Fork crown race fitting

For discussions about bikes and equipment.
hayers
Posts: 125
Joined: 27 Apr 2016, 1:50pm

Re: Fork crown race fitting

Postby hayers » 6 Jan 2020, 8:15am

Maybe just open out.the hole in the adapter by polishing with emery cloth or wet and dry paper until the fit's ok. (Or adjustable reamer if you have one)

Wouldn't touch the fork - hard to put material back on!

Btw calipers will normally under-measure internal diameters by a fraction due to the finite width of the jaws - but not by as this. The effect is worse on smaller holes.

Brucey
Posts: 36756
Joined: 4 Jan 2012, 6:25pm

Re: Fork crown race fitting

Postby Brucey » 6 Jan 2020, 9:01am

Phileas wrote:In another thread, viewtopic.php?t=75026#p655409 :
Brucey wrote:.1mm interference on a headset race fit is over 0.3%. This is too much; something will break. About half or a quarter of that amount is about right.

I’ve got three times this interference.


maybe; it depends how good your measurements really are.

You can get an idea about what your measurements really mean by measuring parts that you do know fit together in an acceptable fashion.

FWIW I'd qualify the statement quoted above with a comment about how stiff the parts are; a traditional (cup and cone type) 1" headset crown race is both stiff and brittle; it is also normally fitted to a fairly thick-walled piece of steel. This combination means that you can't have a big interference without a risk of something breaking. Other headset parts are less stiff or indeed can yield slightly when parts are fitted. In such cases the acceptable tolerances won't be the same.

In practice the steerer will have a lead-in (slight taper on the diameter above the crown race seating) on it. This means that when you try and fit the parts you will be able to gauge the quality of the fit. If your measurements are accurate you won't be able to even start the adaptor over the steerer seat, it'll stop somewhere on the lead-in.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Phileas
Posts: 226
Joined: 18 Feb 2009, 6:12pm
Location: Bristol

Re: Fork crown race fitting

Postby Phileas » 6 Jan 2020, 9:22am

I've just measured the bush again with a calibrated digital caliper and I get 29.93mm dia. So it may not be so bad. I won't be re-measuring the fork so I can only assume it's within normal limits.

There isn't really a lead-in on the race dia, just a small chamfer.

I could try removing the "paint" from the fork using Emery cloth and giving the bush a bit of a rub also.

Brucey
Posts: 36756
Joined: 4 Jan 2012, 6:25pm

Re: Fork crown race fitting

Postby Brucey » 6 Jan 2020, 12:30pm

the paint needs to be removed from any crown race seating that was accurately machined before painting. However occasionally you will find one that is undersized and you are better off leaving the paint on than taking it off.

Arguably there is no point in measuring either

a) both parts using bad instruments or
b) only one part

if you are concerned about the quality of the fit.

NB. crown race (that isn't a split type) and is actually loose on the crown be made good using bearing fit compound ( if it is only a few thousandths) or epoxy resin (if the gap is a bit bigger).

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Valbrona
Posts: 2260
Joined: 7 Feb 2011, 4:49pm

Re: Fork crown race fitting

Postby Valbrona » 6 Jan 2020, 4:10pm

The problem with an exercise like this is that the diameter of the fork crown will likely not match the outer diameter of the head tube at the bottom ... and it will look pants.
I should coco.

Phileas
Posts: 226
Joined: 18 Feb 2009, 6:12pm
Location: Bristol

Re: Fork crown race fitting

Postby Phileas » 6 Jan 2020, 6:19pm

Valbrona wrote:The problem with an exercise like this is that the diameter of the fork crown will likely not match the outer diameter of the head tube at the bottom ... and it will look pants.

As long as it works. :)

Comparing my dial calliper gauge with my mic, it seems the calliper has a 0,1mm error (reads small) on internal measurements but is in agreement for external measurements.

User avatar
Gattonero
Posts: 3641
Joined: 31 Jan 2016, 1:35pm
Location: London
Contact:

Re: Fork crown race fitting

Postby Gattonero » 7 Jan 2020, 8:20am

Phileas wrote:I have a new 1 1/8” fork and a reducer bush that will adapt it to fit in a 1.5” taper head tube.
The fork crown measures 30.1mm and the bush is smaller 29.8mm (using a dial calliper).

If I take it to the LBS and ask them to fit the bush, I assume they will turn down the fork to fit.

My question is, if I later wanted to use the fork in a straight head tube, might I find that the crown race diameter is a bit small?


Make it simple: forks go according to the frame, there's no such thing as "universal forks".

So if your forks are tapered, they are meant to fit in a particular frame (we are also leaving aside the fork height, rake, and all that: we assume is actually fine for the frame they're meant to fit!) so the size of the headtube has to be correct.
Which brings you to narrow down the answer: tapered forks will only fit their matching tapered hadtube (no, they are not all 1" 1/2, there's also 1" 1/4, 1" 3/8, etc.) or a straight "semi-integrated" (also called "zero stack") headtube with 44mm ID. The latter, will use a set-in stemi-integrated cup at the top and an external -much larger- cup at the bottom to accomodate the wider section of the forks, see below an example:
Image
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

Phileas
Posts: 226
Joined: 18 Feb 2009, 6:12pm
Location: Bristol

Re: Fork crown race fitting

Postby Phileas » 11 Jan 2020, 6:16pm

So in the end I removed paint from the crown seat, modified the reducer bush with emery cloth and managed to get the bush on with the aid of copper grease. :)