# Half Rule and 10% Rule Questions



## coopdad (Nov 6, 2010)

I am doing some research into trail building, specifically having our Cub Scout pack modify the walking trails behind their school for not just "hiking" but to also work for the cross country team to run and a few of us to ride on. A lot of good info here and on IMBA's site.

My question: on the IMBA 10 most common Trailbuilding Mistakes page is says:
_use the Half Rule: trail grade, or steepness, shouldn't exceed half the grade, or steepness, of the hillside; and the 10 Percent Rule: overall trail grade should be 10 percent or less_

Question #1: Is it saying that you should keep the grade half the grade if it is less than 10%? And not exceed 10% if the hill's grade is more?

Question #2: Are these percentages used for trails that have traffic going both ways?
The trails we are working with really do not have a way to make it a loop, downhill only. The existing trails currently have many areas that are up to 20% with one or two places that are probably more than 35% (first on the list to re-rout... if it is difficult to walk/run down, it would be impossible for the new-to-mountain-biking cub scouts boys to ride).


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## cjohnson (Jul 14, 2004)

*yes and no*

Average grade implies that at times the half and 10% "rules" are going to be exceeded. Somewhere within the IMBA literature they say is ok to exceed these percentages for short bits of trail.

If exceeded, use other methods to prevent water from running down the trail tread, (ie outslope, grade reversals in key spots...etc). Also, if exceeded, having good site lines can prevent over zealous braking.


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## coopdad (Nov 6, 2010)

So it is all about erosion, not ride-ability? Interesting. 

I will have to get back out there and really look hard in those sections because I do not remember water doing much if any damage.


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## robbiexor (Aug 22, 2011)

_Question #1: Is it saying that you should keep the grade half the grade if it is less than 10%? And not exceed 10% if the hill's grade is more?_

The half rule says that your trail's grade shouldn't exceed half that of the side slope of your hill. If you're building on the contour of a hill with a 25% side slope, no section of your trail should exceed 12%. The 10% rule refers to overall avg grade, this is mostly for usability. a long 10% uphill is going to be rough. Imagine you are building across a hill from point A to point B, and you want to gain elevation. You want ~5% grade on this trail to keep it easy. You'd shoot the grade from A to B, and make sure you're at about 5%. But you're not going to build a straight line from A to B, you're going to incorporate some grade reversals to keep the trail interesting and to shed water, (controlling volume and velocity). Within in these small ups and downs you build, you don't want to go over half the sideslope of the hill you're on. You mark a trail that goes up by 7% then back down at 10% and so on, but by the time you've reached point B, your overall average is 5%. Make sense?

_Question #2: Are these percentages used for trails that have traffic going both ways?
The trails we are working with really do not have a way to make it a loop, downhill only. The existing trails currently have many areas that are up to 20% with one or two places that are probably more than 35% (first on the list to re-rout... if it is difficult to walk/run down, it would be impossible for the new-to-mountain-biking cub scouts boys to ride)._

Yes, these rules will work in both directions. 20% down works if your sideslope is greater than 40%. If it's not, you're building on the fall line and instead of water flowing across your trail tread, it's going to flow right down it.


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## coopdad (Nov 6, 2010)

robbiexor said:


> _Make sense?
> _


_
Yes, perfect sense. Thanks all.
Many of the puzzle pieces of my self-educaiton in trail building are now falling into place.

Now I have to learn how to build a trail that is both fun for biking but not boring for hikers._


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## Walt Dizzy (Aug 18, 2003)

robbiexor said:


> _
> 
> The 10% rule refers to overall avg grade, this is mostly for usability. a long 10% uphill is going to be rough. Imagine you are building across a hill from point A to point B, and you want to gain elevation. You want ~5% grade on this trail to keep it easy. _


_

This, IMO, is an excellent way to look at trail grade.

I built a trail that has several climbs that meet the half rule, but are 10%-15% average grade. They are holding together well, but they are very tough climbs, especially when they are wet. One climb in the loop is in the range 5% to 7%. I like that one better than the steep ones.

