Tweaking a Rigid XC Fat Bike for Trail Riding- Mtbr.com
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  1. #1
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    Tweaking a Rigid XC Fat Bike for Trail Riding

    For clarity: I state "XC" in the title purely based on geometry. My Specialized Hellga has a 70.5 degree head angle and 73.5 degree seat angle.

    So I've taken my fat bike fitted with 29+ tires on two Mountain Bike trail rides and both times I've been sketched out between thinking I'll go OTB going downhill or that I'll lose my front wheel and head off a cliff or bridge. I wanna gain more confidence but the more I do it the more I think that I should change something about my bike. That said between getting a different set of wheels built and all the other modifications that I made to my fat bike I'm kind of limited on being able to fund more upgrades to it.

    The two "issues" that I experienced when I went out was losing traction on my front wheel/losing contact with the ground when hitting an obstacle, and not being able to shift my weight back far enough on my rear wheel when going downhill. The first issue could be fixed by better line choice, or by adding a suspension fork which would be the most expensive option. The second issue could be fixed with a narrower saddle (current one is 155mm wide) so that it doesn't hit my thighs when I try and move back since I'm a big rider, or I can get a remote dropper post and sell my lever dropper post.

    Changing the Dropper is probably the best way to go atm from a financial standpoint, however I am curious if going to a suspension fork might be viable. The fork currently on my bike is tapered, however it has 135mm QR spacing, and an A-to-C of 467mm with a 47mm offset. For comparison an 80mm Bluto has a 491mm A-to-C and 51mm offset. I used a calculator and it would change my head and seat angle to ~69.34 and 72.34 degrees respectively, increase the front center length, decrease my BB drop from 60mm to ~50.8mm, and increase my trail from 83mm to 93mm.

    Some of those things seem like good changes, however I don't know if they are all good changes or if the money required to both purchase a sus fork and new hubs and maybe new spokes would be worth the performance gains. I know it'd still be cheaper than trying to buy a hardtail 29+ bike.

    Wanted to get some feedback on where I should go from here. For now I guess I'll just keep riding and hopefully I'll get better and won't be as sketched out.

  2. #2
    Rocking on a Rocky
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    What tires/tire pressure are you running? If you can not get your butt back maybe the frame is to big for you.
    :thumbsup:It doesn't matter what I ride as long as I ride it Rubber Side Down●~●.

  3. #3
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    Quote Originally Posted by RockyJo1 View Post
    What tires/tire pressure are you running? If you can not get your butt back maybe the frame is to big for you.
    Iím running 29x3.0 Minion DHF/R-II tires at 15-16PSI. That sounds high but considering that Iím 360lbs itís about as low as I can go while minimizing squirm from my rear tire.

    My issue with shifting my weight back isnít due to the bike, but due to my thighs being so big that they hit the sides of my saddle when I move back. If I went with a narrower saddle that would fix that issue but then itíd probably be quite uncomfortable when sitting down.

  4. #4
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    For the purpose of science, head on over to the downhill that is giving you trouble with weight shift. Ride it once as is, then again after manually dropping your saddle all the way down. Get your butt all the way out back so it's just shy of buzzing on the tire. Tip your heels down and arms almost at full superman. Improvement? Yes, then maybe a dropper is for you. No, then we need to look at other body positioning on the bike.

  5. #5
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    Quote Originally Posted by bme107 View Post
    For the purpose of science, head on over to the downhill that is giving you trouble with weight shift. Ride it once as is, then again after manually dropping your saddle all the way down. Get your butt all the way out back so it's just shy of buzzing on the tire. Tip your heels down and arms almost at full superman. Improvement? Yes, then maybe a dropper is for you. No, then we need to look at other body positioning on the bike.
    +1

    Do you have the stock wheels and tires? They are pretty good tires.

    -F
    It's never easier - you just go faster.

  6. #6
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    Quote Originally Posted by bme107 View Post
    For the purpose of science, head on over to the downhill that is giving you trouble with weight shift. Ride it once as is, then again after manually dropping your saddle all the way down. Get your butt all the way out back so it's just shy of buzzing on the tire. Tip your heels down and arms almost at full superman. Improvement? Yes, then maybe a dropper is for you. No, then we need to look at other body positioning on the bike.
    So unfortunately Iím about a good 40 minutes away from where I was riding so I wasnít able to go back right away. I however tried dropping my seat with my current dropper post and I also changed the seat to a slightly narrower one. The seat didnít feel uncomfortable thankfully and with it dropped I felt like I could get my butt back farther.

