I actually thought that I had some toe-in, because I'm starting to see a very slight feathered edge on the outside of the tires, which tells me the tires are toed-in while rolling.

I really would rather have some negative camber in the front, but I'll probably leave it as-is, especially if it changes this much in the real world anyway. I'm one of those perfectionists who like to see the same number on both sides of the car, so I'd work all day trying to dial it in. I have to let that go, I know.

This is a FWD/AWD layout, so the steering rack is mounted aft of the engine, and the tie rods connect to the rear of the knuckles.

 

[IndyIan]

Originally Posted By: mechtech2
When actually driving a FWD car, toe out is pulled straight [or at least straighter].
With RWD car, front toe in is splayed/pushed out to zero [hopefully].

Do you find most fwd strut cars have shorter steering tie rods than lower control arms? I never see my car on a lift, and when laying on my back on a sheet of card board in my driveway I always forget to look...
Assuming 0 toe at rest, shorter tie rods than LCA's would give some toe out on acceleration and toe in on braking I believe.

 

[yonyon]

This actually varies from one setup to another but most are longer. Most, including your Neon, I believe, the tie rods are longer but not by a whole lot. This makes sense because the tie rods are not quite as low as the control arm so the toe change with ride height is not as drastic as you might expect. I think it actually goes toe-in under both braking and acceleration.

Some other, larger, Chyslers of the day had the steering unit mounted up near the top of the firewall with very long tie rods attaching to near the center of the rack unit. This long tie-rod length made sense because they were so high near the strut mount which obviously does not move inboard during a change in ride height.

 

[yonyon]

I think, perhaps, this is also a good thing for many drivers. They can have plenty of toe-out on turns to protect the tires during tight turns at low speed, but when taking the same tight turn at excessively higher speed, the sway will somewhat counter that toe-out so there's more of a tendency to understeer than oversteer or rollover.

Don't take it that I don't love my controlled oversteer, as in reality I love a SWB RWD MT car that will allow the throttle to be used for steering and the steering wheel used for speed adjustments. It's really not a good idea to drive like that most of the time because it's expensive and unexpected conditions can be difficult to deal with, possibly even dangerous. That's not something most people want anything to do with and a lot of people driving on the public roads around here have no clue how to control that sort of thing and they tend to end up crashing when they encounter it unexpectedly. Understeer can be dangerous too, but most of the time it gives an audible warning and makes for nothing more serious than one wheel into a curb.

 

[IndyIan]

Originally Posted By: yonyon
This actually varies from one setup to another but most are longer. Most, including your Neon, I believe, the tie rods are longer but not by a whole lot. This makes sense because the tie rods are not quite as low as the control arm so the toe change with ride height is not as drastic as you might expect. I think it actually goes toe-in under both braking and acceleration.

Some other, larger, Chyslers of the day had the steering unit mounted up near the top of the firewall with very long tie rods attaching to near the center of the rack unit. This long tie-rod length made sense because they were so high near the strut mount which obviously does not move inboard during a change in ride height.


Interesting, I didn't really think about toe changes with suspension travel but its all important for good handling.

 

[mechtech2]

The tie rod length is unimportant.
The flex in the suspension bushings allows the tires/wheels to be pulled forward when under power, in a FWD car.
Not just starting acceleration [torque steer], but cruising as well.
Of course, the reverse is true - braking will splay the wheels out .

We set static wheel alignments to appease dynamic situations.

 

[Hokiefyd]

Originally Posted By: mechtech2
We set static wheel alignments to appease dynamic situations.


What do you normally recommend for a FWD setup?

Do you think the ideal toe is toe'd out nearly to the acceptable limit, so they toe back in to nearly neutral when under power? Or do you like to set the toe as close to zero as you can?

If toward the "out"er edge of the limit, then I guess mine is ideal and I won't mess with it.