help: how to work out forks steerer length & thread lenght

[lex]

Hi

I'm trying to source some rigid mtb forks for an old merlin cycles mtb frame. Whilst there appear to be several available on ebya i'm getting confused as to what length steerer tube i need and how much of it should be threaded.

The head tube is 115mm and is 1" rather than 1 1/8". If I (or lbs) fitted a threaded headset what length steer tube would i need on the forks and how much of it should be threaded?

I'm more used to ahead setups.

Thanks

Lex

 

[marc two tone]

Re:

Yes, it can be an arse to determine as headset of choice' stack height has to be factored in. Get the forks first based on guidance below. then sort a headset.

Personally i insert fork steerer (when forks are on site) and go 20mm over either side of headtube on frame, that should allow for the headset bearing cups, then add maybe 15-20mm for top bearing nut and locknut.

You have no forks but use the same formula. so 60mm extra to your frames' headtube should be ok. Again, it can be disputed based on headset of choice as bearing race, as traditional ones are lower stack than say sealed bearings.
All said and done, if the steerer is then too long by a short-ish amount, say <10mm, then use a spacer under locknut or trim steerer with hacksaw.
If to short then its a case of choosing a headset with less stack....

Atleast 30mm should be threaded, usually 40mm.

Whatever maximum you determine, make that's your limit as you may have to hack saw 20mm or so off, leaving only 10mm of thread to work with.

by the way. when forks are advertised, the length of steerer stated should be the whole length, regardless of threaded amount. the amount threaded should be stated too, if not you can generally tell by eye if it's ample or not. the more thread the better.

 

[lex]

Thanks for the reply!

I've found some forks that have a 152mm steerer (including 40mm threaded). do you think these would work if I got lbs to fit a traditional headset? They are only 37mm longer than the steerer tube.

Cheers

 

[hamster]

A Ritchey Pro Logic headset would fit. Its stack height is 33mm. Get your LBS to fit an extra spacer between the headset nuts rather than cut down the steerer.

Alternatively a Campag Record is 36.5mm stack and will fit spot-on I suspect. These are fabulous headsets, beautifully smooth and durable.

 

[lex]

That's great - thanks for the advice!

 

[FluffyChicken]

I always pick a headset I like and then add that on to the headtube length.
That is the length you will need to end up with, so it should be that or longer.
Make sure the threading starts at (minimum) about "headtube length + half the stack height", that will make sure you can thread the two top art on.

Fit headset on frame.
Fit crown race on forks.
Fit forks and top part of headset on (the locknut might not go on) but set the top race as it would be fitted properly
Mark where the fork needs to end, which is often a mm or a bit under the locknut top as it seals over the top (normally)
You have a bit of leeway here but not a lot.
Cut to length, bevel if you want to do a 'proper job'
Fit top part and off you go.

 

[marc two tone]

Re:

And you will need a 32mm headset spanner/s.