Alignment vs mpg

[johnmyster]

Um, math check on aisle three. Top to bottom of a 16" rim is just under 1/32nd at a tenth.

Another barrier to doing it on the garage floor is that jacking the car up and down to make adjustments will tend to bind up the suspension - preloading a-arm bushings. You can walk most of it out of there, but those roller plates on alignment racks are there for a reason. I've tried toe measurements and found that this alone made it difficult to get repeatable results. Alignment shop said I got within 0.2 degrees, but I felt I was further off than that based on difficulty repeating measurements. If you're multiple degrees out, just dropped your truck, or replaced suspension parts, you can get close enough to get to the alignment shop. However, long, term, I think it's worthwhile to pay the guy with the lasers.

[johnmyster]

Plus - scribing lines on your tires for toe as opposed to measuring from the rim actually accounts for bent rims that would throw off rim-centric techniques. So, actually better than a lazer alignment in this regard if it turns out well.

[dcb]

0.2 is pretty good, and you are supposed to bounce the car before measuring, and I avoid letting other people touch my fleet like the plague, you have to factor in the number of times they goof something up into the results

[johnmyster]

Bouncing, rolling, turning the wheels doesn't quite equal those roller plates.

Yup. I think they actually set mine up to 0.0 caster. Mine has the sport steering box and needs some caster to help it out. Back I'll go to ask questions. Alignments are pretty much the only regular item that gets contracted. We know a great guy back home, but that's the cost of moving away from your resources.

[dcb]

re: math check

a 16 inch diameter rim would scribe a circle of 50.27 inches if rotated around a fore-aft or verticle axis (i.e. toe-in or camber).

50.27 / 3600 = 0.01396 inches per 1/10 degree, or roughly 1/72 of an inch

[johnmyster]

Yes, only if you're measuring edge position relative to the center of your rim. However, I think most of us would measure edge vs. edge. So your 8 inch radius gets doubled.

[dcb]

I showed my work, rim to rim, 2x8xpi=50.27. Where's your work?

[johnmyster]

16" * sin(0.1 deg)

[dcb]

Ah, thx, interesting. I was looking at it geometrically, if you consider the pivot for toe adjustment is located near the center of the wheel, then the basis for making a degree comparison would be the radius (8 inches), even though the tire toe measurements themselves are about 16 inches apart.