I have a couple questions that I hope I can find answers to:

1. Assuming a car has had the alignment set up perfectly, the tires are scrubbed in and in excellent condition, tire pressures are properly set, and the brakes aren't dragging excessively at one wheel, is it possible for the weight of a driver to cause the car to drift slightly to the left when steady-state cruising on a flat road with no crown when the hands are taken off the steering wheel?

2. If a car has it's alignment set with the driver or equivilent weight in the driver's seat, will the setup be different than if the car did not have the driver's weight in it?

The reason I ask is that I just had my '06 JCW Mini at BR Racing for some needed maintainence, and in the process, I had the lower front control arm bushings replaced and a complete alignment. They know what they're doing and they hit the numbers right on. And while I had a slight drift to the left before the work, it seems ever so slightly more pronounced now. Mind you, it's very subtle and many people would likely not even notice it.
The tires I'm running are not new, though, and I'm also aware that even new tires can produce a drift one way or another. (Toyo Proxes 4's I was forced to put on once did this fairly dramatically.)

I haven't talked with BR Racing yet about this, but I wanted to get feedback from the collective wealth of knowledge here before I give them a call.

FYI: I'm 275 lbs.

Thanks much!

 

2) Yes, the setup will be different. And Yes, the alignment should be done weighted. If you have coilovers the car should be corner weighted before the alignment is done. The weight used depends - there is recommended weighting for street alignment. Track alignment should be done for you with 275 lbs in the seat / driver's footwell.

1) Depends on the alignment and whether you have coilovers. What is the toe front and rear? Stock alignment should track straight, but track alignment will make the car wonder on the highway cause it is setup to turn.

 

Don't know yet if weight was used. It doesn't wander, it just drifts to the left if I take my hands off the wheel. Not dramatically, but it's noticable. And there's a sense of the steering wheel wanting to turn to the left when my hand is on it. Most people would NOT notice this, but I'm sensitive to such inputs. The car is setup for the street:

(Cut/paste from a post I made on the autocross forum I just joined):

BR Racing has a "zero clearance" laser alignment rack (Snap-On/Beam Systems). Here are the numbers before and after the alignment. And I'll have to ask if they put weight in the drivers seat when they did the alignment.


Pre Alignment:

Caster Left 4.1 4.5/5.5 degrees
Caster Right 4.5 4.5/5.5 degrees
Camber Left -0.7 -1.0/-0.2 degrees
Camber Right -0.7 -1.0/-0.2 degrees
Toe Left -0.05 0.05/0.09 "
Toe Right 0.01 0.05/0.09 "
Toe Total -0.04 0.11/0.19 "


Post Alignment:

Caster Left 4.2 4.5/5.5 degrees
Caster Right 4.6 4.5/5.5 degrees
Camber Left -0.7 -1.0/-0.2 degrees
Camber Right -0.8 -1.0/-0.2 degrees
Toe Left 0.06 0.05/0.09 "
Toe Right 0.07 0.05/0.09 "
Toe Total 0.13 0.11/0.19 "

('Couldn't get the proper spaces to appear between the numbers. the first is the measured number, the next two are the factory spec mins and maxes.)

 

With the pre numbers the car should pull left, but not with the post numbers.

 

What numbers are you looking at specifically. And what, specifically is it about the numbers that make you say that?

Inquiring minds want to know.

 

toe

-toe means the front of the tire is pointed away from the centerline of the car.
+toe means the front of the tire is pointed toward the centerline of the car.

In the pre numbers you have the left tire with - toe and the right tire with + toe, meaning both tires are pointed to the left.

 

Ah, I understand now.

I've been told that caster can affect drift one way or another. What say you?

Thanks much!

 

Alan is 100% correct, Mini/BMW alignment specs are very specific with requiring the vehicle to be weighted during an alignment. The fuel tank should also be full. The specs you give only reference the front wheels. A true thrust angle alignment references the thrust line from the rear to the front of the vehicle. To accomplish a true thrust angle alignment, you need specs for both front AND rear wheels. A fair amount of alignment shops only perform a "front end alignment", the old “set the toe and let it go” mindset. Our cars have independent suspensions and require a true thrust angle/4 wheel alignment. Caster CAN affect tracking and will tend to drift to the side with more set back (which is the left side according to the specs you gave). You didn’t mention if you replaced the bushings with Polly bushings, which could also affect caster because that is the spec most affected by the position of the lower control arm. Poly bushings are better than OEM rubber bushings as far as handling and durability but, it comes with a tradeoff. The suspension is engineered to flex to a certain degree. Poly bushings are more ridged and modify the way the factory intended for the suspension to flex on rubber bushings. In addition, get the specs from the shop for SAI (steering axis inclination). Without SAI, you can’t be 100% sure if the all the specs are working together to find the true thrust angle/thrust line. If SAI specs are not comparable from side to side, that will cause a drift as well. Bad news is SAI is not adjustable; it’s built into the chassis/unibody. If it’s off, chances are there is a possible structural misalignment. They may be able to compensate or it by repositioning the sub frame and make up for some misalignment but, not alot. Are your towers mushroomed?
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Make sure it is solid and level before doing to much. I'm not familiar with smaller lathes but on larger lathes it will cause a misalignment between the head stock and the tail stock.

 

Thanks for the info. I've kinda forgotten about the drift. It's so subtle that most people wouldn't notice it. All four ends were checked and aligned and the shop specializes in Minis, BMW's, Porsches, Ferraris, Mercedes, and special race versions of all the afore mentioned. The bushings were replaced with polyurethane bushings and there is no mushrooming as JCW cross brace has been installed since the car was new. Weight was not used in the car, though, when the alignment was done and I'm sure the gas tank wasn't full. One of these days I'm going to have coil overs installed along with various upgrades like rear control arms, links, and sway bars. At that time we'll revisit the alignment.