Low tire pressure causing fall in corner?

Question

I recently fell in a corner. My front wheel lost traction (slid out from underneath the bike, what they call a 'lowsider' in motorcycling iirc).

I was running quite low tire pressure (front wheel) to improve front wheel grip, which worked wonders to improve the grip when braking (front brake) in wet weather. But I was wondering if a front wheel tire pressure which is too low can cause one to fall in a corner.

Note: the tire did not roll off of the rim (the tire was still fully on the rim after the fall).

My front tire wasn't worn excessively and is quite new (a year old, plenty of thread left, Schwalbe big ben 26x2.15" folding tire (using inner tubes, so not tubeless). My rim's outer diameter is 26mm (if this makes any difference). My wheel was in good condition before the crash (properly tensioned, no cracks in rim). My bike had some luggage (on the rear rack) on it weighing approx 30kg, the bike with all the luggage weighs approx 45kg. my weight is 80kg. The bike is a hardtail mtb using 120mm travel rock shox reba suspension (in good condition), 26 inch wheels.

I wasn't braking (in the corner). It was dry (temp approx 15 Celcius, surface was tarmac with some loose pieces laying on top (not sure how much, I was distracted by my tacoed front wheel so didn't look very closely, but judging from the pieces of tarmac in my hand there was some loose stuff :P).

I was going approximately 25-30kph (a bit fast for the corner's radius). Here is an image of the corner:

Here's a picture taken standing on the road approaching the corner (sorry I couldn't find a better photo..):

Location of corner on google maps: https://goo.gl/maps/9x4xXEWvxYZk8TwE6

Questions:

• I would expect lower tire pressure to give larger contact patch also when cornering so should equate to more grip in corners (assuming the tire stays on the rim).
• Is this assumption wrong? Does the cornering grip decrease under a certain amount of pressure? (still assuming the tire stays on the rim under this low pressure)?
• if such low tire pressure is detrimental to cornering grip, what is the reason the cornering grip is decreased?
• what could i have done to prevent this accident ? (apart from taking the corner slower)?

tire thread condition:

Note: I just noticed from the last picture that can be seen above (the photo taken from the bridge deck approaching the corner) that the first part of the bridge has an incline of approx 5 degrees but just before the corner starts it appears as if the incline decreases or disappears completely (aka bridge deck becomes horizontal in the corner), which could perhaps be an explanation as to why I lost grip (front tire may have momentarily lifted due to the decrease in incline, which combined with the lean angle required for taking the corner caused the accident. What are your thoughts on this? It could be an optical illusion though, I did not pay much attention to the exact angles of the bridge after the incident occurred. could go back to check once I get a new wheel.

Any input is greatly appreciated. Thank you!

Wheel after crash:

(NOTE: wheel 'folding in half' was caused by front wheel hitting fence posts of the bridge after losing traction/front wheel sliding sideways, as can be seen from the two dents in the rim highlighted in red in the last photo).

Answer 1

I presume you were riding in the direction you came across the bridge and down the ramp.
I doubt tire pressure had anything to do with it. On a moutain bike sized tire, unless rolling tires of rims, too low tire pressure tends to cause imprecise handling and a general 'sloppiness' in the front end. "wiping out" as you did is not typically caused by low pressures. Check the rim for scrapes that would indicate you rolled the tire off the rim and had the rim hit the ground.

The weight on the back of the bike, and the change in angle that coincides with the corner probably unloaded the weight off the front wheel. The amount of friction you get is directly proportional to weigh, so the wheel 'wiped out' from under you because it was not far off hanging in mid air.

When learning to mountain bike one of the first skills you need to learn is to load the shock before the corner and maintain pressure on the front wheel though the corner, letting the shock do its job of maintaining wheel contact with the ground. In this case if you had loaded the front shock, the change in angle coming onto the ramp would have be absorbed by the shock as it extended to maintain wheel contact pressure with the ground.

Things that could contributed to the crash are rider position - too much weight too far back into and though the corner. This means not enough weight on the front wheel to maintain traction and not enough preload on the fork for it to hold the front wheel on the ground. The solution to this problem is enter the attack position, move body weight forward (not lean) to load the front fork with plenty of weight to hold the front wheel down. With 30kg on the rear, there is very little risk of putting so mach weight too far forward you go over the bars. Just how far forward you can go is much further than most people think, and is very unnerving for a novice until they have practiced doing it many times.

