Wednesday, April 18, 2007

Grapefruit from the fridge

And coffee. And sunshine. And mouthmusic. I broke out the vernier gauge last night. Some numbers for you: but first as in any proto~scientific study, a method description.

  1. drink glass of vino tinto
  2. carefully 'zero' and fix vernier gauge (i have an analogue version that measures to the 0.01mm)
  3. take multiple measurements from the steerers of the forks i have in use currently. detail: use flat sections of 'arms' of gauge rather than 'bladed' section in order to try and minimise lack of perpendicularity (my word, and its a good one) and use a 3 point contact on the steerer.
  4. take multiple measurements and average.
Think thats it. Figures:
  1. Black Sheep ti fork 28.55mm
  2. Pace carbon 28.60mm
  3. custom steal Rick Hunter 28.60mm
  4. Project 2 28.60mm
Hmmmm.

You could be forgiven for think Ah ha! the black sheep is undersized, hence the thomson stem issue!

So i went the chris king website in order to try and find out a little more about tolerances etc.

This is a quote: " 8. The steerer tube may be undersize. Steerer tubes should be ±.025mm (.001") of the nominal size. We often see steerer tubes as much as .25mm (.010") undersize. Solution: Replace the steerer tube."

What is that nominal size? 1 1/8" or 28.575mm

So i have one steerer that is *just* undersize, and 3 which are significantly oversized, according to these criteria.

Of course, you have always known tolerances have always been in a range, and the bike industry is seldom a precision industry. I think what i have here is one of those unfortunate incompatibilities occurring as a result of the lack of high precision standards. No ones fault and thus nothing to moan, whine or complain about.

Interesting. Makes you realise why shimano rule he roost so often. They utilised a machine on the initial cartridge bb's to measure to the nth degree the size of the cup race and the bearing size and put together the most appropriate combo for each bb. Outstanding detail work from an industry giant, and well received. The UN-72/3 was always a special bb.

Ok, im done. This is prolly the reason i have 6, or maybe 7, unread magazines lying around the house...

(revised figures after re-reading and re-adding...)

11 comments:

brant said...

When designing products it's good to build in some potential for the mating components being out of tolerance. This is one of the reasons I like Avid brakes so much - their mounting system means that you don't have to face the mount like you do with the older fixed IS systems. Happily most people seem to be adopting a similar (if not identical) system involving post mount adaptors, or just a move to post mounts in general!

Chris King headsets use a rubber O ring to fit to the steerer - this is interesting as it's their way of circumventing the US Patent that Cane Creek hold on John Radar's invention of the "aheadset" which uses a tapered wedge to lock things in place.

Personally I think the tapered wedge is a lovely neat idea.

Thomson stems use their slot-less clamp for reasons of anti-knee knocking I guess. It's also a lovely design to machine. But as you've found, it lacks the capacity to cope with steerers of different diameters outside quite a close tolerance.

I can't remember the last time I banged a knee on my (bolted) stem though.

dRjON said...

good thoughts...i was always concerned about the fact that the kings allow movement due to the rubber o ring, and after badgering them enough the answer was it does but it was allowed for in design by bottom crown race allwing a degree of spherical (?) movement on the lower bearing. this allows a 360 degree contact at all times. its always been in back of my mind but i reckon if i keep the e mail, and warranty would be covered on frame if it rounds out due to this. however, after years of use, it hasnt been an issue. interestingly though when you strip a fork out there is always a wear mark at the top race where it has rubbed due to compression at the o ring. now if you think about it at a small scale that MUST mean the stem is not in line with the spacers and or top cap, which MUST add a small leverage force unless there is the ability to soak up this by compression. this may be what the white washer is for, i dont know. here my thts stop cos its never been more of an issue, except with the thomson wedge type stem which i surmised was being slowly levered upwards...due to the wedge unit being able to rotate in the stem itself, which may have accounted for the noise they always made...

who knows...

Tom said...

The white washer stops massive creaking noise in my experience.

Marty said...

So i have one steerer that is *just* undersize, and 3 which are significantly oversized, according to these criteria.

Surely, your steerers are in/on edge of acceptable range as defined by King (assuming nominal is 28.575mm) rather than over/under.

28.575+0.025=28.6=Pace/Rick Hunter/Kona
28.575-0.025=28.55=Black Sheep.

Or have I missed something?

dRjON said...

no marty thats right...after revising figs i didnt revise words!...i think the issue is thomson is at one end spectrum, and black sheep steerer at other...

ach well...live and learn. at least i had reason to use a vernier gauge? or guage...!

BMac said...

This tolerance stuff is why the precision clobbering hammer was invented...

But I agree, it sucks when bits simply don't fit when they should.

Anonymous said...

cork...

dRjON said...

cork?1! who is this? i feel like i have been handed the black spot...

Marty said...

after revising figs i didnt revise words!...
gah! i spent hours, HOURS, checking and rechecking my sums! does no-one think of the blog commenters?

cork indeed.


:)

dave said...

It's spelt gayj ya gadge.

shaggy said...

watch your palms!