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  1. #1
    _Ed
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    Fork Rake - will reducing rake by 2mm give a noticable difference?

    Hello

    I'm looking to upgrade my road forks. My current bike has a 72degree HTA, and uses a 45mm rake fork. I like the way it feels and don't want it to significantly change.

    The replacement fork I'm looking at comes only as 43mm and 49mm.

    Obviously, the 43mm will increase the trail, and slow the feeling of the steering down to an extent....the question is, will it be noticable?

    Using an online calculator:
    45mm rake gives trail of 61.2
    43mm rake gives trail of 63.3 (an increase of 2.1mm)
    49mm rake, gives trail of 57mm (a decrease of 4.2mm)

    Anyone changed fork rake and noticed much difference?

  2. #2
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    You can really feel the trail when it gets over 60-mm. Note that an increase in tire size will also increase the trail. The fork with 49-mm rake makes more sense to me given your 72-degree HTA.

  3. #3
    _Ed
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    hmm, a little more searching found this..

    http://www.calfeedesign.com/tech-pap...bike-handling/

    "57mm of trail is considered by many to be an ideal combination of stability and agility."

    Perhaps you're right...

  4. #4
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    Went from a 45 to 43mm...I couldn't tell any difference...

  5. #5
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    I would always err on the side of more stability, not less. I have one frame that's unridable hands off with a 43mm fork and fine with a 40mm.
    There's sometimes a buggy.
    How many drivers does a buggy have?

    One.

    So let's just say I'm drivin' this buggy...
    and if you fix your attitude you can ride along with me.

    http://www.youtube.com/watch?v=GekiIMh4ZkM

  6. #6
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    other issues..

    The axle to crown length affects the head tube angle and the trail. If you get the wrong length, it could cancel out or double the expected change in trail. You can find road forks with lengths ranging from 365mm to about 374mm. That's enough to change the HTA by .5 degree.

    Trail values generally vary with frame size. There is no single trail value that's perfect for all frames.

    It's quite common for small frames to have a trail in the 60-67mm range and large frames to have as little as 54mm.

    LOOK uses a 72 degree HTA and 43mm fork offset on their 51cm frame, the size that I ride.
    Last edited by C-40; 02-28-2011 at 07:01 AM.

  7. #7
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    What C-40 said is very true. you really need to find out the a-c length of both your current and replacement fork. Otherwise you can really mess up the handling of your bike. As far as trail is concerned a 2mm difference is considered the limit of perception, so assuming equal a-c length you should not really be able to tell the difference between 45mm and 43mm.

  8. #8
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    Quote Originally Posted by C-40
    The axle to crown length affects the head tube angle and the trail. If you get the wrong length, it could cancel out or double the expected change in trail. You can find road forks with lengths ranging from 365mm to about 374mm. That's enough to change the HTA by .5 degree.

    Trail values generally vary with frame size. There is no single trail value that's perfect for all frames.

    It's quite common for small frames to have a trail in the 60-67mm range and large frames to have as little as 54mm.

    LOOK uses a 72 degree HTA and 43mm fork offset on their 51cm frame, the size that I ride.
    While some bicycle brands do use a the same rake on different HTAs (several Euro brands spring to mind), most American framebuilding luminaries embrace neutral trail (56-57mm) as the "single trail value that's perfect for all frames." That's because it presents the same handling characteristics at all speeds. I'm not sure why a smaller rider would benefit from a long trail and taller rider would need very little trail - and have never read an argument to this end.

    Trail does not "slow the feeling of steering". Trail changes handling feeling at different speeds, rather than cause one effect across all speeds.

    One might even suspect that certain manufacturers do it just to simplify manufacturing - one rake to rule them all.


    It is also good to keep in mind that the difference in fork length can produce a maximum change in head tube angle of .5 degrees - which is the equivalent of 3mm of trail, or 3mm of fork rake. But that is only going to happen if you started with the shortest fork available and replaced it with the longest (or vice versa). If not, the difference will only be 1mm or so of trail difference - much less than the difference in available fork rakes.

