I can’t remember how/when this popped into my mind, I think I was on a particularly wet/muddy ride (so any time in the last 4 months…), but I had a thought. I know, be careful. But bear with me.

Does more speed give you more grip?

Imagine you’re standing on a slippery muddy slope in shoes. If you stand there without moving your feet, you’ll eventually start sliding down the slope.

If you pick up your feet and start walking on the spot (or just walking, if on an off-camber slope), the time each foot has to lose grip and start sliding lessens.

So, if you were to run really really fast on the spot or moving forwards, putting the same pressure on each foot stroke as if you were standing still, you’d theorectically never lose grip, right?

Right?

So, if we take every other factor to be the same (tyre, contact patch etc), if you consider the knobs of an MTB tyre to be invidual ‘feet’ digging into the mud or dirt, the more time that knob spends on the ground, the more likely it is to lose traction and start to slip. So, the faster you ride, the less time each knob spends in contact with the ground and more grip you will have.

Now, obvously the time that each knob spends on the ground at 10mph vs 20mph is probably milliseconds difference, but I still think I’m right, right? Or not?

It’s more that if you’re using your brakes to resist gravity you’re using up grip that you could be using for turning, whilst also reducing the effectiveness of your suspension.

And if you’re going fast it’s easier to pump for grip, so you unweight and then load the tyres as you turn – it’s like how downforce adds grip to a racing car but you only get it for moments in between unweighted moments.

And finally, a more quickly turning wheel sheds mud better.

Wrong.

You exert more force starting to “jump up” on each up-step than static standing, you’ll slip and gall on your arse.

Next!

Imagine you’re standing on a slippery muddy slope in shoes. If you stand there without moving your feet, you’ll eventually start sliding down the slope.

Nope. If the force provided by the friction of your shoe soles on the mud is greater than the force exerted by gravity, you’ll stay there indefinitely.

So, if you were to run really really fast on the spot or moving forwards, putting the same pressure on each foot stroke as if you were standing still, you’d theorectically never lose grip, right?

No, because if you are moving fast each footfall will need more grip because it will have to resist more acceleration from your foot. This is why it’s easier to walk gingerly on ice than it is to sprint flat out…

So, the faster you ride, the less time each knob spends in contact with the ground

No, because whether or not a knob loses traction is not a function of time, it’s a function of the amount of force going through it. If, however, you were riding over a hot surface, then the amount of heat transmitted to your knob (fnarr) IS a function of time, so riding faster would decrease the amount of heat in your tyre. But for traction it’s not the same.

I’m right, right? Or not?

No, sorry 🙂 The faster you are going the higher the forces involved all round. The mud can only provide so much force to resist the forces applied by your tyre, going faster will make you skid more, not less.

And intuitively, this is nonsense. Stand on a slippery slope and think about trying to get to the top. If you start sprinting flat out you’ll fall on your face. The only way is to inch up carefully, we all know this.

I think you fell run? I’m pretty sure you do. And there’s a thing that happens when you go out with slower people than you…

When people are cautious when they’re running, they try and slow down. And that gives the grip on your shoes “too much to do”. The shoes can’t keep you upright whilst also giving “braking force”. And so the people going slow, they slip and slide and fall over whilst the fast people stay upright as they just go with it, staying upright because they aren’t slowing down.

It’s much like when you’re driving hard, and taught not to brake hard or accelerate hard when you’re also cornering hard, as the tyres only have a certain amount of grip to give. Brake or accelerate, or even lift off mid corner and things happen.

Unless you want to impart fun grip-free traits.

So you’re right, but only in certain situations, and only if you’re good and fast.

It’s not speed. It’s a lack of braking.

It’s certainly not “nonsense”.

See, I was thinking that traction/grip (I always get them confused) is the friction between 2 objects, if the force generated is greated than the friction, a loss of grip happens. But, the longer something has to grip onto a surface, eventually it’ll lose grip. I may have been adding an extra element in there, that’s not a factor, as yeah if you clamped 2 things together, the friction would be the same over years and just the passage of time wouldn’t cause the friction to lessen.

Ok, how about when a shoe/tyre knob has exceeded the available grip/traction, and has started to slide? So the cornering/braking/accelerating force is greater than the friction, and so it starts to slide. If the feet/knobs spend a long time in contact with the thing they’ve lose grip with, they’ll have more time to slide, meaning sliding further. Less time in contact, less time to slide (given the same amount of (lack of) grip), and what you’d see on an MTB tyre, say cornering in a flat corner is each individual knob sliding slightly less the faster you go. Ignoring that the faster you go the more force is being generated and the more the knobs will slide… Oh, balls. But still, faster, and no more or less sliding?

No? Okay. 🙁

Added complication could be if mud is thixotropic, like ketchup.

Added complication could be if mud is thixotropic, like ketchup.

You say tomato, i say tomato, you say thixotropic i say non Newtonian.

Could be shear thickening or time dependant in this case.

