rigid fork length question

gsy971

Old School Hero
i have a 1995 trek 8700 shx on the way and would like to install a exotics carbon rigid fork. i am a little stumped on which length fork i need to get to get the geometry and fit correct

i definately do not want the front end of the bike angling upwards and would like a more identical ride specs for the original 8700 shx

the exotics website gives me a chart and there are 3 measurements
42.5/44.5/46.5
the 42.5 is listed at "Geometry corrected for" 0mm to 80mm. i am assuming this is the one i need since the judy that came with the 8700 was a 80mm fork
there is also one listed for 390mm but i am not sure as i don't want the front end of the bike to be lowered
 
gsy971":wm8cmumy said:
...the 42.5 is listed at "Geometry corrected for" 0mm to 80mm. i am assuming this is the one i need since the judy that came with the 8700 was a 80mm fork

You've answered your own question right there... :wink:
 
gsy971":qq99jlj4 said:
the exotics website gives me a chart and there are 3 measurements 42.5/44.5/46.5
the 42.5 is listed at "Geometry corrected for" 0mm to 80mm. i am assuming this is the one i need since the judy that came with the 8700 was a 80mm fork
No, the Trek 8700 came with a Judy XC, which in stock form had 50mm travel in 1995.

To keep the same geometry, you need a 39 or 40cm a-c rigid. A longer fork would slow the handling down a bit. Maybe nothing too drastic, depends how you like slowish handling.
 
OP says above that his choice of fork is listed for '0mm TO 80mm' travel...

...any alteration to the geometry is going to be in the region of half a degree; you could achieve that through tyre choice!
 
Anthony":2ojuo1ak said:
gsy971":2ojuo1ak said:
the exotics website gives me a chart and there are 3 measurements 42.5/44.5/46.5
the 42.5 is listed at "Geometry corrected for" 0mm to 80mm. i am assuming this is the one i need since the judy that came with the 8700 was a 80mm fork
No, the Trek 8700 came with a Judy XC, which in stock form had 50mm travel in 1995.

To keep the same geometry, you need a 39 or 40cm a-c rigid. A longer fork would slow the handling down a bit. Maybe nothing too drastic, depends how you like slowish handling.

thanks Anthony
you're right about that Judy - gosh its been so long since we rode anything less than 80 so i just assumed
anyway i think steveo is correct that my fork will be from 0-80mm and that difference in geo is not going to be that great. i just don't want the front end of my bike sweeping upwards any as i think i am already at my standover limit
 
gsy971":2p7yj2r1 said:
thanks Anthony
you're right about that Judy - gosh its been so long since we rode anything less than 80 so i just assumed
anyway i think steveo is correct that my fork will be from 0-80mm and that difference in geo is not going to be that great. i just don't want the front end of my bike sweeping upwards any as i think i am already at my standover limit
If you look in the 1995 Trek catalogue in the archive section here, you can see the 8700 as well as the 8700 SHX. They will undoubtedly share the same frame, so you can judge what length of rigid fork the frame was designed for by looking at the 8700. Obviously it's impossible to be precise, but visibly it looks like a 39cm fork, which was fairly standard at around that time. The quoted head angle was 71 degrees for size 18 and above, 70.5 for smaller frames.

If you fit a 42.5cm fork, clearly the front of the bike will be 3.5cm higher than it was designed to be.
It won't raise the standover by quite as much, because the standover point is further back, but say by 2.5cm.
The impact on head angle is c0.6 degree for each cm of fork length, so 3.5cm will slacken it off by just over 2 degrees.
That will certainly make a noticeable difference and I personally wouldn't do it - but then I like sharp steering.
If you like slower steering, then maybe a 69 degree head angle would be ok for you, but nevertheless no designer, then or or now, would design a rigid mtb with a 69 degree head angle.

Incidentally, it's really disingenuous for that manufacturer to claim that the fork is suitable for 0 to 80mm travel frames. A 42.5cm rigid fork is designed to be equivalent to an 80mm suspension fork, which would be c45cm long unloaded, or 42.5cm making (generous) allowance for sag. It is nowhere near being equivalent to a sagged 50mm suspension fork, but maybe they've forgotten (or are too young to know) that there was ever any such thing as a 50mm fork.
 
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