The other thing that could easily contribute to the crash is the fork settings - damping, rebound and air pressure are all available on the Reba. Getting this set correctly will make a big difference to hold the front wheel on the ground. With these set correctly, maintain a good weight distribution with you body well forward becomes easier as you learn to trust the fork to work for you. With these set wrong, the fork can feel like its working against you and you are less included to load it.

All that said, the speed you went though the corner with that bike setup and weight was fairly ballsy, and the outcome not entirely unpredictable. If you don't want to crash, you may need to slow down.

Answer 2

• I would expect lower tire pressure to give larger contact patch also when cornering so should equate to more grip in corners (assuming the tire stays on the rim).

As correctly pointed out in another answer, increasing the contact patch does not increase optimal grip.

However, a larger contact patch does give you a form of redundancy: if part of the patch is temporarily disrupted by gravel, oil or whatever, there's more chance that some of your contact is still good.

On rough, rather than loose, surfaces, low pressure can also help your contact patch conform better to the shape of the terrain. Supple, slick(ish) tyres will generally do this better than heavy tyres with a deep tread (although of course they're much worse in mud or snow).

• Is this assumption wrong?

Sort of, as above.

... Does the cornering grip decrease under a certain amount of pressure? (still assuming the tire stays on the rim under this low pressure)?

Sort of. Technically the tyre might still grip the road perfectly well, while being too loosely-coupled to the rim for this to be helpful.

• if such low tire pressure is detrimental to cornering grip, what is the reason the cornering grip is decreased?

If the tyre squirms: on a steep enough turn, it can theoretically deform sideways enough that you're riding on the edge of the rim, without actually coming unseated. That's the (unlikely) limiting case, but you can see how too much sideways flexibility is bad.

• what could i have done to prevent this accident ? (apart from taking the corner slower)?

Read the corner for loose surfaces, slippery surfaces or reverse camber before committing to a line, if possible. If you're braking on the (wide) approach line before the corner, you can scrub off a bit more speed if necessary.

With rigid forks, you have a chance even if your front wheel starts to slide - that is, if it starts feeling squirmy long enough before losing traction that you can notice and react - you can sometimes unweight it, pull it back (inwards) onto a track where it will be closer to your centre of gravity, and then re-weight it. This is unlikely to work with suspension forks though.

Shifting more weight over the front wheel in the first place can help with traction, although not squirm. Using front lowriders for some of your 30Kg of luggage should be possible.

Answer 3

Taking the information in both the original question, and the comments on other answers, there are several possible reasons for the crash.

1. Too fast!!!
2. Loose gravel on corner
3. Unweighted front wheel
4. Tyre pressure MUCH too low (17psi).

I'll focus on point 4 since that's what the question was about.

'Correct' tyre pressure very much needs to be determined on a case by case basis by trial and error, but there is enough data out there to pick a typical pressure.

The factors that effect the correct pressure, will be the tyre/rim width, total tyre volume, rider/bike weight and tyre construction (sidewall stiffness plays a big part).

Most data on low tyre pressures will come from the MTB community, as they are the ones typically running larger volume tyres, and see biggest advantages from running low pressures.

As the pressure reduces, the tyre deforms more, and when under load in corners this can cause a number of undesirable effects (folding, burping or rolling off rim).

A 'typical' front tyre pressure in a tubeless setup for a 75kg rider on a 29x2.25 XC tyre (closest equivalent to what the OP is using) would be around 25psi.

However, the OP has smaller volume tyres, weighs more, and is carrying a load on the bike, so this value would definitely need to increase.

My personal advice would be that for a 26x2.15" tyre with a rider + bike weight of 125kg is that I would not go below 35psi in the front.

Answer 4

Glad to see that only the front wheel got destroyed and you seem quite fine!

The higher pressure the higher toughness of the tyre and vice versa.

For braking/traction lower pressure it is benefical because the tyre follows the surface better than the higer pressurized one and the angular toughness is high even fot flat tyre.

For turning you need tyre to transfer lateral forces, which changes the game significantly. Here the higher pressure is benefical because softer tyre deforms much easier compared to harder one.

Try making shake the handlebars while riding slowly on tarmac. Going from highest pressure the tyre can withstand to flat tyre you will feel:

• Super hard: Bike strictly follows the direction of the stem, you almost can feel all the sand under the tyre.
• Hard: Bike follows the direction of the stem, ride is smoother.
• Soft: You can feel a small delay between changing the stem direction and actual change of ride direction. Ride is very smooth.
• Softer: You can feel significant delay between steering and actual ride direction.
• Even softer: It is easy to find shaking frequency where even high amplitudes results in little to no change in direction. Rear wheel seems to be floating somehow.
• Almost flat tyre: You can feel whether the rim is in the middle of the tyre (soft, nonresponsible) or is leaning towards the tyre wall (harder, pulls your handlebars in or out of turn). You may touch the groun with the rim easily. Braking and accelerating may break the tube valve.
• Flat tyre: You can feel avery single bump in the surface of the tarmac, every single thing on the tarmac. It is very hard to steer at all.

