Frame control measurement deviations - how much is too much

[VWJake]

I’m in the process of setting a new frame rail on one of my ‘02s and the numbers are  off “a bit”.
 

The analytical side of me says any deviation from the blue book spec is unacceptable. There is a another voice in my head asking questions and making statements like, “As long as the front subframe pick-up points are in the same plane, square, and equidistant from the rear suspension points to car should feel fine”. “I’d bet there are a lot of cars out there that are more than 3mm out in a few measurements. Does that translate to an ill-handling car?” “We’re talking about 3mm here... it’s THREE MILLIMETERS” “How many millimeters out is enough for someone to actually notice in road manners from one side to another?”

 

Is this even something that someone has been able to quantify?

 

Perhaps I am being “too German”. This is a 47 y.o. car. I am also working on fabbing up an adjustable tram gauge. So far all of these measurements have been taken with a tape measure so there are extra angles( and lengths) not accounted for in the factory drawings. Because of this I know I may discussing “false” data, but I wanted to start the discussion and get opinions on the topic. 

 

For reference:

 

Where I’m at with my handy dandy metric tape measure:

 

The length from the rear to forward most point always trips me up. Then I recall that I read somewhere that the blue book is off by 20mm on that spec. Looking at this car I believe that is the case. It’d take quite a deformation to lose almost an inch in length.

 

I’ll also add that with the car leveled up via the rockers the subframe mounting faces of both rails are in the same plane and parallel.  The upper strut mounts are in the same plane as well. 

[02Les]

Do you measure 1180mm between the two rear points?

[VWJake]

I believe I verified that previously, but I will check again on Mon.

[jimk]

If my trig is any good the 2336 computes from the diagonals.  You measure the diagonals quiet close to the dwg.

[TobyB]

If you can get the wheels to align the way you want them, it matters not at all what the 

rest of the shell is doing.

 

That's a production drawing, as in, 'Ve must make de shell dis goot for many reasons'.

Keeping the wheels pointed the right way is only one of them...

 

For frame rail replacement, I'd agree with your non-Germanic assessment-

flat, rectangular and the right width, and you should be fine.

As long as the steering and lower pick- up points are where they should be,

the tire doesn't care about the rest...

 

 

t

 

[VWJake]

I was in such a rush to leave the shop last night that I didn’t take any pics, but I managed to get the new rail dialed in “spot on” and burned it in. 
 

I started last night with another chunk of that extruded channel and and some 1” steel round stock. I turned two 1-1/2” tall stand-offs with 16mm dia nubbins (to locate into the forward frame rail bolt hole.  I center drilled both of these slugs using a “W” sized bit for a snug fit with an M10 bolt. I made 5 marks on the channel: center, 340.5mm from center and 590mm from center. Slide the slugs into the channel and bolt the channel to the leading hole in the rails. Now the forward holes are exactly 681mm apart and I have a measuring pads exactly 1180mm apart (same width as the rear suspension mounting points) from which to check the front to back 2336.3+/- 2mm.  I drilled another hole in my shorter tram gauge at 717mm ( the diag measurement for the frame rail mounting holes- not on the blue book cut sheets). Armed with these two new tools I bumped the new rail around a bit and I was elated to see 2338mm on both sides and matching diag measurements rail to rail. The mid and rear rail mounting holes are also 681mm. 
 

It’s amazing what happens when you quit being a Lazy dummy and measure things exactly how the sheet shows- That 2336.3 isn’t hole to hole (as the crow flies), it’s plane to plane (parallel and square; Germanic ?). 
 

Once I saw the rails were truly parallel and   properly located with respect to the rear suspension pick up points I once again fired up the laser to make sure the subframe mounting surfaces were still in the same plane and they were. I couldn’t get my welding helmet on fast enough. ?

 

I’m thrilled to know that that summbuck is in the right place and the car won’t dog track. I’ll get some pics on Monday of all of this jibber jabber. 
 

The car is far from done, but a huge weight has been lifted from my shoulders. 

[VWJake]

As promised-

 

first few are of the fixture made to set the front rail spread and give me the properly located pads for the 2336.3+/-2mm measurement.

 

 

 

 

The rest are just the rail welded in. I’ve left the inner fender loose as I need to recreate the lower few inches of that panel.