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Thread: What happened to traditional cyclocross geometry?

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    Default What happened to traditional cyclocross geometry?

    The old Alan's had 73.5 HTA and 50mm rake.

    Now the norm is 72 HTA and 47mm rake.

    I hate how sluggish the modern geometry feels -- what changed?

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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by jsrieck View Post
    The old Alan's had 73.5 HTA and 50mm rake.

    Now the norm is 72 HTA and 47mm rake.

    I hate how sluggish the modern geometry feels -- what changed?
    73.5 HTA and 50mm rake = 52mm fork trail
    72 HTA and 47mm rake = 64mm fork trail

    You could always try putting a custom high-rake fork onto a shallow HTA bike to reduce the trail, see if trail change alone is responsible for most of the different handeling. I would think that the alan HTA/fork geometry would make it difficult to track strait on rough surface.

    Probably the most informative raod-centric description of the effects of fork trail I have found is on Tom's spectrum site, well worth reading;
    Spectrum Cycles | Geometry

    Interesting to note that Merckx had a preference for low fork trail number on rough roads, seems rather counter-intuitve as most all modern bikes use a relativly high trail for dealing with rough/offroad conditions (for example, production CX and MTB bikes that are most typically in the 65-80mm trail range).

    I noticed a cyclocross magazine pro bike profile recently (maybe it was Johnathan Page's bike??) where he listed that he hauled around a couple of forks with different rake to the races, apparently in order to be able to fine tune the bikes trail to suit different CX course conditions. All I can figure is that on very soft conditions (like sand), the wheel contact point shifts forward as the tire sinks, effectivly making the trail even higher. Is this reason enough to start with a lower trail setting, so that when it sinks a more normal trail is reached? What other CX trail tuning considerations might there be to warrant switching out a fork?

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    Default Re: What happened to traditional cyclocross geometry?

    Trail doesn't mean anything on a road bike. It's a stupid parameter to design a bike around. The difference in wheelbase and front center caused by adjustments in fork take and head angle are far.more..important to.anyone who actually.races a bike.
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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by jsrieck View Post

    Now the norm is 72 HTA and 47mm rake.
    I think your generalization that this is a 'norm' is a false assumption.

    -g

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    Default Re: What happened to traditional cyclocross geometry?

    I thought if your wheel sinks in sand/mud your trail shortens, so you would need less rake to keep the same trail.
    I have a Alan carbon that feels too steep (short trail) for road riding (for which it was not designed).
    Very interesting points Jerk brings up above, about wheel base and front center. Riding a bike before you buy it can show you so much the numbers can't show.

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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by jsrieck View Post
    The old Alan's had 73.5 HTA and 50mm rake.

    Now the norm is 72 HTA and 47mm rake.

    I hate how sluggish the modern geometry feels -- what changed?
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    Default Re: What happened to traditional cyclocross geometry?

    Stock CX bike geometry is all over the place. I think the only norm was when they were all road bikes with canti bosses. The new norm will be ... Fat Tire Road Bike (FTRB) with disc brakes (FTRB-DB).

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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by rabo View Post
    I thought if your wheel sinks in sand/mud your trail shortens, so you would need less rake to keep the same trail.
    I have a Alan carbon that feels too steep (short trail) for road riding (for which it was not designed).
    Very interesting points Jerk brings up above, about wheel base and front center. Riding a bike before you buy it can show you so much the numbers can't show.
    When tire depresses into a soft surface, the tires contact area spreads out over a much larger area to equilibrate tires pressure X contact area with the reduced ground pressure available in order to continue supporting weight of the rider. The center or the contact area dynamically shifts forward while riding, somewhere between the the original contact point for the tire on hard surface and the point where the leading edge of the tire is pushed below the mud/sand surface level (thus reducing effective fork trail). Most of the equivalent trailing edge of the tire behind the original contact point is not in as firm contact with the sand, it just trails along in the groove created by the tires leading contact area. The soft surface reduced fork trail phenomina is even more pronounced with winter fat tire snowbike riding, fatbikes often have even more exagerated (long) static fork trail to compensate for sinking into the soft surface. I would guesse that the effect of the reduced trail is not really important in terms of how it effects the ability of a rider to negotiate turns on sand, but the dynamically reduced effective trail probably contributes toward reduced tracking stability and makes it increasingly difficult to hold a strait line and avoid sideways wash-out of the front wheel.

