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-   -   extending the axle to crown lenght of a fork... bad idea? (http://www.bikeforums.net/general-cycling-discussion/798290-extending-axle-crown-lenght-fork-bad-idea.html)

alex jb 02-12-12 02:53 PM

extending the axle to crown lenght of a fork... bad idea?
 
Hi folks,

I have a nice carbon CX fork with great clearance and a 410mm A-C.
I have a friend with an 'on-one inbred' 29er frame I can get cheap.

I'm liking the idea of building a monster cross type bike with canti's that I can throw some wide wheels on for dirt and my regular 19mm wide 700c's for a group road ride.

My concern is that most 29er forks are 450mm or so A-C so my cross fork is going to drop and tilt the head tube quite a bit.

So... I have lot's of spacers on the top, could I use a steel spacer at the bottom instead to effectively make the A-C 430-450mm?
Anyone done this?

I may look a little funky, but I am not worried about that! This thing will be a mutant anyway!

Seb71 02-12-12 03:23 PM

Yes, bad ideea.

alex jb 02-12-12 03:35 PM

Quote:

Originally Posted by Seb71 (Post 13843140)
Yes, bad ideea.

pray tell why?

increased stress on the steer tube to crown joint?

Clem von Jones 02-12-12 03:47 PM

It's a good idea if the frame is too large but a bad idea if the frame is too small.

If the frame is the right size it could make it a really good climber but a bit treacherous on descents.

Maybe your friend would loan or rent you the frame until you try and decide.

alex jb 02-12-12 03:59 PM

large
 
the frame's a 19.5", if I was building it as a MTB then I'd say it was a bit large for me.
my Klein is a M and my previous hard tail was a 17"

However as a drop bar trail bike I thin it's about right.

I would be adding the spacer, purely to bring up the head tube and re-set the effective seat tube angle.
I'm guessing I would be taking about 10mm off the bike's rake.

Clem von Jones 02-12-12 04:11 PM

You'd have to measure the tube tube to see if it's even viable, maybe the reach would make to too long for drop bars. Compare that dimension to your favorite bike's reach. 19.5 is a huge frame unless you're about 6'3"

LesterOfPuppets 02-12-12 04:16 PM

Quote:

Originally Posted by alex jb (Post 13843064)
So... I have lot's of spacers on the top, could I use a steel spacer at the bottom instead to effectively make the A-C 430-450mm?
Anyone done this?

These give you 2cm, so the 430mm ATC is certainly easy to come by. I think they're Aluminum not steel. I've never done this.

http://www.eighthinch.com/headtube_extenders.html

Clem's point is very important. That frame has a 62cm effective top tube (ETT). That's pretty big when running flatbars, GIGANTIC with drops on there.

I should add that whether you'll be comfy on that ETT has more to do with your torso and arm lengths than your overall height, however.

alex jb 02-12-12 04:17 PM

Quote:

Originally Posted by Clem von Jones (Post 13843206)
It's a good idea if the frame is too large but a bad idea if the frame is too small.

If the frame is the right size it could make it a really good climber but a bit treacherous on descents.

Maybe your friend would loan or rent you the frame until you try and decide.

Ah, are you meaning if I stuck with the 410mm A-C fork?
If so, I see what you are saying, agreed.

I have my carbon fork built into a compact 54 frame now, It was a cheap (scatante R330) buy and I have a lot of seat tube out and a 110mm stem on that build.

I see what Clem is saying too, the bar IS the hand position of a flat bar, on a drop bar you have the swoop of the bar and the brake hood, probably 120mm difference

Seb71 02-12-12 04:26 PM

If you take the spacers from above the headtube and put them below the headtube, under the crown race, then the crown race will sit on the narrower area of the fork steerer tube and you will have play.

At the base of the steerer tube (where the crown race sits), the nominal diameter of the steerer tube is 30mm. Above that, the diameter is only 28.6mm (in case of a 1 1/8" threadless fork).

Metaluna 02-12-12 04:42 PM

Sounds sketchy to me. If you put a spacer underneath, it would have to be inserted between the lower headset bearing and the crown race pressed onto the fork. As a result the lower bearing will not be supported properly and you will probably have a lot of play in the steerer. The only way I could imagine it working is if you got a second bearing cap and compression ring (which are normally used on the top bearing between the spacers and bearing) and inserted it on the bottom. Probably still not strictly safe that way either, as you are applying loads to the fork in ways that it wasn't designed for.

alex jb 02-12-12 08:20 PM

helpfull stuff guys :)

I was looking on on-one's web site and they have a head set with a +5mm bottom cup that would help.
There is also ahedset that slackens the rake by 1.5 deg, I guess it works by having the fork axis not in line with the outer headset axis.
That could also fix the issue of the reduced rake if I stuck with the 410 A-C fork, the short fork steepens the seat tube angle a bit too.

I will go and measure my current winter/trail rig.

alex jb 02-12-12 08:53 PM

Holy moly, I measured up and my eff TT = 515mm then 220mm from there to the hand position, total of 735.
The inbred is clearly perfect for a flat bar but way too long to do what I was planning.

Ah well, back to the search for a cross frame with more than the average 32-35mm clearance.


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