effect of trail and head angle on bike handling - Page 2
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  1. #26
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    Great Info

    This is one of the most interesting and informative threads I've seen. Thanks to all.

  2. #27
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    here's my suggestion...

    I'm not trying to invent criticisms, but I truly think the statement below could use some improvement.

    From the Anvil website:

    "For a given trail dimension, less rake increases steering response, shortens wheelbase (moves CG forward), and lessens shock absorption. More rake causes steering response to slow, lengthens wheelbase (moves CG rearward), and offers more shock absorption."

    This statement is accurate, but it does not tell the whole story. Maintaining a given trail requires a change in the head tube angle (HTA) that generally adds more length to the wheelbase than the change in rake does . Since steering is slowed by an increase in wheelbase the statement neglects the more significant contribution of the HTA change. It also suggests that the shock absorption changes can be attributed solely to the change in rake and says nothing about the change in head tube angle. For the average customer, the above is a very confusing statement. This type of statement belongs under a the heading of Trail, where the contributions of HTA and rake can both be credited appropriately. In this case the HTA is generally the greater contributor, not the rake.

    IMO, under the heading of Rake or Offset, the discussion should be limited to the effect of rake on wheelbase, steering speed and wheelbase and not mixed with an undisclosed change in the HTA.
    Last edited by C-40; 02-06-2004 at 11:37 AM.

  3. #28

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    Quote Originally Posted by weiwentg
    what do you ride now? small Giant TCR / 46.5cm Colnago Dream Plus (destroyed in accident)
    what size frame? 44/46.5 seat tube, 74/75.5 degree STA, 53.5/51.3cm TT, 100mm/110mm stem. Giant's fork had 50mm rake, Colnago's had 43mm.
    how does bike behave leaning into corners?
    - is it stable/climbs out/dives in?
    - at low/moderate/high speed?
    -the Giant dove right into the corners at high speed. as for the Colnago, I sadly never had the chance to really find out. the steering was quick enough that I'd use it in crits, but it was also stable enough.
    how do you want steering to change? - somewhat more sedate, suitable for long road races and stage races, and maybe the odd TT or tri. neutral trail sounds OK to me.
    would you prefer bike with shorter TT? - longer TT
    longer chainstays? - probably, plus a relatively low BB.
    or do you ride no-offset seatpost with saddle forward position? - low offset post (USE Alien) with the saddle forward, and I always end up scooting forward on the saddle.
    which fork are you putting? Headset? Ouzo Aero Comp/Pro, 1" Chris King headset
    will the frame be designed to accomodate this fork/headset? - yep
    something doesn't add up if TCR had 74 HTA and 50mm rake the trail would have been 44mm too short for small frame. Small Giant /or Giant small if you prefer ;) / should have HTA 72 to get 56mm trail. /and that's what I recall from seeing in charts long time ago/

    A few assorted comments:
    - 1cm longer chainstays will make bike marginally slower (equivalent of addin ~1mm to trail).
    - the fact that you want something more stable then Giant and you felt confident to ride Colnago with its longish trail in crit is indication that your ideal trail is likely on longer side of "neutral" 56mm. Perhaps 60?
    - the reason I had asked about fork is that Seven sells 2 rebadged forks Wound Up and Ouzo, and they differ 9mm in height. 9mm would be enough to change angles by .5degree and trail by 5mm. Guess what? I new a guy who had his Seven frame designed around wrong fork height. Headsets have diff stack height, wouldn't hurt to ask when you talk to your builder.

    I just recently replaced fork, went from 56mm trail to 60mm and it fixed all issues I had with handling. Yes bike shows traits of long trail bikes, slightly tends to steer in corner navigating through parking lot, but at higher speeds it corners like dream. Taking 90 corner at 16-25mph it feels I can take hands off and close eyes. Before that it was climbing out of corner, almost felt like front washes out, talk about confidence.

    Here's a formula I had used to calculate "ideal" trail /derived from Bill Patterson for traditional bike/:
    12.5 * B/m,
    where B is the horizontal distance from the rear wheel contact point to the CG and m is mass of bike/rider in kilos. For larger frames/riders or slower steering multiplier can be increased to 13-13.5. You can find B by weighting rear wheel of the bike/rider and measuring wheelbase. If you get number too small or too large choose closest to 56mm.

    CP

    PS. with all posts and references above I believe you got enough information to make an intelligent decision. Think it over, do the math, most importantly test your current bike leaning in corners at different speeds /maybe get out and testride Look or Colnago/ good luck.
    Always Look At the Bright Side of Life Monty Python, Life of Brian

  4. #29
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    confused STA with HTA...