I like riding the shallower slope downhill too. It's fast enough to put a big grin on my face, but slow enough (I hope) that a crash won't automatically mean a serious injury.

The 10% guideline may be mostly about sustainability, but I like it for how it rides too. Steep trails can be fun, but I'd rather have a longer, shallower DH section where I can stay off the brakes.

Walt_


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## robbiexor (Aug 22, 2011)

Walt, you're absolutely right! I don't think people realize how fast a Downhill trail will get at only -4% grade. You might have some ups and downs in there, but if you're overall grade is down, for thousands of feet, 4% will get you going FAST. Too many people associate dh/fr with pointing your bike down the fall line and going straight. (at least it seems that way sometimes). Ideally, you wouldn't need your brakes on a DH trail, as your speed would be controlled by whoops and rollers, and small climbs that momentum will carry you juuuuust up and over.


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## bweide (Dec 27, 2004)

My experience is, above 7% trail grade it gets harder and harder to design in grade dips. This is one of the reasons steep trails tend to end up with constructed dips or water bars. The designed in dips are important both to get water off the trail but also to allow breaks in long climbs or downhills. Riders can sprint up short sections of steep trail if they have a chance to recover on more moderate trail in between. A steep and continuous trail will force riders to walk because they cannot ride aerobically at that level of physical output.

The Half Rule defines the line between a contouring trail and fall line trail (10% Rule then helps determine how steep a contouring trail is acceptable). Following the Half Rule helps prevent fall line trails which are unsustainable. The Half Rule not only applies to steep terrain (max 12% trail grade on 24% cross slope) but also on gentle terrain (max 1% trail grade on 2% cross slope). Fall line trails are a bad idea on any terrain, steep or gentle. On gentle terrain the fall line trails just destroy themselves more slowly.


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## Fattirewilly (Dec 10, 2001)

bweide said:


> My experience is, above 7% trail grade it gets harder and harder to design in grade dips.


It's easy to think that once you're on a 20% side slope that you can build a 10% average slope climb but you shouldn't, and I don't think the current text out there does a good enough job explaining this. The half rule should guide "specific" grade, not "average" grade.

A 10% avg climb for 100 yards might look like this: 12% up for 20 yards, 5% down for 4 yards, 14 % up for 15 yards, 2% down for 6 yards, and so on and so on. Note the steeper the specific segment, the shorter it is. If you want 10% avg grade, you have to be steeper in the climbing parts in order to afford the real estate for the short downhills that create the "engineered" grade reversals.

Now, looking at the example above, to comply with the half rule, you'd need a 28% side slope to build the segment at the "specific" grade of 14%, not a 20% side slope. Does that make sense (more for the OP) but I used to have issues at 10% trying to make it "fit".


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## tyler243 (Oct 28, 2009)

bweide said:


> My experience is, above 7% trail grade it gets harder and harder to design in grade dips. .


i read somewhere that the crew that designed whistler (gravity logic i think) and the upcoming park at timberline found that about 7% is the highest you want to make you trail because it become unsustainable ie braking bumps and too many people skidding down the whole trail


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## atkinson (Sep 1, 2003)

Erosion is not the only factor, rideability is a huge part of the equation. As people have noted, 4-7% grade is really fun for all abilities, climbable and much easier to maintain. 

Erosion might not be a problem now, but add a bunch of traffic from the Cub Scouts and your 20%+ grades are probably going to show maintenance problems.


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## Guitarswheelies (Feb 20, 2005)

My .02 ....
Trails need to have a good feel and flow when they are done. You can build a trail that fits the IMBA guidelines wrt slope but it seems sterile and too linear with lousy turns and bad flow.

I think riders are more interested in feel and flow these days. How this applies to trail building is designing a trail that fits the contours of the land and takes advantage of dips and terrain features to make things interesting. Overall the slopes are mild but they might be steeper for short sections that I mentioned. Overall slope 2-5 percent.If you do it right you have have a good bit of speed going and then hit a dip or whatever and roll right through it. Lotsa fun. Different than the 80's and 90's fire road climbs.

