[RVM]

What frame is it?

My bike had a 80mm fork on it and with that had a headtube angle of 71. I was thinking of putting a fork with 120mm of travel on it. How much will this change the headtube angle.

If the frame was designed for 80mm, the 120 might make you hate the way it handles. The geometry will become slacker/slower steering. But you might like a slower geometry...

 

[fl jay]

My bike had a 80mm fork on it and with that had a headtube angle of 71. I was thinking of putting a fork with 120mm of travel on it. How much will this change the headtube angle.

 

[WarrGuru]

My bike had a 80mm fork on it and with that had a headtube angle of 71. I was thinking of putting a fork with 120mm of travel on it. How much will this change the headtube angle.

The rule of thumb is 1* change for each 1" change in Axle to Crown length. This varies slightly depending on the starting HA and wheelbase.

Here's where you must be careful. The change from 80mm to 120mm can wind up being a 3 inch axle to crown change (depending on brand of forks used). You could potentially end up with a 68* Head ANgle. That's a big change. 68* isn't bad for a trail bike, but if it's a XC racer, you may be disappointed.

 

[Duckman]

To make sure you understand "why" this happens...

Typically, when you go to a longer travel fork, you gain the extra travel by the forking having to be taller at the upper sanctions(but the lowers are also deeper to acct for the extra overlap at full stroke, or you half to have less overlap to start with at the upper/lower junction). So, when its bottomed out, its normally at the same height as it was with the 80mm(when it was bottomed), but it rebounds farther away from the wheel, where it had to gain the extra length...soooo, its sorta like a very short chopper now, in laymans terns. Tihs is an over simplification, but effective nontheless. The longer a fork is compared to the oem setup, the slacker the angle/handling. Will show up in the tight stuff first. Meaning less willing to turn in tight stuff. More "floppy" maybe in extreme instances. You get the idea.

Duck

 

[fsrxc]

Remember the front end will be higher too which you will notice while going uphill, except on a chairlift I guess...

Your bars, stem and seating position will factor in too, so you can compensate a bit if you need to by moving your bodyweight forward. That won't change the steering angle, but it does put a but more weight on the front wheel which still can help steering in the tight stuff (as well as climbing obviously).

 

[fl jay]

update to ??

First off it is an 03 rockhopper. This is my back-up bike my other is a trailbike with a HA of 69.5. I live in florida so i'm not to concerned with the effects on climbing. Here we have mostly short up and downs. I was hoping to put a marzocchi mx pro eta on the bike.
So basically this bike sees limited use and I am already used to a slacker HA so I don't think I will hate the ride if I do this. it's not used for racing just riding when the other bike is down or sometimes as a loner bike. My other bike has a vanilla 125 and I love the extra travel. The rockhopper frame is a replacement for a 2000 that I cracked and the fork was crap on the original frame. I was just wondering if anyone could give me an idea of how much it would change the HA. Thanks for the input so far and hopefully this clarifies the ? a little.