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PostPosted: Mon May 13, 2013 2:51 am 
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Hi... ok, sooo here's my dilema... I have asked around and looked online but seem to get conflicting answers, so I thought I would ask here...

I have a 1996 Rocky Mountain Blizzard that should have a suspension fork with an AC of 428mm

Now, I was thinking of putting a Syncros Powerlite or a Tange Switchblade on, but I have been told that they are too short so the ride will be messed up... fair enough...

I have a factory rigid Rocky Mountain fork off of another Hammer frame so I thought I would try that one...

I tried to measure the AC and... well, here's where I'm confused... I have been told that it's 395mm for the Syncros fork... Is this correct? because, according to my measuring, it comes in at 382... See the pics..

Second question, how long should the AC of a Rigid fork be for it to be ok... i.e. I know that a rigid fork with 428mm AC would be ideal, but that ain't happening =)

So... If I can find one of, say, 400mm, would that work?... I know the only way to know is to try it but It's a hassle to cut threads in it and build it up only to find it's terrible...

I want to know if I'm measuring the SYNCROS one right... then I will use that method to measure the other Rigid Rocky Mountain one to see if it will work...

Thanks!!


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PostPosted: Mon May 13, 2013 6:49 am 
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You're not measuring it right afaik. The way you have it set up you are measuring the long side of a right angle triangle - you should be measuring the diagonal. Just hold it vertically and drop a tape measure from the crown to the floor.


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PostPosted: Mon May 13, 2013 7:35 am 
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from the pic it looks like how I understood it to be, from bottom of crown race to axle ignoring the effect of the rake, I didn't think it was just diagonal, or is this just road frames ?

http://www.sheldonbrown.com/rinard/forklengths.htm


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PostPosted: Mon May 13, 2013 7:44 am 
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daugs wrote:
from the pic it looks like how I understood it to be, from bottom of crown race to axle ignoring the effect of the rake, I didn't think it was just diagonal, or is this just road frames ?

http://www.sheldonbrown.com/rinard/forklengths.htm



If Shelton says that ignore me :wink:


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PostPosted: Mon May 13, 2013 9:46 am 
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Do you not need to factor in a reasonable degree of sag? 25-30%?


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PostPosted: Mon May 13, 2013 9:57 am 
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Bullpup wrote:
Do you not need to factor in a reasonable degree of sag? 25-30%?


wouldn't this be for suspension forks, but the syncros are rigid ? I'm interested in this as well given changes making in forks, although on a marin can always rely on rockstar being right dimensions ?

hopefully someone will be along soon who can explain......................


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PostPosted: Mon May 13, 2013 10:01 am 
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What I mean is if the frame was designed to take a suspension fork of 428mm, a rigid fork or the same length is surely too long as a degree of sag should be allowed to calculate the correct length of the rigid.


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PostPosted: Mon May 13, 2013 12:17 pm 
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Bullpup wrote:
What I mean is if the frame was designed to take a suspension fork of 428mm, a rigid fork or the same length is surely too long as a degree of sag should be allowed to calculate the correct length of the rigid.


Thanks! That's what I figured... my guess is that a 428mm AC suspension fork with 50mm of travel would have a sag from normal riding of about 25-30% of 50mm.. so about 12mm...

so a rigid fork of about 428-12 = 415 would do...

But I assume that anywhere in the 400 to 450 would be close enough that it wouldn't be that noticable... we are talking a couple cm's here...

What does everyone else think?

And Am I measuring that right?

If someone knows the stated value of AC on a Syncros fork then I could verify my method

thanks! =)


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PostPosted: Mon May 13, 2013 12:26 pm 
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If you run a fork that is too long, it will relax the steering angle and make the steering slower (just think long fork chopper motorcycles).

If you run the fork too short, then you are into a whole different ball game, as you will sharpen the the steering and may well make the bike effectively nose down... this is not a good thing.

As for measuring a to c, I always thought it was from the centre of the axle, but to be honest a few mm up or down is neither here nor there and on Switchblades, Pace and by the looks of it those Syncros, you can always adjust this by changing the depth the fork legs are inserted into the crown by a few mm.


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PostPosted: Mon May 13, 2013 12:41 pm 
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NeilM wrote:
If you run a fork that is too long, it will relax the steering angle and make the steering slower (just think long fork chopper motorcycles).

If you run the fork too short, then you are into a whole different ball game, as you will sharpen the the steering and may well make the bike effectively nose down... this is not a good thing.

As for measuring a to c, I always thought it was from the centre of the axle, but to be honest a few mm up or down is neither here nor there and on Switchblades, Pace and by the looks of it those Syncros, you can always adjust this by changing the depth the fork legs are inserted into the crown by a few mm.


True... but would a person who really doesn't ride that much really notice 15mm?


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