ed 

That's what I was going to say...the fork that apclassic9 is installing has a fixed lower crown, so there's no way to slide the tube up/down in the crowns.

Gravity Worx

The lower crown in this case will not move.
That is the whole reason for AP having to cut head tube.
The upper crown is obviously maxxed out too.
The stack height needs to fit in-between.

No magically levitating, just cutting metal to make it fit.
All I'm saying is that the location of where the cut is made can have an effect, and that effect can, in fact be controlled by making the cut on the bottom side or by splitting the ammount of materiel by making 2 smaller cuts, 1 on top and bottom.
This way, the geometry can be slightly manipulated.
Cutting the bottom of the frame head tube just moves the lowest point on the stack to where it fits.
Cutting the materiel from the top of the head tube effectively lowers the top point on the stack in relation to the rest of the frame.

When re-assembling the bike, there will be a slight difference in head angle if the cut is made on the top side of the head tube.

cryptid01

You were good right up to the point you started typing this.^ I am glad you acknowledged that the axle to crown height of the fork is fixed. Let's use the crown race (which rests atop the lower crown) as a fixed point of reference.

Now visualize if you will a bike sitting at rest. If you remove material from the bottom of the head tube the attitude of the frame will rotate forward around the rear axle. This will reduce the absolute height of the bottom of the head tube from the ground, shorten the wheelbase, and steepen the head angle. Got it?

Now visualize removing material from the top of the head tube. What will happen to the absolute height of the bottom of the head tube/crown race from the ground? That's right, absolutely nothing. Which, not coincidentally, is the exact amount of head angle change you will see.

<fingers crossed that you will get it this time around>

edited for clarity

Gravity Worx

Fixed crown height, single crown fork, you are right.
Cutting the bottom will sharpen head angle.

It really depends on your point of reference as to where the frame moves.
Starting with the top of the frame lined up with the bottom of the top crown, there is X head angle.
Cutting the top off the frame, you need to lift the frame to make that top line up with the bottom of the upper crown again.

The same is true if using the starting point of the bottom of the crown as you just stated.
The bottom of the head tube/stack is the same as the top of the bottom crown and you have Y head angle.
Cutting the bottom changes the point of referance on the frame and so you have a sharper head angle.

In reading your post 1 thing did occur to me.
That is that we are starting from different reference points.
I'm starting from everything lined up at the top side and you are starting with everything lined up at the bottom side.
This difference will make the opposite true or untrue to each depending on where you start from and make 2 (or more) grown adults debate pretty harshly when saying the same things with in the same principals.

Think about it.
What was the original head angle and at what height?
We don't know as the bike wasn't assembled yet. The difference is going to be approximately 1/4" to 3/8" and so the angle is going to be altered very little.
Little enough that it will probably never be noticed. A different brand fork with the same travel will have a different height difference than what we are talking about.
So which starting point is correct?

As being a DH rider, I've normally set up as slack as possible making my starting points at the bottom of the upper clamp and lining the head and all up to there.
In "our world" for lack of a better term, this is the propper set up and starting point.
We would line up to the top side and then cut the top to make it more slack by most available means.

As an XC rider (I'm assuming here, so please correct me if I'm wrong) you've ridden the single crowns primarily, and so your starting point is always fixed at the lower crown.
There fore you would want to start at the lower end and either cut the top in that senerio to not effect head angle, or possible cut the bottom to make it more steep for tighter cornering etc.

In short, I'm wondering if we are hammering at the same point, but from litterally opposite ends of the Stack height.

I do know that either starting point does validate my original point that cutting head tube can and will effect head angle. That is what started this and so Thank you, you actually validated my original point, but from the other end.

Fingers also crossed that we can put this to bed now.

Gravity Worx

Yes, moving the triple clamps up and down on the fork tubes is the best way to change or tune head angle.
This technique has been around since before either of us was born.

The possibility of a different point of refference is the one mechanical block that you have on this.
The other block is that you are now trying to argue simply for the sake of arguing, I think you like it.

Starting with the opposite point of refference actually does make the same theory work in reverse.
Fork height will not change with this, so we either move the frame upwards and cut off the top, or move the frame downwards and cut off the bottom, depending on where we start from, if we are just trying to show that it can be done..
The frame can't hang below the lower crown just as it can not rise above the upper.
Either way, it has to fit in the given area and it will be cut or a smaller headset will be located.
Given that we are discussing a 1/4 or so inch difference, this really is just academic.

ViperZ

It would seem to me if you mill the head tube at the bottom, it would change the effective head tube angle, because it's relationship to the fork crown race has changed.

