How does axle-crown lenght affect your geometry ?-
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  1. #1
    mtbr member
    Reputation: Dex11's Avatar
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    May 2005

    How does axle-crown lenght affect your geometry ?

    Can anybody tell some more about this ?? I made a thread about axle-crown lenght regarding a Reba, but there where a some people asking the this Q also.
    So just allround info and maybe some examples of people how changed there fork and did
    notice some effect.


  2. #2
    mtbr member
    Join Date
    Jul 2003

    Okay, this is simple, but it can be tough to explain...

    Most bike frames are designed around a fork of a given travel length, and a fork of that travel will give a specific head tube angle. Head tube angle affects handling. Generally speaking an XC type head angle (steep head angle) is usually between 70 to 71 degrees, Freeride and Down Hill head angles (slack angles) generally run between 69 and 67 degrees. Okay a steep head angle gives snappy quick handling, but is a little unstable on a descent. A slack head angle results in slower handling but is quite stable going down and at speed. Okay, now imagine your frame suspended over the ground exactly like it would be with wheels, with the fork installed only. Now draw a line straight from the rear drop out to the drop out on the fork. The angle formed by the line and the fork is the head tube angle. See the drawing bellow, the head angle is angle A.

    We will assume that angle A is 71 degrees, and that the fork is an 80mm XC fork with an AC lenght of 453mm. You want to go with a 120mm fork from the same manufacturer with an AC height of 493mm. What you are doing is increasing the front end ride height by 40mm. In the attached image the black line between the front and rear drop out is with the 80mm fork. The red line is with the 120mm fork. Now remember the line must be parallel with the ground to get an accurate head tube angle measurement. So you have to tip the bike backwards to make the red line parallel with the ground and get an accurate measurement. This is exactly what a longer fork does on your bike. It raises the front end and in doing so slackens the head angle. The rule of thumb is for every 20mm of additional AC length you slacken the head angle by one degree. So you have effectively changed the head angle in the above example by 2 degrees. So you went from a 71 (a snappy XC angle) degree angel to a 69 degree (slow almost down hill) angle.

    Now as to what you can expect from such a change? Well the handling will slow down, the bike won't react to handle bar input as quickly as before. Depending on how much of a change you make the steering may become floppy, i.e. it will feel like the bike isn't doing anything as you turn the bars, but then will react quite quickly at a given point. It feels kind of like the front end just kind of flops over at a given point. And probably the most noticable would be the change in climbing characteristics. Because the front of the bike rides higher it shifts the riders weight more to the rear. This requires you to move further forward when climbing to keep the front end of the bike down. It can make steep climbs more difficult. On the up side, some bikes need to have the handling slowed down a bit, and descending will be much more stable than before. A couple of factors that will affect how much you will notice these changes in geometry are, the original geometry of the bike, and the size of the frame. Usually with larger frames the effects are less noticable.

    As a general rule I usually recommend that you don't go with a fork that has more that 20mm more travel than what originally came on the bike if you want handling etc. to remain as closse as possible to the original. Either that or you get an adjustable travel fork with a range that comes as close as possible to your original fork travel on the low end of the adjustment range. So if you want to put a 120mm fork on your bike, but are not sure if you will like it, then an adjustable fork with a range of say 80 to 115mm of travel would be good.

    Anyway sorry this is so long, but it is actually tougher to explain than it is to show. Hope this helps.

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