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  1. #1
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    frame geometry experts, please help with specs

    I'm thinking about ordering a new frame modeled after a Thorn eXp, Rohloff specific, sus fork compatible, mtb-ish expedition tourer. I want a custom fit so not looking for an off the shelf Thorn. The BB will be a slightly higher than the LHT to allow for tad more clearance over rough terrain (+5 mm).

    The 2 specs I'm having a hard time deciding on is the 1. Chainstay Length, and 2. Head Tube Angle. Please critique my thoughts:

    1. I like a a smooth ride and stable handling with a heavy load, so I was thinking 46cm stays like the LHT. But any longer and I'm concerned about too much rear wheel skid when climbing on loose dirt. I'm aware tho that I'd have the weight in rear panniers to hold the wheel down to improve traction. Or perhaps 47.5 stays like on the Sakkit 26?

    2. I will use both a sus fork and a rigid fork, depending on the occasion. As for Head angle, I like stable steering, but not too stable because when riding on bumpy dirt with front panniers, I don't want the steering to feel too sluggish.

    I discovered that when I put front panniers on my folding Bike Friday, that the steering actually improved! It felt like the weight actually stabilized the twitchy nature of 20" wheel steering. This makes me think that If I spec'd a slightly steeper head angle (like 72 deg), the extra responsiveness would counter the effects of the mass in the front panniers.

    These 2 specs seem independent from the frame size, but I'll give that if it matters, 54cm Also I picked up a used Gunnar Rock Tour for testing purposes, and will use it as a baseline to help decide. But I also want opinions as well.

    so what chainstay and head angle would you choose? thanks in advance

  2. #2
    Older I get, Better I was velonomad's Avatar
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    With all due respect. The expert you should be asking is Robin Thorn, His shop has built hundreds of frames and he has a good idea of what works on his bikes. I seriously doubt he nor any other established builder will allow you to dictate the angles and chainstay lengths. If you want to go down that road that are plenty of starving wannabe framebuilders on the west coast that will burn together any concoction you want.

    Chainstay length is just one part of the whole design. I have a similar bike to the Thorn, I have 49cm chainstays but I also ride a tall (63cm)frame, wear size 13 shoes and climb sitting down. If I rode a small frame (54cm) and my climbing style was "stand and stomp" I would want chain stays probably around 44-45 cm.

    Head tube angle is just one part of how a bike handles, you also need to know what the fork offset is and how that will affect steering trail (caster effect). On a rigid fork those three parameters also dictate the amount of flex in the front fork. If you want to use a suspension fork you would then have have to consider the fork offset first since most suspension forks come with no choice of offset. Then you have decide what the steering trail will be and adjust the steering tube angle accordingly. http://www.kreuzotter.de/english/elenk.htm

    Sounds like you want a bike to do two diffrent things. What often happens is you end up with a bike that does neither well.


    Last edited by velonomad; 09-12-08 at 05:32 PM.

  3. #3
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    Velonomad,
    I'd be happy to order a bike from Robin (Actually their designer is Andy Blance), but I'm not willing to pay the rediculous price of $5706.72 for an eXXp. But I am willing to buy a frame that has a good chance of achieving something close to it.

    I don't plan on dictating to a framebuilder what angles/specs to choose, but I'd like to give as much input as possible to increase the odds of getting the best frame I can.

    Ideally I'd love to test out a loaded Sakkit 26 or LHT for a day on off-road trails fully loaded, but since I can't I'm just trying to get some opinions from those who have.

    I didn't mention fork offset or trail because I want the option to use a sus fork, and I believe those figures are fairly standardized.

    << Sounds like you want a bike to do two diffrent things. What often happens is you end up with a bike that does neither well. >>
    Currently I'm riding a Gunnar Rock Tour that does both these things quite we'll. I don't see anything wrong with looking for a frame that might do these jobs a little better.

  4. #4
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    "I didn't mention fork offset or trail because I want the option to use a sus fork, and I believe those figures are fairly standardized."

    They are only standard on standard neck specs. You need to pick a fork, or at least go with a fork your builder likes, then just get a rigid fork that will have similar specs. Stability is going to vary more in relation to the trail figure, than the head angle. Also where the bags hang in relation to stearing geometry makes a big difference, and how they are loaded and with how much gear, how high, etc... Personally I would probably not go for a suspension, Sakkit for instance has not really figured that out from what I read in his literature, it can certainly be done but getting something out of it is tough. There are a number of world tourist who do it. One in particular has a website and use a Koga. This requires a suspension capable front rack. I tend to not see loaded touring going hand in hand with the kind of stuff that compresses a shock, and if vide is the issure there are a lot of other ways of dealing with it.

    "I discovered that when I put front panniers on my folding Bike Friday"

    There are a lot of factors at work. I like the way a loaded bike stears, but the whole thing has to be designed to do it, it is about getting ballance in trail of the bike geometry and the load geometry.

    Your chainstay can be pretty long without hurting anything. Someone might come along and say it will hurt something, and the next day we will have a pationate thread on the advantages of cycling the Big Dummy in rural South America.

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