When people are cautious when they’re running, they try and slow down. And that gives the grip on your shoes “too much to do”. The shoes can’t keep you upright whilst also giving “braking force”. And so the people going slow, they slip and slide and fall over whilst the fast people stay upright as they just go with it, staying upright because they aren’t slowing down.

You are talking about running downhill? Your shoes have to decelerate you. The slower you want to go, the more deceleration force you need so the more likely you are to slip. Still holds.

No? Okay.

Get on your bike, give it a try!

There seem to be so many muddled presumptions in your thought experiment, that I wouldn’t know where to start. And I’m sure there are people far better informed to explain why you are likely wrong. But one thing I do know is that, ‘For a given pair of objects static friction is most of the time greater than sliding friction’.

Ps that is not meant to sound as rude as it maybe does. Sorry, if so.

No offence taken, it was a random thought I had while riding along, not a scientific theory I’d spent years on 😁

Phew, good 😁. One thing I love about cycling is that it encourages pondering.

But, the longer something has to grip onto a surface, eventually it’ll lose grip.

Maybe you’re thinking of adhesives, not friction? It’s certainly not true of seatposts in seat tubes.

If you’re standing on a wet clay slope, it might eventually slip, but that’s the only occasion I can think of, and that’s a matter of days, not the sort of time interval involved in tyre contact.

The faster you’re going, the more force it takes to stop or change direction, and the more stress that puts on the soil. But if all you want to do go straight, you don’t need grip – hence “speed is your friend”.

Does more speed give you more grip?

Yes*, but not for the reasons you’re citing. (*Traction, rather than grip, and it depends on the mud.)

At some point, the amount of material being fired backwards by your spinning feet \ tyres provides significant thrust. You’d need good legs though.

(In your case, no, because static friction is usually>dynamic friction as above.)

Passing over slippery roots more quickly is definitely an advantage. The less time on them the less opportunity to slide. So speed often helps I’m sure.

Passing over slippery roots more quickly is definitely an advantage. The less time on them the less opportunity to slide.

Going over roots quickly is better yes but not because there’s ‘less opportunity to slide’ on any given root. It’s because you have more forward momentum, and if it does slide then the sliding event won’t be for as long because roots are fundamentally small things and there’s usually a bit of soil in between and the next door might slope a different way.

If you had a whole load of roots all lined up together with no gaps sloping the same direction, going faster, then more speed would just put you on the deck faster.

Isn’t it the case that the force required to initiate a slide is far greater than the force required for that slide to continue, a very noticable thing with fast cars on a track with all the wizardry turned off – it’ll grip and grip then you’ll have to be very gentle with her when she goes as much less force is required for the slide to carry on. Am no physicist but do I recall this is something to do with the effects of rubber hysterisis?

Also in a completely different take what you’re saying is a bit like when you watch a fast fell runner going down a scree slope, there’s a fast feet thing going on minimising the contact with the ground and preventing the stones sliding underneath as if before the stones get a chance to move the feet are in the air again – whereas to plod down would send all the stones sliding.

is it not a confusion between stability and grip?

the grip is a finite number. The old saying i was taught was you have 10p worth of grip. You can spend that in different ways. A bit on cornering or braking but the more you spend on one the less you have for another.

The other is stability which can increase with speed as your momentum is greater to carry you over things. like the root?

Pretty much as chiefgrooveguru said.

Less braking = less for tyres (or feet) to do = more grip.

Faster spinning wheels (or running) = more mud shed from the tread = more grip.

Also, I would add that you can go at the wrong speed over roots and rocks, even if you’re sat like a sack of spuds and not actively floating over the more slippy bits. IF you go faster and hit something that your wheel slides on (an off camber root or rock), you are on it for less time, so it destabilizes you less. Of course, there comes a point when you’re getting bounced from rock to root with no grip to keep you upright or you need to turn, and you fall.

I would also contend that belief plays a big part in grip in muddy/wet conditions 🙂

Isn’t it the case that the force required to initiate a slide is far greater than the force required for that slide to continue

Yes, this is the same thing that’s been said in a different form: ‘For a given pair of objects static friction is most of the time greater than sliding friction’. Static friction is when your tyres are hooked up, dynamic friction is when they’re not.

Also in a completely different take what you’re saying is a bit like when you watch a fast fell runner going down a scree slope, there’s a fast feet thing going on minimising the contact with the ground and preventing the stones sliding underneath as if before the stones get a chance to move the feet are in the air again – whereas to plod down would send all the stones sliding.

This is because when you are going down, gravity wants to accelerate you at 9.81 m/s per second. This is a bit quicker than most of us would like so we need to retard our progress. The slower you go, the more retarding you need to do each step, which is why the stones get sent sliding (this is a bit of a simplification mind).

Also, I would add that you can go at the wrong speed over roots and rocks, even if you’re sat like a sack of spuds and not actively floating over the more slippy bits.

This is getting to be a pretty complicated physical model. In practice, the trick is to give your tyres the minimum work to do. So don’t try and steer on slippery surfaces – just keep as close as possible to a ballistic trajectory i.e. the line an object would fall or a ball would roll naturally down the hill.