    I am in good company thinking that trail (neutral trail, preferably) and wheelbase are more important predictors of good handling than effective head tube angle. Head tube angles vary as much for fit as handling. Steeper steering angles resist the wheel being turned more for a given front wheel loading. But steeper angles are found on larger bikes intended for heavier riders, so I suspect the real difference is a wash.

    If this were my bike, I'd opt for the 49mm rake fork. It decreases overlap and will steer the most predictably at both high and low speeds. And, at 59mm of trail, is still slightly longer than simply neutral.

  9. #9
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    well...

    Some people will believe anything posted online, like Tom Kellogg's writing about a 56cm trail being "neutral". I say baloney. There's nothing special about that value. For smaller frames it's too small.

    The majority of brands do not use the same trail on all sizes. It's most common to use smaller trails on larger frames and there is a very good reason. Larger frames have longer wheelbases and don't steer as quickly as a small frame. The idea is to make the larger frame not have sluggish steering.

    I've owned frames like the Colnago C-40 that has about 67mm of trail in a 54cm size and did find that it requires more steering input in a high speed hairpin, but I did not find it to be unstable at slow speed, like climbing a mountain at 8-12mph. I've also owned frame with much smaller trails, like a Fondriest MDC and had no problem with it. It's really a matter of what you get used to. The problem comes when you own two frames with substantially different trails, like I did when I owned both of the aforementioned frames at the same time. It wouldn't be a problem with casual riding, but if you're negotiating mountain hairpins at 40 mph, it can lead to trouble if one bike requires far less force to initiate and maintain a turn.

  10. #10
    _Ed
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    Every day's a school day... thanks for your input.

    Digging out the geometry chart, my curent fork has 367 axel to crown.
    The replacement fork would have 369-370 (different search results for 3T Funda).

    So, assuming A2C would increase by 3mm. This would in effect increase trail, by 0.9mm??

    Therefore
    43mm rake gives trail of 64.2 (an increase of 3.0mm)
    49mm rake, gives trail of 57.9mm (a decrease of 3.3mm).

    I guess I'm leaning towards the 49mm rake, as it seems to fall more within the typical range of trail values..

    I bet I can barely notice the difference either way ;)

  11. #11
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    Quote Originally Posted by C-40
    Some people will believe anything posted online, like Tom Kellogg's writing about a 56cm trail being "neutral". I say baloney. There's nothing special about that value. For smaller frames it's too small.

    The majority of brands do not use the same trail on all sizes. It's most common to use smaller trails on larger frames and there is a very good reason. Larger frames have longer wheelbases and don't steer as quickly as a small frame. The idea is to make the larger frame not have sluggish steering.

    I've owned frames like the Colnago C-40 that has about 67mm of trail in a 54cm size and did find that it requires more steering input in a high speed hairpin, but I did not find it to be unstable at slow speed, like climbing a mountain at 8-12mph. I've also owned frame with much smaller trails, like a Fondriest MDC and had no problem with it. It's really a matter of what you get used to. The problem comes when you own two frames with substantially different trails, like I did when I owned both of the aforementioned frames at the same time. It wouldn't be a problem with casual riding, but if you're negotiating mountain hairpins at 40 mph, it can lead to trouble if one bike requires far less force to initiate and maintain a turn.
    Why, exactly, would a 60cm bike with a 99.5cm wheel base need to have a sluggish front end at low speeds (low trail) to make it as maneuverable as a 50cm bike with a 97.5 wheelbase?

    Kellogg is not the only proponent of neutral trail, or the only one writing about it. It's not even a purely bicycle concept - motorcycle designers also recognize neutral trail. In contrast, there is no one I've been able to find extolling the virtues of high trail small frames and small trail large frames.

    Large frames have proportionally shorter wheelbases than small frames (due to increased HTAs). Why would these bikes be better with vague steering at low speeds?

    Mechanically, what is happening on a bike ridden by someone 5'5" and 160lbs. that is so different from what someone 5'11" and 165 that it would require the two to have opposite steering characteristics at low and high speeds?


