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Old 01-06-05, 02:26 PM   #1
Stumpyman
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Why are fork drop outs built the way they are?

I've been paying undue attention to my new mountain bike, and I'm curious why the drop out is positioned forward of the center of the lowers. All mountain bike forks, even the huge double-crowned ones I've seen, all have drop outs like this in some form or another.

In order to reduce the bending at the drops, wouldn't it be better to put the drops directly at the center bottom of the fork and not forward?

Someone I know actually broke their fork at this particular location, but they were doing drops too big for that particular fork anyway. It just seems like a weak point in the design. Is there a factor that is more desirable like trail or something that this affects?
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Old 01-06-05, 03:01 PM   #2
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There are many factors that come into play....I know that Stratos makes a fork with dropouts centered below the lowers.
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Old 01-06-05, 05:58 PM   #3
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Usually it is for tracking and handling.
On road bikes where they have no suspension they count on the give in the fork blades to take the vibration out of the road.

On mountain bike suspension forks they set the shocks to be steeper than the angle from the frame to the axle to reduce brake induced dive effects. Otherwise each time the brake was used the front end would dive(I had one that was like that) and it causes many unneeded endos.
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Old 01-07-05, 07:42 AM   #4
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For one thing, it helps the bike track straighter.

If you take a bike that has a fork with the dropouts offset ahead of the head tube centerline and turn the handlebars, You will notice that the front of the bike raises slightly. When you have your weight on the bike, the wheel will want to point straight. Much better than that floppy wheel on the shopping cart!

Another reason is this: I helps reduce bending in the frame. Mostly, you will see this design in road bikes.

On a properly constructed frame, the trapazoid formed by the head tube, top tube, down tube and seat tube will behave as a triangle. Find the imaginary point where the top tube and down tube would intersect in front of the bike. The front axle should be directly beneath this point. In a static loading condition, the tubes will be in compression only, with no bending.

Before you flame me on this, lay it out and do the calculations. It works. I'll bet my Masters in Mechanical Engineering on it!

Last edited by eubi; 01-07-05 at 07:54 AM.
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Old 01-07-05, 10:40 AM   #5
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Quote:
Originally Posted by Stumpyman
I've been paying undue attention to my new mountain bike, and I'm curious why the drop out is positioned forward of the center of the lowers. ...
Not only on mountain bikes, but road bike forks have this same effect. In the case of a road bike, the fork is curved to position the axle "forward".
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Old 01-07-05, 10:47 AM   #6
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Quote:
Originally Posted by eubi
...Another reason is this: I helps reduce bending in the frame. Mostly, you will see this design in road bikes.

On a properly constructed frame, the trapazoid formed by the head tube, top tube, down tube and seat tube will behave as a triangle. Find the imaginary point where the top tube and down tube would intersect in front of the bike. The front axle should be directly beneath this point. In a static loading condition, the tubes will be in compression only, with no bending.

Before you flame me on this, lay it out and do the calculations. It works. I'll bet my Masters in Mechanical Engineering on it!


Very interesting, regarding all members being in compression. But why is compression in the frame desireable? Compression could lead to failure by buckling. But I guess tension is likely to rip welds apart.
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Old 01-13-05, 09:12 AM   #7
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Originally Posted by MudPie
Very interesting, regarding all members being in compression. But why is compression in the frame desireable? Compression could lead to failure by buckling. But I guess tension is likely to rip welds apart.
Example: For a 1" dia, .060 wall, 36 long steel tube in compression, a very conservative calculation gave me a critical buckling load of 1,270 lbf. The same tube in tension can expect to fail at around 6,000 lbf. Those are some pretty heavy duty loads for a bike!

No problem for high quality material and high quality welds.

Last edited by eubi; 01-13-05 at 09:22 AM.
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Old 01-13-05, 09:22 AM   #8
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Antoher possiblility is for service. The two little allen-head bolts on each side hold the whole mess together, and they need to be right in-line with the stanchions, hence why the dropout could be offset. If you put a dropout there, then there is no room for a bolt.
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Old 01-13-05, 10:59 AM   #9
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Does it not also have something to do with the fact that any bump you hit on a bike not only pushes up on the fork but back behind you slightly too.
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