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    Default Re: What happened to traditional cyclocross geometry?

    on a fat tire roadie with discs is a wonderful place to be.

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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by jerk View Post
    Trail doesn't mean anything on a road bike. It's a stupid parameter to design a bike around. The difference in wheelbase and front center caused by adjustments in fork take and head angle are far.more..important to.anyone who actually.races a bike.
    you really could not be more fantastically wrong, have you actually tried out any of this shit at all?

    the difference between the nervous oversteer of 30-40mm trail and the on-rails understeer of 60-70mm trail is intensely fucking obvious on a road bike the first time you descend through a corner

    it's a property of its own, independent of "where the wheels go"

    and of course you can keep the wheels in the same goddamn place while adjusting other parameters, it just requires telling the customer to STFU about their precious "effective top tube length", the least meaningful measurement on a bicycle

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    Default Re: What happened to traditional cyclocross geometry?


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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by jerk View Post
    Trail doesn't mean anything on a road bike. It's a stupid parameter to design a bike around. The difference in wheelbase and front center caused by adjustments in fork take and head angle are far.more..important to.anyone who actually.races a bike.
    I'd like to know more about this comment. I interpret it to mean that weight distribution is more important than trail, and I assume that this mostly relates to not having the front end slip or wash out in turns. Am I wrong?

    In any event, I was following an acquaintance today; both of us were on our CX bikes and riding a crushed shell (crushed pretty small, well less than 1 square cm...like an old crushed cinder surfaced high school track) multi-use trail; the shell was a little loose in the corners. I was on my own frame, Clement MXP tires at 36psi (rounded vestigial knobs (bumps) in the center section) and 160#, he was on a Chris Chance Chris Cross, Clement Explor tires, 39 psi (reportedly), 35mm (I think) and perhaps 220#. My front end felt like it wanted to slip in the many corners of the trail and he looked like he was riding on rails. My exit lines were typically not as good as his (and so I was often on looser material) but sometimes I think they were and my front end still felt less planted than he appeared.

    Mine: 72.5x72.5, 52mm trail, 75 drop, 430 chainstays, 555 TT, 537 ST (my build book).
    His: 72.5 HT x 73, trail unknown, 67 drop, 425 chainstays, 570 TT, 563 ST (product info).

    I have a limited knowledge of typical CX trail numbers but based on what I'm seeing I'm guessing that his was at least 52, maybe 62, so by my figuring the bikes aren't very different in the basic geometry arena; unless that potential 10mm is a bigger deal than I would have thought (affecting both the CG and how the tire engages the surface).

    Photo of my tires here: Flickr

    Build sheet and other photo are in the same album.

    When I shifted my weight forward the front end felt more securely connected (of course...it always does, as I always feel loose on this trail). I didn't think to ask him how his front end felt but he looked solid.

    My question is this: If my front end really was less secure than his on the turns I got right, to what would I attribute that?

    He's just a better rider and the difference in exit lines made all the difference?
    His CG was farther forward (heavier torso and maybe more geometric trail) and so the front end hooked up better?
    Something else?

    I am curious to know what folks who have fiddled with this stuff think?

    Does anybody know what the trail of that bike is? Other CX rigs? It would be easy to build another fork to a different rake and see what happens.
    John Clay
    Tallahassee, FL
    My Framebuilding: https://www.flickr.com/photos/21624415@N04/sets

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    Default Re: What happened to traditional cyclocross geometry?

    All I know is we are all faster on the new bikes than the old ones around a CX course. 20 years ago I wasn't even trying bunny hopping the barriers unless those were tiny ones.
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    Default Re: What happened to traditional cyclocross geometry?

