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  1. #1
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    Methods for Sizing Corners

    So I'm currently building a short technical trail on my property, nothing too crazy, just enough to build skills on bike handling and build confidence on jumps and other features. I'll post pics when I get enough posts but, I have a problem...

    I tried riding it today after I was working on berms in a switchback section. While I was tired and rode terribly, I'm concerned my corners may be actually too small for a good smooth turn on the bike. How do you more experienced trail builders "measure" these turns? I will probably end up rebuilding the berms and make the turns wider but for future reference, what's the best way to get it right the first time?

  2. #2
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    A good general rule of thumb is that you need an 8' radius for a switchback to ride naturally without feeling like you are having to do any funky body english to get around it.

    Two armspans of flagging tape is roughly equivalent to 8' (this isn't rocket appliances). Pin one end of the flagging where you want the center of the turn to be, and use the flagging tape to establish the radius of the turn. The steeper the cross-slope, the more valuable this is, as there is a tendency not to cut the tread far enough into the uphill side, and at the same time mistaking downhill spoils for rideable trail tread. Once you can pull the tape taut and walk solid trail tread all the way around the turn, you are where you want to be. It will seem huge when you are walking, then magically shrink once you get on bike.

  3. #3
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    Turns are definitely deceiving. When I asked a group how big they thought a specific turn was, they reference to the length of 1 table (6'). Someone said maybe two tables, but the others were very skeptical of that. They all imagined they were riding it by putting their hands out in front of them and turning around the table. I though it was even bigger than the 2 tables. When I took the tape measure to the turn, I found out it was bigger than even I thought. It was a 19' radius! That's 38' across! Not the 6' that everyone else thought.

    The speed you enter a turn will be the deciding factor in the radius. For example, 12' might be okay for a climbing turn. But if you have a fast and flowy trail coming down to that turn, you'll find out how much it sucks when you have to slam on your brakes to make that tight turn. It's what we call an "instant sped killer".

    Unfortunately, there's no correct answer to your question. Experience will help you figure out how wide to make a turn. Ttyl, Fahn

  4. #4
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    I can measure the inside radius of an "average" turn by holding a shovel stuck in the ground in my left hand, hold a rake at arms length in my right hand, then describe an arc on the ground with the rake. That makes about a 10' inside radius.

    As mentioned, it depends on the expected speed.
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  5. #5
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    Quote Originally Posted by Trail Ninja View Post
    I can measure the inside radius of an "average" turn by holding a shovel stuck in the ground in my left hand, hold a rake at arms length in my right hand, then describe an arc on the ground with the rake. That makes about a 10' inside radius.

    As mentioned, it depends on the expected speed.

    This might be the most helpful post I've read in a long time. Simple, makes sense, and sounds effective. Will certainly employ it this weekend when we go out to add onto the trails we've built on High School property here. so thanks!

  6. #6
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    Thanks a lot guys! I feel like as far as trail building goes corners are definitely my weakness. Makes sense because my only prior experience has been with hiking trails. Anyways, look's I'll be out there this next weekend rebuilding my berms! Many thanks again!

  7. #7
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    Do use TN's technique, but make sure you size the inside radius of the turn and then cut the berm from outside that line back towards it. Does that make sense? Think of standing on the line of the inside edge of the turn and then cutting a berm back towards your feet. Not only does it allow for a natural berm-like arc with the mattock or hoe, but the outside trail edge radius will be wider again and therefore easier to ride.

  8. #8
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    20 feet is a good size for a general berm. size it bigger (24') since you are going to back fill and sculpt it.
    use a string and a stake in the ground to get the radius correct. marker paint every couple of feet as you work the radius.

    here's some berms that size i'm working on recently in my backyard...

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  9. #9
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    Results

    So I finally made it out today to do some work! Everybody's advice was incredibly helpful. It was surprising how when I started working, I could see exactly what was wrong and how it needed to be fixed. I still have some more work to do on them. I will post a couple photos in my next post since I need one more post to upload photos.

  10. #10
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    They aren't that impressive as they aren't needed for high speed but like I said, they still have a little work that needs to be done too to get them where I want them. I need to find some more dirt.

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  11. #11
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    It is possible you may not need as much dirt as you think. It looks like your entry/exit is pretty flat, suggesting that there may not be that much speed through the corner. If you are able to identify where the trail tread surface of most effective riding will be, and make sure that portion of the tread is adequately banked in (a gentle, curving radius to the tread v. a flat tread w/ a sharply angled berm), you may find that you can get away w/o having to build a Wall of China berm.

