markjenn

Hi, I'm building up a new CX bike from a reputable framebuilder and have noted that the rear triangle appears to be laterally displaced about 1.5-2.0 mm right of frame centerline as measured at the center of the rear dropouts. (The problem looks worst at the seatstays since the bike has fairly wide tires - with 35mm tires, the clearance on one side are about 8mm while they're 12mm on the other side, so it's visually noticeable.)

The builder says this is within normal production tolerances and that I can recenter the wheel if I like by simply redishing it slightly.

This is my first from-scratch build, so I'm unclear what's acceptable tolerances here. Any guidance?

- Mark

burk

I would be wary of assessing alignment based on the position of the wheel/tire, as any irregularity in the axle or dropout could cause the wheel to not sit properly. Having a shop check alignment with an alignment table or (more likely) a ***-2 is a better route. If it is a steel rear triangle, the alignment is easily fixed with a little leverage.

kenhill3

Of course the rear wheel needs to be accurately dished. A frame builder who would tell me to redish for this- I would call bull*****.

San Rensho 

How did you measure the rear triangle? String or frame guage or are you just eyeballing it.

2mm would not bother me, although it is quite a bit. If the frame were steel, the fix would take about 5 min.

markjenn

Couple ways. I've confirmed the (new) wheel is correctly dished (clearances are unchanged flipping the wheel in the opposite direction), has negligible lateral runout, and the rim is centered in the hub. Bolting up the wheel there is about a 3mm side-to-side discrepency between chainstay and seatstay clearances side to side. Halving give you the discrepency with respect to the centerline. As someone else has pointed out, this doesn't really tell me if the problem is a laternal displacement of the rear traingle to one side or perhaps the wheel plane is out of line with frame plane due to dropout positioning.

I've also used the string method, running it from the dropout faces around the steering head and checking clearances with the seat tube. This is also slightly off too, but it is less than the stay clearance discrepency.

I'm inclined to think that I'm probably being overly picky. 1.5mm isn't much.

- Mark

AndrewP

Adjusting the dish to move the rim to centre will even the tension between drive side and non-drive side, resulting in a stronger wheel.

nitropowered

I wouldnt sweat it. most industry tolerances for frames is 5mm

markjenn

Re-measured this evening even more carefully and I think it's actually within a mm, more or less. I've decided I'm probably worrying about nothing.

- Mark

ParkingMeter

Measuring rear triangle alignment is easy to mess up. The string method will tell you if it's moderately to really badly out of alignment, but isn't accurate enough to actually get it back to within acceptable tolerances (the measurement isn't taken at the inside of the dropout, which is what matters). A good frame should be aligned with less than 1mm of deviation from center (again, taking the measurement from the inside of the dropouts, which determines where the wheel actually lines up w/ the frame).

If you know your wheel is precisely dished, that can at least indicate that the frame needs alignment, but a frame alignment gauge and dropout gauges are needed to correct it (center the rear triangle affects dropout parallel alignment, and vice versa, so you have to check both with each adjustment and correct accordingly). I've seen lots of mechanics screw this up however, because they forget to measure the dropout thickness and factor that in or forget to check parallel alignment of the dropouts with each other. A lot of frames with replaceable dropouts have a thicker right side dropout (most are aluminum however, which can't take a coldset); if you don't factor that in, you can misalign the frame by as much as 3-4 mm because you inadvertently align the outside of the dropout with the center line of the frame (which is what the frame gauge references from). I'd take it to a shop that can do this for you; make sure it's a good shop that won't make the above mistake, I've found it surprisingly common. If the frame can't take a coldset (IOW, is not steel), you can still have the misalignment measured and adjust the dish of the wheel accordingly; this will achieve identical results. A good shop will do all the math and measuring right, then use a set of feeler gauges to determine the exact error. HTH.

San Rensho 

I would go with the string method, its usually pretty accurate. The string method I use is to run the string from the head tube back past the dropout and tie it to a weight. Then I adjust the weight until the string just touches the seat tube. You can then measure very accurately the distance from the string to the plane of the dropout with a caliper, and compare to the other side.

johnny uprizing

string method is generally not reliable. Park FAG2 is just an overpriced string. None of the frame alignment "standards" people quoted is really correct, because there are two standards. Any good framebuilder will have two alignment tolerances, one for how far off of the centerline line a part of the frame can be (but parallel to the centerline), and one for how twised it can be relative to the centerline. The twist is the tigher tolerance. The important thing is that when you look behind the frame - are the wheels parallel (and perpendicular to the seat mast / BB) or do they make an "X"? The frame can be out of alignment quite a bit but as long as everything is parallel to the centerline it will handle fine. When the front and rear wheels make and "X", then you start have problems. The issue with the string method is that it measures down the center of the "X", which can read perfect but your wheels may possibly point in completely different directions. The true test is, take your hands off the bars - does the bike pull in one direction? Now turn hard left and right, does is want to dive into the turn going one way, but fights you going the other way? Then you have an alignment issue that can only be corrected by measuring and setting on a precision table. If it handles fine, then forget about it and don't mess with your wheel dish either.

San Rensho 

So to test for twist, what can you use, spirit level, plumb bob?

sfrider 

Another good indicator is uneven tire wear (on opposite sides) plus severely shortened tire life.