Mountain Biking - NEED A NEW FORK! determining axle to crown and rake for an older steel frame
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01-27-09, 10:09 PM
I recently bought a cool older steel Ibis Mojo frame off eBay, and I was hoping to get some opinions on how to determine what fork to get for it. It did not come with a fork, but the person I bought it from thinks the frame was from 1995 or thereabouts, and seems to remember 60mm of travel on the original suspension fork.
Okay geometry whizzes, here's the question I need help with: based on the geometry of the frame, can I determine the ideal axle-to-crown measurement? and the rake?
Thank you BikeForums!
01-27-09, 10:45 PM
thanks for the reply! i was leaning toward rigid, too--what i don't know is, are "suspension corrected" forks designed to correct for differing axle-to-crown measurements (i.e. correcting for different amounts of travel)? or, if i am looking at a suspension corrected fork is it going to be a good fit for this frame for the most part...? shows what i know!
You could look up the 60mm travel forks of the day (did it have a Judy?) and check the axle to crown.
To preserve geometry, you want a rigid fork to have a similar axle to crown as the fork would have when properly sagged. Of course, you can go to a longer a-t-c to slacken out the bike and raise the bottom bracket, or even a shorter one to quicken up the steering. Most people would choose the former option with an older bike. Mountain bikes have generally gotten a bit slacker over the years to increase technical capabilities. Suspension travel has often increased as well, and you won't really find new forks with more than 80mm travel.
But 80mm would probably work pretty well, and there are lots of good XC race forks that would be a great match for that frame. Just depends on your budget and preferences (lightweight? Stiff?).
and you won't really find new forks with more than 80mm travel.
01-28-09, 09:43 AM
the surly fork i bought was suspension corrected for 80mm i believe. i dont think that would ruin your geometry
01-28-09, 01:35 PM
Thank you for the reply, and the advice on how to measure axle to crown.
I was checking out cyclofiend's scans of the old Ibis catalog (http://www.cyclofiend.com/ibis/94/index.html) from around that era--it looks like if I want to troll around for an older suspension fork, I might be looking for “RockShox Mag 21” or “Manitou 3” if I wanted to find a match to what the bike was built for originally.
I might do a search for the manuals for these, in order to find the axle-to-crown measurement “when properly sagged” (thanks M_S for that bit, I would not have thought it!) but it looks like I might be able to bypass this step and just assume that a rigid fork that is suspension corrected for 80mm travel might actually provide a slacker ride and raise the bottom bracket.
This would be a benefit, too, because I have been kicking around the idea of running drop bars (a la Midge) on this bike and the raised height of the head tube (and therefore raised handlebar height) might even be a little easier to attain if I used a fork with longer a-to-c dimension…
Thanks for the tip on the Surly fork—I will check for it—and it sounds like 80mm suspension corrected is the direction I might want to go.
At the risk of overstaying my welcome, here’s another question: will changing the a-to-c dimension—therefore changing the head tube angle—affect what rake I should be looking for? It would stand to reason that I would be looking for a fork with a lesser rake in order to compensate for the change in headtube angle…any opinions on this?
Thanks again to all of you.
01-28-09, 06:46 PM
i did notice a tad less ground clearance for my pedals when switching to my new fork. but i switched over to some fatter tires and it all evened out.
07-21-12, 12:34 AM
the other way around slacker head angle with more rake or steeper head angle with less rake
more rake = quicker steering
steeper head angle = quicker steering
then there's an issue of wheel flop or something like that
i haven't tried this out but supposedly something like a 72 degree head angle with 40mm of rake would be close to 71 degree head angle with 45mm of rake
i haven't confirmed but i seems like the rock shox mag 21 had 1.5" of offset and was for 80mm or 60mm of travel (would need to confirm this that's what i'm trying to figure out) the original suspension adjusted frames from 1995 seemed to allow for a 20mm higher axle to crown measurement
so like originally forks were around 390mm a to c and then they went to 410mm a to c in the mid 90s
can't speak on bianchi though
also bottom bracket height also affects the handling and the ability for the bike to lean into turns
yeah so in the example above the 72 degree head angle with 40mm offset will have less wheel flop than a 71 degree head angle with 45mm offset (so i think that's why that creates fast handling without veering off course so much)
then a low bottom bracket seems to keep the bike steady and easy to steer in fast descents
it's hard to get this stuff correct and you end up trying a lot of different forks (with mismatched head tubes fork offsets and bottom bracket heights resulting in strange handling and motion sickness)
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