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I have an average cross-country bike (Cube AIM), with stock parts on it. I have no problems descending at high speeds on a rocky road/singletrack. But when I have a long, steep descend with big roots (10cm in diameter) hidden partially in the ground, I feel like my bike is going to fall apart under me and I begin to brake more than I would like to, because I'm scared.

On such rooty (don't know if it's the right word) descend the front fork is working really hard, hitting the travel limit all the time. The chain is clanking furiously over the roots. The overall jitter is so strong that I can't sometimes keep my feet on the pedals.

Is it safe to drive on a hardtail CC bike in such conditions? If not, what kind of bike should I use for it?

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    From what I can tell, the Cube AIM is actually an entry-level MTB. Entry-level bikes are typically the same frame as more expensive versions of the same bike, but with lower quality parts. It looks like the fork on that bike is one of the places where they saved a lot of money. I didn't really dig too deeply, but that fork may not be designed for the type of riding that you're describing. – jimchristie Sep 19 '14 at 18:14
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    Safe is relative. You could hook a root, go head-over, and end up riding a wheelchair. – Daniel R Hicks Sep 19 '14 at 20:59
  • @Daniel - I don't think the bike makes it more or less safe - with a better bike the rider goes faster, and where I live, the road police are always telling us "The faster you go, the bigger the mess" – mattnz Sep 20 '14 at 8:40
  • Unless you have a death wish the bike will out last your willingness to be scared. You'll stop before it stops you even with basic stock parts. In recent seasons many World Cup xc courses have featured roots and rock gardens. Hardtails with steep headtubes (as many people refer below) are the origin of mountain biking and will do most things. Other bikes such as FS trail bikes and DH bikes is improved control and therefore speed. – DWGKNZ Sep 21 '14 at 10:44
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The scenario described is safe a certain number of times, however, if this activity is repeated enough something will fatigue and break or wear out.

The fact that you can travel such a rock/root section at speed with a given bike doesn't mean you should. A Cross Country bike is normally optimized for weight and have a geometry that is more oriented to be efficient when pedalling. This is important because of the fork angle and head tube structure. Your bike has a "more upright" angle compared to a Downhill bike. When the wheel hits a root, the impact is very close to perpendicular to head tube, putting a lot of stress on the crown, particularly the crown-steerer joint, and on the headtube, specifically the joints with the top tube and the down tube.

The bike can survive a number of abuses, but eventually material fatigue will kick in and one of the joints will develop a crack. Once this happens the piece is lost, the crack will expand progressively more with each hit (even the smaller ones) until catastrophic failure.

A DH bike in contrast will have a less steep fork angle, which in part allows it to absorb (a bigger) part of the horizontal impact through it's travel, and this impact won't be as close to perpendicular to the angle of the head tube. Both differences help reduce the load the parts have to bear with. Add to this that a DH bike is reinforced in key parts, including joints (they are beefier, thus heavier) and will usually have a dual crown fork.

An All-mountain bike would be somewhere in-between and would be better suited for root sections like the ones you describe, because they aren't as heavy as a DH bike and are still designed for better pedalling than a DH bike; they are heavier than XC bikes though.

If in your habitual routes there is only a few root sections, the you may be better simply slowing down before the beginning of the section, and traverse at a controlled speed at which you feel confident on both your technique and the bike's endurance.

You can also improve your technique. I suspect you are already doing this, but for other riders that get here: Position yourself centred on the bike, stand up on the pedals but keeping the knees bent (i.e. just lift your butt four-six inches from the saddle, do not fully extend legs), pedals at the same height, Arms bent with elbows pointing outwards. This position allows you to complement the suspension, effectively using arms and legs as a suspension. You must have a firm grip but not be tense.

Another point is: If you are braking when you hit a bump, you increase the load on the parts. It is better to release the brake and apply it when the tire is on a more even patch of terrain. Of course this is not possible if the roots are too close one another, that's why I recommend regulating the speed before the root section, so you keep brake usage to a minimum on the most critical segment.

