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  1. #1
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    Effect of Steep Head Tube Angle on a Fat Bike

    What would the effect of Steep HTA have on a fat bike handling? i.e. 72 degrees. I have a basic understanding of geometry and HTA, but I wondered if there is anything about a fat bike specifically that would handle differently with a 72 degree angle. Would the fat tires and extra contact patch be compounded by the angle when it comes to handling? Would this effect stability or self steer, under-steer or over-steer more dramatically than a "normal" width mtb tire?

    There has to be a reason that all the mainstream manu's are using 68 degrees up to about 70 degrees.

    Any insight would be greatly appreciated.

  2. #2
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    Quote Originally Posted by Venturewest View Post
    What would the effect of Steep HTA have on a fat bike handling? i.e. 72 degrees. I have a basic understanding of geometry and HTA, but I wondered if there is anything about a fat bike specifically that would handle differently with a 72 degree angle. Would the fat tires and extra contact patch be compounded by the angle when it comes to handling? Would this effect stability or self steer, under-steer or over-steer more dramatically than a "normal" width mtb tire?

    There has to be a reason that all the mainstream manu's are using 68 degrees up to about 70 degrees.

    Any insight would be greatly appreciated.
    I believe there are many advantages to a very steep HA on a fatbike, but it has to be accompanied with an appropriate alteration to offset to allow a decent amount of trail. My belief is that this makes the bike handle better when in ruts or running edges. I have done some experimentation on this. It incidentally gets rid of most of the self steer effect of fat tyres.

    The market however has been conditioned to believe that slack HAs are necessary for good handling. And they are - on long travel suspension bikes. This is because when the travel gets used then the working range of the fork is close to a steep HA rigid. Also a telescopic fork is more likely to bind or stutter if it is at a steep angle. So really slack angles are all about suspension.

    If I was building a frame for myself, I would go very steep.

    If I was building for the market I would use slack angles because it's never wise to argue with the market, and it also allows for the later fitment of suspension forks.

    BTW there's are many more variables but won't go into them here.
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  3. #3
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    if you ride mostly flat trails, it's probably alright, given enough offset and trail; if steep terrain is in the picture, slack angles are better.
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  4. #4
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    Thanks for the replies. I live on the Colorado Front Range so steep terrain is in the picture everywhere. What kind of offset and trail are you looking for to be a good compliment to the 71.5 or 72 degree angle? Do you want more or less than a more slack HTA?

    I am sure this is complicated and not an easy answer.
    Thanks!

  5. #5
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    There's no flat land around me.
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  6. #6
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    It all sounds so impressive.

    Head tube angles, fork slack. 68-70 degrees vs 72 degrees and what not.

    For the average rider might never notice a difference. Unless something is totally out of wack, they will rapidly adapt.

    If something is totally new to you, like riding a fat bike. After, the first few minutes of a ride your mind and body have automatically compensated for the new sensation.

    Like when you learn to ride a bike in the first place. How was the handleing then?

    My first moments on a fat bike weren't so great. When, I took the fat bike out for its first ride. In order to get to the trail, I needed to ride on a paved suffice. I was going too fast downhill that turned sharply around a corner.

    Guess what?

    It handled like jelly and not like a road bike. Yesterday, I took the bike to a trail that I've ridden plenty of times before with other bikes. The bike was everything I expected and more.

    Just a nubes, perspective.
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  7. #7
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    Quote Originally Posted by Solo-Rider View Post
    It all sounds so impressive.
    ...For the average rider might never notice a difference. Unless something is totally out of wack, they will rapidly adapt....
    That's true. For most circumstances you can adapt to anything. If you look at the head angles of bikes over the last century or so, HAs have been slack, steep, and anywhere in between.

    It's at the extremes that the differences are obvious. When you hit the unexpected it's good to have the bike react safely in that space between the incident and when your own reactions kick in, and that's what my experiments were about.
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  8. #8
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    I love my bikes form the 90's with uber steep angles - road bike handling!


