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If I replaced the fork on my cyclocross bike, surly cross check, with a track fork, what would be the effect on the bike geometry? What would be the effect of the modified geometry on bike handling.

I'm assuming the track fork will be a bit shorter and with less offset than the current fork. The crosscheck typically has a 400mm long fork with 44mm offset. I considered swapping it with a surly steamroller fork which has a 375mm axle to crown measurement and 38mm offset Or a Milwalkee bruiser fork, which is the same length 400mm with 32mm offset, which would give me a trail at about 75mm on 32mm tires.

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Current geometry at: surlybikes.com/bikes/cross_check –  Benzo Feb 28 '13 at 4:25
    
I think the hubs would be a bit different as well. –  Ken Hiatt Feb 28 '13 at 6:42
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It would be wise to get the actual dimensions of the fork you plan to purchase and compare them with your current setup. Maybe the fork is not like you assume it to be. –  heltonbiker Feb 28 '13 at 14:07
    
Considered the surly steamroller fork. 375mm axle to crown, 38mm rake. –  Benzo Feb 28 '13 at 17:08
    
Or a Milwalkee bruiser fork, which is the same length 400mm with 32mm offset, which would give me a trail at about 75mm on 32mm tires. –  Benzo Feb 28 '13 at 17:29
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3 Answers

up vote 1 down vote accepted

As mentioned in other answers, trail does affect how the bike rides. I don't necessarily agree that a higher trail number means more manoeuvrability. Most bikes have trail numbers between 60-66 these days. Much older bikes had low trail numbers between 35 and 50. There are a number of reasons for the change. One advocate of Low-Trail handling is Jan Heine, the publisher of Bicycle Quarterly.

Also, Cross Bikes have a certain geometry to allow lots of clearance for mud and big tires. Going to a straight blade that is much shorter will mean smaller tires, less than 28mm probably, and difficulties putting on fenders.

http://janheine.wordpress.com/2011/02/10/a-journey-of-discovery-part-4-front-end-geometry/

Here is a trail calculator that can be helpful: http://yojimg.net/bike/web_tools/trailcalc.php

Update:

If you know your old angles, you can figure out the new ones based on some formulas from Sheldon's website: http://sheldonbrown.com/rinard/forklengths.htm

You can usually get your current angles from the vendor website, you can figure our your new head tube angle. You can also guess! But there is also a formula:

  [change in head angle] =  arcsin [(old length - new length)/(wheel base)]

I wanted to do something similar on my Aurora Elite, my fork is 400mm (43mm offset) and I don't like the handling (not stable enough on long-ish rides, fatiguing) These were my calculations:

  Google tells me: 
  arcsin((400-375)/1024) in degrees
  arcsin((400 - 375) / 1024) = 1.39896174 degrees
  Old HT + 1.40 = 73.4 New HT
  New Rake: 50mm

  Results:
  Tire Diameter  686mm
  Wheel Circumference 2154mm
  Trail 50mm
  Wheel Flop 14mm
  Mechanical Trail 48m

  So, how do I figure out the new wheel base? The old rake was 43, the
  new is 50, so we can guess the new wheelbase will be about 7mm
  longer. Not quite, probably more like 5mm. So, we might get 10mm of
  extra room infront of the toe-clip, as the fender is not up at the top
  of the fender currently, and it will only move a couple of mm closer
  to the tire, maybe 4mm? So, a real increase of 7-8mm infront of the
  toe-clip, but that will be enough to get rid of the overlap.

  Now, going with a 55mm rake, takes us into completely different
  territory: 

  Results
  Trail 45mm
  Wheel Flop 12mm
  Mechanical Trail  43mm

  This is probably too much for a wide range of speeds, and speedy
  decents.

  Another factor that will change bike handling by using a shorter fork
  will be a lower bottom brack and less trail, putting the bike closer
  to a classic touring geometry. 

  http://www.cyclingtips.com.au/2011/02/the-geometry-of-bike-handling/
  http://davesbikeblog.squarespace.com/blog/2007/5/4/trail-fork-rake-and-a-little-bit-of-history.html
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Can't really use the trail calculator without knowing my head tube angle which is going to change with the shorter fork. I guess I could bust out my geometry book and figure out the new headtube angle. –  Benzo Feb 28 '13 at 17:10
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If you know your old angles, which is usually easy to find from the vendor, you can figure our your new head tube angle. You can also guess! But there is also a formula: [change in head angle] = arcsin [(old length - new length)/(wheel base)] –  Doug Denny Feb 28 '13 at 20:08
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See my lengthy update above. –  Doug Denny Feb 28 '13 at 20:15
    
This should be all the info I need to calculate the new trail on the bike. I can use the calculator to compare this to other bikes. I'm thinking to acheive my goal of a more responsive, but less stable bike, I'll want a shorter fork with about the same amount of rake to increase the headtube angle, shortening the wheelbase, and reducing trail. A road, not track fork, would probably be more appropriate for this goal. –  Benzo Mar 1 '13 at 14:49
    
Yes, I agree. A road fork would be better than a track fork and will give you room for fenders and bigger tires if you want. –  Doug Denny Mar 5 '13 at 16:19
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Less fork offset on it's own means more trail. The fork length and wheel size will also affect this though, so you'd need to get all the numbers and maybe draw it out to work out the final difference in trail.

More (longer) trail makes the bike more stable (especially at speed) but less manoeuvrable. Touring and downhill bikes tend to have more trail, while city commuter bikes or sprinting/criterium bikes have less.

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But it should be noted that the fork rake is "tuned" to the head angle. I would guess that there tends to be a significant difference in head angle between the two bike styles, so the way a given fork performs on one bike is no indication of how it will perform on the other. –  Daniel R Hicks Feb 28 '13 at 12:34
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In a perfect world, every bike would be assembled from a FRAMESET, which is a frame-fork pair designed for one another. With a frameset, very relevant parameters can be actually DESIGNED into the system, instead of derived from it. The main parameters which depend on fork-frame interaction are, in my perception, bottom-bracket height, seat-tube angle, head-tube angle, and of course trail.

In the real world, everytime a fork is replaced in a bike, all of these design parameters, which are independent at design time, are changed at once. This so happens because they are physically coupled to one another after each frame and fork is built.

Replacing a stock fork for a shorter one (smaller distance from axle to lower cup of headset) decreases bottom-bracket height, increases seat-tube and head-tube angle and decreases trail. That would be generally associated with a more performance-oriented setup.

Another thing is that stem length has a significant influence in the responsiveness of handling, since with longer stems the force applied by the arms have a self-centering effect which is similar to the caster effect associated with fork trail. If you change seat tube angle, getting a more forwarded torso position, further adjustments in stem length might "feel" convenient or even necessariy, and these can have an effect on handling, greater yet than the fork geometry per se.

In the end, this is not only a very subtle and personal matter of cyclist-bicycle interaction, but the actual ability to tune these parameters is more often than not determined by the limited amount of available fork options.

Hope this helps!

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