I think that you were too fast in the corner so your tyre folded aside therefore wasn't able to support the trajectory you wanted to go. It also lost traction, because thread was not in the contact but ste wall was, so it slipped - worsening your turning abbility further.

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One of the information on the tyre wall is minimal recommended inflation. Below this pressure the tyre is too easy to fold in turn (changing the traction behaviour abruptly) or hit the surface with the rim (punching up to four holes in the tube, losing the rest of the air and changing the traction behaviour abruptly). Keep te pressure safely above this value. The faster you go, the sharper you turn, the higher minimal pressure is reasonable for you.

I've also learned that "slow in - fast out" works even for bikes. If you want to take corner faster, lern going it slower and speed slowly up and feel the behaviour of the bike. If it starts wobbling, slow down a bit - you are close to the limit.

Answer 5

I think the main culprit (apart from the speed and corner) is the heavy luggage on the rear wheel. On bicycles which are not intended for luggage on a rear rack it’s sometimes necessary to mount the rack very far backwards to avoid heel strikes. In severe cases this can even put the luggage’s center of gravity behind the rear wheel. Personally I’ve had two very surprising crashes with only ~15kg luggage on the rear wheel. For best handling characteristics you should usually try to distribute the weight evenly across both wheels.

You should also pick your path through the corner properly. At a constant speed your path should be a circular arc with the widest radius possible.

Low tire pressure helps on rough surfaces. On perfectly smooth surfaces it doesn’t make a difference at all, as pointed out by Sam.

Answer 6

How low, exactly?

You're right, low pressure increases the size of the contact patch, which increases grip. If the pressure is so low that the tire is flat and the rim is riding on the ground, then this rule ceases to apply. But if that was the case, you would probably have noticed it earlier.

The cause was probably either the change in incline, or the loose gravel, or both - nothing to do with your tires.

Answer 7

If the tire pressure is low enough then the rim can contact the pavement.

Riding aluminum or carbon on pavement (concrete, etc) is like riding on ice. There is virtually no grip. Been there and done that. In my case and that of a couple friends who I also witnessed, the cause was due to a slow leak from a thorn picked up during the ride.

As previously mentioned, too low of pressure in a tubeless tire can lead to the tire burping and loosing much of the air in a corner. A tubed tire will simply have the tube explode (I've seen this happen too many times in criterium races)

A bicycle with very soft tires will tend to wallow (flex) in corners. This is not a reduction in grip so much as a loss of precision in steering. One of the methods for determining optimal tire pressure for gravel racing is to start at a high pressure and do laps of a tight course. Every lap or two drop the pressure by 2 to 4 psi. At the point the bike feels like it is losing steering accuracy you have passed the optimum pressure. Cyclocross racers typically go for the absolute minimum pressure which will prevent burping. Lower pressure increases grip from the tire's lugs - this is not to be confused with basic "friction" which remains unchanged.

Answer 8

Q: Does a larger contact patch increase "grip"?

A: No.

The "grip" one object has on another is a function of the coefficient of friction, and the object's weight. Let's look at an example to better understand this....

Imagine a wooden box filled with 10 bricks sitting on the pavement. Pushing this box along the pavement requires a certain amount of "force".

Now imagine 2 more wooden boxes of the same size, each filled with 5 bricks. These boxes each weigh half as much as the first box, but the total surface area of the boxes in contact with the pavement has doubled. Pushing both these boxes at the same time requires a certain amount of "force".

Which would it be easier to push? The 1 box with 10 bricks, or the 2 boxes with 5 bricks?

Physics and common sense tells us they will require the same force.

Back to the tire question. The larger your contact patch, the more your weight is spread out over the patch, but your "total grip" stays the same. This is how cyclists descending down Alpe d'Huez can whip around the corners at high speeds while riding on 23c tires at 80psi (5.5bar).

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Edit: I see that some people still do not find my explanation convincing. Here are some questions from Physics SE which back up my answer:

• Why is the equation for friction so simple?
• How does friction not depend on surface area?
• Does the contact area affect friction forces?
• Dependence of friction on area