  12. #12
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    Thanks Hank! Like I said, they are just meant to be to help maintain speed in the turn. I'm not looking to build giant berms, I just want a little extra dirt to add more structure and smooth things out.

  13. #13
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    Pics of berms can be difficult to interpret. It always seems there are straight lines and odd angles.

    Link, in that first turn, it looks like there are 2 half turns with a flat spot/lower berm height in the middle? If so, does that mean you have built for your own favourite line - enter high, square-off mid turn and berm out? The reason I ask is that building for the best line by allowing room to find it is a good technique. However, it looks like there may be drainage issues on that berm with water collecting mid turn and then maybe running over the exit where you need that last bit of traction.

    What you want on your trail first and foremost is longevity. If you can adapt the IMBA omega turn to that berm, you may be able to get your water to drain away from the berm, plus allow the rider (you) to set for the turn by jumping into it. Best thing about a berm is that when you have done the beta, you can ride, re-dig and evolve it. In this case it may mean digging deeper into the current wall and adding the diggins to the back of the berm to make it a lot bigger (deeper) overall, but not far off the current outer line. Just means more hard work, as usual.

  14. #14
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    Sorry to hijack a bit and go back to the thread title, but it is a really good question:

    The answer I think, is too complicated because it is what is in the mind of the most experienced builder available, modified by circumstance. Circumstance includes intended use, available materials and labour, climate and timeline etc.

    However, the one most important thing is what riders like (rather than want). Having an eye for potential lines does not require building skills as such. It may be more difficult if you are building only for yourself. Catering to others can alter outcomes hugely.

    Here are some berms you would ride without thinking of them as berms.

    http://forums.mtbr.com/attachment.ph...&d=13557373800

    Within this simple right-downhill-left section are 2 berms. They are not big but they allow an alternate, faster line with a bit of jump/float between, plus ascending lines and a view ahead. So, it is really hard to size a corner just right for everyone, but don't stop trying.
    Attached Thumbnails Attached Thumbnails Methods for Sizing Corners-p1090573.jpg  

    Methods for Sizing Corners-p1090574.jpg  

    Methods for Sizing Corners-p1090625.jpg  

    Methods for Sizing Corners-p1090628.jpg  


  15. #15
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    Thanks for the advice Paradise! That dip in the middle of the first one is just a mistake on my part and will be fill in with more dirt when I get back out. When I built it I was envisioning riding the top pretty much all the way around, thats how I like to ride berms myself. In the end the single purpose of the berms I'm building right now is to help whip you around the corner without losing too much speed. I would say your concerns about erosion are valid and I'll see what I can do about it when I get back out there.

    To prevent myself from hijacking my own thread, I may make another post to provide updates and get feedback from you all. I'll need all the crowd-sourced help I can get since this is my first bike trail.

  16. #16
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    Idea!

    We all have general things to consider when building turns in the woods, and having to deal with trees, rocks, roots, and other terrain challenges we all have to adjust, and work with what is there. Unless your landowner let's you have total control to remove any object or a track type trail build where you can haul in dirt and run measurements and build to them.

    Some of the most fun trails I have ever built/ridden. I have roughed in by Motos first. Years of racing Motos in the woods, I have learned either your on the throttle or dragging the brake in a turn. If allowed I will use a small underpowered Moto like a Honda CRF150 to test my theory on my planned trail layout. If I can't use the Moto I will rake the planned route of the turn and ride it with my singlespeed MTB, considering how the trail will carry more speed once hard panned in.

    Most turns are either increasing radius or decreasing radius. Next I split the turn into "Entrance", "Apex, and Exit" when building them.

    "Entrance" I envision entrance speed from the previous connecting trail. If the previous connecting trail is a downhill grade I consider more open radius, where you can get back on the pedals quicker. If it's fairly flat entrance then a tighter radius can work.

    "Apex & Exit" If a tighter radius is the choice I usually follow with a "S" turn next, that can be pumped for fun and to carry speed into the next trail section. If a larger radius is the choice from the apex to exit, hence runout on the exit of the turn, I will let it flow.

    I do no build what I consider a "dead area" of a turn where a straightaway is added to the apex.

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