    I realize these geometries are the bread and butter of your favorite bikes, but have you ever actually read anything to suggest why they are doing the opposite of so many US builders? I think this is a very interesting topic, and I have been completely unable to locate anything advocating for these geometries. I'd love to read a different perspective.

  12. #12
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    again...

    The idea of a "proportionally shorter" wheelbase on large frames is nonsense. The wheelbase has nothing to do with the vertical size of the bike, it's an unavoidable result of a longer TT, required to fit the rider. Large bikes, with their riders aboard, also have a much higher center of gravity.

    Since you're fixated on American designed bikes being different, check out a C'dale. The trail values range from 62mm on the smallest size to 56mm on the largest.

    http://www.cannondale.com/bikes/road...mod-team-16556

    The wheelbase on the largest frame is about 5cm longer than the smallest. The 1.5 degree steeper HTA on the largest frame only shortens the wheelbase by about 5mm.

    The bike with the shortest wheelbase would be considered to be have twitchy handling if it had a 56mm trail.

    Cervelo tried that short wheelbase and small trail geometry on their Cervelo R3, in the 49 and 51cm sizes. I owned a 51cm, just long enough (in 2006) to ride it for 200 miles on mountain routes and thought it sucked, so I tore it down and sold the frame. Fast forward to 2011 and you'll see a new geometry that falls in line with the norm - the same HTA as a LOOK. For some reason Cervelo fails to list the fork offset or trail.

  13. #13
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    Quote Originally Posted by C-40
    The idea of a "proportionally shorter" wheelbase on large frames is nonsense. The wheelbase has nothing to do with the vertical size of the bike, it's an unavoidable result of a longer TT, required to fit the rider. Large bikes, with their riders aboard, also have a much higher center of gravity.

    Since you're fixated on American designed bikes being different, check out a C'dale. The trail values range from 62mm on the smallest size to 56mm on the largest.

    http://www.cannondale.com/bikes/road...mod-team-16556

    The wheelbase on the largest frame is about 5cm longer than the smallest. The 1.5 degree steeper HTA on the largest frame only shortens the wheelbase by about 5mm.

    The bike with the shortest wheelbase would be considered to be have twitchy handling if it had a 56mm trail.

    Cervelo tried that short wheelbase and small trail geometry on their Cervelo R3, in the 49 and 51cm sizes. I owned a 51cm, just long enough (in 2006) to ride it for 200 miles on mountain routes and thought it sucked, so I tore it down and sold the frame. Fast forward to 2011 and you'll see a new geometry that falls in line with the norm - the same HTA as a LOOK. For some reason Cervelo fails to list the fork offset or trail.
    So, you can't find any references, either? Okay.

    I wasn't implying that ALL US made road bikes hold to this - most probably do not. It requires using more expensive forks that come in more rakes, like Enve.

    And wheel bases are not proportional to frame size. A frame size with 6cm of extra TT and 8cm of extra height has only 1 or 2 cm extra wheelbase. That's the main reason HTA get steeper - to preserve a short wheelbase. If the two bikes were truly "proportional", they'd be constructed of the same angles, just like two triangles of the same proportion.

    As for cervelo, they didn't just change the front end, they lengthened the chainstays, too.



    Anyway, you make neutral trail out to be a myth. Many motorcycle and bicycle designers believe in it, describe it mathematically and write papers about it. And those who don't design bicycles with this in mind don't have anything to say about their reasoning. So this discussion becomes more like Darwin vs. the Creationists - one side has reasoning and facts, the other blind faith.

    No point in arguing with your faith.

  14. #14
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    facts, not faith...

    Quote Originally Posted by rx-79g
    So, you can't find any references, either? Okay.

    I wasn't implying that ALL US made road bikes hold to this - most probably do not. It requires using more expensive forks that come in more rakes, like Enve.

    And wheel bases are not proportional to frame size. A frame size with 6cm of extra TT and 8cm of extra height has only 1 or 2 cm extra wheelbase. That's the main reason HTA get steeper - to preserve a short wheelbase. If the two bikes were truly "proportional", they'd be constructed of the same angles, just like two triangles of the same proportion.

    As for cervelo, they didn't just change the front end, they lengthened the chainstays, too.