No suspension is capable of doing ALL the work, you've got to help it, think of yourself as a form of "active smart" suspension. This means for example that when you know you are going to hit a root, you prepare your arms and try to move the wheel up just before the hit, and then down to the ground again after reaching the top of that root. The same can be done with the legs. This is an ability that takes time but is really useful for riding bikes with little or no suspension on rough terrains.

However, there are combinations of terrain and speed that simply exceed a bike's designed capacity. If possible, upgrade to a full suspension XC bike or an All-mountain one. You'll find yourself riding more confidently on more technically challenging trails.

Suspension travel used as you note, is a good measure, assuming the suspension is well adjusted and your technique is right. 10cm (100 mm, 4 inch) is a good travel for an XC fork, thus a 10 cm root completely out of the ground is practically the most it can take without help from the rider. An AM bike should have a fork travel in the 130mm-170mm range.

In my case: I'm an XC and DH rider, and have a hardtail, a full-suspension and a DH bike. I can ride any of the DH tracks I know even with the hardtail bike, (except jumps and drops) but to do it safely, I have to keep a moderate speed on technical sections. The first signal I get when riding to hard on a too light bike are the pinch flats, (and I ride them to 40psi minimum) so at that point I know I am close to the bike's limits.

If you are interested, in this answer I cover more of the technique for descending faster and more confidendly: What skills/drills should I learn to descend faster and more confidently? (for mountain biking)

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It's safe as long as you use your body properly. Learn about:

  1. proper neutral posture while descending:
    • light hands, heavy feet (stay above BB, mostly with your feet)
    • don't sit
    • knees bent
    • elbows out
    • back straight
    • look ahead (read the terrain), not on your front wheel, nor the obstacle itself for too much time
  2. dealing with obstacles (roots, rocks etc):
    • choose a good line, since you are already looking ahead and not down
    • compress/weight before an obstacle
    • uncompress/unweight while hitting the obstacle

If you've never heard, thought or done any of the above before then give it a try. It'll improve your safety, confidence and handling on the bike. If you've been riding long enough not doing the above then it may be difficult to adjust. But it'll eventually happen. It needs practice and it'll not come overnight.

If you find this stuff interesting and you are really interested in improving your technique I suggest finding a skills lesson/course to take and also reading http://www.amazon.com/Mastering-Mountain-Bike-Skills-Edition-ebook/dp/B004D2BD1Q

  • +1 - Can you expand on this - other answers are talking about fixing the bike, yet it is almost certain the bike is more than capable of being ridden very fast, safely, by a competent and skilled rider. – mattnz Sep 20 '14 at 8:44
  • @mattnz: updated my answer. – cherouvim Sep 21 '14 at 6:39
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If your fork is regularly bottoming out, you need to either increase the spring rate or modify the loading.

In the latter case, achieve this by moving your centre of mass rearwards, reducing your speed or modifying your route such that your inertia has a greater component away from the surface... use the first root to jump over, or "float" over, the subsequent roots, depending how much surface-normal inertia you can handle.

Most modern forks contain hard elastomer or plastic bumpers to absorb the force experienced when the fork exhausts its available travel, but these bumpers can fail under extreme loads or repeated heavy loads.

As for the chain-slap, rear mechs containing clutches such as Shimano's Shadow+ models can help reduce the motion. (Note, Shadow models, without the plus, have no clutch mechanism)

  • tl; dr: It's probably safe in the short term but you should modify your technique or fork so it doesn't happen frequently. – Emyr Sep 19 '14 at 11:56
  • A chain guide is another option. – Vorac Sep 19 '14 at 12:12
  • Although at the price level of the bike, a chain guide will be too expensive. Also, the fork will soon (within a couple of months) develop movement in the stanchions, relative to the fork legs. This is normal for Suntour XCM. – Vorac Sep 19 '14 at 12:41
  • None of the the mechanical issues you discuss are safety - the shocks won't fail catastrophically under this abuse. Chain slap is nothing to worry about (wrt safety) unless its causing you to drop chains (usually something else is wrong in that case). – mattnz Sep 20 '14 at 8:47

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