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Recently had a custom Black Sheep 29er frame made for me & am now contemplating having James B also make a matching Ti fork. The frame was originally designed w/ the idea that it would be able to accomodate either rigid or sus fork. Currently leaning more toward the rigid option; though if I did decide to try a sus fork, it would more than likely be the White Bros BW 0.8

Below are a couple of the alternatives that have been proposed:

Suspension (w/ 80 mm travel)
Fork Length: 465mm, Rake: 45mm, Head Angle: 70.8, Tire profile: 50mm, calculated trail: 78.4

Rigid
Fork Length: 450mm, Rake: 45mm, Head Angle: 71.5, Tire profile: 50mm, calculated trail: 73.7

After running across some postings by PeT & Walt (-thx to super informative postings by PeT & Walt I have moved from ground-zero, to perhaps the level of knowing-enough-to-be-dangerous re head angle, rake, trail, fork length, etc), I got to thinking that I should probably be shooting for a trail value of around 60-65mm.

I realize that if the rake increased that it will reduce the trail. For example, for the Rigid option, if the rake is increased to 50, it changes the trail to 68.1 (assuming that this does not change the head angle?). I guess my question is - should I look into only having the rake increased?; or should I also be looking into the possibility of changing the fork length? Any constructive suggestions will be greatly appreciated!

Live long, ride far,

Blackbeard
 

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Make it the same as your sussy fork...

At the very least, the *length* should stay the same as the (sagged) length of your suspension fork. If you get something shorter, you'll pretty radically change the weight distribution over the wheels (steepening the seat angle, lowering the bars, etc).

Second, don't shoot for a trail figure totally blind - 60-65mm is VERY little trail by MTB standards, and I'd be fairly shocked if you were anywhere near that with a suspension fork (given the slacker head angle and less rake, you are probably around 80mm of trail with that setup, in fact). If you do a shorter, 50mm rake rigid fork, you will in effect have 2 totally different bikes when you swap forks. For most people, that's a bad thing. If you want a little quicker handling with the rigid (I like the opposite, but I digress...) go with something the same length and perhaps an extra 5mm of rake. That's a pretty big difference already, believe it or not.

Good luck!

-Walt
 

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Another note...

Both the WB and Reba forks are around 470mm and 38mm of rake. There is not a 45mm rake suspension fork available, as far as I know. So the trail number in the first example would be 85, not 78.

-Walt

Below are a couple of the alternatives that have been proposed:

Suspension (w/ 80 mm travel)
Fork Length: 465mm, Rake: 45mm, Head Angle: 70.8, Tire profile: 50mm, calculated trail: 78.4

Rigid
Fork Length: 450mm, Rake: 45mm, Head Angle: 71.5, Tire profile: 50mm, calculated trail: 73.7

After running across some postings by PeT & Walt (-thx to super informative postings by PeT & Walt I have moved from ground-zero, to perhaps the level of knowing-enough-to-be-dangerous re head angle, rake, trail, fork length, etc), I got to thinking that I should probably be shooting for a trail value of around 60-65mm.

I realize that if the rake increased that it will reduce the trail. For example, for the Rigid option, if the rake is increased to 50, it changes the trail to 68.1 (assuming that this does not change the head angle?). I guess my question is - should I look into only having the rake increased?; or should I also be looking into the possibility of changing the fork length? Any constructive suggestions will be greatly appreciated!

Live long, ride far,

Blackbeard[/QUOTE]
 
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I also have a question about running a rigid fork on my 29er. I have a Rig w/ a 80mm reba and I want to put on a rigid fork. The karate monkey fork is around 475mm and most other 29er forks (Vicious, Kelly, Pace) are 440mm. What is the correct length to run and why?

Thanks in advance.
 

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470mm

You need something that will maintain the basic geometry of your frame - which was designed for a ~470mm fork (KM is 468, Reba or WB w/15-20mm of sag are around 470mm). If you use a shorter fork, the following will happen:
-You'll steepen the head angle considerably.
-You'll lower the bottom bracket considerably.
-You'll steepen the seat angle (which you can fix by moving the saddle back further on the rails if necessary)
-You'll reduce trail (assuming you have a fork with the same rake).

Basically, the bike will get much twitchier, and you'll have more of your weight over the front wheel. These *could* be good things in some circumstances, but if you like how the bike rides with suspension, you'll want a fork of a matching length.

Some builders use premitered unicrown fork blades for the sake of convenience, and these *until recently* haven't been available in lengths sufficient to do a 470mm fork. Hence the 430-440mm Kelly/Vicious/etc forks.

-Walt
 
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