    Anyway, you make neutral trail out to be a myth. Many motorcycle and bicycle designers believe in it, describe it mathematically and write papers about it. And those who don't design bicycles with this in mind don't have anything to say about their reasoning. So this discussion becomes more like Darwin vs. the Creationists - one side has reasoning and facts, the other blind faith.

    No point in arguing with your faith.
    You got one thing right, Cervelo did increase their chainstay length from their prviously stupid 399mm, so it's also more in line with the majority of other brands, like LOOK.

    You don't seem to understand geometry very well. If you don't change the STA or the HTA, 6cm of extra TT length will add the same 6cm to the wheelbase. The effect of a 1.5 degree increase in the HTA is a trivial 5mm reduction in wheelbase. If the STA is changed, then you comparing apples and oranges, since the reach is reduced with a slack STA on a large frame. That's where reach really is a more valid dimension.

    I just posted a clear example of a large frame having a 5cm longer wheelbase than a small one.

    The principle of using less steering trail on the large frames should be obvious. A longer wheelbase results in a larger turning radius and a slower feel. Reducing the trail counteracts some of that.

  15. #15
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    Quote Originally Posted by C-40
    You got one thing right, Cervelo did increase their chainstay length from their prviously stupid 399mm, so it's also more in line with the majority of other brands, like LOOK.

    You don't seem to understand geometry very well. If you don't change the STA or the HTA, 6cm of extra TT length will add the same 6cm to the wheelbase. The effect of a 1.5 degree increase in the HTA is a trivial 5mm reduction in wheelbase. If the STA is changed, then you comparing apples and oranges, since the reach is reduced with a slack STA on a large frame. That's where reach really is a more valid dimension.

    I just posted a clear example of a large frame having a 5cm longer wheelbase than a small one.

    The principle of using less steering trail on the large frames should be obvious. A longer wheelbase results in a larger turning radius and a slower feel. Reducing the trail counteracts some of that.
    Duder, changing the HTA by 1 degree will change wheel base by more than 1cm. That's from the top of a 100mm headtube, where TT numbers are measured. You should be familiar with that number, since it is almost identical to correcting TT length for different STAs.

    So if you want to try your math again, we can talk about this. Make a picture, look at a chart, try the math. Something.

    That last paragraph is particularly silly. You realize that one way to reduce trail is to increase rake, which makes the wheelbase longer, not shorter? Increasing HTA mades the wheelbase shorter, which you'll see once you fix your math above.

    "You don't seem to understand geometry very well." Hmmm.

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    oops...

    I goofed and put in sine instead of cosine in my calculation. I've got homebulding on my mind these days, rather than bikes. Using your example, a bike with a 100mm head tube measures about 45cm from the HT/TT intersection point to the center of the axle. Each degree of HTA angle change will increase the wheelbase by (cosA-cosB) x 45cm. Use angles of 72 and 73 degrees and you get .75cm per degree. A frame with a 220mm head tube gets close to 1cm per degree.

    My comments are all still valid. A large frame might have a wheelbase reduction as large of 1-1.5cm from the use of a steep STA, but the wheelbase is still 5cm longer than the smallest frame. The steeper STA reduces the trail, with NO change in the fork rake required.

    The C'dale example shows the SAME 45mm fork rake on all sizes. Only the HTA is different, as the frames progress from the smallest to largest size.

    Using smaller trails on larger frames is extremely common. Both American and European designs follow this trend. Colnago, for example, does the same thing as C'dale. Colnago uses a 43mm rake on all sizes, but they also have larger trail values on all sizes, due to a more slack HTA. The smallest frames use a 71.5 degree HTA and the largest is about 73.4 degrees. Unlike C'dale, Colnago retains a slack HTA of under 72 degrees, all the way up to the 57cm size. It's only in the 58-65cm sizes, where the HTA change escalates.

  17. #17
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    Quote Originally Posted by C-40
    I goofed and put in sine instead of cosine in my calculation. I've got homebulding on my mind these days, rather than bikes. Using your example, a bike with a 100mm head tube measures about 45cm from the HT/TT intersection point to the center of the axle. Each degree of HTA angle change will increase the wheelbase by (cosA-cosB) x 45cm. Use angles of 72 and 73 degrees and you get .75cm per degree. A frame with a 220mm head tube gets close to 1cm per degree.