    The small Giant has a 72 degree HTA, which produces a trail of 56.6mm with the specified 50mm rake.

  5. #30

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    Maybe its the length of the fork ...

    Quote Originally Posted by PT
    That is a possibility, but I do know things got worse in some ways when I went from a SID with a rake of probably 42 mm (but I'm not sure about my particular model) to a Manitou with a rake of 38.1 mm which was also too tall for my era of frame. There was a big increase in trail going to the Manitou and now the front end "drifts" when climbing and doesn't dive through corners. I have an old Bridegstone MB1 that I ride a lot (rigid) with a trail of 66 mm that is pretty quick. Both bikes were endo machines until I raised the bars to only about an inch below saddle height. The front center of the Moots and the wheel base are large in comparison to the cyclocross bike or the MB1, both of which are much better bikes on the trail when suspension isn't an issue. So maybe the Moots is too large...
    Hey PT,

    "which was also too tall for my era of frame" offers evidence to your problem.

    Your steering issues on the YBB may be substantially caused by the longer fork when switching from the SID to the Manitou, than from the shorter rake. Particularly if the SID was a 68mm travel version, the crown-to-axle length is relatively short. Not knowing which Manitou you have, I can't speculate on the exact crown-to-axle length, but I am guessing that you (like many of us) went to a longer travel fork when you changed, 80mm or more. If the crown-to-axle increased by 25mm then your HTA slackened by 1 degree. As an example, a 25mm change in fork length impacts trail by 6mm, while the 4 mm shorter rake your mention would only impact trail by 4mm.

    I guess its "all trail", regardless of whether it was defined by a longer shock or a shorter offset. In your case, both variables (rake and length) were accumulating against you, so its no surprise that your steering went to crap. But I think you may do better to seek a fork with shorter crown-to-axle length than your Manitou. An older Marzocchi Z-2 (45mm rake if my memory serves me) set-up at 70mm travel would put both variables back into your favor, and would probably improve your steering substanially. You can also find these on eBay for pretty cheap, parts (springs to match your weight) are readily available, etc.

    My 2 cents,

    IM

  6. #31
    PT
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    Good point: looking forward to experimenting with rigid fork

    Quote Originally Posted by InertiaMan
    Hey PT,

    "which was also too tall for my era of frame" offers evidence to your problem.

    Your steering issues on the YBB may be substantially caused by the longer fork when switching from the SID to the Manitou, than from the shorter rake. IM
    As you noted, it all adds up to "trail". The Manitou is about an inch taller than the SID, and has less rake to boot, all of which adds up to more trail. I've been trading rides on my rigid singlespeed (trail=66mm) and the Moots (trail=approx. 77mm): there's a huge handling difference. I had a rigid fork built for the Moots and expect it anytime now. I had to go custom (it will be a nice steel fork) because stock bikes were either the wrong rake or the wrong height. I look forward to the chance to try out the rigid fork and see if it will work for me -- I anticipate a more forgiving ride than straight rigid as the YBB tail should amerliorate some of the bumps. It may turn out perfect for me, even for long trail rides. I want a better climbing/handling bike for this summer's 100 mile mtb race (the Durango 100).
    Hmm... Can I think about that?

  7. #32

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    Quote Originally Posted by Henry Chinaski
    Is there any chance that your YBB is just too big? Maybe too much wheelbase?

    My Bontrager mtb, which had a lot of trail with his special crown, was incredible in tight singletrack--so that goes against that theory. But the bike was designed to put your weight forward, so while it was great for technical climbs, it was an endo-machine on steep, technical descents. So I guess I'm saying you have to look at the whole package, not just one element. For me an Ibis Mojo fit me right and had the whole package I was looking for. There seems to be a lot more voo doo with mtbs--you can do a lot with setup, but if it's too big or too small it's probably not gonna work for you.
    just what i was thinking since moots are known t have sharp handling. however, get a big that's too big and ... there goes the handling. anothe thing: i find long front centers and short stems bad on twisty fast trails. my mtn bike is a tad too small, wheelbase is small by a 1/3 of a in but i ride it w/ a 130 stem and it feels like i can ride it in the palm of my hand.
    another trend in mtn bike that doesn't fit me is short chainstays.. they are uncomfortable. shifting is compromised and i don't like the resulting balance on high speed but it's not moots case.
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