    What I noticed with that same setup is that I started feeling my front tire lose traction and it was deflecting a bit more than when Iím seated. I donít know if lowering the pressure would be a good idea but I can try that and see if it still deflects as easily. Iím currently running 15 psi front and 17psi rear. 14 in the front is about as low as I can comfortably go w/o worrying about rim strikes or burping.

  7. #7
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    Quote Originally Posted by DarKris View Post
    What I noticed with that same setup is that I started feeling my front tire lose traction and it was deflecting a bit more than when Iím seated. I donít know if lowering the pressure would be a good idea but I can try that and see if it still deflects as easily. Iím currently running 15 psi front and 17psi rear. 14 in the front is about as low as I can comfortably go w/o worrying about rim strikes or burping.
    ďFeeling your front lose tractionĒ is a little puzzling. If this is when going down hill then shift some of your weight back forward. Not lean your shoulders over and beyond the front hub, but rather imagine moving your belly button towards the front hub. Get low.

    ďDeflecting a bit more than when seatedĒ. Again, ??? Meaning it gets bounced off course and not tracking straight? Or itís bending? Assuming the first, shift some weight back towards the front and take care to choose some better lines.

    When lowering the tire pressure think about being light on your contact points as you hit obstacles. Absorb the bump with your arms and legs rather than bashing through all stiff. Think about making a little hop type move with your body and sucking the bike up to you as you pass over.

  8. #8
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    Quote Originally Posted by bme107 View Post
    ďDeflecting a bit more than when seatedĒ. Again, ??? Meaning it gets bounced off course and not tracking straight?
    This is more of what I meant

  9. #9
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    Since you mention not being able to get behind your saddle on DHs, getting a remote activated dropper seems like a no-brainer to me as a first step.
    15mm is a second-best solution to a problem that was already solved.

  10. #10
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    Quote Originally Posted by DarKris View Post
    This is more of what I meant
    The front tire deflection thing...

    Here's a practice drill for you.
    Find a (reasonably sized) parking block. Practice riding straight over it. You can do absolutely no technique, and your bike will still go right over it.
    Then add in a front wheel lift, then a rear wheel lift.
    By "lift", I do not mean actually raising it up in the air. I mean to unweight it, just enough, so it can smoothly follow the path over the parking block. In a few tries, you will already be smoother.
    Now for the fun part.
    Start approaching the parking block at an angle. If you mess up your lightening of the front wheel, it will deflect. If you do it right, you will ride a straight line over the angled block. It's a timing thing. Increase the angle over and over until you are almost riding parallel to the parking block and pretty much "changing lanes" across it. This will improve front wheel traction on the trail, and greatly reduce the effect of those angled front tire hits. The weighting/unweighting will pay off immediately on the trail.

    -F

    PS - you can also do this in a circle/turn.
    It's never easier - you just go faster.

  11. #11
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    So as a quick update I did practice with unweighting the front tire, however in an effort to try and slacken my front end I revisited the idea of running a "smaller" rear tire. Before getting 29+ wheels built I played around with potentially running a 26x4.8 front tire and a 4.0 rear. I abandoned that as I wanted lighter wheels for trail riding, however I think pairing a 26x4.0 Minion FBR with a 29x3.0 DHF/R-II (When the DHF wears out) might be a good short term solution for making the bike more slack.

  12. #12
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    So I did some practice with body positioning and I'd have to say I'm feeling a bit more confident on the bike. I did however go forward with the "mullet" bike strategy and I definitely feel like the bike is a bit more slack which is also improving my confidence. I'm also going to experiment with running a 2.6 rear tire when I'm riding less technical trails but so far I'm looking forward to trying this setup out on some trails



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    Tweaking a Rigid XC Fat Bike for Trail Riding-img_2792.jpg  

    Last edited by DarKris; 2 Weeks Ago at 05:51 PM.

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