As an example, there’s a narrow gulley/path in my local woods (Castell Coch, you’ll know it if you’re from Cardiff). The bottom of it is rocky and full of leaves, so you need to ride on the side of the gulley. But it’s slippery, so if you try and ride a straight line traversing the side of the gulley you’re fighting gravity all the way and you’ll slip down sideways. So the technique is to drop in then roll up the other side, then cross the middle and roll up the other side. This way you aren’t fighting gravity at all so you won’t slip. I call it Pinball Alley for this reason.

However this isn’t related to speed, really. If you go faster then the wavelength of your oscillations across the gulley will just be longer 🙂

Am intrigued with this. So are we saying that to ride/ run down a slippery surface there may be an optimum speed at which grip is maximised, and that would be very close to a speed of 9.81m/s (vert speed or trajectory speed?) and that anything below this speed lessens grip due to a braking force being required….edited…realised this is nonsense…

Is this how downhillers make it look so easy…just go faster?

As an example, there’s a narrow gulley/path in my local woods (Castell Coch, you’ll know it if you’re from Cardiff). The bottom of it is rocky and full of leaves, so you need to ride on the side of the gulley. But it’s slippery, so if you try and ride a straight line traversing the side of the gulley you’re fighting gravity all the way and you’ll slip down sideways. So the technique is to drop in then roll up the other side, then cross the middle and roll up the other side. This way you aren’t fighting gravity at all so you won’t slip. I call it Pinball Alley for this reason.

This is another factor – there’s a tree down on a pretty wide path near me, you need to hold your speed and just bank it in to get around it using the lie of the land on the side where there’s a gap. Go slow and you might make it but it’s much harder.

So are we saying that to ride/ run down a slippery surface there may be an optimum speed at which grip is maximised, and that would be very close to a speed of 9.81m/s (vert speed or trajectory speed?)

It’s not speed, it’s acceleration (m/s per second). That’s the rate at which an object accelerates downwards due to gravity. That defines a ballistic trajectory.

Isn’t this similar to the theory that jumping is safe because no-one ever crashes in the air.
It’s landing that causes the issues

Well, falling isn’t the thing that hurts. Hitting the ground does.

Not sure how that relates though! 😁

Isn’t this similar to the theory that jumping is safe because no-one ever crashes in the air.
It’s landing that causes the issues

Yes and no. It’s not about not braking, it’s about braking in appropriate places, which are usually the bits you don’t feel like you need to brake on.

So are we saying that to ride/ run down a slippery surface there may be an optimum speed at which grip is maximised

Well, I said it was simplified.. according to the simple Newtonian physics model, if your weight is 700N retarding force from each step is 650N, you’d continue to accelerate down the hill indefinitely anyway. Having thought about it a bit, I think the deceleration forces both in the upwards direction to stop you crumpling to the floor and the backwards direction to stop you accelerating down the hill are the same regardless of speed. I think the fact your feet dig in and move the stones when going slowly and not when going fast is due to how your foot interacts with the stones.

Similar to the way water can be like concrete if you hit it fast enough, I think the slow placement of a foot on scree allows the forces to shovel stones out of the way, whereas a quicker foot-fall does not. Like one of those lizards that run on water.

Which, after all that, is kind of what the OP was saying except I don’t think it apples to bike wheels on gravel because tyres aren’t hitting the ground in the same way. The fact your tyre rolls onto the gravel means it’ll always be pushing it around not hitting it. Just maybe though it might actually work on deep gravel if you went really fast.. any rally drivers want to weigh in here?

I have been able to ‘hydroplane’ in a foot or more of snow (on a normal bike) by riding really fast so that my tyres are levitated about 6″ above where they would be if I were stationary. Flipping hard work though! And there was almost no lateral grip.

I suppose that while a lot will be known about calculating grip forces for something like a racecar on a flat track, once we throw in the effects of steep gradient combined with on a top surface with a hard to calculate adhesion to whats below it – then we have a very complicated model.

Even if more speed did equal more grip it would surely be irrelevant anyway as you don’t need that much grip to fall down a mountain in straight line. Once you get to the point where you want to change the trajectory by making a turn then the effect on the grip requirement to do so will be overwhelmed by the effects on energy of the increase  in velocity…

Yes, and far more grip is also often available by using trail features. E.g. the first corner on a fast local descent you can drift your nuts off into it, there’s a small bank on the outside which acts as a berm and points you in the right direction regardless of grip.

This video is nice:

ah well, surely centrifugal force from higher speeds throws the tyre into the ground harder resulting in greater traction/grip!

….then also, more speed creates more gyroscopic force in the wheel which results in keeping the bike more stable on its trajectory does it not?, like a spinning top the faster it goes the harder it is to topple which makes the bike harder to turn? Lots of physics involved with riding a bike down a hill…think i’ll stick to not thinking about it ‘n’ just doing it…