However if you milled the top of the head tube, no geometry changes take place, except stem height.

santiago

I've learned that bearded surveyors know their geometry.

Gravity Worx

This is the perfect explination Pete.^^

Now lets take this 1 step further and I think you will see where I've been coming from on this, and hopefully we can put this to rest.

Visualize the same bike, but with a triple clamp fork.
Now cut some materiel off the top of the head tube, then loosen the clamp bolts on the lower clamp and slide them upwards until the head tube sits where it should in relation to the top triple clamp as we did not move it.
Obviously there is more to it than just this. Cutting the top of the steerer tube or spacing the stem, etc, but it shows the same theory at work.

When we slid the lower triple clamp upwards, what did it do th the frame?
It rotated around the rear axel in an upward direction.
The only difference is that I started from the upper starting point as that is normally where I've set up my forks with the triple clamps, as slack as possible, and thus, the top side is the chosen starting point my general use.

There fore, Gastro did actually validate my original point with his description here by making a visual of the theory at hand.

ViperZ

It had nothing to do with your picture, rather what I carry around in my head

Gravity Worx

I actually hope I am wrong on your motivation.

misterC

yep.

ViperZ

If you are moving the fork crown race up, then sure, geometry changes may not take place, however it is assumed the fork crown race is fixed, and constant, if that is the case head tube geometry does change, if the bottom of the head tube is milled.

ViperZ

Maybe... That was a pretty technical drawing packed with a wealth of information

ViperZ

OK, here is my kick at the cat

ed 

That's the whole point!!! We're not dealing with a dual crown fork with crowns that you can move up/down the tube. We're dealing with a dual crown fork that has a fixed crown on bottom and a double clamp crown on top. No triple clamps. Only one crown can move.

Gravity Worx

Exactly my point.
I'm not saying that I want to make head angles any different by cutting, just that there is an effect.

Raising the lower crown or making the axle to crown distance greater raises the front of the frame and gives a more slack head angle.
As would the starting point that I was using.
The top of the head tube lined up with the bottom of the upper crown, and then cutting the top off and then re-alligning the top after the cut will make it more slack.
The triple clamps on the fork tubes get raised and then tightened to hold it in this position, or in this case, the smaller area to fit the head tube is what holds the position, and the fixed lower clamp did not move.

Cutting the bottom off did not move the frame because I started with the top already lined up and just cut the bottom to fit in my scenerio.
just as when starting at the lower refference point (bottom crown), cutting the top off also did not move the frame or head angle.
It's stil X and we have actually been going off of the same theory, but from opposite ends.

ed 

But your scenario doesn't apply to the thread. She's not putting an 888 on her boy's XC bike. She's using a Lefty with a FIXED lower crown.

AfterThisNap

ed 

Yeah...yeah...oh well. What were we talking about?

Oh yeah...apclassic9, you're hosed unless you mill down the top of that head tube a bit.

Hey! Here's a thought:

Try to find a stem with a short stack height too.

Gravity Worx

That is true about being hosed until you make it fit.
The rest is academic.

Either way, make it fit by what ever means you have to,
Get Cai to the races and to be able to practice on the race bike, and the rest will pretty much take care of it's self.

For the record,
I have only cut 2 head tubes (other than just a VERY minor facing here and there for assembly that would normally entail just taking powder coat or peoples paint off where they didn't mask it)

1 was for a guy who dropped a brand new frame on a concrete curb and left a 1/16" mark in the upper face, and the one on my bike as described a page or 2 back where I made more clearance to fit the desired equipment being big tires on an old school bike.
The first was easy, just reface it and he was happy. 1/16" is not much to worry about and it came out perfect.
The second was mine and the bike is still ridable and in my garage today.
No adverse effects were noticed. I still use this bike quite a bit for urban assault rides and at the local BMX track to play on the jumps and pump sections.
Yes, it did make it more slack, only a little, but more slack none the less, lower crown got moved up,
and I think from there, we've beet this thing to death.

cryptid01

Sorry it took me so long to get back - I had to go find a new keyboard after seeing Pete's sketch while drinking a glass of water.


You know what they say about "assume?" Well, in this case, they're half right (not my half, either).

wewe

i hav inquiries about tubing patchinesses if my thing that you peeps talkin bout hear gets hole will patchinesses of glue type work for plugging of heedtubes. sometymes my bike luvs to hit rocks crashy-like and want to make it back if bad things happens and i alwhys have the carrying of patchinesses of glue type anwaays.

thx peeps

dminor 

Gastro, the mods frown on creating second usernames and playing around making postings like this

santiago

Meme's writing style had a certain something that really, really made him stand out.

CaptMatt15

Flowbee!