It’s strange having this blog come up more than any other site for framebuilding information when I Google anything from welding to mitering questions. I KNOW other builders blog about this stuff but maybe Google likes WordPress blogs better than Blogger blogs. That’d be funny since Google owns Blogger. I hope some people find this stuff interesting other than me!
This is a almost straight copy of my text from the mtbr framebuilding forum since I don’t want to write it again but wanted to include it on this ‘archive’. It’s what i learned about building a custom frame for a Lefty since I found very little out there on this subject plus a little more.
I have a Lefty build coming up and he’ll be using a 2013 29er XLR 100mm. Finding specs online is extremely difficult, at least I couldn’t find anything except conflicting info on mtbr. Eventually my friend found the information HERE, look for the “2013 axle to crown measurements” link. So for the 29 XLR the ATC is 500mm and the offset is 45mm. Add/subtract 10mm for each different travel version (i.e., 120mm travel would be 520mm atc). The Lefty manual recommends 20-30% sag for the length of travel (20-30mm for a 100mm fork) with XC builds going in the 20-25% range and All-Mtn going 20-30%.
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 (can’t move them) 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 inset 7 (i7) headset for a tapered steerer (14mm lower stack height, 8.2mm upper) that would give me 137.2mm total length. That would fit within the regular size Lefty’s clamps but with 0.4mm 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 slightly 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 cut to length some MHT-44 from True Temper.
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 for it to be cut to 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. I’m not exactly sure of the best headtube, headset, & steerer combination for a Lefty — longer head tube and straight steerer or a little bit shorter head tube and tapered steerer?
For this build I’m going to use a 115mm Paragon head tube and a King i7 headset. I’m pretty sure the MHT-44 stock would be plenty strong for this application, but for a double-crown fork and a 180lb rider that plans to do some bikepacking and general Rocky Mtn riding…I just feel more comfortable with going beefier on the headtube on this frame. So we’ll try out the tapered steerer option from Project 321 and report back later with any issues or praise.