How Straight Should My Frame Be?

Originally published in 2002.

How gray is gray? For builders I think it comes down to what you feel comfortable producing. For rider’s, if you’re comfortable with the handling of your bike does it matter if your bike is aligned within 1mm or .1 mm if you can’t tell the difference? I’ve been told time and again that the “industry” standard for alignment is 2mm. I’ve verified this to my own satisfaction by measuring every frame I get chance to and I’ve seen some very expensive “factory” frames that were a long way from being straight. For builders, you need build them as straight as you can to start with so you don’t have to bend the piss out of them when you’re done. Remember that cold setting is about taking a material past its yield point to remain in a new position. Do you really want to do that to a frame any more than you absolutely have to? Do you really even want to try bending a Foco mega tube or other heat treated thin wall tube for that matter? When you’re working with those materials if you don’t have your system down before you start, you’re going to be losing some frames.

If you want a number to shoot for, no-one is going to produce a frame that is closer than .005″ across any significant distance and have it stay that way for long. It always tickles me to hear someone saying how a frame was perfectly straight or aligned perfectly. No such animal. The fact of the matter is when you start working with tolerances approaching .001″ or greater, you need to have highly accurate measuring equipment and very true standard to start with. I don’t know of a single bike shop or mechanic or builder who has the calibrated equipment and standards to do it.

An example: say you align off the BB shell and check alignment by working with theoretical tube centerlines, if the shell face is out of parallel by .0005″ (5 ten-thousandths) across the width of the shell, a head tube centerline 24-inches away will be off by .009″. That 5/10,000ths of an inch is communicated and multiplied over that 24-inch distance. Does it help if I tell you that most BB facing tools can’t maintain parallelism to better than a thousandths or two? Hold by the head tube? Same problem in the opposite direction to BB and dropouts. Do you want to take the chance of cold setting a frame to correct for .009” when it’s the BB shell that’s .0005” off? The easiest way to think about what you’re trying to achieve here is that if you align a frame to read true and then place one side of the frame where it’s facing the sun and re-measure it 15 minutes later, it’ll be considerably off again. You can spend all day chasing your tail here.

So when that pimply faced mechanic down at the LBS is putting your custom creation on some makeshift table and tells your customer it’s .5mm out of alignment and insinuating that that’s poor, somebody should be there to measure his dental work with their knuckles unless he can verify for you his measuring process and equipment calibration standards.

I’m of the mind that if you have a frame that is more than a millimeter or two out when you’re done joining the tubes, you’ve done something wrong to start with. It’s certainly not the end of the world and the frame will probably still ride like a dream, but it’s still some little thing you missed, a miter that was off, a little gap here, missed your weld sequence or didn’t follow one. If you find that each frame you build is out of alignment in the same fashion, do a little detective work and figure out what’s causing it and how to correct it.

As far as what misalignment means to performance, I don’t think it means much unless the problem is severe. One only has to sit on a bike, grab some brake and apply moderate pressure to a pedal while watching the BB and rear tire to see that a bicycle frame under power spends more time misaligned than aligned but that certainly doesn’t mean it shouldn’t be aligned to start with. It should definitely track straight and be rideable with your hands off the bars and certainly shouldn’t be the root cause for any handling “quirks.” Build them straight; let your customers bend them otherwise.

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