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I was wondering how changes in fork height effect frame geometry and handling. I have a GT I drive FS (2001) that came with a 80mm travel fork. I have a 100 mm travel fork that I might put on it. The new fork is about 1/2" taller than the old fork. If I put the 100 mm on the i'drive I was thinking about putting the 80 mm on a 94 steel hardtail I have that has a RS quadra 21 with hardly any travel and quite a bit shorter. Will changing the fork height-travel noticeable effect the feel of the bikes? I know this is the case trying to put a suspension fork on and old bike designed for a rigid fork.
 

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bike moron
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What I've heard is that every 1" in fork height difference = 1 degree head angle difference. For example, going from a 3" fork to a 4" fork (roughly 80mm to 100mm) will slacken the head angle of the bike about 1 degree.

In this example, that would tend to slow down the steering a bit, help high speed stability and control, and improve descending. The flip side is a negative on climbing ability and a loss in handling quickness.

Whether or not the change is positive for you depends on what you're looking for. In my case, I run a 100mm on a bike designed for 80mm & the change is really beneficial since I don't race & like control and good technical ability.

You also don't want to do anything extreme and you should check the manufacturer's specs. Too tall of a fork will place excess leverage on the head tube & could lead to problems. Results will vary bike to bike; but I think the changes you're thinking about could turn out pretty well.
 

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mojo2367 said:
I was wondering how changes in fork height effect frame geometry and handling. I have a GT I drive FS (2001) that came with a 80mm travel fork. I have a 100 mm travel fork that I might put on it. The new fork is about 1/2" taller than the old fork. If I put the 100 mm on the i'drive I was thinking about putting the 80 mm on a 94 steel hardtail I have that has a RS quadra 21 with hardly any travel and quite a bit shorter. Will changing the fork height-travel noticeable effect the feel of the bikes? I know this is the case trying to put a suspension fork on and old bike designed for a rigid fork.

Longer / shorter changes the head tube angle. Slack = slow steering, steep = faster steering. What I do is find a head angle that I prefer and try and apply it to suspension component selections. I like 69* so I measure the current angle w/ an angle gauge finder ( Home Depot or hardware store less than $20). Once I find out what my starting point is I raise or lower the front according to the new forks a-c height. Raising can be done by placing wood shims under the front tire. Lowering can be done by deflating the tire then take your new measurement.
 

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fork height vs. steering angle seems to vary with preference

I have installed a Z1 MCR on my 03 I-drive, and it is a 130mm travel fork. The bike was designed for an 80mm fork, and it has made the steering angle way more slack. I happen to love the change, as it has inspired way more confidence on the descents, and with the ECC on the fork, the climbs are a snap. Personally, a change of one degree either way doesn't make a huge difference in feel to me, but some have said it is unbearable. If you want the extra travel and the cool features in today's forks, but you love your frame, sometimes you gotta compromise. I love the longer fork on my GT, it made it feel like a new bike. :)
 
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