5

I'm trying to take my hybrid bike with slick hybrid tires down a short (maybe 200 foot) steep gravel descent with a gradient of around -25 percent (according to Strava). How should I ride this to reduce the risk of falling over? Is modulating the brakes around the limit of gravel (before it starts shifting around) the solution, or do I take the standard descent approach of letting the bike speed fluctuate?

3
  • 2
    There is always the option to walk down and not to ride it, however controlling the bike on steep sloop is not always trivial.
    – mattnz
    Commented Nov 12, 2022 at 19:46
  • 2
    @mattnz don't rule out tyres having better grip than shoes. I'd rather ride that on slicks than in road shoes, which is why they're in the wardrobe and I wear mountain/touring shoes. And I've certainly got off to walk after finding myself on ice, and had far better grip on my tyres than MTB shoes.
    – Chris H
    Commented Nov 13, 2022 at 21:59
  • How is the road after your descent? Are there any obstacles? If not, just YOLO it! Going at speed and touching the brakes will increase the chances of wheels locking and will do more harm than good Commented Nov 14, 2022 at 7:32

5 Answers 5

9

Going very slowly, modulating the brakes, using not only front brake but as much rear brake as you can is the best idea.

I'm assuming it's not -25 degrees (-47%) but rather -25%. That's because -47% would be outright dangerous if it's really gravel. Minus 25 degrees would require a friction coefficient of 0.5, or else you'll crash. With gravel that friction coefficient might not be achievable, at least if it's the type of loose gravel that could act as ball bearings between your tire and the road.

if you go very fast, then air resistance will help with your braking, but the problem is, to get any reasonable amount of air resistance you would be going at very significant speed. For example, -25% (as opposed to -25 degrees) and cyclist weight of 70 kg and bike weight of 15 kg requires braking of 21.25 kg at -25% road. If you take a really large posture, maybe your Cd*A is 0.57 square meters. To get even half of your braking (10.625 kg) from air resistance, then you need a speed of 63 km/h. But the trouble is, going at 63 km/h would be unsafe. If you suddenly notice that the gravel is getting looser and need to quickly stop, at 63 km/h that's impossible. You will crash. Hard.

However, if you go very slow, so slow that you can stop with your feet, you always have an option of digging into the loose gravel with your feet, and can stop without crashing even if the gravel is so loose that your brakes can't stop you.

So your only practical source of braking is your brakes, and your feet. Use the brakes primarily, both front and rear, while going so slowly that you can always stop with your feet.

Remember that if your front brake locks the wheel, you will crash, but if your rear brake locks, you won't. So use the rear brake at the limit of locking and the front brake somewhat below the limit (which is larger for front brake). You will need both brakes, however, at steep downhills using only the rear brake is a terrible idea because the front wheel has practically all of your weight.

2
  • 3
    Concur - gravel wouldn't stay in place on 47 % grade - the road would have been made of concrete. Even asphalt won't stay in place on that kind of slope. The world's steepest street is 35% at worst and had to be concrete. en.wikipedia.org/wiki/Baldwin_Street
    – Criggie
    Commented Nov 12, 2022 at 20:01
  • @juhist Thanks for the advice! Tried it out, and shifting the weight and using the rear brake more did the trick without feeling too dicey> Commented Nov 14, 2022 at 16:52
9

If you have concerns, there is absolutely no shame in getting off and walking down this descent.

There is no reason to ride anything that makes you feel unsafe, so just don't. Always an option.

2
  • 1
    Strongly agree that there's no shame to getting off the bike to avoid dangers. However, on smooth slopes I actually often find staying on the bike the safer option. Certainly when it's knobby tyres vs clipless shoes. I don't ride slick tyres, but with the small-profile tyres on my gravel bike I'd definitely prefer to ride a 25% gravel descent over walking, unless it leads straight to an abyss without safety rail. Commented Nov 13, 2022 at 22:31
  • 1
    While I agree that walking may be a perfectly viable option, I can think of at least one reason to ride something that feels unsafe - to overcome that fear and to become a better cyclist as a result. Naturally, the decision is still up to the rider.
    – user622505
    Commented Nov 14, 2022 at 2:34
6

In addition to juhist's answer, if it is a steep descent, then shifting your weight backwards by sliding back in the seat can help as well. This shifts a little more needed weight on the rear tire which will help with traction there.

Be conscious that your front tire will lose some weighting with this weight shift so modulate your front brakes accordingly as to not lock them up. A rolling wheel can steer. A locked up wheel cannot.

3
  • 3
    And unweight the saddle to avoid a little bounce breaking your grip. On poor surfaces I often descend almost as if I'm on a mountain bike, even on my tourer
    – Chris H
    Commented Nov 12, 2022 at 19:34
  • 1
    If it's a steep descent (at least on anything other than good tarmac) then the weight shouldn't be in the saddle at all. Not only is it more comfortable and safer to have the legs acting as suspension, it also helps with grip – unsprung mass makes the tyres bounce around instead of following the surface. And for really steep sections it's actually good to move your back behind the saddle to keep the center of mass really far rear and also low, which is always good for stability. Of course, a dropper post helps a lot with this, but even without one this holds. Commented Nov 13, 2022 at 22:38
  • @leftaroundabout I can agree with that. My answer is somewhat general. Moving back in the saddle does weight the rear tire more. Moving even further back in the saddle (like off the back of the saddle) weights the rear tire even more. Knowing that shifting one's weight to the rear tire is the common thought on this answer, and every slope, riding surface, tire, rider experience level comes into play as to what is the best position to be in. A rider who is not familiar with this dynamic needs to experiment with this to find the position they are most comfortable executing.
    – Ted Hohl
    Commented Nov 14, 2022 at 4:44
5

When looking at such an obsticle from a mountain biking perspective, we have some things to consider. Your hybrid bike should be more than capable of traversing this problem, no special modifications necessary.

While the slope is quite steep, it is not absurdly steep. Look at the descent and think about your exit plan. If it is straight down with a clear flat exit straight ahead, maybe you can go straight down with the right body position and even if you feel out of control, you will probably still end up where you want to go, maybe without using the brakes at all. The bike will shift around under you on a loose surface but everything likes to go straight down most of all so aiming for the bottom is relatively straightforward.

Your body position should be as far back as possible with your bottom hanging over the back of the saddle, the saddle dropped down if possible and your knees bent to absorb bumps. This puts plenty of traction over the rear wheel for braking and feels a more natural orientation for your body on a steep descent so you will feel more confident and in control. The front brake can be used only gently when descending like this.

If you consider there is a risk of losing control and the descent is technical or has a sharp turn at the bottom, it doesn't hurt to plan ahead and think where might be most comfortable to crash and looks like a possible escape route; maybe going back up again somewhere or into a softer sand or vegitation area, all of which slow you down.

1
  • 2
    Yes, though my crash a few years ago is a cautionary tale (slicks, wet gritty tarmac, brake trouble). I knew I couldn't make it round the next bend, with rocks and barbed wire on the outside so headed for the grass, trying to turn uphill or at least parallel to the contours. I did dump some speed but was still doing 24km when I side-slipped into a ditch.
    – Chris H
    Commented Nov 12, 2022 at 19:37
1

In addition to juhist’s answer: Lowering your tyre pressure will help a lot with grip and control.

Also, be careful when braking in turns. Keep in mind that you have a limited “budget” of grip. Turning and braking both eat into this budget.

It can be okay or even necessary to ease up on the brakes in some sections – gaining speed in the process – if you can see that the terrain will get better and that you’ll be able to reduce your speed again.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.