    Does that sheet on Flickr imply 55mm of rake OR 55mm of trail?



    Quote Originally Posted by jclay View Post
    I'd like to know more about this comment. I interpret it to mean that weight distribution is more important than trail, and I assume that this mostly relates to not having the front end slip or wash out in turns. Am I wrong?

    In any event, I was following an acquaintance today; both of us were on our CX bikes and riding a crushed shell (crushed pretty small, well less than 1 square cm...like an old crushed cinder surfaced high school track) multi-use trail; the shell was a little loose in the corners. I was on my own frame, Clement MXP tires at 36psi (rounded vestigial knobs (bumps) in the center section) and 160#, he was on a Chris Chance Chris Cross, Clement Explor tires, 39 psi (reportedly), 35mm (I think) and perhaps 220#. My front end felt like it wanted to slip in the many corners of the trail and he looked like he was riding on rails. My exit lines were typically not as good as his (and so I was often on looser material) but sometimes I think they were and my front end still felt less planted than he appeared.

    Mine: 72.5x72.5, 52mm trail, 75 drop, 430 chainstays, 555 TT, 537 ST (my build book).
    His: 72.5 HT x 73, trail unknown, 67 drop, 425 chainstays, 570 TT, 563 ST (product info).

    I have a limited knowledge of typical CX trail numbers but based on what I'm seeing I'm guessing that his was at least 52, maybe 62, so by my figuring the bikes aren't very different in the basic geometry arena; unless that potential 10mm is a bigger deal than I would have thought (affecting both the CG and how the tire engages the surface).

    Photo of my tires here: Flickr

    Build sheet and other photo are in the same album.

    When I shifted my weight forward the front end felt more securely connected (of course...it always does, as I always feel loose on this trail). I didn't think to ask him how his front end felt but he looked solid.

    My question is this: If my front end really was less secure than his on the turns I got right, to what would I attribute that?

    He's just a better rider and the difference in exit lines made all the difference?
    His CG was farther forward (heavier torso and maybe more geometric trail) and so the front end hooked up better?
    Something else?

    I am curious to know what folks who have fiddled with this stuff think?

    Does anybody know what the trail of that bike is? Other CX rigs? It would be easy to build another fork to a different rake and see what happens.

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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by e-RICHIE View Post
    Does that sheet on Flickr imply 55mm of rake OR 55mm of trail?
    55 rake, 52 trail
    John Clay
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    My Framebuilding: https://www.flickr.com/photos/21624415@N04/sets

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    Default Re: What happened to traditional cyclocross geometry?

    Considerably heavier rider on a similar size tire, with greater tread area, at very similar pressure?

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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by jclay View Post
    55 rake, 52 trail
    (Way) too much rake.
    And I looked again - you sit too far back, assuming that design sheet is accurate.

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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by e-RICHIE View Post
    (Way) too much rake.
    And I looked again - you sit too far back, assuming that design sheet is accurate.
    Thank you Richard.

    Sheet is accurate.
    I'll make a new fork and adjust position.

    John
    John Clay
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    My Framebuilding: https://www.flickr.com/photos/21624415@N04/sets

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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by e-RICHIE View Post
    (Way) too much rake.
    And I looked again - you sit too far back, assuming that design sheet is accurate.
    I'm thinking 42 rake and move the saddle forward 5 to 10.
    And maybe treat myself to some fresh rubber.
    If that doesn't put me in a happy place do some more homework and build another frame/fork.
    John Clay
    Tallahassee, FL
    My Framebuilding: https://www.flickr.com/photos/21624415@N04/sets

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    Default Re: What happened to traditional cyclocross geometry?

    Quote Originally Posted by jclay View Post
    I'm thinking 42 rake and move the saddle forward 5 to 10.
    And maybe treat myself to some fresh rubber.
    If that doesn't put me in a happy place do some more homework and build another frame/fork.
    That's a pretty radical change, 55 mm to 42 mm.

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