    What I am finding is that before I hurt my back I liked to have a lot of weight on my front tire with bars lower than my seat and a longish top tube.

    I just put the enabler on my pugs (shhh do not tell Surly) the longer AC really does slow down the steering - but it also puts a lot of weight on the back tire. It has a longer steer tube than my pugs fork - so I can put my bars up quite a bit higher as well.

    I had to add air to my back tire, but now the bike is a little slower turning but it is much harder to wash out the front tire.. with Larry at 10psi, it grabs everything and no self steer! We only have dirt/rocks/sand and NO snow (or mud) in San Diego). I can roll better and climb better. Slacker seems to be better for me. But I am not trying to ride fast - just never putting my feet down. (slow fat trials if you will).

    I am going to try to puff up a steep hill tomorrow (Cowels Mt. 1 mile road up 1000 feet with a 300m flat section) and I cannot wait to see how the slack works and lets hope I do not have to set foot on the mountain!
    My bike is heavier than yours - it does not have Carbon or Titanium parts - I love it!

  9. #9
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    I sold my Puglsey frame after testing a 9:zero:7; I kept finding my Pug uncomfortably steep afterwards and kept wishing for the slightly lacker, lighter front end of the 9:Zero:7. The frame will show up next week and I figure that, along with the 'fork of many names' will be the perfect all mountain, hard-tail fat experience for me.
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  10. #10
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    I like a steep HTA on my rigid 29er singlespeed (designed for a 505mm A-C fork but running a 465), but its geometry is now considered old-school 29. Originally, I matched my 29er position on my fatbike. My fatty has a slacker HTA, which I like, and I have also found that I'm happier on that bike in an aft position. I'm running a shorter, higher stem and layback seatpost, and the bike seems happier. Both frames were built at the same time and with matching angles, with the fatty originally as a 26er. I later replaced the bottom bracket and rear end to fatten it up and it has been great. Interestingly, when I get back on the 29er now, I want to lower my bars and get into a more classic low XC position, while the fatty is happy being driven from the back seat. On both bikes, the ti seatpost has been wonderful, and yes, even at 7PSI on dirt, you can 'feel' the liveliness of titanium.

    My fatty has a slightly slacker(1.2*) HTA and 5mm more trail than the Pugsley it replaced, and I think it steers better. This may be because the steering is slower, causing the bike to tend to track a bit better. Certainly on a road bike, 2mm of trail is dramatic, but on giant tires, 5mm is not easily discernable. So, given my preference for fast-steering XC and road bikes (I had an '08 Orca, and loved it), I am very happy with the slacker and slower steering on my fatty.

  11. #11
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    Fatbikes are introducing challenges to frame design because the the feedback/resistance of the fat tyres mean the frames need to be stiffer than the conventional diamond frame. This has not been much of a problem with the adventure bike bias of the first generation, but now that there's a growing interest in them for trail park use, speeds are going to rise, and downhill performance will be important to owners.

    The likely result of a change to a trail park bias will be stiffer frames, and a more downhill optimised geometry.

    Regardless of the HA I'd expect the On-One Fatty to have better feeling steering simply because it has what looks like the best braced steering head I've seem on a fatbike. It would be interesting to do deflection tests on the various fatbikes to see which had the least deflection of the steering head and compare this to perceptions of steering precision.

    Put crudely, my opinion is that the further away you can put the hinge (steering head) from the back wheel, the better. If you want to keep the wheelbase reasonable, then a steep HA helps. But there are disadvantages to a steep HA. If we assume that the riding position isn't changed, then a longer toptube is necessary. In turn a longer toptube makes it difficult to keep the frame stiff.

    Riding position can make a huge difference to feel as well, as anyone can test simply by changing the length of their stem and shifting their saddle.

