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  1. #1
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    front end geometry Bikepacking

    Im a new frame builder and Ive already tackled my first frame. The first frame I brazed up was an all road bikepacking/touring bike with low trail geometry (35mm). Its great on the road, and ok on the trails.

    This summer I'll be brazing up a new frame designed around more modern parts. Im trying to make it as versitile and low mantanance as possible. This time around Im looking to design more of an off road oreantated rigid bikepacking rig. The thing im stuck on is the front end geometry. Ive ridden my low trail bike on some single track and its a little sketch. Ive been reading that 60mm is a common trail number for bikepacking rigs. What head tube angle and fork offset combo is the best? Any help on this?

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  2. #2
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    Typically low trail numbers for touring bikes are intended to fight the sluggish feeling of having a fully loaded front pannier and to combat wheel flop at low speeds (which is again exacerbated by having a loaded front pannier).

    If you don't have a front pannier (ie you're using a frame bag, bar bag, seat bag, etc) then you should just do whatever trail number you usually prefer for the most technical terrain you plan to ride. Generally speaking carrying big loads on the front axle is always going to be bad for trail riding no matter what the trail number is, though. So if you can avoid putting lots of heavy stuff on the front fork/axle that will help a lot.

    Hope that helps!

    -Walt

  3. #3
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    Low trail works because you're trading stability from trail for stability from a mass damper (handlebar bag or panniers). But they're different in how the stability works. For trail, the restoring torque is proportional to the steering angle, for a mass damper it's proportional to the steering velocity. In plainer English,with high trail the farther the front wheel gets twisted off a straight line the more it wants to fight back. In low trail, how far the front wheel is turned sideways doesn't matter as much as how fast it gets knocked off line. So that's one reason why off-road low trail bikes are weird - they want to go in whatever direction they are going and if they get knocked off-line they don't try to recover, they just merrily go in a new direction. They also have less lean-steer so you can't use as much body english. In really rough situations on a high trail bike you can focus on controlling the handlebars and steer with your hips. On a low trail bike it's all from the handlebars so in rough conditions you have to both steer and control the front end.

    So I think it's reasonable to design a backpacking bike with less trail. What I'd probably do is get this number with a custom fork with higher offset and all the bikepacking warts. Getting a trail number with a slacker head angle and larger offset means you get a little less straight-line stability (made up for with your bags) and a little more wheel flop, which helps you hip steer and gives you some "power steering" at low speeds.

    Then when it's not loaded, run a fork with less offset to get some of that stability back.

  4. #4
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    The previous two posts are spot on, so Iíll just add some numbers that Iíve found works for me.

    Donít go with 60mm trail, thatís cross bike territory IMO (regular trail weíre talking here). Iíd say 75 at the very least.
    Get the fork you want to use, or make it, and slacken the head tube angle to the point where youíre not sure if itíll be too slack. Make it a fun bike to ride, gear will change how it rides but not by much unless youíre carrying too much crap anyways.
    I never put too much weight on my fork and bars since it makes it hard to ride trails and pick the bars up and messes with fork sag, etc.
    So design the frame how you want it to ride on trails without gear but with a little longer stem (more weight on front axle, more room to attach straps and bags) and shorter front center than on something like what PVD would make

    68-70 hta, 80-90mm trail, no need to go crazy with offset either on the fork, itíll just make it weirder without weight on the front.




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