How much Steerer can be left on a carbon fork?
 

I'm looking at purchasing a new, uncut Van Dessel Fork to put on my Leader 725. The frame has put me in an awkward position. My current fork is far too short and it's leaving me with around 4 inches of drop between my saddle and the flats of my bars. With the drop section of my bullhorns (RB-021) being even more extreme. I'd ideally like to cut that drop in half. But that will leave me with a bit of a stack on my Headset. I'm hoping to run a full carbon fork. What's the longest steerer i can safely run?

You can leave it all on if you like - you`ll just have to add enough spacers above the steering stem to let you cap the tube and preload the bearings.

So I guess you really want to know what the maximum recommended stack height is below the steering stem. And that depends on the steering tube diameter and material - which you haven`t posted.

Recommendations vary from manufacturer to manufacturer and since you seem to want an accurate maximum permitted suggest to check the specs on the make and model of what you`re looking at or contact the manufacturer.

A full carbon fork with a carbon steering tube would have a shorter permissable stack height than carbon fork with an alloy steering tube if you haven`t already made a purchase.

Pictures, but if you need that much steerer / stack height, thinking you have a frame which is too small

My personal rule, which is conservative, is the stack below the stem shouldn't be taller than the diameter so I prefer to stay below 25mm. Others say up to 40mm is fine and it probably is, but beyond that may be pushing a good thing too far.

It becomes a question of bending moments at the place where the steerer exits the top of the headset. The total moment is based on the straight line distance from the handle bar to the top of the headset, so those with long extensions should be more conservative than those with only 90-100mm extensions.

How far you can fudge also depends on your weight, strength and how and where you ride. If you're heavy or a hard climber or sprinter, who tends to pull on and work the bars a lot, stay conservative, but of you're a light flat ground spinning type rider the stresses will be lower giving you more room to play with.

Seek out a steel steerer fork, then the issues of full carbon steerer are not there .

I tend to agree with this thinking. It sounds like it might be a not badly sized frame IF you were able and comfy about getting into the correct racer's crouch. A 4 inch drop between the saddle and the top of the bars is not at all unusual for a bike being ridden in a proper racing crouch. If you're not comfy riding it that way then you most certainly do have the wrong size of frame AND the wrong style of frame. It sounds like you'd be happier with more of a touring style or cyclocross style setup where the rider's cockpit setup was more upright. Or at least a bigger road frame that lifts the bars up higher. But then that would open up the top tube as well.

Another option, depending on what you have now, would be to switch the stem for one which has a higher angle and keep the fork you have now instead of ending up with a huge stack of spacers. Either way the bike is still going to look odd with the high mounted saddle and bars though.

The biggest issue in terms of fit comes from weird geometry on the bike (Leader 725TR) It's a 58CM, i'm 6'2. I run 170mm crank arms and while I aimed for a more aggressive positioning on this frame the shorter crank arms sent my seat post up about another 2-2.5 inches from where it is positioned on my other fixed gear bike with 175mm crankarms. The fork i'm looking at is a full carbon van dessel. Any additional input is appreciated. I will also contacted Van Dessel about this. I'd love another inch or two on the fork (Right now i'm guessing I have a little more then a CM of stack on the fork)

This makes no sense. You are saying changing to 5 mm shorter crank arms caused you to raise your seat by 50 to 65 mm? There must be something else going on.

perhaps, I confused MM with CM.

But I have a massive difference in the amount of seat post on this bike. It feels like it fits me more like a 55CM, when comparing it with a 55CM Javelin Vigorelli. I'm stumped.

Your seat should have gone up 5 mm which is 1/2cm which is about 3/16 inch.

If your seat post is that much longer with the new bike it must be because it's a totally different type of bike - for instance a compact frame with a shorter seat tube, or because it's a smaller frame.

If your saddle to bar drop changed a lot from the one bike to the other, it's because the head tube on the new bike is smaller - either because it's the way it was designed or because the new frame is a lot smaller than the old one.

What are the seat tube, effective tt and head tube measurements between the two bikes?

I don't believe there is any practical way to make up 2" with a new fork with a carbon steer tube. Assuming your current fork has no spacers above the headset (nothing - not even a conical spacer), and you want to max out the new one, you'll can only gain 40 mm = 4 cm = which is a little more than 1.5 inches. This is because the maximum number of spacers between the headset and the stem that I've ever seen for carbon steer tube (1 1/8") is 40mm. Even if you add a high rise stem, you'll only gain another inch or so.

My bet is that you really need a larger frame or one that is specifically designed w/ a larger head tube for a give size.