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  1. #1
    mtbr member
    Join Date
    Jan 2011

    Building for Lefty

    When building a frame for use with the Lefty fork what are the advantages and/or disadvantages for various head tube specifications? Do you do anything special in the case of a lefty?

    One option is to use the Project 321 lefty adapter and a standard 1.125 head tube. I think this option will work but want to understand the pros/cons and hear what everyone else is thinking. Are there advantages to using a 44mm headtube in the case of the lefty and if so why?

    Also do you worry about crashes where the lefty strut will impact the head/down tube?

    And please share photos of your lefty builds.

  2. #2
    mtbr member
    Join Date
    May 2012
    Use a 44mm ht. That way you have the option of using the tapered project 321 steerer or a tapered fork from other brands later on.
    Quote Originally Posted by meltingfeather View Post
    If I told you I saw a unicorn ****ing a leprechaun trail side, you'd probably be suspicious.

  3. #3
    mtbr member
    Reputation: mbeardsl's Avatar
    Join Date
    Sep 2009
    Quote Originally Posted by febikes View Post
    Also do you worry about crashes where the lefty strut will impact the head/down tube?
    Do you mean like when I endo'd on the last ride with you and the bar spun around to the left? There is a huge "bumper" on the fork to soak up the impact and spread any force over a wider area. I don't think I'll make the ride tomorrow but you can check it out at next weekends race if you want.

    Given I'm not a builder, take my opinion with a grain of salt, but I think an impact that would cause damage to the DT/TT from the fork would likely be serious enough that you wouldn't be worried about your bike at all.

  4. #4
    mtbr member
    Reputation: Meriwether's Avatar
    Join Date
    Jul 2007
    So i was Googling looking for information on this subject and am kinda surprised there's very little out there and nothing on this thread so here is what I've learned. Any feedback would be great.

    I have a Lefty build and he'll be using a 29er XLR 100mm. Finding specs online is extremely difficult, at least I couldn't find anything except conflicting info on mtbr.
    I'm pretty sure the 100mm version of the 29 XLR is 500mm axle to crown and has 46mm offset. Add/subtract 10mm for each different travel version (i.e., 120mm travel would be 520 atc).

    You need to supply your own steerer for the fork. Cannondale sells a kit with a straight 1-1/8" steerer and their own zero stack headset (different stack heights than Cane Creek's). Project 321 also sells a couple of steerer kits - one with a straight 1.125" steerer and one with a tapered 1.125" to 1.5" steerer. It appears to me the Project 321 is higher quality and you get to use your favorite headset as well.

    The Lefty comes in two versions - one has 137.6mm between the fork clamps and one has 163mm (XL) between the clamps (#'s from the 2012 Lefty Manual Supplement). Those clamps are set so you need to have the headset cups and head tube fit within that distance. For example, suppose I am using a 115mm Paragon head tube, and a King i7 headset for a tapered steerer (14mm lower stack height, 8mm upper) that would give me 137mm total length. That would fit within the regular size Lefty's clamps but with 0.6mm left over. With that same head tube, you could also use a non-tapered steerer and a Zero Stack headset (4mm lower stack height, 8mm upper) and that would give you 127mm so you'd have to fill the 10.6mm gap above the upper headset cup and the Lefty clamp with some spacers. Obviously you could also NOT use the Paragon head tube and cut your own to 125.6mm (with a ZS44) and not have those spacers permanently lodged there. For longer head tube frames same issue but the total stack height and head tube length has to be less than 163mm.

    A straight steerer/zero stack headset would allow for a longer head tube. If you use the tapered steerer option, you need the same headset you'd use on any other tapered build - Cane Creek's EC44 bottom or the King i7 - both have the same lower stack height. This makes for a shorter head tube (since you have to stay within the clamps).

    You could also turn a 1.5" head tube and use the oversized Cannondale steerer and stem combo but I didn't do any research into that since I want to use either Paragon's or MHT-44 from True Temper.

    My questions are similar to Mark's. Because the Lefty is a double-crown and puts a lot of torque and stress on one side of the head tube, I would think an oversized 44mm headtube is preferred and cut to be the max length it can be with the zero stack headset and using the straight 1.125" steerer. It seems like a longer head tube would be more important to front end stiffness than using a tapered steerer and a shorter head tube with a lower external cup. But maybe the same reasoning holds for regular forks using tapered steerers and a stiffer steerer would help handling.

    What's your opinion of the best headtube, headset, & steerer combination for a Lefty? Longer head tube and straight steerer or shorter head tube and tapered steerer?

    Also, are there any watch-outs with Lefty's that I've missed?

  5. #5
    mtbr member
    Join Date
    May 2007
    According to bike design legend Mike Burrows, champion of single-sided forks, Lefties pose no greater stress to a head tube than an ordinary fork. So just go with what you'd choose for any other bike.

    Realistically, the difference in head tube length between a zero-stack 1.125" and a tapered 44mm (around 10mm) probably won't make a huge stack of difference; you could put the top and down tubes in exactly the same place (relative to the steerer clamps) on either, and that's where the difference is when it comes to front-end stiffness (the length of the tube is a resultant of this, and you're limited by the gap between the steerer clamps). I'd make it as long as possible with space for a 0.5mm shim for fine-tuning.

    Now we move on to straight versus tapered steerers (see a hundred million other posts for the pros and cons of each). Since you've got the fork leg and top clamp making a closed system with the steerer and bottom clamp, my guess is a 1.125" steerer would be stiffer than a single-crown fork with a tapered steerer anyway (unless anyone has evidence to the contrary).

    So it really comes down to headset and head tube choice. Prefer the look of a 44mm head tube and Chris King purple anodized bottom cup? Cool. Rather have a neat straight tube and clean hidden bearings? Awesome.

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