    At the end of the day it all depends on what sort of riding the bike is used for. My riding seems to consist of an inordinate amount of climbing at low speed on lumpy tracks where floppy steering kills the climb.

    Downhills take very little time in a days ride, so I prefer my steering to be optimised for the bulk of my riding time. However that is all on natural trails (or no trail at all), so there's never any "flow".
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  12. #12
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    A couple of other considerations:

    -Mike Curiak's custom setup uses a very slack head tube angle with a high offset fork in order to achieve a "auto-centering" effect when on slick or deep snow, while retaining the same overall quickness of steering. If he says it works, I'm inclined to believe it. Also, my experience on my old pug indicated that I was having to make tons and tons of corrections with the front end to track straight on difficult snow that changes every 10 feet.

    -The same head tube angle that might work for some won't work for all. I used my pug extensively for trail riding on tight New England singletrack. One of the things I loved most about it was how quickly it steered, and how it was not suspension corrected. It could navigate tight turns even better than my 29er. Plus, one of the failings of fatbikes is that they tend to understeer on flat slippery surfaces. Having more weight over the front tire helps with this. You are hearing others complain about those very characteristics in this thread.

  13. #13
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    I think the slacker angles lend greater stability at slow speeds - like in the snow. Less need for steering corrections due to bumps, and just plowing through snow. I find the 70* Pugsley head angle to be just about perfect for xc trail riding, could be a bit slacker/slower for deeper snow. The downside to slacker angles is some floppiness in the steering - it takes more effort to return the wheel to center after a good crank on the handlebars, but in general the wheel sits in center-range without much effort until you turn hard. I think the heavier wheels amplify this effect. Greater offset helps with this, as do steeper angles.

    Different strokes for different folks when it comes to steering feel IMO.

  14. #14
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    Quote Originally Posted by Velobike View Post
    Fatbikes are introducing challenges to frame design because the the feedback/resistance of the fat tyres mean the frames need to be stiffer than the conventional diamond frame. This has not been much of a problem with the adventure bike bias of the first generation, but now that there's a growing interest in them for trail park use, speeds are going to rise, and downhill performance will be important to owners.

    The likely result of a change to a trail park bias will be stiffer frames, and a more downhill optimised geometry.

    Regardless of the HA I'd expect the On-One Fatty to have better feeling steering simply because it has what looks like the best braced steering head I've seem on a fatbike. It would be interesting to do deflection tests on the various fatbikes to see which had the least deflection of the steering head and compare this to perceptions of steering precision.

    Put crudely, my opinion is that the further away you can put the hinge (steering head) from the back wheel, the better. If you want to keep the wheelbase reasonable, then a steep HA helps. But there are disadvantages to a steep HA. If we assume that the riding position isn't changed, then a longer toptube is necessary. In turn a longer toptube makes it difficult to keep the frame stiff.

    Riding position can make a huge difference to feel as well, as anyone can test simply by changing the length of their stem and shifting their saddle.

    At the end of the day it all depends on what sort of riding the bike is used for. My riding seems to consist of an inordinate amount of climbing at low speed on lumpy tracks where floppy steering kills the climb.

    Downhills take very little time in a days ride, so I prefer my steering to be optimised for the bulk of my riding time. However that is all on natural trails (or no trail at all), so there's never any "flow".
    Having had this discussion before, it is interesting to see it again with new info/opinions added. Steeper HTAs are of an interest to me also.

    Recalling off the top of my head...having the axle mounted off the rear of the legs, (trailing axle) also helps with getting out of ruts? If so, that might be another way to approach the desired end?

    VB, you state that in your opinion, the greater the distance that the rear axle is from the HT, the better, which brings to mind why some prefer a Jones truss fork on a pugs. While not the same as your example, as the distance referred to is not changed, but the FC of the latter example is increased, suggesting a relationship?

    As always VB, I find your opinions noteworthy.
    Just like Fat Bikes...the posters on here can go anywhere...and do.

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