Effect of lessening the lower headset stack height?
 

Going from 19mm down to 12mm on my headset will create a top tube that slopes down because of the 7mm loss.
This will cause the bike to handle a little like a time trial bike?

Look at it this way. The front is dropping 7mm, rotating around a pivot point at the rear axle. So that, 7mm over 38", or 965mm or so. To see the angle change we need to find an angle with a tangent = to 7/965, or .007

That's 0.4° or so. If you think that's material you'll need to compensate, maybe with a larger tire up front. OTOH- you could pretend you never thought about it, and enjoy your bike. Of course, if you later find the handling to be funky, you know the cause and van buy a new headset.

Damn! So that's why my 89 Peugeot Versailles commuter got so twitchy after I staggered the tire diameters. ;)

http://i.imgsafe.org/4d4b3c5.jpg

I got totally lost in FBinNY's math. Thats why I leave em the way their suppose to be.

I tried to keep the math as simple as possible, and only went through it to support the conclusion that the difference was less than 1/2 degree.

But in all fairness, the only hard thing is knowing that there's an easy way to convert a drop of 7mm over 38" (the wheel base) to an angle change. That's just simple trig, and knowing that there are sites that do it for you.

Simple Trig, never heard that before. I guess thats why I'm a firefighter. But why would you want to change the angle?

To figure out how change like this is may negatively effect handling, it would help to know the size of the frame and geometry. On a fairly small stiff frame with tight geometry short wheelbase slight changes an cause significant changes in handling ride, while on larger frames with fairly long wheelbase and slightly relaxed fit you an make some fairly big changes in setup without much effect. Typically less than 20mm change in stack stem hieght won't have much of a negative effect on handling.

Nobody suggested that they wanted to change the angle. It's just a consequence of dropping the front end 7mm by using a lower profile headset.

The OP wondered whether it would make a difference, and I provided the calculation to offer some basis of comparison so he could make a judgement for himself.

Ignoring the details of the math, the change will be somewhere between imperceptible and almost imperceptible. As a result the bike may handle better or worse, but you won't be sure which; and it may be a little more or a little less comfortable, but you won't be sure which. I wouldn't worry about it.

Which is why I concluded my original post here (no.2) with the suggestion that the OP forget he ever noticed the "problem".

There are times that we could be well served by one of those memory zappers the men in black carry.

Using [MENTION=158672]FBinNY[/MENTION]'s trig answer, the trail will be lower by a couple of millimeters.

Plug in some reasononable numbers in this:

Bicycle Trail Calculator | yojimg.net

Then add 0.4 degrees to the head tube angle.

I did a similar thing for one of my bikes. Bike came with a high end after market fork, and I didin't really like the way it handled. I changed the fork, dropping the front end by about the same amount 7-8 mm through a combination of headset stack height and axle-crown length. It changed the handling enough that I like the way it rides better now.

The change is small enough that that it would probably not be noticed by the majority of cyclists. However, for those that can detect it, is is a small enough that the rider should easily adjust to it.

Some of the change that will be felt will be due to the altering of the rider position. The saddle to bar height differential will be larger. The saddle will be slightly farther forward relative to the pedals. Angles of the bars and saddle change very slightly. Re-establishing the baseline position by adjusting these parameters will attenuate any noticeable effect.

I've done enough fork rerakes to know that the drop in handlebars is easily adjusted by raising the stem a few mm. Reraking a fork changes the head tube and seat tube angle relative to the ground as well. I haven't noticed the relationship of the saddle to pedals changing, althought that too can be adjusted out. I'm more sensitive to saddle tilt, having pulled a seatpost and saddle out of one bike to move to another, the difference in seat tube angle dropped the nose slightly, and I noticed that I was sliding forward while riding just a bit. Again, a simple change of the saddle angle fixed that.

For the OP, the 2mm change in trail would be difficult to feel.

I am completely confused! Doesn't stack height apply to both above and below the head tube? I don't think I have ever seen a difference of 7mm on the bottom side of the head tube1

What am i missing?

Just add fully loaded panniers to the rear to compensate for the front end drop...:twitchy:

Every headset has an upper and lower stack. I'm pretty sure he is referring only the lower stack height (i.e. crown race plus bottom cup), as he mentions going form 19mm down to 12mm. I don't know of any headsets with a total (upper and lower) stack height of only 19mnm, let alone 12mm, unless we were talking a modern, threadless, integrated headset. Having said that, I don't think I've ever seen a C&V headset with a lower stack height as tall as 19mm, though around 12mm is very common.

Now that's a good point! Typical framebuilding rule of thumb is to account for 13mm of headset between the fork crown and the bottom of the head tube. Obviously no amount of headset height above the head tube would affect the head tube angle.

[MENTION=20650]T-Mar[/MENTION] [MENTION=381793]gugie[/MENTION] - Thanks. I was missing the meaning of the numbers somehow. Guess I have difficulty translating from text to graphics! I tend to be a visual person and couldn't process the information correctly.

Often the discussion get into some real detail that I have not thought about. My only concern with HS have been overall stack and diameters. Overall stack to address steerer length and diameters as it pertains to crown races.

I never considered that there would be 7mm of difference between lower stack of HS's. My expectation for C&V HS (non integrated or hidden) would be no more than 3mm. Having said that, I really don't know what the range of differences are.

Most of us learn trigonometry in high school. You may have solved problems like this then. Since then, you haven't had need to apply it, so you forgot it. This thread might show the practicality of trigonometry, but we have people here who put it in perspective. Basically, it occurred to [MENTION=163198]avhed[/MENTION] that a new headset may make a difference because he knew that head angles affect bike handling. [MENTION=158672]FBinNY[/MENTION] and others point out that it is a measurable difference but probably not a significant one. With the knowledge of how to use trig, we can predict when changes are likely and unlikely to be significant.

The point isn't that we all have to be wizards in all things we learn in school, but it is helpful to know what arts are out there and what they can achieve.

Very well put Tom, but I was a slacker in high school and didn't take Trig. I guess thats why I ended up in the military instead of college. Very interesting thread though, thanks for the input. Math was always my weak subject and now I'm going through it with my oldest daughter. My youngest takes after my wife who took Trig in junior high.

You don't have to know trig, only that it exists. Then it's a matter of blocking out the issue on paper or in your mind.

This is simple if you open your mind to it. Imaging the bike supported on the two wheels separated by the wheel base, say 38" (965mm) or so. Now one end drops by 7mm. You can now imaging a narrow triangle formed by the original line and the new one dropped at one end.

So, in considering the effect, one of the questions is what kind of change to the frames geometry happens, specifically to the head and seat angles. This is where the trig come in, and fortunately, all that work is done already, and if you can frame the question, you'll get the answer on the internet.

In any case, this isn't about showing off, it's simply an attempt to quantify the effect so the OP has a basis to consider the possible effect on the bike.

BTW - there's a certain irony here, if you don't mind me pointing it out. Folks here on BF often talk about all the tools they own, and the importance of using the right tool. Well, a basic understanding of math (very basic) is also a tool, and is the right tool for questions like then OP's.

It has been decades since I was friends with trig equations.
So, I threw a bike profile into a CAD program. 39" wheelbase, 16" chainstays, 2" of fork rake.
I came up shy of a .4 degree change, ( using that instead of minutes and seconds for the sake only of keeping what has been discussed congruent) the bottom bracket dropped less than an eighth of an inch.

Changing the fork rake will do much more than the variations of lower headset "stack" heights will effect in a perceptible way.

How far from horizontal is your top tube now?