need help in mountain bike geometry?
What does a slack and tight head tube means?
and roughly what are their angle?
- Ride!UrbanLv 71 decade agoFavorite Answer
Sfr1224 is correct. Head angle is an important part of frame geometry that you're more likely to feel when you ride. It's only one aspect of frame design, but it's a biggie.
Slack head angles do more than slow the steering response, though.....they also have the effect of slightly shifting the rider's weight rearward, further behind the front axle. This, combined with the typically longer fork travel that accompanies slack frame geometry, helps with rider balance and impact absorption when your bike is pointed downhill. Another effect it has is to lengthen the wheelbase, which provides some stability at speed and in cornering.
The steeper the terrain that the bike is meant to handle, the more slack the head angle. You can feel two degrees of difference if you know what you're looking for. Serious downhill bikes may have heads as slack as 64°. Light freeride and 5"-6" trailbikes will hover around 66°-69°, and xc/race frames will stick to the more traditional 70°-71°. Road bikes, in comparison, are usually 71°-74°.
Steep head angles are really responsive, and there are some dirt jump and trials bikes with a steep head that are really nice to maneuver. They make riding through tight, twisty trails a little easier, but if you're in rough terrain or steep terrain, they make it a tad easier to wash out the front wheel in a corner or to go over the bars when you get surprised by obstacles (however, slack head angles won't cure endo-sickness....fair warning. LOL) :o)
In contrast, if you put a slack head angle bike on flat ground, the steering will feel sorta dead, and if you turn the bars hard, there's a "breakover" point where the wheel meets resistance and then snaps past it....kinda sucks until you get used to it. As an example, a couple years ago I bought a freeride hardtail frame....very burly, and I wanted to set it up for urban drops, etc. I was used to xc frames with 70°ish heads, and this was a 67° bike designed around a 130mm fork (5"). It felt bad to me, and wasn't what I wanted for riding the ledges on square planter boxes, quick maneuvers on stairs and tight cornering.....or for trackstands, pivots, etc. So, I put a 100mm fork on the bike which had the effect of steepening the head angle by about 1°. Changed a couple other aspects on the stem/handlebars, too, but it improved the handling for my purposes. You don't want to vary a whole lot from what the manufacturer designed (and putting longer forks on bikes designed for shorter ones is generally a bad idea for safety/durability).....but there's a little room to play.
There is a lot more to frame geometry and what-affects-what, how the bike feels and handles, etc. Chainstay length, seat tube angle, and bottom bracket drop (height) are all important numbers to pay attention to if you want a particular type of frame/handling. It gets a little complex, but manufacturers have tweaked geometry into different variations that work very nicely for their applications. It's funny, though......in a couple hundred years or whatever, the initial design of the bike frame hasn't changed very much at all. Whoever came up with it was either lucky or pretty intelligent! :o)
- sfr1224Lv 51 decade ago
Slack vs. tight refers only to the geometry of the bike. It has nothing at all to do with how parts fit together.
A bike with slack geometry will tend to be less responsive to steering input. A bike with tighter geometry will tend to be more resonsive to steering input.
Typically, bikes designed for big hit type riding like freeride and downhill bikes will have pretty slack geometry. When you are getting bounced around a lot, you don't want the bike turning sharply in response to every little twitch of your muscles.
Cross country (XC) bikes will tend to have much tighter geometry. This makes the bike highly manuverable for negotiating twisty, turny single track type courses.
If for example you were to look at the Turner DHR bike which is designed for downhill work you'll see it's head tube angle is 65 degrees. Turner's XC bike the Nitrous has a 71 degree head tube angle.
Hope this helps.
- Anonymous1 decade ago
Slack or tight head should be reffering to how your stem and fork fit into the head tube. But it shouldn't be slack, that would be a problem.
I can't give you an exact number of degrees, but it should be angled slighly outward on the bottom. Every bike company makes different angles for different bikes.
- How do you think about the answers? You can sign in to vote the answer.
- McGLv 71 decade ago
You have expert answers there.
I clicked on this question half thinking it was going to be another homework assignment, LOL .
- 4 years ago