Its helps to have variable terrain. Vermont seems to have a good bit of variation in the mountains and the trails there take advantage of it. Here in Pennsylvania the slopes are a bit more uniform. When the ridge is linear with a constant grade you have to be careful how you do the slope so it doesn't seem too mechanical and boring. One thing I did when designing a trail was to mark two large trees with tape that were maybe 200 feet about. These would represent the 10 percent slope reference. I would alter the trail so that it would move around but never go over the 10 percent line I marked out for very much if at all. .


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## coopdad (Nov 6, 2010)

I am learning so much from this thread! Keep posting please!

This past weekend I rode a trail that was clearly designed by someone that understood flow. It was a welcome surprise to finally feel and now understand what all the above discussions deal with. Now, highly inspired, I am taking what I read and translating it to our system. Much work to be done.
Thanks!


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## slocaus (Jul 21, 2005)

Yes, as stated earlier, the half "rule" and 10% "rule" are all about shedding water off the trail and therefore, sustainability. They should be used as guidelines and not hard and fast rules, but if you really understand them, you know when you can violate them. 

Laying out a trail for "flow" requires another layer of skill, achieved with lots of trial and error experience, and the "guidelines" and "flow" trails are not incompatible like so many believe. Some of the best trails I know are designed by a group that tries to use a 5% average grade. They are very flowy and you would never know they are designed with such a low grade in mind.

The best trails are mostly art, based on some good sound science.


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## 9.8m/s/s (Sep 26, 2005)

It doesn't look like anyone mentioned rock yet, so I'll add it. If you can incorporate rock sections, you can push the steepness much higher (rock won't erode, and it's fun to ride). If these sections can quickly drop you even 5-10 feet in elevation, it will be much easier for the rest of your trail to stay under the 5% guidelines.


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## sambs827 (Dec 8, 2008)

Listen to the text to attain sustainability. For fun and flow:

Step 1: Scout out your area and determine a general corridor.

Step 2: Go ride awesome, fun, ripping, sustainable trails around you. Pay attention to how they do stuff.

Step 3: Emulate what you see elsewhere when you build.


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## Skookum (Jan 17, 2005)

robbiexor said:


> Walt, you're absolutely right! I don't think people realize how fast a Downhill trail will get at only -4% grade. You might have some ups and downs in there, but if you're overall grade is down, for thousands of feet, 4% will get you going FAST. Too many people associate dh/fr with pointing your bike down the fall line and going straight. (at least it seems that way sometimes). Ideally, you wouldn't need your brakes on a DH trail, as your speed would be controlled by whoops and rollers, and small climbs that momentum will carry you juuuuust up and over.


Yah really many of the rollers, step downs etc in our build are oversized grade reversals. Practical in drainage, but of course practical in riding experience to jump or pump for speed.
Our line is less than a 2% sustained grade, and in this video (start at 1:53) i don't pedal except at the start. We are shooting for the highest standard in sustainability, and so far our drainage and flow (how riders ride and react) are super dialed, both integral factors to that end.
Skip forward to 1:53





And yes our trail does bend/break some of the rules, but to me rules and standards are there for a reason. i don't lend credence to learn anything from anyone who would be so stringent to the rule, but also anyone who completely disregards them either. So in many circumstances deliberate thought and pre-planning were given to overcome problems, and many times that manifests itself into alot of hard work. But we go by the notion if you understand enough to anticipate the problem instead of reacting to it after a year or 2 of heavy use, you're ahead of the curve.


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## robbiexor (Aug 22, 2011)

Skookum, that's awesome. In trying to pitch the idea of more progressive trails here, we face the popular opinion that they have to be down hill, when clearly, they don't need to be. I just might use your video! 

Do you experience any water problems in any of those corners?


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## Skookum (Jan 17, 2005)

robbiexor said:


> Skookum, that's awesome. In trying to pitch the idea of more progressive trails here, we face the popular opinion that they have to be down hill, when clearly, they don't need to be. I just might use your video!
> 
> Do you experience any water problems in any of those corners?


Cool, hope it helps.

No water issues at all, berm pinnacles are high and never low to catch water. It's all in the routing.


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