Bicycle Mechanics - I'm glad I'm not a toolmaker

I need to replace my bike's forks. So this morning I removed the headset & stem, dropped the forks out and put vernier calipers around the tube, at the threads. I measured outside, inside and the tube length. It's an italian bike (Olmo) so I assume measurements are in mm. I get 30mm for outside, 26mm for inside and 190mm for length.

So I'm feeling confident when I phone the bike shop to place the order. The guy asks me what diameter and tube length and there's a pause when I tell him "30mm"

"With respect" he says, "I think sir might be mistaken. Its most likeley to be 1 inch - 25.4 mm" I confirm that my headset is Campagnolo Mirage, so it definitley is 1 inch.

How could I be out by nearly 5mm on my measuring? I must have been looking at the wrong marker on the calipers or something. I can't wait to get home and check it again.

Like I say, it's a good thing I'm not a toolmaker or working for NASA! Nothing would ever fit together!

FWIW I bought clerance LOOK forks, a new Campag headset, the shop are going to press on the lower bearing for me.

Stew

You don't sound like you know very much about forks. If you did, you would know that threaded steer tubes for road bikes are nearly always 1" (except ancient English/French bikes).
Does the new fork have the same rake as the original? If you aren't careful, the wrong $200 fork can make a $1500 bike handle like a $100 Huffy.

You're right, all I know about forks are the pair I have on the bike now.

I never have been pleased with the way the thing handles, riding hands-free I always have to lean over to the right. The headset always gave a knock and would never tension quite right, which is why I replaced the headset about 2 months ago. This Campag Mirage replacement means there's only about 2 turns of thread left for the locknut. It keeps coming loose.

Hence the requirement for whole fork replacement.

I take your point about rake. Too steep could make the bike as twitchy as . . . a twitchy thing. But the fork cost is only £20 ($29USD) for my commuter/winter hack. Worst case I'll have to put the old forks back on.

Stew

I, on the other hand know a lot about forks. Have used them to eat almost all my life.

However, to your problem. Even though I had used calipers a lot in an earlier life, I fell prey to just such a mistake recently. What I did was to take my reading at the end of the moving carriage, and not at the zero mark on said carriage, which is some distance away. Even after I looked again, it took me a while to realize my error. Check to see if that is what you did.

Sixty years old, and I learn something new - often a lot - every day. :)

Cheers...Gary

I think I also should have been measuring the tube where there were no threads, Does the thread cutting splay things a teeny bit, or just cut into the tube?

Stew

You were only off by .6mm. Not bad, really, but every tenth of a millimeter counts.