Fork crown race fitting

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Phileas
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Fork crown race fitting

Postby Phileas » 5 Jan 2020, 8:01pm

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?
Last edited by Phileas on 5 Jan 2020, 9:50pm, edited 1 time in total.

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100%JR
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Re: Fork crown race fitting

Postby 100%JR » 5 Jan 2020, 8:05pm

Phileas wrote:So 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 20.8mm (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?

I don't understand.
1 1/8" is smaller than 1.5" so you don't need a reducer bush you need a tapered head-set bearing.
The tapered headset should come with the correct sized crown-race.
The fork steerer tube remains the same.There is no need to "turn down" anything to fit!

Phileas
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Location: Bristol

Re: Fork crown race fitting

Postby Phileas » 5 Jan 2020, 8:18pm

100%JR wrote:I don't understand.1 1/8" is smaller than 1.5"
The tapered headset should come with the correct sized crown-race.
The fork steerer tube remains the same.There is no need to "turn down" anything to fit!

It’s a straight steerer that will be used in a tapered head tube, so I have a reducer bush to adapt it.

My question is sort of a general one about what prep usually goes on with forks at LBSs. Perhaps the crown race is not usually reduced but the bush ID seems a bit small although I can’t measure it very accurately.

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100%JR
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Re: Fork crown race fitting

Postby 100%JR » 5 Jan 2020, 8:27pm

I'm still at a loss.
Phileas wrote:It’s a straight steerer that will be used in a tapered head tube, so I have a reducer bush to adapt it.

To fit a straight steerer tube in a tapered head tube you need a 1 1/8th top bearing and an 1.5" bottom bearing/cup/reducer.As I said the bottom bearing will come with the correct sized crown race.Are you sure the "bush" you've bought is correct?
Phileas wrote:My question is sort of a general one about what prep usually goes on with forks at LBSs. Perhaps the crown race is not usually reduced but the bush ID seems a bit small although I can’t measure it very accurately.

No LBS would ever "turn down" a fork.
As stated in my posts they don't need to any way because the steerer tube is smaller than 1.5".

No LBS should do anything to a steerer other than cut it to size when fitted with stem/top-cap etc!

Phileas
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Location: Bristol

Re: Fork crown race fitting

Postby Phileas » 5 Jan 2020, 8:33pm

100%JR wrote:I'm still at a loss.

No LBS would ever "turn down" a fork.
As stated in my posts they don't need to any way because the steerer tube is smaller than 1.5".

No LBS should do anything to a steerer other than cut it to size when fitted with stem/top-cap etc!


You might be right about the LBS but I can assure you I do need a reducer bush to use my existing tapered headset on my new fork. :D

Brucey
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Re: Fork crown race fitting

Postby Brucey » 5 Jan 2020, 8:48pm

if the sizes are as you describe in your post

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 20.8mm (using a dial calliper).


then you have not got the correct part at all.

I would expect one of these

https://www.wiggle.co.uk/hope-15-headset-reducer-crown-converter-1/

to fit without a lot of palaver.

It ought to be a five minute job to fit the correct part, and it is more like a five hour job to fit the wrong one.

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

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100%JR
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Re: Fork crown race fitting

Postby 100%JR » 5 Jan 2020, 8:51pm

Phileas wrote:You might be right about the LBS but I can assure you I do need a reducer bush to use my existing tapered headset on my new fork. :D

If everything else is present and correct then the "bush" is obviously the wrong size.Can you provide a link to the item you've bought?
Whenever I've used a 1 1/8th fork in a tapered headtube(MTB) I've always bought a replacement headset kit as it comes with the correct race/reducers etc.

Phileas
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Re: Fork crown race fitting

Postby Phileas » 5 Jan 2020, 9:05pm

Brucey wrote:if the sizes are as you describe in your post

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 20.8mm (using a dial calliper).


then you have not got the correct part at all.

I would expect one of these

https://www.wiggle.co.uk/hope-15-headset-reducer-crown-converter-1/

to fit without a lot of palaver.

It ought to be a five minute job to fit the correct part, and it is more like a five hour job to fit the wrong one.

cheers


That’s the part I have, although I bought it from SJS, pt no. HS136S.

Maybe it’s not as small as it measures, but I doesn’t feel as though it would fit without a lot force.

I could take the bush to work and get it measured properly. I could probably get it adjusted if required.

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100%JR
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Re: Fork crown race fitting

Postby 100%JR » 5 Jan 2020, 9:12pm

I take it you have the Hope headset?
Fork crown converter that used in conjunction with the crown race supplied with Hope's 1.5" lower headset so that you can fit a straight 1' 1/8" fork in to your frame.


Your OP states:-
The fork crown measures 30.1mm and the bush is smaller 20.8mm (using a dial calliper).


That's almost 10mm difference!
You'd never get that to fit the steerer.
Are you sure it's 1 1/8th steerer?

ElCani
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Re: Fork crown race fitting

Postby ElCani » 5 Jan 2020, 9:21pm

The internal diameter of that part should be very slightly more that 30.0mm, if it’s approx 20mm, you have a different, wrong, part.

Phileas
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Re: Fork crown race fitting

Postby Phileas » 5 Jan 2020, 9:22pm

:lol: :oops:

I meant 29.8mm.

slowster
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Re: Fork crown race fitting

Postby slowster » 5 Jan 2020, 9:25pm

The internal diameter of a crown race should be fractionally smaller than the diameter of the fork crown to provide a good press fit - Park Tool advise 0.1mm to 0.2mm smaller. I would expect the same guideline to apply to your convertor.

Phileas
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Re: Fork crown race fitting

Postby Phileas » 5 Jan 2020, 9:31pm

Yes, I was expecting a press fit. It’s probably fine but I had been hoping not to need an LBS but I’m not sure I can press it on with my home made “tool” (bit of plastic pipe).

tim-b
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Re: Fork crown race fitting

Postby tim-b » 5 Jan 2020, 10:31pm

Hi
On a 1 1/8 steerer (which is a legacy diameter), the SHIS standard for the fork crown seat is nominally 30mm. The actual dimension should be 30.015mm to 30.075mm (https://blue.canecreek.com/resources/products/headsets/PDFs/canecreek-shis-poster.pdf)
Regards
tim-b
~~~~¯\(ツ)/¯~~~~

Phileas
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Location: Bristol

Re: Fork crown race fitting

Postby Phileas » 6 Jan 2020, 5:01am

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.

I’m going to see if I can get the bush measured more accurately although I wouldn’t expect my measurements to be that far out. The fork measures about right, perhaps top limit.