    My comments are all still valid. A large frame might have a wheelbase reduction as large of 1-1.5cm from the use of a steep STA, but the wheelbase is still 5cm longer than the smallest frame. The steeper STA reduces the trail, with NO change in the fork rake required.

    The C'dale example shows the SAME 45mm fork rake on all sizes. Only the HTA is different, as the frames progress from the smallest to largest size.

    Using smaller trails on larger frames is extremely common. Both American and European designs follow this trend. Colnago, for example, does the same thing as C'dale. Colnago uses a 43mm rake on all sizes, but they also have larger trail values on all sizes, due to a more slack HTA. The smallest frames use a 71.5 degree HTA and the largest is about 73.4 degrees. Unlike C'dale, Colnago retains a slack HTA of under 72 degrees, all the way up to the 57cm size. It's only in the 58-65cm sizes, where the HTA change escalates.
    If you only adjust HTA, you'll get the 5cm range you've noted on the Cannondales. But if you maintain a fixed trail number by adjusting rake, the number drops to less than 4cm:
    http://www.trekbikes.com/us/en/bikes...done/madone65/
    This is the opposite of what you're claiming, since Trek is reducing the range of wheelbases by sticking close to 56/57mm of trail. (They have also drank the neutral trail Kool-aid.)

    In any case, the use of a single fork rake across a product line does not validate the practice's geometric advantages. Using one fork rake is cheaper and more expedient, which is more likely to be the reason for the practice than some secret geometric truth that no one wants to talk about.


    Summary:
    1. Fixed trail imparts the same handling across a range of sizes, despite the need for varied head tube angles to maintain reasonable wheelbase/front center.

    2. Fixed trail geometry uses long rake forks on the smallest sizes, increasing front center and decreasing toeclip overlap, and short rake forks on large sizes which shortens the wheelbase. With a range of steerer angle between 72.3 and 74, optimal neutral trail rakes run between 50 and 40mm, keeping the wheelbase range smaller by nearly 1cm more than fixed rake geometries.

    3. There is no explanation available, outside of economics, for fixed rake/variable trail geometries. The arguments you have offered are actually opposite reality, since fixed trail better produces the short wheelbases on large frames that you are in favor of. There may be an explanation, but you and I are not privy to it.

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    last comment...

    Using a very limited or single fork rake makes perfect sense. It's much easier to change the HTA than offer many different fork rakes.

    Trek, which you mentioned, only uses two fork rakes (45 and 40mm) on some high end models, but they do subscribe to the smaller trail value. That's a matter of opinion, that other brands don't agree with. Having owned both types, I have no problem with either, as long as I don't own one frame at one extreme and another at the opposite extreme.

    http://www.trekbikes.com/us/en/bikes...e/madone69ssl/

    It makes perfect sense to use a slack HTA and a little more fork offset on a small frame in order to reduce toe ovelap. At the large extreme, just the opposite works.

  19. #19
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    Quote Originally Posted by C-40
    Using a very limited or single fork rake makes perfect sense. It's much easier to change the HTA than offer many different fork rakes.

    Trek, which you mentioned, only uses two fork rakes (45 and 40mm) on some high end models, but they do subscribe to the smaller trail value. That's a matter of opinion, that other brands don't agree with. Having owned both types, I have no problem with either, as long as I don't own one frame at one extreme and another at the opposite extreme.

    http://www.trekbikes.com/us/en/bikes...e/madone69ssl/

    It makes perfect sense to use a slack HTA and a little more fork offset on a small frame in order to reduce toe ovelap. At the large extreme, just the opposite works.
    I don't disagree with any of that.

  20. #20
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    Interesting thread, I'm curious as to what bike you are replacing the fork? And what fork did you end up going with?

  21. #21
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    If you want to stay with a 45 fork rake, Pedal Force sells all-carbon forks for under $150. Does your bike have an integrated headset?

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